poor off-idle throttle response
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Senior User
Joined: Nov 2001
Posts: 294
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From: Calgary Canada
poor off-idle throttle response
I have a 302 in a '72 F-100. It has the stock 2 barrel carb, headers, and Pertronix ignition. My problem is that when I step on it hard, at idle, in drive, it does not respond but just sort of bogs. When I step on it like in easy normal driving it moves out OK.
I have played around with the ignition timing. With initial timing at 10º BTDC it pings badly if I mash the gas at idle. If I back it off so it doesn't ping then it feels weaker all around.
I'm starting to think that it's the carb. The accelerator pump linkage is set for maximum squirt and I do get a stream of gas when I work the throttle but it is more of a stream and not a spray...could this be the trouble? Would a carb. rebuilt help?
The engine feels strong once it gets rolling but the off-idle throttle response leave a lot to be desired.
Any help will be greatly appreciated.
I have played around with the ignition timing. With initial timing at 10º BTDC it pings badly if I mash the gas at idle. If I back it off so it doesn't ping then it feels weaker all around.
I'm starting to think that it's the carb. The accelerator pump linkage is set for maximum squirt and I do get a stream of gas when I work the throttle but it is more of a stream and not a spray...could this be the trouble? Would a carb. rebuilt help?
The engine feels strong once it gets rolling but the off-idle throttle response leave a lot to be desired.
Any help will be greatly appreciated.
Senior User
Joined: Oct 2002
Posts: 194
Likes: 1
From: united states
poor off-idle throttle response
Sounds like it could be a fuel mixture problem. Does it blow out blake smoke when it bogs? Get a vaccum guage and set your mixture and see if that helps. My one and a half cents.:-X12
Mountain Pass
Joined: Jun 2002
Posts: 230
Likes: 0
From: SW Maryland
poor off-idle throttle response
Check to see if your accelerator pump is clogged. My 2100 seems to clog up in between the pump and the jets...tough to get the crap out again. Test by removing the carb and working the pump with water or something
(if you use water, make damn sure you blow-dry the piping afterward. Water can crust up in the small passegways of the carb if you let it evaporate. It also doesn't mix with gas).
(if you use water, make damn sure you blow-dry the piping afterward. Water can crust up in the small passegways of the carb if you let it evaporate. It also doesn't mix with gas).
Elder User
Joined: Aug 2002
Posts: 662
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From: CA
poor off-idle throttle response
[updated:LAST EDITED ON 22-Dec-02 AT 09:34 PM (EST)]I'd bet this is almost certainly the accellerator pump worn out. You say with the timing up it detonates when you kick it (which it should if the timing is WAY to high), but at 10 btdc, this could also be an example of a lean condition, which is exactly what will happen with a shot pump diaphram. The fact that it runs strong at speed (I assume a steady speed) is because it's running on main metering and doesn't require nearly as much from the accellerator pump. Try running at about 10 mph and then kicking it HARD. If it bogs it's the pump. Rebuilding these carbs is easy, and that's the cure.
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Thread Starter
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Senior User
Joined: Nov 2001
Posts: 294
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From: Calgary Canada
poor off-idle throttle response
Is this normal: if I look down into the carb when I work the throttle there is a stream of gas from the accelerator that hits the round venturi thing. This is not a spray but a narrow stream. When it hits the venturi thing it gets atomized somewhat. Is that how it is supposed to work?
Anyways I am going to rebuild that carb. for peace of mind if nothing else.
Thanks for the great advise. Have a Merry Christmas :-X12
Anyways I am going to rebuild that carb. for peace of mind if nothing else.
Thanks for the great advise. Have a Merry Christmas
:-X12 FTE Stories
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Thread Starter
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Senior User
Joined: Nov 2001
Posts: 294
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From: Calgary Canada
poor off-idle throttle response
OK, I did a rebuild on my carb. and that seems to have done the trick. When I had it partially together without the top on, I filled the float bowl up with varsol and tested the accelerator pump. Even after cleaning, one side was squirting less than the other so it took the venturi booster off and blew it out again. Then both sides worked the same.
When I put it back together I put the accelerator link into the hole recommended in the instructions, which was the second one from the bottom. I previously had it in the top hole thinking that that would give me the most squirt.
The float setting was also way too high (by about 1/8") which may explain why I alway smell gas when the truck is in the garage.
Anyways, when I took it for a test drive it definately is much more responsive. I even increased the initial timing to 15 BTDC and no pinging. I might experiment with opening up the vacuum advance and play around with the accelerator link position to try and get the maximum off-idle snap .
Thanks for the help guys. Any suggestion on fine tuning this little beast.
When I put it back together I put the accelerator link into the hole recommended in the instructions, which was the second one from the bottom. I previously had it in the top hole thinking that that would give me the most squirt.
The float setting was also way too high (by about 1/8") which may explain why I alway smell gas when the truck is in the garage.
Anyways, when I took it for a test drive it definately is much more responsive. I even increased the initial timing to 15 BTDC and no pinging. I might experiment with opening up the vacuum advance and play around with the accelerator link position to try and get the maximum off-idle snap .
Thanks for the help guys. Any suggestion on fine tuning this little beast.
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