Cutting out/stalling at low revs

[petermigreen]

My 500 starts and runs fine, Ticks over no problem, belts along 70, 80 + no problem but.
Just lately started cutting out or stalling after slowing or stopping at a junction/roundabout whatever just as applying revs to pull away again.
Bike restored after import by me last year, carbs off, ultrasonically cleaned, reassembled. New coils/leads/caps/plugs.
Thoughts please.
Peter

[hairygit]

Did you fit new carb to head rubbers? Are they the proper clips (jubilee clips are trouble!) Air leak is usually first suspect.

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[petermigreen]

Would an air leak cause it to cut out? I know it will cause erratic running.

[hairygit]

It certainly can do, as well as erratic running, often getting worse as the engine gets hotter, as rubber gets softer and meatal parts expand, air leaks cause more problems. 

[deltarider]

IMO it's unlikely that an airleak causes this. It would take multiple airleaks. The last time I had what you describe, the idle mix was far too rich. But ignition always comes first in the checklist.

[matthewmosse]

How's the battery voltage? I once had a cb550 that struggled to run at low revs, or for prolonged n8gjttime riding, turned out only 2 of the 3 charging coils were doing their job so it was fine at high revs but just could not keep up demand. Other thing I would check is plug coulor, see if it's running lean, maybe spray carb cleaner near the inlets and see if revs rise, does possibly sound like running lean might be an issue.

[zebedee85]

My CB550 would cut out when warm and be difficult to start, it would especially do this after a spirited run or in traffic. A new battery fixed it, the old one just wouldn't hold charge, and the weak spark would be an issue. iridium plugs are a bonus, as they withstand long idle sooting up or flooding pretty well if I'm careless.

If you're getting it cutting out when opening the throttle when warm and under load but it idles or revs ok I would be sceptical about air leaks though always worth checking. it is almost as if you're flooding it (or just giving it more fuel that it can burn) suggesting weak spark.

any problems getting it going again?

[petermigreen]

Okay, thanks to all who commented.
New rubbers have been fitted with genuine clips and no evidence of air leaks that I can find.
Battery is new and charging.
Spark is good with BB EI fitted.
It is easy to start again and always does straight away.
It's rich, plugs are sooty but cannot seem to influence this. Same problem on the 400 and haven't been able to dial this out no matter what I try.

Side note. I went out on Wednesday evening around 1800 and it was stalling every junction I came to. Spent a few hours at a bike event and dreading the ride home set off and was surprised that it didn't do it at all on the way back. It was much cooler in the later part of the evening and think heat may be a contributing factor.

I will keep my eye on it.
Thanks again all.
Peter

[paulbaker1954]

I had a similar issue with my 500 when I first rebuilt it - just stuttered off the line and running badly at low revs.

This sounds similar to me and in my case it was absolutely traced to blockages in the idle circuit once I cleaned the  carbs ultrasonically and renewed the pilot jets it ran like a different animal altogether.

This definitely sounds a carb/over rich mixture issue.

You said you had the carbs rebuilt by Gerben and I know he does outstanding work and I am not throwing any rocks here don't get me wrong but I would pull the carbs and check the idle  circuits these are VERY fine bore holes and easily blocked.

Below is a write up I did a while back on how to check the idle circuits

The idle circuit consists of 3 holes and the pilot jet.

hole 1 can be seen on the carb wall on the inlet side throat at 90 degrees from the top before the throttle slide, this actually joins in to the hole from the air screw by the way.

Hole 2 is a VERY small one at the base of the throat on the exhaust side at 180 degrees from the top just in front of the throttle slide. This is where the idle mixture is fed in to the cylinder.

Hole 3 is where the air bleed screw goes in.

At idle air is drawn from hole one and the air screw then adjusts the amount of incoming air, this is then mixed with fuel from the pilot jet To provide the idle mixture.

The resulting mixture then squirts out of the very small hole (hole 2) at the front of the throat in to the cylinder.

I found the best way to check for blockage in the whole pilot circuit is to remove the pilot jet and air screw and blow down the air screw hole and at the same time cover the pilot jet hole and the other 2 holes with some combination of fingers and thumbs. You can then lift a finger or thumb from each hole in turn and hear if you get air blowing through.

It's a bit like mastering playing the bagpipes but an effective way to check the circuit for blockage.

This may sound complicated but if you find the holes and study the plenum chamber in the base of the carb it's pretty obvious how it works.

[petermigreen]

Just need to correct you on one point there Paul. I didn’t say the carbs were done by Gerben, as they indeed weren’t. I wouldn’t want to perpetuate any doubt as to his work.
I do feel increasingly though this is an over richness issue and will be pursuing this line when I can muster the energy to pull the carbs again.
Thanks, Peter

[paulbaker1954]

Here is a picture that hopefully helps explain where to find the pilot air circuit holes

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[petermigreen]

Thanks Paul and Oddjob for the extra information.
Peter