help with jetting, please

[t20sl]

Bill:  (hrbay) is giving good advice.  You need larger (richer) jetting at lower levels.  Don't go to a 125 try 135/140 on main.  This may not be your problem, like Ron sugests,  but if all other things like temp/humidity/ignition are same then lower alititudes require richer jetting.  What was temp at the 1k compared to your usual temp at 4K?  Did choking help or hurt starting? When we ice raced here in Ohio I usually jetted 10-15 percent richer and temp was only difference, otherwise detonation occured. Ted

[Doug Wilford]

Just two cents worth but all the Pentons that we use to race all over the US  the jetting was 135 main 276 needle jet and a 35 pilot.  It really sounds like your pilot is plugged and your float level may need tweeking.    Also like Larry said:   The slide may be worn also.

[tomale]

Billy, I thought I would say a few things, since you and I have been down this road already. I think that Doug and Larry are right. Leaning out the bike is the last thing you want to do. It will not make the bike start easyier. The Main jet does not effect the air/fuel mixture until 1/2 throttle. the needle jet and Pilot jet do. This is were you should concentrate your efforts. first take the carb apart and find out just what you have. make sure that the carb matches what the jets that Doug gave you. That will give you a base line. If the bike runs clean and the trottle is crisp but does not start easy then either the needle jet needs to be moved up one or you need to go to a larger pilot. If it starts really easy but does not run clean etc. then it is too rich. Changing out jets could be the way to go but first make smaller adjustments to see where you are, if you can not get it to where you need by doing small adjustmenst then it is time to changs a pilot. to richen it up with out changing jets move the needle up a notch and screw in the air mixture screw a quater turn at a time. If it is too lean... drop the needle a notch and screw out the airmixture screw a quater turn at a time.
keep at it you will get it, confussion is a good thing because it is the path to knowledge. Keep your head and keep a log it will be invaluble.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5

[bildough]

You guys are great. I have tried to do what you wanted. You may be right in that the morning of the race it was foggy and cold and at 1000 feet. It was very damp out. Up here it is 75 and dry. I looked at the needle and of the 4 slots it was on the next to top position. I lowered it one, I think that was Toms message on the phone. Now I am having trouble starting it here after making that adjustment. i have ordered additional jets from Al and they will arrive soon I hope. Should I move the needle position back to next to the top where it was?

[tomale]

Billy, where is the airmixture screw set at? I would have to say that if the airmixture screw turned out more than two full turns and with the needle getting harder to start because of moving the clip down one notch I wonder what the size pilot jet is.It could be too big. Or there is another problem.  Did you check for leaks around the carb on the intake side. what about the slide, does it look like it is worn? If there is alot of scuff marks and there is a cresent shape on the air box side of the slide then I would say that the slide is suspect.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5