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Cadillac 500 DD Airboat

Greetings from Oregon! Being an Airboat owner in a State that may have but a dozen or so in its entirety, finding answers to Airboating questions is virtually non-existent. Finding this Forum looks to be a God Send. Anyway, 4 years ago I purchased a 2005 Rivermaster hull powered with a DD Caddy motor. It is utilizing the 72" three blade Whirlwind Super snapper prop, seats two rear (including operator) and 3 below. Pancake cage, 3/8" poly bottom, 15' 3" plus grass rake and a 7' 3" beam. Fiberglass hull. The engine had been overheated and needed to be rebuilt. Should have done my due diligence at that point, but greed/excitement won the day (price was $5000). Sooo, I rebuilt the engine (.040 over), but the kit I purchased came with a stock cam (due diligence should have started at this point.....). So now the problems/questions. At first, I had overheating issues, but finally solved that problem by adding an overflow tank that sat ABOVE the highest engine coolant level. Ran great, but would always hesitate when stomping on the throttle, thus taking getting up on a plane a bit longer and making steering at slow speeds iffy at best. Started replacing things. First, replaced the Carb. (original Rochester 4brl had been replaced with a Rochester 4brl for a 350 chevy). No difference. Went crazy and had the remanufactured Rochester I'd just received rejetted by a professional carb guy, also had him test the new HEI distributor for correctness, and while this was being done, went ahead and upgraded the rockers and installed the aluminum intake manifold from the CAD Company. Now the hesitation is worse, AND, it has a second hesitation at 2000rpm.
All this upgrading started because while cruising along merrily on our local Reservoir, alongside a biking path only 30' away, I was being PASSED by a guy on a bike! Holy Crap! So pulled out my phone and GPS'd my speed....32mph! (turning 2750rpm). Naturally, can't be slower than a bike, so had to do something about it. Now I don't know where to turn for advice. Instead of throwing 1000's of dollars at this engine, I should have moved on to something newer and stronger. Too late now. I've been in contact with Buddy Branch's widow, and she still has some of his custom cams for the 500, and even one of his last 800cfm thunder carbs. Would this be chasing the uncatchable dream? If I could lose the hesitation AND reach 50mph I'd be happy. Any ideas, thoughts, or comments will be greatly appreciated!
 

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Slidin Gator

Well-known member
I was being PASSED by a guy on a bike! Holy Crap! So pulled out my phone and GPS'd my speed....32mph! (turning 2750rpm).
So a few things.

#1, The guy on the bike was riding an Ebike, he wasn't doing any work! Yep, them things are cool, if bikes can run where you run, best you sell the boat and get an Ebike, it will save you plenty of money, time and agrivation.

#2, you were turning 2,750 RPM's at that point and it does not sound like the boat is up on a plane. What RPM do you get WOT on the trailer? 3,000 RPM is pushing it.

#3, how much does that boat weigh? 15+ is a big boat and there is only so much a direct drive can do. It sounds like some setup or weight distribution issues might be in play here.

#4, Swamp can take it from here.
 

John Fenner

Well-known member
Ok, firstly the stock cam installed won't hurt as long as it was degreed about 10/12° advanced.

Secondly, the aftermarket intake may be lighter but is likely a street performance piece harming the low end torque curve due to less velocity through the plenum and intake runners, you stated that you went to a carb off of a 350, better move but I would suggest going back to the factory intake, adapter to fit a 390 holley, 68 primary jets, 72 secondary plate or install metering plate kit that allows jet change.

Thirdly, ignition timing,, install a curve kit in that HEI, one spring very light, the other medium,, do not use the vacuum advance, set total ignition timing "not initial" 32° max It will wake up.
 

One Eyed Gator

Well-known member
Backing up a bit, you do know that a caddy thermostat is different than a SBC? If you do not then the thermostat has a plate on the bottom that blocks a hole below it when the thermostat opens.

