jonesey

Hi all, need to pick the collective brain....
I've just fitted a 2 into 1 exhaust to my vtr, which is finished by a very short carbon race silencer(!)
I have already run the 2 into 1 with a full length can with no problems.
The current issue is this: with the short can, when the revs reach 6-6500k, there appears to be quite a large flat spot.
Any suggestions that i can try myself before taking it to someone who actually knows what they're doing??
Cheers all, Chris.

captainchaos

I can tell you that I ran into the EXACT same issue with a gsxr600 I had. It had a full akrapovic header and muffler and it ran fine. I tried a shortened vance&hines muffler on the same header and there was also a terrible flat spot, also at 6-7k rpm, that would completely dissappear when I put the akrapovic can back on. I suppose the easiest would be to swap the full can back on? But it's up to you I guess it the extra noise is worth the tuning headaches. Good luck.

gboezio

Right on the torque peak, I would check the plugs after a few pulls and if pale ot white, I'll richen the offended cylinder(s) by increasing the main jet(s) one size up.

jonesey

I have a pair of 190 jets, what about just putting those in?

Tweety

That would depend entirely on what you have in there now... Going from stock to two 190... Nah... Not a solution really...

What seems to be tha basis of the problem is to little backpressure... Try a washer welded on the end of a bolt in the endpipe of right before going into the muffler (same but more discreete look) then you can turn it to finetune... It's an old trick when running old hardleys and old british with straight pipes...

gboezio

Check the plugs first to make sure that you have a lean out, not to make a bad condition worse, then wait for the dealer to be open, tear the carbs apart and check what's in and make +5 on the cyl that show lean

JamieDaugherty

Whoa, that's way too big. VTR setups rarely need more than a 182 or 185 main jet.

I'd say the lack of backpressure is causing a cylinder phasing issue at the collector. Keep in mind that since these engines are a 90° V you can never get perfect exhaust phasing over the entire rpm range - it must be tuned for a "band". A well designed system will have only a little drop off from the theoretical perfect rpm. Systems that are intended for race use won't care about midrange and can be designed just for the top end. I think the latter is basically what you've created. My suggestion is to run a longer pipe and muffler and see what that does.

jonesey

Ok, update. I've since put the scorpion full length can back on, and what do you know? back to normal.
I have had a chat with a well known local guru (Mick Whitnell of Total Traction here in the UK) and he has confirmed what you have all said, that it is a combination of the short can causing pulse/fuelling issues, and that basically I've upset the poor old thing by putting this can on!!
So, I've booked him for a whole day, where you pay a certain amount, and he keeps the bike on the dyno, and tunes and fiddles until it's spot on.
It'll be nice to have it run right with the short gp style can on, just for the sound if nothing else!!!!! BTW, the can was made as a one off by british company JAC,
who sell mainly on ebay over here, but do have a site in the works.
www.jacexhausts.co.uk.
Thanks for all your input everyone, it's much appreciated.
Chris.