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Hello, I have been trying to get my 81 750f super sport to running a bit better. Bike seems to idle ok, the issue is when I get the bike up to 3500-4000 RPMS the bike bogs out and the RPM will not advance much. The Bike is all stock. The bike had sat for a few years, the fuel delivery is good and there was spark issue but it was resolved after replacing the spark plugs they all look to be firing now and have strong spark. When its idling can hear some popping sounds like maybe from the exhaust, its also running incredibly rich so much so it burns your eyes if you are around it long enough.
I hooked up my air pressure carb sync to it got the carbs pressure to be all about the same (did the adjusting after it was warmed up) but the gauge shows them all in a red section, reading late timing or leak at intake manifold or heat riser. The carbs all look to advance in pressure about the same and look to synced. I checked the rubber boots on all the carbs does not appear to be any leaks. I am not convinced this is a timing issue but i don't have a lot of experience with timing except on cars and you really know when the timing is off.
I have a lot of experience running on mid to late model 2 stokes but not so much on these engines. I would like some direction on what i should go with next if i should go down the path of checking and adjusting the timing or if it could be another issue all together.
If its a timing issue I would love a write up I have read some previous threads here and there but some of them where not very clear to me, I do have a timing light, again from timing cars(looks like a silver gun almost) so nut sure if it will apply. Appreciate any help!
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Administrator
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Sounds like it's flooding. Why it's flooding is the question. Over jetted, flooding needles, weak battery creating a weak spark (these bikes really need a strong,healthy battery), or just dirty carbs. Hard to say without being there. Probably just have to use the good ol' process of elimination.
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Administrator
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If they're flooding, there should be dripping carbs at the overflows.
Timing wise, I'd just mark the backing plate at the motor, scribe it with an awl or something,
and move it a real little bit. Advance it a degree or 2. See how it runs. Leave the light on the bench.
Noise wise, check timing chain tensions.
Me?
I'd clean the carbs again.
The passages behind the main jets might be a bit clogged,
starving when you get to the upper rpms.
Sitting there at idle, the pilot jets and their passages downstream may be
fine, even over adjusted to compensate for the lack of main jet fuel,
which would be good only to a point, the higher rpms, where it would starve
and kinda fall on itself.
I know you don't wanna hear it, but yeah, I'd go over the carbs again,
making sure fuel is making it from behind the main jet to the throttle body,
squirting into the airstream. much luck guy.
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Administrator
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Maybe the main jets are switched with the pilots?
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Administrator
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Easy to do. The last set I went over, the guy had 3 out of 4 right.
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thanks for the help so far, did not have a lot of time to take a crack at it this weekend Ill let you guys know how it pans out after I get the carbs off and go through them again
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Hey finally able to find some time to work on the bike. Went threw the carbs again and made sure to get the jets really well. It did not seem to eliminate the issue with the bike dogging out at 4000-5000 rpm. I also pulled the timing cover off after having to drill out one the screws I was able to make sure the bike was properly timed. Also adjusted the fuel mixture screws but also did not seem to make any difference. The Carbs are in sync but the gauge shows them to be underpressure.
Not sure what else to look at, plugs have been replaced and confirmed they are all firing and the gaping should be correct on them not a 100% sure. Pretty sure its not a issue with the battery or charging system I even at one point hooked it up to a car jumping system to make sure it had the right amount of juice. The only other thing is I can think of is this whole time the air box (where the filter and such is that goes on the carbs) is off but that should not make the bike run so poorly.
The bike does seem to start easier now, so maybe making progress I dunno. The bike still seems to be running rich can defiantly smell it. When the bike is choked it will go super high up in the RPMs like it should so i dunno maybe the choke is setup wrong? If that's even possible, if it is i have no idea how to adjust it. Any further help be great starting to reach the end of the line of what I know
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Administrator
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Now that you say it does not have the airbox on, and it runs better with the choke on this points to two problems. These carbs will not work right without the airbox because they need the vacuum created by it to function properly, and without it they will run really lean. Also the running better with the choke on points to a vacuum leak. This also is a lean condition. It is probably caused by the intake boots not sealing around the carbs. Both of these will cause the running problems you mention.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."
1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)
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Good news, it was the airbox :D. I cant believe I spent so much time fiddling with it. After I put it all together started right up. Took it down the road sounds great now, runs like a top. Never herd a bike that sounds like this either. Thank you every one for the help and to get the bike going and have a good rest of the year.
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