Tuning R90/6 Bing carburetors - idle mixture.

I have been forewarned that I may never get them matched.

I rebuillt both of them, following Bings own How-To DVD. Fairly straight forward.

But I've always had problem with idle mixture. According to maintenance manual - while idling, I shud be able to adjust idle mixture screw where the RPM's will peak.

I can do that with the right side, but not the left.

However, standing behind, listening to the mufflers - both sides sound the same. It's idling at about 1000, and runs smooth, no hesitation at any speed, or upon sudden acceleration.

After a few hundred miles, I will pull the plugs to see if there's any difference.

Any thoughts?

topoutat76 said:

I have been forewarned that I may never get them matched.

I rebuillt both of them, following Bings own How-To DVD. Fairly straight forward.

But I've always had problem with idle mixture. According to maintenance manual - while idling, I shud be able to adjust idle mixture screw where the RPM's will peak.

I can do that with the right side, but not the left.

However, standing behind, listening to the mufflers - both sides sound the same. It's idling at about 1000, and runs smooth, no hesitation at any speed, or upon sudden acceleration.

After a few hundred miles, I will pull the plugs to see if there's any difference.

Any thoughts?

Click to expand...

How many turns out is the left idle mixture screw?

fuel mixture screw

I rebuilt my carbs and had a heck of a time getting them to sync at low speed.
My problem was that I wasn't getting good flow through all the fuel passages.

There are three of them. One just above the mix screw (big one). That one was open. There are also two smaller ones located just at the butterfly. One of them was plugged. Had to take a fine copper wire and run it down the orifice to clean out.
Then plug off the big orifice and spray with carb cleaner. When I got flow through both the tiny orifices I reassembled and all my problems went away.

Some kind soul pointed this out in the Boxerworks forum and hope it helps you.

Disclaimer, I am not a mechanic. Don't pretend to be. But the Bing carbs are simple enough for even someone as challenged as me. After I had removed them a half dozen times or more it got pretty easy.

My r75/5 had this same issue.... the idle mixture screw on the left carb had absolutely NO effect on the left cylinder's idle speed. After dissembling, rebuilding and cleaning the carb passageways for the 3rd time, I noticed it was jetted differently than the right carb!!! Just a thought, you never know what to expect on a bike with crafty POs. Made the switch to Mikuni and haven't look back.

Thanks one and all.

I guess I really wanted to make sure it wasn't my imagination. Now that I know, I will rebuild - I must have overlooked something simple.

Thanks.

Thank you, PMonk!

Yep - that was it. The two holes by the butterfly. One of them was clogged.

What helped a lot was screwing the pilot jet back in before giving the orifice at the air intake side of the carburetor a blast of air. I could then really discern a small blast of air coming out from each of the holes.

So what was I able to do?

Following Snowbum's advise, first rode the bike for 10 minutes. Then put it on center-stand with fan blowing.

Then adjust idle stop screws so it was idling above 1000, and sort of sounded even from the rear. Then, adjusted idle mixtures, left and right, so that RPM's peaked. Finally able to do it on the left carbuertor.

Idle RPM now at 1500. So, went back to idle stops - backing them off a bit. Listened at rear of bike for balance. Then adjust idle mixtures for peak. Went back and forward in this manner til it was idling at 1000 RPM, exhaust sounded same on both sides, and I could still peak RPM on both sides using idle mixture.

Did not use Snowbum's shorting method. May try that later.

So, it's running smooth - no problems when accelerating. I think I'm pretty close to where it shud be.

Thanks again.

The plug-shorting method is really great for getting the low end and idles just right. A shameless plug for NorthWoods Airheads: he has a lot of little goodies for tuneups, and for my $8 or so, the shorting leads are the cat's whiskers/pj's or whatever...

North Woods Airheads

To make your postage $$ go farther, order some other tool as well, the points adjuster is a keeper, and he has lots of goodies on his site. Great guy to deal with.

A similar sounding problem was happening on my R90/6.

