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full throttle lag on 351c 

 

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 Post subject: full throttle lag on 351c
Posted: Tue Mar 22, 2005 8:07 pm 
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everytime i take off rapidly from a set of lights, i get a stutter, then the car takes off. It also seems to be struggling to rev over 5000rpm.
it's a mildly worked 351 with a c4.
is there any logical reason?
any help would be appreciated.

 

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Posted: Tue Mar 22, 2005 8:46 pm 
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More info?

What's been done? Any aftermarket ECU? Sounds like it's running lean in a part of teh fuel map where you engage the croutch and the revs drop but the manifold pressure is still very high.
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Posted: Tue Mar 22, 2005 8:50 pm 
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it's carby, it's auto, and has a mechanical fuel pump that was replaced when the motor was rebuilt.

 

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Posted: Tue Mar 22, 2005 9:02 pm 
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Is it coughing every time you mash the throttle, or just when you take off from the lights?

If it's every time you hit the throttle hard, or even just intermittently, it's most likely an accelerator pump problem.

Edit: Also, if it's struggling over 5000rpm, it could be running too lean, it could just be the cam, it could be the fuel pump isn't big enough, it could be a restriction in teh fuel system, it could be needle selection.

Easiest thing to to first is put it on a dyno with a sniffer and check out the AFR.
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Posted: Tue Mar 22, 2005 9:13 pm 
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unclewoja wrote:
Is it coughing every time you mash the throttle, or just when you take off from the lights?

If it's every time you hit the throttle hard, or even just intermittently, it's most likely an accelerator pump problem.

Edit: Also, if it's struggling over 5000rpm, it could be running too lean, it could just be the cam, it could be the fuel pump isn't big enough, it could be a restriction in teh fuel system, it could be needle selection.

Easiest thing to to first is put it on a dyno with a sniffer and check out the AFR.

yeah i wanna dyno it soon.
it won't do it if my foot is partly on the throttle, then mash it.

 

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Posted: Tue Mar 22, 2005 9:15 pm 
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Hmmm, have a look at the linkage to the accelerator pump... if you know where it is.

It might be binding somehow @ the top of the stroke and giving you grief the you mash the throttle from zero, but not binding in the middle of the stroke and not giving you the grief.

It also might be something else completely!!
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Posted: Tue Mar 22, 2005 9:20 pm 
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what is the accelerator pump?
i know what the fuel pump is

 

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Posted: Tue Mar 22, 2005 9:26 pm 
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Accelerator pump is a little cylinder with a little piston located on the carby somewhere.

It fills up with fuel and is controlled by a linkage connected to the throttle.

What is does is it pumps extra fuel into the air/fuel charge going into the engine when the throttle is opened fast so that the mixture doesn't lean out.
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Posted: Tue Mar 22, 2005 9:29 pm 
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ah ok, i got it re-jetted when i rebuilt the motor, with a new float, now im wondering if the accelerator pump got done.
might look into the carby a little more.
cheers for ya help.

 

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Posted: Tue Mar 22, 2005 9:41 pm 
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Check the fuel level in carb it may be too low also ??
But it sounds like fuel delivery problem to me..
Check plugs after a drive???

 

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Posted: Tue Mar 22, 2005 9:51 pm 
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how do i check fuel level in the carby?
and i checked plugs/timing today, didn't fix it.

 

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NEW RIDE - FPV FG F6 6spd Auto. 12.26@114MPH, 1.88 60FT on Cheapy Tyres with 38PSI - Stock except K&N filter!

R.I.P. BF XR6 Turbo - You will not be forgotten :( 12.06@120MPH

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Posted: Tue Mar 22, 2005 10:13 pm 
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smoke_rubber wrote:
how do i check fuel level in the carby?
and i checked plugs/timing today, didn't fix it.


Ahh I mean read the plugs.. If they look new and white in colour its way too lean....
What type carb ?
Sounds like power jets are too small...
Carbs if you don't know much about them.Be carefull, you could end up in more trouble as there are spings etc that can full out and go missing....
A leak on inlet can cause this also??

 

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Posted: Wed Mar 23, 2005 8:33 am 
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I've had a similar problem on a 351 with a vac' secondary carby the vacum springs were too light so they were opening to early/much and dumping to much fuel causing the engine to stutter. Take the car in to a mechanic even without a dyno a descent mechanic with carby experience should be able to sort out any problems.
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Posted: Wed Mar 23, 2005 1:49 pm 
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I had the exact same problem, but got the cabie cleaned by the local pro, $50 later, att better havent had a problem since. I did find that my problem went from that to a jet that wouldnt stop letting in fuel in about 6 months

 

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Posted: Wed Mar 23, 2005 8:08 pm 
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hey, you got electronic igniton or points? if u got points then it sounds like ya spark might be breakin down, get new points and set em properly with a dwell meter.

James

 

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