Stock jet sizes for BS38 on a 76 KZ750 Twin

btkz650 wrote: ...Did a compression test and it read 90....

If not already doing, supposed to hold throttle wide open during the compression test.

Good Fortune!

On these twins, that kind of low compression is usually a valve problem. On mine, the valve seals would fail and leak oil down through the valve openings. Eventually, the accumulated burned oil would keep the valves from seating fully.

On these twins, that kind of low compression is usually a valve problem. On mine, the valve seals would fail and leak oil down through the valve openings. Eventually, the accumulated burned oil would keep the valves from seating fully.


^^^^ i think that might be why mine smokes only untill its warmed up real nice??

Do you know where I can download a service manual for adjusting the valves?

bountyhunter wrote:

btkz650 wrote: Got my new jets in today. 50 pilot and 130 main. Fired her up and she smoked like a chimney (black smoke). Did a compression test and it read 90. I don't know if that's good or bad. I haven't checked the valve clearances yet. I'm hoping that's it.

90 compression is terrible. Stock is 155 and service limit is about 115 with not more than about 15 variance side to side. Check valve clearances. Spec is .002" - .004" but up to .006" is serviceable.

I found a service manual and checked my valve clearances. They were all .05mm except #1 which was about .04. When I bought the bike, I rode it home about 25 miles and it ran ok but was on half "choke". It smoked a little when I got on the throttle then but not before. I thought it was jetted wrong because he took the muffler off so I increased the main and pilot jets. Could that have made it smoke that bad? I did the compression test when the bike was cold and with open throttle, the best reading on either cylinder was about 110. Guess I'll put the stock jets back in and see what it does again.

bountyhunter wrote: On these twins, that kind of low compression is usually a valve problem. On mine, the valve seals would fail and leak oil down through the valve openings. Eventually, the accumulated burned oil would keep the valves from seating fully.

A leakdown test could help diagnose which components are causing weak compression.

Good Fortune!