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1981 CB750 wont rev out.

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Guitarguy1996

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Hey guys! So I'm working on my friends 81 cb750 and running into some really weird issues. I've adjusted the valves, rebuilt the carbs, put in a new starter clutch, and done some electrical work on it. It's got good spark, at least 150 psi of compression in all 4 cylinders (the highest is like 160-165), all the carbs are getting fuel and are clean, everything looks great! But once it's up to temp, it won't rev out past like 1/4 throttle or so. It's driving me nuts, I got it to hit 60 today but barely. It feels like one or two cylinders are trying to rev out but the others don't so you'll get little spirts of full power and then back to falling on its face. It's got a very open exhaust with a very small baffel and pod filters. Running stock carbs with 72 pilot and 120 main jets. Choke does seem to help it rev for a sec but then it's getting too much fuel and falls on its face again for a whole other reason. Could this be ignition timing? Is it still to lean? I don't get it. Stock jets are 68 and 102 so it should be getting plenty of fuel right? It idles beautifully! Any advice would be great. Thanks guys.
 
I'll chime in before amc shows up (the guru). I did a ton of research, out of necessity, when my bike did a similar thing. I was all stock, though. Here is the main mechanical function required for these stock carbs: vacuum. If/when you put pods on these carbs, the down stroke of the cylinder needs to pull a vacuum across the carb throat (maybe a poor choice of words, but go with me). The pods provide so much air to the carb, there is very little vacuum across the carb throat. Therefore, the slides do not lift. The greater the vacuum across the carb, the higher the slide lifts.
People have wrapped tape around a portion of the pod to help create that vacuum. I think that looks like crap and defeats the purpose of the pods. Others use an entirely different carb that is less sensitive to that vacuum and easier to mod.
Long story short, if you put the stock air box and filter back on the bike, it will run perfect like nothing was every wrong. I say that with the caveat there is also a free flowing exhaust on the bike lacking back pressure.
 
Oh that makes perfect sense! I can't go back to the oem air box, it's my friend's bike and it's got a pretty aggressive cafe bike look to it. But I could definitely tape up the filters a bit to see if that does it. How much of the filters should I tape up to start? Like half?
 
I've gotten you as far as I can. Lol. I have no idea how much they're taping. I heard about it, saw a picture, and left it there. Icve heard the black uni filters are more restrictive but they don't give you the same look.
Keep us updated!
 
So I've messed with taping up the pods and can't seem to get it right now. The bike seems like it floods out and still won't rev out. Is a 120 main too much for the main? I took a video but apparently it's too large of a file.
 
Would a busted ignition advance cause the weird no rev issue? Just found these pieces loose behind the cover.
 

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Behind the timing cover? Looks important! If you are running on two cylinders (1 and 4, or 2 and 3) then that might be the issue. If all cylinders are firing but the bike isn't revving, that isn't your issue. It is a second, unrelated issue.
 
Yeah it's weird. So about a week ago, I did the test where you let it idle and pull one plug at a time and see if the idle changes to determine if that cylinder is making a difference. They all were, one less so than the other 3 but still. I tried it today and the bike died when I pulled cylinders 1 and 4 but no change for 2 or 3. Ya check spark on them though and it's nice strong blue spark in all 4. That got me thinking ignition timing...maybe. Pulled the cover and the ignition plate and those 2 pieces fell out.
 
Guitarguy1996 said:
Yeah it's weird. So about a week ago, I did the test where you let it idle and pull one plug at a time and see if the idle changes to determine if that cylinder is making a difference. They all were, one less so than the other 3 but still. I tried it today and the bike died when I pulled cylinders 1 and 4 but no change for 2 or 3. Ya check spark on them though and it's nice strong blue spark in all 4. That got me thinking ignition timing...maybe. Pulled the cover and the ignition plate and those 2 pieces fell out.
Click to expand...
The timing is set for cylinders 1 and 4, and then 2 and 3. You maybe it was on the bike but worked itself loose. I forget the name of the thing that piece attached to, but you will need to reinstall. Plenty of videos out there about timing.
 
Ok so I finally got around to replacing the spark advance unit and that all went fine but it's still not running on 2 and 3. Checked timing with the light and it's dead on for both sets of cylinders, it's got 150 psi of compression in all 4, I shimmed the valves so that should all be good (thus the good compression, 2 cylinders were low before), I've cleaned the carbs and re-jetted them so that all should be good. I'm kinda stumped. If I pull the plug wire for either 1 or 4, while idling, the bike dies, if I do it for 2 or 3, no change.
 
Double-check that the wiring between the spark unit pickup for 2,3 is actually hooked up to the wiring harness. It's possible a pin slipped into the plug while you were plugging it in or something like that.
 
SenseAmidMadness said:
Double-check that the wiring between the spark unit pickup for 2,3 is actually hooked up to the wiring harness. It's possible a pin slipped into the plug while you were plugging it in or something like that.
Click to expand...
I can double check it but it does have spark in 2 and 3 and I checked the ignition timing on 2 and 3 as well and that's on too.
 
Could this be a carb sync issue? I noticed at idle with the filters off, the slides for 2 and 3 bounce up and down quite a bit and the ones for 1 and 4 don't. All 4 come up when you rev it and none of them stick but those middle 2 are kinda bouncy.
 
It could be a carb sync issue if the carbs are massively off from each other, yes. More likely than that, unless you really know what you're doing with carburetors, you missed something like a clogged air passage. It's very strange that it's specifically 2 and 3 though. It's a long shot, but another guess I'd have is that if you're not running an inline fuel filter, some junk from an old fuel tank flowed into the fuel rail and jammed up the main jets for carbs 2 and 3, and didn't go far enough horizontally to get into 1 and 4.

You're in Nashville? I'm in Murfreesboro and I know my way around carbs.

I will say, though, you are fighting an awful battle here. The CV carbs on these bikes are incredibly sensitive to vacuum and will never, ever run right with pods no matter how much tuning you do. They will always make less horsepower than stock. The only way to properly run an intake system that isn't stock, off the shelf, is to pony up the $1200 for a set of Cycle X round slide carbs or the $650 for a pair of Murray's. Or do all the work they already did yourself and try to adapt a set of off-the-shelf Mikuni VM34's or another direct-pull carb...but at that point you've gotta have a ton more time than money.
 
SenseAmidMadness said:
It could be a carb sync issue if the carbs are massively off from each other, yes. More likely than that, unless you really know what you're doing with carburetors, you missed something like a clogged air passage. It's very strange that it's specifically 2 and 3 though. It's a long shot, but another guess I'd have is that if you're not running an inline fuel filter, some junk from an old fuel tank flowed into the fuel rail and jammed up the main jets for carbs 2 and 3, and didn't go far enough horizontally to get into 1 and 4.

You're in Nashville? I'm in Murfreesboro and I know my way around carbs.

I will say, though, you are fighting an awful battle here. The CV carbs on these bikes are incredibly sensitive to vacuum and will never, ever run right with pods no matter how much tuning you do. They will always make less horsepower than stock. The only way to properly run an intake system that isn't stock, off the shelf, is to pony up the $1200 for a set of Cycle X round slide carbs or the $650 for a pair of Murray's. Or do all the work they already did yourself and try to adapt a set of off-the-shelf Mikuni VM34's or another direct-pull carb...but at that point you've gotta have a ton more time than money.
Click to expand...
I thought maybe fuel flow too so I cracked the drain screw on all 4 and they all had gas leaking out as soon as I got about 1 turn out. Not to say there still couldn't be a blockage but you'd think if that were it, cylinder 4 would be down too.
 

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