Gsxr750m cold cylinder

[Airwolf222]

Hi guys I'm sure thus has come up before but I'm new to this forum and looking for help ! I have a 1991 gsxr 750 m  barn find  all sorted except cyl 1 and 2 cold exhaust eg 100 deg;  3 and 4 250 deg and runs rough new plugs and coils spark at all plugs  coil 1 feeds 1 and 4 coil  feeds 2 and 3  carbs been off and cleaned in ultrasonic bath as many jets and orifice cleaned as possible  going for valve clearance next compression equal at 125 psi cold ! Had one of these bikes back in 1992 so would like this old girl rideable again  thanks fellas  andy 

 

[Oilyspanner]

Could still be blocked pilot passages on 1 and 2, leaking pilot screw seals on those carbs or  float heights wrong on them. You seem to have checked other likely possibles. You probably aren't far away from having it running properly, but most likely you'll need to take the carbs off again and go through the carbs really carefully - it doesn't take much to muck things up.

Is the bike fairly standard, or running pods etc ?

[Airwolf222]

Hi thanks for the reply  yes bike is standard with a unknow exhaust can  so off with those carbs again  oh well one things you may be able to shed some light on when

 You look at card from airbox side there are to holes facing you ,the jet on the right has a hole in it the one on the left seems blocked on all carbs s this the case  

Thanks andy 

[Fula28]

Air correctors, in lower bellmouths should be on standard carbs open! I.e not blocked my advice use a nice clean (single ) bristle from wire brush very fine and hold it in pliers an have poke about , 

[Captain Chaos]

I once had the same problem on my 750M, 1 and 2 not working correctly. As soon as I lifted the fuel tank a bit it was ok. Turned out the fuel pipe for carbs 1 and 2 was a bit too long and got kinked when the fuel tank was mounted in position and thus restricting fuel flow to those carbs. Easy to overlook this and easily sorted.

[boilerdude]

slingshots and their silly 2 hoses from the tank. 

Also all of these old bikes are prone to having the intake boots leak from the carbs to the engine. Over time they warp and shrink and crack and lose seal.   

[Airwolf222]

Thanks for your thoughts bioler dude   I'm testing on a temporary tank during the fault finding and the robbers are all ok  cards are off again for another check through can't find a lot wrong  getting pissed off now lol 

[Oilyspanner]

I take it the plugs on 1 and 2 are sooty/wet ? and the carbs balanced, 1 and 2 are a pair....just trying to think !

[Airwolf222]

Hi oily spanner  the plugs out of cyl 1 3 and 4 shows signs of fireing number 2 still looks white,  there all new plugs, I have swoped plug leads over from 2  to 3 as there fed from the same coil  i have a spark at all plugs but number 2 exhaust is cold compared to the other 3 carbs are in bits again but I can see nothing amiss  starting to think valves or head gasket next

[Fula28]

Airwolf,  Sorry if you have allready been though the carbs few times allready it's frustrating for sure. Back to float needle valves have you replaced the o-rings on the outer brass valve body?  And new needle valve too. 

I would change these as mater of course if there fresh new items it takes this out of equation. Mixture screws I would remove and check the pointy tips (taper) as these can break off blocking !!! idle mixture from getting through. Been there done that. Set all four mixture screws 1-3/4 turns out as base line, if you have done all this ignore my waffling on.

Then bench Syncronise them. Easy job with 4 new clean drill bits as feeler gauge type of thing. Use idle adjuster to open your throttle up, to a point where all four drills can be slipped under butterfly's, then adjust 1-2 carbs until they feel same dragging on drills, then 3-4 same way, and finally each pair together nicely.

and unscrew your idle adjuster screw to original  Postion, 

not cv carb in piccy but you get the  idea.

 

[Airwolf222]

Hi guys thanks for all your thoughts carbs off again all checked out ok bench set throttle butterfly's as suggested all good moved the throttle open by moveing the part where the cable connects all good move it with the cable and right pair open 12 mm before the other pair caught up  found a spring missing  looked at pics I took when first had bike it was missing then so  I did not notice  sod it but sorted now 

[Captain Chaos]

Good

[richyrich]

where did you get that spring from ? theres one missing on  my 750L bst38 but ive never missed it, as the thing seems to run well enough now.  did it make a difference for you maybe i should get one.

pics of the bike ?

[Airwolf222]

The spring came from a packet of odd springs bought in home depot  in usa where I am at the moment I have a spare ill post it to you when I get back to uk in march if you like ! That spring made a massive difference to my carbs as it's need to keep both pairs in sync , with it missing the right hand pair butterfly's were open 12 mm ish whilst the left pair had not opened at all ! 

[Fula28]

Nice one. Had same issue, on my latest build.