New to me XS750...need some ideas

Put a timing light on it, see if it's advancing smoothly between 1000 and 3000 RPM.
 
See how many volts are going to the coils. You should get the same reading at the coils as at the battery, or nearly so. If not, you have corroded connections in your wiring. Start with the kill switch. If it's nasty, you'll get a voltage drop there.
 
With new caps the coils all tested within spec primary & secondary.

I'm not seeing any advance with a timing light.
 
Fleksta said:
With new caps the coils all tested within spec primary & secondary.

I'm not seeing any advance with a timing light.
Click to expand...

You have voltage to the coils in excess of 11 vdc? I'm not suggesting caps and/or coils are bad. I'm saying you need to check for voltage drops due to corrosion.
 
Could be an air leak somewhere, cracked or ill fitting rubber manifolds, vacuum barb not blocked off?
How old is your fuel?
 
Fleksta said:
My meter says that all three coils see about 11.8 volts
Click to expand...

I like to be positive that my carburetors aren't having an electrical problem before I futz with them. With the coils at 11.8, what is the voltage at the battery? This could possibly be due to a weak battery. What is the difference between the battery voltage and what you're reading at the coils? That would be your voltage drop. You want that number as small as possible. I'm not really familiar with your bike, but I suggest you get some contact cleaner and remove any corrosion from all of your connectors. Assuming 11.8 to 12 volts at the battery, I'm suspicious of it.
 
We may have found success! Swapped the ignition unit out with another one off ebay and we have revs...love the way a triple sounds :bike:

Revs steady until about 5k then rockets up to redline (have to see if this feels right for the uni-pods, Rick upped the mains to 140)...will have to get at least the front brake working ok before doing a short road test. Checking with a light it appears to be advancing properly now.
 
jetmechmarty said:
So, it was electrical.
Click to expand...

Not sure...seems my issue is still present. When I swapped the units I had let the bike run longer than usual and got better throttle response. The day after, with a cold engine, it does the same thing as before. If I let the bike warm up a bit, I become able to rev it. Though when I took it out on the driveway it seems hit/miss, I can ride it a few feet then it cuts out like usual. Rolling off throttle will return it to idle, then I can rev it again (rinse & repeat).

Sprayed the carb boots & vacuum barbs with cleaner to check for a leak, found none.

I found a how-to on youtube to check the pickup coils and they read 1 (infinite) while they should be about 560 ohms give/take 20%. Perhaps that test isn't performed the same way on my bike though (bike on youtube was an XJ750)...as how would it run at all?

I'm stumped once again...:banghead:
 
When I had an open pickup coil on mine, the bike would not start. Since there are two coils in there, perhaps it matters which one is open. You should read some resistance on each leg. Infinity is bad in this case.
 
jetmechmarty said:
When I had an open pickup coil on mine, the bike would not start. Since there are two coils in there, perhaps it matters which one is open. You should read some resistance on each leg. Infinity is bad in this case.
Click to expand...

That's what is confounding me, all three pickup coils read open. There are three wires leading to the pickups, one ground, and a fourth white wire for the neutral switch. With a meter's negative to ground & positive to a colored lead I get an infinite reading. I tried hitting the pickups with a heat gun and testing them hot, same reading. Yet if I plug them back in, I can start up the bike.

Is this sufficiently weird behavior to warrant buying & trying another pickup coil assembly?
 
Do you have a wiring diagram? We need one. Remind me what bike we're working on. I signed on this afternoon thinking XS650, which will not run with an open coil. My XS1100 drops two cylinders if I lose a pick-up coil. We need to make sure the coil is open. If it is, then you must replace it. It's a pity the triples site is so lame. This ain't no XS650.
 
Yeah, the triples site is no help at all unfortunately.

The bike is a '79 XS750F, I have a copy of the overall service manual & the addendum for the later CDI bikes which has a wiring diagram.

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OK. Unplug pick-up coil from the ignitor unit. Assuming you understand how to use the ohm meter. Orange to Black should read 560 ohms. Yellow to Black = 560 ohms. Grey to Black = 560 ohms.

Take the harness apart and find the place where the coils are tied together. This is represented by the dot on the Black wire. Make sure that connection is good.

If any of those legs has failed, you will require a replacement. If all three show open, the bike won't run, so we need to visit the meter.
 
It's finally spring! Meaning the garage is warm enough to work in again :D

Pulled off the ignition pickup coil and found the advance unit on the backside of it was gummed up with old grease. It would not snap back when twisted, gave it a couple shots of cleaner, spritzed it with lithium grease and it seems fine again. I should have realized that Yamaha would still be using a mechanical advance :doh:

Put it all back together & set the timing at idle. It will now rev past 2000 RPM! But only on the full enricher setting (not a choke on these). I had sent these out to oldskool for cleaning but apparently there is still a problem with the pilot circuit so I'll have to dig into them again. It puttered around the yard under its own power with the enricher out, so I think we've made progress :bike:
 
Whelp...may have been chasing my own mistake around with this one. After pulling the new pilot jets out again I realized they looked different than the originals. Apparently the rebuild kits I bought came with non-bleed style pilots, while these carbs actually need the bleed type (holes in the sides). Float height was fine, and there was plenty of gas in the bowls; so this probably explains why the bike seems to starve for fuel when the enricher is closed. I have a new set ordered up as I didn't want to put the nasty old pilots in the spotless carb. Fingers crossed this gets it going!
 

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