Runs great cold, but revs high when warm

[mil1638]

Hi again all

After a complete rebuild my '79 D runs great when cold and also runs great when warm. My problem is when i rev the engine above 4500rpm it seems to stay there. I can rev anywhere up to that and the bike will return to idle (1450rpm ish) and it will rev right to max but will then return to 4500rpm.Stop the engine, restart and idle will be normal again.

I am confident that carbs are clean and fitted with 'repair kits' and bench balanced, mixture set to 3 1/2 turns. Timing is good as is petcock and vacuum pipes. Petrol (gas) is new and good.I have had throttle cable off (new cable), oiled and fitted 'loose' without effect
As said, bike runs and drives great, nice and smooth but seems to want to run at high revs when warm.
I have messed with carbs settings when the motor is running and managed to draw idle down to 3500rpm but that seems to be the best i can get.

All suggestions gratefully accepted

 

[levdir]

Have you checked your carb diaphragms for leaks? Hanging idle can be a symptom of that. Also if there are cracks in your carb boots those can cause weird leaning-out symptoms.

 

[mil1638]

Hi thanks for reply. Diaphrams have been checked and i am confident both are fine. carb boots are new and are air tight.
It has to be a carb issue, just cant find what it is. Guess im gonna have to pull them...again

 

[rugbywarrior89]

Check your butterfly seals. I changed mine twice (once with mikes, once with Yam OEM) and both times they still leaked a bit. I packed in both areas with grease and greases up the seals themselves and that seamed to fix the leak I think. chokes leaking can also be a easily missed leak.

If you have aftermarket exhaust/air intakes with stock jets, that can cause a lean issue because the jets are too small.

I have also seen people on here that swore up and down their diaphrams didn't show any light through them during testing and replacing them still fixed issues so if they are stock, you may want to consider changing them anyways.

At the very least, if the problem isn't too bad, you can still ride it. If you are stopped, put it in first gear hold the brake, and let the clutch out slowly. That will force the engine down. If you are moving, just keep it in gear until you are almost stopped and it will force the engine down. Be careful though, pulling away from a stop can be a bit tricky cuz if you pull the throttle too hard, it can take off on you and give you some serious whip-lash. DISCLAIMER: Fixing it is the best coarse of action and I'm sure this method wears out other parts faster (clutch, cylinders,brakes, etc.) and I'm not responsible if it breaks.

If the problem is bad, then it can be dangerous to try that. Believe me! I once pulled the throttle and the engine immediately red-lined, popped a wheelie and threw me off the back. Thankfully I was okay and it only did some cosmetic damage but it could have been a lot worse.

 

[xschris]

Sync the carbs with a manometer You also may be a little lean in the pilot circuit but my guess is a proper carb sync will fix most of it. Also make sure the idle mix screws have not broken off in the carb bodies.

 

[Lou Ranger]

Have you verified the air tightness using the carb cleaner spray test or just visually?
Without the former you can't be sure.

 

[mil1638]

thanks again for the replies.

I am 100% confident air tightness and have 'spray tested'

Working nights for next couple of days but will pull carbs at weekend, investigate and post results.
As for drivability, it is possible but not very pleasant so will need to be sorted out. this is all a bit embarrassing giving i was once a mechanic

 

[xschris]

Manometer, trust me This issue has happened to a lot of " bench synced carbs".

 

[mil1638]

Just a quick question...

Pulled carbs again (second time this weekend) and thought to pay a bit more attention. got main jet out and looked at numbers stamped on it ( or rather, the kid did, way too small for my eyes). the number on it is 271 then under it X-6. Having trawled this site, these numbers do not seem to fit in with any ive seen on here.

Any thoughts??

 

[xschris]

These came with a few different numbered ones. 271-Y-2, X-4 and a X-6. They all look the same. So as long as you have a pair of them your good

 

[mil1638]

Ok thanks for that. So if i were to go for a size, or two, up what should i look for?
My mechanical knowledge is more toward fuel injected cars

 

[xschris]

The only way to change the mid-range carb setting, witch is what the needle jet and needle do, is to raise the needle up to rich en it or to drop it to lean the mix. You need to rich en the pilot circuit if the engine idles high and after the carbs are synced and adjust to best they can be. Usually one size up on the pilot jet is all you need for a stock bike.

 

[mil1638]

Ok, firstly thanks for all replies.
So here's where i'm at....
-Carbs pulled and cleaned to within an inch of their life
-Re-fitted and balanced with manometer
-Intake system spray tested for leaks..all good!
-Valve clearances re-checked (ok)
-Throttle cable checked for free movement (ok)
-Points and timing re-checked, again ok
And.....problem still there Revs still running away when hot 4-5000 rpm and can only be brought down by force, i.e into gear and slipping clutch. One other thing i've noticed is a backfire through left carb , only occasionally.
I have fiddled with mixture screws (originally 2-3 turns out) to the extent of full in to full out in small increments.
Only thing i did find during rebuild was that one diaphram is very slightly faster than other closing the slider, Im talking fractions of a second. So believe the only way forward is new diaphrams (a very sore hit here in the uk.£75 per carb if i can find any stockists)
Any thoughts would be greatly appreciated

 

[xschris]

It could be a small pinhole in one of the diaphragms or a loose slide in one of them. If a slide is sticking a bit this will also happen. Did you check and make sure the idle mix screw holes are open and the tips have not broken off inside? If turning them made no difference that would be why.

 

[mil1638]

New mixture screws have been fitted as part of the repair kit and holes are very clear Adjustment does affect running a little (not as much as i'd expect)but not high revs
Ive got an old pair of BS32's kicking about is there any chance diaphrams/slides are interchangable (long shot) ?
Also just out of curiousity, is there another carb out there somewhere that can easily be fitted. I am a mechanic to trade (albeit 20 yrs ago) so can tackle most alterations,although i am starting to doubt my abilities after working on this bike!!

 

[mil1638]

Should maybe add, exhaust is 2 into 1 with 'megaphone' style end can, which probably is pretty much straight through.
Main jet is 142.5 and air jet 170 according to my kids eyes

 

[mil1638]

Oh yeah, and stock air filters (brand new)

 

[XS400NJ]

Is your "throttle stop" bolt is out all the way? I had same problem, bike would idle high when warm or after long highway run, but i was able to bring idle down by unscrewing the bolt a little. Of course next day it would make my bike die at idle until I add few turns back.

After carb sync and setting idle screw 2.5 turns out (I played with them while doing carb sync) this problem became smaller, and by now I am almost fine where the bolt is, my idle rpms are set low (bike will not idle for too long without help from throttle) but I do not have those "high rpm" problems anymore.

I think idle screw playing during carb sync made the difference, the last half-tun had a big effect on rpms. Also, I did carb sync with home-made differential manometer(bottles with water in them), I think it is the most accurate way to sync carbs.

 

[Lou Ranger]

... So believe the only way forward is new diaphrams (a very sore hit here in the uk.£75 per carb if i can find any stockists)
Any thoughts would be greatly appreciated

Hopefully it won't come to that, but if diaphragms are in your future, my Scottish half owes it to you to share this link with you (http://www.jbmindustries.com/). Apparently a few guys on here have used and been very happy with his product.

Best of luck.

 

[CDNTX650]

Does that bike have a mechanical advancer? They can stick and cause this. Take the weights off and grease the pivots. Sometimes the springs can weaken too.

You said 79, it should, behind the points plate.