Cylinder Misfire on Cold Startup

I see that I had a similar post about this problem last year. But I have not fixed the problem. 1976, stock, with stock BS38 carbs.

My last compression check, done a year ago with warm engine and wide open throttle, was 145 right cylinder and 140 left. That said, the left cylinder has a bad misfire/non-fire on cold startup that has existed since day one. The engine rattles quite a bit on a cold startup due to the problem. It's always the left cylinder. I can easily tell it's missing, and at times, not firing by putting my hands over both exhausts. This scenario will last until I ride off, at which time the miss completely goes away. After a cold, rattling start, I can ride a few hundred feet down the road, stop, and the engine is idling perfectly... both cylinders smoothly firing, and even at a low rpm. The bike runs beautifully and smoothly at all speeds (within legal limits!). Both plugs are a identical beautiful tan color (inspected after a long ride with little stopping). As I said, this condition has always existed. After the poor, rattling cold starts, subsequent starts throughout the day are immediate, with no choke required (as it should be). The idle is perfect all day long. One thing I've noticed, and the original owner commented on, is that the left header has a very slight yellowish discoloring to it, while the right does not. He pointed that out, but also said he didn't know why it was like that. Oh... one last thing... hitting each cylinder with the infrared thermometer shows acceptable and very close temperatures on each cylinder. For the cold startups, choke is required, but after firing, must immediately be taken off or the engine will die.

In anticipation of changing the pilot jets (which I have not done), I removed the float bowls and made sure they were entirely clean. I sprayed through all passageways, and nothing was plugged up. I also used compressed air. Float heights, checked previously, were fine. I have not yet removed the carbs for a complete cleaning.

Sorry about the long post! Your ideas and suggestions are most welcome and appreciated!

Have you checked the adjustment on your valves?

Wingedwheel said:

Have you checked the adjustment on your valves?

Click to expand...

Yes... and I have carefully adjusted them from day one. This problem has been occurring since day one as well. The valve train noise is acceptable and consistent from cylinder to cylinder. As soon as I ride off, and for the rest of the day, this problem does not occur. The bike runs and sounds great!

Looking at your first post on this thread you stated you checked the compression with your engine warm. Have you checked it cold?

Sounds like a "too lean" condition on cold startup that resolves itself after it's warmed up a little.

Either not enuff fuel, or too much air.

Heat expansion may be closing off air leak(s). If so, may not be able to detect air leaks (with the spray tests) after warm-up.

There IS a hidden air leak potential from the starter (choke) plunger if it doesn't fully seat.
It's detectable by doing the diaphragm drop test, both cold and warm, see if there's a difference.

There's also the seal quality of the float bowl gasket in the central zone where the pilot jet fuel intermixes with the air (bleed) jet. A warped carb body would be suspect.

There's more, but it's so much easier to diagnose with non-linked carbs...

I had that exact problem with one cylinder not firing when cold but ran great when warmed up. It turned out to be the little hole\passage in the float bowl that feeds the choke circuit was plugged up.

"Looking at your first post on this thread you stated you checked the compression with your engine warm. Have you checked it cold?"

I just checked compression cold, throttle held open. It was 130 on both cylinders. "Diaphragm drop test".. I'll give that a shot. Thanks guys.

I'm not sure how the petcock is set up on your year but on my 81 there is a vacuum line that goes from the carbs to the petcock. If this is loose or not perfect the left carb will suck in air.

TwoMany... I believe you may have hit the nail on the head.

"Sounds like a "too lean" condition on cold startup that resolves itself after it's warmed up a little."

I have checked for air leaks in the past, but never on the cold engine. Immediately after I started it the other day, I sprayed some brake cleaner into the vicinity of the left carb... mainly choke area, manifold, etc. I did notice a change in the sound, and the engine (left cylinder) did smooth out much quicker. Whatever the leak is, it must be fairly minute, because it disappears after just a minute or so. That would explain why it only happens when cold, and starts perfectly any other time. Now to find the source of the leak. Thanks! It might be wise to replace all items that are prone to air leaks.

Uhlaf... there are no petcock vacuum lines on my 76. And Grinder, those passageways (in the bowl) are clear.

Thanks to all for you responses!

smg65 said:

TwoMany... I believe you may have hit the nail on the head.

"Sounds like a "too lean" condition on cold startup that resolves itself after it's warmed up a little."

I have checked for air leaks in the past, but never on the cold engine. Immediately after I started it the other day, I sprayed some brake cleaner into the vicinity of the left carb... mainly choke area, manifold, etc. I did notice a change in the sound, and the engine (left cylinder) did smooth out much quicker. Whatever the leak is, it must be fairly minute, because it disappears after just a minute or so. That would explain why it only happens when cold, and starts perfectly any other time. Now to find the source of the leak. Thanks! It might be wise to replace all items that are prone to air leaks.

Uhlaf... there are no petcock vacuum lines on my 76. And Grinder, those passageways (in the bowl) are clear.

Thanks to all for you responses!

Click to expand...

smg65.... I have a '77 that has EXACTLY the same problem with the left cylinder at startup. All that you describe is identical... I know the above discussion was a few years ago but I'm wondering if you were able to sort out the issue? If so, I would really like to know what you did. Thanks!

I too would like to know the answer.

The original post said "For the cold startups, choke is required, but after firing, must immediately be taken off or the engine will die."

Mine does exactly that. Cold motor, choke on, second kick starts right up, then choke must immediately be taken off or bike will sputter and die. Soon as choke is taken off it idles and runs fine, ready to ride.

I was thinking it's a rich condition because it doesn't need any choke while cold, but earlier posts say it's a lean condition.

I'm a little bit confused. Any clues?

Smg65 hasn't been on here for a while.
He *did* report a manifold leak.
I suppose that he fixed it...