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Grrrrr Flat Spot!!!!

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Hi Guys,

Just registered, and first off can i just say i am stoked that OS is back!! I have missed this site and the guys and girls here :-)

Ok - I have a 1993 GSXR 750 WP, which came to me in a right state and i have been building a 'fighter out of her for the last 2.5 years. She is now on the road, and running almost beautifully again. I say almost, because since I got it its been running rich. Black smoke from the exhaust, thick black soot on the plugs, and smells go-awful if you run it in the garage and not outside!

On initial investigation, it had a Scorpion race can so i checked it had been jetted for this. Looking inside the carbs it seems that it has been Dynojetted and I believe they only ever did a stage 3 for this model of GSXR (?). All looked good in the carbs, but i cleaned them out while i was there and gave them a balance - still running rich.

Checking the fuel/air screws, and reset to factory - still running rich.

So i checked the airbox. Standard box and standard aftermarket filter! So i bought a K&N filter for the airbox - still running rich.

Fitted one grade hotter plugs - still running rich....


So, then i bought some cheapie secondhand K&N pod filters from (that popular online auction site), and fitted them. Whilst they didnt fit quite right and leaked a fair bit, this seemed to improve the richness a fair amount. Happy that i think im now on top of the richness, i invested in a new set of RAMAIR pod filters.... fitted and finally, the richness is abating!!!! At least it isn't blowing black smoke unless i give it a fistfull, and the plug ends are starting to go tan - wayhey!!

Or so I though.....

Now i seem to have developed a flat spot, and quite a big one. I cant tell you the exact rev range of this as i have an aftermarket KOSO clock fitted, but it is the low rev range, i would say around 4K, and it seemed quite a big flat spot. It feels like its getting too much air now, if i open the throttle quick it hits the flat spot and struggles, if i gentle roll the power on it does better, and applying the choke a little seems to help. So is this fuel/air mix do you think? If i adjust this, wont the bike go back to running too rich again?

Am I missing something obvious? I dont really want to take this to a garage, as I am currently priding myself on doing this by myself - but without a rolling road, this is starting to get tough to diagnose!

Any suggestions, or anyone that has experienced similar?


Edited by Iceycold
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Stage 3 DJ is probably way to fat for just a can and filter! Changing plugs won't change mixture so revert to std and get the AFR sorted. Doesn't matter how free flowing the filters are, if its to fat to burn its too fat!

Internet tuning is never really successful - book a few hours on a dyno and get it right!

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Thanks for the advice - much appreciated... Just re-checked this and I was wrong (its been a while since i looked into it!).. They never did a Stage 3, only a Stage 1. Which is now making me wonder if i might have stumbled upon the problem here.... From the Dyno website :- 

"Stage 1 Kits
Intended for motorcycles with a stock engine using the stock airbox, air filter and stock pipe. Stage 1 kits are fully adjustable to allow the use of a well designed aftermarket pipes and stock replacement air filters like K&N. Stage 1 kits are designed to improve throttle response and driveability and in most cases power increases of approximately 5% throughout the entire power range can be achieved."

"Stage 3 Kits
Intended for motorcycles with stock or mildly tuned engines using a well designed aftermarket pipe and individual air filters. In most cases power increases of 10-15% can be achieved. However driveability may be compromised particularly with short stroke, high rpm motors."

Now - if i remember correctly, the GSXR 750 is a short stroke, so have I gimped it by adding the RAMAIRS!! Im gonna have to take it to a dyno aren't I :-/ Oh well, it'll be money well spent. :-)

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Hi Guys,

Thank you all for your advice. I guess I will have to bite the bullet and book her into a proper dyno. Its the only way to be sure and would be well worth the money making sure its right and theres no need to worry. Now to find local dyno's :-) ;)

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