Its been a while since I had to rejet my quad and now that its getting drastically colder here (by Texas standards anyway), I've noticed that my quad tends to run poorly at high speeds (30 mph and up). For example at my last XC race in December, it ran poorly at 30mph and up while I did a practice lap (8 miles long) early in the morning while it also ran perfect the rest of the time so long as I didn't go over 30mph. Later that afternoon when my B class race finally started, the temps rose at least 10 or 15 degrees and I had no more problems at all when going over 30mph and I made absolutely NO ADJUSTMENTS! This has happened a few times when the weather turns cold, so I believe my main jetting is not correct for cold temps.

Can someone please remind me of the general rule of thumb for jetting? In other words, as it gets colder (everything else being the same), do I typically need richer or leaner jetting (main jets) to compensate for the denser/colder air?

Happy trails...[img]i/expressions/face-icon-small-smile.gif[/img]

 

the colder the bigger (jets) is needed, but you have to make sure it's completely warmed up.
also when it runs crummy over 30 mph try putting the choke on slighty. there would be your answer.

 

yep... just what he said... MIKEY... a four stroke is much more stabil as far as jetting goes than the 2 strokes, & the more air you are moving through the engine, the more effect that temp will have on your engine... I also commented about the choke, the same as SCH, on another forum...

 

Thats what I thought. Thanx for the feedback. I'll try the choke trick next time I ride/race in cold weather to see if it helps or hurts.

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Mikey, Haven't you tried adjusting your DAJ's?

 

Yamihoo,
I might try adjusting the DAJs this weekend at my local XC race if its really cold as expected. However I'm a little reluctant to do so b/c I've done timed runs at different times in the summer (in much warmer weather) and the middle DAJ setting always performed best no matter what my main size was. But I don't think it would hurt to try it now that its cold here (by TX standards that is). I've heard of someone who used the middle setting (#3) on the DAJs in the summer and used the next richer setting (#4 I think) during the winter months. So maybe thats what I need to do.

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Yeah here in MN we can go from 60 to 30F in one day in the fall, or right now its -30, 2 days ago it was +25. Of course I won't ride when its -0, its just stupid, stuff breaks. BUT, I was playing around with them a couple weeks ago when it went from 25 to 0 in one afternoon, I went a click up when it got colder and I noticed a difference for sure. Starting was better, and 30mph+ was better. I actually believe I read somewhere that theoretically one click should accomodate a 30degree change in temp. Now of course thats probably not exactly true depending on humidity and actual temp, weather its 0 or 30 or 60F out the air temp is not the only issue, its also more dense of course.
Let us know what you find. I didn't have any timed runs, but I'll do that next time I get the chance, but seat of the pants feel was noticable.

 

yamihoo,
Thanx for the feedback on your recent DAJ adjustments for temp changes. In theory, I definitely agree that adjusting the DAJs to a richer DAJ setting should help it run better when the temps drop...However, I am not sure why your quad seemed to start easier after you adjusted the DAJs b/c they should not be flowing fuel to the carbs at startup. As I understand it, the DAJs only kick in when the motor is under a heavy load (like WOT acceleration for example). When the motor is at idle speeds, the DAJs do absolutely nothing as I understand it. Did you happen adjust the idle mixture screws at the same time as your DAJ adjustments...b/c those idle mix screws can make a difference in how easy the motor starts?

PS - Yes, I will report back after my race on Sunday to let you know if messing with my DAJ adjustments helped out. I just found out that the weather report for Sunday (at the races) has changed and its now supposed to be in the 50s on Sunday...so I may or may not need to mess with the DAJs at all (just depends on how well my motor runs in the coolish temps on Sunday).

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Mikey, yeah I thought they only work with a certain amount of vacuum or "load" as well. Yes I have adjusted my pilots since I have installed 130, 135 mains, prior I had 136 and 138 (dont ask where I got the 136, it was in it when I bought it). Anyway, I have my pilots at 2 1/4, so does my buddies pure stock machine (Same one). Well one morning when is was -10 degrees, My machine started quite easily, his would not start until we cranked on it for about 2 mins. and had to jump the battery. So it must make a difference on start up as well. I can't see any other explanation because his is pure stock, exact same machine. Well we both have TwinAir, but I doubt that matters.

 

Yea Mikey.....cold air is more dense and contains more oxygen molecules.Thats why some performance cars are intercooled and that the maufacturers started finally plumbing the air intake OUT of the hot engine compartment.
I can really tell the diff in winter time when I fly.Since the air is more dense the engine makes more horsepower and the wing and prop are much more efficient.If you can imagine a chunk of 30 degree air and a chunk of 90 degree air just remember the 30 degree air has more oxygen..
02 650 camo