Murray Carbs?

[johnstewart]

I had some time to wrench on this last night.

What do the spark plugs look like on 2 and 3?

I didn't pull the plugs (I forgot you posted this), but they are brand new.

I did put the ohmmeter on the coils, and they tested good (my recollection is 750 ohm was about right on both pickup coils, and the ignition coil primaries were within spec... on the primary leads they were about 4 ohms instead of spec 1.5 ohms... but I think that's "normal" - these are aftermarket coils I put on many many years ago and I think they just have higher primary resistance (my chicken scratching in the Clymer manual indicates this is what they always tested at).

I swapped the ignition coils (I actually left them physically in place and just swapped all the wires - NOT easy to extract given how they are mounted).

The problem stayed on cylinders 2 and 3.

So the issue is NOT with the ignition coils (good, because in looking at how they're mounted in there, I don't ever want to have to touch them. The maniac who installed them also used non-metric nuts, oy) (that's me).

So, the problem is not the ignition coils. The pickup coils tested good, though I'm not sure that's sufficient to say they're "good".

Where ARE the pickup coils? I don't see any info in the Clymer or Yamaha shop manual... I think they're under the left cover along with the ignition advance mechanism?

I'm now tending to think maybe the lead wires for the coils in the ignition advance mechanism are flakey for one of them... THAT would explain these symptoms, no?

Many years ago I had the wire fixed by the mechanic. I wonder if the repair has flaked out, or maybe the other one has gone.

Am I barking up the right tree?

Edit - found this:
https://web.archive.org/web/20160504...pictorial.html

I'll try diagnosing this next.

[bikerphil]

Yes, the P/U coils are under the left engine cover. The 2/3 is the lower one, try tugging on the wires and if they stretch you have a broken one.
A broken wire would cause intermittent spark and could be your problem.

[johnstewart]

I think the lower one is the one that was NOT fixed years ago, so this leads more credence to this theory, too.

[johnstewart]

I'm now completely mystified.

I opened up the cover for the pickup coils to poke, and no wires appeared to be broken. I even started up the bike and poked/pulled at all the wires; no change to the running of the bike.

I then put the ohmmeter on the wires to the pickup coils (both around 750ohm, which is "good"). And with the meter on, poking and prodding, they were both solid at around 750... so it does NOT seem like a broken wire.

What the bloody else could it be?

What else could affect only cylinders 2 and 3 like this?

[Schming]

I'm now completely mystified.

I opened up the cover for the pickup coils to poke, and no wires appeared to be broken. I even started up the bike and poked/pulled at all the wires; no change to the running of the bike.

I then put the ohmmeter on the wires to the pickup coils (both around 750ohm, which is "good"). And with the meter on, poking and prodding, they were both solid at around 750... so it does NOT seem like a broken wire.

What the bloody else could it be?

What else could affect only cylinders 2 and 3 like this?

Have you checked the resistors inside the plug caps, and/or the ends of the plug wires that the caps screw into, for corrosion ?

Have you pulled the plugs (2 & 3) after a bit of running to see if they are wet,dry ?

[johnstewart]

I think the plug wires themselves have to be fine.... I swapped the input and output wires to the ignition coils, and the problem remained on 2&3... so the plug wires were swapped, effectively.

I can look at the plugs, but I'm certain they are going to be wet - I can see there is gas coming through to the air filters; I presume unburned gas in 2&3.

[Radioguylogs]

Ignore this post - I forgot these were Murray carbs.

[Justin]

Any update on the Murray’s? I’ve been eyeballing these. Thanks!

[johnstewart]

I haven't dug back in since September. I'm stuck on why I don't have any spark on my middle cylinders, since everything seems to test fine.

The Murray carbs themselves I think are probably a good choice; I've just got other problems.

[DiverRay]

Pull the connector off the ICU that the pickup wires go to. It's the 4 pin connection. Check your pickup coil resistance at this location static. Then advance the plate for the ignition at the left side and hold it with a zip tie or something while you check the resistance again. White is one pair, Green is another pair on the wire colors. If this checks out good or bad, get back to us if you need to. Let us know what you find.

[pbowes]

I picked up a 79 xs1100 about 3 months ago and have been chasing my tail with the same issue you are having. Bike ran fine when testing ,put the tank on properly with the seat and now intermittent firing with pretty much the same heat difference in the 2/3 cylinders Take off the tank and seat and jiggle some wires and she would be up and running again. .Did all the steps you have done (except the pick up wires),rebuilt the carbs 3 times thinking it was a fuel starvation issue (hey don't shoot me im a guy who doesn't read manuals that say 2/3 are on the same coil) ran straight gravity fed petcocks. Even changed the ecu from a parts bike and still the same issue. There is a break somewhere and i am thinking of stripping the loom outer to find the issue.as i have narrowed it down to the head stock wires back to the box (lol so not at all). Once i get her running will look at the Murry Carb conversion again but that steep price when i got 3 bikes for double that stings.

[johnstewart]

pbowes if you get anywhere, please post here. I will, myself. But it's likely spring before I get around to bike work again.

[bikerphil]

Has anyone actually run these Murray carbs for a decent amount of miles? Maybe give us a report?

[Radioguylogs]

I picked up a 79 xs1100 about 3 months ago and have been chasing my tail with the same issue you are having. Bike ran fine when testing ,put the tank on properly with the seat and now intermittent firing with pretty much the same heat difference in the 2/3 cylinders Take off the tank and seat and jiggle some wires and she would be up and running again. .Did all the steps you have done (except the pick up wires),rebuilt the carbs 3 times thinking it was a fuel starvation issue (hey don't shoot me im a guy who doesn't read manuals that say 2/3 are on the same coil) ran straight gravity fed petcocks. Even changed the ecu from a parts bike and still the same issue. There is a break somewhere and i am thinking of stripping the loom outer to find the issue.as i have narrowed it down to the head stock wires back to the box (lol so not at all). Once i get her running will look at the Murry Carb conversion again but that steep price when i got 3 bikes for double that stings.

It quite possible (likely?) your problem is the pick-up coil wires. It's a common problem, especially '79 and earlier models.

Put an ohmmeter on the pick-up connector terminals and pull on the wires next to the pick-up coils.

[Bax]

I'd suspect this 2 carb setup is the cause of the 2,3 cylinder EGT imbalance. Firing order is 1-2-4-3 so the carb AFR flow time and inlet pressure would result (explained split for left carb): Intake 1, Intake 2, Pause 4, Pause 3, Intake 1, Intake 2. Basically this setup will provide a significently better AFR cylinder charge for the intake 1 or intake 4 time after the pause and imbalance the charge to the following cylinder.

---Bax