GT380 Needle Position

[miked914]

‘Afternoon all,

I’m performing surgery on my GT380 carbs (1974 model) now and had a question. I’ll be running 3 into 3 Jemco pipes and UNI double-foam filter pods (I know, the stock airbox is better for tuning, running, etc, but I’m a glutton for punishment…)

I ordered rebuild kits for the carbs from Jets R Us and have all new brass installed. I’ve gone with stock pilots but have bumped the mains up to 85’s.

Question is, where should my (initial) needle setting be? When I pulled the carbs from the stock bike, they were set at the second clip slot down from the top of the needle—is this correct or should I move them while I have them out?


These carbs came out looking really great after lots of individual soaking in carb dip and some heavy Simple Green and toothbrush work.


Also, aside from the specs in the manual, does anyone have any advice for setting the float height? Just want to make sure there isn’t something I’ll overlook.

Thanks!

-Mike

[Alan H]

Go with the manual regarding float height. After that you're on your own!
Post how you go on as everyone has different ideas on jetting a particular bike.
It's better to start rich and work leaner - you'll not melt anything with a rich mixture.
Start off with the needle clip in the centre slot and remember that pilot from tickover to about 1/3 throttle and main from there to full.
Get the thing sorted at bottom end first

[tz375]

Just keep in mind that carbs should NOT be upside down when you set float height. They should be tilted until the tang on the floats just contacts the spring loaded tip of the float needle.

[Alan H]

Now I thought they should be upside down and was told many years ago that it was because of the head of fuel in the tank caused the float to need some 'lift' on the float valve to hold it on the seat against the pressure of the fuel above it when the tank is full. That's why they are drawn upside down in the service manual.

[Suzukidave]

Just keep in mind that carbs should NOT be upside down when you set float height. They should be tilted until the tang on the floats just contacts the spring loaded tip of the float needle.

+1

[tz375]

Now I thought they should be upside down and was told many years ago that it was because of the head of fuel in the tank caused the float to need some 'lift' on the float valve to hold it on the seat against the pressure of the fuel above it when the tank is full. That's why they are drawn upside down in the service manual.

They are drawn that way because it's simpler. If the carbs are upside down you will most likely set floats to the wrong height - especially where the spring loaded pin is weak. A better way is to set fuel level at 3mm below the gasket surface but that requires a modified float bowl.

[Alan H]

Well, I must admit to doing mine upside down, but the float tang had hardly any 'push' on the spring loaded needle valve - less than half a mill.
I'll keep an eye on the mixture when I finally get it on the road, and there's no signs of carb flooding (yet!)

[tz375]

Setting float height the wrong way (upside down) will result in low fuel level when running and higher probability of meltdown.

As the float presses down on the spring loaded needle tip, it will naturally be too low (as measured) and will be "corrected" to bring it back to what appears to the correct height and when the carb is the right way up, the float will be lower than spec and so will fuel level.

[Alan H]

I shall keep my eye on plug colours.