carbs? mileage is KAKA

[jcofwmburg]

My bike is a '74 R90/6 102000 miles
Just got back from the NC airhead rally and I have questions on carbs.
My mileage on my last two tanks were 33 and 34MPG.
Bike ran well no problem.
Saturday on a hot day at idle the engine would slow down and die.
On returning to camp, gas off, saw some dripping from right side carb, turned gas on a little later, no leak.
Right side carb has a lot of dry black stuff on carb body below the dome cap, also on right side air cleaner near air tube to carb.
Both exhaust are black and getting black deposit on bottom of saddle bags.
My wife says she can smell the unburned hydrocarbons when she is following me.
No smoke from exhaust either speeding up or slowing down.
Last carb rebuild 10+ years ago but don't get a lot of miles per year.
Bike had a major maint. at 100K but no carb work as I can recall.

So, carb guru's, is it rebuild time? Dare I do this myself, not a great mechanic? Can I get step by step instructions? Thanks, JC

[Rod Sheridan]

JC, I'd start by checking the operation of the floats.

Do they float well in gasoline or are they in need of replacement?

Are they properly adjusted, and is the needle valve/seat in good condition so that the fuel flow does stop as required, and the float level is set correctly.

If that doesn't cure the problem, I'd buy a rebuild kit and replace the gaskets o-rings, jets/needle and diaphragms etc.

Regards, Rod.

[88bmwJeff]

I'm no guru, but I would say it's time to do a rebuild. It's possible the floats are getting heavy and causing the bike to run rich. Also, some of the passage ways could be blocked or partially blocked. Cleaning and rebuilding is not that difficult. Clymer's has a good step by step instruction. Also, I would search online.

If you do decide to rebuild them yourself. Here are a few tips.

Do one carb at at time. That way you don't mix left carb part with right carb parts. Also, it will give you a good example of how things go together.

Be careful of the throttle shaft/butterfly screws. They are peened; as such, the threads on the screws need to be filed/ground down or you will strip the threads on the shaft.

[lkchris]

Rod has stated the sequence of diagnosis perfectly.

Floats are definitely a first priority.

Next should be verifying that the enricheners (chokes) are indeed shutting off as designed.

There's really not that much associated with the jets and o-rings that will cause much in the way of rich running and these are the 3rd step for sure.

Go ahead and check both carbs, as since there is a crossover in the exhaust system, both cylinders exhaust through both mufflers and it's therefore a bit difficult to find the culprit looking at color of pipes.

It's always good in any such process to verify valve adjustments and FWIW I'd note that when my bike once suffered horrendous fuel usage, the culprit was loss of compression in one cylinder.

[AnnapolisAirhead]

+1 on the carb rebuilds and doing them one at a time. I'd start by buying the Bing carb book for $10. A good investment, good setting info and exploded views of how things go together for each carb they make. It'll tell you the default fuel mixture setting. Here is a link (I think you have to call them). http://bingcarburetor.com/manual.html

I run my carbs a bit richer to eliminate popping on deceleration by turning the mixture screw out 1/8 of a turn more than the book recommends. YMMV and I'm sure there are many opinions on this. In hot weather, I'd rather be more rich than lean. That's probably what your wife is smelling, but it could be from the float settings that others mentioned too. The leaking gas might be some gunk in the float needle(s) that prevents the fuel from shutting off too. Bing sells alcohol proof floats, but there are mixed reviews from what I've read and the claims of greater fuel economy hasn't seemed to come true. However, with more and more ethanol in today's fuels I'm reconsidering giving them a try. I think you need to order their bowls too as the float mechanism is different. If you call Bing for parts (not the manual), have your carb numbers handy--they are located on the webbing between the carb body and the dome. Should be something like 64/32/325 or at least in that format. The Bing folks will need that to get parts to you.

I had the black mess on my carbs too. Turned out to be (of all damned things) brake dust. Cleaned my carbs, serviced my brakes (squeaky at the time) with new pads and the black stuff went away. Although I also replaced the rubber tubes between the carbs and heads. If you spray carb clean on the tubes between the carb and the head (not the air filter tubes) while the bike is running and the idle changes, then you have a leak. At 100k miles, you might want to make the investment in new tubes.

Good luck. Lots of info on carb rebuilds in here. They are 'handed' so doing one side then the other is advisable. If the butterfly is functioning well, I'd suggest not replacing those particular o-rings especially if this is your first rebuild.

[jcofwmburg]

Thanks, how do you determine if the floats float properly?
I think the choke is ok going by the change in engine sound when applied at startup and when warmed up a bit.
The bing link didn't work. Will google them.

[AnnapolisAirhead]

The bing link didn't work. Will google them.

The link works, but their site is down. I just got off the phone with them.

[mymindsok]

You can test the floats by dropping them into a beaker of fuel but after 10 years? Spring for new floats and needles too. IMHO, you're going to need em.

The age, by-the-way, is not the deciding factor. Usually its related to mileage but after 10 years, I'd say that it's time for new seals and o-rings everywhere and you might need needles and jets as well.

After that, a carefull tune-up will tell the tale.

[jforgo]

yes, definitely new floats and float needles after all this time. You will be doing this more often with the new ethanol fuel

[88bmwJeff]

Bing sells alcohol proof floats, but there are mixed reviews from what I've read and the claims of greater fuel economy hasn't seemed to come true. However, with more and more ethanol in today's fuels I'm reconsidering giving them a try. I think you need to order their bowls too as the float mechanism is different.

Bing sells TWO types of alcohol resistant floats. There's the independent floats, which you've described above. This is a kit and comes with new bowls. More recently, Bing created a hinged alcohol resistant float, which is a direct replacement for our "white" foam ones. I would recommend getting these newer ones over the independent float kit. Although I haven't tried either.

[Polarbear]

We're ALL going to cry more about poor mpg's with more ethanol, as its already made its mark on poor gas mileage, nearly everywhere. I've heard from numerous Airhead owners and even my new BMWs getting poor gas mileage, due to the ethanol. NOT so quick to rebuild a perfectly good carb, when its the GAS:(. I ride cross country several times a year and its(ethanol) a factor indeed. Wonder how much longer we put up with that crap, subsidized up the ying yang by our tax dollars!!! Randy.....PS; not too far off topic, but its NOT the "carbs" anymore:)....its what we're buying to run in them!

[AnnapolisAirhead]

Of the two types of alcohol proof floats that Bing sells, only one requires a new float bowl. The other is a direct swap for the OEM floats, only black in color. You still use the float needle but at $35 per float, I'm giving my float more time since they are only 10 months old.

+1 on the '..how muich longer we put up with that crap....' comment from Randy. Turning our rides into AA members.

[lkchris]

I run my carbs a bit richer to eliminate popping on deceleration by turning the mixture screw out 1/8 of a turn more than the book recommends.

You're only adjusting idle mixture, of course.

This has little effect at bigger throttle openings when diagphragm is pulling needle jet out of main jet.