Yamaha 1979 XsS750 - one thing after another

Rymurf

New Member
Hey all!

First time post here. New-ish to the site and the world of old bikes. I have a 1979 Yamaha XS750 Sepcial currently sitting in my buddy’s garage that we just haven’t quite gotten to fire.

While I’m quite green my buddy is experienced with bikes and he’s verified compression, clean carbs and such. But unfortunately his shortcoming is electrical and that seems to be where most of my problems are. Had lots of help from Matt at Sparck (thanks again!) but still haven’t fired this thing up.

So far I have replaced reg/rec with a modern one from Sparck, replaces the fried ignition unit (CDI?) with a pristine OEM off eBay, brand new Spark plugs, swapped the old corroded fuse box with a modern universal blade style and fresh fuses. I think that’s everything thus far?

For a while we were having issues with popping fuses as the PO did some interesting wiring and stuff but that’s all been sorted. Just before I resorted to ripping out the whole harness we got that sorted and now the bike is turning over regularly. I have spark on the second and third cylinders. Ive just heard from a cafe racer instagram that the first cylinder not having spark could keep the bike from firing?

So I’m looking to replace the first coil for sure. Wondering if it’s a good bet to replace all three while I’m at it?

Also, looking online and worried about the coils I’m seeing. Everything that comes up on Mikes XS and similar sites that says it’s for the 750 says it’s for the 650 in the headline and seems to be for points ignition. Will that work with my solid state ignition?

Any other thoughts on what may be wrong? It seems like every time I say “this will definitely be it!” There’s another part or repair. I love it!

Appreciate the site and it’s been invaluable to prowl these forums. Thanks y’all.

Rymurf
 

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I think 650 and 750 coils are the same. It should still fire but only run on 2 and 3. Very rough. May have the first coil wired wrong as well.
 
I'd double check your wiring against the stock harness. Be sure that the orange, yellow, and gray wires are all running to the correct slot in the ignition unit and to the correct coil. It's pretty easy to mix up the order if you'd taken everything apart. One of the symptoms you'll get from mismatched coils is very loud backfires if you've been trying to start the bike for more than 30 seconds or so.

If you don't have a strobe light for testing/setting ignition timing, but sure to grab one. This will help a lot in diagnosing your problems.
 
Sonreir said:
I'd double check your wiring against the stock harness. Be sure that the orange, yellow, and gray wires are all running to the correct slot in the ignition unit and to the correct coil. It's pretty easy to mix up the order if you'd taken everything apart. One of the symptoms you'll get from mismatched coils is very loud backfires if you've been trying to start the bike for more than 30 seconds or so.

If you don't have a strobe light for testing/setting ignition timing, but sure to grab one. This will help a lot in diagnosing your problems.
Thats exactly how mine was on first start, had swapped 1st and 2nd coil wires. Bangs almost made me shit myself!
 
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