Commando backfiring on the overrun

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Dec 30, 2003
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I've complained about my '72 750 backfiring with throttle just open on overrun. During last weekend's ride with the club it was worse so yesterday I pulled the carbs to check and clean the idle passages and replace o-rings and fuel bowl gaskets. What I discovered during this was the fuel bowl flanges were warped. I suspect air was leaking through at this point. Checking some other spare bowls, they also were warped. Easy enough to sand flat again, but I'm wondering why this happens. If it's from over tightening the two screws that hold the bowls on, I know if you don't tightened them the vibration will loosen them. I've seen several Commandos with these coming off.

Others have commented about the backfiring. Check the bowls on a flat surface. Do they rock?

One down, still working on crankcase oil going into the primary and will work on the oil pump to cut down on wet sumping this winter.

I always coat both sides of the carb bowl gasket with a healthy amount of Gasgacinh. I let the stuff dry completely before installing the gasket. I tightening the bowl screws just to the point where the lock washer becomes flat and no more. This provides and leak-tight seal without warping the bowl.

Backfiring on overrun may not be the result of a lean mixture. Instead, it could be a rich fuel mixture combined with a exahaust leak.

I doubt that air leaking at this point would cause the backfiring. After all, the bowl always has a headspace at this point. I have found that backfiring on the overrun is usually caused by the slides out of synch, a clogged air passage on one carb, or an air leak at the exhaust. I'm sure these are not the only causes, just the ones I have found.
Speaking from my side of the paddock, I use this same symptom as a warning that the joint between the header pipe and the muffler has worked loose ~ again ~

As the muffler is rubber mounted , the whole pack of cards tends to work loose and I simply remove the offending muffler.. bung in a bit of (blue) silastic.. (any ~ silastic will work!!) , refit and off I go.. NO more back fires...

(Not guaranteeing this is solely your problem .. but it works for me !! )
At a recent race I saw a 750 with lots of backfiring in his last heat. Talking with him later, he pointed me to his carbs, Right had the slide down where it should be with the bike not running. Left was stuck almost wide open.
Heat had warped the slide throat badly enough to bind the slide up.
Not sure I'm quite on subject here, but the body bones connected to the bowl bone....

The right exhaust ring was a bit loose, so tightened that up. Once the engine is warm the backfiring diminishes, but not completely gone. I going on a club ride tomorrow and will get it up to running temp. May be the mufflers are getting blown out and not giving enough back pressure.

The carbs were s/s sleeved eight years ago during the entire rebuild of the bike and the slides still work great after 15K miles. The mufflers are tight on the exhaust pipes. I have no reason to believe the RITA is the cause. It's fairly new and these never fail in my experience. I replaced the plugs which helped a bit. I think the not so old ones got oil fouled...NGK seem to be succeptable to fouling.
backfire on over-run

I had a similar problem recently, looked at all of the usual suspects, nothing showed up, Frank Holmes of "Franks Brit. Bike Barn" in New Hampshire --great guy ,pointed out that 89 octane gas, which I was running, is too low I filled up with 93, since then not a pop or bang, plugs are much cleaner too, the Norton Tech. Digest suggest that modern 89 oct. gas is similar to the "old" 94 oct. -I am having serious doubts about the accuracy of that statement, any petro/chem. engineers out there able to shed some light?. Good luck and ride safely. James.
illf8ed/ David

I too had stainless steel inserts put in when I rebuilt the bike way back in the early /mid 90s. They have been a pain in the butt .
Recently I pulled them apart again and this time I made a 'perfect' fit 'buffer' wheel form a piece of hard wood, and mounting it in the drill gave the carb bodies a very short (3-4 second) hone job and presto ~NO more jamming up on cold.. and the bike goes like a branded stallion on race day!! ~

one more point I have is that fora long time now ~The right hand Amal actually spits back after about 30 or so miles..

I have standard 3 1/5 slides and standard jetting ~ altitude has nothing to do with this though. Since honing the carbs the problem has been dramatically reduced, but it still does it ..

James has a valid point too as I have horrendous problems with shoddy fuel. Living on top of the Great Dividing range of Australia at <2,000> ft and then descending to the coast / sea level , even just 'sliding around the surrounding countryside # , I believe has a radical effect on the tune.

(# Local surrounds has a high degree of altitude change as well!)

I prefer Caltex Vortex fuel as it performs better and the fuel maintains its volatility longer..

After the club ride yesterday the backfiring is for the most part gone. I think Stuart SS was the closest with a loose right side exhaust ring. The remaining few pops are likely due to a blown out muffler on the right.

I was getting some rattling sound toward the end of the ride just off idle. When I pulled up to the garage I saw the left rear turn signal dangling by the wire. The bracket on the huge rear numberplate (fastback) had fractured at the turn signal mount. It survived nearly through one riding season.

I have the "lock' or locating plates on the head exhaust flanges my bike, but even then mine worked loose and required removing and resealing.

The joint at the muffler and pipe is the main offender/s .

As for trafficators I was so frustrated with mine I was ready to ditch the original and go for after market japo types. but I have resolved this to through sheer perseverance.

But the bottom line is ` always apply the "kiss method" first.. don't go complicating the issue at hand till the simpliest resolve is covered.. first ! 8)
If the "backfiring" is in the pipes, look for air leaks in the exhaust system, or a tight exhaust valve. If it's " backfiring" in the intake, look at the inlet valve clearance or lean mix at idle....or non-synchronized cables. If these are ok, look for sticky advance mechanism or funky IE box/loose wiring connections.
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