I have had the engine and crankshaft rebuilt on my MC21 SE due to leaking crank seals. Previously the bike would refuse to idle and drop down onto one cylinder at idle consistently, along with some other crank seal related symptoms. Since the bike has been rebuilt, I have noticed something strange, which only happens under certain conditions; hopefully someone can shed some light on this? The bike seems to be running perfectly apart from this one issue.


It pulls through the range without a problem and there are no flat spots or bogs. The bike starts first kick both hot and cold. The problem is that when the bike is ridden hard under load and the clutch is disengaged whilst the bike is still rolling, the bike will hang at 3,000 rpm for a couple of seconds (sometimes up to 5) before dropping back down to idle. The revs will hang at 3,000 rpm only, until this point they drop as expected. Once the bike eventually drops down to idle it will idle perfectly. This appears to get worse when the bike is hot and does not happen when the bike is stationary being revved in neutral.

It also only seems to happen when the bike is being ridden hard. The bike is a totally standard MC21 SE apart from having a HRC box fitted. The bike runs totally standard jetting. The only thing different is the needles are for the other model of MC21 carb, should this cause this type of problem? The carbs have already been checked and cleaned, there also doesn’t seem to be any signs of air leaks. The plugs are a chocolate brown colour. RC Valves have been correctly set and are functioning properly. The clutch has been checked to make sure there is no drag. Any advice would be appreciated.

I had something similar in my R5K
It sounds like air runing in the carbs.
I noticed in my bike that the air could enter before the filter in the two hoses that attach the filter box to the carbs.
I sealed the tubes with heat silicone in the outer part of the air box were the tubes attach to the filter box, so the air enters only through the filter holes fitted the top cover.
Also check all the air hoses, maybe one is blocked or maybe is leaking.

first thought is air leak, but you said you've ruled that out. Another, simple possibility, is that your idle is set too high. Mine at one point was a little too high, maybe 1500, and the rpm would hang up for a while around 3k when I would come to a stop. Make sure idle is set at 1250. Worked for me! Good luck.
_________________
-Jim
'93 MC21


Last edited by Jim on Fri Aug 15, 2014 1:55 pm; edited 1 time in total

as above i had a 350ypvs doing it a few weeks ago and it was the idle settings
this one had a mark on the slide where it hits the idle stop ,bit of mega fine wet and dry and its perfect now