1981 CB750 wont rev out.

[Bo82CB]

Check your carb sync, keep in mind your lead carb is #2 and adjust other carbs to the #2 values, granted #2 is working good.
Also check operation of your slides, since you don’t have OE air box just stick screw driver in there and lift slow and feel for resistance.

 

[Guitarguy1996]

Check your carb sync, keep in mind your lead carb is #2 and adjust other carbs to the #2 values, granted #2 is working good.
Also check operation of your slides, since you don’t have OE air box just stick screw driver in there and lift slow and feel for resistance.

All the slides move up and down freely, checked that yesterday.

 

[brettp]

If cylinders 2 and 3 are still down, check the ignition coils, ignition wires, and plug caps for resistance and continuity.

 

[JohnCoulson]

Its the pod filters that are the problem. I had exactly the same problem.
Put the old airbox back and the problem will be resolved.

 

[brettp]

Or use different carbs.

 

[Guitarguy1996]

Yeah unfortunately, neither are an option here. And the weird thing is, I can see the slides all moving up and down if it's sitting there in neutral with no air filters on. Also, I put my hand over the carbs for 2 and 3 while it was running just to try something and force fuel through (good ol redneck clean out) and that's when it started revving out all the way now but once you're going, it's like you've still only got half the motor and 2 and 3 still have no change if you pull the plugs while idling.

 

[brettp]

Wow. That is really odd. I have to wonder if your increased air and fuel in 1 and 4 can carry the momentum for 2 and 3, hence no change when you pull the plugs on 2 and 3.

But let's do one at a time. If you're revving in neutral with carbs 2 and 3 restricted, you probably need to increase that restriction to do the same under load.

As for plugs on 2 and 3, if they have spark when the bike is cold AND warm, forget about those. They're working as designed (as long as you see the spark under both conditions).

 

[Guitarguy1996]

Yeah I've always had spark in all 4 no matter what. When I covered either of the carbs (2 or 3, couldn't really do both at the same time) whichever cylinder I was covering would kinda come on for a sec, then flood out pretty quick and the bike would die. And like I said, the bike started revving out all the way after that, but it feels like my wife's cl350. 63 mph was the best I could get out of it.

 

[brettp]

How do the spark plugs in 2 and 3 look?

 

[Guitarguy1996]

All 4 look pretty lean but I haven't run it much and the guy that owned it before my friend had like 45 pilot jets in it or something really low like that.

 

[Guitarguy1996]

I've also put new plugs in it since then which are I think too new to tell.

 

[brettp]

I should be able to read the plugs pretty quick. How many miles are on them?

 

[Guitarguy1996]

Maybe 3 or 4 at most. I've just been riding it around the block after adjustments.

 

[Guitarguy1996]

So went through the 2 middle carbs again and found a clogged tube under the pilot jet! I was thinking it wasn't removable like my wife's cl350 but wrong I was! It's still not great though. Sounds like it's running a whole lot better but it's just got no go. Topped it out at 71. Best yet but even that was a fight

 

[brettp]

Some progress! I have a feeling your pods are the issue. You're going to have to find a way to get the slides to open all the way, then you can check your jetting again.

 

[Guitarguy1996]

So now I'm not so sure. Came back and just to see, I did the same let it idle, pull plug wires and see which cylinders make a difference test, and same thing. 2 and 3 made no change, 1 and 4 were it. Tried to start it with only 2 and 3 plugged in (1 and 4 unplugged) and nothing. Choke on or off, difference t throttle positions, didn't matter. I'm thinking I'll swap the coil from 1 and 4 to 2 and 3 and see if that makes a difference.

 

[brettp]

So now I'm not so sure. Came back and just to see, I did the same let it idle, pull plug wires and see which cylinders make a difference test, and same thing. 2 and 3 made no change, 1 and 4 were it. Tried to start it with only 2 and 3 plugged in (1 and 4 unplugged) and nothing. Choke on or off, difference t throttle positions, didn't matter. I'm thinking I'll swap the coil from 1 and 4 to 2 and 3 and see if that makes a difference.

Brilliant idea.

 

[Guitarguy1996]

Same story. 1 and 4, no 2 and 3.

 

[brettp]

Ok, could it be those spark plug cables, then?

 

[Guitarguy1996]

It's gotta be valves right? Carbs are clean and the middle 2 cylinders are consistently not firing regardless of which coils are on them. The only thing I can think of was maybe the valves are out of spec even though they have the same compression as the running cylinders?