74 R90/6 Backfring and stalling on downshift/deceleration

[leigh]

Hi everyone. New member, new owner here. Thank you for all of the info I've gained lurking the forums. I've been riding my 74 R90 for a few months as I work through the areas that need attention and enjoy the bike for the summer before I take it off the road for the winter and do a refresh with it. Most of the systems are in good working order with a few things being quirky but all signs point to those being worked out with the refresh. One thing that is happening and seems to be getting worse is that on down shifts to a stop or slow down, i.e. coming off a highway or up to a stop sign after a stretch at speed the bike will backfire and stall. I have tried to approach this situation by pulling in the clutch and gliding to the exit or stop without down shifting and it will stall and backfire as well as down shifting at appropriate speeds and once it gets in to first it will do the same. The bike will sputter and backfire if my up shifts are not timed accordingly and I attribute that to operator inexperience. I just came back from an afternoon ride from Wallingford Ct to Gillette Castle and on the way back the problem was noticeably worse that whenever I came to a stop sign or similar decrease in speed it was stalling and backfiring. If anyone can point me in the right direction for diagnoses or resolutions it would be greatly appreciated. THank you again for the information and I look forward to hearing what all of you have to say. I am really enjoying this bike and want it to run as smooth as possible. Thanks.

 

[dieselyoda]

Hopefully someone will weigh in here with specific details but it sounds like your bike isn't returning to the idle circuit on decel. I would do a "carb" tune-up, curb idle, idle enrichment and then cable balance.

You also didn't mention how old the carb diaphragms are. I would guess if you haven't got relatively new diaphragms, they look like a well used condoms as they have been subjected to ethanol.

One tank with 2% ethanol will destroy the diaphragms.

 

[20774]

Welcome to the forum! Certainly doing some carb work seems prudent. Be sure that the floats are set properly. If the floats are set such that there's not enough gas in the bowl, coming to a stop will shift the fuel forward, making the already lean situation worse. Also check the clamps on the carb-to-head rubber tubes...if they're loose, they will also lean the mixture.

Even before doing that, you should probably be sure the valves have the correct clearance. Tight valves make for a poor idle. And also you should confirm the ignition timing. S-mark in the window at idle RPM (say 1000-1100 or so) and that the F-mark comes into the window around 3000 RPM and stays there when the RPM is advanced beyond 3000.

 

[leigh]

Thank you guys. Good starting points for me. Rainy day here so this seems like A good activity. I'm sure I'll have more questions along the way. Appreciate the help.


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[jagarra]

Before you adjust valves, check head torque first. Made mistake of adjusting valves then torquing head, then had to readjust valves.

 

[happy wanderer]

Hopefully someone will weigh in here with specific details but it sounds like your bike isn't returning to the idle circuit on decel. I would do a "carb" tune-up, curb idle, idle enrichment and then cable balance.

You also didn't mention how old the carb diaphragms are. I would guess if you haven't got relatively new diaphragms, they look like a well used condoms as they have been subjected to ethanol.

One tank with 2% ethanol will destroy the diaphragms.

Say What??? Ethanol may speed the decay of carb diaphragms but one tank DOES NOT destroy them. Torn diaphragms usually present as a bike that won't make full power or does not accelerate properly.

Backfiring on Deceleration has nothing to do with the diaphragms and everything to do with the mixture. Usually it is caused by carbs that are set too lean and unburned gas is blowing up in the exhaust. Yes too lean. Sounds backwards but it isn't. Join the Airheads club and read the Airlist or go to Snowbums extensive information on this subject.

Another thing to look at is the main jets being worn and also the needle being worn. If the OP has not refreshed (rebuilt) the carbs in a long time another problem that will affect mixtures is worn O Rings throughout. Take the main jet out and look at that O Ring. If it is cracked, hardened up or worn flat then the rest of the O Rings will be in the same if not worse condition usually. Time to get out the Bing Carb Manual and learn how to rebuild them. It's not too hard to do.

 

[pglaves]

Yes it would seem to be a lean condition at idle or just off idle. So a couple of questions and comments.

If the bike is warmed up and stationary how does it idle? Does it still idle well on the sidestand?

If the idle is rough with the bike stationary I would remove the idle jet from the bottom of each carburetor and clean the jets. Then I would use spray carb cleaner up into the passages from where the jets were removed.

If the idle seems OK with the bike vertical but not on the side stand then correct the fuel level in the float bowls. You should check this anyway. I mention this because if the fuel level is too low the engine will fuel stall as the fuel sloshes forward on deceleration.

That is what the few clues are telling me.

 

[leigh]

Yes the bike idles just fine on side stand and center. I'm going to rebuild the carbs as they are in need anyways. I've been planning on tackling them and this is as good a time as any. Even with a week of good weather ahead. So hopefully I can get them back together and operational in time. Wish me luck and thank you for the tips.


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[lmo1131]

I'm going to rebuild the carbs as they are in need anyways.

Don't forget the throttle shaft "o" ring(s), a lot of folks skip replacing them because they're a PITA to get to. You'll have to pull the butterflies to get to them; so don't forget to re-install the butterflies pointed in the correct direction.

http://bmwmotorcycletech.info/bingcv.htm

 

[guenner]

Mechanical ignition advance system slow to retract?

/Guenther

 

[dieselyoda]

Say What??? Ethanol may speed the decay of carb diaphragms but one tank DOES NOT destroy them. Torn diaphragms usually present as a bike that won't make full power or does not accelerate properly.

Two years ago, on my R80RT that had the Bing Ethanol kit, bowl, floats, diaphragms, I fueled at a very well known Western Canada fuel distributor. About 30 miles after fueling, lost power, and starting acting very strange.

At the time, I was heavily involved in this whole bio-fuel fiasco and had the fuel tested. Advertised to be premium with no Ethanol at the pump but it had 2%. Distributors in Canada only have to report if Ethanol content is ABOVE 2%.

Pulled the carbs and sure enough, diaphragms felt like well used condoms. Exact same thing happened to the diaphragms in my KZ 440 and now my buddy's KZ 650. The 440's diaphragms were only a couple years old and I think the 650's diaphragms were ancient.

 

[cycleman2]

Check your timing, if it is stock you'll have points. If it was points it would seem that the gap had closed up. If that happens bike will backfire and be down on power. That is the first thing I would check before I messed with carbs. So disconnect the battery first and take the front cover off.