Xs650 misfire at idle

[bazz]

Looking for suggestions to stop the left cylinder misfiring at idle on my bike
The carbs are bs34
I've played with the pilot air adjusting screw
It has Boyer ignition so it's a wasted spark so I swapped the plug leads with no difference
I've checked the float height
Cleaned all the passage's and jets
The rubber bung is in pilot jet hole
I've checked the valve clearances
If I remove the left plug it'll idle fine on the right cylinder
If I try that with the left it'll run for a few seconds then then stumble and stop
I was wondering if the O ring on the pilot jet adjusting screw was worn and pulling in air
I have new O rings on order but to test the theory I tried blocking the top of the pilot air adjusting screw with blue tack and this made no difference
I've checked for air leaks at the manifold by spraying carb cleaner around the joint
Is there anything else I could check ?
I've only just got this engine running so I don't know it's history
It's not a daily rider that's just started messing
Cheers

 

[Its been a long time]

Try spraying carb cleaner onto the throttle shaft. Could be pulling air across the throttle shaft seals? If the engine note changes, you know that's happening.

 

[member43148]

Spraying carb cleaner into those rubber seals may tell you a clue but carb cleaner will only worsen the seal failing.
Drip motor oil into the crack where the seal is.
That will tell you the same result and actually help swell the seal to live until another day.

 

[bazz]

Try spraying carb cleaner onto the throttle shaft. Could be pulling air across the throttle shaft seals? If the engine note changes, you know that's happening.

That's an idea

 

[bazz]

Spraying carb cleaner into those rubber seals may tell you a clue but carb cleaner will only worsen the seal failing.
Drip motor oil into the crack where the seal is.
That will tell you the same result and actually help swell the seal to live until another day.

I'll give that a try
Should be getting the new seals very soon

 

[bazz]

I've been considering moving the carbs to the left
So the right carb is on the left cylinder and if it runs correctly as a single like the right cylinder does I'll be 100% certain it's the left carburettor and nothing else

 

[Jim]

Spraying carb cleaner into those rubber seals may tell you a clue but carb cleaner will only worsen the seal failing. Drip motor oil into the crack where the seal is.

Spraying carb cleaner to look for air leaks won't hurt a thing. Carb cleaner evaporates pretty fast leaving little or no residue. Not really any chance of it harming anything... it evaporates too fast for that. And the beauty of it is there's no cleanup after.
Motor oil on the other hand is gonna have a bit of a cleanup after.... OK....a lot of cleanup.

Carb cleaner, brake cleaner and electrical contact cleaner... all 3 are flammable, fast evaporating and leave no residue. This is the way.... it's tried and true and has worked for me since I was a kid 50 odd yrs ago.

Spray away gentlemen.

 

[Melnic]

Sync issue?
How low are you idling?
what do plugs look like after idling a long time?
BTW, just doing a dry float height measurement does not mean the float level wet is going to be perfect. (learned my lesson)

 

[Wingedwheel]

Did we ever find the air leak?

 

[bazz]

Sync issue?
How low are you idling?
what do plugs look like after idling a long time?
BTW, just doing a dry float height measurement does not mean the float level wet is going to be perfect. (learned my lesson)

It's never been ridden by me
It's a project bike
I bought the engine/carbs/Boyer ignition together so it's unknown to me
It's a little smokey but it's smokey on both cylinders
I've balanced the carbs with vacuum guages
I did a compression test and it's 132 psi on both cylinders so I suspect the the valve guide seals have gone hard
Hopefully I'll get these carbs sorted
If not I have a pair of VM 32s on the shelf although I do quite like single carb setups

 

[bazz]

The new pilot jet adjusting screw O rings turned up today but they made no difference when fitted
I'm fairly convinced there's something wrong the pilot air passage
Maybe some debris in there somewhere that I've missed
I tried spraying carb cleaner around the butterfly spindle of the carb but no change in the running

 

[bazz]

Well I did my test today of fitting the right carb on the left cylinder
And it still misfired
So I tried the right inlet manifold on the left cylinder fitted upside down and the cylinder ran fine right down to a low idle
So the only conclusion is there must be a leak on the left inlet manifold
What I can't understand is why the carb cleaner sprayed around the joint didn't pick anything up ?

 

[jetmechmarty]

What about the spark plugs? Did you swap them one side to the other?

 

[bazz]

What about the spark plugs? Did you swap them one side to the other?

Yep many times
I also tried swapping the leads as it runs a wasted spark
I also fitted new plugs
And I also tried champion plugs because they cope a bit better with oil and carbon compared to NGKs
And this motor is a little smokey

 

[Wingedwheel]

What I can't understand is why the carb cleaner sprayed around the joint didn't pick anything up ?

I never knew you could flip those with any success?

 

[Jim]

So the only conclusion is there must be a leak on the left inlet manifold
What I can't understand is why the carb cleaner sprayed around the joint didn't pick anything up ?

Usually it's a crack/split in the rubber and not the "joint."
Unless you sprayed the carb cleaner directly on the split, it's not likely it'll show up.

 

[bazz]

I never knew you could flip those with any success?

Do you mean the inlet manifolds?
If so you can't do it successfully because there's a bevel built into them so putting the right manifold onto the left cylinder angled the carb up slightly

 

[bazz]

Usually it's a crack/split in the rubber and not the "joint."
Unless you sprayed the carb cleaner directly on the split, it's not likely it'll show up.

I shall check it out properly tomorrow

 

[JAX71224]

Well I did my test today of fitting the right carb on the left cylinder
And it still misfired
So I tried the right inlet manifold on the left cylinder fitted upside down and the cylinder ran fine right down to a low idle
So the only conclusion is there must be a leak on the left inlet manifold
What I can't understand is why the carb cleaner sprayed around the joint didn't pick anything up ?

Sounds like an intake valve to me Can you do leakdown test? That might tell