Performance at low speed

[Guest]

Went for a short run today for the first time this year. Starts OK with choke. But when warm and choke pushed in, the bike pops and splutters at low speed. If I open the throttle the bike accelerates fine. But between 1 and 2.5k revs it coughs/pops and splutters. Could this be the pilot jets or something else?
Regards
Pete

[Nessism]

Most likely pilots but hard to say for sure. You may want to very carefully check each header pipe after a cold start to see if they are all getting hot.

[peterd123]

All four headers getting hot. Backfiring at a constant 2.5k revs

Most likely pilots but hard to say for sure. You may want to very carefully check each header pipe after a cold start to see if they are all getting hot.

[Steve]

Can't really diagnose by engine speed, need to know the throttle opening.

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[peterd123]

Throttle just open and coasting it seems to pop and splutter. Idle is fine although a bit fast. Acceleration is fine.

Can't really diagnose by engine speed, need to know the throttle opening.

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[rapidray]

Pull the carbs & clean them, paying CLOSE attention to the pilot jets. The holes are small enough that they get blocked, or partially blocked, fairly easily.
Ray.

[GregT]

Pull the carbs & clean them, paying CLOSE attention to the pilot jets. The holes are small enough that they get blocked, or partially blocked, fairly easily.
Ray.

I'd go a bit further and drain the tank to give it a clean out. OP is in the UK in the spring - plenty of water about, LOL. Condensation is always a problem too.

[KiwiAlfa156]

BS or VM carbs?

Check the needle jets. These can wear oval or can corrode if the the carbs have been left for years with fuel in them.

A blocked pilot jet or bypass orifices would cause leanness which given the popping and spluttering and the fact it runs well when you accelerate sounds rich rather than lean. Run it for a minute with it spluttering, kill the engine, pull in the clutch and read the plugs. You'll soon know whether its rich or lean.

The needle/needle jet starts flowing fuel just off idle. If you have just cracked throttle richness that you can't seem to adjust out with the mixture screws, I'd look at the needle jets. A blocked air jet can also cause richness. I had this very problem on set of VMs and adjusted air and mixture screws using a colortune and couldn't lean it out just above idle (around 2,500 rpm). It required 4 new needle jets and all suddenly I could adjust the idle mix.

[peterd123]

Well after all the help from you guys, today decided to go for a run to run the tank down a bit. then to remove tank to repair fuel gauge and then remove and service carbs.
So the run went like the bike just came out the showroom. Not one pop or splutter, and I really tried, at all speeds. So looks like it has cleared. So i'm going to remove tank, clean thoroughly, replace inline fuel filter ,repair gauge, refit with new fuel and see how it goes. have ordered a Colour tune kit as well. Was actually looking forward to playing with the carbs. But seems a shame if working fine now.
thanks for all your help. Great forum.

[limeex2]

Did it run fine last year before storage? Before you go to the trouble of cleaning the whole tank, drain the carb float bowls, pour the removed fuel into a glass container. let it sit. If there is water present you should be able to see it. The water is always at the bottom of the tank. In the spring with hot/cold days/nights condensation can be a issue. Keep the bike covered and the garage door closed. This is a big issue in Mn.

[Steve]

.... have ordered a Colour tune kit as well. ...

Hopefully your luck with a Color Tune is better than some of us have had.

I borrowed a ColorTune and used it on a set of VM carbs on a 77 750. Worked PERFECTLY, I was really impressed. Took them home to do my wife's '82 850L (with BS carbs) and was rather disappointed. The color never changed. As I turned the mixture screw in, the blue flame remained a blue flame, but started sputtering a bit, then more regularly and finally went out. As I turned the mixture screw back out, the BLUE flame started sputtering to life again, becoming more and more regular, finally becoming a steady flame. The same color blue the whole time. I just set it by leaning the mixture until the flame started sputtering, then backed out about 1/8 turn. Works very well.

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