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Another VM carb question

Wallowgreen

Wallowgreen

Forum Mentor
Past Site Supporter
Following the strip down and rebuild of the VM carbs on my '79 850, the gas level in carb 1 and 2 bowls is obviously too high, gas is slowly seeping out from the gasket and dripping from the drain plugs onto the engine case. I'm not sure exactly when this is happening, it may be only when the engine is running but the flow is so slow it's difficult to tell.

Before stripping them down again, I'd like to make sure I understand how things work in this area and consequently where I have to focus my effort to fix it. Here's what I think:

When the bowl is empty gas flows into the bowl through the float needle valve seat past the float needle valve. As the float rises it makes contact with the float needle valve which then also rises in the seat until the gap between the seat and float needle valve is closed and the flow of gas stops. Is that it as far as the level of gas in the bowl is concerned?

I'm confident the float levels were set correctly when I reassembled them so what else do I look for? I've read a number of things through the search feature but in the rebuild notes it says if the float needle valve needs to be replaced, replace both the valve and the seat. Ok, but how do you tell if they need to be replaced? Last time I inspected them for what I thought might be excessive wear but didn't find anything that concerned me - did I miss something?

I also read in the pertinent section of the rebuild notes a comment about the spring tension on the jet needles as aftermarket versions are weak and will allow the gas level in the float bowl to be too high. Is that really talking about the spring on the jet needle in the slide or the spring force in the float needle valve itself?

Can one of the carb guru's help me out? Tear down starts again this weekend. Thanks as always.
 
Gas would normally go up into the carb throats. Maybe one bowl gasket is bad and it is splashing onto the next one in the wind. Also possible is a leak at the fuel tee that runs between those 2 carbs.
 
If you set your carbs up off the bike in a vice and carefully pour fuel down the fuel pipe you'll soon see if your gaskets are seeping.

If they do leak fuel either your float levels are set wrong or your needle valve needs replacing. Worth re-checking those float heights - it's easy to get them wrong.

A very quick and dirty check on the condition of your needle valves is to remove the seat and the valve, apply your best estimate of the pressure applied by the float and try and blow through the thing with you mouth. If it let's air through it's definitely faulty - they should seal very easily.

This method isn't 100% foolproof and will not pick up every faulty valve though. Check with a magnifying glass to see if you've got any muck or corrosion in there and look for pits. Any doubts - replace as a set. In my skinflint days (are they over? My wife doesn't think so :oops:) I reground seats with fine paste with some success but it's a lot of hassle and a bit hit and miss.

You can also check the actual fuel level by removing the drain plug in the bowl and attaching a bit of clear tube which you hold up against the side of the carb. I think the workshop manual shows this method and should give the heights.
 
Thanks for the help chaps.

Don't think the gas is getting blown around in the wind the pattern on the bowl suggests more an at rest seepage. I discounted the fuel tee for no other reason than #1 has gas leaking down the outside of the bowl but maybe that was a bit premature. Definitely worth a look.

So Hampshire, from what you wrote I conclude my understanding of how petrol get's into the bowl and the mechanism for level regulation is correct. I appologise but being a poor electrical bloke, I get easily confused when there's mention of a "needle valve" with at least two "needle" thingy's present. I assume you refer to the float needle valve with your breath test - right?

Wondering if I should just get new floats along with new float valve and seats for both carbs? Not cheap I know but doing the same job over and over will get old pretty fast. Much rather be riding it now the sun's coming out anyway.
 
I've read here that you can check to see if the floats are good by taking them out and putting them in a liquid and submerging them to see if they fill with the liquid. After letting them go and if they don't sink then they're not letting fuild leak into them and the floats are good. If they sink then they're leaking b/c they have filled with the liquid.
 
Wallowgreen said:
So Hampshire, from what you wrote I conclude my understanding of how petrol get's into the bowl and the mechanism for level regulation is correct. I appologise but being a poor electrical bloke, I get easily confused when there's mention of a "needle valve" with at least two "needle" thingy's present. I assume you refer to the float needle valve with your breath test - right?
Click to expand...

Right.

I presume you've got plastic floats and it's very rare that they leak, though the test that Tas850g suggests is worth trying if you suspect them - use petrol as the liquid, water won't work for finding very fine leaks because of the surface tension. I bet you a pint they're ok though (if you're ever in the Wheatsheaf I'll buy you one).

The other needle by the way is called the jet needle (at least in Queen's English:D). Its job is to meter fuel delivery / mixing with air on part open throttle
 
You're on mate, Wheatsheaf it is!

You're lucky I'm not still living in Gloucestershire or I'd be bugging someone (probably you as you seem in the know) to let me trailer the thing down for some on the spot help in trade for a few pints.
 
Are you CERTAIN its not leaking out of the overflows? VM carbs have overflow vents, unlike the BS (CV) type that would simply dump it into your crankcase and airbox. Those damn EPA bastages (shakes fist menacingly) Sure, if my bike is having issues, let it get ruined, but god forbid a drop of refined oil fall to the earth from whence it came :rolleyes: At anyrate, the overflows are also a good place to look. If they are in fact overflowing thru those vents, your floats are indeed set for the fuel level to be too high. This will cause improper running, besides the gassy mess all over the bike.
 
Anyone found a way to test the choke circuit on these VM carbs before putting them back on the bike?

I cleaned that thoroughly last strip down but was still doubtful #1 choke was working correctly.
 
As long as the plunger lifts and pops back down cleanly enough (you need to make sure that everything lines up properly) and the fuel passageway is clear the choke mechanism won't go wrong.
 
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