For 10 years I ran a 15' rivermaster with 472 that was .020 and 10.2:1. Well the rivermaster hull only lasted 4yrs in the gulf. But I could pull up of the dry turn around and then pull off. With 2 people it ran great add a third at 200lbs and you noticed it. Ran 42 wot. Best prop was an old FAP wood 72lxl38. But ran a 72" 2 blade Q, 72" 2 blade stump puller, 1 other wood prop and a 72" 3 blade H.

As for the thermostat I used a restrictor plate and a freeze plug with 1/8" hole (to allow air to get out of the system) i the bypass hole below the thermostat.
My ignition was set up similar to what John described above. I would say to verify TDC as there are a number of timing pointers and pully combinations. When I checked mine they did not agree.
So I got rid of the cast steel pullies that were on my motor and got the aluminum pullies (help the snap a bit)
I did run a custom cam with 4 degrees of advance.
I did run 5 different carbs and none ran any better than the others until they were tuned. I tuned mine with a wideband O2 sensor. In my case the best carb was a custom 600 holley with annular boostersi built. Never tried the 390 though.

I did run an aluminum intake with a welded plenum and 1" split riser Phenolic. I also blocked off the exhaust crossover in the intake
Looking at the picture it appears to be heavy in the rear.

My rigging was a front drive with a double rear. As silly as it sound a put it on pipe and balanced a 1/3 point from the transom. ( I was told that by and old timer, figured he was BS's me But I tried it and my boat run much better).

I do know that it takes attention to the small details for DD caddy to stand out. I know mine surprised me a few times.

Good Luck and enjoy.
 

SWAMPHUNTER45

Well-known member
As you read the stories and advice you should quickly be able to determine that there are a whole slew of things that “could be an issue” and the overall performance of a 15ft loaded up DD may never exceed a E-Bike or high performance skilled cycle rider in deep water.

Most machine shop or back yard Cadillac rebuilds won’t survive 100 hours running on a direct drive airboat !

The aluminum intake generally hurts low rpm power. If you ended up with some custom large port intake it’s worse than anything these other guys have experience with. Most have only seen a Edelbrock #2115 and then as stated custom modification (welded plenum) need to be done to make them perform worth a flip not to mention the castings are 1/2 degree off and most guys just crank down instead of taking it to a machine shop and getting the new intake milled.


Timing set ? Cloyes Race set?
Crank key way worn?
Cam degreed properly?
Distributor used ? Chinese billet ?????
Type of timing set up? Mechanical or vacuum
Which intake exactly did you buy ?

Those Hong Kong Phoey distributors were well known not to give much advance and the spring pins pull out.

Overheat as stated typically wrong stat or no stat. Easy fix but damage may be already done.

OneEye, Fenner and Deano all are good guys and can point you along, There are one or two Cadillac airboat guys out your way but it’s been years since I spoke with them.


Ever wonder why the Branch engines ran so strong well there is the start.
 
As you read the stories and advice you should quickly be able to determine that there are a whole slew of things that “could be an issue” and the overall performance of a 15ft loaded up DD may never exceed a E-Bike or high performance skilled cycle rider in deep water.

Most machine shop or back yard Cadillac rebuilds won’t survive 100 hours running on a direct drive airboat !

The aluminum intake generally hurts low rpm power. If you ended up with some custom large port intake it’s worse than anything these other guys have experience with. Most have only seen a Edelbrock #2115 and then as stated custom modification (welded plenum) need to be done to make them perform worth a flip not to mention the castings are 1/2 degree off and most guys just crank down instead of taking it to a machine shop and getting the new intake milled.


Timing set ? Cloyes Race set?
Crank key way worn?
Cam degreed properly?
Distributor used ? Chinese billet ?????
Type of timing set up? Mechanical or vacuum
Which intake exactly did you buy ?

Those Hong Kong Phoey distributors were well known not to give much advance and the spring pins pull out.

Overheat as stated typically wrong stat or no stat. Easy fix but damage may be already done.

OneEye, Fenner and Deano all are good guys and can point you along, There are one or two Cadillac airboat guys out your way but it’s been years since I spoke with them.


