M750 Running Poorly

[HOMUTH]

Hi...I have a 1999 M750 which is carbureted- a few weeks ago we were on a group ride & one morning it started running really rough...no power, missing, wouldn't rev....a few minutes down the road all of a sudden it instantly starts running fine again - the guy riding behind me commented that it belched black smoke when this happened

Since then I've tried a few times and exact same thing happens....runs like a toilet until it warms up a bit and then it's like a switch it starts running fine - I put a new set of plugs and didn't help - on start up the exhaust from the front cylinder doesn't feel as warm as on the back cylinder

is their likely something messed up in the ignition or

my son has a 2002 M620 (fuel injected) it has always ran better & been more responsive than my M750 with carbs....he can pull me in a drag race even though I have 20% more displacement and my bike is mint and his bike has a ton of really hard miles on it

is there really that much difference between carb & FI - the carbed bike has always been super sensitive to choke position and been cold blooded

Anyways any input would be greatly appreciated...Thanks!
Sheldon in Canada

[Speeddog]

Here's a bit of info on 620 vs carbie 750:
http://www.bikeboy.org/620sport.html
Your bigger motor is held back by the same valve sizes and less valve lift.

I'd say carb or ignition issues, tough to pick from here.

[ducpainter]

Here's a bit of info on 620 vs carbie 750:
http://www.bikeboy.org/620sport.html
Your bigger motor is held back by the same valve sizes and less valve lift.

I'd say carb or ignition issues, tough to pick from here.

In addition to what Speeddog said, 'a 99 750 likely needs new needle jets, and fresh plugs because the worn jets have sooted them up.

[koko64]

The plug on the failing cylinder black and sooty? Ovaled out needle jets won't help that's for sure. Also occasional carb flooding can happen with dirty fuel. Search for carb flooding solutions using the search function. If your bike still has a vacuum tap then bin it and fit a manual tap. You have to plug the vacuum fitting on the manifold and fit an in line tap like a Briggs and Stratton or Motion Pro type. The vacuum taps either fail shut starving the carbs or leak down the vacuum line flooding into the cylinder. Many threads here with info to search through.

[HOMUTH]

The weird thing is it doesn't want to rev & bogs pulling out from a stop light and then it belches a bit of black smoke and instantly revs up and runs like normal the rest of the day.....the next day it repeats the exact same pattern....replaced the plugs and the old ones looked OK

This bike has always been cold blooded and finicky with the choke....is that typically of carbureted monsters?

Anyways appreciate any direction in diagnosing the problem...THANKS

[Howie]

If the carbs are at factory idle settings and stock jetting, quite cold blooded.  My old 750 ran DynoJet stage 2, open air box,  Factorypro needle jets.  Warm day, no choke, hit the starter button and ride off.  Cold day, half choke, choke off in a few seconds, ride off.

Belching black smoke makes me think intermittent flooding.  If the needle jets are original or have mileage on them you want to go through them anyway.  Note the jetting and report back.

Oh, also pull off the vacuum line to the fuel pump and look for signs of gasoline.