Flat Spot

[thunderpantz]

Hi all! After taking the head off and replacing the downpipes for stainless, and replacing the carb rubbers, I now seem to have a 'flat spot' around 6k. When hitting 6k with the throttle open fully it seems to bog down and refuse to get through the 6/7k mark (seems fine in 1st, 2nd, 3rd though) yet when using about 3/4 throttle it goes past no issue.

Any ideas?

[darrsi]

I had that a few years back, and after a lot of money later it turned out to be number 3 carb inlet rubber had folded in on itself when my mechanic refitted them.
And because it was number 3 it wasn't spotted until the carbs were removed again by another mechanic. 


So basically there was an air leak.

[thunderpantz]

After some more riding and fettling, I don't have the flat spot in first and second. At full chat the bike just sails through the rev range. 3rd is a little hesitant then 4th, 5th and 6th aren't happy at all.


In 5th at 70, upto 1/2 throttle bike accelerates smoothly, any higher it's like i've driven into treacle and it slows right down. Took it to a local mechanic, who had a quick look and listen(he was very busy). He doesn't seem to think it's timing as the engine revs so freely, he measured the temps of the collector pipes and all equal, he lent me his carbtune and have balance the carbs but the issue is still there.


All very odd :/


Any ideas chaps?

[thunderpantz]

No one?

[hightower]

Were your carbs much out of balance when you synced them?

[thunderpantz]

Numbers 1 and 2 were fine, Number 4 was out the most. Balanced up nicely and idles a lot smoother. Really dont get it

[hightower]

And your rubbers on the intake are seated right as Daz has said? With the throttle position having a say (1/2 on is the problem right?) would that not suggest a problem with the jets? Perhaps one of your jets has gotten some rubbish in somehow while it was off? Perhaps spray a bit carb cleaner in the intake and see if that cleans it out a bit.


Not an expert, just throwing some ideas out until one of the resident experts comes along.

[thunderpantz]

Yeah they appear to be. That said. There wont be any harm reseating them to be sure.

[hightower]

I know when I refitted mine, they were as tricky to get seated as the block was over the pistons. It took me a few goes, one time I was happy with them - tightened 3 & 4, looks good, came round to tighten 1&2 and 2 has totally tucked.

[darrsi]

Fuel tap switched fully on?
Kinked fuel pipe?

[thunderpantz]

Right. Reseated the carbs, changed a couple of the clips,  and the issue is still there. No kinks in the fuel pipes. Just very flat at 6/7k and loses power when the throttle is fully open.

Getting annoyed now