Ever wonder why the Branch engines ran so strong well there is the start.
Well, let's take this one at a time:
1 - It is a 2115 Edelbrock, stock. Nothing done to it other than having it ported before they sent it to me. No correction to 1/2 degree issue.
2 - Timing set is a CLO-C- 3034K (I assume this is Cloyes set, but racing....not sure) from Summit racing equipment.
3 - Crank key looked and fit good.
4 - New Distributor is a CRT Performance HP HEI
5 - Mechanical timing spec'd at 20 degrees at 3400 (done on a test engine) and initial timing at 16 degrees (taking this info off the data sheet given to me by the guy who checked/changed the Carb and Distributor) Also states a vacuum timing 14 degrees - 14" The Quadrajet is a rebuilt unit I purchased to replace the smaller Quadrajet that was on the boat (supposed to been from a chevy 350).
One thing I never mentioned in my post....I'm not a fine tuned mechanic, but merely can follow directions and use common sense to tear a motor down and reassemble. Talking about degree advances and the such is over my head, thus I use local machine shops and speed shops to do the fine tuning.
Attached pics are of the tilted spacer that was on the boat when I got it. I assume it was to allow the Carb to ride level more or less when the stern of the boat was sitting in the water. Have been continueing to use it.....toss it or keep it?
Thanks, Frank
 

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Backing up a bit, you do know that a caddy thermostat is different than a SBC? If you do not then the thermostat has a plate on the bottom that blocks a hole below it when the thermostat opens.

For 10 years I ran a 15' rivermaster with 472 that was .020 and 10.2:1. Well the rivermaster hull only lasted 4yrs in the gulf. But I could pull up of the dry turn around and then pull off. With 2 people it ran great add a third at 200lbs and you noticed it. Ran 42 wot. Best prop was an old FAP wood 72lxl38. But ran a 72" 2 blade Q, 72" 2 blade stump puller, 1 other wood prop and a 72" 3 blade H.

As for the thermostat I used a restrictor plate and a freeze plug with 1/8" hole (to allow air to get out of the system) i the bypass hole below the thermostat.
My ignition was set up similar to what John described above. I would say to verify TDC as there are a number of timing pointers and pully combinations. When I checked mine they did not agree.
So I got rid of the cast steel pullies that were on my motor and got the aluminum pullies (help the snap a bit)
I did run a custom cam with 4 degrees of advance.
I did run 5 different carbs and none ran any better than the others until they were tuned. I tuned mine with a wideband O2 sensor. In my case the best carb was a custom 600 holley with annular boostersi built. Never tried the 390 though.

I did run an aluminum intake with a welded plenum and 1" split riser Phenolic. I also blocked off the exhaust crossover in the intake
Looking at the picture it appears to be heavy in the rear.

My rigging was a front drive with a double rear. As silly as it sound a put it on pipe and balanced a 1/3 point from the transom. ( I was told that by and old timer, figured he was BS's me But I tried it and my boat run much better).

I do know that it takes attention to the small details for DD caddy to stand out. I know mine surprised me a few times.

Good Luck and enjoy.
Yes, I did read about the Caddy thermostat being different, so purchased correct one. Also, read about drilling a few small holes in the thermostat base to bleed off trapped air, and did this too. I still seemed to get air trapped in pockets, so added the small water reservoir higher than the engine, and that took care of the problem (I still watch the temp gauge!).
Are the cams cut for these advances (4 degrees), or do you merely advance the cam a tooth while installing it? (I'm NOT a mechanic).
I am heavy in the rear, as the engine sits over the 35 gallon fuel tank, and I sit just in front of the engine (of course, I don't add that much more weight at only 163 pounds :cool: ). BUT, I seem to get up on a plane easy enough, even with the slow throttle response.
I do enjoy this airboat, even when it isn't running up to snuff. To me, it is a leisure get away from all the bothers of life. I only go to the local lake/reservoir to skim along for a little while, then shut it down and just drift with the breeze while I read. Not a thrill seeker, don't fish, and enjoy chatting with everyone who comes over to ask about the Airboat. You get that kind of attention around here when there is probably only a dozen airboats for a State of 4 million people. I retired last Christmas and hope to get in a few years of this chapter of my life before moving on to something else. I did build an Airboat 42 years ago when I graduated High School. It was built out of plywood and I powered it with a Volkswagen engine. Pretty hokey, but fun. Guess once you have tasted airboating, it never goes away......
Thanks, Frank
 

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Ok, firstly the stock cam installed won't hurt as long as it was degreed about 10/12° advanced.

