Hi all. I have a 1991 Cbr1000f that has an annoying stumble between idle an 3000RPM. I just recently bought the bike and it has a V&H 4 into 1 exhaust on it. I don't know if it has been re-jetted or if it even needed to be. would a carb synch help? any thoughts or ideas would be appreciated.

 

My bike had the same thing when I put on the V&H 4-1. When I get the $$$ I will get a jet kit for it which should fix the problem.

 

I've seen alot of people talk about this "kit". Is this something that I would get from the dealer, and if so is there more than one? Also, do you have any idea how much it might be? I've read that jetting is somewhat of an art as well as a science and that it can be screwed up quite easily.
Sorry to ask so many questions but I'm a big believer in learning as much about a subject before tearing into it as I can. Thanks in advance!

 

Before you buy a kit, check out the following items that the Haynes manual says to check for poor running at idle and low speed. Ignition: Check for low battery, fouled plugs, plug wire shorting, ignition timing, faulty coil and faulty spark unit. For carbs: check for intake air leaks, mixture adjustment screws, carb sync, pilot screw clogged, air cleaner clogged or omitted, choke stuck on, float level wrong, fuel tank air vent clogged, and sticking carb pistons. It also says to check valve clearance. It doesn't say, but we know CBR's, so also check for a stretched cam chain, as that retards valve timing and hurts low end performance.

 

I found out today that it used to have a cheap Kerker system on it and so it wasn't rejetted after the V&H was installed. The prior owner (the sales manager of the local honda dealer) says that he was told by his shop guys that they though it just needed synch. Does that sound right?

 

I've had my '91 1000F for a couple of months now. It has Supertrapp slip-ons and, according to the seller, a jet kit, but he couldn't remember what kind, so I assume it's a stage one of some sort, installed together at about 10K by a dealer. My bike: vibrates through the bars, numbing my throttle hand in a few minutes; stumbles at idle slightly; and surges ever so slightly even when I'm holding the throttle steady. So: balance shaft? Carb sync? Plugs? Help?

Craig

 

Check the balance shaft adjustment.

Synch the carbs.

Also just a not on crappy running at Idle. When I pulled my coils off last time I found a ****load of gunk buildup on the 3 pin connector that goes to the coils as well the the spade terminals on the coils them selves. A blast of Contact cleaner and the bike was noticebly smoother.

 

It might need a sync, but it's more then likely going to need a re-jet. It sounds like a classic example of the mixture going a bit lean caused by the ability of the exhaust to scavenge more of the charge out of the combustion chamber (as it should). With the stock exhaust there is a bit of the previous charge left in, and the amount of new air is less, thus you are getting a richer mixture. When that leftover charge is completely removed you will need to fatten up the mid range mixture to eliminate the stumble.

Edited because I can't spell.......

 

my bike developed an exhaust leak, now it runs pig rich... it also has slightly dented #1 and #1 header pipes.. it got rejetted but had the stock cans on it... apparently richened a little

 

I think the low rpm stumbel is typical with the V&H 4-2-1 system. Any free flowing system could have the same problem though.
If you run the dynojet jet kit you could richen the needl all the way. Atleast thats wat they recomend at dynojet. http://www.dynojet.com/pdf/1126.pdf
I don`t even have a K&N airfilter and mine still leans out under 4000rpm.