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Discussion Starter · #1 ·
Hello all,



I recently overhauled my carbs, and of coarse replaced my accellurator pump, while i overhauled the carbs i checked and double checked that all the tolerances were correct.(choke linkage accel pump etc,) I am having fuel starvatin issues at sustained speeds of around 55mph and up. (I have ignitech so it's not the stator or cdi.) So i guess my question is did i maybe set the accel pump wrong? I really don't want to pull my carbs off again but it's looking that way... Any help is greatly appreciated.
 

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I had a similar issue a few years back and it turned out to be rust in the tank. Going faster than 55 would eat up all the fuel that was provided and the bike would choke out.



Does your tank look clean? Have you have the petcock off lately or pulled the fuel strainer.



A similar problem that causes the bike to die at 55 would be if you had the carbs off for cleaning and put the jets in backwards.



Can you go faster than 55 at all?
 

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Hello all,



I recently overhauled my carbs, and of coarse replaced my accellurator pump, while i overhauled the carbs i checked and double checked that all the tolerances were correct.(choke linkage accel pump etc,) I am having fuel starvatin issues at sustained speeds of around 55mph and up. (I have ignitech so it's not the stator or cdi.) So i guess my question is did i maybe set the accel pump wrong? I really don't want to pull my carbs off again but it's looking that way... Any help is greatly appreciated.


Is your vacuum petcock connected correctly?



Also is there a chance you put the jets in the wrong places,,the 115 goes in the brass fitting.



edit; Stitch beat me to it.
 

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Discussion Starter · #4 ·
Is your vacuum petcock connected correctly?



Also is there a chance you put the jets in the wrong places,,the 115 goes in the brass fitting.



edit; Stitch beat me to it.
Yes fella's, i cleaned the tank and fuel screen inside the tank. Yes all lines are hooked up correctly. Yes jets are in correct places. yes i can go over 55, It's any sustained speed at around 55 and up key seems to be sustained speed. Before you ask, i'm sure it's fuel starvation. It will spit sputter like it's running out of gas. Until i let off the gas and give the bowls time to refill sufficiantly.



I have the carbs off now and appears the little tang that the accel pump is supposed to hit. It's only traveling far enough (when i engage the throttle) to just make contact and isn't pushing the actual rod up at all. Both tolerances are correct, .004-.012 on accel pump rod, and at the other tang it's .12-.13 which is all one piece and it has a tang on each end, hope that made sense.



So my next question is should the tang be pushing up far enough to actually push the accel pump rod? I'm pretty sure it should be, but am confused as why it's not with both tang's adjusted accurate. Might it be that the piece which has the tangs on it, is just bent in a funny way.
 

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I am not sure off hand how the accelerator pump linkage is set up,,but I don't think that if it were not working correctly that you would have the symptoms you are describing, you may notice the bike was a bit slower to accelerate but should not matter at a sustained speed.



What you are describing sounds more like a float level problem or some other fuel flow issue.
 

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If you have a vacuum shut off are you sure it is working properly?



It could have a vacuum line that is cracked or the rubber bellows inside the shut off might be cracked and it only shows up with high speed vacuum.
 
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It's any sustained speed at around 55 and up key seems to be sustained speed. Before you ask, i'm sure it's fuel starvation. It will spit sputter like it's running out of gas. Until i let off the gas and give the bowls time to refill sufficiantly.


What model do you have?

Is it a vaccum petcock?



I would start there. If they get fine holes in them, they will act exactly that way. They open up the piston valve far enough to let some fuel through, but not enough for full throttle. "81 D or custom? Did the '80's have the vaccum petcock also.



Check the vaccum feed to the petcock for cracks also and for mud wasp nests.
 

