Test your troubleshooting skills

[Hawkins]

80' SG with 79' F engine had intermittent stumble @ 2500 rpm, but otherwise ran excellent. Last weekend went for ride on rough road, and noticed bad backfire when I got home. Got progressively worse, so that below 4000 it hardly runs at all and won't idle. Above 4000 it's perfect. All four plugs fouled black. Clean plugs quickly foul up again. I checked wires from pick-ups to igniter, from igniter to coils and checked coils. Air filter is fine. Gave up and put it in the shop. Should know today or tomorrow what's wrong, so here's a good chance to make your best guess and find out in the next few days if you're right or wrong.

My guess is igniter. I don't see how all four carbs got dirty at the same time, or changed mixture on their own. My 2nd guess would be valve timing jumped due to bad chain.

[randy]

Pickup wires

BTW, What is the prize?



Randy

[Hawkins]

I just replaced the wires. I think I'll make that an annual routine. i'm trying to source fatigue resistant wires from an aircraft mechanic buddy.

Prize? If we were all close, I'd buy beers for the winner(s). I guess personal satisfaction will have to suffice!

[ratbyk]

Diagnosis

I second Randy on the PU wires. Recheck your previous work on them.
If not that then my second guess would be dirt from a dirty fuel tank holding the needles off their seats and minor flooding causing the rough running below 4000. Clean out tank, carb bowls and needles and seats. Install in line fuel filters. If tank is really rusty cream it and carry spare fuel filters.
I would rather have screech but if not available then Moose beer thanks.
Ken/Sooke

[Czekus21]

I would have to agree with the pick up wires.
Lienenkugals honey wiese
second guess, dirty carbs./pluged low end jets.
Hienekin

[RS Thornton]

I think you are right about the electrical, having all 4 carbs go funny in exactly the same way. The most likely answers are taken so I will pick a less likely answer and go for the prize. Have you ever cleaned the connectors in wiring harnesses? I think that these can corrode and foul over time, increasing resistance.
If I win I will take the Screech, but no more kissing that stupid fish!

[skids]

It seems unlikely that all four would foul at the same time with bad ignition pickups. That would mean that both sets failed simultaneously. Maybe you are sitting on the snorkle...uh, I mean the seat is squashing the intake hose to the airbox. Or, maybe you got some crappy gasoline. I can't remember the model, but if you have breather hoses to the airbox, make sure you didn't leave them unhooked. If you have individual air filters, make sure the breather hole is not being blocked.

[tommystinson]

Check the flux capacitor.

[mitch]

HMMMMMMM

My XJ did the same thing it was water in the gas try a can of gas line antifreeze ! worked foe me. Kiss lol ..MITCH
( Keep it simple stupid )

[Ken Talbot]

My guess is you get your bike back from the shop with new sparkplugs and a bill for $200, then when you take it out for a ride it will foul the plugs again and you'll be back where you started. Then, I would have a closer look at some of the other multi-pin electrical connectors and the main battery connections to see if any of those have wiggled loose with the rough road vibrations. For your sake, I also hope I'm way high on my guess on the $$ at the shop......

[tfm3]

Sounds like you know your way around a multimeter so I'll chime in with this bit of experience. Nowhere on any old bike, truck or car I've had have I found true marine grade wiring,especially where multipin connectors are concerned.As a matter of preventive maintenance I use a generous amount of contact cleaner and an old toothbrush. Electrolysis is a given with copper and aluminum- simple point to point continuity tests can yield some startling results and help correct some bizarre gremlins.

[pgg]

I gradually phased out all those plastic pin connecters with screw connecters on my bike. A few I just twisted and taped. Prefer a solid connection over a 'convenient' plug any day. Hawks, giving your bike to the shop is a very sad desperate situation!

[scalded dog]

If you are calling the CDI box, the igniter, I will have to agree, my 79 did the same thing, I would do the old "hit the TV move", and she would fire up again, run a few days then quit again, I replaced that with one off e-bay fo around $35, and everything is fine now. Later 'Dog

[randy]

You know, the igniter can be repaired. PM me if you want it fixed.

Randy

[Hawkins]

No answer yet. The local Yamaha shop charged me $130 to clean the carbs. Wife paid for it Fri. afternoon. I rolled into town 10:30 pm, eager to get he bike, went down to the shop (they had it outside for me) and .... they had her idling nice and revving nice on the stand, but still working like a POS when trying to ride. She refuses to start again now. I swapped the CDI box, no change. In another shop now. It's 400 km from home, and I have no time or tools to fool with it much. I'll give this guy a couple of days and then I'll go bring it home and try my luck again. I'll keep you guys posted.