Commando won't run with Mikuni "choke" off

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stuck amals

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My 1974 Norton with a single Mikuni will start and run with the choke on. ( I know it's not really a choke) When I turn the choke off engine will die. I can't find anything wrong with the carb. In fact I took the Mikuni carb from my Combat and I get the same results.

I sprayed carb cleaner around the rubber carb mount and 2 into 1 manifold to see if it was sucking in air from somewhere. No Luck.

Any suggestions?
 
Hi, Welcome. Got a compression gage? Give a quick check. Report back please
 
Compression is about 130 for both cylinders. The carbs have been soaked in cleaner and blown out with compressed air. Jets look good. Plenty of fuel in bowl. I thought I might be sucking air from the rubber carb mount. But it seems okay.

This bike has not been ridden since spring. I took it apart for to replace the swing arm bushes. I removed the carb and emptied the fuel. The carb has been in dry storage. The bike ran good until then.
 
Starving for fuel. Ckeck flow to the carb from tank.
Click to expand...

yes

I would take the gas line off the petcocks and open them fully into a cup to verify you are getting
a good flow of gas

petcocks get gummy and restricted when they get old
 
Most likely not your problem but I had a similar issue and found one of the cap screws that hold the two into one manifold on had come out and was sitting on top of the inlet valve. It was one of the internal cap screws that had come out and the bike ran a lean as buggery with an 10mm hole direct from the manifold to the atmosphere. Bike would only run with the choke on or above 3500rpm no where in between. I ran like this for 30 klms and it was an interesting ride coming over a winding mountain road with switchback corners and steep drop offs. Lucky I didn't know at the time what was wrong. Went through everything pulling my hair out trying to find what was wrong until I discovered one of the screws missing and it was sitting on top of the inlet valve. No damage was done, compression and clearances normal. Screws are now fixed in with Loctite (no washers).

I am not the first one nor the last I expect to suffer from this problem.

All the best
Peter R
 
stuck amals said:
Compression is about 130 for both cylinders. The carbs have been soaked in cleaner and blown out with compressed air. Jets look good. Plenty of fuel in bowl. I thought I might be sucking air from the rubber carb mount. But it seems okay.

This bike has not been ridden since spring. I took it apart for to replace the swing arm bushes. I removed the carb and emptied the fuel. The carb has been in dry storage. The bike ran good until then.
Click to expand...
You've got TWO carbs with blocked pilot circuits, easy to do with modern fuel, goes bad quicker than before. The jets are only part of the story. You must thoroughly VERIFY flow through every passage. I use aerosol brake cleaner NOT for it's solvent values, just an easy way to see a jet stream through a drilled passage. You had the needle jet out of the main well, yes?

Study this Mikuni tuning manual, pages 3 and 7 show diagrams about fuel circuits. http://www.mikuni.com/pdf/vmmanual.pdf

You don't have a weak battery or trying to run without one or something weird like that?

Nice user name, BTW.. I had one keep sticking open while ascending http://mtwashingtonautoroad.com/ on a '69 BSA Lightning riding 2 up.
 
Fast Eddie said:
Blocked pilot jet.
Click to expand...

That's what it sounds like to me too. Soaking and compressed air does not get them clean. Need to put a small wire in there (smaller than the orifice).
 
dennisgb said:
Fast Eddie said:
Blocked pilot jet.
Click to expand...

That's what it sounds like to me too. Soaking and compressed air does not get them clean. Need to put a small wire in there (smaller than the orifice).
Click to expand...


Remember gents, the OP said "Mikuni", and "the jets all look good" the pilot jet is removable, easily inspected for blockage. No chance he just looked at the MAIN jet??? :oops:
 
Vacuum leak.

Oh, sorry, I guess I said that already.

In any case, I'd check for a vacuum leak.

:)
 
Switching working crab from another bike and giving same signs and symptoms implies its not the carb or carbs. Would be a knee slapper if turns out to be electrical.
 
Update,

Fired it up with the choke on. It idled good. I let it idle for quite a while then turn the choke off. It would run for about 30-45 seconds as log as I pumped the throttle every few seconds.

Then I got the crazy idea to adjust air adjusting screw from it's recommended 1&1/2 turns out. The further IN I went the better it got. I'm now all the way in and it idles smoothly with the choke off.

I"m running a #25 pilot jet. It seems I need to go to #30 or #35.
 
Steve is right ,substitution is what I practise all the time to isolate and nail down sources of issues. Electrical the next phase of considerations ,battery connections ,fuse oxidations,loose corroded grounds,dying condensers,crabs in harness. :) .... O.K.,you're on top of it now, Size 25 is too small for a MIK.
 
stuck amals said:
Update,

Fired it up with the choke on. It idled good. I let it idle for quite a while then turn the choke off. It would run for about 30-45 seconds as log as I pumped the throttle every few seconds.

Then I got the crazy idea to adjust air adjusting screw from it's recommended 1&1/2 turns out. The further IN I went the better it got. I'm now all the way in and it idles smoothly with the choke off.

I"m running a #25 pilot jet. It seems I need to go to #30 or #35.
Click to expand...

Before you change it, check properly that it isn't partially blocked.

My 850 has a single Mik with 35 pilot. It was partially blocked. Now I've unblocked it, and it is too rich. So Id say, 35 is too big.

I soaked mine in thinners and sprayed carb cleaner through it, and blasted it through with a compressor... all with zero result.

I had to unblock it physically, using weld tip cleaners.
 
The choke on my Commando is off when it is on and on when it is off if you know what I mean.

When the choke is fully engaged, that is the choke cable is tightest, it is off.

The choke works back the front to what you would think. Well mine does and I think that's how they are supposed to operate.

Frank
 
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