Another Carb Jetting Thread

[Jack G]

Hello all,

Sorry for the essay, but for the sake of being thorough, it is a necessary evil. Here goes:

I have a 1985 200X with a SuperTrapp exhaust (10 discs & closed end cap), UNI air filter, and brand new XR200 carburetor and intake manifold. Everything else is stock performance wise including running with the air box lid on. When I still had the stock carb and exhaust on, it was a strong runner, but does have low compression at 80-90 PSI and smoked very little.

From all of the other threads I have read regarding carb jetting with these performance parts, and carb jetting in general, I expected I would be running a 112 or 115 main jet. This is not what seems to be the case in my situation. While I do understand all machines are different, I expected similar results given similar modifications.

I have main jet sizes 108-122 and slow jets 38 and 40 (all Keihin brand). It idles best with the 38 pilot, pilot/mixture screw out 2 turns, and idle/throttle stop screw out about 1 and 3/4 turns. Needle clip is in the center and I have no air leaks. I started with the 122 and worked all the back down to the 108 main and I'm still running super rich. By super rich, I mean my plugs are black and wet. I can't even get to 1/4 to 1/2 throttle without a serious bog, full throttle is not even in the equation. There are no pops/backfires. Removing the air box lid didn't seem to make a difference. I ride below altitudes of 1,000 feet.

Without a doubt, it bogs the least/runs best with the 108 main jet but does not even come close to running well by any standard. Any throttle and it bogs and smokes considerably. On the current trend, I am gonna end up with a main jet under 100 with these modifications.

Does this make sense to anyone else? Please correct me if I am wrong, but I have more air flow and therefore should need larger jets to increase the fuel flow. Why would my engine run better with a smaller main jet?

Or, is the low compression causing my problem?

I have read every relevant thread I could find and have followed the jetting procedure on Jetsrus. Criticism is welcome nonetheless.

Thanks in advance.

[shortline10]

Your compression is way to low , it should be double of what you have now and by adding more carburation it will make it worse .

[Jack G]

Your compression is way to low , it should be double of what you have now and by adding more carburation it will make it worse .

Yeah I can't believe the thing ran as good as it did considering it should have 164 psi of compression. I had a feeling that would be an issue, just wanted some feedback from others before I took my next step (which will be finding out if my cylinder needs bored and ordering a matching 10.25:1 Wiseco piston)

Thanks for your input, I appreciate it!

[350for350]

Doing a top end isn't all that horrible and you'll LOVE the fresh engine. With the set up that you have, I'll take a wild stab and guess that you'll be somewhere around a 115 main jet with a fresh top end.