Not Enough Carb??

jughead · Nov 10, 2009

Hi Guys,
Been running a 34 Mikuni on my Commando and love it. One kick starts, cold or hot. Running a 140 main and 27.5 inter and plugs are tan brown. The bike idles great and she pulls hard through the gears.
Here's the rub, high speed she breakes down after a few seconds at 90+ mph like its starving for fuel , back off for a few seconds and its fine.
I'm thinking I need a larger carb or go with twin Mikuni's. (size?)
Dont want to go back to Aminals.
Its a a fresh build (well run in) 750 Roadster with some port work done. Standard cam. Running a belt drive 34/70
Any advice on carb set up?

MEHAVEY · Nov 11, 2009

That doesn't sound like carb size, that sounds like you are emptying the float bowl faster than it's being fed by the gas lines through the needle valve.

Unfastened from the carb, how fast will your gas line fill a cup w/ fuel (i.e, what's your max fuel flow outta the tank)?

B+Bogus · Nov 11, 2009

MEHAVEY said:
That doesn't sound like carb size, that sounds like you are emptying the float bowl faster than it's being fed by the gas lines through the needle valve.

Unfastened from the carb, how fast will your gas line fill a cup w/ fuel (i.e, what's your max fuel flow outta the tank)?

+1

geo46er · Nov 11, 2009

Greetings,
You wrote that you are using a 140 main jet. In all of the suggested Mikuni/Norton references I have read the main jet range is usually given as 230 to 260 you probably need a bigger main.

GB

grandpaul · Nov 11, 2009

A plug chop will tell you how your top end jetting is, but it sounds like fuel starvation alright...

Cookie · Nov 11, 2009

I'd go with everybody here. Sounds like too small a jet, maybe low float level, and maybe not enough fuel getting to the carb. I'd start by seeing how much was coming out of the petcocks.

chopped850 · Nov 11, 2009

If I remember tonight when I get home from work I can check my Mikuni to see what is in there.

Cookie · Nov 11, 2009

I don't recall what you have for a tank but if it's glass or has a lining a look in might be good.

B+Bogus · Nov 11, 2009

If it was just running lean I'd expect it to pick up a little when the throttle is rolled off.....just a thought

jughead · Nov 11, 2009

Hi, I read all the replies and am incline to think its emptying the float bowl and not the main jet size. The symptoms are not those of too small a jet. And a plug chop reads perfect on the main. Is there a larger float bowl or reserve reservior that can be attached or do I go to twin mikuni's (size needed) BTW tank is a steel roadster.
JUG :?

kommando · Nov 12, 2009

No need for a bigger float bowel, just increase the flow into the float bowel, some fuel taps have a rubber insert that expands reducing the hole size through the middle and slowing the flow, this can be fixed by drilling the hole back to size. As you are drilling rubber it only takes seconds.

splatt · Nov 12, 2009

Strange, mine does exactly the same thing, only at full throttle after about 30 seconds,what sort of air filter are you using, small pancake :?: I have heard they are to small, tried turning on both taps :?:

Triton Thrasher · Nov 12, 2009

jughead said:
Is there a larger float bowl or reserve reservior that can be attached or do I go to twin mikuni's (size needed) BTW tank is a steel roadster.
JUG :?

You don't need a larger or twin anything- you need to fix the problem you have, which may be fuel flow restriction in tap, pipe, filters, tank vent or float valve.

Does a Norton tank have two taps and do you open both?

MEHAVEY · Nov 12, 2009

In order....

1. Check that you have free fuel flow out of both taps, and that either tap will fill a cup in 30-40 seconds max. (Mine will do it in less than 25 sec even when near empty.)

2. Then check that the gas line connection/flow to the bowl can do the same thing.

3. Drop the bowl and remove the float/needle combination to check the fuel flow through the fully open orifice. You should be able to fill a cup through it in less than a minute (based on a conservative 90 mph/25 mi per gal consumpion rate).

4. All that done, check that the float is set high enough not to cut off fuel early.

Last but not least.... you may find that one needle orifice/needle/float system just can't handle the fuel flow necessary. Then you could need a second carb for those kinds of speeds.

jughead · Nov 12, 2009

Thanks guys, all good feedback. Will let you know the results.
JUG

Not Enough Carb??

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