The thing that made the carb balance take so long for me was the fact that the bike would perform great after it had been shut off for a minute (and the bowl filled back up). I could tune it fine, then after it started starving again, I got no response from the mixture screw. The solution involved setting the main needle to a different click, and adjusting the float level. This finally let the mixture screw have a range of performance during the balance (because of the right fuel delivery rate), rather than just one "bleh" feeling idle.

Now it runs like a top.

dht7nine said:

My r75/5 had this same issue.... the idle mixture screw on the left carb had absolutely NO effect on the left cylinder's idle speed. After dissembling, rebuilding and cleaning the carb passageways for the 3rd time, I noticed it was jetted differently than the right carb!!! Just a thought, you never know what to expect on a bike with crafty POs. Made the switch to Mikuni and haven't look back.

Click to expand...

out of curiosity, is this a big issue?

i've got a 135 in one carb and a 130 jet in the other (at least i think those are the numbers)

my bike takes FOREVER to warm up and run at a normal idle even in the city in the summer and i feel things are not as smooth as could be.

I don't think that the difference between 130 and 135 would be noticeable.

Your forever-warmup etc may be due to running lean at idle, which it will not do very well until the engine is well warmed up. Try turning your idle mix screws to a richer setting. (iirc OUT for the CV carbs, as it is a fuel screw, IN for the slide Bings as it is an air screw)

Smooth is a function of getting the carbs balanced/matched: do it at idle and then at above-idle. Lots of threads/posts on the various ways to do this. Makes a HUGE difference in smoothness

rpeckham136133 said:

I don't think that the difference between 130 and 135 would be noticeable.

Your forever-warmup etc may be due to running lean at idle, which it will not do very well until the engine is well warmed up. Try turning your idle mix screws to a richer setting. (iirc OUT for the CV carbs, as it is a fuel screw, IN for the slide Bings as it is an air screw)

Smooth is a function of getting the carbs balanced/matched: do it at idle and then at above-idle. Lots of threads/posts on the various ways to do this. Makes a HUGE difference in smoothness

Click to expand...

yeah, actually, the "smoothness" once warmed up isn't really an issue, it's mostly just how long it takes to warm up before i can turn the choke off fully. and it's still idling around 1250 or so rpm. i'm worried if i set the idle any lower it won't start

long warm up

Have you raised your needle?

My 90/6 is supposed to be set with needle in first (leanest) position.
I could not get it to accelerate without the similar symptoms until hot.
Raised needle to second notch (richer) and it improved.

However all problems were not solved until I got all orifices cleaned on fuel mix
circuit. see above post.

Also was having issues with timing. It was too advanced at speed when timing set correct at idle. Lots of pinging. Installed Omega ignition and that helped also.

Seems like there were several things contributing to my problems.
Just keep plugging away, when it gets right it is really sweet!

BubbaZanetti said:

yeah, actually, the "smoothness" once warmed up isn't really an issue, it's mostly just how long it takes to warm up before i can turn the choke off fully. and it's still idling around 1250 or so rpm. i'm worried if i set the idle any lower it won't start

Click to expand...

If you try the method in the Bing book you may have a simple fix.

Engine warmed up take a ride, try turning the idle mixture screw in or out (Out I would think in your case) one screwdriver blade width at a time, this should correct your stumbling until HOT problem. I surmise that your mixture is just a tiny bit lean which finally runs reasonably well sans enrichener only when the engine is hot.

The good thing is that this will only cost you time to try. Don't forget that the "choke" really isn't a choke, and while set to anything other than off only has an effect when the throttle is completely closed. The enrichening is giving you residual fuel during the transition from idle to regular running, a job that the idle mixture screw and circuit is there to perform when properly adjusted/operating.

The main bad thing in this is that you are to some degree fuel/carbon fouling your plugs piston heads and combustion chambers and putting out extra hydrocarbons from your exhaust.

amiles said:

If you try the method in the Bing book you may have a simple fix.

Engine warmed up take a ride, try turning the idle mixture screw in or out (Out I would think in your case) one screwdriver blade width at a time, this should correct your stumbling until HOT problem. I surmise that your mixture is just a tiny bit lean which finally runs reasonably well sans enrichener only when the engine is hot.

