backfiring 23hp Vanguard (Copperhead Duck boat motor)

[RMR]

Ladies and gentlemen, do you ever have one of those months, not days, months, ok it's been a couple weeks, that just won't let you be the mechanic that you have proven to be?? Yep, I'm smack in the middle of it.

I have a customers mud motor that came in with the complaint of running bad and one side of the exhaust header glowing. I bolted it down to the bench, threw a little power and fuel to it and fired it up.
What I found was a really bad backfire at mid-high rpm and a related small miss at idle. After doing the routing troubleshooting- compression, leakdown, valves, carb, plugs, coils, etc, I thought I had it narrowed down to a burnt valve. So I took the heads off and did see some, not much, but some evidence of pitting on a couple valves. So rather than trying to lap them, I took them to the machine shop and had a true valve job done on them. After all this is a high hopped up performance engine with some sweet aftermarket parts in it.

Got the heads back, put it all back together, and it is doing the exact same thing. I even took some header temps and discovered that one side was about 100˚ hotter than the other. Pulled the new plugs expecting to see signs of a lean condition, I found lots of carbon. More on the colder cylinder.

Thinking it could be a weak coil, I swapped them to see if temps followed- They did not, now it ran worse than it did before. Thinking I had maybe 2 weak coils, I swapped in a couple used coils off a 26hp v-twin that I had. I know, not the right coils, but dimensionally the same and theoretically it should have at least solved the issue if that was the case...It did not. Runs exactly the same.

At this point I'm reaching for anything. I pulled the carb off and opened it up- Spotless. Nevertheless ran it through the Ultrasonic anyway. Still same issue.

I'm sorry I have no model number off this engine as the plate was removed by Copperhead, probably due to the internal and external upgrades made to the engine. One being a motorcycle carburetor and SWEET stainless crossover header/muffler.

Anyway, I am here once again even in the same week reaching out for some help. Sorry for the long post, I didn't want to miss any details, and quite frankly just needed somebody to vent to that understands the topic.

Thanks in advance everyone.

[KE4AVB]

A couple of weeks of Mondays. Been there lately too but it been problems with vendors.

Have you checked the valve lift to see if they are the same. I haven't had a v twin with bad cam lobes in a long time but I did run into one about 8 yrs ago.

[bgsengine]

tried swapping plugs and see if it the problem changes sides with the plug swap? Tried a power balance test? (see which cylinder is actually giving the problem)

[RoyM]

Also try disconnecting the primary wire from one coil, if it smartens up you have a bad diode in the harness. You will need to hook it back up to shut the engine down.

[RMR]

A couple of weeks of Mondays. Been there lately too but it been problems with vendors.

Have you checked the valve lift to see if they are the same. I haven't had a v twin with bad cam lobes in a long time but I did run into one about 8 yrs ago.

Good point, and No I have not done that. I didn't even think about it. maybe because I've never seen a lobe go flat on a Vanguard, but I'll darn sure give that look. That really should have occured to me seeing how I have a Kawasaki in the shop with a bad cam right now. DOH!

[RMR]

.....Tried a power balance test? (see which cylinder is actually giving the problem)

Well, Kinda, I did do a start up on it once forgetting I had one wire still off. When I put the wire back on, It did come up in rpms. I have done that test before on a weak coil and only having 2 cylinders, it's a little tougher if its a weak coil over a dead coil. I'll do it again but both this time.

[RMR]

Also try disconnecting the primary wire from one coil, if it smartens up you have a bad diode in the harness. You will need to hook it back up to shut the engine down.

I did swap the diode harness at the same time I swapped in the other coils.

[Arkie]

.....Tried a power balance test? (see which cylinder is actually giving the problem)

Well, Kinda, I did do a start up on it once forgetting I had one wire still off. When I put the wire back on, It did come up in rpms. I have done that test before on a weak coil and only having 2 cylinders, it's a little tougher if its a weak coil over a dead coil. I'll do it again but both this time.

Just a couple hints:
Keep a heads up on this balance test. May have to have a tach attached if not apparent by ear because the weak cylinder is not completely dead, just weaker.
The cylinder that drops the most rpms when it's disabled is the strong one. (and may not be due to just ignition) might be cam, valve, one side of carb, suction air leak, etc.
This balance test is to get you looking at the weak cylinder stuff.
Very hot exhaust from one is sign of lean running. (too much air)

[RMR]

Well I checked the valve lift and all was well. One valve had .001" difference, but that was it. Load test off idle was pretty insignificant as well. 1360rpm with both cylinders. cyl 1 dropped- 1048rpm, cyl 2 dropped 1068rpm. The miss/backfire is so random I think a balance test can't pick it up accurately.

So I'm still at square one. Also I have sprayed carb cleaner around every single possible vacuum leak aft of the carburetor.

I am going to try and post a youtube link of it running so it's more clear what I'm working with....

https://youtu.be/EV6w12pbGUQ

[bgsengine]

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..