car launch stumble
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car launch stumble
hi everyone
i have a question well here's what i am working with i have a sbc with a tunnel ram with 660's now when i lauch with the trans brake the car feels like it runs out of fuel for a split second and recovers with out any wierd flubber or anything and when i leave off the foot brake it is fine.could this be the fuel shifting to the back of the carb and leaving the front jet to go dry now the car lifts the wheels pretty violently on the trans brake. would raising the float level and longer vent extension fix this or does anyone have other reasons this could be happening to many friends giving me input that makes no sense
i have a question well here's what i am working with i have a sbc with a tunnel ram with 660's now when i lauch with the trans brake the car feels like it runs out of fuel for a split second and recovers with out any wierd flubber or anything and when i leave off the foot brake it is fine.could this be the fuel shifting to the back of the carb and leaving the front jet to go dry now the car lifts the wheels pretty violently on the trans brake. would raising the float level and longer vent extension fix this or does anyone have other reasons this could be happening to many friends giving me input that makes no sense
Have you considered going to a 50cc accelerator pump? Usually when it goes for a split second then falls on its face a smaller squirter will help by extending the length of the pump shot. With the tunnel ram you may need more of a pump shot that lasts longer than a standard accel pump can provide.
Just a thought. Hang in there, I'm going through something similar with my Camaro.
Just a thought. Hang in there, I'm going through something similar with my Camaro.
E85 racer and E85 carb builder
www.horsepowerinnovations.com
68 Camaro 427" E85 powered small block, 9.95 @ 133 mph best motor ET through the mufflers. 1.319 best sixty foot.
www.horsepowerinnovations.com
68 Camaro 427" E85 powered small block, 9.95 @ 133 mph best motor ET through the mufflers. 1.319 best sixty foot.
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Well mine is doing the exact same thing -- goes for 10 feet then dies, picks up, then dies again . . . then finally takes off.
I'm putting jet extensions on mine . . . not sure what to do about sideways carbs though.
I'm putting jet extensions on mine . . . not sure what to do about sideways carbs though.
E85 racer and E85 carb builder
www.horsepowerinnovations.com
68 Camaro 427" E85 powered small block, 9.95 @ 133 mph best motor ET through the mufflers. 1.319 best sixty foot.
www.horsepowerinnovations.com
68 Camaro 427" E85 powered small block, 9.95 @ 133 mph best motor ET through the mufflers. 1.319 best sixty foot.
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Center-pivot floats were designed to handle high g cornering with normal fore/aft carb mounting, since it doesn't take too much to starve the engine on a long, hard turn with the side pivots. This would mean that with similar longitudinal g forces, i.e. less than 1.0, they could starve on launch when mounted sideways. Therefore, center pivots should be the cure...
Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things.
Happy is he who can discover the cause of things.
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Chris,
If you're still having trouble tuning out the bog, maybe a few of these thoughts
can help you further:
From reading your post, I can assume you have quite a high stall speed
(~4000 RPM?) against the transbrake which means the throttle blade angle
is pretty much wide open.
The squirters dump fuel according to the accelerator pump arm position
on the pump cam. My guess is...with your foot that far into the throttle,
the acceleator pump arm has gone to full extension on the cam and there's
nothing left to squirt out.
Mark the pump cam with ink to determine if there is additional travel remaining on the pump cam.
Have a friend look at the throttle position (linkage) as you stall up to the
regular launch RPM (don't move the accelerator pedal any further than this point!). Remove the cam and check how far the linkage rubbed out
the ink.
As you sit idling, the fuel is getting pulled through the main circuit.
When the transbrake is released and the load of the car hits the motor,
the RPM will drop causing a higher pressure in the manifold.
This lowers the fuel from discharging into the venturi as the pressure rises.
If the engine bogs after ten feet, it might be more a float level issue, or
jets getting naked with the fuel slopping around as mentioned above.
By chance, there might be some squirt left in the accelerator pump if
you depress the accelerator pedal down further after releasing the transbrake.
If this is the case, the bog is caused by the mechanical "injection" of fuel
through the squirters when the RPM drops. This creates a sudden rich
condition.
I would try removing the pump cam totally. This will disable the accelerator
pump circuit. From this point, you can isolate the bog as float level/jet
extensions.
If the problem gets worse, then pay attention to the accelerator pump
circuit.
If you're still having trouble tuning out the bog, maybe a few of these thoughts
can help you further:
From reading your post, I can assume you have quite a high stall speed
(~4000 RPM?) against the transbrake which means the throttle blade angle
is pretty much wide open.
The squirters dump fuel according to the accelerator pump arm position
on the pump cam. My guess is...with your foot that far into the throttle,
the acceleator pump arm has gone to full extension on the cam and there's
nothing left to squirt out.
Mark the pump cam with ink to determine if there is additional travel remaining on the pump cam.
Have a friend look at the throttle position (linkage) as you stall up to the
regular launch RPM (don't move the accelerator pedal any further than this point!). Remove the cam and check how far the linkage rubbed out
the ink.
As you sit idling, the fuel is getting pulled through the main circuit.
When the transbrake is released and the load of the car hits the motor,
the RPM will drop causing a higher pressure in the manifold.
This lowers the fuel from discharging into the venturi as the pressure rises.
If the engine bogs after ten feet, it might be more a float level issue, or
jets getting naked with the fuel slopping around as mentioned above.
By chance, there might be some squirt left in the accelerator pump if
you depress the accelerator pedal down further after releasing the transbrake.
If this is the case, the bog is caused by the mechanical "injection" of fuel
through the squirters when the RPM drops. This creates a sudden rich
condition.
I would try removing the pump cam totally. This will disable the accelerator
pump circuit. From this point, you can isolate the bog as float level/jet
extensions.
If the problem gets worse, then pay attention to the accelerator pump
circuit.
I ran into a simular problem some time ago. What was happening was on hard launch the side hung float was actually cutting off the fuel due to the g force. Once I changed the carbs over to center hung floats the problem was gone.
Sorry if I ask alot of questions, but you never stop learning if you ask questions
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Chris,
Are you running the side hung float bowls on your carburetors or have they been changed over to center hung float bowls? How high does the front end come up in the air when you let go of the button?
If you're leaving off the transbrake with your foot flat on the floor tuning the accelerator pump circuit is most likely not going to be any help to you.
Try raising your float level. This may not cure the problem, but should tell you what the engine is looking for. It's always safer to add fuel.
Are you running the side hung float bowls on your carburetors or have they been changed over to center hung float bowls? How high does the front end come up in the air when you let go of the button?
If you're leaving off the transbrake with your foot flat on the floor tuning the accelerator pump circuit is most likely not going to be any help to you.
Try raising your float level. This may not cure the problem, but should tell you what the engine is looking for. It's always safer to add fuel.
Doug Schriefer
I had the same problem with my boat, it would leave the line hard at the intial hit of he throttle then fall on its face then fall off and then back on and do this for about 2.2 seconds according to the data logger.
I went to a jet extention in the rear of the carb (carb is mounted with floats in front/back) and I installed a relay on the fuel pump and it was fixed.The fuel pump was not getting the voltage it needed, according to aeromotive the pump can see 40% loss in efficency when it does not have what it needs as far as voltage so make sure its got what it needs or it may not be able to keep up.
I went to a jet extention in the rear of the carb (carb is mounted with floats in front/back) and I installed a relay on the fuel pump and it was fixed.The fuel pump was not getting the voltage it needed, according to aeromotive the pump can see 40% loss in efficency when it does not have what it needs as far as voltage so make sure its got what it needs or it may not be able to keep up.