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0470 overheating and running rough

AlaskanOutdoorsman

Well-known member
Boat is running fine for 5 or 10 minutes then the temp starts increasing and the engine starts running rough as if it's either not getting fuel or is fouling plugs. (Which it doesnt). Engine loses power so I limp at 2000 RPM to the bank to let oil temp cool down from 250 degrees. Oil pressure never goes over 60 or under 50. After a few minutes it runs fine and starts all over again only to repeat the process.

Other than an oil change I changed my plugs recently and upgraded my Champion's to D21's. Curious to anyones thoughts: is it a spark issue, to hot a plug, air leaks or a fuel issue? I've had no issues with this boat otherwise and aircraft ships are all closed for the extended Labor Day weekend.
 
Sounds like a lean condition to me, d 18 is all you need, d21 for oil burners in bottom plug holes. What carb?
 
No go; neither were the issue. Found two small airleaks kn the manifold booties; going to attempt to resolve and try again. The overheating makes me wonder if it's two issues but one step at a time. Need to spend time with the manual tonight to see if anything stands out.
 
Gary S said:
Air leaks will make it run lean. They don't have to be big leaks to do it.
And running lean will make it run hot, which explains both issues. You will torch the exhaust valves if that is the issue. It sounds like time for new rubber on the intake system.
 
My 520 is doing the same thing, I have resealed all of the intakes and re clamped all of the boots. I can not find the leak. Look for a black oil like substance in the intake. Im told that is a sure sign of an intake leak.
 
After a leak test it appears I have a bunch of air leaks around the rubber sleeves. Hopefully that was the only problem. Hope to know more tonight.
 
Update:

Been swamped with work so just got the boat. Ran it today after I ordered/installed new wires on both mags. Fixed all the air leaks on the engine. Installed a new carb heat cable, P-nut on the carb heat 'foot' and adjusted the carb throttle rigging per a mechanics instructions.

Checked torque on prop. Started engine and it initially ran great. Smooth with a lower idle than before at 800 RPM. It used to be 1000 RPM. Wondering if it's the new plug wires but so far I'm a happy camper..

5 minutes run time on trailer with new prop at 2700 RPM. Smooth.. no issues. This is where it acted up before on the river so I wanted to see if it would duplicate on the trailer.

Oil pressure running 50-60 PSI steady. Normal.

Temp started to climb (expected). At about 180 on gauge and five minutes run time it started slowly losing power. When RPM fell to 2500 the power continued to drop but it started immediately running rough then I shut it down. Grayish white smoke coming from the exhaust.

Back to the drawing board.

Have not done a compression test or drop leak test. Don't have the tools for it. Aircraft shops are a minimum of three weeks out to look at it.

I'm going to pull the plugs, look at them then go back to D16's like I originally had. I don't expect a hot plug could be the issue but I may be wrong.

Could this be a stuck valve? Broken spring? Bad rocker? I don't have any exhaust obstruction. Carb works fine.

Moose season has another week for the area I have a permit in. Any suggestions would be helpful.
 
Ok, you obviously have a timing issue here, pull #1 plug, get to compression stroke tdc and back up crankshaft to 22° btdc if it is a true o470 there are degree marks on bottom edge of flange to align with split line of case to determine the degrees, pull sight plugs from mags and point covers, the gear in sight window should be on a beveled tooth dead center of said sight window, if not then timing is likely too high, if it is in center of both, proceed to check that both sets of points are just barely beginning to break open, be sure that the tungsten contacts are not pitted, and still crowned and thick.
If all the above checks out good, you may have some bad fuel, assuming you are running 100ll avgas.
 
Im trying to sort through your suggestions. A bit over my familiarity.

This boat has only ran premium pump fuel. Would hotter plugs throw off timing? I noticed it as after the change or is it just coincidence?
 
There lies the problem, it is detonating, you need to back off timing for 1, or go run some av gas through it 2 being you noticed a difference with the plug heat change, you need the d15 plugs back in it. Connies are very temperamental with timing and detonation you will have cracked cylinder heads and hammered ring lands as well as rod bearings if this continues.
 
Another option is using a real octane booster. I ran ACES IV from bnd automotive in my continental e 185 with pump gas. It’s not cheap, but it works very well. You also have to call directly the first time you order as they do not have an online store (they don’t want people purchasing the wrong product), but you can order via email after that.
 
Tonight I'm swapping the plugs and going back to the original D16's like it originally had. I'll gap them at .020. This much I can do on my own.

I'll report back later tonight after the engine run. Thanks!
 
One of the great things about this site is the wealth of knowledge available. The trick is identifying it when it is given. You should follow John Fenners advice.

Running avgas and making sure timing is right is cheap and simple compared to the damage detonation causes.

Avgas not only has higher octane to burn slower. It has lead to lubricate the valves. Also is more tolerant of sitting in a tank for extended periods of time. I don’t know all of the differences, but there are 3 reasons it is better for your motor. Not to mention the obvious: you motor was designed for it.
 
Agreed mojoe. I've not done the timing on these 0470's before, going to have to learn how. After the advice here and speaking with a few mechanics (the river version of a friend of a friend telephone troubleshooting) the list of what it 'could be' has grown to include:

air leaks.. (resolved- fixed leaks)
bad cables.. (resolved- installed new cables)
hot plugs/timing issue..
carb ice/lack of carb heat..
weak or wandering magneto..
carb issues..
bad cylinder(s)..

Tonight's run up didn't provide any answers so the boat's on it's way to an aircraft mechanic's shop tomorrow. Hoping to have some solid answers in the next 24-48 hrs.

Fingers crossed I can get her running and install the new set of blades in my garage before the end of moose season.
 
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