Help me with Carb tuning!!!

[West]

You are still starving your carbs somehow. You probably have a vacuum operated petcock, have you checked that you are getting vacuum on the line from the carbs to the petcock?

I am assuming that you did set the floats correctly, that was covered in earlier responses.

Try setting up an alternate gasoline supply - a suspended funnel or water bottle (make sure it doesn't dissolve with gas in it), and some spare gas line straight to the input on the carbs. If it runs OK, you have a petcock/vacuum/boot leak issue.

Or you could just disconnect the petcock vacuum line from the carb it's connected to, plug the carb connector & suck on the vacuum line. You should not end up with a mouthful of gas - if you do, you definitely have petcock issues.

Just for funzies, release the gas cap before you start it - if the tank cooled off overnight and your gas cap bleeder is clogged, you might have a vacuum in the tank.

These are just some possibilities, I guarantee nothing, but I hope this helps eliminate a few items.

[akreider]

I have already eliminated the petcock as a problem, tested the vaccum by sucking on the vaccum line. I also have made sure that there are no leaks around the intake boots, which would lean the mixture. And the gas cap is works good, the carbs get the right amount of gas. The only thing left is the pilot jet because the bike runs great once I get it out of the lower rpm range that is controled by the idle circuit of the carbs.

adam

[West]

O.K., so it's not the petcock. Still doesn't make sense that it would be your pilot size. I mean, the size may actually be wrong, but your theory that the high RPM's are getting you into the needle/main jet range doesn't make 100% sense - that is not controlled by RPM's but by throttle opening/slide position. Admittedly, the higher RPM's you have at a given throttle opening, the higher the slide is going to be, but with no load, 6000 is not probably more than 1/4 throttle.

And unless the pilot size is wayyy the hell off, (like 250 pilots in a 750) it should make it lean, but not so much that it won't run at all without the choke.

I don't have much more to suggest except to look for obstructions in the pilot circuit, diaphragm leaks, or your float level being way off.

I am sure that other (better than me, which is not hard) expertise here will put you on to something, though.

Good luck.

[chef1366]

Did you get it straight?

[akreider]

Still trying, just got the pilot fuel jets in and have to try them out. I'll let you know how it goes.

Adam

[akreider]

Ok I got it running pretty good (no poppinp during decel) with #50 pilot fuel jets, 138 main jet, stock air jet, mixture screws at 1.5 turns and the eclip on the 5th groove of the needle. But now the bike only wants to idle at around 2000 rpm and I occassionally get little pop from the #4 carb. When I try to turn the idle adjuster down the bike will drop in rpms then die. I cant just turn it slowly to hit 1100 rpm. I made sure that the floats are set properly and that the throttle carble isnt hanging upl. Does anyone know how to solve this?


Adam

[akreider]

Also I checked the plugs and they are all nice and tan.

[chef1366]

Did you try the 47.5 or go straight to the 50? Seems like quite a jump for pilot jets.

[akreider]

I tried the 47.5 but it was running way to lean. it was backfiring a lot on the decel.

Adam

[akreider]

I think that I will try them again because I adjusted the needles later to the 5th notch down. When I had the 47.5s in I had them on the 3 notch. Now that I think about it that backfiring would be attributed to the needle not the pilot fuel jets.

Adam

[chef1366]

Keep plugging at it and you'll get it. The reward is oh so sweet.

[gmcpheter]

That is one thing I made sure not to do is to tighten them in, I only set them right when they touched.

Ok I have to admit my major lapse in thought while installing the jet kit, I didnt read the fine print that says to only change the air screws if it is running to rich. So I reinstalled the stock one earlier today and that solved the problem of the engine dying when twisting the throttle. The bike was just getting to much air. But I still have the problem of having to use the choke to keep the engine running. It will only run around 2000 rpm, I cannot back the choke off enough to get it to normal idle speed. I currently have the e-clips in the 2nd groove up or 5th from the top on the needle. I also have the 138 main jet installed.

My 1981 GS 850 is having the same problem and seems to be getting worse. Won't run at all unless the choke is out. Runs great as RPM's increase while driving. Dies when I pull up to a stop sign. Hard to restart.

[akreider]

The problem that I found on my bike that caused the same symptoms was a small pilot fuel jet. But it was the wrong one because I had modified the intake and exhaust which leaned the engine out where the stock jets were useless. I solved the problem by switching in 47.5 pilot jets.

Have you done any modifications to the bike that would cause it to run lean? And can you idle the bike with the choke on?

Adam

[chef1366]

Did you get it?

[akreider]

I have to check one final thing tonight but it fires up right away when I press the starter button. And it runs great, but I did swap the 47.5s back in because it was idling way to high with the 50s. I will let you know how it goes.

Adam