jetting question

I have a '76 xs650, and its not runing right at higher RPM.

The bike has a 750 big bore, straight pipes ending before the rear tire, and pod air filters.

What jet size would you recomend?

You need to check out the "carb guide". www.amckayltd.com/carbguide.pdf
All bikes are different and there is no set recipe.

Also, vacuum balance on these engines is very important. The motor will not be right without proper balance

This doesn't surprise me. The '76-'77 carb set came with the smallest main ever put in the 650, a 122.5. I would bump the mains up to a 130 or 132.5, go up one on the pilots, and lean the needles a step.

Did it run well as a 650 with that exhaust and pods? Then it should run the same with the 750 kit.
I did the 750 kit on my 75. Using the same air filters, exhaust. I thought it would need carb work, spent almost two weeks of tuning and ended up right where I was with it as a 650.
In the carb guide it give the procedures for testing your jetting and how to adjust the jetting to fix the problems the testing shows.
All these bikes are a bit different. What works well on my bike might not work on your bike. You need to test then asdjust as the testing dictates.
Leo

hi i,ve got cv38,s carbys ,,they are linked together,,,, i put a mikesxs kit in and also up a couple of sizes in jets ,,,,easy to do ,,not complicated ... make sure you do strip and really clean the carbys before putting them back together regards oldbiker

5twins said:

This doesn't surprise me. The '76-'77 carb set came with the smallest main ever put in the 650, a 122.5. I would bump the mains up to a 130 or 132.5, go up one on the pilots, and lean the needles a step.

Click to expand...

very good info "5twins"

XSLeo said:

Did it run well as a 650 with that exhaust and pods? Then it should run the same with the 750 kit.

Click to expand...

Er... No. You can't go up 15% in displacement and run the same jets *over the whole rpm range*. If you're sticking below 5500rpm, i could buy it, for at least mains/needle position.

I did the 750 kit on my 75. Using the same air filters, exhaust. I thought it would need carb work, spent almost two weeks of tuning and ended up right where I was with it as a 650.

Click to expand...

750 rephase with a 71 cam, 78 bs38's, mac 2 into 1, and pods and i couldn't get the plugs to come off stark white until i was at a 150 main and a 40 pilot. Which it seems happy at.

Anyway - OP, you can't use my numbers for yours unfortunately - because the E series (78) carbs are radically different than the D series. But where i started was taking stock jet size and multiplying by 750/650, or 1.15ish. That's where i started, and it was fairly close. I'm not sure that jet size is a linear measurement, but the fact that it more or less worked for me seems to indicate so.