Idling issues!

[Vloukole]

Hi guys

My motorcycle wont idle. The carbs are ''synced''. I tried using the pilot mixture screw to get the motorcycle to idle as high as possible(3 turns out). That made a difference but not enough. The only way the motorcycle would idle is by adjusting the idle speed screw at the bottom of the carbs but then when I rev the motorcycle the rpms would hang at 3000rpm. Any ideas?

One thing I don't really understand: a Vacuum leak causes high idle because of the added air. Why does the rpms increase when you increase the fuel at idle using the pilot mixture screws?

Would a overly rich motorcycle struggle to idle?

 

[Sesty]

what year is your bike?

what does "synced" mean? bench synced?

have you taken you carbs off and inspected and cleaned them?

my 78 had trouble idling recently had to clean the carbs several times over and even when fully clean all passages clear including the idle circuit i still had issues and had to mess with the mixture screw settings

 

[16VGTIDave]

If you can't get your bike to idle, how did you sync the carbs? If you had the idle speed screw adjusted too far, you weren't operating the carbs on the idle (pilot) circuit, so your sync isn't valid.

If the engine is hanging at 3000 rpm, then the idle mixture is too lean and the idle speed screw is turned in too far. I'd strongly suggest cleaning the carbs. It is common for the tiny pilot passages to be blocked with corrosion or debris.

 

[Vloukole]

It's a 81 model. Yes I bench synced it. When I use the idle speed screw to get it to idle it idles at +- 1250rpm but when I touch the throttle it will increase and hang. I have cleaned it but seems to me I did not do a very good job. I will clean them again. Do you think that one size up on the pilot jets will improve the idle?

 

[Vloukole]

So what you are saying DAVE is that when I open the butterflies to much the pilot circuit will be bypassed and the main jets will come into play? Something like that?

 

[Recreateme]

If you have bench synced your carbs, and have screwed in the throttle stop screw all the way then you definitely have a leak or a plug somewhere...

Hanging at 3K is probably caused by something wrong with your pilot jet. Most likely its blocked or partially blocked. So what is happening is if you try to idle it can't because of the blockage, but the pilot, needle and main jets all have an overlap a bit so when you get the throttle opened to a point where they all overlap the needle jet and main jet take over and it revs, but when you let off the throttle it would hang at 3K because its trying to go back to the pilot jet but it can't so its leaning out whatever fuel is left and then it would die.


This could be the case if your pilot jet is undersized too, like if you put on pod and shortys and upped your main jet a bunch but not your pilot jet. It would be running lean in the 'pilot zone' and would hang high becuase of that leanness..

So i would reccommend cleaning your carbs again, and if you did any mods maybe revisit your pilot jet sizes..

Also if you have an air leak it could mimic this behaviour a bit too depending on where the leak comes from..



EDIT: when you say increase and hang.. do you mean increase to 3k and not pass it even with WOT, or it will rev all the way through the range except when you let off it falls but sticks at 3k a bit longer then the others?

 

[Vloukole]

Thanx. Yes it will pass 3k no problem but hang when coming down. I bought the bike with after-market exhausts(very loud). Will that add to the problem? Or is that the problem?

 

[Recreateme]

You could always put in a larger sized pilot jet and see if it helps. It's probably running lean so it sticks when the main jet turns off. It probably couldn't hurt to put on new jets all around rather then having the mixture screws cranked open.

Chances are the exhaust is messing with the factory carbs. You'd be better off upsizing the jets and cleaning the carbs.

Theses bike can run with open headers so your exhaust is just changing the setup so your gonna need to adjust the carbs to match the exhaust.

What goes in the engine has to match what comes out, your exhaust is letting stuff out faster then your carbs are letting things in.

 

[Vloukole]

One size up on pilot and main jets?

 

[Recreateme]

Maybe, you might wanna order a size up and two sizes up and see.

But first clean your carbs because it might just be a block pilot jet passage or something you can fix for free

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[xschris]

Sync them with a manometer or vacuum gauge. Make sure there are no holes in your diaphragms also. The main jet is only for 7k+ running. I would start with a 45 pilot jet.

 

[16VGTIDave]

So what you are saying DAVE is that when I open the butterflies to much the pilot circuit will be bypassed and the main jets will come into play? Something like that?

Not exactly. The pilot circuit is from closed throttle to about 1/4 throttle. Then the needle and jet. The main jet is for above 7000 rpm, typically wide open throttle. These 3 circuits overlap each other a lot, so changing one effects the others. Fine tuning these carbs starts with the main jets and works down to the pilot screws. But that can wait until you have it running decently.

The pilot screws control idle mixture and the transition to the pilot circuit and it's jet. If you have the throttle too far open, you will already be operating on the pilot circuit. This would also explain why the pilot screws had little effect for you. Or your carbs are dirty, or the o-rings on the pilot screws are bad or missing, or the throttle shaft seals are leaking, or...

When you remove the carbs, take a look for the holes at the top where the throttle plates close. The one for the pilot screws is down stream from the throttle plates. There are 3 more holes at the top where the throttle plate closes. Only part of one hole should be exposed by the throttle plate. It sounds like you have the throttle open to far and need to close it with the idle speed screw.