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1980 GSX750 Jetting Advice

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I've got a 1980 GSX750 with OEM Mikuni CV 32mm carbs, Pod filters, 4 into 1 exhaust and cam/port work to the motor.

Its currently running rather lean (see plug photo)

Current settings are:

135 mains, 45 Pilot, Fuel screws 2.5 turns out, standard 5C32 needle on the 5th (lowest) clip.

Where should I go from here? I was going to richen the needle but its already at its richest setting, what needles do I need?

Shall I go higher on the mains as well?

Any advice would be greatly appreciated!


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7 minutes ago, Gixer1460 said:

How many hours are on that plug? Looks fairly new - is it taken directly from a plug chop - at what rpm? How does the current jetting compare to std. specification?

Yeah new plug, maybe had around 1 hours use. No this is after a 'roll into the drive and turn off' check. 

Standard jetting for a GSX750 is: 112.5 Main, 42.5 Pilot, 3.5 turns out on the fuel screw and a 5C32 Needle on the middle clip #3. So it's already a fair amount richer than standard.

The bike rides well on the whole other than a flat spot at around 1/4-1/3 throttle.

Could it be worth just putting more time on the plug, and performing some various RPM plug chops?

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Definitely - with the filers you are using, most people experience the 'hole' off idle and into the mid range - most solve it with a Dynojet kit but I don't think one is available for the 750ET (wasn't when I had mine!) Either get some miles on it with plug chops or get a dyno pull to see what's happening under load. The std. needle is the problem - jet changes either side help but it is metering the fuel in that critical phase!

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Just had a quick look on the DynoJet UK site and it does appear as though they do them. Not in stock and £138 though!

I'll have a play and see if I can improve it a little. But it's not the end of the world if it isnt 100% perfect. I was more concerned with it being potentially damaging, but the exhaust tip colour is a very dark brown, and the motor runs well so it doesn't suggest its dangerously lean?

Thanks for your help

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6 hours ago, banoffee said:

Have you read the Dynojet troubleshooting guide? I found it handy to get reasonable settings before a dyno run.

I've not no, but I've just found it and read through it and it looks helpful!

I'm getting a carb balancing kit tomorrow. I know I should have done that at the beginning, but I've been impatient and couldn't wait to ride the bike xD

I'll get the carbs synced then follow the guide from there. I've also read that people don't seem to have much luck with the gauze style tapered cone filters like I'm running. I may ditch them for a set of foam filters instead, which will hopefully help with the flat spot. 

Edited by Russ750ET
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3 hours ago, Russ750ET said:

I've also read that people don't seem to have much luck with the gauze style tapered cone filters like I'm running. I may ditch them for a set of foam filters instead, which will hopefully help with the flat spot. 

Unlikely - its the increased / turbulent airflow with K&N / S&B / Foam types that screws up carb function not the material. Some carbs like dual pods but some work ok with individuals - I had individual cone S&B's on my 750 ET but used a Ledar jet kit and they worked great. 

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Made very good progress with the 750 and now have it running 99% perfect.

The vacuum sync kit confirmed that the carbs were quite far out, and the bike ran noticeably better as the carbs came into balance. I spent quite a while getting them as perfect as possible. So to anyone else reading this with carb issues, balance your carbs first!! xD

I have now changed back to 42.5 Pilot Jets, as it was virtually impossible to achieve any kind of sensible idle with the 45's. 2 turns out from closed seemed to achieve the highest idle and the low end response is pretty good.

Now the interesting bit.... chatted to Grumpy Gary 1260, he suggested taping the filters as a way of proving if the carbs are generally too lean. So I wrapped a loop of tape around the base of each pod filter. What a difference! The flat spot is all but gone and the plug is a much healthier colour. Doing 5th gear roll ons from low speeds and the bike pulls clean through the rev range, with maybe the slightest small flat area that it quickly gets past and revs out nicely.

The dilema I have now is do I ride with tape around my filters, or do I attempt to iron out the lean flat spot by playing with different settings? I've not found a richer needle the stock 5C32 for this carb. So I may have to bite the bullet and get a DynoJet kit at £138




Edited by Russ750ET
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49 minutes ago, Gixer1460 said:

Oiling the filters using K&N oil may provide sufficient resistance to allow the tape removal?

And here's a thing - needle files aren't called needle files just because they are shaped like needles ! ! ! ;)

Good idea, thats got to be worth a go!

Never thought about that possibility with the needles either, pretty sure I have a spare set of 5C32 needles so I will have to experiment

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