'78 R80/7 backfiring on deceleration

[r67boxer]

Background: carbs have been rebuilt using existing jets etc., carbs balanced and valves properly set. Air/fuel mixture is set to just under 0.5 turns out. The bike starts easily with the choke and has lots of power. Acceleration is smooth. The PO installed a Boyer Bransden MK 3 ignition. The problem is that when decelerating I can hear a popping sound which I presume is backfiring. Thoughts?

Kevin

[gspd]

Warmed up and idling, the bike should stall a few seconds after solidly blocking both exhausts with gloved palms. If it doesn't stall, fix the exhaust leaks.
You can also try a bit richer. Tune your mixture screws to max rpm and drop it back down with the throttle stop screw, repeat if necessary. The 'correct' mixture is achieved when turning the mixture screw 1/8 of turn in or out causes the idle speed to drop slightly. The specific figures published for the mixture screw position are for general setup and in most cases need a bit of tweaking. This can vary from bike to bike and often from side to side.

[r67boxer]

Thanks. The exhaust system is tight (no leakage) so clearly that is not the issue. It's been raining here quite a bit but I will try the richer fuel/air suggestion next. I'm also going to take a close look at the diaphragms to ensure that they are properly seated. They were replaced during the rebuild.

Kevin

[barryh]

On the basis that carb problems can turn out to electrical, I'd check the ignition system over carefully. If there are no obvious loose connections and it was easy to return to points I'd try that to eliminate ignition as the cause of the problem.

[Seth]

Is the exhaust stock?
Changes to the exhaust cause change to back pressure which can impact mixture. Less restrictive exhaust often causes popping while decelerating.

[SteveD]

When the weather can accommodate, I'd tweak it by a 1/4 turn more open then take it for a ride, bring your screwdriver. If that solves it, I'd be inclined to leave it there. That'd make it 0.75 acw turn from closed and isn't a huge leap.
If it doesn't fix it, progress accordingly.
None of those new o-rings were damaged were they?

[r67boxer]

Is the exhaust stock?
Changes to the exhaust cause change to back pressure which can impact mixture. Less restrictive exhaust often causes popping while decelerating.

Good question/point. The exhaust system is a stainless steel Keihan so not stock and quite a bit noisier than I like. It's fine when not accelerating hard but when I 'give 'er' it's quite noisy.

Kevin

[r67boxer]

When the weather can accommodate, I'd tweak it by a 1/4 turn more open then take it for a ride, bring your screwdriver. If that solves it, I'd be inclined to leave it there. That'd make it 0.75 acw turn from closed and isn't a huge leap.
If it doesn't fix it, progress accordingly.
None of those new o-rings were damaged were they?

I will try a bit richer to see if that helps. Also, the mixture screw o-ring was breaking down (slimy black crude) so I replaced same. That definitely helped on starting. Not sure why this happened though as the o-ring was replaced during the rebuild last winter.

Kevin

[r67boxer]

Further to the exhaust point, I did a quick search and found this post back in 2016 by pmonk:

"back to the muffler question. I put mac whispertone mufflers on my 90/6. They are louder than I like. Bike didn't run quite right either. Went to the Bing seminar at Billings and they suggested I go up one size on the needle jet, of course they had them for sale. It really did improve the performance. Just saying, If you put less restrictive mufflers on, you may need to rejet."

The Keihan's have one crossover at the front.

Kevin

[Rob Frankham]

Stock Kiehans are designed to have the same characteristics as the OEM mufflers... unless, of course, you have the special exhausts produced by Kiehan for the british tuner Jim Cray. You can tell if you have these because the end holes are visisbly larger than the standard version (or the OEM silencers).

Rob