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Hey guys & gals,

So, I'm in the tuning phase. Since this is my first bike, I am now getting to realize that this is much more tricky than getting the bike re-worked and running.

The bike runs well I think. Tends to stall once in a while if I have the idle lower than 1100 but not too bad. My problem is that once I get cruising and let off the throttle say when rolling down a hill, I get all kinds of popping/backfiring. If I hold the throttle ever so lightly so it's getting gas, the popping goes away immediately. Now:

1.) I haven't checked the timing with a strobe gun but I just borrowed one and will check it soon. Will messed up timing cause this?

2.) If the timing is ok, what else could be causing this?

Any help would be great. Thanks!

-sf

Dude, this could totally be the wrong answer but I'd have to say it's "somewhat" normal, albeit not acceptible for a perfectionist. It's a lean condition for sure, but "slightly" lean. Frankly I wouldn't mess with it. You're not going to cook a valve when it pops and burbles as you lurch down a hill. Now, if you give it a good rev and it REALLY backfires consistantly, then I'd be a little worried. The only reason I say this is because my bike used to do it too.

Try checking your points gap, or the overall condition of the points. Is your bike a little cold-blooded? Do you have to mess with the idle speed for the first few minutes of running or choke the heck out of it for the first start? If you are satisfied with your bike's performance overall but are concerned with the downhill/decel popping, I would definately take a look at the points gap, could be at the lower engine revs they are arcing a bit and the plugs aren't hot enough to light the mix, but the exhaust is. I'd at least check the points gap and dwell before messing with the timing.

You're on the right track though...def don't mess with the carbs/mixture until you know your ignition system is set up right. And I wouldn't bother with checking the timing until you "check the points."

Sometimes a less than perfect exhaust gasket seal will allow air to be drawn into the head pipe during part of the cycle. When this air mixes with a rich spray from a pinched off intake tract you get popping and backfiring during deceleration.

KZCSI

richen the fuel mixture a little bit
check for exaust leaks at the head
put a baffell in the muffler
those are the first things I can think of to do first,after that it gets a bit more tricky and a little more info is needed

more modern carbs have deacceleration valves that richen the mixture when the throttle is closed to avoid the popping noises

Just so you know you're not alone, my '78 750 twin does the same thing.
I had it toned down and in the past couple of days it's started the popping up again, only on my right side though. I'm checking my exhaust this weekend, then going after the carb boots. If they are letting air in after the carb you can get the popping.

If I come up with the solution I'll post it. My friends that ride Triumphs say it's fairly normal for their bikes. Just hate that coming to a stop having everyone look at you like you just farted in church.

Peace

Yeah it's fairly normal as long as it's not excessive. It's just the nature of these old carbureted bikes. Are you running pods and a 4-1 exhaust?

Popping could be several things:
1. air leak to combustion chamber; either cracked vacuum plug, hose, intake manifold or exhaust gasket

Any of these things could potentially damage the engine as cold air can focus on the piston crown and the problem should be rectified.

2. incomplete combustion due to lack of valve clearance or coils not powered sufficiently

These problems, if not extreme, are not particularly dangerous to the bike but can be annoying. I suggest you have a look at the write ups on these subjects on my web site.

3. pilot circuit lean

Most late 70s/early 80s bikes relied on lean pilot mixture to comply with emissions testing requirements of the era. Put one size larger pilot jets in OR adjust your the air/fuel mixture towards a bit richer by adjusting your air screws or pilot fuel mixture screws (not sure which you have)

4. aftermarket pipes lack backpressure

Some aftermarket pipes lack sufficient backpressure and will pop when the throttle is shut. The only cure is to repack them for more baffling OR replace them.

Post edited by: wiredgeorge, at: 2006/08/03 09:07

george, if the seal between the fore-pipes and the extension pipes is not tight, could this effect the bike in the same way as a leak at the exhaust gasket?

If I understand, you are talking about a seal in the exhaust system NOT at the cylinder head/head pipe juncture? If so, I doubt this would cause a backfire this far away from the cylinder head but I can't be sure. I would plug the leak there or put in new seals to be sure (if those seals are available). Many folks try and reuse Kaw crush gaskets and they don't work well if reused.

Is it possible to make new gaskets out of gasket material, or is it just easier to go buy new crush gaskets?

buy new ones it's easier.

Cool..thanks.

Could increased back-pressure cause the popping as well, or is that not possible? I have occasional popping when simply riding along now, whereas earlier it was only on decelleration.

Though maybe because the exhaust is quieter now, I can hear it..and it was always there