1980 450 Carb Synch

[Greg]

Anyone got any ballpark numbers for the carb synch screw on a 1980 GS450?...I cant seem to even get it in the ballpark.

Thanks!

[terrylee]

Are you trying to bench sync. if so look on Basscliff's site and there's info on there about bench syncing and then sync. with the right tool. No real number of turns with sync. screws since each carb. can be different. If you talking the mixture screws start with 2 to 2 1/2 turns out. terrylee

[pete]

I got no idea... I used the Morgan Carbtune to sync. them and it made a *huge* difference to the mixture between the carbs once they were in sync.

There are ways to bench sync. your carbs which I didn't bother with, I just waiting until I got the Carbtune and did it then.

[Steve]

Anyone got any ballpark numbers for the carb synch screw on a 1980 GS450?...I cant seem to even get it in the ballpark.

Are you trying to bench sync. No real number of turns with sync. screws since each carb. can be different. If you talking the mixture screws start with 2 to 2 1/2 turns out.

Yep, first things first, let's make sure we are talking about the right adjustment here.

The "carb sync screw" is the adjustment on the throttle shaft between the carbs. There is no "set number" for it, you adjust it until the two carbs have the same opening. When you get the carbs on the bike and the engine running, you connect vacuum gauges to the ports on the intake tubes and adjust the sync screw so you have equal amounts of vacuum in the two intake runners.

Terrylee mentioned the "idle mixture adjustment screw", which is an adjustment on the top side of the outlet of the carb. In your case, it is a horizontal screw that is just above the throttle shaft. On the four-cylinder bikes, the screw is vertical, but in the same general area. That screw is NOT used to sync the carbs, it adjusts the fuel mixture for idle and very low throttle openings. That one can be preset to about 3 turns to get the engine running with a slightly rich mixture. Then, when the engine is fully warmed up, adjust it for highest idle speed. Try turning it in (clockwise) SLOWLY until the engine speed starts to drop, then back out about 1/8 turn. You may end up at the 2 to 2 1/2 turn setting that was suggested, but starting with a richer mixture at 3 turns makes it a bit easier.

.

[Greg]

These carbs done have 2 seperate vacuum lines. Ive got home made guages buuut....i cant use them.

[pete]

These carbs done have 2 seperate vacuum lines. Ive got home made guages buuut....i cant use them.

Yup they sure do!

Have a look at the cylinder head just in front of the intake boots and you will see a Philips head screw there. That's where the vacuum gauge needs to go in.

You can see the Carbtune vacuum line going to it just in front of the intake boot in this pic:



Apologies for the oil around the place, this was before I replaced the dodgy Athena valve cover gasket with a genuine one and stopped the constant weeping.