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Acceleration

2K views 17 replies 8 participants last post by  92367 
#1 ·
Hey guys, don’t know if I’m on the right form. I don’t know if this is or isn’t normal but I can’t my 1999 Honda Rebel over 40-45mph. I feel like I’m pushing it at that speed. I thought maybe because it’s an older bike?
 
#2 ·
When was the last time you changed your air filter?
take for a short quick ride without filter..
my 87 will do 75mph no problems.. sounds like it's going to explode from rpm..
air cooled Honda's like high rpm
 
#5 ·
Sounds like another case of the no good diaphragm on the carburetor's vacuum piston.


Sparker, looks like these carburetors have a (rubber / rubberish) diaphragm that deteriorates over time and doesn't seal properly. And when that happens, it doesn't allow enough pressure difference on either side of the diaphragm to cause the vacuum piston to slide all the way up in the carburetor when needed. Therefore, the engine doesn't get as much airflow as is possible. In essence the airflow is choked off, but the air fuel ratio is good because that piston is also controlling the jet needle within the needle jet holder and main jet, so the bike runs good at it's new airflow limited speed.


That's what I'm guessing here...
 
#6 ·
@7milesout could be correct.. Not knowing of any carb history anything is possible.

Specific instructions for vacuum piston assembly:

"Hold the vacuum piston up to almost full open so the diaphragm is not pinched by the chamber cover"
"Install the chamber cover with the spring, aligning it's tab with the hold in the carburetor, and secure with at least two screws before releasing the vacuum piston."

been negligent myself with this instruction..
but i would check the air filter first.
 

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#8 ·
Valves and Air filter

I’m back! Okay, so air filter was replaced but I haven’t adjusted the valves (a link was provided). That’s not the issue atm. After I do adjust the valves, what else could be causing my Rebel 250 to only be going at 45mph? It will go any faster and I’m in the highest gear and still nothing. I feel like there’s such short times between changing gears and then I’m at the highest. Help meh please! Should I do a video as well?
 
#9 · (Edited)
If the person who rebuilt the carb didn't notice any tears in the diaphragm, that doesn't mean there weren't any, and would explain your problem.

Please put your bike information in your signature so others will know what year and model you have. See my signature for how-to. I'm going to merge your latest thread with this one since it deals with the same issue.
 
#10 ·
Do ya mean adding in a message when I type it as the signature? Just making sure. And thank you. So they rebuilt the carb but the diaphragm could be torn? Forgive me, I'm not familiar with the terminology. Do you have a photo I could see for reference?
 
#12 ·
Rubber diaphragm is attached to the vacuum piston
It's found under the carb top cover..
In post #6 I addressed the proper installation method..

if the sealing lip should leak or a tear develop in diaphragm, vacuum will not be strong enough to lift piston.. thus you will not have mid and upper range power..
 

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#14 ·
Just a suggestion... could it be that your clutch is slipping? If you changed the oil, and used a 10W/30 car oil, that can cause it. Maybe a quick change to motorcycle-specific oil might still help.

If your diaphragm is damaged or rotted (mine was, a 2005), there's a guy in London, England that makes exact replacements at a fraction of other (over-priced) costs. You can find him on eBay, and his products are good and shipping is fast.
 
#15 ·
Once the wrong oil gets on the clutch plates, there's nothing that will fix it other than replacing them. I tried extended soaking with Sea Foam, then gas, then good oil. Still slipped.

I recently helped a guy with his Rebel carb issues, and we discovered his diaphragm had two very small tears. Ordered this one, which includes the slide and needle too, and the bike runs well. I suggest using the needle from the original diaphragm as the one with the new diaphragm is slightly different. Regular price is $25.99. I offered $24 and he accepted with no haggling. Might sell for even less. https://www.ebay.com/itm/Membrane-D...e=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
 
#17 · (Edited)
check the compression for both cylinders. check it dry...and then add a little (spoon full) of 2 stroke oil to each cylinder and check it again. (prepare for some smoke) Compression should be in the 160psi range...even 180 depending on generation.

my Reb runs great through 3 gears...no major problems beyond a little bogging when cold, needing choke to get going...but 4th and 5th gear top out immediately at 50mph with throttle pinned. zero torque in 4th gear and 5th. barely maintains speed.

I replaced plugs and air filter and then did a compression check. 80psi in both cylinders when dry. 170 when wet. Suggests the rings are toast or the cylinder wall itself. either way, I'll never go faster than 55 until the top end gets rebuilt. I'm not sure why it runs so well in low gears at high RPM if the compression is half what it should be but it might be a testament to the quality manufacturing. it takes a lot to actually kill a rebel. I'm sure this problem is due to owner neglect. it's totally adequate in-town transportation with half the head compression.

more info here
do a compression check with a basic compression gauge. I noticed the harbor freight gauge that screws in doesn't seal well enough to give an accurate reading, but the gauge that I just hold against the plug hole while firing the ignition works fine.
good luck
 
#18 ·
I hate to chime in and try to blindly diagnose a problem.


If your carburetor has been off and back on there is the possibility of a missing o-ring between the carburetor and the intake manifold. Also, the o-ring can be reused and usually is, but overtime they compress and can cause an air leak with a lean condition.


I have seen compressed o-rings cause this same problem on a few other bikes.


This should be a very quick and easy check that can be done without removing the carburetor completely.
 
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