78 gs400 leaning out

[Guest]

Hello everyone, new to the site as a member, but have been lurking for some time trying to figure this thing out.

A little back story -
I picked up the bike, 78 gs400 from a friend for doing some labor on his car. The bike had sat for probably 10 years or so. Changed the plugs, oil, and cleaned the carbs and it fired right up, but would not rev over about 2100 rpm unless i choked out the carbs with my fingers in their throats. Upon checking other things, i found 150 psi compression in one cylinder and 90 psi in the other, which went up with oil in the cylinder, so i had a stuck piston ring was my guess. Anyways, i replaced the rings, honed the cylinders, and got it all running again yesterday.

Bike still would not rev over 2100, but compression is good now.
I took the carbs apart again and re cleaned. I adjusted the throttle needle down one notch to richen the mixture, and am now able to get about 5500 rpm out of it, but with redline at 9000, this is not ideal still. Even though it revs to 5500 now, it seems like it takes much longer to get there than it should.

As far as i can tell, there are no intake leaks, i installed new o rings when i put it all back together. There are no holes in the CV diaphragms, and the carbs are spotless. One thing i did notice, was as spring was missing from one of the adjustment screws on the side of the carb. The o rings on the adjustments are still intact and in good shape. Adjusters are turned 2 turns out from seated.

Bike has factory everything, and round about 5700 miles on it. I have not converted to pods, still stock air box. I feel that if i were to richen the mixture further with the needle, i would probably see better results, but i dont believe i should have to have the bike at its richest setting at sea level altitude.

Does anyone have any other suggestions for me to check out?
I have also installed new points, and played with ignition timing, and also replaced the condensors.

[Nessism]

Did you adjust the valves? Is the air filter in place and clean? Stock mufflers? How can you choke the carbs with your fingers over the carbs if the airbox is installed?

[jreedpete]

I did not adjust the valves. I would think compression would be down if they were the issue.
I choked the carbs with my fingers while the air box was off.
Air filter is in place, and clean. Mufflers are stock.
Thanks for the quick reply by the way.

[jreedpete]

A quick update -
I have tried the richest setting on the needles, and had zero improvement. Disassembled again and moved back to stock setting, and am back to 2100 rpm limit. I have double checked with propane that there are no intake leaks.
As an ASE certified mechanic, i can make anything on 4 wheels run, but this bike is testing me viciously. I suppose it is possible that jetting may get the results Im hoping for, but I feel I should not have to jet a bike that is 100% stock.
Any input is appreciated.

[Nessism]

Did you fully break down the carbs and soak the bodies and parts in carb dip?

Adjust the valves. They tighten with mileage and failing to adjust them will result in them hanging open after the engine gets hot.

Using propane will only find gross air leaks, not small ones. That said, air leaks won't cause the issues you mention.

The pilot screws should be open to 2.5-3 turns open. I doubt that being open only 2 turns would cause the issues noted though.

[jreedpete]

I did fully break the carbs down, however I did not soak them. They were relatively clean to begin with, and I made sure that every passage was clear with compressed air. I am unable to adjust the valves, I do not have any feelers that are within the .03-.08mm specification, but I can say that the bike will run the same whether cold or warm. I have tried the pilot screws from 2 turns all the way out to 5 turns with little difference.

[jreedpete]

I have checked the valve clearances, and I am able to fit at least a .06 mm feeler on each one.

[John Park]

I'd verify that the ignition advance mechanism isn't rusted closed before tearing any more hair out. It's a common problem on bikes that have sat for that long. It will make the bike peg out at about 5K.

[jreedpete]

Im not sure what mechanism you are talking about. Is that maybe something that the bike would have if it had electronic ignition rather than points and condensers?

[John Park]

I see you're in Lincoln City which, as I recall, is on the Oregon coast, thus a major fog area. So a rusted advance is a good possibility.

If you remove the screws that hold the points plate - mark its position first - and swing it out of the way, you can grasp the cam that opens the points and see if it twists one way and snaps back. If it doesn't do that freely and willingly, remove the 10mm bolt that holds it on and soak it in WD40 or whatever to free it up. Clean it up well after so that it doesn't throw oil onto the points.

The high carbon steel used to make the cam wear well tends to rust easily, and condensation inside the cover will tend to rust it over time if it sits immobile. I always shot some lube on it and wiggled it in whenever I service the points. You may be able to free it up without taking the points plate off if it's stuck. Don't ever try to rotate the crank with the 10mm bolt; always use the 19 behind it.

80 on used the same mechanism but with a magnetic end to trigger the trigger coils; seem to rust less. Then they went to a fixed magnet type and the advance was done electronically, mid 80's. I guess if you're going to go electronic, you may as well go all the way.

You could also unscrew the plug caps and cut 1/4" off the ends of the wires and screw them back on; common refresher trick.