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Discussion Starter #1
recently picked up another '86 750, started going through the bike to find that there's '86 1100 carbs on there, unmolested and all factory settings. I ditched the airbox and went with K&N pods that I had laying around, figured it would run better with those (plus the airbox made it a royal PITA to remove the carbs, reinstallation would've been near impossible). the motor is bone stock with a full aftermarket exhaust. compression readings on all 4 cylinders are pretty close to each other, so a simple bench synchronization on the carbs was sufficient. all jets and orifaces are clean, all o-rings replaced, jet needles and emulsion tubes are in great shape, good floats and float valves, float levels at recommended service heights, etc. the carbs are in good shape. the bike ran but was very sluggish and hesitant to throttle response. I was able to rev it out all the way, but the revs climbed slowly. started on choke just fine but still sounded like it was loading up. I had to give it throttle off of choke to keep it running. no big clouds of black smoke, in fact there was no black smoke at all. the carbs were hooked up to an auxiliary fuel tank that was gravity-fed. something is telling me its not getting enough fuel.

I know others have ran these carbs on their 750's, what jetting are y'all using? mine is all standard, mixture screws 2 turns out so far
 

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Discussion Starter #3
As previously mentioned, throttle response was very sluggish and slow to rev. Wouldn’t idle without some throttle, but started up with choke on. I came to the conclusion that the bike was running lean after looking at the spark plugs. I eventually ended up with mixture screws 4 turns out, went up on pilot jets from the std. 32.5 to 37.5, and plugged the pilot air jets entirely. Bike runs better now and idles, but revs are still pretty slow and throttle hangs for a little, then settles to idle. Went out on mixture screws another 0.5 turns and shortened the diaphragm springs by 1 inch, should help with throttle response. Haven’t ran the bike yet because I plan on replacing the orings behind the intake manifold boots, hopefully they don’t crack in the process. I can’t think of a vacuum leak coming from anywhere else that would cause a hanging idle momentarily
 

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i have two different first gen 750's with 34mm carbs, but they are the 750 34mm carbs, and "i think" i'm running 135-140 mikuni main jets. it's been a while, but i remember having to reduce the MJ because the previous owner had 145. both bikes run excellent with the current set up and k&N pod filters.
 

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Discussion Starter #7
the 1100 34mm carbs are different with the air jets being different sizes and the jet needles & emulsion tubes are also different, but not by a whole lot. I was surprised to see that the stock settings for 1100 carbs were pretty lean on this bike with pod filters and full exhaust. there seems to be a slight vacuum leak present with a hanging idle, only place i can think of are the manifold orings flattened and rotted out. I don't think this would affect jetting by a considerable amount though
 

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Can't help with settings, as I haven't used those carbs. One thing I noticed though 'shortened the diaphragm springs by 1 inch, should help with throttle response' - if you remove several coils, you actually make the spring firmer, so the slide won't move as quickly. I wasn't convinced when I first heard this about springs, so I experimented on an old spring ……. and sure enough !
Something I did do though.
I measured up the nitrile seals in the carbs, pilot mixture screw, emulsion tube and float valve seat, bought them in bags of ten for not a lot - got spares now too :thumbup
 

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Discussion Starter #9
Can't help with settings, as I haven't used those carbs. One thing I noticed though 'shortened the diaphragm springs by 1 inch, should help with throttle response' - if you remove several coils, you actually make the spring firmer, so the slide won't move as quickly. I wasn't convinced when I first heard this about springs, so I experimented on an old spring ……. and sure enough !
Something I did do though.
I measured up the nitrile seals in the carbs, pilot mixture screw, emulsion tube and float valve seat, bought them in bags of ten for not a lot - got spares now too :thumbup
if you cut then stretch them to length im sure they will become more stiff, I just shortened mine, they lift easier. every seal in the carbs have been replaced.
 

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Discussion Starter #10 (Edited)
replaced the manifold orings and went up to size 40's on the pilot jets, it revs a little faster from shortening the diaphragm springs but its still doing the same stuff. I recall checking float heights when I had the bowls off for the first time and all floats were set pretty high, I lowered them down to the factory recommended 14.7mm height. I'll try raising them and see if it does anything, but I'm convinced that you just can't make 1100 carbs work on a 750, not without changing the needles and emulsion tubes and air jets, therefore making them 750 carbs
 

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if you cut then stretch them to length im sure they will become more stiff, I just shortened mine, they lift easier. every seal in the carbs have been replaced.

nickr is right, the springs will become stiffer if you clip them. Yours will now jump up a little sooner, but get progressively slower to open due to the increased tension in them.

Think of it this way, a torsion bar gets it's strength from the size of the bar and the length of it. If it's shortened, it gets stronger. Or if you leave it long and make it fatter it gets stronger.

A coil spring is just a torsion bar that has been coiled around a screw to keep its size within certain parameters.

Curiously enough, a first gen 750 uses the same spring as a first gen 1100. :wink2:
.
 

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Discussion Starter #12
I just shortened my diaphragm springs by zip-tying about 1" of coils together, I DID NOT cut and stretch them because I know that would in turn "stiffen" the springs. I did this so I could just cut the zip ties and the springs would be back to normal, a step to returning the carbs back to factory settings in case I were to exchange them for 750 carbs. Everything I've done so far on the carbs is easily reversible
 
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