Secondly, the aftermarket intake may be lighter but is likely a street performance piece harming the low end torque curve due to less velocity through the plenum and intake runners, you stated that you went to a carb off of a 350, better move but I would suggest going back to the factory intake, adapter to fit a 390 holley, 68 primary jets, 72 secondary plate or install metering plate kit that allows jet change.

Thirdly, ignition timing,, install a curve kit in that HEI, one spring very light, the other medium,, do not use the vacuum advance, set total ignition timing "not initial" 32° max It will wake up.
Thanks for the reply. I have no idea how to degree in a 10/12 degree advance, but will check around to see who can do it.
May go ahead and drop my old intake manifold back on, at the very least to see if changing carbs will work. By the way, it's the other way around.... I took off the chevy 350 Carb and exchanged it for the original stock Carb. Both were Rochester Quadrajets, so only had to pay the difference for the rebuilt one. I do have two more Carbs sitting here that I can test out too. One is a 650 cfm Holley (still new in box from 2002,
so don't know if age has taken a toll on anything internal....looks new from the outside though), and the other is a used Demon that has been rebuilt and tested by a local Carb. guy that wants me to give it a try.
The Carb guy also checked out the new Distributor, but will have to call to see if he did anything performance wise with it. As I recall, he set the timing at 32 degrees total, with the vacuum disconnected, but again, will double check with him on that. Trouble with living in the country is everyone else is miles and miles away. Carb guy is 42 miles, so lots of time passes between visits. Hope to get answers to questions via phone, but even that can take time as most people are busy with their own lives.
Frank
 
So a few things.

#1, The guy on the bike was riding an Ebike, he wasn't doing any work! Yep, them things are cool, if bikes can run where you run, best you sell the boat and get an Ebike, it will save you plenty of money, time and agrivation.

#2, you were turning 2,750 RPM's at that point and it does not sound like the boat is up on a plane. What RPM do you get WOT on the trailer? 3,000 RPM is pushing it.

#3, how much does that boat weigh? 15+ is a big boat and there is only so much a direct drive can do. It sounds like some setup or weight distribution issues might be in play here.

#4, Swamp can take it from here.
1 - the guy was on a regular 10 or 15 speed. Granted, he was wearing a racing bike outfit with helmet, but still.....
2 - Get up on plane, even with 4 people, pretty easy, but it will start to sink again if I throttle back below 1900 rpm. Used to achieve WOT of 2700 on the trailer, and 2750 on the water, but then I increased the prop pitch by 2 degrees and that kicked me down to 2450 on the trailer, and I haven't been back to the lake as yet.....figure if I can get the HP on the engine up, I can get the rpm back too....maybe wishful thinking.
I know more pitch will give me more speed, but just gotta get more power to get there!
On the other hand, if I just cured the hesitation issue, I'd still be happy.
Frank
 

SWAMPHUNTER45

Well-known member
Starting at the most basic

If you bring the engine up to 2700 rpm what is the total timing ?

You want to be at 32’ degrees (all in)

The timing set you used is the “street” set not the roller race set.
 

John Fenner

Well-known member
Ok, let me reiterate, ignition timing,, use zero vacuum advance,, period!! The vacuum falls off under throttle and renders it useless, it is designed for passenger cars for fuel mileage, plug off port on carb, let dirt wasps build a nest in the servo on the distributor, have timing set to total 32° wide open throttle static run up.
Go back to stock intake, adapt to 390 holley as I stated above.
You can't use racecar theory on a direct drive, you have to use tow truck theory, meaning, torque curve should fall off where you are stalling the engine at just below the rpm say 3200, prop should stall engine at 3000.
If you could get your hands on one of those custom ground camshafts, do it!!
 

