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Motorcycles » rec.motorcycles.tech » Honda CT90 tuning
| Honda CT90 tuning [message #517274] |
Sat, 19 November 2005 22:50 |
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I am having a little trouble with a '70 Honda CT90 I have been working
on, hopefully somebody can offer some advice.
It runs fine at low rpm's, but sputters and surges at higher rpms.
I've checked out the main jet, its clean. The plug looks normal. Also
set the float to what I believe is correct. Carefully adjusted points
gap and timing. Adjusted valve clearance. Cleaned fuel screen. New
air filter, plugs, fuel lines, fuel filter, no rust in tank.
The slide and needle were from a donor bike, maybe the needle is wrong.
If I look in the air end of the carb, the needle doesn't come all the
way out of the seat at full throttle, is this normal? It only has one
groove to set the height. Maybe get a carb kit with a new needle?
What else to check? Could it be something other than carburation,
maybe exhaust or ignition?
Thanks for any help..
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| Re: Honda CT90 tuning [message #517276 ] |
Sun, 20 November 2005 00:34 |
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blackford wrote:
> It runs fine at low rpm's, but sputters and surges at higher rpms.
> I've checked out the main jet, its clean.
Could be fuel starvation. Does choking help? If choking helps, it
suggests the fuel level in the float bowl is high enough to provide
enough gas.
Surging indicates lean mixture or air leaks.
Don't forget about the slow jet. Is it clean? Carburetors keep sucking
gas through the slow jet up until about 7/8ths to full throttle. You
might try turning the idle mixture screw clockwise 1/2 or 3/4ths turn
to see if that helps.
> Also set the float to what I believe is correct.
Find a manual and verify the setting dimension. It's normall done with
the carb upside down on the bench and the correct setting will be xx
millimeters, plus or minus 1 mm from the bottom of the float while the
adjustment tang is just resting on the end of the float valve to the
aluminum gasket surface *on the carb body*, NOT the surface of the
gasket..
> The slide and needle were from a donor bike, maybe the needle is wrong.
Wonderful. The designer of the carb selected a needle jet (the brass
tube the jet needle goes into) that was big enough to flow more fuel
than the main jet could pass whenever the tapered jet needle was about
7/8th to all the way out.
IOW, you have two restrictions in series. The main jet is a fixed
restriction, the jet needle/needle jet combination is a variable
restriction. At 7/8ths to 100% throttle opening, the variable
restrictor has to provide less resistance to fuel flow than the fixed
restrictor.
If you put the wrong diameter jet needle into the correct diameter
needle jet, you don't know how far you have to pull it out of the
needle jet to get the flow you need for 1/2 to 7/8ths throttle.
Yup. That could definitely cause surging.
> If I look in the air end of the carb, the needle doesn't come all the
> way out of the seat at full throttle, is this normal?
That's not a seat. It's the needle jet. If the jet needle did come all
the way out of the needle jet under full throttle conditions, what
would guide it back into the needle jet?
> Maybe get a carb kit with a new needle?
www.oldbikebarn.com has carb kits with new parts.
> What else to check? Could it be something other than carburation,
> maybe exhaust or ignition?
The top of the carb might be loose and you might have an air leak
there, or the carb might be sucking air where it bolts onto the intake
manifold.
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| Re: Honda CT90 tuning [message #517277 ] |
Sun, 20 November 2005 00:35 |
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blackford wrote:
> It runs fine at low rpm's, but sputters and surges at higher rpms.
> I've checked out the main jet, its clean.
Could be fuel starvation. Does choking help? If choking helps, it
suggests the fuel level in the float bowl is high enough to provide
enough gas.
Surging indicates lean mixture or air leaks.
Don't forget about the slow jet. Is it clean? Carburetors keep sucking
gas through the slow jet up until about 7/8ths to full throttle. You
might try turning the idle mixture screw clockwise 1/2 or 3/4ths turn
to see if that helps.
> Also set the float to what I believe is correct.
Find a manual and verify the setting dimension. It's normall done with
the carb upside down on the bench and the correct setting will be xx
millimeters, plus or minus 1 mm from the bottom of the float while the
adjustment tang is just resting on the end of the float valve to the
aluminum gasket surface *on the carb body*, NOT the surface of the
gasket..
