Fine tuning carb setup

[syvrider]

92 R100GS. Dual plugs. 9.5 pistons. Wedgetail ignition, timed correctly (confirmed with Mark in Australia). Valves set properly. Carbs cleaned with all new o rings. Butterflies installed and working correctly.

Needle jet 2.68. New needles on third notch down.

Only remaining issue...lots of popping closing throttle at and under about 3000rpm. True with both 145 and 150 mains.

As an experiment I set idle high...2000 rpm. No popping. Comes back at normal idle setting.

Thoughts on what to do next?


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[20774]

What are the carb numbers? Have you done a carb synch? Not sure how to do it with dual plugs, but I suspect you would need to pull the lower plug wire and stick a sacrificial plug in there with the plug threads resting against the cylinder fins. Most won't consider the plug shorting method to synch carbs with the later electronic ignitions but it can be done correctly. Otherwise, you're going to need something like a TwinMax or ColorTune (or is it CarbTune?).

Sounds like you don't have the idle mixture correct. Where is the idle mixture screw set at? Before trying anything else, you could try resetting them to the factory spec and then adjust accordingly, trying 1/4 turns each direction to see if the problem is eliminated.

 

[stevenrankin]

Ignore it

Ignore it, it causes no harm. I have had this issue for the life of my 84 R80RT, my original dealer/mechanic said it happens. 230000 miles and many years, I am still riding the bike. I had the heads rebuilt at 50K miles due to the problem with valve seats that cropped up in those bikes from time to time. I did rebuild the jugs with higher compression pistons and a cast iron liner after the "wonderful" nikasail units started slapping the cylinder walls. That was at 95K miles. Since then, I am riding.

Set the carbs to factory settings and ride it.

By the way, I installed a wedge tail point replacement system on my 78RS. I was impressed with it, sadly I never got to put more than 500 miles on the bike before I totaled it in a stupid accident. The company from my experience lives up to the marketing promises.

If the stock system in the 84 Ever fails, I will replace it with the Wedgetail. St.

 

[syvrider]

I've set idle mix per Snowbum's method. ie, start at two turns out. Slowly turn in until you hear a stumble. Back out about a quarter. Do that for both sides then sync again.

Sync done with a Carbmate unit. At idle with the screws. At ~2000rpm with the barrels.

Is this popping really just part of life with these? It's loud and annoying.


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[syvrider]

From what I understand, there should be plenty of vacuum at 3k to be pulling fuel up the needle jet. And that's where the popping starts on a closed throttle.

Idle jet should be moot anywhere past 1500 or so.


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[20774]

I've set idle mix per Snowbum's method. ie, start at two turns out. Slowly turn in until you hear a stumble. Back out about a quarter. Do that for both sides then sync again.

Hmmm...basically correct, but if I recall correctly, you turn it in until you hear a stumble, then turn it out until you hear it stumble, then turn the screw back to the mid point between the two...with a little hint towards the rich side which is a bit of turn out from the mid point.

Idle jet is in play up to 25% throttle, so I think it could be an issue. Bottom line, all of the ranges of the carb - idle, mid range, wide open throttle - interact and need to be adjusted accordingly. Not to mention that carb adjustments need to be made when the engine/carbs are at operating temperature.

I didn't see you mention the carb numbers. It would be interesting to compare your settings with the Bing book.

 

[syvrider]

Carbs are stock to this bike.
L - 64/32/357
R - 64/32/358

I got the bike with all stock jetting (per Haynes) except for 150 main (stock is 135). Was popping on decel. Consensus among Airheads and carb experts was...you're running lean. Too lean a mixture means it's not all burning, and the extra fuel ignites in the exhaust header.

Advice also included removing pulse air system. Did that. No change. I tried 135, 145 and 150 mains. No change.

So I overhauled the carbs and bumped up to 2.68 needle jets.

Still popping. Although...it was popping at higher rpms before. Now it starts at about 3000. Close throttle at 5k - no popping until it comes down to 3k. Then pops all the way down till I pull in the clutch or open the throttle back up.


