Won’t idle

Production K Models

Won’t idle

Postby Otis » Mon Nov 28, 2022 7:06 pm

Hello, I’m asking this on behalf a big twin flatty ULH but I always get great advice on this site and I hope no one minds. I’ve been through everything on this bike and can’t figure why it won’t idle and is difficult to start. I’ve adjusted the valves and checked compression. Spark is very strong and timing is dead on factory specs. Just to be redundant I did replace the condenser. I replaced manifold seals with PEEK and bubble tested the manifold. I’ve completely dismantled and cleaned the carburetor with an ultrasonic cleaner. Replaced throttle shaft bushings. Nothing else is worn, idle circuit is clear and everything looks correct. Modern float is set at 1/4”. This problem has been ongoing for quite awhile and everything has been checked at least twice. I can’t think of anything else. Any suggestions? Thanks.
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Re: Won’t idle

Postby Ferrous_Head » Mon Nov 28, 2022 7:56 pm

If the compression is OK and the spark and timing is OK, all that's left is the A.F ratio...
i'm presuming the bike runs ok off idle(mid range and top end) but simply won't idle.

That points to an air leak and as you have checked the intake manifold seals I would suggest it might be a worn body where the throttle shaft goes through. A more common problem on bigger engines.

Easy option is to do a carb swap. If the problem goes away you can begin troubleshooting the carb,
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Re: Won’t idle

Postby wz507 » Mon Nov 28, 2022 8:02 pm

Could you please tell us more about “it won’t idle and is difficult to start”.

Is the idle circuit totally absent, i.e., runs with more throttle and revs up, so long as you keep it off the idle circuit, but when you try to slow it down by returning the butterfly to the idle position it just looses fire and dies? Or is the idle circuit pig rich or too lean as your try to ease it from higher register operation to the idle circuit?

Regarding hard starting, does it perpetually pop and fire when you poke away at it, or does it seem short of fire as you attempt starting it? If you squirt a teaspoon full of fuel into the induction system and kick it through once thereafter with no ignition and the throttle nearly closed, then provide ignition and kick it, will it start right up? I ask because I’m trying to better understand if it is starving for fuel or getting too much.

Battery or magneto ignition?
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Re: Won’t idle

Postby Rubone » Mon Nov 28, 2022 8:14 pm

Is the low speed needle assembled correctly? Spring on top of the needle? Correct washer for the air bleed?
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Re: Won’t idle

Postby 44UL » Mon Nov 28, 2022 9:53 pm

I would also like more info, but would suggest making sure the float/needle is not sticking closed. Just because it's new doesn't mean it works.
Also like to know if there is excessive throttle disc wear in the body/disc.
will it idle with the choke closed or close to closed?
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Re: Won’t idle

Postby Otis » Tue Nov 29, 2022 8:41 am

I’ll answer in order of the replies:
Throttle shaft bushings were replaced and carefully reamed
It seems as there’s no idle circuit and it dies as soon as the throttle backs off. Depending on where the needles are set it runs rich or lean. Good suction pulls plenty of fuel into the body
Stock battery ignition
I’ll check low speed needle assembly but I believe it to be correct
The bowl fills with fuel and I’ve checked operation extensively on and off the carburetor
It runs as though the fuel air is totally out but no combination of settings will get it even close. Usually 2 turns on the high and 4 on the low will at least let it run even if a little rich.
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Re: Won’t idle

Postby Otis » Tue Nov 29, 2022 8:44 am

Also the throttle disc fits very well. Nothing appears worn on the carburetor
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Re: Won’t idle

Postby Otis » Tue Nov 29, 2022 9:49 am

The choke doesn’t improve how it runs. The low speed needle, spring and collar are installed correctly.
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Re: Won’t idle

Postby panic » Tue Nov 29, 2022 2:31 pm

What model Linkert?
Is this a completely original carburetor, or assembled from parts?
[url="http://victorylibrary.com/L-BK.htm"]The Linkert Book[/url]
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Re: Won’t idle

Postby Ferrous_Head » Tue Nov 29, 2022 2:36 pm

Obviously, this is not your first rodeo so I will not try to tell you how to suck eggs.

But clearly, this one has you scratching your head or you wouldn't be asking here.

But with things like this I always take a systematic approach. Top down, beginning with the basics. Compression, a spark at the right time and the correct A/F ratio. If those three areas are right the engine will run. I never assume anything and I test with the correct tools. This breaks the problem down and stops you wasting time on the wrong area.

Basic questions first.

Did the bike come to you in running condition ? ie was this an existing problem before you started on it ?

What work have you done on this engine ? Obviously you've worked on the carb but are we looking at say, a fresh re bore/pistons ? As part of any work did the ignition timer come out ?

What were the actual numbers on the compression test ?

Has the timing been set with a strobe light or just static timing ?

I'm not trying to be rude, merely making sure we're looking in the right area first.

You haven't mentioned what carb you are looking at. I'm assuming an M51, but is that right ?

And was that carb on this bike and working at some point before this ?

I chased a similar problem with a pair of new Amals. Spent days on them before finally learning they had not had an internal passage drilled by the factory. A defective batch. Frustrating to say the least. And I hate Amals.
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