TOPIC:

I am trying to help my buddy fix his 79 kz650. Last year(fall time), he had the carbs rebuilt. Since then, the bike has not run right. I have tried everything I can think of to fix this, but nothing seems to work. Basically, when you open the throttle, the engine bogs down and dies. Opening the throttle is basically the same as turning it off. It will start and idle, but once you touch the throttle, it's done for.
Things that I have done since:
Installed new points & condensors, set timing w/ gun
checked valves(all in clearence)
checked compression - 150-145-160-155
disassembled the carbs many times - blew out all passages & cleaned throughly.

The carbs have a 102 main jet, and I think 16.5 pri.(I will double check in the morning). The only mods done are pod filters. I have literally set the needles in every position there is, with the 5th yeilding the best result - It would rev, but at/near full throttle, it would run so rich that it would begin to fire down the exhaust. When you shut it off, the plugs were litterally wet with gas. the 4th clip and it won't rev. I'm tempted to pick up a set of 95 mains and put them in to see what it does. Any thoughts?

Does the bike have the stock exhaust or aftermarket headers? This will make a difference as it may need even larger main jets with aftermarket headers, 107.5 perhaps.

The stock jets for a '79 650 with the accelerator pump (has the extra pump at the bottom of #2 carb) are 97.5, so you're already up two sizes. The stock pilot jets are 15s. For the record the stock needle is 5CL16-3 where the 3 is the clip position which is the middle groove.

What do the plugs look like?

Where are you? Someone nearby may be willing to help you out.

- Nate >>

The plugs are sooty black, which is making me think its rich.
The pipes are stock.
The needles say Y6-3 on them....that wouldn't cause it, would it?

Just having gone through a little of this myself, I would add to make sure there are no intake leaks, around the boots where the carbs are connected, the vacuum hose from the carbs to the petcock, and possibly relating to the air injection system if you have it (I don't know if this affects intake or not, my bike does not have it). Maybe you already checked this. Also make 100% sure everything in the carbs is assembled correctly and isn't sticking.

If you give it the choke after warming up does it rev high with no problems?

I have checked all the boots, and found nothing(no cracks that go through. The air injection system was long ago removed and the reed covers were capped off with rubber caps. A couple of the boots have cracks on the outside, but it doesn't look like they go through.

Oh, and the pri. jets are a 15
We had it revving on the 5th clip, but it was so rich at the top end, that we were getting flames out the exhaust pipes. We could rev it to red-line, but you couldn't go to full throttle. When you hit full throttle, it would spit, stumble, and backfire. The timing is working correct, and full advance is hitting somewhere around 4-6k RPM(hard to tell when the bike doesn't run right)at idle, it is dead on.

I'm seriously thinking that this is a fuel problem. Talking to my buddy, he said that it ran right(carbs just leaked gas everywhere)before he rebuilt them. Now, the only thing that changed are the main jet size and the needle numbers. He got the kits online, so the possibility that the needles are NOT the right ones is fairly high. Which brings me back to my previous question. The needles, being Y6-3 and not 5CL6-3 would cause this? The main jet sounds about right, as that is what is in my 78 650 with 2-1 exhaust and pods.

I have checked all the boots, and found nothing(no cracks that go through. The air injection system was long ago removed and the reed covers were capped off with rubber caps. A couple of the boots have cracks on the outside, but it doesn't look like they go through.

Oh, and the pri. jets are a 15
We had it revving on the 5th clip, but it was so rich at the top end, that we were getting flames out the exhaust pipes. We could rev it to red-line, but you couldn't go to full throttle. When you hit full throttle, it would spit, stumble, and backfire. The timing is working correct, and full advance is hitting somewhere around 4-6k RPM(hard to tell when the bike doesn't run right)at idle, it is dead on.

I'm seriously thinking that this is a fuel problem. Talking to my buddy, he said that it ran right(carbs just leaked gas everywhere)before he rebuilt them. Now, the only thing that changed are the main jet size and the needle numbers. He got the kits online, so the possibility that the needles are NOT the right ones is fairly high. Which brings me back to my previous question. The needles, being Y6-3 and not 5CL6-3 would cause this? The main jet sounds about right, as that is what is in my 78 650 with 2-1 exhaust and pods.

Sounds like your problem could be float related? I'm thinking your floats are not closing to stop the additional flow of fuel.

Well, I feel stupid....
Just for the hell of it, took some carb cleaner, and sprayed the rubber manifolds between the head and carbs. The engine immediately flared. Does it for all 4 boots. Thinking now, I need to find a cheap replacement set. My local dealer wants 35 bucks each! thats $140 for all 4! I'm going to try some RTV on the rubber and see if that will work, but I'm going to guess that this is the source of my problems...

RTV'd the boots, the engine doesn't race anymore when carb cleaner is sprayed on them. However, The bike will only start with the choke on full. The needle is on the 3rd clip, and it won't rev. I'm going to try the 4th clip and see if that does anything.
The bike won't even run if I turn the choke off. This is driving me nuts at this point. I think it's down to the mixture at this point, but I don't know which direction to go.

BTW, when it starts and the choke is on, the bike revs to ~8K RPM. Half choke gives about 4.5K RPM, and no choke give 0 RPM. Can't rev it, bogs down. The way its acting with the choke make me think its real rich, but it won't even start or run without it, and the choke on these only makes the mix richer.......

ran the bike for about 10 min with the choke on, and shut it, and it died. Pulled the plugs out, and to my suprise, they are turning tan(were sooty black). Well, I decided to pull the carbs off again(not sure what I'm going to change this time, think I'll drop the needle to the 4th clip). Well, anyways, while I had the carbs off, I decided to play with the motor a lil. I put my hand over the 1st boot on the motor and cranked it(make sure it was moving air), did nothing but crank. When I did it to the 2nd cyl., the engine would fire on the 2nd cyl. Weird, I thought. Same thing on no. 3. Holding my hand over the boot would cause the cyl. to fire. no. 4 did nothing. A-HA!
pulled the plugs and laid them on the block. well, they are firing a yellow-blue spark. Grabbed my scope, hooked it up to the coils wires, and instead of a nice square, it looks like a slope. Would go straight up, level off, and go at a 45 back down. Checked coil no. 2&3, straight up, level off, straight down.
I think the only thing that would cause this is a bad condensor. I tested the points with my scope, and they had a nice square wave. The condensor is bad. I have a new set for my bike that I am going to donate to my buddys bike. Will install them first thing in the AM. I will definately post back and let you know how it goes. This bike is becoming a headache......

At least you're getting somewhere.

I was going to add that you need to be sure that the bowl vents aren't plugged. #2 and #4 carbs both have nipples just ahead of the slide which admits atmospheric pressure to the bowls (#1 and #3 connect via a bit of rubber hose between #1/#2 and #3/#4 carbs). The #2 carb has another nipple recessed toward the engine that provides vacuum to the fuel tap.

Just so we're on the same wavelength, the needle clip positions are numbered 1-5 from the blunt end toward the pointed end.

Keep plugging along.

- Nate >>

If you have the accelerator pump on #2 carb, depress the pump arm and make certain that you can see a spray into the carb. This could be one reason for your initial problem with the engine bogging.