Hi fellas. Im going to probably have a few questions about this bike!

Ive just bought it, its a race bike only, and looks like it has been this way for quite a while. no history or anything with it.

IVe taken it for two short - ish shakedown runs and have noticed the same issue.

From a standing start, bike takes off fine. but gets a dead spot, where it doesnt seem to want to rev any further at about 8k rpm.

If I shift into 2nd, the same thing happens.

I then drop it down a gear, and it tales off just fine all the way to redline, and if I keep it above that 8k rpm, no issues.

This occurs regardless of what gear I am in, and it happens both when the bike is cold, and when it is warmed up. It idles very nicely every time its fired up though.

I do plan to remove the carbs and dismantle and put them in an ultrasonic cleaner, but I thought I would see if anyone else has exerienced this issue, and can tell me what I might specifically want to be looking at?

 

Hi Tim does sound like carbs and a good clean will work wonders , have you seen this if your going to use it for track
and if you want bike specific
400GreyBike ? Index page

 

I have not seen the video, but it came as a racebike, and has been one for quite some time, and Im pretty sure this has already been done, just from glancing at it, but i will double check, as anything that helps me go a little quicker will be welcomed!

 

well i just got back from a few days away, and finally had time to take a look at the bike.

I think I fund the reason for the dead spot, the bottom half of the airbox has three screws at the rear end, above the carbs - well, these 3 screws were missing!. This would have caused a vacuum leak, and, I assume is the cause for the flat spot. I wil find out when I put it back together I guess.


There are quite a few bits and pieces on this bike that are a bit rough.

Im going to look at making a new stay for the fairing screen / blade, the stock one that goes up at the front of the tank annoys me, and being a tall-ish guy, I think it wil get in the way a bit when Im tucked in, so Ill make something to replace it myself when I go back to work in a week or two.

I also ned to sort out a catch can and some other stuff too, like tidy up the wiring.

Its running total loss currently, and theres a few scres i need to undo to remove the seat, and get at the battery, so Im going to figure out some kind of quick release system to make that a bit less of a hassle as well.

 

well, after pulling the bike apart to tidy up a few other bits and pieces, I got it back together after having replaced the three missing screws at the rear of the airbox - however this appears to have made no difference whatsoever.

guess I will have to completely remove the carbs after all

 

Hi Tim, I know racers usually drill holes in the airbox, affects low rpm, but have a look at a fellow Kiwi racer, who knows you might even know him
400GreyBike ? Information

 

wont let me see your link,

There are holes about an inch and a half in diameter towards the front of the top of the airbox, i did wonder of those were original or not, so that might have been done.

This bike has been a racebike for a pretty long time, so i wonder if those holes are a race mod.

Interestingly, it otherwise runs, and idles absolutely perfectly, it just hums, its just right around 8-10k it stumbles like its starving for fuel, then its away again after about 11-12k

 

been back at it again today.

The holes I mentioned in the top of the airbox are towards the front, and are stock by the look of them.

I removed the carbs, and stripped them, and put them in an ultrasonic cleaner. there was a bit of crap in the bottom of the bowls, which I thought might have been a contributing factor. however after cleaning, and reassembling, it turns out that the whole exercise was a waste, as it made no difference.

runs fine up to about 8krpm, after that it starts stumbling, it wil now stumble up to about 10krpm, then take of until 11krpm, then stumbles again, and struggles to get to redline.
This thing is really starting to **** me off.

I think the next thing to do is replace all the plugs. I havent looked at them yet, but it seems like the next logical step.

So far, I have sealed up any vacuum leaks, stripped and cleaned the carbs, checked the fuel pump is working properly, so plugs are the next step.

After that, I really dont know what to try, anyone have any ideas if plugs dont fix the issue?

 

Hi Tim sorry that didnt work, its only the 400greybike site, I know from experience when doing the carbs , 2 of the bikes I am doing have had the jets swapped to the nc29, are they the right jet size?

 

I downloaded the manuals you sent me, and checked, yes theya re 35's, which is correct for the K model.

Tomorrow I will remove the carbs again, and check the pilot screws are set properly, as that is one thing I did forget to do today. I also didnt measure the float heights, but they wouldnt be too far off the 7mm recommended in the manual, so I will check that as well.

My friend is also bringing over his manometer so we can balance the carbs. I am praying that makes a difference, as, after that, Im running out of things to check next!!!