iklukowski None
1/30/12 12:34 p.m.

Ok, so I've been going through hell trying to tune my current bs 38's so the bike will idle properly. I've tried everything under the sun to get those stupid cv carbs to work right and I just can't figure em out (These are the 2nd set of bs38's i've tried). I recently picked up some vm34's and I'm about to install them. I'm wondering if there is a simple way to figure out jetting. I have the cv carbs jetted properly but I know that the vm's are different from the cv's. If I run a #135 main in the bs38's what would be equal in size and flow rate for the vm34's? They are going on a 1976 Yam XS500. Any info will help. Thanks in advance

Curmudgeon SuperDork
1/30/12 3:23 p.m.

Jetting is an involved part science/part witchcraft thing. This manual may help: http://www.mikunioz.com/download/pdf_files/vmmanual.pdf

The first thing you need to do is make real sure there are no vacuum leaks, the ignition is in good shape and that (good points/plugs etc), the cam chain is adjusted and the valves are properly adjusted.

The float levels have to be right, this is THE most critical carb adjustment!

Then the carbs have to be balanced. To do that, you will need to make a manometer.


That one has 4 tubes for a 4 cylinder, all you need are 2. He used water, I used engine oil. I figure engine oil will just make the thing smoke if it gets sucked in. ATF will work also, the color is probably better. The top level of the fluid will 'pop' up and down at idle. if this makes it hard to read the level, you can put a restrictor in each of the tubes, an old main jet works well. bBbe sure to use the same size restrictor in each side.

You need to set the balance at idle first. Start the engine, get it warm. Then connect the manometer (there should be barbs on the intakes for this) start the engine and let it idle. If the carbs are balanced, the fluid level will be even in both tubes. If it isn't, the tube that is low needs to have the idle screw turned in. If the idle is now too high, lower the other side, then check it again. What you want is the correct idle speed (IIRC it's 1200 RPM) and the fluid level equal from side to side.

Once that is done, open the throttle slightly (~1/16 turn) and watch the manometer. The sides should not change. If they do, one slide is rising before the other and the cable(s) need to be adjusted to fix this. Hint: the side that is NOT rising needs to have the slack adjusted out.

Once this is done, now you get to look at jetting and that's where you refer to that manual. On VM's, if you get a nice smooth idle with no exhaust smoke and the air screws between 1 1/2 and 2 1/4 turns out, chances are the idle jets are just fine.

If you call Sudco, they may have some jetting numbers you can start with. http://www.sudco.com/

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