Cylinder #3 not firing 78 GS750

[ledman_70]

I've been working on this bike on and off for several months, but I'm at the end of my expertise now. Motor has 32k and only runs on cylinders 1,2 and 4. I have tried everything I know, so here's the list. When the motor is running pulling wire 3 makes no difference, yet putting a plug in that wire and grounding it gets a great spark. I can put my hand over the other 3 carbs and kill the motor, but on 3 it sucks my hand against the carb, but has no effect on it running. Compression is 135-145 on each cylinder. Had tried 3 different sets of carbs, all have been cleaned, same result, no 3 not firing. Pull the plug and its wet. Pulled the head off, nothing obviously apparent.  Reground the valves, new seals removed all the carbon on head, valves, pistons. With pistons 2 and 3 at bottom, I poured 2 inches of Marvel into the cylinders and 4 days later it was all still there. That tells me the rings are sealing. Put it back together, tried a different set of carbs which have been cleaned, no difference.  Even changed the coil with no effect. I am really stumped here, I have worked on bikes most of my life and never ran into a problem yet that defies trouble shooting like this one. So tell me what an idiot I am and how I overlooked something obvious!

[ledman_70]

Also forgot new electronic ignition.

[Arttu]

Sounds like a mystery indeed...

• Even compression and sucking air through the carb are quite sure indicators that all is good with valvetrain and piston/cylinder.
• Trying different carb sets and having a wet plug pretty much outrule fuel side problems.
• Having spark and trying different coil cover a good part of potential ignition issues too.

Based on symptoms I would still look on the ignition side. I guess you have changed the plugs too? And plug wire / boot wth coil? Even if the plug gives good spark outside of the engine it can fail under the compression.

For fuel side you could still try spraying starting spray into carb inlet when the engine is running and see if the cylinder gets alive. Bad enough air leak could possibly prevent firing even if the carb is feeding fuel.

Out-of-box idea: Have you checked that the exhaust header isn't blocked?

[Dezza]

If you switch the HT leads between 2 and 3, does it still fire on 2? Or does the problem move?

[Rijko]

The fuel tap vacuum hose is on nr. 3, is it connected to the petcock ?

[Dennisgs1085]

I would agree with the vacuum pipe from carb 3 to fuel tap also. Block that pipe and turn tap to prime to see if it runs on 4 cylinders. Don't leave tap on prime without engine running. Check the tap works as it should and shuts off with engine off.

[Captain Chaos]

Still not clear if the spark plugs are new

[Rijko]

You did not mention replacing the base gasket and o-rings.
These will often develop a leak when disturbed by taking off the head.

[Gixer1460]

Seems OP doesn't want to engage - hasn't been back since !

[ledman_70]

Benn very busy with the bikes guys, but I appreciate all the input. Yes, new plus, yes swapped coil and wire. Tanks is removed, feeding from small can and vaccum hos on #3 is capped. Here's the kicker, I pushed it aide and dragged the 78 GS1000 in to work on it and it's doing the same thing on #4. Head has not been off, but coils swapped, new plugs, carbs swapped. Put my hand on the carb and the motor doesn't die, but it does if I cover 3. I will try swapping the 2 and 3 leads tomorrow as suggested. This whole thing is really blowing me away. I have wrenched on bikes since the 70's and never ran into this mess before!

[ledman_70]

I didn't replace the base gasket, after letting it sit for 4 days with cylinders full of marvel, I figured the rings had to be good since none leaked out.

[Dezza]

What's marvel (apart from us comic book enterprise)?

[Gixer1460]

Marvel Mystery Oil - seems to cure / fix a multitude of problems - https://marvelmysteryoil.com/pages/our-story

[ledman_70]

So are you guys as baffled as I am? I have no idea which direction to go here, the problem is obviously not the carbs. Compression is not the best, but certainly good enough to run.

[ledman_70]

So I pulled the exhaust off #3 and ran a snake through it, no obstructions. I have no idea which direction to go now. This is the same issue as the 750 has, just a different cylinder. If I can fix this one then the 750  should respond to the same repair and both bikes will run. The 750 has been restored except for this issue.

