What is it this time?

[Urban_Legend]

Hi All

Diagnosis help needed.

About a week ago my bike developed a misfire. The missfire only occures under acceleration when the bike is still cold (the first few miles of riding), and only in the higer rev range (above 4500 RPM) It came on suddenly. It disapears after the bike is at operating temp. The Bike is smooth at idel.


What has the bike had done to it you ask?
So it is an 84 FJ1100, running pod filters and RPM 4-2-1 exhaust. The carb have been extensively cleaned over the last 2 years with a new petcock and filter installed during my first build (2 1/2 years ago) and the inside of the tank is still like new.
Carbs are running 42.5 pilots, 115 mains with the fuel/air mix at 4 turns out. It has FJ1200 slides and the diaphrams are in good condition. New O rings and carb intake boots installed 3 months ago. I have done the relay upgrade to the coils.
The plugs are 2 years old. I have know idea if the valve shims have ever been done, and i have asked the previous owner of the motor. He said he never did them, but was unsure of previous history. The motor has about 80 000km on it (50000 miles)

Any help is appreciated. I am going to re tension the carb intake boots and check for air leaks as time permits in the next day or two.

Mark

[Pat Conlon]

Yea, get those valves checked asap. I assume your carbs are sync'ed.

As general practice, I don't wind my engine out when it's cold. I'm running heavier 20w-50 oil needed in the hot summers here in the low desert, so warm up takes some time...I've noticed it takes 20 miles or so of moderate riding before my oil gets up to proper operating temperature 170-180*F

Does the misfire feel like an ignition problem? Does the tach do anything wonky? Does it feel like 1 cylinder is dropping out or more than one?

A cold problem with the carbs that gets better when it's warm can mean a mixture problem, but at the 4500 rpm range you are on your needle jets...outside of the air idle circuit and pilots.
It sounds to me like your carbs are sorted, good jetting, so I'm leaning towards an ignition issue.

Go ahead a gets those valves checked

[Urban_Legend]

Thanks Pat.

Yes carbs sync'd and needles raised to the 4th clip.
You jogged my memory. Yes the RPM needle does jump around when the missfire occures. Which is making me think electrical. I as leaning towards a bad plug or lead. Possible a coil not working at its best (they are 30 years old after all).
Mark

[aviationfred]

One thing that I caught in your post..... You are running a fuel filter. The gravity fed/vacuum actuated petcock equipped FJ's typically do not respond well with inline fuel filters. Have you checked the condition of the installed fuel filter?

Fuel starvation at higher rpm's with the vacuum actuated petcock with no fuel filter is why Yamaha went to a fuel pump in 1988.


Fred

[fj1289]

Good list of what's been done on the bike.

Sounds like three months since the last thing has been changed?  How often has it been ridden in that 3 months?  When was the last time it was ridden before the issue began?  How long did it sit before the issue?

[Urban_Legend]

Fred - The filter mentioned is the OEM I tank one that is attached to the petcock.

The bike has done over 2000km in the 3 months since I did the last bit of work to it, and is my daily ride to work.

[fj1289]

Runs better when warm - IF fuel delivery related, then it's something causing a cylinder (or two) to run a little lean.  I'd guess some trash got past the fuel filter or maybe got some gas with some water in it.  Pull the carbs and check the bowls.  While your there spray some carb cleaner through the pilots to make sure they are clean. 

Maybe plugs — I've cleared up more than one "carb issue" with a set of new plugs.   

If the battery is starting to lose a cell, I've seen them cause some intermittent ignition issues too   

[Pat Conlon]

Mark, as we know, intermittent electrical problems can be a bitch
The tach needle going wonky is a clue.

1) It could be a 12v power supply problem to your a) coils or b) igniter box.
Disassemble and clean all your connections. Perhaps something is loose when cold but makes better contact when heated up.
The power circuit to your igniter box runs from the fuse box thru the run/stop switch on the handle bar. That switch gave me problems years ago.
Check your keyed ignition switch....hopefully you don't ride around with a lot of keys on your key ring.

2) Like you said, it could be a bad coil. If a coil goes bad, it's obvious...2 cylinders drop out.
Warm them up with a hair drier and bench test them. I found the oem coils to be pretty durable but some have reported a bad coil.

3) It could be a signal problem from your pick up coils. Unplug the pick up coil lead at the igniter box, warm up the 2 coils (hair dryer) and ohm check them at the prongs on the plug.

4) It could be something as simple as a bad spark plug or the resistor plug cap. I doubt it's a plug wire problem. The Yamaha oem stainless wire core plug wires are damn near indestructible (not like the aftermarket carbon core plug wires) However...The resistor plug caps have been known to go south from time to time.

Don't give up. Be methodical. Resist buying new stuff until you know what the problem is....throwing $$$ at electrical problems can quickly get expensive.

Cheers   Pat

[Urban_Legend]

Thanks all.
Pat. Keyed ignition switch is new. Replaced after a wire came adrift in my old one in march.
Your input has given me somewhere to start. And so far all are cheap fixes. Thanks again.
The irony is I held a shed day last Saturday and didn't get to look at my bike while I had 6 other people there.
Mark

[balky1]

After the valves - change the plugs. I started having issues with NGKs recently that are about 2 years on the bike now. One cylinder wouldn't fire when cold, but ran fine when warm. Plug itself looked fine, but something went bad inside it.

[Urban_Legend]

I think I have found the issue.... With my bike running richer now since I changed the pilot jets, because the bike was running too lean with the pods, and I have been doing lots of short, round town runs. The plugs have been progressively getting worse. Took the bike for a run out of town yesterday and got it up to proper operating temperature. Started the bike and rode to work this morning. No hint of the issue. Might turn the fuel mix screws in A bit though and new set of plugs on order. The things and a few more longer rides should do the trick...I hope.

Now back to yesterday's ride...another issue popped up. This one I knew was coming as had peeped through a few times. Clutch slippage. I changed down from. 5th to 4th for some spirited overtaking and and started out OK, until I went for WOT. The revs jumped up to 9000rpm and not a lot of change in speed.
I was going to purchase the RPM 7 disk upgrade and cover gasket, but with the exchange rate and postage this was going to cost me $230 au.
A few phone calls today and I have a clutch pack co.
Ming fro. Yamaha for $130
Just have to ride like a nanna  for a week or two until I can install.

Mark