> The slide and needle were from a donor bike, maybe the needle is wrong.
Wonderful. The designer of the carb selected a needle jet (the brass
tube the jet needle goes into) that was big enough to flow more fuel
than the main jet could pass whenever the tapered jet needle was about
7/8th to all the way out.
IOW, you have two restrictions in series. The main jet is a fixed
restriction, the jet needle/needle jet combination is a variable
restriction. At 7/8ths to 100% throttle opening, the variable
restrictor has to provide less resistance to fuel flow than the fixed
restrictor.
If you put the wrong diameter jet needle into the correct diameter
needle jet, you don't know how far you have to pull it out of the
needle jet to get the flow you need for 1/2 to 7/8ths throttle.
Yup. That could definitely cause surging.
> If I look in the air end of the carb, the needle doesn't come all the
> way out of the seat at full throttle, is this normal?
That's not a seat. It's the needle jet. If the jet needle did come all
the way out of the needle jet under full throttle conditions, what
would guide it back into the needle jet?
> Maybe get a carb kit with a new needle?
www.oldbikebarn.com has carb kits with new parts.
> What else to check? Could it be something other than carburation,
> maybe exhaust or ignition?
The top of the carb might be loose and you might have an air leak
there, or the carb might be sucking air where it bolts onto the intake
manifold.
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| Re: Honda CT90 tuning [message #517278 ] |
Sun, 20 November 2005 00:58 |
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"blackford" <blackford [at] usfamily.net> wrote in message
news:1132437017.778253.143700 [at] g47g2000cwa.googlegroups.com...
>I am having a little trouble with a '70 Honda CT90 I have been working
> on, hopefully somebody can offer some advice.
>
> It runs fine at low rpm's, but sputters and surges at higher rpms.
> I've checked out the main jet, its clean. The plug looks normal. Also
> set the float to what I believe is correct. Carefully adjusted points
> gap and timing. Adjusted valve clearance. Cleaned fuel screen. New
> air filter, plugs, fuel lines, fuel filter, no rust in tank.
>
> The slide and needle were from a donor bike, maybe the needle is wrong.
> If I look in the air end of the carb, the needle doesn't come all the
> way out of the seat at full throttle, is this normal? It only has one
> groove to set the height. Maybe get a carb kit with a new needle?
>
> What else to check? Could it be something other than carburation,
> maybe exhaust or ignition?
>
> Thanks for any help...
Sounds like you have a leak at the intake.....to check, spray some carb
cleaner to see if the surging smooths out....until you can pinpoint where
the leak is...
Good LUck...
Fwed
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| Re: Honda CT90 tuning [message #517288 ] |
Sun, 20 November 2005 04:02 |
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question: at hi revs does it run lean or rich?
can you use the whole carb from the donor?
there are other calibrated parts than the needle.
good luck, sammm
"blackford" <blackford [at] usfamily.net> wrote in message
news:1132437017.778253.143700 [at] g47g2000cwa.googlegroups.com...
>I am having a little trouble with a '70 Honda CT90 I have been working
> on, hopefully somebody can offer some advice.
>
> It runs fine at low rpm's, but sputters and surges at higher rpms.
> I've checked out the main jet, its clean. The plug looks normal. Also
> set the float to what I believe is correct. Carefully adjusted points
> gap and timing. Adjusted valve clearance. Cleaned fuel screen. New
> air filter, plugs, fuel lines, fuel filter, no rust in tank.
>
> The slide and needle were from a donor bike, maybe the needle is wrong.
> If I look in the air end of the carb, the needle doesn't come all the
> way out of the seat at full throttle, is this normal? It only has one
> groove to set the height. Maybe get a carb kit with a new needle?
>
> What else to check? Could it be something other than carburation,
> maybe exhaust or ignition?
>
> Thanks for any help..
>
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| Re: Honda CT90 tuning [message #535885 ] |
Fri, 02 December 2005 05:09 |
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Thank you everybody for the advice. Unfortunately the weather finally
turned here in MN and I had to put the bike away for the winter. I'll
save these ideas and try a few things in the spring. It should be fun,
my kids are getting just old enough to ride around the yard on thier
grandpa's old Honda 90.
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