Fuelling problems after rebuild

redrocks · Post by **redrocks** » Mon Sep 12, 2016 7:41 pm

I've done the valve clearances which were very tight all round. After the rebuild I'm having fuelling problems. Ive cleaned the carbs several times but the revs are "hanging"....they stay high after blipping the throttle. On a short run the bike was stuttering as if it was fuel starvation. I'd not changed anything on the carbs !

I can see any leaks. The float height is 12mm at the moment and I've cleaned the jets.

Not sure what to try next?

The carbs were off for 4 weeks but I had drained them.

Would an ultrasonic clean help?

I had the hanging issue a while ago but at that time a good clean sorted it out. This time it's much worse.

Andy

Post by **Stig** » Mon Sep 12, 2016 9:50 pm

Hanging revs is usually down to an air leak. Try spraying carb cleaner around the inlet area, when the revs rise you've found the fault

andy88 · Post by **andy88** » Wed Sep 14, 2016 8:54 pm

Hi.....when you cleaned your carbs, did you make sure the little o ring rubber is still in on the diaphragm lid area.. Just a thought.

redrocks · Post by **redrocks** » Mon Sep 19, 2016 5:34 pm

I've cleaned the carbs again and put them back on. This time I tried the carb cleaner and when the revs "hung" i sprayed each carb in turn and on cylinder 4 the revs rose straight away. I've looked at the connecting rubber and it looks OK, but i've ordered a set of new ones anyway. The carbs have been on and off quite a few times but i'd assumed the clamps would have made the seal good. Anyway once the new ones come i'll try those.

Could air get in anywhere else -do i need to check anything else?

The diaphram o-rings are all Ok - maybe a bit worn but I only sprayed towards the carb rubbers !!

Thanks, Andy

andy88 · Post by **andy88** » Mon Sep 19, 2016 5:56 pm

If you've ordered new inlet rubbers, i personally would wait untill ive fitted them and then check other areas... You may get a "false" positive air leak check..best to iliminate one area, and move on to the next..

Ps,,,,with air box off,,,,have you checked your diaphragm rubbers, and made sure your slide is returning properly on the cylinder in question...
Lift slide up to top and let go,, it should re sit with a kind of whooshing clunk kind of noise.. (can someone please describe that noise any better

) "..

Also make sure all your vacuum hoses are reattached and clipped up.

Hope that helps...andy

redrocks · Post by **redrocks** » Fri Sep 23, 2016 9:58 pm

Well......new rubbers have not made any difference. So i've tried spraying carb cleaner on others parts of the carbs and i've found that if I spray near the coupler connected to the overflow pipe between carbs 3 and 4 the revs rise! I know that the fuel inlet pipes couple close by too. There are no fuel leaks as far as I can see.

There are O-rings on the overflow coupler but would they cause the problem? They also seem to be very expensive for simple O-rings.

I will try the same test tomorrow and see if its exactly the same.

Andy

redrocks · Post by **redrocks** » Sun Sep 25, 2016 5:00 pm

Yet another disassemble and rebuild but still the same. Blip the throttle and the revs stay about 2000 then go down slowly.

I did notice what could be a hairline crack in the carb 4 top. Ive tried to do a temporary fix with epoxy but the cap doesn't sit very flat around the o-ring area. There's a possibility air could be getting in. All other caps are OK - just this one on carb 4 is odd.

I've never seen carb caps for sale though and I'm considering a second hand set.

Andy

redrocks · Post by **redrocks** » Fri Sep 30, 2016 9:31 pm

The problem is still there with a second had set of carbs ! The fault seems to be carb 4 so I've focussed on the intake area. I've replaced the o-ring in the intake stub (???) between the carb and the engine. Yet to see if its the problem !

redrocks · Post by **redrocks** » Tue Jan 03, 2017 8:15 pm

Hi all, believe it or not I still have a problem and cant get the bike to run right at all. Ive checked for leaks using carb cleaner but cant find any. I have new carb rubbers and clamps, new air filter and plugs. Float height has been set to 14mm. I have to set the mixture screws to 3 1/2 turns out otherwise the revs "hang" at about 1500rpm. On a road test about 2 weeks ago the bike was very "woolly", not wanting to accelerate and felt lumpy from idle to about 4000 rpm but better at motorway speeds once spinning at higher revs. This is about the best I've got so far but its still far from good. I checked the plugs after the run and they looked OK - slightly grey if anything.

Does anyone have any other ideas or know of a Mikuni expert who can help? Its very frustrating since all I did was to check and set the valve clearances !
Andy

Post by **Stig** » Tue Jan 03, 2017 9:12 pm

is the exup valve sticking/stuck?