Irratic idle

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maylar

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We had a lovely day here in Connecticut and I managed to take the 850 out for a good shakedown run after rebuilding the engine and installing new isos. I have new valves and guides in the head and a new cam. Only one issue needs to be sorted out - irratic idle.

The left cylinder is misfiring at idle, but nowhere else in the throttle range. There's no backfiring, even on decel. Part throttle at 30 mph is smooth with no surging like you get with a clogged pilot jet, but I've poked them clean anyway with no effect. New plugs, no change. Carbs are new Amals with about 5000 miles on them so I doubt the slides are worn already. Both air screws are at 1-1/4 turns out and are happy there. No air leaks at the manifold.

If I set tickover to 1000 rpm when the engine just warms up it'll race to 1500 after riding for a while. If I dial it back down when it's hot, it'll sometimes stall when I come to a stop. Very frustrating. I hate having to blip the throttle at a stop light to keep it running but 1500 is just too high when standing still.

I'm going to try swapping coils and see if the misfire switches to the other side, but I can't remember ever having a coil that misfired only at idle - usually the crap out at speed. Not sure what else to try.

BTW at the first gas stop we'd traveled 85 miles and I only needed 1.4 gallons. Notbad for a big twin.

drc
 
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You mention that left cylinder is missfiring at idle.
First thing I would do is to hook up a strobe light to the left side and run the engine at idle and part power. By doing this you will see if the strobe light is flashing properly or not and this tells you if you have an ignition problem or not. As far as I know this is the easiest method to figure out if you have an iginition problem or not.
If ignition is OK you have a carb problem or eventually it could also be lack of proper compression (you did a valve job, maybe leaking??).

If you want a solid idle you should consider re-sleeve your Amals, that makes wonders with your idle and part power smoothness. I have never compared new Amals to re-sleeved ones but been told the re-sleeved are much better than new.

Best regards,
Per
 

maylar

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Both excellent suggestions, thanks. Valves not seating typically give you a backfire, which i don't have. Besides, the machinist that did the head work is top notch so I'm inclined to look elsewhere first. I did fuss with float level on that side last year, not realizing at the time that I had a bad cam. So I'll revisit that after blinding myself with the strobe light.

Thanks
 
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The strobe light test will guide you and my guess is that in this case it is not ignition (when only on idle) but it is still worth to get this confirmed with the strobe check before starting to check/adjust the carb.

/Per
 
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Sounds like fuel delivery to me. Take off the air cleaner and squeak a little propane in the suspected side and see if that smoothes it out.
 
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Hi Maylar
Could it be a manifold leak? Try spraying WD40 around the manifold to see if it idles better, or faster...to give you a hint.
As Per says, amals wear terribly. My new Amals idle fine. How long that will last I will find out...
Stuart.
 
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Dave compare the plugs to see if they colored differently. Easy check and might tell you more. If carbs were ok before you did the work it's unlikely they are causing it.
 
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