Long story on carb(?) problem GSX750S

[jimbo22]

Hi, Second issue on the table...

My GSX750S doesnt run right.

First started - carb setup (original = carbs just cleaned and inspected. Main jet - stock. Pilot jets changed as 1 was badly damaged (but the size original) . 1 mixture screw changed as it had a broken tip. etc - carbs after "rebuild".

Running rich on idle. And doesn't rev when half throttle during cruise (IMHO also rich).

So I lowered the needle. But during some testing - turned out that 1st cylinder is not working on idle (taking the coil wire of the sparkplug - doesn't do anything). The other 3 when tested - make the engine stumble. All sparkplugs seem to be rich.

So I changed the sparkplugs (new ones). And tested again. 1st sparkplug completely clean. the rest - rich.

So I changed the wires - 1 and 4. Nothing changes - still first cylinder not right. (wires not guilty).
Tested the resistance in coils - 100% fine values (so it's not the coils).
Tested the spark when cranking - blue spark, regular (seems fine).
Checked the compression (fairly worm engine) - all cylinders nice 9,5-10.
Oil doesn't smell like fuel.
Checked if there is fuel in the bowl - all carbs have fuel (around 60-65 ml each).
Synchronized the carbs - result was fine (before sync it wasn't very bad, after was way better).
Tested for air leak (as the idle is not as stable as I would like and needs corrections) - but no leaks found.

When adjusting this first mixture screw (turning out) - exhaust starts to pop, so there is some feedback. 

Opened the carbs once again yesterday (switched airjets, tried to blow some more air into carb holes etc - blind shots on what can it be).

My next step will be:

1. trying some engine starter spray straight into the first carb. To see if the engine reacts to "fuel".

2. To Switch the wires again (4 with 1) to check if I missed something.

3. Changing the wires and spark plug "connector" to new (blind shot)...

4. Putting on the BSR36 Mikuni Carbs from GSXR (slingshots) to confirm that the reason is in the carbs (waiting for the courier, I bought a set from a guy who took them from his Bandit as he's switching to 40's). - biggest hopes here...

But what else can this be? Airleak on the butterfly throttle valve axle?

 

 

Edited May 23, 2017 by jimbo22

[Sheep]

No fuel from carb??hmmm....floats all set correctly?..float needles not sticking?..and when were the valve clearances last checked out of curiosity?

[Gixer1460]

'Mixture screww with broken tip' - is the tip still there blocking the hole?

[jimbo22]

18 hours ago, Sheep said:

No fuel from carb??hmmm....floats all set correctly?..float needles not sticking?..and when were the valve clearances last checked out of curiosity?

floats set. float needles checked (they are new - together with needle jets (emulsion tubes)). valve clearances i did correct before starting the engine  so no mileage since then.

17 hours ago, Gixer1460 said:

'Mixture screww with broken tip' - is the tip still there blocking the hole?

ofc not. took it out, and put a new screw.

 

But as for today - Long story for me, but not so long for this forum   But it was definitely a carb issue.

There is significant improvement after last inspection/cleaning. I don't know what was it that I did 2 days ago, but it worked. Yesterday the bike started on choke nicely, got worm and after quick adjustment of mixture screws - it runs very nice on idle now. Revs nice, stays on RPM. Reacts to disconnecting the plugs. So it's stable now...

Still chokes on 1/2 throttle. but it's an improvement. Ill keep this post updated Next step I'll change the main jet and rise the needle a little. 

As it's now running lean on acceleration - I'll also take off few discs from exhaust.

 

Also new Slingshot carbs just came Yey.

Edited May 24, 2017 by jimbo22