Troubleshooting acceleration issue help needed

[jabcb]

Fuel flow is controlled by the size of the jet & the pressure drop across the jet.

For externally vented float bowls, the float bowl is at atmospheric pressure.
For internally vented float bowls, the pressure in the float bowl is different & depends on exactly where the float bowl is vented to.

Might seem to be a minor difference, but it has an important impact on the pressure drop between the float bowl & the air flow in the carb venturi.
The big difference in main jet size allows both venting approaches to provide the same flow rates with very different pressure drops.

[garncarz]

Interesting. So, an externally vented VM32 has a greater pressure differential between the float bowl and the venturi where fuel is drawn into the airstream and thus requires a smaller jet to deliver the same amount of fuel. I have new 97.5 mains on order.

When I get all the new brass it will look like:
main - 97.5
needle jet - R: P-4, L: P-4
jet needle - 5FP17
slide - 2.5
pilot - 30

Thanks to all for the great intel.

[garncarz]

Happy Spring to all! I was hoping that by letting the ole T500J rest for a few months, the acceleration problem would have gone away by itself and I would be ready to ride! No such luck.

She starts on the first kick but the left cylinder is not firing at all (cold down pipe). Here's what I know/think/have done:

1. Strong blue spark on both cylinders: (1) New Denso W24FS-U plugs gapped at 0.5mm, (2) Points cleaned and gapped at 0.3mm, (3) Timing set to 3.4mm BTDC, (4) New condensers.
2. New GT pistons and rings, compression = 145R/135L, retorqued head bolts, arrows on pistons pointing to exhaust.
3. Fuel (unburnt) on left plug after running

I don't know what I am missing. Thoughts? TIA.

[karl pa]

Does applying choke have an effect?
Does increasing throttle have an effect?
If you have good spark, I would look for blocked carb passages, ultrasonic cleaning doesn't always clean blocked passages. I always use carb cleaner and compressed air to verify all passages are open, sometimes it takes several attempts.
If carbs are not the issue I would look a little deeper, If you remove carbs and place your hand over intake boot, you should feel sucking pressure when you kick the engine over. If not then you could have an air leak in the bottom end.

[garncarz]

I am thinking of swapping the carbs tomorrow to see if I can make the issue move to the right cylinder.

[garncarz]

I swapped the carbs and now the right cylinder does not fire. Also the left side (now running) blew out about a cup of unburnt fuel from the muffler. I guess it puddled up as it was not burning. So, somehow way too much fuel (enough to prohibit combustion) was getting into the left side. Any thoughts on where to start troubleshooting the suspect carb? TIA.

[karl pa]

At least you are getting the issue narrowed down to 1 carb.
When you say it cleared out unburnt fuel, was it fuel or could it be oil?
Does your fuel petcock stop fuel flow when off? Does your float shut off the flow? Or could the float allow fuel to flood the engine? If I let the manual petcock on, my 250 floods the bottom end without leaking out the carb drains.
If you pull the effected carb cable to raise the slide, does it try to fire?

[garncarz]

At least you are getting the issue narrowed down to 1 carb.
When you say it cleared out unburnt fuel, was it fuel or could it be oil? - Definitely fuel.

Does your (vacuum) fuel petcock stop fuel flow when off? - Yes.

Does your float shut off the flow? Or could the float allow fuel to flood the engine? If I let the manual petcock on, my 250 floods the bottom end without leaking out the carb drains.

The float valve/needle seals up correctly, at least it did when I tested it last year. Does the float close the valve properly? I'll need to check.

If you pull the effected carb cable to raise the slide, does it try to fire? Hard to say, likely not. It starts up, runs drives on the right cylinder only.

I'm definitely flooding the left cyl. It has to be float, float height, float valve and/or float needle related, no? Still wondering if I have all of the right pieces/parts for the externally vented carbs, as what I got from the PO was a mish mash.

[garncarz]

I figured it out! Running strong on both cylinders now. I hate to even say it though: The pilot jet was missing from the left carb. It was on the floor of my shop, apparently for 4 months. I can't imagine how I repeatedly missed that. What a dumb ass. Sorry for wasting everyone's time.

[Alan H]

Oh we all make mistakes, trust me on that one.