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[Omega Man]

FWIW, my F800GS pops on deceleration- (exhaust). Never been concerned about it.
Good luck.
OM

 

[20774]

Thanks for the numbers. Looks like you have all the settings starting right. The main jets really don't do much as they don't become affective until you go wide open throttle maybe around 65-70% throttle...most never really see that unless you're constantly running at very high speeds. You said you have the needle jets at the 3rd notch. Just to be sure, the notches are counted from the blunt end of the needle, not the pointy end. Note: I just reread your first post and you have counted notches properly.

I still think that a complete carb synch needs to be done to reset the situation so you can make fine adjustments from there.

The Bing book says that the initial idle mixture screw setting is 1/2 a turn out. You said you are starting at two turns out. As I said I think it would be better to go with the book settings and try syncing the carbs from there. Remember the idle mixture screw is controlling a gas circuit, so further out only richens the mixture.

 

[manicmechanic]

That could be just what is known as a lean afterburn, possibly due to an air leak. Time to get out the ether, carb cleaner or even WD40 and spray all around the intake from the carbs to the heads, then even the exhaust system. Spray a connection and watch for a rise in RPM, then go from there.

 

[Omega Man]

Supposedly, the reason lawn tractors and related have wires to the carburetor is to instantly shut off the fuel preventing the “bang” at shutdown.
This is related to the aforementioned “lean after-burn”.
OM

 

[syvrider]

Went back to first principles. I would have sworn the carb-head boots were tight. Sprayed some carb cleaner on there...engine sped up. Took them off and cleaned thoroughly and cranked the hose clamps really tight.

Then the air mix was very sensitive to small changes (it wasn't before). Got that set right.

No popping!

Sync with throttle open was then way off. Got that right and a little stumble off idle that I had been tolerating went away.

It's now running perfect.


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[stevenrankin]

Again

Okay drive yourself crazy trying to eliminate a non problem. You have a BMW 95 airhead, and I hate to say it this popping on deceleration is normal for the bikes.

You have the bike set up to run properly, ride it and enjoy it. Or, LOL, go crazy trying to fix something that is normal.

I bet if you do a forum search form the past, this topic has been covered and the same answer comes up. St.

 

[syvrider]

...I hate to say it this popping on deceleration is normal for the bikes.

...

It doesn't do it anymore. It's not supposed to. That's why I wanted to chase down the issue.

A few pops is normal. Waking up the whole neighborhood is lazy bike ownership.


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[Visian]

Useful thread! I am having somewhat the same issue with my R80G/SPD+... with the added feature of stumbling/hesitations as the bike revs up from idle while underway (sometimes). This is not the famous CV-carb flat spot. I say sometimes because it doesn't happen every time.

I've tried everything, I have the bike running and idling well, so I am assuming that my idle circuit on one or both carbs is/are fouled (bike sat for two years during its refurbishment).

So, I shipped the carbs to a friend for ultrasonic cleaning and a rebuilt. Hope this fixes it.

fwiw, the Bing Manual is very good for carb setup, if not somewhat weirdly written.

photo & retouching by Chris Parker, BMW RA OTL Editor

ps -> I have also learned that I did not mount my carburetors level, which could possibly be part of the problem, but I doubt it.

 

[pglaves]

"Simple" by design they say! Ha!!!

As for loud bangs, not little pops, K75s do that when excess fuel that didn't burn gets into the exhaust and then ignites. With Airheads it might also be a fuel mixture issue.

 

[stevenrankin]

Ah

LOL, I over reacted, I thought you meant burbling, when you meant popping. Sorry one of the reasons I would rather talk about issues on the phone or in person, better communication.

I do thank you for the information on the fix and am happy you did. Sorry for the misunderstanding. St.

 

[Omega Man]

As an aside-
This is my favorite BMW. It has the appearance of go-anywhere simplicity.

OM

 

[Visian]

As an aside-
This is my favorite BMW. It has the appearance of go-anywhere simplicity.
OM

imo, between this and the R80ST, I don't think there's ever been a more suitable BMW motorrad! No wonder it was the choice of so many world adventurers...

 

[pglaves]

As an aside-
This is my favorite BMW. It has the appearance of go-anywhere simplicity.

OM

That is what I rode to and from Alaska in 1991 while Voni rode her R80ST with a raised front fender (better clearance to dig out mud) and we both had 50-50 dual sport tires. It was a great bike which I traded in when I bought Voni Big Red (R1100RS) for Mother's Day in 1993. She still has and rides Big Red. I wish I still had my R80G/S.