Slidin Gator

Well-known member
Used to achieve WOT of 2700 on the trailer, and 2750 on the water, but then I increased the prop pitch by 2 degrees and that kicked me down to 2450 on the trailer, and I haven't been back to the lake as yet.....figure if I can get the HP on the engine up, I can get the rpm back too....maybe wishful thinking.
I know more pitch will give me more speed, but just gotta get more power to get there!
Frank
More pitch gives you less speed and more problem getting on a plane. Take pitch out to get trailer RPM higher. This will have the engine making more Hp, in turn applying more Hp to the wind. For a 72" prop, 3,000 RPM is about the limit as your prop tip speed is getting close to the speed of sound (at which point prop efficiency drops like a rock and eventually the prop blows up). Do note that it will get much louder as you go to the higher revs.

If you end up getting the engine power up, then you can always add some pitch to limit revs.

Keep in mind the concept on carb choices. The auto hotrod guys just don't naturally think this way. You have a 500 cubic inch engine. At 3,000 RPM that engine draws around 500 CFM (similar to a 350 at 5,000 RPM). Putting too large of a carb results in less velocity through the carb and intake manifold. This results in poor fuel atomization/mixing and decreased torque (i.e less power) at the slower engine speed you are limited to.

Keep the spacer, that helps make torque. A 500 CFM Qjet is a good option, but will need bigger fuel jets vs. a 350 setup, otherwise it will run lean. You can use epoxy to decrease the size of the flow path through the intake, which will increase velocity and low end torque.

Advancing a cam moves the torque band down (where you need it). Retarding a cam moves torque to higher RPM.
 

John Fenner

Well-known member
500 CI x 3000 rpm = 1,500,000 ÷ 3456= 434.02 cfm needed, now, there are other variables that come into play, you want to round back on the cfm of the carb for better booster "venturi" charge for better response, if you are adamant about using Rochester Quadrajunk, find a carb off of a 305 chevy truck, will have to change jets and needles if mixture is too lean, broadband O2 sensor and gauge is your best bet to fine tune to at least 14 at wide open.
 
500 CI x 3000 rpm = 1,500,000 ÷ 3456= 434.02 cfm needed, now, there are other variables that come into play, you want to round back on the cfm of the carb for better booster "venturi" charge for better response, if you are adamant about using Rochester Quadrajunk, find a carb off of a 305 chevy truck, will have to change jets and needles if mixture is too lean, broadband O2 sensor and gauge is your best bet to fine tune to at least 14 at wide open.
Not stuck on Quadrajets, in fact, never heard of them until I bought this boat. More than willing to purchase a new Carburetor that fits the bill,
and can have the local Carb. guy jet it to reach that 14 level. When I took the motor apart when I first purchased the boat (it supposedly just had a blown head gasket), I discovered that one of the intake valves had melted into the shape of a cup. You could literally drink out of it! It had been kissing the top of the piston, but hadn't broken it. Was told this was caused by the mixture being too lean. Always found this hard to believe, especially since discovering the motor overheating issue due to air pockets.
Attaching a photo of the 3 cams that are still on the shelf in Branches shop built for airboats. Which one would you recommend? Margaret said she sold out the original 212 cams.
Thanks, Frank
 

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SWAMPHUNTER45

Well-known member
STOP

None of those cams will help you. They will only make for problems.

Just leave your cam as it is. If it is a stock replacement or mild RV if will be fine.

The cams left over and all way to big for your needs.

If you were building a budget gear drive 1.77 ratio or 2.12 or street car engine they would be a good fit.
 

SWAMPHUNTER45

Well-known member
All of the Branch Series 1 or Series 2 Direct Drive engines that left the shop in the past 10 years had Edelbrock 650cfm Thunder AVS carbs.
The Series 1 was a relative stock build with a few upgraded items specific to airboat use.

