Carb help needed
 

My 1990, cbr1000f,

I did a stage 2 jet kit 2 years ago, 4 into 1 exhaust, uni air filter, cleaned, oiled, new plugs.

below 3,000, rpm, bogs bad, hardly takes off. Or dies. A real peta, taking off. From dead stop.
idles fine, above 3,000, runs fine.

what should my a/f, screws be at, ? What should I look at ? To resolve issue.

Did you replace the copper seals when the 4-1 was installed? Sounds like a head pipe leak.


Also did you turn the pilot jets out 2 1/2 turns after re-jetting?

No, bike came with the 4 into 1. 3 years ago. I have felt no air leaks there, I have checked that area, don't think this is the issue.

I think there at 3.5, that was two years ago.

You may not feel any leaking head pipe. The 4-1 works by creating a low pressure behind the high pressure wave of a different pipe. This low pressure helps pull the next high pressure wave along. The low pressure in the head pipe can suck outside air into the head pipe where it meets the engine if not sealed properly. The outside air, when it collides with the hot spent exhaust gas, expands many times its volume. The expanded air blocks the head pipe like stuffing a potato in the end of the pipe. At higher RPM, with a small leak, the air doesn't have enough time to get sucked in. The problem is not so noticeable with a more restricted (stock) system where there is some back pressure stopping outside air from entering. With a 4-1, you got to get them sealed properly.

From your description, you have signs of a head to engine leak. You can try to tighten them down more evenly and see if that helps.

Here is a LINK the the parts you need. If you pull the radiator back to get to the head pipe nuts you might as well replace the seals. They are not much money. I didn't have to look up the part number as I have 4 in hand while typing. I suspect I have a very small leak myself as I have a slight stumble while taking off from a dead stop only on hot days when the fan is running. Last month I had a similar problem with a bike I'm rebuilding. I put a 2-1 on and it had a hard time coming off idle but would rev. I thought I had them on even but had to even them out. Now its OK.

You have to go by feel. Tighten them down by hand, then use a ratchet slowly 'til you feel resistance on one. Then turn the other until you feel the same resistance. Then tighten them down evenly with the same feel.

If you find the problem, please report back. I'm putting money on head pipe seal.

Tim, I may take a look into this, I appreciate the info, it be a week before I have tgst kind of time.