Desperate for Help!! — 1978 CB750F keeps dying

WaddleDee72

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I'm at the end of my rope with this bike.... I've been chasing this problem for a year, and in that time I've spent countless hours reading through forum posts, replacing parts, and taking it to different mechanics, all to no avail.

Here's the issue: My bike starts up and runs fine, but on a longer ride (45 minutes or more), it will seem like it's running out of gas. It will sound like one or two cylinders drop out, and if I keep pushing it for a minute or two, the engine dies completely. If I pull over and wait for 10-15 minutes, it will start up again (roughly), and I will be able to go another mile or two before it dies again.

It's more likely to act up under higher loads. If I'm cruising around at 40 mph on surface streets, it's usually fine. But if I'm on the freeway going up a hill in 5th gear at 5k+ RPM, that's usually when it gives up on me. Another point: if I give it full throttle when the cylinders start to cut out, they come back to life and the bike runs perfectly (for a bit before it dies). <That makes me think it's a carburetor issue, but my Honda mechanic is adamant that's it's a problem with the ignition.

The previous owner installed a Dyna S electronic ignition, and I've replaced both coils, thinking that would help, but it made no difference.

Any ideas from the CB750 owners' knowledge base?
 

kirky

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is it getting enough petrol flow for all the carbs when it's working hard, you say if you rev it when it's dying it picks up because it's getting the extra juice through the jets. is there some gremlin moving about in the tank? as daft as it is the petrol can absorb moisture and a water bubble might be hiding in the filter, sounds crazy but i had this with a chainsaw and it drove me mad until i found it hiding in the filter. every time i wanted the saw to rev it bogged out because of the water bubble not letting enough petrol through...other than that i don't know. my problems pointed to electrical when bogging out at 5000rpm ,cracked coil and melting boxes. but it was carb trouble. i asked the builder of the carbs to put 102 jets in instead of the 75's i took out because all my books said that, then i found out it's a JDM and needed the 75's to correspond with the smaller air intake tunnel than the other models. i sorted it by taking the rectangular filter cover off allowing it to pull the air in around the side panel. don't give up and think of the things you would say were impossible, good luck
 

BAHNST

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Does sorta sound like ignition...but could be fuel related;).
Are you running a inline filter? How is it routed is so...any sharp bends in the hose?
 

dirtdigger

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There was a time when the Dyna ignitions did have a chance of failure. Normally the pickups would die but I had one that quit then started working again. So dont rule out the ignition entirely.
 

madmtnmotors

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>What kind of credentials does your "Honda mechanic" have? Is he a mechanic for a Honda dealer, or just a guy that works on motorcycles?
>Have you checked/replaced the fuel filter inside the tank?
 

Scootch

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Have you checked the alternator/charging system? Might be a long shot. Check the battery voltage with the bike running and at a few thousand RPM's. At higher RPM's the battery voltage should be around 14+ volts. At idle the voltage will probably drop. Check the voltage with the headlight on and off. What does the battery voltage read with the bike off? I have a bike that had one or two rectifier diodes go out. The bike would partially charge unless the headlight was on for a period of time. The longer the bike was ridden the more the battery would get drained...
 

WaddleDee72

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Does sorta sound like ignition...but could be fuel related;).
Are you running a inline filter? How is it routed is so...any sharp bends in the hose?
Yes, there is an inline fuel filter. The filter itself has a 90º bend, but there aren't any sharp bends in the hose.

madmtnmotors said:
>What kind of credentials does your "Honda mechanic" have? Is he a mechanic for a Honda dealer, or just a guy that works on motorcycles?
>Have you checked/replaced the fuel filter inside the tank?
He's been the head mechanic at a Honda motorcycle dealer for decades and owns a CB750 himself. He's very knowledgable, but then again, even the best mechanics are wrong sometimes.
I replaced the petcock last year (which the tank filter is attached to), so it's fairly new. To be sure though, I drained the fuel tank onto a paper towel, and the towel had no dirt or debris on it. The tank also had a steady flow.

