tm40 strange happenings

hi all

i have a 1990 dr650 to which i have fitted a mikuni tm40 carb,but trying to set it up is proving to be a nightmare.

i can get the bike running but when opening the throttle sharply from tick-over [1500rpm] the engine either bogs and stalls or sometimes backfires through the air filter. above 2000rpm the engine seems fine.

i have also noticed that if i set the tick-0ver to 1500rpm when the bike is stood on the sidestand and then stand the bike up dead vertical the tick-over goes up by about 500rpm. float hight too low?.

any got any ideas please. 

Your accelerator pump is not working, or is squirting way too soon.

You have to set the float height to stock, apump to squirt for 1 sec and miss the slide, and fuel screw for max idle.

To add, no one rides at idle then hits the throttle like that. Other than reving it up in the driveway or at a stop light, how is it?


Idle should also be higher, more like 1,850. Idle speed should not change between being on the kick stand or vertical, just not that much of a change in tilt. More likely it is a throttle cable issue.

+1 --  RPM rise -- likely it is a throttle cable issue -


I managed to get in the garage this afternoon. Two things i found to get the bike running better were

1 the arm that works the rod on the accelerator pump had approx 4mm gap i reduced this to aprox 1-2mm the bogging disapeard.

2 the difference in tickover speeds i traced to my side stand. It was loose and kicked out too far. After fitting another one i found the bike stands more upright and no change in tickover speed weather the bike is held upright or is on the sidestand.

Many thanks for all the help.


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