'78 XS750 no start - popping and backfiring

[beard_thicc]

Bought this bike as a non-runner. Previous owner said he had it running great for a week, then it just decided not to run anymore. I figured he'd clogged the carbs, as he wasn't running in-line filters, but I think I've been way off the mark.

I have verified compression on all three cylinders, as well as spark. When I spray starter fluid into the cylinders (no matter if carbs are on or off), and try to start, the bike will pop and backfire, sometimes shooting flames out the exhaust.

My first line of thinking was the timing jumped, but when I pulled the top cover, I found the chain to be in place and tight. Valve timing appears to be all good as well.

So as far as I can tell, this is an ignition timing issue. Problem is, no matter what I adjust or fiddle with, the symptom doesn't change.

So far I have done the following:

• Adjusted the timing plate using a timing light.
• Played with the timing plate in all sorts of positions.
• Pulled the governor assembly and verified the pin on the crankshaft is in place, and the governor is seated on it (as suggested by a few posts here)
• Flipped the governor 180 degrees (also as suggested by a few posts here)
• Cleaned out the entire timing plate / governor assembly

At this point I'm completely stumped, and I could really use some insight. Hopefully there's just something I've missed here.

Thanks in advance for any help!

[LAB3]

I'm betting it's fuel related, a stuck carb float would dump gas out into the exhaust and a lean condition could make it backfire through the carbs. Another often overlooked part on bikes from this era is the elbow on the spark plug caps, they have a resistor in them that should read about 5k ohms but you might want to verify that for your model. As mentioned, carbs is the first place I'd be looking. Drop the float owls and see if there's any crud in them and if there is then a full teardown and cleaning is in order. Dipping the car bodies in Berrymans Carb Dip or Gunk is well worth the effort and the only thing you REALLY need out of a carb kit is the float bowl gasket and o-rings. Carb kit parts are usually full of knock-off parts and if the old parts can be cleaned you're best off using those.

[beard_thicc]

I'm betting it's fuel related, a stuck carb float would dump gas out into the exhaust and a lean condition could make it backfire through the carbs. Another often overlooked part on bikes from this era is the elbow on the spark plug caps, they have a resistor in them that should read about 5k ohms but you might want to verify that for your model. As mentioned, carbs is the first place I'd be looking. Drop the float owls and see if there's any crud in them and if there is then a full teardown and cleaning is in order. Dipping the car bodies in Berrymans Carb Dip or Gunk is well worth the effort and the only thing you REALLY need out of a carb kit is the float bowl gasket and o-rings. Carb kit parts are usually full of knock-off parts and if the old parts can be cleaned you're best off using those.

Normally I'd agree with you on the floats, but I'm just spraying starter fluid through the tubes at the moment, and the condition stays the same with or without the carbs on. I don't even have fuel hooked up to them at the moment.

I will check the spark caps, but all three spark plugs are getting a nice strong blue spark.

[red bandit]

I'm not familiar with the 750. Are you sure you have the correct plug wire on the correct cylinder?

[beard_thicc]

I'm not familiar with the 750. Are you sure you have the correct plug wire on the correct cylinder?

Yes, I've verified that as well. I also did try switching around the #2 and #3 cylinder wires just to try ssomething. The wire for the #1 cylinder is unmistakable, as it's much longer than the other two to make it around the cam cover on that side.

[Radioguylogs]

If you checked the compression in all 3 cylinders, confirmed the spark in all 3 cylinders, adjusted the ignition timing, and checked the valve timing, then the problem seems to be the carbs.

You already have them off, so it's easy to look inside them.

I wonder if it meaningful to spray lighter fluid into intake without the carbs.

[beard_thicc]

If you checked the compression in all 3 cylinders, confirmed the spark in all 3 cylinders, adjusted the ignition timing, and checked the valve timing, then the problem seems to be the carbs.

You already have them off, so it's easy to look inside them.

I wonder if it meaningful to spray lighter fluid into intake without the carbs.

The carbs are not presently part of the equation as I don't have the fuel hooked up - I'm spraying starter fluid into the carb bodies, which bypasses the inner workings of the carb (aside from the choke and throttle).

I've also pulled the carbs off and sprayed starter fluid directly into the intake tubes, as I stated before.

The carbs may be clogged or malfunctioning, but we're not there yet, as I'm not trying to get the bike to run off fuel yet.

[oldyam80sg]

When you spray starter fluid in the intake the bike won't stay running. I saw that on a you tube video -I believe that is just to see if the bike will turn over. It is not for a long-term effect.

[beard_thicc]

When you spray starter fluid in the intake the bike won't stay running. I saw that on a you tube video -I believe that is just to see if the bike will turn over. It is not for a long-term effect.

Right, the bike will not run on starter fluid for a long time, but it will start and run for a second or two.

It's not running at all. Not even for a second. It's only backfiring and popping.

[DiverRay]

Starter fluid can cause engine failure trying to use just it to run an engine! Try spraying a carb cleaner, Berryman's B12 or Seafoam instead. They are volatile, but much closer to gasoline than just ether. I have had an engine running on just B12 with no carbs while testing with no problems.

[beard_thicc]

Starter fluid can cause engine failure trying to use just it to run an engine! Try spraying a carb cleaner, Berryman's B12 or Seafoam instead. They are volatile, but much closer to gasoline than just ether. I have had an engine running on just B12 with no carbs while testing with no problems.

Oh good advice. Thank you.

[oldyam80sg]

Please update us on this-too many threads leave us hanging. I see either Gumout or whatever AutoZone has in terms of carb cleaner.

[beard_thicc]

Please update us on this-too many threads leave us hanging. I see either Gumout or whatever AutoZone has in terms of carb cleaner.

I'll definitely update on the outcome! I'm having the same problem...many threads about this issue but none with a viable solution (at least in my case)

[beard_thicc]

A small update: I've discovered through other threads that the ignition modules on these bikes for different years can look and fit the same, but not be completely compatible.

With this in mind I looked at mine and found the left side connector has 8 pins on the box side, but only 5 occupied pins on the plug side.

The ignition module is the last variable in this timing system, so I've ordered a replacement that has 5 connectors on the box side. I'm hoping either the wrong box was put in, or maybe just the existing box is bad.

Fingers crossed! I'll update with my findings.

[beard_thicc]

UPDATE! I got my '78 specific CDI box in the mail and threw it on today. The bike wanted to start up the second I cranked it. Even got it to run for a second off starter fluid.

Tried to get it running off fuel but I've got a little work to do on the carbs.

Once I get it running off fuel I'll update here, but I'm very confident the box was the issue.