Help - Poor running after carb rebuild

[bluesmokeracer]

Guys

Bought a GSXR400 1993 model that has been standing for 15 years.

It ran, badly, with a jump start.

Drained tank - fuel was yellow.

Stripped carbs - replaced main jet, feed (orange thing!) and carb top o-rings. New needle valves. Checked and set float heights at 20.4mm.

Cleaned carbs out with carb cleaner and air line.

Pulled diaphragms out. Replaced one with a spare. All appear good.

New air filter.

Fresh fuel.

Re-assembled. Took ages to start - in fact left it five minutes and came back and it started. All Float bowls have fuel (used prime function on tap). Have loosened drain to check fuel present.

Starts easily but revs very very unsettled - as if it running really lean.

Changed plugs (figured they may be old/fouled). No change.

If you touched the throttle the revs shoot up and slowly fall. Found throttle cable slightly trapped - sorted.

But still won't rev through clean - misses like buggery in the mid-range, coughs. Very very woolly. Very poor power output. Stammering/stuttering when under power. Basically unrideable. If you hold throttle when station it will cough and splutter and then scream to 15k rpm.

The bike started and ran when I collected it - but was very very rough in the mid-range, hence I rebuilt the carbs - o-rings were perished on two mains.

Problem could be more fundamental (I don't know why it was laid up for 15 years).

Have put carb balance kit on - yes they are out a bit but not enough to make this difference.

Any ideas before I push it into a ditch?

Thanks

[Maelstrom]

My two cents.
Compression, spark, fuel.
Bike is 25 years old and spent 15 years with rotten fuel in it.
Test everything and when you are absolutely certain it has passed all the tests, move to the next item.
Anything less than a full overhaul on the carbs and you are just being optimistic.

[Suzuki416]

Stripped carbs - replaced main jet, feed (orange thing!) and carb top o-rings. New needle valves. Checked and set float heights at 20.4mm.

Cleaned carbs out with carb cleaner and air line.

Starts easily but revs very very unsettled - as if it running really lean.

Sounds like you are pulling the carbs apart again.
Did you clean it all the jets / mains ?
Is that the correct float height ?

Sounds like not all the carbs are getting the same amount of fuel - highly likely you have crap somewhere in the carbs somewhere.

[bluesmokeracer]

My two cents.
Compression, spark, fuel.
Bike is 25 years old and spent 15 years with rotten fuel in it.
Test everything and when you are absolutely certain it has passed all the tests, move to the next item.
Anything less than a full overhaul on the carbs and you are just being optimistic.

Thanks

What am I missing on the carbs though?

I have done:

O ring on main
O Ring on incoming fuel supply
O ring between carb top and carb body - underneath where you attach vacuum gauges
All the jets and channels are clear.

The guide on here talks about an O-Ring thus:

- around the slide holder base, the large plastic piece that the plastic slide runs in. A leaky O ring here will bugger up differential pressure on the diaphragm and mess up throttle response and mixture progression

It could be this - and I bought the 10x1 O-Rings specified but I cannot for the life on me work out where is goes because the 'large piece of plastic the slide runs in' does not appear to come out - unless someone can advise how without breaking it? I cannot see the O-Ring on the Suzuki parts diagram either. How do I remove without breaking it?

The bike is running a little better now - appears to have settled - but the tickover is completely erratic - it will sit at 1,500 then rise to 3,000 and then drop again without anything being touched. My guess is differential air pressure across the slides. I hooked up the carbtune and they are out but not that badly.

Thanks

[bluesmokeracer]

Stripped carbs - replaced main jet, feed (orange thing!) and carb top o-rings. New needle valves. Checked and set float heights at 20.4mm.

Cleaned carbs out with carb cleaner and air line.

Starts easily but revs very very unsettled - as if it running really lean.

Sounds like you are pulling the carbs apart again.
Did you clean it all the jets / mains ?
Is that the correct float height ?

Sounds like not all the carbs are getting the same amount of fuel - highly likely you have crap somewhere in the carbs somewhere.

Thanks

Going to pull them again and absolutely triple check everything.

Will update

[Maelstrom]

Your mixture screws are set too lean.
10 x 1.0 is not the correct size for the slide housing o-ring. That is just an over the counter size that someone deemed to be close enough. Very few o-rings on Keihins & Mikunis are standard sizes.

[bluesmokeracer]

Thanks for your help all.

Pulled the carbs again. I took the float bowls off and stripped jets again. Ran compressed air through everything again.

Pulled the slides out. Check needles and position. All clip three. All diaphragms in good order.

This time I pulled out the air screws. I replaced the O-rings on the air screws. My son (it's His bike) found an article saying that rough running/tickover/poor mid-range is linked to poor air screw settings/o-ring condition. So. did these, and blew compressed air through the circuits.

Morbidly expected no difference. I can advise that the bike is now running beautifully. Tickover was settled immediately. Engine responsive. Balanced the carbs and hey presto, its running creamy smooth at the bottom, no stuttering and screams through to the red line.

Thought I was going to have to send it to a specialist!

Got another one to sort out next weekend! Got two GSXR 400's in the garage!!!!! Joy!

My son is taking out on the road tomorrow for a really decent shakedown. Will update.

Thanks

BSR67

[Maelstrom]

Good work. Here is my guide to setting mixture screws.
https://litetek.co/Guide_FuelScrews.html
Cheers

[Suzuki416]

Don't feel bad - I cleaned my wifes CBR400rr fuel tank, replaced the filters & then had to remove & strip the carbs 3 times - due to crap in the lines...
It just happens & you get better at doing it each time - lol

Glad to hear its fixed.

[bluesmokeracer]

Thanks for your help all

Bike running well. Some fine tuning needed but overall very pleased with the results.