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HELP: DR650 Running Too Hot At High Speeds

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I was supposed to leave on my South America trip yesterday but didn't get a chance to do any test runs until the night before leaving and that was the first time riding the bike after rebuilding the engine and installing the GSX-R exhaust. What to do; I know it's not the proper way but it's been snowing like crazy and I'm trying to beat the rains in Central America by leaving now.

I have a Vaportech dash with an engine temp sensor on the spark plug and the bike usually runs around 170-180 F. Now, it's running around 200F and shoots up past 220F when I go above 50 mph. When I slow down, the temp drops and when I speed up past 50 again, it shoots up. Running at 45 mph or slower, the temps stay between 190-210.

This means that it's lean, right? The bike was running great at the end of last season and the only major change has been installing the GSX-R exhaust and bad on me, but I didnt know I had to tune the jets for this exhaust. I read that it wasn't necessary.

The bike is a 98, with a Mikuni TM40 Flat Slide (non pumper), K&N filter with foam pre-filter and airbox mod (top cut) and now GSX-R exhaust. It starts easily and idles nicely at 1500 rpm and runs smoothly.

I had a 270 main jet in there before the exhaust mod and if it's running lean at higher rpms, I was told by a mechanic to put in a larger main jet. So I ran a 300 and noticed no difference. It was still heating up and going past 220F above 50 mph. I then put in a 340 main jet and no difference again.

Spark plug with 270 main jet was lightly coated in black. Spark plug with 300 was white-ish. Does that mean it's running leaner with a bigger main jet? I'm confused on what to do now. :thumbsup:

I checked the connection between the carb and the engine and the carb and the airbox and it looks tight with no air leaks. Sprayed WD40 on it to see if it would get sucked in while running and nothing there. I dont see any damage, holes in the airbox besides the cut in the top of the airbox.

The next thing I'm going to try is putting the stock exhaust back on and returning it to the spec it was in at the end of last season where it ran great on a 2 week trip with no issues.

Any other ideas on how I can reduce the lean condition at higher rpms?

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220F at the spark plug is not too hot. It is not even close. I don't know about your jetting, but the temps you are getting are not a sign of a problem. Hopefully someone with a similar setup to your can give some jetting advice.

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220F at the spark plug is not too hot. It is not even close. I don't know about your jetting, but the temps you are getting are not a sign of a problem. Hopefully someone with a similar setup to your can give some jetting advice.

+1

220 at the plug is certainly not an indication of a lean running engine... I'd go back to the standard jetting that works for the TM40 with a performance exhaust system. The white plug with a 300 main is a puzzlement.... even though your plug temps aren't saying you are lean I'd be sure you do not have a fuel flow restriction to the float bowl which could be running the float level low at higher throttle settings. Sounds as though you've checked for vacuum leak issues..... It could be a "too much information paranoia thing"

Those dang high tech instruments can be dumbfounded by the overly simplistic DR. :thumbsup:

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Ok, good to know. Yeah too much info causin unneccessary worrying, lol.

Also, these are the first runs after the engine has been rebuilt with new piston and rings. Does it run hotter while it's breaking in?

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Those are pretty low temps. I would only be concerned if temps were to run over 350 for an extended length of time.

We try to have the head temp between 260 and 290 when doing dyno runs.

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Don't confuse safe water/coolant temperature ranges with safe aircooled/cylinder head temperatures.

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Did a 100 mile run today and bike's running fine. Temps hovering near 250 while cruising at 60. It's running smooth.

Yup, do know that air cooled will run hotter but thought 220 was a hot zone and 260 a danger zone, but looks like I was wrong.

Thanks for all the help. I'm good to go :thumbsup:

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Modern gas makes plug reading a real challenge. Whether there is ethanol or not in the gas can change what your plug looks like.

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JamminJay , i would almost bet me left knacker youll be fine

i have an oil temp gauge , today it was 60-66 Celcius on a 30 C day

on a 40- 45 C it reads 88 Celcius

i also have a TM40, but the Pumper, used to have the side of the air box off was 80 C max

after i did the airbox mod ansd replaced the side cover it runs 8C hotter same jetting, im hoping its telling me its running leaner and i get better fuel econmy as i was getting 25o k for 17 L

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