Float jets?

Hi All,

I don't know much about carbs so I'm looking for advice.

I have a single 32-01 Mikuni Flat-slide on my otherwise stock '71 750. (I know, but it came that way...) I have a new leak and I have a problem on the *extremely rare* occations I run full throttle. Could they be related?

After 10-15 seconds at full throttle it begins to crap-out, feeling as if it's running out of fuel. Closing the throttle for 3-5 seconds seems to fix it when I go back to WO, then the cycle repeats itself. I deduce, right or wrong, that the float bowl is running dry and that backing off the throttle gives the float bowl a chance to catch-up, until it starves out again.

Other day I noticed gas dribbling out of the carb; engine off, petcocks open. The bike had been parked for about 10 weeks (due to a different NOW FIXED problem) I removed the float valve, cleaned and found it had no effect. Since a replacement (with a modern Viton tip!) is $17. I thought I'd replace the whole thing. The current float valve is a "3.3". I can also get a "3.5" so my questions are...

First, is there a better (or correct) way to fix the leak? Should I just figure out how to change the float level? And second... If I go to the larger "3.5" size valve, will it cure the "full throttle" problem or will it just cause problems in some other regime?

As you can tell, I'm confused. Any guidence or suggestions deeply appreciated.

Regards,

-Danny
Oakland, CA, USA

Danny, I don't know anything about these specific carbs, so can't help much. But I will add a little:

I would say you have correctly deduced the float bowls are emptying at WOT. This can only be because they are not filling fast enough. It could be the valve you mention, it could faulty petcocks or blocked filters etc. With carb issues, you MUST always check the simple stuff first, never assume (I once started stripping a carb down... when the bike had run out of fuel... I only realised when I noticed, carb partly disassembled, that I had not turned the fuel off)!

You can prolong your carbs ability to run at WOT even with an inadequate flow by ensuring the float height is set correctly, a low float height will empty more quickly.

Finally, modern carbs like these are super sensitive to dirt, even fine dust on the tip of the float valve could easily be enough to allow it to leak. A good quality fine filter (like Pingel) is pretty much essential.

Fuel flow test from the petcock first, stopwatch & coffee can. (the expected usage/required flow has been calculated & is posted here somewhere if you search).

http://www.sudco.com has the parts you need and info too.

Crack open the gas cap latch and see if that impacts it, quick/simple acid test for vent viability.

When running WFO I always make certain both petcocks are open.

GTCarlos said:

Hi All,

I don't know much about carbs so I'm looking for advice.

I have a single 32-01 Mikuni Flat-slide on my otherwise stock '71 750.
-Danny
Oakland, CA, USA

Click to expand...

That carb is just too darn small.

The carb is not to small, the float needle jet is too small. A 3.5 or even larger would be the right move.
Of course the rest of the fuel system should be checked for flow. Jim

comnoz said:

The carb is not to small, Jim

Click to expand...

Oh ya?

pete.v said:

comnoz said:

The carb is not to small, Jim

Click to expand...

Oh ya?

Click to expand...

ya

I always put in the biggest float valve (needle and seat combo) as a matter of course.

Problem #2... Leaking carb. If I have a problem with an overflowing carb, overflowing for no apparent reason, the first thing I try is to shut off the fuel then start the motor. Let it run for a few seconds then turn on the tap. Quite often this solves the problem. Works with Amal...Mikuni...Keihin etc.