Dead Cylinder, '75. 115

[tboltmike]

Had her out seveal times this summer and everything normal. Ran it on hose before taking it out a week after the last outing and it just sounded strange. Cranked up ok and idled and didn't sound like a miss. Started pulling plugs and #2 was room temp. Inspected plugs, gapped and swapped around, #2 still cold. Spark jumped a good 1/2" and very blue.
Hadn't had a chance to check compression.
I understand that each bbl of the carbs goes to a dedicated cylinder. If so, could this be a carb/fuel problem or a reed problem?
If a broken ring, does this mean a major tear down and involve all cyls?
This engine has never had a screw turned on the power head or anything above the lower unit.
Thanks, Mike

 

[emdsapmgr]

Re: Dead Cylinder, '75. 115

A compression test will tell you a lot about the condition of the engine. You may want to do that first. No point in putting time and $ into the engine till you know it's worth it. I'd be looking for compression in the 125 range.

 

[emoney]

Re: Dead Cylinder, '75. 115

What emdsapmgr said....plus, the test will help narrown down where the issue might be.

 

[rhaser]

Re: Dead Cylinder, '75. 115

If comp. checks out good. remove the front air cover, start it, squirt a lil carb cleaner in the cyl. if it picks up that side of the carb is dirty. thats a place to start.

 

[tboltmike]

Re: Dead Cylinder, '75. 115

I did a compression test using a borrow gauge from the parts house, so I can't vouch for the calibration.

Got:
#1- 90
#2- 95 This is the one not firing
#3- 90
#4- 85

Could not find in my Service Manual neither specs for compression nor a procedure.
I put a good charge on the battery, removed all of the plugs, misted in a quick bust of Royal Purple in each cyl, opened the cold idle lever all the way to fast, attached the hose and spun until the gauge leveled.

Thought about squirting in a little oil/fuel in the carb throat to see if that would make it hit. Does the top carb, port side throat feed #2?

Any other ideas?
Thanks, Mike

 

[emdsapmgr]

Re: Dead Cylinder, '75. 115

Though the compression is low, the engine should run with those numbers. If you observe that the #2 plug is dry when cranking, then the fuel test you described is a good one. If you have no spark on that #2 plugwire, the only easy test you can do is to swap the bad coil with one of the others. If you still have no spark on #2 (likely) then you have an issue with either the stator, timer base or the power pack. All expensive parts and worth some test time on your part to find the defective one. You can find some good ignition troubleshooting information at cdi.com.

 

[tboltmike]

Re: Dead Cylinder, '75. 115

Thanks for the suggestions. Yes, I am getting good spark and the plug did look wet. Is there any way #2 spark timing can be off and the others still in time?
I'm suprised that the Service Manual does not mention compression testing or give a pressure range, or did I just not see it.

 

[emdsapmgr]

Re: Dead Cylinder, '75. 115

Most service manuals don't include compression specs. (yamaha does on some engines.) The compression varies on models, depending on the heads, yet the factory manuals covers many hp ranges. The timing on the other 3 should be insync with #1. All of the timing is dependent upon a magnet ring in the center of the flywheel. Typically, that timing relationship does not change over time.