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Discussion Starter #1 (Edited)
Patient:
'89 GSX-R 750 where has been swapped '90 GSX-R 750 engine with carbs.
Carbs are '90 stock Mikuni BST38SS with dynojet stage 3 jet kit. (I have drilled one lift hole larger on the slider and drilled main air jet, as listed on instructions)
Airfilters are K&N RU-2922, as listed on dynojet stage 3 jet kit. They have been washed and oiled.
Exhaust is full vance&hines SS2R.
I have never been able to test these 38mm carbs with stock jetting.

Dynojet kit is E3335.
http://www.dynojet.com/pdf/european/E3335.pdf


Problem:
There is constantly sudden cut-offs when accelerating. First gear goes fine, but after that problem gets worse. See video and you get what I mean.


Tried so far:
- Ultrasonic cleaned carbs.
- On first start, bike didn't want to rev at all. Changed needle to the second most richest position (step by step) (instructions says to put to the second most leanest position).
- After changing needle position, adjusted mixture screws.
- After mixture screw adjustment, synced carbs.
- Changed main jets. Instructions says that 150 is the correct one.
-> Started with 150 main jet. Stuggling a little bit to rev past 9k and also same cut-offs.
-> Changed to 155 main jet. Struggling badly to rev past 9k.
-> Changed to 146. This is the current state and the best one so far.
- Changed new spark plugs. Tried with NGK CR9EK and CR10EK (current).
- Tested spark. It seems good strong blue.
- Measured coil resistance, both are ok.
- Fuel lines are replaced.
- Battery is charged.
- No visible air leaks.
- Carbs diaphragms seems ok.
- Float level adjusted.
- Fuel tank filter cleaned.


Does this seems more like fuel issue or spark issue? :sad
 

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Have you inspected the fuel petcock for proper fuel flow? In some cases, vacuum operated OEM petcocks are known to have fuel flow issues as they age and it frequently shows up under high rpm/heavy acceleration and/or with re-jetted carbs that are flowing more fuel than stock.


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Discussion Starter #4
With that I mean step 6. in the instructions.

6. Locate the main air corrector (Fig.C). Using the main air jet drill (DD #3/64) provided, drill the
main air corrector. Greasing the flutes will keep all of the shavings from getting into the carb.


I ordered new OEM fuel tap, lets see if that helps.
 

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Discussion Starter #6
Hi,

Side stand switch is shorted. No help at all.

Yes I have seen that guide. Will try even smaller main jet after replacing fuel tap.

Instrument panel is Koso RX1N.
 

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Discussion Starter #7
Fuel tap replaced and made sure that there is no kinks on the fuel lines. Problem still persist...

This same cutting (always only 1 cut) happens also sometimes when accelerating from low speeds on half throttle at around 3-5k rpm.

Is there any possibility to be like diaphragm issue? Even tho I looked them once already...

If someone got any advice, that would be appreciated.
 

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Discussion Starter #9
I have tried to install '89 intake boots, but they don't fit anywhere closely. The flow is blocked very drastically.

'89 has 34mm carbs and '90 has 38mm.

Is there any differences on the ECUs?
 

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Squid
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I think your transmission is slipping. I had to pull my 93 750 apart and back cut the gears to solve this problem. Reason I say that is that you're good in first and then it starts jumping once you shift up. You'll see what I mean once you get it apart.

If you don't want to machine your gears you can just buy new ones.
 

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Discussion Starter #13
Did your bike do the exact same thing?

You are correct, it feels like it's skipping a teeth. It can't be drive chain, because I just replaced sprockets and chain.

The guy from which I bought the engine, said that he also disassembled it and checked transmission and cylinder head condition.

I'll test next with the ECU which is disassembled from the same bike where the engine is from.
 

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Squid
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I owned that bike 17 years ago so its a bit foggy, but yeah it was pretty much the same performance. Undercutting gears is pretty common in high performance applications but it can also be used to correct the issue you're having. It stems from worn shift forks, then the dogs of one gear start to cam out the pockets on the adjacent gear and they'll start slipping. Each time a dog slips from one pocket to the next you lose about a quarter turn of shaft rotation in the transmission, that's probably the surging you are feeling.


Look at this video, the first two gears that he pulls off show you the dogs on the side of the gears and the pockets on the adjacent gear that they engage with. The divisions between the pockets on that gear look pretty decent. Mine were noticeably cammed out.





This isn't nescessarily what's happening on your bike so I'd rule out cheaper failure modes before ripping your engine apart, but keep this in the back of your mind while you're diagnosing the problem.


You may also find this link helpful: https://www.pistonheads.com/gassing/topic.asp?h=0&t=704477&mid=76995
 

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Discussion Starter #15 (Edited)
High rev problem was solved with new ECU.

32900-17C00 (old)
32900-17D00 (new)

After that, problem with the second gear got worse. It started to cut/slip the whole rev range from 4k to redline. All other gears worked perfectly.
Here is a vid to show how that second gear was working.


After a couple of accelerations second gear started to make grinding noises. So lets take transmission apart.



Tip of the day. It's not always the best solution to buy new engine. Think for a long time before you decide if you want to repair your old engine or buy a new one...
 

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Squid
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Glad you got the issue pinned down man, you're going to have a feeling of a job well done when you do your first pull after the repair.

If you're still around after this please post some pictures of what you find in the transmission.
 

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Discussion Starter #17
Finally had time to take engine apart.

Here is video what that 2nd gear dog pockets looks like


Extremely worn-out. I have a replacement transmission from 1989 GSXR750. Difference is that driveshaft is 3mm shorter, I might give it a try.
 

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Discussion Starter #18
Now im struggling with loss of top end power. With original engine I got top speed of 230kmh.

DJ146 (same as mikuni 137)
- Very poor top end power, no power on fourth gear. Max speed 140kmh.

DJ150 (same as mikuni 141)
- Little bit better top end power. No power on fifth gear. Max speed 180kmh.

DJ155 (same as mikuni 146)
- Stuttering on top end. Max speed 160kmh.


What can I do next?
 
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