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Turbocharged, Supercharged, and Hotrod Motorcycles: Re: [turbobike] turbo gsxr

Re: [turbobike] turbo gsxr

Tim Drew (timdrew@cix.co.uk)
Mon, 14 Sep 1998 22:19 +0100 (BST)

> hello everyone,
> I would like to get some info from other turbo gsxr owners who have a 
> Mr.Turbo draw thru system.


 I am using a modified MR.Turbo draw-through kit with an S&S Super E 
shorty carb on my 92 GSX-R11. I can't comment on the Mikuni HSR as I 
haven't tried one, but I can tell you about my experiences with the S&S.

Firstly, I'm only running about 14psi of boost at the moment so my 
settings my not be relevant to you if your going to use significantly 
higher boost.

As you have already said the transitioning on these carbs leaves a lot to 
be desired, and you will probably have to compromise a bit on the jetting. 
When I set the bike up on the dyno I found the best settings were:

Main jet = .095
Pilot jet = .038
Float = level
Air Screw = best fast idle
Accelerator pump = not very much
Ignition = 36deg BTDC off boost and 28deg BTDC on boost
Plug gaps = .018

With these settings the idle was a bit erratic but acceptable, varying at 
worst between 1,000-1,300 RPM, you could wack the throttle open under load 
in any gear from low revs and the engine would respond cleanly. These 
settings gave a nice clean power curve that looked great on the dyno and 
would have been fine for racing or hacking round the lanes. However when I 
rode the bike on the road at a steady speed and at low throttle using any 
rev's between 3,000 and 7,000 it would jerk and misfire (almost 
un-rideable!). So back to the dyno. The problem turned out to be that we 
were having to run the pilot circuit rich to get clean transitioning.

I should point out at this point I have a compression ratio of 8:1. You 
may find that a higher compression engine is less susceptible to this sort 
of problem. Although I doubt you will be using high compression if your 
looking at 28PSI boost!

For road use I have now found that a pilot jet size of .029, the 
accelerator pump set quite high and the ignition advanced to 40deg off 
boost and 28deg on boost works well, but I have to be gentle with the 
throttle below 3,000 RPM or the engine will die lean. Looking at the spray 
pattern from the accelerator pump nozzle I feel it's a bit weak at small 
throttle openings, so I'm going to try fitting a stronger spring to the 
pump. I hope this will cure the problem. (Fingers crossed).

One other thing I found was that cylinders 1&2 tended to run richer at 
times than cylinders 3&4. I took a tip from Joe Haile's book and welded a 
divider plate in the plenum chamber, this seems to have cured that 

As for fuel delivery, I haven't had any problems with the S&S. At the 
boost level I'm running at the moment I've not even found it necessary to 
fit a fuel pump. I'm just gravity feeding the carb via a Pingle high flow 
fuel tap without any sign of leaning out.

While I have got the attention of the [turbobike] mailing list I have a 
question. I'm currently running my bike with stock cams and cam timing. I 
wonder if anyone else has experimented with cam timing on a turbo M or N 
engine, if so what did you find were optimum cam settings?
Tim Drew

> hello everyone,
> I would like to get some info from other turbo gsxr owners who have a 
> Mr.
> Turbo draw thru system. The kit I just purchased came with a Mikuni HSR 
> 42
> mm carb. From speaking with other people most prefer the ss super e 
> carb.
> Now from what Ive been told from Mr.Turbo the mikuni carb offers better
> transitioning thru the power band but has a smaller float bowl. The ss
> suffers from poor transitioning but has a bigger float bowl. Both are 
> good
> carbs for this application but if there is someone out there who has 
> tried
> them both please let me know. I can return this carb for exchange if 
> needed.
> This bike will be seeing 20-28lbs of boost  so if either carb has a 
> problem
> with high boost fuel delivery Id like to find out before its put on the
> motor. Thanks.    sebastian