cutting out at idle

I've had a quick search through the forum but can't seem to find an entry that reflects my symptoms.

Bike starts, idles( 900-1100 when warm) and runs fine, battery voltage when running is up near the 15v mark and drops to around 13-13.5 at idle so low voltage Boyer problems shouldn't be an issue.
Single 32mm Mk1 concentric with a 3.5 slide, 270 main and 106 pilot with needle in the top groove.
The problem is when coming to a stop, as I'm slowing down and coming down through the gears it'll cut out unless I keep a bit of throttle on, once I kick it back into life it'll idle ok again, this doesn't happen all the time and I'm convinced it's a fuel thing, I've had the air screw out and blown it through with carb cleaner which I can clearly see coming out of the holes in the carb body. I thought it might be a fuel starvation problem but then that would show up elsewhere.
Anyone experienced this sort of thing and found a remedy?

Sounds to me like the throttle slide is dropping too far down and starving the engine. (if the carb is shagged out enough air will be getting past to give you a tickover but maybe not on a closing throttle) If you screw the throttle stop screw in (up) by about 1/8 turn that will give you a higher tickover but if you then don't have the cutting out, it will prove the diagnosis. try to re-adjust the tickover with the air bleed screw, probably in (clockwise). I piddled about with single Mk1 and Mk2 amals for years. Both would freeze in cold damp weather, not good heading for a set of lights. Best option if you want to stay with a single carb would be a 34 or 36mm mikuni IMHO fit and forget, get on with riding the bike not fettling it. Steve Doran at Motocarb Ltd is the best bet in the UK. www.motocarb.co.uk or 01512 384170 he's in Merseyside.

Single 32 mk1?
3.5 Slide?
Clip in Top groove?
Is your bike on a diet?

Seems like the only way you are going to get enough fuel in there would be at WOT.

pvisseriii said:

Single 32 mk1?
3.5 Slide?
Clip in Top groove?
Is your bike on a diet?

Seems like the only way you are going to get enough fuel in there would be at WOT.

Click to expand...

The bike pulls like a train and like i said only does this now and again so the set up mentioned seems to be ok. Have you anything useful to add that might throw some light on the subject of my post?

gripper said:

Sounds to me like the throttle slide is dropping too far down and starving the engine. (if the carb is shagged out enough air will be getting past to give you a tickover but maybe not on a closing throttle) If you screw the throttle stop screw in (up) by about 1/8 turn that will give you a higher tickover but if you then don't have the cutting out, it will prove the diagnosis. try to re-adjust the tickover with the air bleed screw, probably in (clockwise). I piddled about with single Mk1 and Mk2 amals for years. Both would freeze in cold damp weather, not good heading for a set of lights. Best option if you want to stay with a single carb would be a 34 or 36mm mikuni IMHO fit and forget, get on with riding the bike not fettling it. Steve Doran at Motocarb Ltd is the best bet in the UK. http://www.motocarb.co.uk or 01512 384170 he's in Merseyside.

Click to expand...

I'll have a bash with the air bleed, it's a new carb with less that 1000 miles on it so I'm loathed to let it beat me, but it may just be one of those times to bite the bullet and go for a hassle free mikuni, like you said better to be riding that fettling.

willy mac said:

pvisseriii said:

Single 32 mk1?
3.5 Slide?
Clip in Top groove?
Is your bike on a diet?

Seems like the only way you are going to get enough fuel in there would be at WOT.

Click to expand...

The bike pulls like a train and like i said only does this now and again so the set up mentioned seems to be ok. Have you anything useful to add that might throw some light on the subject of my post?

Click to expand...

Fair enough. I did not intend to be elusive. It just seems to be under carbed and the carb seems to be set up lean other than the main jet. You may be on the verge of starvation all the time but only show up once in a while. What kind of banjo do you use? Single, double, 5/16, 1/4"?

you are in Scotland, so basic mikuni settings with no altitude issue


I ran a single Mikuni for 16 years, but here is my remembrance

I think you may have a slide issue, as in the wrong slide

you say you have a 3.5, I am trying to remember but isn't that a more cutaway, hence a "leaner" slide number?

why do I want to remember that a 2.5 is standard for sea level?

someone will quickly confirm, but I would first verify that you the correct slide

willy mac said:

I've had a quick search through the forum but can't seem to find an entry that reflects my symptoms.

Bike starts, idles( 900-1100 when warm) and runs fine, battery voltage when running is up near the 15v mark ?

Click to expand...

Clean the idle jet with a probe (#78 Drill). Set the idle when hot. If the slide is sticking up a tiny bit, that can cause the issue. You say carb is near new. Have you checked float level ?

I find a stable idle around 700 rpm ( I use 2 amals) . I do not know if the idle jet can fuel properly much beyond this. The electronic ignition I use has idle stabilisation, which helps a lot.

13.8 to 14.2 is the best voltage for lead acid batteries. 15v is too high.

Had the exact same issue, drove me nuts trying to track it down and I went through a complete rewiring and new PWK flatslides from JS before I cracked the head to discover a resin fouled exhaust valve.

Since your carb has only 1000 mi or so a worn slide is unlikly to be the problem but that was the exact cause of the same behavior on my single carb'd BSA. Another thing it did, was, at it's normal idle, if I opened the throttle ever so slightly, the engine would lose some rpm. Does your's ?

Had another think about the problem and looked back at all the settings I had played with while trying to find the cause, my only assumption was that the float height would have been set at the factory! WRONG. Instead of the 2mm from the top of the float bowl I had 6, so a couple of warm up and tap sessions on the valve housing got me to the desired float height and the cutting out problem seems to have magically disappeared, amazing !
Reading some of your comments about carb settings has got me thinking about my set up though, I've just covered 500 miles since the build and it had it's first oil and filter change yesterday, so I'm still limiting the revs to a Max of 5k, once I get a few more miles on the clock I can increase the revs and should be able to see if I've got enough of the go juice going through.
As always a problem shared on this forum is a problem solved .

Cheers Fellow Nortoneers.

Nortiboy said:

willy mac said:

I've had a quick search through the forum but can't seem to find an entry that reflects my symptoms.

Bike starts, idles( 900-1100 when warm) and runs fine, battery voltage when running is up near the 15v mark ?

Click to expand...

Clean the idle jet with a probe (#78 Drill). Set the idle when hot. If the slide is sticking up a tiny bit, that can cause the issue. You say carb is near new. Have you checked float level ?

I find a stable idle around 700 rpm ( I use 2 amals) . I do not know if the idle jet can fuel properly much beyond this. The electronic ignition I use has idle stabilisation, which helps a lot.

13.8 to 14.2 is the best voltage for lead acid batteries. 15v is too high.

Click to expand...

Hi there Fella

As it was a brand new carb the float level was assumed by me to be correct, however that was not the case and it's now been reset from 6 down to 2mm and all is well. When I say 15v mark I'm just going by what the analogue voltmeter on the bike is saying I don't think it's a particularly accurate device but it'll give me something more to check when I get back home in 3 weeks time.

Cheers
Mac.

I had a phantom cut out problem that I thought was fuel. It turned out to be a very simple loose connection at the ignition switch. Do a wiggle test while running.