Maxes-out around 55mph. 81 CB750 DOHC

Have you eliminated the possibility of the simple stuff? Intake leak, timing, fouled plug, mis-adjusted valves, etc.
 
deviant said:
Have you eliminated the possibility of the simple stuff? Intake leak, timing, fouled plug, mis-adjusted valves, etc.
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Some, just replaced that plug, just synced carbs without leaks. But Ill do a WD40 leak test in case.

Haven't looked at the valves though. Ya think a valve needing adjustment could cause a cylinder with a spark not to fire? I don't have experience working on the head admitibly. My garage is a sidewalk...


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Joseph New York said:
Haven't looked at the valves though. Ya think a valve needing adjustment could cause a cylinder with a spark not to fire? I don't have experience working on the head admitibly. My garage is a sidewalk...


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Absolutely, if it's not scavenging then it's not sucking fuel/air into the chamber. Intake opening too late causes lower fuel/air mixture, too early causes reversion and big power loss. They may not be opening at all. They may be staying open all the time, which could explain the loss of vacuum. You could've bent a valve at high revs too, so maybe it's jammed or leaking beyond the seat.
 
Joseph New York said:
Some, just replaced that plug, just synced carbs without leaks. But Ill do a WD40 leak test in case.

Haven't looked at the valves though. Ya think a valve needing adjustment could cause a cylinder with a spark not to fire? I don't have experience working on the head admitibly. My garage is a sidewalk...
Click to expand...

what is a wd40 leak test?

if your valve adjustment is too tight [no clearance] it will not allow the valve to close . . this will cause lower compression . . if your compression is around 100, it wont run no matter how much spark and fuel you have in it.

so you have an "open" garage?
 
barnett468 said:
what is a wd40 leak test?

if your valve adjustment is too tight [no clearance] it will not allow the valve to close . . this will cause lower compression . . if your compression is around 100, it wont run no matter how much spark and fuel you have in it.

so you have an "open" garage?
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A few drops of oil, not wd40, through the spark plug hole is what I was talking about. Rules out bad rings if the reading doesn't change.
 
Joseph New York said:
Damn, Im on the bike now and the first cylinder isn't firing again. I'll bet the plug is sparking. Should I pull the carbs??


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Yes. It is entirely possible that jets or drilling in that carb are clogged. Pop them off and clean the jets and spray carb cleaner through the drillings - try to avoid breathing too much of that stuff though. It's not healthy..
 
barnett468 said:
what is a wd40 leak test?

if your valve adjustment is too tight [no clearance] it will not allow the valve to close . . this will cause lower compression . . if your compression is around 100, it wont run no matter how much spark and fuel you have in it.

so you have an "open" garage?
Click to expand...
Deviant mentioned intake leak, so I was referring to spraying the manifold boots with WD40 to check for vacuum leaks.

Makes sense about the valves... I'm going to do a wet/dry compression test before diving into the head for now. I need to grab a torque wrench first anyways. I'm also going to hook up the carb sync if compression is OK...

Yeah, I have an open garage lol. The busy streets of SoHo is no place to wrench, but in nice weather and no tourist questions, I can be productive. And the eye candy... Oh man.

BTW, shes not running as good as when I first posted this thread. When all four cylendars are back in working order again, I'll get back to the original issue. Sorry for the detour!
 
teazer said:
Yes. It is entirely possible that jets or drilling in that carb are clogged. Pop them off and clean the jets and spray carb cleaner through the drillings - try to avoid breathing too much of that stuff though. It's not healthy..
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Yeah, I literally just guitar-stringed a clogged slow jet (f**ing pushed-in pilot jets) on my CB650 today, on the street, without unhooking the cables. Idles like a top now.

My 750 has the same carbs so I'm going to do the similar if my compression are OK.
 
An update on my slow progress:

I didn't have time to do a wet test, but I got in a compression test today to try to resolve my first cylinder not firing issue. It's exhaust is warm while other very hot. The plug is sparking BTW. The results are very confusing and I'm going to do it all over again because of it:

1) 135psi (not firing)
2) 60psi
3) 115psi
4) 130psi

I'm not convinced the test on 2 worked because it shouldn't be firing if so.

I'll let ya know when I test again, and also get a carb sync too.


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60 psi sounds like a Harbor freight Compression tester. I have tired two that read really low at random and eventually read low all the time.

It will still run like an old 30's low comp motor at less than 100 psi but barely. Spark and low heat suggests that carbs still needs to be cleaned.
 
teazer said:
60 psi sounds like a Harbor freight Compression tester. I have tired two that read really low at random and eventually read low all the time.

It will still run like an old 30's low comp motor at less than 100 psi but barely. Spark and low heat suggests that carbs still needs to be cleaned.
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Yep. Being that I've been busy cleaning my 650's rack, and knew I had a clogged slow jet and no time to pull it, so I lazily dumped in some Seafoam last week, and as of a few days ago, blamo!! Running on all 4! I never believe in magic potions but damn that stuff can really surprise you! And make you lazy :/

Now that I'm running all 4, I can finally get back to the fact that my 750 is still my slowest CB. Even slower than my 500...

I notice that the warmer she gets the more typewriter sound effects I hear from the head. That sound means I may need a valve adjustment? Its kind of hard to hear, but listen: https://www.flickr.com/gp/125586034@N04/e8f811 It's not the sound of my cam chain though, been through that. Regardless, perhaps I need to get a torque wrench and cam removal tool and pop her head off?

Other tests piling up are 1) Recheck #2 pressure. (I agree with what ya said that it consistently underestimates it's readings) 2) Pop off the clutch cover and play sprocket dentistry.




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barnett468 said:
The name is "Brand"?
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hah, no. I don't know what brand the garage put in. I checked the bill and they didn't charge me for it either as it's not itemized. I can call em up when they open tomorrow and ask. Why are so many moto shops and garages closed on Mondays??
 
Because they're open on Saturday. And Mondays are typically shit for business anyway.
 
Joseph New York said:
hah, no. I don't know what brand the garage put in. I checked the bill and they didn't charge me for it either as it's not itemized. I can call em up when they open tomorrow and ask.
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Yeah, it wouldn't hurt to ask . . Shops often use "average" quality oil and there are some oils that don't loose viscosity [get thinner] when they get hot and this type will protect your engine better but they are more expensize then average oil.
 
barnett468 said:
Yeah, it wouldn't hurt to ask . . Shops often use "average" quality oil and there are some oils that don't loose viscosity [get thinner] when they get hot and this type will protect your engine better but they are more expensize then average oil.
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Yeah, prob good to know! Haven't changed the oil myself on this bike.

Others have ran it and say it should be more ballsy, but could it be possible I'm not pushing it hard enough? I'd love to hear some other gear/rpm/mph reports to compare to mine: 5/4.5/55. It has pods/4-1 BTW.


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Joseph New York said:
Others have ran it and say it should be more ballsy, but could it be possible I'm not pushing it hard enough? I'd love to hear some other gear/rpm/mph reports to compare to mine: 5/4.5/55. It has pods/4-1 BTW.
Click to expand...

we need to know what your gears are.

it sounds like it is geared low so it doesn't have a lot of top end which should make it really fast when accelerating.
 
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