Scootch said:
Have you checked the alternator/charging system? Might be a long shot. Check the battery voltage with the bike running and at a few thousand RPM's. At higher RPM's the battery voltage should be around 14+ volts. At idle the voltage will probably drop. Check the voltage with the headlight on and off. What does the battery voltage read with the bike off? I have a bike that had one or two rectifier diodes go out. The bike would partially charge unless the headlight was on for a period of time. The longer the bike was ridden the more the battery would get drained...
I don't think it's the charging system because a couple weeks ago, I went to start the bike and it wouldn't crank. (The battery was dead because I hadn't ridden in months). I kickstarted it to life, and the lights were really dim. However, at the end of my ride, the lights were at full brightness, and the bike had plenty of power to crank the starter. That would indicate to me that the alternator is in good shape.

Tonight, I just replaced the Dyna S ignition plate and set the timing. I'll keep you posted if that fixes the problem. I also noticed something unnerving—after running the engine for a minute, I poured water on each exhaust pipe. On cylinders 1-3, the water turned to steam, but on cylinder 4, it didn't. That makes me think cylinder 4 isn't firing. And since I've replaced the Dyna S and coils, I would guess there's a problem with the #4 carb. What do you guys think?
 

madmtnmotors

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Yes, there is an inline fuel filter. The filter itself has a 90º bend, but there aren't any sharp bends in the hose.

Are you are running an inline filter and the in-tank filter?


I replaced the petcock last year (which the tank filter is attached to)

The fuel filter is not "attached" to the petcock. There is the reserve tube that extends out the top of the petcock and the filter slips down over that. The filter often stays inside the tank when the petcock is removed and usually takes deliberate effort to extract it. If you installed a new filter in the tank, then the old filter would have had to be removed


In your original post it sounds like it's starving for fuel at higher demand. Had the exact same symptoms with a clogged filter, acted like it was running out of gas. Could be a plugged vent in the cap, or the inline filter not rated for the flow required under higher demand too.

In your later post you have narrowed down part of the problem to the #4 cylinder which could be a bad plug wire, a bad plug, clogged carb, or any combination of these three. Are you still having the stalling at highway speed under high demand load? If so, then it sounds like multiple issues.
 
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WaddleDee72

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The gas cap is definitely not the problem because I've ridden without it and the bike still dies.

My petcock broke awhile back, and since Honda doesn't make them anymore, I got one of these. Here it is inside the tank:
in-tank.png

Fuel couldn't flow more quickly out of it, so I don't think it is the issue

The inline filter however had some crap in it, and the flow was slightly impeded.
IMG_3576.jpg

I just ordered a new one, so hopefully that helps.


But more importantly I found out why cylinder 4 wasn't firing...

float-bowl.jpg

jet.jpg

How do I keep this from falling out again? It doesn't have any threads!
 

dirtdigger

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The idle jets are a press fit in the carb body. take a plastic hammer or even a heavy screw driver handle and tap it back into place. If its loose you will have to try a new jet see if it stays.
 

Geraldth

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Yes, there is an inline fuel filter. The filter itself has a 90º bend, but there aren't any sharp bends in the hose.


He's been the head mechanic at a Honda motorcycle dealer for decades and owns a CB750 himself. He's very knowledgable, but then again, even the best mechanics are wrong sometimes.
I replaced the petcock last year (which the tank filter is attached to), so it's fairly new. To be sure though, I drained the fuel tank onto a paper towel, and the towel had no dirt or debris on it. The tank also had a steady flow.


I don't think it's the charging system because a couple weeks ago, I went to start the bike and it wouldn't crank. (The battery was dead because I hadn't ridden in months). I kickstarted it to life, and the lights were really dim. However, at the end of my ride, the lights were at full brightness, and the bike had plenty of power to crank the starter. That would indicate to me that the alternator is in good shape.

Tonight, I just replaced the Dyna S ignition plate and set the timing. I'll keep you posted if that fixes the problem. I also noticed something unnerving—after running the engine for a minute, I poured water on each exhaust pipe. On cylinders 1-3, the water turned to steam, but on cylinder 4, it didn't. That makes me think cylinder 4 isn't firing. And since I've replaced the Dyna S and coils, I would guess there's a problem with the #4 carb. What do you guys think?
Is # 4 the carb that attaches to the fuel peacock (only 1 attaches to petcock )to provide vac to pull the rubber diafram open to allow gas to flow to carbs also if diafram rots to won’t allow fuel to carbs or will send crazy amount of fuel to the carb which provides vac would certainly keep that cylinder from running?
 

user8086

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Double check plugs,wires, coil etc... If you have a multimeter you can check your coils etc, I think you found your main issue right there.. when something cuts out or stops working in electrical it's normally bad (loose) grounds etc... What about a compression test? Has that been done on all cylinders?
 

madmtnmotors

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Is # 4 the carb that attaches to the fuel peacock (only 1 attaches to petcock )to provide vac to pull the rubber diafram open to allow gas to flow to carbs also if diafram rots to won’t allow fuel to carbs or will send crazy amount of fuel to the carb which provides vac would certainly keep that cylinder from running?
The 78' CB750 (and earlier models) do not use a vacuum diaphragm petcock.

