Topic: 74 CB750 sputtering and choking when running and idling (update from last year)

[jembers]

I had previously posted here (http://forums.sohc4.net/index.php/topic,184964.msg2141616.html#msg2141616) looking for some advice and have since explored a few things to no improvement. I'm hoping with some explanation and video that you guys may have some advice on where to go next.

Here is quick rundown

• end of last riding season here in Ontario, bike started acting up while on a longer ride — started choking when riding at a decent speed in 3rd or 4th gear. Riding partner switched plugs 2 and 3 and I don't know if it was coincidence but I managed to get home without any problems

• the next day I attempted to go on another ride and only got about 10 minutes into the ride and it started acting up even worse than the day before — same choking — barely made it home and had to keep flicking the throttle at stop signs and lights to stop from stalling out

• last year before getting ready to store, I pulled the caps and cut a small amount of the wire, rethread the caps and installed new plugs. I also removed the coils and switched them out with coils from another CB750 I have. Also had someone help me check the timing. Even after these I had the same issue

• topped up fuel and put some seafoam in it over the winter and didn't start it once

• just last week pulled the bike out, had to swap out a fuse because the 15A blew, and took the bike out to see how it acted. Before storage I couldn't get down my road without the sputtering and choking setting in, last week for the first ride it managed to go a few km's in my neighbourhood, but then randomly started acting up again

• quite often when going up and down my road it'll produce a high pitch pop from the exhaust (I suspect it is from 3) and smoke

• at idle, I did notice pipe 3 sputtering and coughing — it was definitely acting different than the rest.

• pulled the carbs, cleaned them, reinstalled but am still having the same issue

Here are some vids of the bike sitting at idle to see what's happening. This same thing is happening when I ride, especially when I slow down to a stop sign. If I don't keep flicking the wrist the bike will stall out. When giving it some throttle going down the road it produces the same issue – even when flicking the throttle the rpm's will drop — it almost feels like it's gasping for fuel or something. I don't know...

1. Any opinions on what the problem could be?

2. Any opinions on what to check next? I think someone is going to help me with some valve clearance, compression testing, carb syncing and timing, but these were all done at the start of last years riding season.

3. Anyone have any recommendations on guys in the GTA that work on old bikes out of their garages or unlicensed home shops? I wouldn't mind getting it to someone to ride it and tell me what they think and see if it's worth them exploring further.

Thanks in advance, everyone.

[jembers]

Sorry, videos of bike idling here:

https://streamable.com/o724mq

https://streamable.com/xm43ht

[newday777]

1st thing that comes to mind is a dirty or rusty fuel tank or if it eas lined, failing liner causing plugged carbs.
2nd do you have an aftermarket ignition switch on the bike? They are known for failing, hitting bumps can make a loose connection fail, momentarily at minimum.

[jembers]

1st thing that comes to mind is a dirty or rusty fuel tank or if it eas lined, failing liner causing plugged carbs.
2nd do you have an aftermarket ignition switch on the bike? They are known for failing, hitting bumps can make a loose connection fail, momentarily at minimum.

Yes, the tank is a bit rusty on the inside and my plans are to clean it out with some CLR. I figured to do this after I get the situation sorted out. I'm assuming cleaning the carbs would have solved the issue, at least temporarily if it was a rusty tank, but it started making troubles right away after cleaning the carbs. It does have a Dyna Coil on it and I will take a look at the electronics to see if anything is loose or disconnected. Thank you.

[Ujeni]

From the video, it sounds like a fuel issue indeed. It sounds like you are running three cylinders.

First things first, I would check the easy stuff. Track down which cylinder is running strangely. Confirm good spark in all four cylinders. Swap out your inline fuel filters. Check your float height. If you have clean fuel, good float heights and good spark, then it is your timing.

[caluser2000]

Had a similar thing happen to my Suzuki XF650 single. Carbs (2 carbs on these) whre gunked up and I couldn't even remove the float bowl drain screws. At first I thought it was bad fuel so ran the fuel tank completely dry( had a 5ltr container of gas to get me to a fuel station  to fill the bike up again). I then tossed twice  The recommended amount of carb/injector cleaner in the fuel tank full of petrol and went for a 400km You could feel the engine running better and better as the kms increased. By the time I got home it felt like a totally different bike. More responsive and more power.