Have you eliminated the possibility of the simple stuff? Intake leak, timing, fouled plug, mis-adjusted valves, etc.
Maxes-out around 55mph. 81 CB750 DOHC
- Thread starter Joseph New York
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Joseph New York
Active Member
deviant 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...
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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:
barnett468
Coast to Coast
Joseph New York 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?
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.barnett468 said:
barnett468
Coast to Coast
.
yes and the heavier the oil the better like straight 30 or 40 engine oil but most any engine oil will work.
yes and the heavier the oil the better like straight 30 or 40 engine oil but most any engine oil will work.
Joseph New York
Active Member
Deviant mentioned intake leak, so I was referring to spraying the manifold boots with WD40 to check for vacuum leaks.barnett468 said:
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!
Joseph New York
Active Member
teazer said:
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.
Joseph New York
Active Member
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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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.
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.
Joseph New York
Active Member
teazer said:
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
Coast to Coast
.
you may need a valve adjustment but your oil is thinning out also . . exactly what oil are you using?
you may need a valve adjustment but your oil is thinning out also . . exactly what oil are you using?
Joseph New York
Active Member
10w40... Brand?barnett468 said:
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barnett468
Coast to Coast
Joseph New York said:
The name is "Brand"?
Joseph New York
Active Member
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??barnett468 said:
Because they're open on Saturday. And Mondays are typically shit for business anyway.
barnett468
Coast to Coast
Joseph New York 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.
Joseph New York
Active Member
barnett468 said:
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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barnett468
Coast to Coast
Joseph New York said:
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.