The good thing is that this will only cost you time to try. Don't forget that the "choke" really isn't a choke, and while set to anything other than off only has an effect when the throttle is completely closed. The enrichening is giving you residual fuel during the transition from idle to regular running, a job that the idle mixture screw and circuit is there to perform when properly adjusted/operating.

The main bad thing in this is that you are to some degree fuel/carbon fouling your plugs piston heads and combustion chambers and putting out extra hydrocarbons from your exhaust.

Click to expand...

thanks a bunch, i feel that you're probably right as i hamfistedly adjusted the bike the same way i adjusted the oilhead, but substitute the mixture screw for the big brass screw on the oilhead.

then i read the snowbum article

reset my misture screws so they were out just a turn or so, then did it "properly" using the mixture screws and the throttle stop screws, and since they, the bike has been slow to warm up. i bet the screws DO need to come out more.

but riddle me THIS: i put in new floats right before i went to the rally. they came with other parts that i'd ordered and figured they were the last step in truly overhauling the carbs (had already done the diaphragms and rubber bits a month ago) and now the bike is even harder to start/keep running when cold.

i've been told that old sinky floats can make the bike run rich, so these new floats might be enhancing the lean condition, but WHY is this???? if for a given vacuum pull and jet size, why would it matter how much fuel was in the bowl, as long as you're not running out?

i'm learnin' here, thanks guys

The float level affects the "height" of the fuel in the idle circuits: there is not a tapered needle, just an idle jet, so if the fuel level is low, it is harder for the fuel to get sucked out.... but having said that, the most likely culprit is a bit of crud in the passages... those little holes are teeny-tinyand it doesn't take more than a spec to at least partially block them. Consider that there is a tiny fuel passage AND a tiny air passage for the idle circuits, and it is no wonder that they give us all fits. The idle circuit alone is the reason that I run those little inline fuel filters....

""but riddle me THIS: i put in new floats right before i went to the rally. they came with other parts that i'd ordered and figured they were the last step in truly overhauling the carbs (had already done the diaphragms and rubber bits a month ago) and now the bike is even harder to start/keep running when cold.

i've been told that old sinky floats can make the bike run rich, so these new floats might be enhancing the lean condition, but WHY is this???? if for a given vacuum pull and jet size, why would it matter how much fuel was in the bowl, as long as you're not running out?""

My best guess would be that the floats as they came to you and as installed are not properly set to give you the correct fuel level in your application. I doubt that the manufacturer sends them out with any more than a ball park adjustment. Very much like spark plugs, reliance on the installer to double check adjustments and fine tune the parts after the sometimes not so gentle shipping process is taken for granted. The correct fuel level in the bowls is important to the proper function of the carburetors.

Another tip.

I had tried Snowbum's shorting technique when my left carb's two-or-one ports by the butterfly was clogged.

When I shorted the right cylinder the engine died. When I shorted the left cylinder, the rpm's reduced a bit, but it did not die.

After using copper wire to clean those ports, when I shorted the right cylinder, RPM's drop to about 500, but the engine did not die.

Likewise, when I short the left cylinder, RPM's drop to about 500, but did not die.

In other words, if you short one side, the other side shud still be able to keep the engine running. If not, something's not right - could be as little as those two ports.

Wow - just like Snowbum said it would. And you can really match the sound and RPM's so they're pretty much the same.

Just don't know how satisfying it was to solve this problem, and have everything operaing like it shud.

Great forum - great mechanics.

Actually.......when I pull the plug wire/short it to the head, I like for it to just chug a few revolutions and then die.............I end up with an idle around 700 or so.....just a thought........Dennis

Fuel level in the Bowl

Amiles, The pressure(atmospheric) on top of the fuel in the bowl is constant. The pressure(vacuum) between the butterfly and intake valve stays the same as long as the b'fly in on the stop. When you change the height (float level) the fuel has to be pushed to get into the carb bore, more fuel flows with a higher float level. In other words, more or less resistance to flow causes less or more flow through the same size jet. This is why float level has an effect on idle mixture.