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Discussion Starter · #9 ·
I fixed the accel pump issue, and still had fuel starvation only now it was more like sustained speeds to 65-70 before it would happen. Finally after checking everything for the fourth or fifth time (i lost count). I finally found the problem. It was the vacuum diaphram, the larger outermost one with the plastic washer that attaches to it and the spring butt's up against. Where the plastic washer attaches to the rubber diaphram (it's a little rubber knob that protrudes up and the washer fit's over the knob) I didn't realize that washer was removable and there was a large crack around the rim of the rubber knob which was hidden by the washer!



Thanks again to all that replied.



I was almost certain it was just the accel pump. As i understand the pump only increases the amount of fuel being supllied to the bowls under heavy accelleration or high speed to account for the increase in fuel consumption, and has nothing to do with actual increase in felt accelleration as suggjested. Please correct me if i miss understood the pumps function, also it may help clarify for later user's.
 

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I could well be wrong here but I think all the accel. pump does is deliver fuel on opening the throttle but needs the throttle to be closed again so it can reset itself for the next fistfull of throttle and hence should have no bearing on sustained throttle openings.



Somebody please set me straight if I'm wrong, but this is how the mechanism looks to me when I work the throttle shaft on an accel. pump pair I have here.



Australian market carbs don't have accel. pumps, these were an ebay buy from the states.
 

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The accelerator pump does not supply the float bowls with fuel,,it draws fuel from the bowls and squirts it into the throat of carbs below the choke plates when accelerating.



I think it provides extra fuel for acceleration when the throttle is opened quickly. I have bikes with and without the accel. pump and really do not notice much difference in the performance between them.



Some people with the accel. pump carbs use it to aid in cold starting. Quickly cracking the throttle wide open a few times before trying to start the bike will squirt a little extra fuel into the throat of the carb.



edit; if using the cold start trick, I think it is best to do this before pulling the choke on because the choke plates may block the fuel from getting through the carbs.

ie. crack throttle 2-3 times, apply choke,,start bike.
 

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My understanding of the accelerator pump's function is to provide a "shot" of fuel to the carbs when the throttle is quickly applied. There is usually a lag until the vacuum piston catches up with the demand of increased throttle, and the accelerator pump provides a momentary burst of fuel to counteract this. The pump draws a bit of fuel out of the float bowls and shoots it directly into the carbs.



I don't think there is a reason for the accelerator pump to cause any high speed RPM issues. Now perhaps the split diaphragm in the pump could cause possible fuel delivery issues. So maybe you did correct something that was related.
 

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Discussion Starter · #13 ·
The accelerator pump does not supply the float bowls with fuel,,it draws fuel from the bowls and squirts it into the throat of carbs below the choke plates when accelerating.



I think it provides extra fuel for acceleration when the throttle is opened quickly. I have bikes with and without the accel. pump and really do not notice much difference in the performance between them.



Some people with the accel. pump carbs use it to aid in cold starting. Quickly cracking the throttle wide open a few times before trying to start the bike will squirt a little extra fuel into the throat of the carb.
Thank you for the clarification.
 

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Discussion Starter · #14 ·
My understanding of the accelerator pump's function is to provide a "shot" of fuel to the carbs when the throttle is quickly applied. There is usually a lag until the vacuum piston catches up with the demand of increased throttle, and the accelerator pump provides a momentary burst of fuel to counteract this. The pump draws a bit of fuel out of the float bowls and shoots it directly into the carbs.



I don't think there is a reason for the accelerator pump to cause any high speed RPM issues. Now perhaps the split diaphragm in the pump could cause possible fuel delivery issues. So maybe you did correct something that was related.
Yes the split in the diaphram was the issue. Back to raising hell on the street's!
 

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Finally after checking everything for the fourth or fifth time (i lost count). I finally found the problem. It was the vacuum diaphram, the larger outermost one with the plastic washer that attaches to it and the spring butt's up against. Where the plastic washer attaches to the rubber diaphram (it's a little rubber knob that protrudes up and the washer fit's over the knob) I didn't realize that washer was removable and there was a large crack around the rim of the rubber knob which was hidden by the washer!


I have no idea what part you are talking about.
 

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