Not sure your expectations or experience but regardless of the carburetor brand or features if you try to stab a DD engine with a big prop pitched hard its gonna stumble. Bring the rpm up slowly then around 1200-1500 roll into it
 

John Fenner

Well-known member
Get a stock cam, send it to Powell machine Inc, 412 Wilson Creek Rd
Iva South Carolina 29655
Have them grind it 178 intake duration, 194 exhaust duration, .233 intake lift, .256 exhaust lift.
Have them put 10° advance into it as well.
Stock intake, 390 holley tuned as I stated above, curve kit in the hei dizzy, 32° total advance, no vacuum advance at all.
Only had a fleet of 10 dd Caddys running this configuration with 80" wood props on tour boats 24' long x 12' wide at bottom, twin engine on each tour boat, circa 1989 through mid to late 90s carrying 25/30 people on tours, if you took one out empty, it would try to blow over at 60 mph. That was fun standing on both pedals to drive "fly" it back on to the water, cleaning your shorts back at the park.
 
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SWAMPHUNTER45

Well-known member
For a fellow who has a running engine together my opinion is revisit all the basics and see if the current build will fit the need.

Engine timing curve, fuel delivery are cheap basics. (fuel, air and spark) then play with pitch to get the rpm right. I have zero Super Snap experience so can’t offer much more than with any Whirlwind I have been around they like to run at a higher rpm. Do your research and take pitch out and spin it up.

If your not happy and are going to jump into tearing into it so be it, but from what you have said I feel you can tune in a gain.

Good luck
 

SWAMPHUNTER45

Well-known member
So the CRT distributor appears to be a Hong Kong Phoey from what I’m seeing in ads. Nearly every one that came into the Branch shop had some type of physical interference with the timing plate. Mr Branch would totally disassemble them and grind away the interference.

Then put them back together with the vacuum Advance delete. You then use different weights and springs to establish a timing curve. As the engine gains rpm the springs and weights allow the internals to provide timing advance. As stated before you want 32” total timing at come in as you reach your cruise. I would say make sure your timing is all in by 2400 - 2500rpms.

Some of these units would only give 10” advance prior to being altered so that is definitely something you want to examine. See what your initial timing is at idle. Then add 200 rpm intervals and watch your advance respond. You should see the timing advance as you add rpm. The “curve” is the rate of advance to the engine rpm increases.
You can chart it out on a piece of paper.

Using different springs and weights alters the rate and point where the timing “comes in.” Having a good curve and getting the 32” advance where it’s needed really makes for good performance.
 
So the CRT distributor appears to be a Hong Kong Phoey from what I’m seeing in ads. Nearly every one that came into the Branch shop had some type of physical interference with the timing plate. Mr Branch would totally disassemble them and grind away the interference.

Then put them back together with the vacuum Advance delete. You then use different weights and springs to establish a timing curve. As the engine gains rpm the springs and weights allow the internals to provide timing advance. As stated before you want 32” total timing at come in as you reach your cruise. I would say make sure your timing is all in by 2400 - 2500rpms.

Some of these units would only give 10” advance prior to being altered so that is definitely something you want to examine. See what your initial timing is at idle. Then add 200 rpm intervals and watch your advance respond. You should see the timing advance as you add rpm. The “curve” is the rate of advance to the engine rpm increases.
You can chart it out on a piece of paper.

Using different springs and weights alters the rate and point where the timing “comes in.” Having a good curve and getting the 32” advance where it’s needed really makes for good performance.
This is the kind of info I like. Every response is helping me wrap my head around what to look for, what to do, and what not to do. Plan on contacting my distributor/carb guy and set up a time to tweak things a bit. He'll have the springs and such to dial in the timing advance. But first I'll reset my blade pitch until I can reach 2900 WOT on the trailer to reduce the strain on the engine and get the rpms up. Will then play a little with the different test carbs I have to see if I can gain anything. Will check into Mr. Fenner's suggestion of a new cam grind too.
Should have joined this Forum back in the Fall so I could have been messing around with the motor during the Winter months. Here it is, the July 4th weekend, and I haven't been out on the boat ONCE! Now the summer chores are upon me (repainting the house, building raised planting areas for the wife, and the list goes on.......) and those days snuck in here and there for taking the boat out are slowly slipping away.
Oh well, there's always the Fall season.
Thanks, Frank
 
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