Stumped: 1 cyl dead

[Shuriken]

I've had various vintage Yamaha's over the years and know what dropping a cylinder feels like. A couple weeks ago, the bike lost power. I suspected a bad cylinder.

So... pull each plug boot at a time and rear left makes no change in engine rpm. So that's the offender.

Yank the plug and it is black and fouled. Rest are fine.

Test primary and secondary ignition coil resistance on that one. Within spec.

Swap coil to the rear right cyl. Problem does NOT follow the coil. So coil appears to be fine. In fact both rear coils measure in-spec.

Swap only the plug wires and boots. Problem remained the same regardless of plug lead/boot positions!

On to spark plug tests. Yank fouled plug and ground it to the engine and crank the bike. Good consistent spark.

Take a break. Come back to cold engine. Swap in a new plug. Start bike. Yank boot while running and engine rpm drops like it should. Awesome.

Take it up the street and it's gutless!

Pull back into driveway while warm and running and yank the boot. No change in rpm!

Turn it off, disassemble and get to all harness connectors around bike, includingat the ignition module. Reseat all connectors. No difference.

I can leave a plug in the cylinder and disconnect boot from that plug. I can then use another spark plug grounded to engine and I can observe good spark and rev engine and the spark advance appears to also be working.

All signs are that there is spark!

So that leaves me to ponder other possible issues. Poor compression on one cylinder all the sudden? Carb issue on one cylinder? Or ignition board fault on one cylinder?

The only thing I noticed was when it was a cold engine, yanking the boot on the bad cylinder did create the drop in rpm. So it seems a cold engine and new plug worked as expected for a couple minutes.

Telltale sign of overly rich fuel is usually indicated when bikes run worse when hot. But I swear this seems like a spark issue....

Some things left to try:
Plug chop test at mid/full throttle to make sure only 1 cylinder has the issue.

Check for fuel in the float bowl of the bad cylinder.

Compression test

VBoost does work, I can observe the mechanism opening at speed.

Anybody have any further ideas?

 

[Traumahawk]

It can be a carb issue. It's easy to get junk caught up in a various circuit.

You could try a shotgun, and see if something is caught up in the pilot jet. I've had better luck with taking out PAJ2 to get a full blast of air into the pilot jet circuit.

https://vmax.lvlhead.com/tips/shotgun.htm

 

[Shuriken]

I noticed that I have 3 plug caps of the same type and 1 different. The front plug cap is an NGK-SB05F, specified for 5k ohms resistance. The other 3 are TD T-139. Ngk measures 5k ohms and other 3 measure 8k ohms. The ngk makes a more robust spark to my eye.

However, moving the cap around did not solve the issue.

The bike is running on only 3cyls when warm, and 4cyls when cold.

If is a carb issue, what carb issue would affect all 3 circuits: idle, mid, and full?

Only thing I can think of would be a plugged main jet or no fuel in the bowl but that would not be affected by engine temp, IMO.

I would like to have 5k ohm resistor caps as per spec so time to go hunt but that doesn't explain the sudden issue.

This one has me stumped, this time...

 

[MaxMidnight]

The good news is that you have ruled out one possible cause.
You have correctly concluded that if you have spark but no fire then that leaves either compression or fuel.
Next step would be to check compression to rule out another possibility.

Assuming that is OK then it is a fuel issue.
From what you describe you imply that the cylinder does not fire up regardless of throttle position?
In that case I would suspect either too much or too little fuel.
My next step would be to check the float levels.

 

[RempageR1]

If it`s an `85 like in your signature, the original CDI (actually TCI) could be a problable cause same as the four pick-up coils the original one has. Especially since you mention it fails when hot. Perhaps you can do a longer test on rear-right with a test plug. Perhaps the engine needs to be a bit hotter before the issue occurs. Been on your position 8 years ago and it drove me mad. In my case the TCI was bad and I replaced it with an Ignitech.

 

[maleko89]

My thoughts is if carb related, not a fuel jet problem. It would be running lean. Check float level, shot gun the air jets and see where that leaves you. Check for stuck choke plunger too. Otherwise, sounds like ignitor issue. If p/u coil was flaking out, it most likely would be both rear cylinders since it's an early gen 1. There are two p/u coils...one for the front cylinders, one for the rear.

Mark

 

[Shuriken]

I ran out of time yesterday before I could get into it further.

I will do a compression test. That's easy and quick to rule out. I would initially think compression would improve as the engine warms up, so I would be shocked to find that compression is the issue. Stranger things have happened, though.

I did the simple check for fuel. Loosening the drain screw for that carb revealed fuel.

I do not have a spare TCI but I thought about that as a possible issue. I just had to replace the VBoost controller module but it was because it was flaking out and appeared to have a bad trace or solder joint I couldn't find (flexing the board revealed that issue).

Since I don't have a spare TCI to try (I had to swap the ignition board on one of my old XS1100s in the past), I will have to go back to the carbs again. The effort is the same it seems to tear into the TCI or front coils vs the airbox and carbs.

I did fully disassemble, clean, and rebuild the carbs to start the summer because I had let the bike sit too long and you know the rest of the story. That brought back full performance for the summer. The current problem emerged just a couple weeks ago.

If it isn't a sneaky TCI / ignition problem, which I cannot see while riding (I can observe the spark presence, intensity, and timing while statically revving the engine), and since there is fuel in the float bowl, I'll do a compression check next. If that looks good, I will yank the carbs and see if I can tear down the possible offender and look for a bad float, stuck float valve, plugged main jet, and float level height. If all of that checks out.... I may need to try a TCI or just give up and buy the 2003 I hope to look at this week.....

 

[Shuriken]

I just did another spark test with a harbor freight spark tester. Spark looks fine and advances with rpm too so... on to compression

All 4 cyls ~100psi cold.

Manual says 128 minimum at sea level. 142 normal. 5700ft asl here.

The bike has run and with power and all are about the same. It's low for sure but with altitude correction it would rise somewhat. Previous old bikes I've tested here have been around 108-112 cold.

Fuel in the float bowl so I have to think it's way to rich to fire.. stuck float valve... etc... carburetion...

 

[Shuriken]

https://www.vmaxforum.net/threads/how-to-measure-wet-float-levels.17274/page-2

Aha.... this thread has a much better pic than the manual does...

And there is room for ingenuity as well...
https://xjbikes.com/forums/threads/...312-tool-and-adapter-for-mikuni-carbs.129884/