• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

1978 cb750k cylinder 4 not always firing?

Mikejk

CB750 New member
Messages
2
Reaction score
0
Points
1
Location
Detroit
I've been tinkering with this bike for years (cosmetically). PO put pods, 4 to1, and dyna ignition (that i know of). I'd like to sell it but i want it to run right before i put it up. Couple years ago i fired it up to take it for a ride and it was running rough, chalked it up as dirty carbs and it has been sitting since. Made my decision to pass it along so i took it to a "carb guy" over the weekend. Long story short, it runs better but backfires and pops around 5k rpm. I checked timing, carb sync, spark, all good. Haven't done a compression test yet, didn't have the right adapter. But i noticed #4 exhaust is not as hot as the rest. Pulled plug, double check spark, good. Put plug back in, fire the bike up, don't hook up the plug wire and the bike runs even better with no backfiring. So it's running better on three cylinders instead of four? What is going on here?
 
Compression test before you waste another cent. It's always funny the numbers of people that think stone age bikes have nothing wrong with engines while tearing the carbs down over and over. Then the carb work commonly makes them run worse because the best carb work on the planet will not touch basic engine condition issues so then the whacky 'cures' begin.

If they run ethanol in the local fuel the carbs could well be fragged, it eats carbs when the bikes sit. The 'couple years' thing............
 
Yes do compression test to confirm, betting it is still a restricted circuit in carb. Could have sucked up junk from the tank or didnt get it clean the first time. Also...if using dyna coils they do go bad...just because you have spark when you check it doesnt mean you have spark when it is under a load. Carb sync after carb "rebuild"?

Cant backfire if you dont have spark in the cylinder you didnt hook back up
 
'So it's running better on three cylinders instead of four? What is going on here?'

It's a perception issue that drags the person into what was before a purely mechanical discussion. It as well cannot be true.

As well, just to throw another wrench in there, if wire stayed close to plug even though not on the top of it then a strong ignition coil can make the spark jump right through the rubber to the top of plug to then be a higher volt than before due to the bigger gap and fires correctly then when it didn't before with wire on correctly. I used to place plug wires on an old Kaw 3 cylinder like that when the plugs fouled to get it back running right long enough to drive a bit and then plug the wires back up the right way and good to go again. Cheap man's plug change.
 
'So it's running better on three cylinders instead of four? What is going on here?'

It's a perception issue that drags the person into what was before a purely mechanical discussion. It as well cannot be true.

As well, just to throw another wrench in there, if wire stayed close to plug even though not on the top of it then a strong ignition coil can make the spark jump right through the rubber to the top of plug to then be a higher volt than before due to the bigger gap and fires correctly then when it didn't before with wire on correctly. I used to place plug wires on an old Kaw 3 cylinder like that when the plugs fouled to get it back running right long enough to drive a bit and then plug the wires back up the right way and good to go again. Cheap man's plug change.

Definitely ran better on three instead of four without plug wire even close to plug. Not saying it ran great but the backfiring went away and things seemed to smooth out. That being said, I couldn't acquire the correct adaptor for my compression tester so I figured I might as well adjust the valves... # 4 seemed a little off. Made the adjustments and it seemed to clear up the backfiring and roughness. However, it still bogs around 5k rpm under load.
 
Checked coil resistance? Plug wire resistance? The dyna coils do go bad. Have you checked and verified the ignition advance works and works correctly? Carb boot condition? Checked for vacuum leaks...common. Compression check still top of the list....especially if valve adjust seems to have helped...may have done some damage. Bike has pods and 4 to 1, have you jetted for this condition? Mainly needle position and main jet but low end bog with pods and exhaust sometimes require a small cut be taken off the bottom of the slides to get it perfect. FIRST make sure engine mechanically sound, then go threw and do all tune up items then adjust jetting to aftermarket components...pods and exhaust.
 
Back
Top