Topic: 74 CB750 Sputtering and choking when running and idling

[jembers]

I had a previous post about some problems I was having with my bike. I feel like I've narrowed it down to it being a fuel problem. Bike fires up fine and when I get out on my street in first gear it runs fine. It's when I give it some throttle in first and jump into second that it starts choking and sputtering like it's not getting fuel. I pulled the bike back into the garage and let it sit and idle and it did so with no problems. I let it idle and turned off the petcock and the way it acts at the very end of the fuel when it's choking for fuel is pretty much exactly what it's doing when I'm out riding.

I replaced the fuel line with some new 5.5mm line and checked the petcock to make sure the flow was ok and it is. Bike is firing all cylinders as well as all exhaust headers are super hot.

Suggestions on what I could do next? I've had the bike two seasons now and I've not pulled and cleaned carbs myself. I honestly don't even know when they could have been cleaned last.

TIA

[BenelliSEI]

When sitting in the driveway, does it “choke and sputter” when you rev it up in neutral too? When was the last time you cleaned, gapped the points and checked ignition timing and full advance timing? Might not be fuel at all. If you are riding it regularly, not likely plug jets ......

[jembers]

Yes, when sitting in driveway it chokes and sputters, but not nearly as much as when trying to ride it. When I rev it and the rpm's pull back, it chokes and sputters every so often.

Early this summer a nice gentleman helped me work on valve adjustment, compression test, timing, carb syncing.

[BenelliSEI]

If you are riding it regularly, and this happened in the past few days, it is unlikely to be carburation. Check points gaps and timing first. Pull the points cover off in a dark place (in your garage or in the evening). Are the points sparking all over the place? Both or just one set?

Alternatively; did you just add fresh fuel? Any chance you have water in it? Or old gas?

[jembers]

I added fresh fuel just before a long ride, when it first started acting like this on the way back from the ride. The next day I attempted to go on another long ride and a few km's in it started getting really bad so I brought it back home. I've been tinkering with it since then thinking it's fuel related. Replaced fuel line, checked flow from petcock, cut off .25" from spark cables and rethread them, put in new plugs, tested battery. My problem is I'm a bit green at this and I don't have all of the necessary tools for points gaps and timing.

I'm going to put some seafoam in it and let it idle for a bit and see if that happens to help at all. I may have a winter project on my hands here...

[BenelliSEI]

Understand totally. Still, pull off that points cover and start the bike up, in a dark place. If the points are sparking and putting on a real light show in there, we’ll walk you through it. I’m betting the problem is in there....... read my post from a few weeks ago, “chasing shadows”. There are several members in Toronto. One visited me just yesterday. Where are you?

[jembers]

Ok, sounds good. I will get around to that but it won't be until early next week. As for my location, I'm in East York Toronto right close to Woodbine subway station.

Here are some videos I just took of it. First is it idling and you can see it randomly chokes out. Second and third are me revving the engine and you can see how it acts there as well. I also managed to get a nice loud pop from the exhaust in the third video. Thanks for all of your advice so far.

https://streamable.com/a1wt27

https://streamable.com/njm7cg

https://streamable.com/b6qk57