flat off idle through midrange - 1980 GS550

[Anonymous]

I am working on a 1980 GS550 and it just isn't coming together. I pulled all the carbs and squirted them out. They weren't bad to begin with but I cleaned everything anyway (no dunk) and reassembled. The performance is same as before. No guts off idle. It almost feels like timing. It is slow to rev and just doesn't have any "punch". No backfiring or anything like that, just no juevos.

I drilled the caps on the air screws but changing them doesn't have too much of an effect. The carbs appear to be in good shape, I didn't notice anything out of the ordinary. Looking through the intake ports with a flashlight, I did notice some oily buildup on one or two intake valves, nothing scary though.

Any ideas?

And where are the vacuum ports on these carbs? They are 32mm diameter. I can only find one vacuum port on them to sync with.

Thanks for the help.

 

[Motorcyclemodeller]

Sounds like a carb sync problem, you could also check ignition, I believe that year did not come with electronic ignition so..........points, condenser, coils, wires, sparkplugs, and all electrical connections inbetween. While you are at it, a compression test should be done first to make sure all cylinders are close in pressure.

Back to carbs.

The vacuumm line you are talking about is probably for the fuel petcock.

There should be four small allen head cap screws on the intake side of the head just after the rubber carb mounts.

These need to be removed and the threaded sync gauge adapters screwed in.

Now, one carb is factory set.....you just have to sync up the other three to match.

Hope this helps!

Dr. Dre

 

[Anonymous]

Ok, I've seen the threaded ports - gotcha. I made a threaded adapter and put a vacuum gauge on the number 1 cylinder and it bounces all over the place, smoothes out and comes up with more throttle.

When I hook up a carb sync gauge to these ports, do I have to run the rpm's up a bit to do the sync? Aren't the intake pulses offset from one another? How do the gauges compare the vacuum signal between cylinders when those signals are not happening at the same time?

 

[Motorcyclemodeller]

Dont get too technical, raise the idle screw up to about 3000 RPM.

Make sure all vacuumm hoses are plugged with bolts or something similar.

It could take a while so put a fan in front of the bike or take a break if it gets too hot.

You must sync the carbs at normal operating temperature, so let it warm up good.

Adjust the three carbs to the unsetable one, if the RPM goes up/down while adjusting a carb reset to 3000 RPM.

Once all 4 carbs are 1 cm of each other at 3000 RPM you are pretty much done.

Put her back together and go for a blast.

Hope this helps,

Dr. Dre

 

[Anonymous]

Re: flat off idle through midrange - 1980 GS550

I pulled all the carbs and squirted them out. They weren't bad to begin with but I cleaned everything anyway (no dunk) and reassembled. The performance is same as before. No guts off idle.

Synchronizing is always a good idea, but for off-idle problems my first guess would be the pilot jets.

They supply fuel at idle and off-idle (up to 1/4 throttle), and can be responsible for low end stumble or weak response.

They plug up easily in old carbs. Sometimes they're difficult to remove. If you can get them out, the best thing is to replace them. Otherwise, the small holes can be cleared with a fine wire, or "special tool".

A full tear-down isn't necessary to check the pilot jets. You can just remove the float bowls. However, if you have only "squirted them out", it might be worth the time to do a full clean. Probably the best online article on cleaning/overhauling Mikuni CV carbs is right here on GS Resources. That's the source of mosto of the pictures I've referenced.

http://www.thegsresources.com/gs_carbrebuild.htm

FWIW, I usually skip the carb separation steps. Most of the cleaning steps can be accomplished with them connected. I've found that separating them can allow leaks through the 'O' rings on the connecting fuel tubes.

Good luck, and let us know how it works out!