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hesitation on heavy acceleration on 400
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fred8
New User
| Posts: 15
| Joined: 06/09
Posted: 09/20/09
04:20 PM
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Hello, my 1977 pontiac 400 has a Q jet which was just adjusted and the engine runs pretty good except when you floor it it hesitates alittle then takes off and runs good. It does this in drive but not when its in park and if you ease into the throttle it works fine. We lowered the timing to about 8 BFT and like I said it only does this when you punch it. Thanks for any help or ideas on this.
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Posted: 09/20/09
05:00 PM
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How long since the q-jet has been rebuilt? The older kits have accelerator pump rubber cups that are not compatible with the new high ethanol blended gasoline. These cups fail and cause a hesitation on acceleration just as you describe. Replace the cup with a new one from a new carb kit (I think the new ones are blue, I'll have to check on it) and the problem should go away. the issue is not enough volume from the accelerator pump on heavy acceration which causes the bog or hesitation.
Steve
A little help... 'cause we don't all have to learn the hard way!
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fred8
New User
| Posts: 15
| Joined: 06/09
Posted: 09/20/09
05:12 PM
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The carb was rebuilt about 2 years ago but Ill check to see if the cups are blue. Thanks
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Posted: 09/20/09
05:22 PM
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Yes, the blue cups are the good ones. Cut and paste this address into the address bar, it is a link to an HPP article on the new fuels and Q-jets.
Steve
http://www.highperformancepontiac.com/tech/hppp_0909_ethanol_fuel_and_vintage_vehicles/vintage_vehicle_ethanol_fuel_conversion.html
A little help... 'cause we don't all have to learn the hard way!
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Posted: 09/20/09
10:16 PM
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this is a picture of where a mid to late 70s qjet has a hidden adjustment.. that will totally affect drivability
http://www.hotrod.com/techarticles/engine/edelbrocks_q_jet_carburetor_replacment/photo_06.html
there is a full article there also..
the 3 photo up from the bottom is something that you want to make sure that you don't adjust while you have the carb apart... many people make that mistake... and then it is a drive and turn it a quarter turn... drive and turn it a quarter turn...
http://www.vetteprojects.com/kstyer/quadrajet_3.htm
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Posted: 09/21/09
04:52 PM
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Lowering your timing is part of the problem as well.Bump it up to 10 or 12 degrees advanced at idle this should help the acceleration alot.Hopefully your mechanical advance is in good shape as well giving you at least 28 degrees advanced in by 2500 rpms.Every car is a little different so you have to play with it a little bit.
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