Odd VM Carb Question???

[Footy.]

Hi all.

Had a great first season on my rebuilt 78 GS750E. Thanks to all the good folks here for their good advice that helped me rebuild a trouble free and reliable bike.

I only have one issue concerning the choke, and with hindsight i think i may have slipped up when i rebuild the carbs?

Living in England i do need some choke on cold starting. I give it full choke and it fires up first time, on firing up i close the choke to about half, and she runs OK for a few seconds. But then it starts to smoke and stutter, and is obviously too rich.

Problem is if i close the choke even a little, the engine stalls. I can keep it running to warm up point only by opening the throttle a little. It is like the choke system air/fuel ratio is way too rich?

Now this is my question for the experts.

The choke fuel jet as you know is just a restriction in the brass tube that extends down into the float bowl. But this goes into it's "own" seperate little "chamber" feed by a small hole right at the base of the float bowl.

Why this seperate arrangement? why not just have the choke tube "dip" directly into the float bowl?

When i rebuilt my carbs, i opened up the feed holes in the float bowls to the choke "chamber" with a small drill, as i thought it would ensure a better fuel feed, and the control "jet" itself was in the brass extended tubes.

With the choke problem i am now having, i am begining to think maybe the seperate choke feed chamber exists, and is restricted for a reason?

This would allow fuel to fill the choke jet tube while the bike is not running. This may supply an overich supply for first firing? Then as the engine runs, the fuel is quickly used in this small chamber, and the size of the hole feeding it from the main float bowl dictates a weaker mixture untill the choke is finally closed?

If this is so, i had better start looking for some new float bowls :roll:

This is just my thoughts, any one any further knowledge on this one??

Thanks a lot.

Footy.

 

[arveejay]

In my opinion, the choke tube sits where it is so that fuel will only seep in to the sock for the tube. If you opened the hole size you have allowed the carb to pull too much fuel when the choke is engaged. That would explain the rich condition when choking. Your only solution may be to replace the float bowls with original bowls and clean them with cleaner rather than a drill bit. Others may have a better explanation.

 

[Sandy]

In my opinion, the choke tube sits where it is so that fuel will only seep in to the sock for the tube. If you opened the hole size you have allowed the carb to pull too much fuel when the choke is engaged. That would explain the rich condition when choking. Your only solution may be to replace the float bowls with original bowls and clean them with cleaner rather than a drill bit. Others may have a better explanation.

That is also my theory on how the choke works. The small choke reservior will allow a good rich mixture for initial cold starting and then a metered amount for proper mixture during warm up. With the holes drilled you would be getting too much fuel flowing up the choke pipe. You might try filling the holes with something resistant to gas and redrilling them to the proper size.

 

[Footy.]

Thanks for the replies.

The more i think about it, the more i think it was a mistake to drill the feed holes.

I know of a spare set of carbs at a breakers, so i will see if they will let me measure the original hole size, and take it from there.

Thanks again.

Footy.