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Added mufflers and now my boat backfires.

Ol Yeller

Ol Yeller

Well-known member
I did the right thing and added mufflers to my 350 hopped up chevy airboat. Really quieted my boat down. But on hot days with a warm engine when I turn my engine off I get one backfire from each exhaust. I never had this problem before I got mufflers. This happens after my engine, like most car boats, goes through the mandentory 5 seconds of run-on after turning the engine off. I checked my timing and it's fine. I would like to try an "idle stop relay" on my Holly Carb. in order to slam my throttle plate closed when I turn the key off. I've heard this can eliminate run-on. I'm sure others will have this problem as mufflers become more popular. Does anybody know where I might find this relay for a Holly carb. By the way I get the same RPMs with mufflers as without.
 
The backfire might be an indication of unburned fuel in your system. You may be able to save gas if you get your carb jetted to put the right amount of fuel in there. I am not a motor head but mine did the same thing and the jetting solved it.
 
YOu might consider puting a switch into your fuel pump so you can kill the fuel 30sec beofore you want to cut the engine off. You should also let your engine idle for a full min. before shutting down the engine. There is also a dieseling selonoid you can add.
But as far as your mufflers go, I have installed hundreds of sets of mufflers on airboats and do not have that problem as a result of mufflers.
You should also make sure your idle is set to about 600 to 700 RPM's.
Good Luck,
Faron

We do use 3" in and 3" out on all of our 350's I assume that is what you have.
 
billalvord,

I believe Waterthunder has posted those part numbers for the solenoid a few months ago on the site. If you can't find it here using"solenoid" in the above search engine, PM waterthunder.

Basketcase
 
that shut off you wanted was used years ago on quadrajet carbs I got mine off a car in the junkyard but I have been told you can still get them from a parts dealer PS they do work. COPPERHEAD
 
Thanks for all the good tips. It's great to know I'm on the right track with the solenoid. I have noticed my backfires are much less intense if I idle for while before cutting the engine. Because of my cam, I have to idle at about 1000 rpm in order to keep my engine running. Also, because my cam has a bad studder at idle, it causes my belt drive to really make a bad "rumble" at the lower rpms which is normal, but almost as unnerving as a couple of cherry bombs going off at shut-down. I'm thinking about backing the idle adjust screw out all the way, so when my foot is not on the pedel the throttle plate will be all the way closed. It's very seldom I'm not in the seat when the engines running anyways. I'll keep you all posted.
 
when you put the fast idle solenoid on your boat you should back the idle screw all the way out and set the idle with the solenoid energized. when the key is off the solenoid will open and close the carb completely, stoping the run on.
 
something easy you might want to try. Turn in (lean out) your idle mixture screws 1/4 to 1/2 turn. Most people set the mixture screws and the rich drop off, turn the idle mixture screws out (richer) until the engine falters and turn them back in till it smothes out. This leaves the engine on the rich side at idle leaving unburnt fuel in the exhaust and could cause backfires. If you set the engine on the lean drop off, you wont have the unburnt fuel in the exhaust. Don't know if it will help but it's something that is easy to try and if it doesnt work it wont cost you anything and can put it back easy.
Good luck
Tim
 
Every Chevy ZZ 350 I have driven would run on at the end if you ran anything less than 93 octane and you didn't let it run at 1000 RPMs or less for a minute or two before shutting off. Especially if ran hard. What octane are you running. I have seen the shift to 93 octane stop it on several boats.

But I know that the option you are looking at works as well.
 
You're getting some good advice here. Every suggestion has been a good one. The MAIN reason that it backfires is becasue the mufflers made the engine run richer. And hotter weather compounds the situation. Leaning out the idle mixture and the suggestions of a dashpot type solenoid as well as letting the engine run definitely help. Any time you run mufflers the mixture will richen across the entire RPM range, and actually more under load/RPM. So even if you cure your idle situation, you really should consider rejetting. One step (jet #) at a time is best. Just my $.02

Felber
 
After adding mufflers I had a problem with backfiring when I turned the engine off. I finally fixed my problem by adding a fast idle solenoid to my engine. So after I let my engine idle for about 15 or 20 seconds and power off, no more backfires and my engine doesn't run on like it used to. I got these Holly parts from JEGS for about $90 including tax and shipping. The part numbers are 510-46-74 for the solenoid, 510-26-57 for the solenoid nut, and 510-20-9 for the solenoid bracket. The only wiring I had to do was I got 12 Volts from my distributer wire, so my solenoid is energized only when my key is on. When I turn my boat off, the 12 Volts goes away and my throttle plate slams down cutting off fuel and air to my engine. It works like a champ. Don't forget to remove your idle screw from your holley carb.
 
Billavord ... all great advice. You're just a little richer now because of the mufflers. They're actually doing your exhaust valves a favor. They'll be running cooler.

BF
 
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