Topic: CB750 not Running Right

[Bsawyer1]

Hello everyone,

So i finally got my 1978 CB750 Running today. I've been troubleshooting throughout the past couple months on my free time on why i couldn't get it started. i succeeded Wooot Wooot.

Now i ran it for 2-3 minutes, and i noticed that 3 of the exhaust pipes were really hot, and one was semi-warm. and its running little ruff, Now in hind sight i have not synced the carbs. im going to try doing that next,but  im just wondering if that could be the reason 1 of the cylinders isn't running right or might there be other reasons its not running right.  want to possible figure this out before fully getting everything buttoned up , to just find out i need to pull the engine lol.  (Im slowly learning working on this bike im no mechanic)

Now a list of things Ive done to the bike to give you guys some helpful knowledge, to help with weening out questions.

The Exhaust pipe that is semi warm is the far right pipe on the kick starter side.
Everything's brand new.

Wiring harness, Coil packs, Regulator/rectifier, fuse box, all handlebar switches, Condensers on contact points, new spark plugs, Carbs have rebuilt with new floats (only thing i didn't replace was the Needle jets)

Ty everyone that helps with my question =) Love you all!

[steeldoncb550]

My first guess would be something with the carbs. What carb rebuild kit did you use? Did you just change the jets and left the needle and the needle seat? Sometimes needles might be a tiny bit longer or shorter but the seat is matched to that needle. So if you just changed the needle but used the stock seat or vise versa that could be an issue. Also you put in new floats did you set the float heights right?

[Bsawyer1]

My first guess would be something with the carbs. What carb rebuild kit did you use? Did you just change the jets and left the needle and the needle seat? Sometimes needles might be a tiny bit longer or shorter but the seat is matched to that needle. So if you just changed the needle but used the stock seat or vise versa that could be an issue. Also you put in new floats did you set the float heights right?

i just used a stock carb rebuilt kit dont know the brand. and i dont see how the needle would be a problem when i didn't change any of them. and yes all the floats were gaped properly before install

[Scott S]

 Fuel, air and spark.

 Check that plug. Is it dry? Is it wet with fuel? Do you have spark on that plug?
 Is there fuel in the bowl of that carb?

[Bsawyer1]

Fuel, air and spark.

 Check that plug. Is it dry? Is it wet with fuel? Do you have spark on that plug?
 Is there fuel in the bowl of that carb?

i pulled all the plugs after i ran it to check if there was anything diffrent between them all and they all look the same. all smell like gas but not wet. bowl def has gas in it, checked the bleeder and came draining out fine. don't know if it makes a difference but im running all 4 Pod Filters

[disco]

I had a similar situation with a CB750 that just felt “not right”. Often dropping the No1 cylinder. Finally tracked it down to the reproduction slow jet. When I put the guitar string thru it I could feel a ‘step’ in it. Like it wasn’t machined properly. Replaced it with the original Keihin #40 slow jet and it now runs beautifully!

[Spanner 1]

Does the 'warm' cyl. pick up when you rev the motor, or did you just test it at idle ?  If sync is off @ idle then very possible 1 cyl. carb slide is bottomed out = no run/ barely run.

[Keith]

I would check all specs first: valve adjustment, dwell, timing, vompression tedt if you have a tester. This will save hours of frustration.

[Bsawyer1]

I had a similar situation with a CB750 that just felt “not right”. Often dropping the No1 cylinder. Finally tracked it down to the reproduction slow jet. When I put the guitar string thru it I could feel a ‘step’ in it. Like it wasn’t machined properly. Replaced it with the original Keihin #40 slow jet and it now runs beautifully!

Ill have to look into that, guess ill pull the carbs and check all the jets just to make sure

Does the 'warm' cyl. pick up when you rev the motor, or did you just test it at idle ?  If sync is off @ idle then very possible 1 cyl. carb slide is bottomed out = no run/ barely run.

Honestly cant tell, and i haven't sync'd the carbs yet that's next on my list of things to do. Ive been rebuilding this bike from frame for past year. so its been a learning process. just got it running today

I would check all specs first: valve adjustment, dwell, timing, compression test if you have a tester. This will save hours of frustration.

I will be checking the compression but i gotta wait for my compression tester to come in. still in shipping

[HondaMan]

You have the PD42 carbs on that bike, if OEM.
Inside the idle air screws' holes there is a tiny washer, and above that is a tiny(er) O-ring. When you put this back together, did you make sure the O-ring went in first, and then the washer, before the air screw? This can make the idle adjustments uneven.

The PD42 carbs are 'lean-burn' carbs, often with #37.5/#38/ or even #35 pilot jets. The air screws are stated to be at 1.5 turns out from stock, but this is to ensure California CARB standards, not happy idling. You might try 1-3/4 or slightly more turns out for #35 jets, if you have those.

Another tip: if the jets are from a Keyster carb kit: they are about 10% leaner than the number stamped on the jet. Other than that, they work OK. Their jet needles are WAY, WAY off: for reference, if you have their needles over #110 mainjets, you must go to #150 mainjets to get the same air-fuel mix ratios, but then it goes very rich above 7000 RPM.

The pods will NOT help you...

[Bsawyer1]

You have the PD42 carbs on that bike, if OEM.
Inside the idle air screws' holes there is a tiny washer, and above that is a tiny(er) O-ring. When you put this back together, did you make sure the O-ring went in first, and then the washer, before the air screw? This can make the idle adjustments uneven.

The PD42 carbs are 'lean-burn' carbs, often with #37.5/#38/ or even #35 pilot jets. The air screws are stated to be at 1.5 turns out from stock, but this is to ensure California CARB standards, not happy idling. You might try 1-3/4 or slightly more turns out for #35 jets, if you have those.

