backfiring 23hp Vanguard (Copperhead Duck boat motor)

[38racing]

Even though plugs are new you might want to do balance test and then swap plugs and balance test again. I've had a case where a new wasn't firing. I use an inline spark tester on both cylinders. Since they have exposed metal I clip a jumper wire onto them and alternately ground each one .

[KE4AVB]

On week 4 of Mondays here.

Just a thought. Have you tried swapping coils around to see if problem follows one of coils?

[RMR]

Only other thing I can think of - bypass the fuel tank, and test with a bottle (or jug, or whatever) of fresh, known quality gas - try different octane ratings also - a difference in performance by increasing octane rating of the fuel may indicate a pre-ignition or detonation (in which case, I'd be taking a closer look at the flywheel key) or a change (or elimination of the issue) by using known fresh quality gas would tend to indicate a problem in the fuel supply or storage..

Thanks bg. It currently is pulling fuel straight from a new can of ethanol free gas. I believe it's 89 octane. I'll check the flywheel key this morning to see if maybe it's at like a half shear point.

[Arkie]

If flywheel key is ok try doing a change in the gap of the plugs. Change them by at least .010-.015.

Might even try changing the gap on just the cylinder that has the hotter exhaust pipe.
That cylinder is running lean for some reason.

cylinders are not really balanced if the exhaust temp is different by 100F.

[RMR]

If flywheel key is ok try doing a change in the gap of the plugs. Change them by at least .010-.015.

Might even try changing the gap on just the cylinder that has the hotter exhaust pipe.
That cylinder is running lean for some reason.

cylinders are not really balanced if the exhaust temp is different by 100F.

So I went to check the flywheel key and discovered it has a timing advance key in it. And I'm pretty sure that Copperhead also puts a hotter cam in it. That made me question. The valve lash should still be the same as stock, right? I tried calling them twice now and I don't get an answer or even a VM.

[RMR]

Well, dang-it! I finally got ahold of copperhead and according to ole Gill, their 23hp engines are stock. I know for a fact that this one has a timing advanced keyway, and aftermarket valve spring retainers and shims. So there is no telling what I'm dealing with now.

[KE4AVB]

Well it looks like someone has taken on the challenge of trying to make the engine into a race engine. If it is a twin barrel setup they might have swapped the jets and got them in the wrong order. That would make it run leaner on one side.

What is the Briggs numbers (model and type)?

[RMR]

Well it looks like someone has taken on the challenge of trying to make the engine into a race engine. If it is a twin barrel setup they might have swapped the jets and got them in the wrong order. That would make it run leaner on one side.

What is the Briggs numbers (model and type)?

The Briggs number plate has been removed, so that has been a shot in the dark since the start. I did however just run down the manufacturer of the performance parts that are on it. They are called Backwater Performance. They also have info on their site in regard to valve specs. In my case, even if running a perf. camshaft, it still has the stock rockers, which call for stock specs. So that idea is out the window. However I am going to loosen them .001" just to see if it makes a difference. as well as maybe open up the plug gap a smidge. Then run it back up to temp at just an idle and check the temps again.

Oh and it's a single barrel carb fit to the stock intake with an adapter. I moved the clip on the throttle slide needle one up and one down from where it was originally and didn't get any change with the backfire issue.

[RMR]

Low and behold, I found some model info hidden under some black paint on the valve cover 386447 3065-G1

Here is the latest run down of what I have done.

Swapped in a new set of plugs, started it, brought it up to temp at idle. Currently the slide needle is at its leanest setting. 5" off the exhaust manifold I read 360˚ off cylinder 1 and 500˚ off cylinder 2. Pulled both plugs, still black as can be. Swapped coil packs side to side, and richend up the needle back to its original location (middle). Cleaned plugs, ran again. Temps dropped a hair, but still hotter on cylinder 2 than cyl 1, indicating not a coil/spark issue. Plugs still showing a rich condition.

I don't have a spare keyway in the parts box, so I'm off to rob one off something. However, seeing as how this engine ran fine previously to when the issue started, I don't foresee a change with a stock keyway.

[bgsengine]

any backfire or sudden stop of the driven implement can cause a sheared key.. in the case of boat motors (assuming these are the type with the long shafts or such, clutch driven) I could imagine getting the prop tangled in some rope/weeds/etc whatever and jamming up the propeller suddenly - even being clutch driven that sudden stop can be enough to shear a key simply from the momentum of the flywheel acting against crank. Other thing that makes me think of is possibility of a twisted crank, or a worn out/sheared crank gear key (internal crankshaft gear that mates with cam gear) which would throw off valve timing. (not to mention possibility of worn cam lobes?)