Hi all, I am the original owner of this bike. (33 years). It has 16,500 miles on it. I recently pulled it out of a 2 year sleep and did some maintenance to make it dependable (new fork seals, boots, and fork oil, new brakes and fresh brake fluid, rebuilt carbs, new fuel filter, new battery, tune up, etc). After 2 weeks of trouble free riding the engine began to stumble and died as if it ran out of gas. (engine was warmed up and I was going 35 mph). I pushed it home and began trouble shooting it. (Bike would not fire up). Here's what I checked:

Fuel was full and fuel delivery to carbs normal.
There was spark to points and plugs ( I thought spark looked a bit weak, but went on to check some other things first).
Plugs were gapped correctly (30). They were black sooty.
Points gapped correctly. (.014). Timing was set right. (but I noticed the
I decided to check primary and secondary coil outputs- primary was good, but secondary resistance was low. I replaced the coil and plug boots and the spark to plugs looked good. I figured I found the problem. NOT!

Now the bike will fire up and run poorly. If I give it gas it stumbles and sometimes backfires out right side pipe. It will idle fairly well, but a bit stumbly. After a few minutes to warm up no improvement. Pulled plugs-dry but black and sooty. Checked timing while at idle- I noticed that the timing was fluctuating back and forth a bit on the mark. I checked the timing advance mech. and it seems fine.

Here's where I'm getting stuck. I took the top cover off the engine to look at the valves and timing chain. The chain and cam chain tensioner appear fine, but I don't know what fine really is. Nothing seems loose or out of place. Oil is flowing up to the cam. I don't know if the fluctuation is normal, or the cause of my poor running engine, and if it is the cause, then what is making this happen.
I also looked in each cylinder to see if the pistons were in time with valves on the timing "T" mark. They seem to be dialed in ok. The piston on the right has a rainbow colored ring around the top edge, left side doesn't.
Well, that's about it, hopefully someone can help me figure something out. I don't want to take it to a shop and be at their mercy! Thanks, and Happy Riding!

 

Sounds like a plugged jet to me. I suggest pulling the carb that feeds the cylinder that is messing up and pull the jets, particularly the main jet. It idles, messes up when it goes off the idle jet onto the main.

Before I pulled the carb off, I think I'd make sure that there was a good seal around the intake manifold. I find spraying WD40, carb cleaner, some aerosol canned petroleum product around the intake manifold while running the engine will detect air leaks. The engine RPM's should pick up if there is an air leak. If all seems well there, my su****ion is the main jet has become contaminated.

ETA: S-U-S-P-I-C-I-O-N ..........is verboten? Your hall monitor best not get close to the shed when I have a wrench slip.

 

I checked seals between carbs and heads as well as the seals behind carbs with start fluid and found no leaks as one of my trouble shooting checks. I can definitely look into the carb as a culprit. That could make sense. I still wonder about the timing fluctuating. I don't know how much it should waiver off the mark? Thanks! I will be checking in soon.

 

33yrs of the original ride. Impressive and welcome to KF

yeah, pull the rack, clean and synchronize after install would be my first thought as well since you have already changed the air filter, fuel filter and verified that you had no vacuum leaks.

.030" plug gap sounds abit large, but I do not have an '80 manual and it sounds like you have that covered as well. NGK D9EA plugs?

When you had the valve cover off did you happen to check your valves? Just curious, 16k miles and no mention of it.

 

Sounds like carbs to me. Hows the fuel tank look inside? Some old gas or rust broke loose inside and made its way into the carbs.

 

I wish I were perceptive enough to diagnose over the internet. I find I sometimes struggle when the bike is parked in front of me. Anyway. A dim light flashed in the recesses of my cranium this A.M., a reminder not to over look the possibility that the diaphragm may not be seated, sealed, or possibly deteriorated or damaged, particularly since this carb was recently disassembled and cleaned.

It might pay to pull the diaphragm cover and take a peak.

 

Yesterday (Sunday) I pulled the carbs off and completely disassembled them looking for contamination (Tank is clean inside- I have kept it in good condition-everyone who sees my bike can't believe it's 33 years old). I don't separate the carbs-I use a service manual for the bike and it suggests not to. I didn't see anything yet, but I have to take a break from working on it for a short spell. When I get back to it I will use carb cleaner and compressed air and put them back together and see what happens. Thanks for all the great advice, I'm using it! I will check in after the next try running it and let you know what's up.(diaphragms look OK, no tears, but they do feel kinda thin).

 

P.S. I'm running NGK B7ES (always have) per manual. Plug gap can be between .028 and .032 in.. I run them right in the middle (always have). Being the only owner and rider of this bike gives me the advantage of knowing it's complete history, but right now that's not helping. The motor has never needed work and I have kept bike in decent shape as far as maintaining it. The bike has always served me well up until now.
I'm just a "LITTLE FRUSTRATED". I'll be back after the next try! Thanks again...

 

Welcome to the forums, I agree, it sounds like something got into one of the jets. Time to pull and clean again.

 

Hi, I went back and cleaned, blew out and double checked carbs etc. I also made sure fuel supply was fresh and delivering to carbs properly. The engine still runs bad, no change from before. I am ruling out a carb issue.
I checked compression again and it's 150 both sides with throttle held open. Spark is great. Engine will idle fairly well, but still a bit stumbly. When I apply throttle engine stumbles and I hear backfiring through air box and out exhaust pipes.
I took the rocker cover off again. I line the arrow mark on the cam timing chain sprocket up with edge of engine case per the manual. This is supposed to correspond with the timing marks down in the case where the points are. These marks there also line up so I know the cam and valves and pistons are moving and lined up correctly. I can turn engine by hand( using a socket on crank bolt per manual) and see all components are lined up together and should make a running engine.
The points appear to be opening and making spark when piston(either side) is at top of compression stroke. I can't figure out how all the components seem to be in place to make a running engine, but it won't run good, Maybe I should threaten it with a for sale sign!
This is the first time I couldn't figure out the problem and solution and make something run. Live and learn! (I'd rather be living and riding!)