Diagnosis help

[droyce]

Okay, I really need some help diagnosing why I can't get this thing running right. I have a '77 GS400 that I recently bored out to 448cc using GS850 pistons/rings.

Now that its reassembled I can get it started and idling just fine but the right side fires and runs hot, the left side MUCH cooler. At first I thought it wasn't even igniting the fuel at all. Driving it in first gear, it has quick short bursts of power but any throttle and it dies or tries to die (but now I'm wondering if I was running out of gas when riding it yesterday)

I put a Dyna S ignition system, new wires, new plugs on it and getting spark is no problem. Timing is spot on. I've tried taking the carbs apart and giving them a good cleaning, didn't help. The valves have been checked and adjusted using proper shims. Compression is in the mid 150's (psi) for both cylinders, the left side actually a few psi higher then the right side.

I've noticed I can play with the right side carb adjustment screw to tune, however, the lefts side screw does absolutely nothing to the idle. This is what confuses me.

On another note, and this may or may not be related, I have a weird sound coming from behind the left side cover, I pulled it off and there is nothing in there. Its kind of a "rattling in an empty metal can" type of sound.

Let me know what you guys/gals think. I have some videos I'll try posting later if possible.

 

[koolaid_kid]

If it is the left side as you sit on the bike, there had better be a stator bolted to the cover and a rotor bolted to the crankcase.
Have you checked the 3 screws that hold the stator on? They have been known to rattle loose.

 

[Steve]

If adjusting the left mixture screw does nothing, look for a broken tip that is stuck in the carb body. If the tip of the screw is nice and sharp, I would suspect a blockage in the pilot mixture passages inside the carb body. Time to clean the carbs (and replace the o-rings while you are in there).

.

 

[Guest]

What do the plugs look like ?

IS there any gas in the left carb bowl.

Did you do anything to the jetting yet?

 

[Gorminrider]

"On another note, and this may or may not be related, I have a weird sound coming from behind the left side cover, I pulled it off and there is nothing in there. Its kind of a "rattling in an empty metal can" type of sound."

This seems like a good description of the noise the starting motor's clutch can make. Is it possible it's not disengaging?

 

[droyce]

The three screws on the stator are tight .

The carb adjustment screws are intact, I cleaned out all the orifices with carb cleaner and compressed air already, there's no blockage.

I haven't touched the jetting yet and I haven't checked the level of fuel in the bowls but it poured out then I took the carbs apart for cleaning.

Left Plug

Right Plug

How do I check if the clutch is operating properly? The sound only occurs when I rev the motor. I can't pin point it, but its toward the front of the motor right at the top of the side cover.

 

[droyce]

"On another note, and this may or may not be related, I have a weird sound coming from behind the left side cover, I pulled it off and there is nothing in there. Its kind of a "rattling in an empty metal can" type of sound."

This seems like a good description of the noise the starting motor's clutch can make. Is it possible it's not disengaging?

My major concern is that it might be the springs in the counterbalancer. Any chance they can make this sound?

 

[koolaid_kid]

Right plug good.
Left plug bad. (Too rich.)
Time to start digging into the carb. Did you check the idle mixture screw as Steve suggested?

 

[droyce]

Right plug good.
Left plug bad. (Too rich.)
Time to start digging into the carb. Did you check the idle mixture screw as Steve suggested?

Yes, adjustment screws are fine and I blew out the passages with cleaner and compressed air when I cleaned the carbs. The problem has still persisted.

Adjusting the left side screw does nothing to the engine speed.

 

[tkent02]

Did you vacuum synchronize the carburetors on the engine? If one cylinder is doing all the work, the carburetor adjustments on the other cylinder will do nothing.

 

[droyce]

Did you vacuum synchronize the carburetors on the engine? If one cylinder is doing all the work, the carburetor adjustments on the other cylinder will do nothing.

I don't have the proper tools to sync the carbs yet. Working on it though.

I'm wondering if the rattling is actually coming from the exhaust, maybe something is lodged in there rattling around, could this cause a rich condition?

 

[tkent02]

Poor man's twin cylinder synch…

Warm it up, screw the idle speed screw to a high idle, 2000 or so.
Unplug one spark plug wire and see how much the RPM drops. Plug it back in and unplug the other. The RPM will drop the same amount if the carbs are synchronized well.

If one cylinder is doing all of the work, that plug will prop the RPM a lot more when it is unplugged. The other won't change the RPM much, maybe not at all.

Adjust the synchronization until the drop is the same on both sides.

Then see what your other adjustments do.

Usually the rattling is a baffle inside the muffler coming loose.
Could be a mouse or a child filled it with rocks, too.
Can't help you with that.

 

[Guest]

Use a big screwdriver against your ear and place it against the left side cover and exhaust etc. to see if you can nail down the location of the rattle.

You might need a friend to rev it for you if it doesn't happen at idle.

Also, if it's the left plug running rich, someone on here just recently worked out a leaking fuel tap was causing his to run very rich, so have you verified your fuel tap is turning off correctly when the engine stops? Easy way to check is remove the fuel hose and vacuum hoses from the fuel tap and place a little container under there. See if there is any fuel in it after a while. There should be absolutely none.

 

[Gorminrider]

I recall somewhere that a bolt screwed into a piece of clear hose can be a stethoscope...You would need to wear muffs or plug ears to block out the overall noise. Could this help pinpoint this rattling noise? Or just a microphone in a tube to an amp to earphones...?
(that poor-man's synch is a keeper! I'm gonna try this!)

added; ah cross posting. well, plug the other ear anyways, using a screwdriver.

 

[droyce]

Poor man's twin cylinder synch?

Warm it up, screw the idle speed screw to a high idle, 2000 or so.
Unplug one spark plug wire and see how much the RPM drops. Plug it back in and unplug the other. The RPM will drop the same amount if the carbs are synchronized well.

If one cylinder is doing all of the work, that plug will prop the RPM a lot more when it is unplugged. The other won't change the RPM much, maybe not at all.

Adjust the synchronization until the drop is the same on both sides.

Then see what your other adjustments do.

Usually the rattling is a baffle inside the muffler coming loose.
Could be a mouse or a child filled it with rocks, too.
Can't help you with that.

Pulling the left plug wire does absolutely nothing to the rpms. Right side cuts the engine.

 

[droyce]

Use a big screwdriver against your ear and place it against the left side cover and exhaust etc. to see if you can nail down the location of the rattle.

You might need a friend to rev it for you if it doesn't happen at idle.

Also, if it's the left plug running rich, someone on here just recently worked out a leaking fuel tap was causing his to run very rich, so have you verified your fuel tap is turning off correctly when the engine stops? Easy way to check is remove the fuel hose and vacuum hoses from the fuel tap and place a little container under there. See if there is any fuel in it after a while. There should be absolutely none.

Pete, I haven't noticed any fuel leaks anywhere. I work on the bike almost daily and never noticed any fuel, or even the smell of it. By "fuel tap" do you mean the petcock or the line between the carbs? Both are tight.

 

[droyce]

What is this brass nipple for?

 

[Gorminrider]

...possibly a fuel filter. That looks like an outboard motor fuel-line you have there, by the way. In any case, it looks haywire and I'd have it out to see. I don't mind haywire, if I did it, but to look at someone else's is an agony.

 

[droyce]

I'm talking about the nipple on the carb body, not the fuel filter. Directly behind the fuel filter in the picture.

 

[Thumper88]

That's the overflow drain, should be one right next to it on the other carb.