Back in July 2007 I was after a cheap track bike as I didn't want to use my K6 600 roadbike. I found a 1990 GK76a GSXR400 locally on ebay that looked OK and put in an impulse bid not expecting to win the auction. I got the bike for £550 but it had no tax or MOT.

I then later found on this site that I had bought an SP model.

The little 400 did run though and didn't need a lot of work. It had done 30 miles since its last MOT but that was 2 years back. A lot of the work I did was removing rust and giving it a bit of general maintenance.

In 2008 it passed an MOT but wasn't running great (stalling a lot and not revving cleanly). It went off to a dyno place to investigate more. Every now and again it was running on 3 which was diagnosed as a dodgy spark plug! They were brand new NGK's. After changing it the bike made 57 or 58bhp with a Scorpion end can and K&N. This was still on standard jetting as the tuning place advised me it was going to get expensive having the fuelling corrected and was only slightly lean but OK to run.

It did run fine for one day and then returned to the stalling probs. In March of this year I decided to sort the carbs out myself... This is where things took a turn for the worst...

A lot of the screws holding the carb tops on were stuck. The heads of said screws were a mess anyway. A relative with bike experience offered to help and proceeded to use impact on the screws. Whilst not giving 100% attention they used too much force and snapped the plastic carb top and also the cast itself!!! Bollox! These were rare SP carbs that used different casts!!!!! Gutted.



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We used araldite to bodge it back togther but I wasn't confident with it.

Onto part 2

Part 2

The same cast also had further complications. I was unable to remove one the mixture screws as it was stuck in place... I very carefully tried to drill the little bastard out. Because the screw was soft as cheese I had just made things even worse:



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Working on old bikes was tuning out to be a nightmare. There was no way I could use this cast now. The thread had gone and the hole was blocked where the screw once was. I don't think I have the patience for these things

I contacted Suzuki and many others for info on these SP carbs. I scoured ebay UK, USA, Germany and Japan but nothing. Suzuki called me back and confirmed this was the ONLY bike to make use of these carbs - calling it a bit special.

Someone on here sent me a PM about selling some SP carbs they had. Thought my luck was in but tehy decided not to sell in the end. Booo!

Other options I had was to try and use the standard 33mm Mikuni's. Now I could either try and run them pretty much like that or have them machined to try and make them more like the 35mm's.

Of course never straight forward -



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* The SP uses a TPS where as the non SP doesn't. So either I change the loom and CDI to do away with the TPS or I have bespoke 33mm's with a TPS.

* The 33mm's have one correct mount point for the TPS with thread. Unfortunately the other mount point on the 33mm cast is shorter and had no thread. This is the SP one with screw in where as the 33mm is much shorther;



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* The SP cast also had an opening for the TPS shaft. The 33mm one doesn't have this.

Well I took the plunge and bought the readily available Mikuni 33mm's carbs. The internals were the same as the SP - needle (inc height) and jets. The butterfly's were obviously different sizes - 33mm v's 35mm.

The easy bit for me was to decide what to do. The more difficult part was the engineer friend of friend who was going to machine them for me...

Part 3

Well I had decided to have the 33mm casts bored out to 35mm. I also wanted to run the TPS and keep the SP loom and CDI.

The engineer had to extend the mount point on the 33mm's for the TPS and put a thread in it. He did this buy drilling a little into the cast and making a piece to extend out with. A special bolt was then made to fit inside. Yay.



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The 33mm bored out to 35mm with the 35mm butterfly's:



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Now the major difference remaining between these bespoke carbs and the SP ones is shown in the pic below. Non SP's do not have this intake and due to that they run less hoses.



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Since I'm running the K&N and Scorpion end can I'm going to try 112.5 main jets. I have no idea how the bike will run with these differences but I'm hoping to try it out tomorrow or next weekend.

ps thanks to variablevalves who has been a great resource on this site and offered to look at my SP cast which unfortunately I think is beyond repair.

I also crashed the K6 600 on 08/08/08! Round Gerards at Mallory park I lowsided at around 100mph.

Doh!!! If the 400 was running well I would have used that but I got talked into using my K6! Anyhow walked away having only suffered a broken thumb.

Pics from 04/07/2008 at Mallory;



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Pics from 08/08/08!



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Repaired the bike myself at a great cost!

28/09/2008



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I used the bike to go to NW200 and Assen this year

Part X'd the K6 600 after Assen. I was never happy with it after crashing it...

Staying with GSXR's bought this and put a Racefit on it;

K8 750



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The 400;

Pics from the ebay ad;



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I fully expect this bike has been abused by all forrmer owners!

How it looks now;

(not much different!);



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^^ Well spent the day working on the 400.

What a bitch getting the carbs back in. Hopefully will be quicker if I'm ever stupid enough to attempt this with a next time

I can bench a fair weight down the gym and I only got the carbs on the rubbers when I lost my temper and pushed the things as hard as I could. Oh and a smidge of washing up liquid assisted.

Exciting moment getting ready to press the ignition button. This bike hasn't run since March. Well it eventually fired up and idled at 4k rpm lol. Adjusted the throttle cable and now 1k. I think that I may need to cut the spring down on the idle control as I have no adjustment at present.

The little 400 idles fine but as soon as I even touch the throttle it dies. Hopefully a balance will be the answer? Have a carbtune here so will check next weekend.

Certainly progress though. I thought I may have to break the bike after the carb cast got damaged.

Now that was a post and a half!
I can't really offer much advice, other than to suggest you heat the airbox rubber up (I used a hot light), they slip right on I hear you on the temper front, I've had my carbs out a couple of times and skinned a lot of knuckles!
I'm impressed you went to the effort of having the carbs bored out, I would have just got a pair from an RF600....
If it's bogging down when you open the throttle, you should check your main jets are secure, you may have cracked the threads and/or damaged the o-rings. Also check the spring seats are on top of the throttle needle, if not then you will have similar symptoms. Also, check the hoses under the tank aren't kinked or trapped. I did that too

It certainly was a loooooooooo oooo ooong post but thanks for replying + posting info.

I had considered RF600 carbs as a friend had one and said how similar the Mikuni's were. The original idea was to run 33mm with a TPS but I then decided to try and get things a bit closer to the SP spec. That could be my downall but total cost of the engineers work was £20 - result

Main jets are secure and in correctly with new o-orings throughout the carbs. Everything is as it should be so far so I'm pinning my hopes on a carb balance. This weekend I'll know if it is.

When putting the carbs back in, it's a bit of extra hassle, but if you take the seat, battery and airfilter off, you can pull the carb rubbers into the airbox, then slip them back on when the carbs are back in place.