quick jetting question. Proper size?

[section8joe]

Im trying to jet a 76 with bs38's. Stock jetting for that year is 25 and 122.5. I'm running 1.5 straight pipes with velocity stacks. I was going to try and bump up the pilots and mains up 2 sizes. Do you think going up two sizes on both the pilots and mains will be good for this set up?
Thanks

 

[5twins]

Pilots probably only need one size up but mains will most likely need more than 2. Try something in the low to mid 130s (132.5 or 135). Also drop the needles a step. The needles will need that leaning for the large increase in main jet size.

The '76-'77 carb set came stock with 122.5 mains, the smallest ever put in any 650 carbs. When you start modding with this carb set, those small mains quickly run out of steam. While one or two sizes up on the mains will often work on some of the other 650 carb sets, it's usually not enough for this set. All the other sets had mains 2 to 5 sizes larger to begin with.

 

[hotrdd]

5twins - also dealing with 76 carbs but wondering as we move up the mains what are the tell tale signs that I've gone to far? Or are still running lean? I'm assuming that this needs to be tested on the road and doesn't work sitting in the shop on a stand?

 

[section8joe]

Pilots probably only need one size up but mains will most likely need more than 2. Try something in the low to mid 130s (132.5 or 135). Also drop the needles a step. The needles will need that leaning for the large increase in main jet size.

The '76-'77 carb set came stock with 122.5 mains, the smallest ever put in any 650 carbs. When you start modding with this carb set, those small mains quickly run out of steam. While one or two sizes up on the mains will often work on some of the other 650 carb sets, it's usually not enough for this set. All the other sets had mains 2 to 5 sizes larger to begin with.

Thanks, I went to my locally indy shop yesterday and they didn't have the the sizes that I wanted so I tried a 30 and 135. It's running too rich with these jets. I'll have to order some from Mike's and swap them out.

 

[5twins]

The test I use most for main jet selection on these CV carbs is to watch for upper midrange break-up. The mains bleed over into the upper midrange. As you increase the main jet size, eventually it will make the upper midrange too rich and create stumbling. You lean the needle to fix this but this stumbling is a pretty good indication that you've gone as big as you can on the mains. Going up another size usually brings the stumbling right back unless you lean the needle another step. I'm not a fan of running the needle at one of it's extremes (1st or last slot) so I go for the largest main that will work with it leaned one step.

These CV carbs are very forgiving and can mask over minor jetting glitches unless pushed hard. So, to test the upper midrange, you need to use full throttle. In one of the middle gears (2, 3, 4), from about 3.5K, grab absolute full throttle and run the bike up to near redline. Watch for stumbling in the upper midrange around 4 or 5K. If you get none, go up one size at a time on the mains until you do. That's pretty much your main jet size limit unless you lean the needle some more.

The reason you can't go as big on the mains with this carb set as on most of the other ones is because of the rich Z-8 needle jet. It's the biggest ever put in a 650 carb set and is going to help cause that upper midrange break-up with mains smaller than what may cause it in the other sets.

 

[5twins]

Section8, did you lean your needles a step? You have to do this when increasing your mains or you won't get far.

 

[section8joe]

No, I didn't lean the needle. I will when my buddy brings his bike back down to my house. I believe he ordered some new jets today.

 

[5twins]

What you have to realize about your carbs are that the 3 circuits overlap and have some effect on the one next to them. You richened the idle and main circuits but didn't lean the midrange so of course it's going to be too rich.

 

[section8joe]

I kinda figured that the 30 and 135 would be too large. The carb guide also suggests replacing the orings for the needle. I did not have replacement orings so I just left it alone. The bike in question is not mine. It's a friend that lives and hour and a half away. When he brings the bike back down I'll try to adjust the needle, replace the orings and change the jets.

 

[5twins]

The o-ring in question is on the needle jet. Place your finger on the needle jet and see if it wiggles in the hole. If it does, it needs the new o-ring. If it's tight, it may still be OK.