Another tip: if the jets are from a Keyster carb kit: they are about 10% leaner than the number stamped on the jet. Other than that, they work OK. Their jet needles are WAY, WAY off: for reference, if you have their needles over #110 mainjets, you must go to #150 mainjets to get the same air-fuel mix ratios, but then it goes very rich above 7000 RPM.

The pods will NOT help you...

ty for the words of wisdom and i do have the  pd42 carbs, i did put the washer in after the o-ring so atleast i did that right lol, on a side note whats bad about the pod air filters? to much air flow??

[scottly]

Does the 'warm' cyl. pick up when you rev the motor, or did you just test it at idle ?  If sync is off @ idle then very possible 1 cyl. carb slide is bottomed out = no run/ barely run.

1+ Try reving the motor a bit: if the #4 cylinder's pipe suddenly gets a lot hotter, you have a carb sync problem.

[Bsawyer1]

Does the 'warm' cyl. pick up when you rev the motor, or did you just test it at idle ?  If sync is off @ idle then very possible 1 cyl. carb slide is bottomed out = no run/ barely run.

1+ Try reving the motor a bit: if the #4 cylinder's pipe suddenly gets a lot hotter, you have a carb sync problem.

I will give it a try soon going to read the manual on syncing the carbs tonight, i got the syncing tool so im in good water there ty for the help ill let you know what i find

[ekpent]

 While your at it on the carbs swap a good known plug in the cylinder that is not burning hot to make sure you do not have a dud in there.

[BenelliSEI]

#1 to ekpent! I’ve beat this to death, but I’ve had several bad NGK D8EA Plugs (real NGKs, not fakes). Swap that plug out first........ probably OK, but start there first.

[Keith]

Me too! I listened to Hondaman, and switched to Denso...ahhhhhh!

[Airborne 82nd]

You have the PD42 carbs on that bike, if OEM.
Inside the idle air screws' holes there is a tiny washer, and above that is a tiny(er) O-ring. When you put this back together, did you make sure the O-ring went in first, and then the washer, before the air screw? This can make the idle adjustments uneven.

The PD42 carbs are 'lean-burn' carbs, often with #37.5/#38/ or even #35 pilot jets. The air screws are stated to be at 1.5 turns out from stock, but this is to ensure California CARB standards, not happy idling. You might try 1-3/4 or slightly more turns out for #35 jets, if you have those.

Another tip: if the jets are from a Keyster carb kit: they are about 10% leaner than the number stamped on the jet. Other than that, they work OK. Their jet needles are WAY, WAY off: for reference, if you have their needles over #110 mainjets, you must go to #150 mainjets to get the same air-fuel mix ratios, but then it goes very rich above 7000 RPM.

The pods will NOT help you...

ty for the words of wisdom and i do have the  pd42 carbs, i did put the washer in after the o-ring so atleast i did that right lol, on a side note whats bad about the pod air filters? to much air flow??

He may be talking about the wrong type of air flow

[scottly]

The use of pods does not explain why one cylinder runs differently than the other three.

[Bsawyer1]

#1 to ekpent! I’ve beat this to death, but I’ve had several bad NGK D8EA Plugs (real NGKs, not fakes). Swap that plug out first........ probably OK, but start there first.

I'll have to do that. ive been battling my carbs all day today been 1 stupid thing after another taking a break for the day hopefully i can get it running and sync the carbs tomarrow when im refreshed and battery is recharged.

On a side note i got my compression tester today cylinders were as fallowed.

1. 145
2. 140
3. 135
4. 135

This was all done doing kick starter (so values might be more even i did 12 kick each cylinder), i cant seem to get my electric start to work atm. (still another thing to do on my list lmao)
I'll keep everyone posted on my progress tomarrow will be another long day working on it since i got the time

[PeWe]

Air vents have been up before on this kind of carbs. If blocked not good.
I'm sure these are easy to miss.
Clean the air vents in carbs and hoses connected to them  not blocked/kinked. (This is not the overflow hoses)

I guess it is possible to carefully grab the vent tube and blow into it to verify flow or blocked.

I did that with the older carbs (K6) afterwards with compressed air after tubes had got a cleaner injected. (Drive chain cleaner spray work really fine)

Air box got some fuel when flow worked with compressed air, even not that hard pressure used
Upgraded the airbox with small holes in each corner to let fuel out 1.5-2mm. This will evacuate in case of overflowing carbs and make that visible.
An airbox with fuel feels not safe.

[Bsawyer1]

Run into a new problem for some reason my #2 carb the main 1 is leaking from somewhere and i cant find the source. the only time it starts to leak is when i pump the gas with the throttle. kinda seems like its leaking from the accelerator pump which i never replaced so its very possible.

what do you all think?

Air vents have been up before on this kind of carbs. If blocked not good.
I'm sure these are easy to miss.
Clean the air vents in carbs and hoses connected to them  not blocked/kinked. (This is not the overflow hoses)

I guess it is possible to carefully grab the vent tube and blow into it to verify flow or blocked.

I did that with the older carbs (K6) afterwards with compressed air after tubes had got a cleaner injected. (Drive chain cleaner spray work really fine)

Air box got some fuel when flow worked with compressed air, even not that hard pressure used
Upgraded the airbox with small holes in each corner to let fuel out 1.5-2mm. This will evacuate in case of overflowing carbs and make that visible.
An airbox with fuel feels not safe.

My air vents are fine i cleaned them all out and they have no blockage i tested it when i first cleaned them. they soaked for like a week with carb cleaner and then i spent a day cleaning all the air vents and ports out to make sure there was good airflow. Possible something came loose and blocked 1 but i dont think air is my problem atm