[Gixer1460]

Using the old Sherlock motto " when you have exhausted the plausible and practical solutions, whatever is left however implausible or impractical will likely render the solution" So, do you have a rodent issue / problem in Ohio ? The little critters seem to love chomping on standing wiring looms and other automotive parts! 

Edited November 22, 2023 by Gixer1460

[ledman_70]

If only, but no evidence of that, either.

[Arttu]

Have you tried to swap #2 and #3 plug wires already? That should cover pretty much all the electrical faults. And that starting spray into non-running cylinder carb throat is worth of trying too if you haven't done that.

I think at this point all remote diagnosis starts to be pretty much blind guessing. But sometimes that can help too...

[ledman_70]

I have tried ALL of these suggestions to no avail. I have swapped the car bank between the 750 and 1000. Carb #3  from 750 bank won't fire on 750, but does on 1000. Same with carb #4. That tells me it's not a carb issue. Swapped coils and wires between both bikes with no impact, either.  Also sprayed starting fluid into the non firing cylinder with effect.  Covering the back of the carb on either bike will kill the engine, except for the non firing cylinder. The vaccum will suck my hand against the dead cylinder  without changing the engine, but it will get gas on my hand like the carb is blowing instead of sucking!  This just beats the hell outta me. I've wrenched on cars and bikes for almost 50 years and NEVER been baffled like this before!

[TonyGee]

5 minutes ago, ledman_70 said:

I have tried ALL of these suggestions to no avail. I have swapped the car bank between the 750 and 1000. Carb #3  from 750 bank won't fire on 750, but does on 1000. Same with carb #4. That tells me it's not a carb issue. Swapped coils and wires between both bikes with no impact, either.  Also sprayed starting fluid into the non firing cylinder with effect.  Covering the back of the carb on either bike will kill the engine, except for the non firing cylinder. The vaccum will suck my hand against the dead cylinder  without changing the engine, but it will get gas on my hand like the carb is blowing instead of sucking!  This just beats the hell outta me. I've wrenched on cars and bikes for almost 50 years and NEVER been baffled like this before!

time to pull the top end off for more investigation me thinks !!!!!!  

[Rijko]

Since we're kinda guessing and you have already taken off the head, reground valves etc : assuming you set valve play and rechecked compression ....

Did you replace the o-rings on the carb-to-cylinder boots ?
Maybe test the seal on number 3 by spraying brake cleaner or starter fluid on the boot.
Any change in rpm, up or down, indicates a leak.

Are the 4 carb sync bolts in the inlet manifolds present ?

Got a strobe light ?
Neat use for that thing is connect it to the problem cylinder and just look at the flickering light, no need to point it at your timing marks.
Multiple issues like a broken spark plug, cap, bad connection to the cap, etc show up clearly.

Either by not flickering at all, or clearly irregularly.

Stuff i would not have noticed because i saw a nice spark were clearly visible this way.

Got a 'known good' situation where the bike ran on all 4 ?
Maybe with the points setup ?

Got good 12V at the ignition and coils ?

[Rijko]

How did you spray starting fluid into the non firing cylinder ?
These VM carbs do not like to run without airbox much and you sayin' hand against the carb makes me ask.

What happens if you start the bike and spray some fluid into the nr. 3 vacuum hose with the airbox on ?

Does applying the choke on and off and giving it a bit of throttle make any difference ?
 

Edited November 27, 2023 by Rijko

[ledman_70]

New o rings on the intake boots, all 4 sync screws tight in the boots,  tried starting fluid in the back of carb and around the boot. Starts on choke, but never runs smooth. Got the bike from the guy that bought it out of a closed shop with no history.  New electronic ignition, i DO have a timing light, so I will try that suggestion. Bike hasn't ran smooth since I've had it. The only thing I haven't seen with my own eyes is the rings, and when the head was off I poured a couple ounces of Marvel in cylinders 2 and 3 and let it sit for 4 days. None of it leaked out, so that tells me the rings must be OK. I will also try the staring fluid in the #3 hose suggestion, too. The carbs are all open in back and 3 of them run, so I don't think that's the issue.

[ledman_70]

I really appreciate all you guys pouring over my problem since I have obviously exhausted my abilities here.

[redgs1000]

I would check the valve clearances as they might have tighten'd up on no3 if your getting fuel spitting back . As above if you've tried easy start and you've got a spark there is no reason it shouldn't run ,