83 750 twin backfiring

My bike has taken up the habit of backfiring when you let off the throttle and allow it to use engine breaking. I put new plugs and wires and that didn't help.I blocked offthe crossover and it is as I suspected it is all coming from the left side. When I changed the plugs (again) the one from that side was badly coated in carbon and/or soot. Also it spits and sputters under hard acceleration in the 6K RPM range.
Where do I need to start? This sounds like a carb or possibly a timing issue to me but this is the first multi cylinder anything I've ever worked on.

Weird. Backfiring on deceleration is a sign of lean-ness, or perhaps an air leak. Carbon sooty plugs is generally the opposite.

Blocking off the exhaust x-over? That's funny. I'm doing everything I can to return the crossovers to their original state on both my 750s, after I learned what a performance benefit it provides to the one bike.

Well, there's a few pieces of advice I give to every twin owner who asks:

1.) set/validate your float levels,
2.) replace the coil/wires, and
3.) do the well-known coil-power mod.
...Oh, and inspect your starter clutch, but that's a different topic.

In your case, from the sounds of it, you should also check for exhaust header leaks, cracking carb boots, and that kind of stuff....

My did that and it was a bad float. You might as well replace both of them while your at it. I replaced one that was obviously bad and then had the problem and had to replace the other.

Thanks I'll start with your ideas and work from there. I blocked the cross overs so I could seperate the pipes coming from each cylinder. That way I could be 100% sure that it was just 1 side going screwy on me. I rebuilt the starter clutch when I first began working on this thing and it is in fine shape.

Post edited by: Wildh2oskier, at: 2007/08/30 06:27

Has your bike backfired on decelleration since you have had it or is this a new thing?

Of the three things mentioned above I'm not clear of the last two as I just got an 82' CSR and will be getting an 83' belt drive this weekend. There's no need for long explanations if you can direct me to a previous forum discussion that covers the changes listed. Thanks for any help and advice.

wiredgeorge wrote:

Has your bike backfired on decelleration since you have had it or is this a new thing?

It just started it in the 1,000 miles. Also it seems that the hotter the engine is the worse it does it.

This seems like a simple enough issue but the backfiring might be caused by any number of things.

FIRST cleck valve clearances, then timing. Yeah, I know it is a pain, but either of these issues could cause the problems you have. Check the voltage at your coils... I think you have two coils. See the article I have on our website on COIL REPOWERING... this could well be an electrical issue... bad condensor, pitted points or poor coil voltage.

If these basic tune up issues are sorted, then step two is to look for an air leak. Air can leak in around re-used exhaust gaskets, cracked vacuum hoses/caps or cracked or ill-sealing carb holders. The Mikuni butterfly shafts are sealed to prevent air leaks but lots of the BS series carbs seemed to have been smacked on the sides and this damages the little pot metal cups that seal the shafts. If these are badly damaged, air can be leaking in.

I suspect the backfiring is from the FIRST paragraph list of issues rather than the second but it isn't possible to determine without the bike in front of me. Get a manual and start your tune up stuff...

wiredgeorge wrote:

... Get a manual and start your tune up stuff...

Honestly, out of everything said here, this is the most important line. Get a Kawasaki Shop Manual and set everything to spec.

BTW George, the 750 twins have only one coil.

Thats the first thing I did when I got the bike. It took me a while on ebay but I found one that was affordable (those things were going for upwards of $75.00) I got it and got the bike on the road, life was good. Then when this problem popped up I go to look what the big book of all needed knowledge said and ... its missing. I have no idea where it is. I'll find it soon or break down and get another. Till then I have the collective genius of this site to help me muddle through. I'm a dumbass when it comes to keeping up with things.

The 750 twin have got one dual coil that fires both plugs at the same time, and if I remember correctly, the 83 750 twin also have an electronic ignition system that is fixed in one position.
Since the problem seems to be that one plug is getting sooty,it is more likely that the problem is carb related than ignition system related, but I would still check that the mechanical timing advancer is working properly.

Backfire is an explosion in the intake manifold, carburettor/throttle body or air cleaner of an internal combustion engine. Although an explosion in the engine's exhaust manifold or exhaust system, is often referred to as a backfire, it is actually an after fire.
Usually, backfiring occurs in carbureted engines that are running lean. After fire occurs in engines that have an emission system malfunction , an exhaust leak or unburnt fuel in the exhaust system.

In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where ii fails to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust pipes. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust pipe causing the backfire, or more correctly and after fire. You can try to set the pilot screws a little richer (by 1/2 turn) and see what happens.
And if you have got after market pipes on the bike they often increase backfiring, probably because they cause a slightly leaner mixture than the stock system.

Post edited by: ibsen22000, at: 2007/09/01 07:45

Post edited by: ibsen22000, at: 2007/09/01 09:14