Low speed response

[Mike Horton]

How much low speed response can I expect from a 1973 JP ? I have been working to get mine running right. It is much better, but still seems a
abit sluggish from idle to 20% throttle. Its been a long time and I can't remember what to expect. At the 20% mark the bike pulls like gangbusters (lifts the front wheel). I am thinking it still is a little rich. Any thoughts ?  Thanks Mike

[tomale]

Mike, That maybe true but lets see... Is it hard starting, If timing is right and you have good spark then the problem is the pilot is too lean. A lean pilot will cause the bike to not respond very well down low. To check this where is the airmixture screw... how many turns out is it? Find this out This is important. turn it back to where is ws and then after the bike is warmed up turn the air mixture screw in a 1/4 turn does it responded better, try another 1/4 turn. If not try a 1/4 turn farther out. The air mixture screw controls how much air will be mixed with the fuel from the pilot jet. By turning it in your are reducing the amount of air thus enrichening the mixture. Turning it out adds more air and leaning out the mixture. To put it another way the airmixture screw changes the ratio of air to fuel. This is only true for the pilot jet. ON my 400 I was having trouble starting the bike. The colder it was the harder it was to start. It seemed to respond pretty well on the bottom end of the trottle but was still not as good as I thought it could be. I changed the needle setting and wham, It was alot better. I then thought that if i had moved the needle to the top notch.... enrichening the mixture then my pilot must still be too lean. I changed it out and it made a big difference. By the way. colder weather will change the way the bike needs to be jetted. cold air is denser than warm air. What that means is that you need bigger jets to keep the ratios the same. As the temps warm you may find that you will need to go to smaller jets. not just the Pilot but the needle and the main. Also, too rich of jets is not dangerous but too lean is. By the way, I left the needle in the top notch and went up one more size on the main. That helped too, It actually reved better and it pulled better on the bottom because of the richer pilot and needle. And the bike starts better. If it is too rich the easiest way to check that is to try to start it cold with out the choke. if it starts pretty easy then it is too rich.

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

[Larry Perkins]

Jackpiners are a little bit pipey but if it is a bog down low be sure the slide has no smiley face and be sure your needle jet is new.  Either of these will cause a low bog and then a hard bang of power with the Bing carb.

Larry P

[Mike Horton]

Thanks Thom and all for the info. The timing on the bike is ok and it starts and runs without choking, therefore I still think it is a bit rich. Spec calls for a #30 idle jet and it now has a # 35 changed from a #40. I must of been lucky with my 125 30 years ago. Never touched a thing for 3 years and rode year round in Mo. Thanks again for all the advice. This is what makes these bikes so fun.