car launch stumble

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chris yellow camaro
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car launch stumble

Post by chris yellow camaro » Sat Apr 29, 2006 8:24 pm

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

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Post by Eric68 » Sun Apr 30, 2006 6:54 pm

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.
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chris yellow camaro
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Post by chris yellow camaro » Sun Apr 30, 2006 9:39 pm

well the car lauches goes about 10 feet and then feels like it shuts off and right back on
thanks

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Post by automotive breath » Sun Apr 30, 2006 9:43 pm

chris yellow camaro wrote:well the car lauches goes about 10 feet and then feels like it shuts off and right back on
thanks
It sounds like fuel slosh is uncovering the jets, try jet extensions.

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Post by NoMoore » Mon May 01, 2006 12:22 pm

Are the carbs sideways mounted? If they are, perhaps increasing the float level will be enough to get you going.
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Post by automotive breath » Mon May 01, 2006 12:25 pm

NoMoore wrote:Are the carbs sideways mounted? If they are, perhaps increasing the float level will be enough to get you going.
I thought about that after my post. Is there some sort of jet extensions used for sideways mounted carbs?

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Post by Eric68 » Mon May 01, 2006 1:46 pm

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.
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Post by automotive breath » Mon May 01, 2006 2:08 pm

Eric68 wrote:Well mine is doing the exact same thing -- goes for 10 feet then dies, picks up, then dies again . . . then finally takes off.....
If the jet extensions don't cure the bog, try larger squirters, some times necked down squirter screws are needed.

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Post by MadBill » Mon May 01, 2006 2:43 pm

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...
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Post by chris yellow camaro » Wed May 03, 2006 2:08 pm

thank you so much for all your replies i am going to start working with float level and float issue
thank you again
chris

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Post by Procision-Auto » Wed May 03, 2006 5:10 pm

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.

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Post by crazycuda » Wed May 03, 2006 5:21 pm

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.
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Post by Doug Schriefer » Thu May 04, 2006 8:25 am

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.
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Post by cs19 » Sun May 07, 2006 10:09 pm

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.

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