zen and the art of tuning a carb

[dp63130]

Having a little trouble with the carbs on a 1975 r90/6
Here are the symptoms.
bike is a little hard to start but usually starts on the second time...
once I turn the choke off it tends to race around 2,000 rpm with the idle screws turned all the way down.. infact the idle screw isn't even touching the throttle support arm.

I have the mixture adjust screw out about 1/4 of a turn --it seems to like it there..
the jet needle is set to the first position and the idle jet is out about 1/2 a turn..

seems like the engine is getting to much fuel but I checked the choke housing, all the gaskets, and o rings -everything looks fine..

any ideas what would cause the engine to race that high when the idle screw is all the way out.. -the bike has been restored with all new parts.

valves are set to spec.
Thanks in advance.

[jgr451]

Wild guess,no claim to expert status:floats in the bowl too high?Carburetors not level?

[dp63130]

will check that... thanks

here is something else. one fo the jugs is running extremely hot. The plug is just as white as can be. The pipe on that side is extremely hot as well.

[jgr451]

Further inexpert guess:sticking exhaust valve;dirty oil(will not circulate freely around all the galleries;some galleries on the hot side are blocked by crud)?;inadequate oil pressure/failing oil pump(though the whole engine would be hotter if the last 2 were the case.
Used to have a 1971 R75/5,now have a 1984 R100RT,plus shop manuals.

[jgr451]

did not mean to use the Stick smiley,but there it is!!
I meant sticking exhaust valve..

Are the plugs the correct temperature range and both the same?

[russbritt]

The side that is running hot is possible lean, check that carb first.

[dp63130]

its a little on the lean side after some inspection. but still does not explain why the idle is so high... maybe an o-ring vanished and fuel is getting through some where?? -choke housing?

[russbritt]

Maybe the lean carb is sucking air ? Leak in carb or carb boot etc.?

[sgborgstrom]

How long has this been going on? Is it something that started after a certain procedure or over time?

Some long distance thoughts on possible causes include:
Lack of freeplay in the throttle cables keeping the plates open

Improper rebuild of carbs with throttle plates not centered in bore

Choke cables improperly adjusted

The problem might be in one carb sticking "open" the bike won't idle very well but it will run.

[dp63130]

I thought it was an air leak too, but everything is nice and tight. The problem just presented it self about a week ago after some moron decide the best place for his unfinished lunch was all over my heads and carbs. So I gave the bike a good cleaning then broke down the carbs just for the hell of it... just to be safe. Now it idles around 2,500 and the right pipe gets red in no time. The mixture screw is out 2 full turns.. The idle mixture screw is out 1 full turn now as well... Still seems that the right side is staving for fuel but the bowl is at the correct level. I think the only thing left to do is break the carb down again and check for junk in the smaller areas. It's actually pretty nice to be working on the bike again.. renews my faith in bmw machines to run even when completely out of tune.
Thanks for all the help.

[Win3855]

Originally posted by dp63130
after some moron decide the best place for his unfinished lunch was all over my heads and carbs. That really deserves a more through explanation. Please elaborate.:confused:

[dp63130]

to explain.. I work ata' University in downtown Saint Louis so not only do I have to contend with the drunken student getting on my bike and pretending to be riding (yes, I have caught kids sitting on my bike) but my cylinder heads also serve as a hot plate for the homeless...

[camfarm]

I agree with sgborgstrom on the order of possible causes. My most common carb problem is sticking throttle cables -- alot there to check from grip to carb.

Added note, the 1/4 and 1/2 turn out you mentioned -- from where? My manuals and notes indicate there is a 'baseline' setting of 1 turn out for idle and 1 1/2 turn out for throttle mix (just barely touching the throttle stop = zero turn out).

Also, notes from wherever (Airhead list??) indicate that over time (meaning 25-30 years) it is possible that your carbs just won't sync that well.


[Hijack]
Meanwhile, to hijack your thread just a little, the Dellorto's on my R90S are wacky (technical term). Right side hot (too lean perhaps, sticking cable probably) and left side very cool but won't run up the rpm - tends to stall that side.

I plan to adjust valves in the morning followed by the check ride and the rebaslining of the carb settings and a re-synching effort using manometer.

Any tidbits of advice before I pursue that?

Thx [hijack over]

[camfarm]

For some reason I cannot edit my own post. Please ignore typo's and spelling errors.

[mthelmet]

ave you checked the diaframs for a pin hole? Had that problem on an R75/5 with high miles, [55,000 there abouts]. Dealer said he never saw a bike run with pin holes in a carb diafram like that, so anything is possible!!!:dunno

[lkchris]

Originally posted by mt helmet
ave you checked the diaframs for a pin hole? Had that problem on an R75/5 with high miles, [55,000 there abouts]. Dealer said he never saw a bike run with pin holes in a carb diafram like that, so anything is possible!!!:dunno

Diaphragms are important, but have little to no effect on idle.