The idle jet falling out would certainly cause drivability issues. If, after tapping it back in place, and it still won't stay, you can source a new one:


If the new one turns out to be a loose fit, then the carb body may be stretched ever so slightly at the idle jet mounting point. If this turns out to be the case you may have to get creative. I'm thinking maybe a small punch point angled downward about 45 degrees in the side of the brass to help provide more of a "friction fit", just don't get carried away. The idea would be to create an ever so slightly raised dimple in the brass. Everything you are working with is very soft material and the mantra "less is more" would certainly apply here. ;)

Maybe even "tin" the press fit portion of the brass with solder? Again, don't over do it, and the solder could even be sanded down a little for a perfect fit.
 
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WaddleDee72

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The idle jet falling out would certainly cause drivability issues. If, after tapping it back in place, and it still won't stay, you can source a new one:

Thanks for the link @madmtnmotors! I ended up resorting to the easiest (laziest) solution, which was to put a few drops of blue threadlock on the jet before tapping it back in. I waited at least 24 hours before starting the bike, and after taking it for a long ride today, the jet stayed in. If it falls out again though, I will surely be taking your advice

daveyshreds said:
Double check plugs,wires, coil etc... If you have a multimeter you can check your coils etc, I think you found your main issue right there.. when something cuts out or stops working in electrical it's normally bad (loose) grounds etc... What about a compression test? Has that been done on all cylinders?
I've checked all the connections from the Dyna S to the coils. I'll check the plugs again. The coils are new. I can also borrow a friend's compression tester and see if that yields anything.


What I am dismayed to report is that even with the jet secured, a new Dyna S ignition, and a new inline fuel filter, the bike still suffers from the same rough running/dying problem.

After limping it home, I hooked up a timing light to each cylinder and found that 2 & 3 looked fine, but 1 & 4 were jumping around—meaning that when I pointed the timing light at the timing marks, it flashed at a different point each cycle, giving a jittery sensation. Even though the coils are new, I swapped them just to be sure, but that resulted in no change (2 & 3 still looked stable and 1 & 4 still looked jittery). It could just be that timing lights are somewhat erratic.

Anyways, I ordered a set of carb rebuild kits today, so I will rebuild the carb as a next step, but if that doesn't resolve the issue, I'm out of ideas and the bike might find better use as a boat anchor 😜
 

user8086

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There aren't.. there should be a S...a, F...and a T.... When piston 1 and piston 4 are at top dead center on the compression stroke.. timing is correct when the projection on the pulse generator and the rotor lines up... The f is your timing mark running at about 1000-1200 RPM.... That is for double overhead cam 750s I believe up until the eighties I would have to check my service manuals
 

WaddleDee72

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There aren't.. there should be a S...a, F...and a T.... When piston 1 and piston 4 are at top dead center on the compression stroke.. timing is correct when the projection on the pulse generator and the rotor lines up... The f is your timing mark running at about 1000-1200 RPM.... That is for double overhead cam 750s I believe up until the eighties I would have to check my service manuals
E39D1642-A404-4D8F-9E01-25C0CEAC2E31.jpeg

This is from the service manual for my bike. Perhaps yours is different?
 

dirtdigger

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There aren't.. there should be a S...a, F...and a T.... When piston 1 and piston 4 are at top dead center on the compression stroke.. timing is correct when the projection on the pulse generator and the rotor lines up... The f is your timing mark running at about 1000-1200 RPM.... That is for double overhead cam 750s I believe up until the eighties I would have to check my service manuals
This post is about a 78 750 with dyna ignition which is not dohc so they are correct 1-4 and 2-3 are time separately.



View attachment 12214
This is from the service manual for my bike. Perhaps yours is different?
You are correct in assuming you need to check timing on both 1-4 and 2-3. They are timed separately. Your timing light should work the same on both....the dyna pickups are generally reliable but they do fail.
 
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