Carb Question
Thread Starter
Burning Brakes
Joined: Sep 2005
Posts: 1,139
Likes: 109
From: B'ham Alabama
Carb Question
I recently acquired my 71. She's an all original 270hp automatic with the numbers matching Q-Jet. She starts up and runs GREAT.
However, if I stomp down on it...the car spits, coughs and bogs down. It slowly picks up and reumes running well. After this, when I top and shut her down...the gas smell is terrible. No signs of leaking....just a terrible gas smell.
As long as I don't try to stomp down on the accelerator hard it does fine. It cruises at 75 to 80 with no problem. I just can't kick it down hard. I assume while it's running on the 2 barrels she's fine.
Any ideas or suggestions?
However, if I stomp down on it...the car spits, coughs and bogs down. It slowly picks up and reumes running well. After this, when I top and shut her down...the gas smell is terrible. No signs of leaking....just a terrible gas smell.
As long as I don't try to stomp down on the accelerator hard it does fine. It cruises at 75 to 80 with no problem. I just can't kick it down hard. I assume while it's running on the 2 barrels she's fine.
Any ideas or suggestions?
Thread Starter
Burning Brakes
Joined: Sep 2005
Posts: 1,139
Likes: 109
From: B'ham Alabama
But I started wondering if maybe the Q-Jet was opening up and dumping so much fuel in that the old plugs couldn't burn it, (thus bogging it down for a moment).
I'll try replacing them.
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
First, do a general tune-up with new points, condenser, plugs and wires. Second, get ready to rebuild the carb. The accelerator pump is shot-get one that is ethanol resistant.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I don't know, man. I would think that if the accelerator pump was bad, you'd have problems at part-throttle, too. If you give it 1/2 throttle with no accelerator pump, it's going to puke and stumble a bit. From the limited info, it sounds more like the secondaries are mis-adjusted. A general tune-up and carb rebuild is still a great idea, though.
Scott
Scott
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Most likely problems are: (in priority order)
1. The seal at the bottom of the accelerator pump (in the carb) is worn out or damaged by ethanol in your gas. You can replace just that by removing the top cover of the carb {and the other stuff required to get the top cover off}.
2. Your ignition timing is excessively retarded. Check timing with a timing light. Also determine if the vacuum advance can (on the distributor) and the mechanical advance system in the distributor are working properly. [this can be determined with your timing light by the action of the timing mark when you alter throttle position]
3. Ignition coil or condenser problems providing a weak spark.
1. The seal at the bottom of the accelerator pump (in the carb) is worn out or damaged by ethanol in your gas. You can replace just that by removing the top cover of the carb {and the other stuff required to get the top cover off}.
2. Your ignition timing is excessively retarded. Check timing with a timing light. Also determine if the vacuum advance can (on the distributor) and the mechanical advance system in the distributor are working properly. [this can be determined with your timing light by the action of the timing mark when you alter throttle position]
3. Ignition coil or condenser problems providing a weak spark.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
I recently acquired my 71. She's an all original 270hp automatic with the numbers matching Q-Jet. She starts up and runs GREAT.
However, if I stomp down on it...the car spits, coughs and bogs down. It slowly picks up and reumes running well. After this, when I top and shut her down...the gas smell is terrible. No signs of leaking....just a terrible gas smell.
Any ideas or suggestions?
However, if I stomp down on it...the car spits, coughs and bogs down. It slowly picks up and reumes running well. After this, when I top and shut her down...the gas smell is terrible. No signs of leaking....just a terrible gas smell.
Any ideas or suggestions?
- Incorrect plug gap
- Bad plug wires
- Incorrect dwell
- Bad points
- Retarded timing
- Vacuum advance control unit stuck
- Sticking centrifugal advance
- Loose secondary airvalve spring tension
- Sticking secondary airvalves
- Ruptured choke pulloff
- Misadjusted or missing secondary airvalve rod
- Low float level
- Incorrect float
- Secondary fuel transfer tubes fallen out of airhorn - laying in float bowl
- Low float level
- Incorrect secondary rod hanger height
- Incorrect secondary rods
- Incorrect primary jetting & rods
- Commercially rebuilt carb with incorrect air bleeds installed
- Accelerator pump rod installed in wrong hole in pump lever
- Bad accel pump
- Choke misadjusted - not opening fully
- Secondary lockout lever not fully retracting
Lars
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Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
That's the point. It's absolutely, positively impossible to troubleshoot drivability issues as described via Forum posts and e-mails. I've listed 23 items that all can cause, or contribute to, the issues noted. Most likely, it's not one of the issues listed - it's likely a combination of several of those issues combined with an issue that's not even on the list that nobody could imagine (I had a guy try to install a Q-Jet backwards once, and he couldn't figure out his throttle linkage. I spent hours with him on e-mails and phone calls until I realized he actually had the carb backwards. I'm not kidding). It's probably not an accel pump issue, or the car would die as he tried pulling out from a standing stop. But maybe it is.
Bottom line and fix:
The car needs to be given a complete proper ignition system setup, and then the carb needs to be torn down, inspected, cleaned up, corrected, set up, and tuned.
Lars
Bottom line and fix:
The car needs to be given a complete proper ignition system setup, and then the carb needs to be torn down, inspected, cleaned up, corrected, set up, and tuned.
Lars
Last edited by lars; May 11, 2014 at 08:30 PM.
Thread Starter
Burning Brakes
Joined: Sep 2005
Posts: 1,139
Likes: 109
From: B'ham Alabama
Do you do carb work for the public, (forum members)?
If so...would you be interested in working on mine?
What is the process of getting it to you?
What is an average turn around time?
What would an "estimated" rebuild / setup cost run?
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
Just drop me an e-mail with a request for my Services Info Sheet. You might also ask me for my timing papers: "90% of all carb problems are ignition problems." Set up your timing and ignition curve before ever touching the carb, since you cannot compensate for timing issues with carb tuning. Once timing is known to be good, you can tackle the carb, and I'll be glad to set it up for you.
Lars
V8FastCars@msn.com
Lars
V8FastCars@msn.com
Thread Starter
Burning Brakes
Joined: Sep 2005
Posts: 1,139
Likes: 109
From: B'ham Alabama
Just drop me an e-mail with a request for my Services Info Sheet. You might also ask me for my timing papers: "90% of all carb problems are ignition problems." Set up your timing and ignition curve before ever touching the carb, since you cannot compensate for timing issues with carb tuning. Once timing is known to be good, you can tackle the carb, and I'll be glad to set it up for you.
Lars
V8FastCars@msn.com
Lars
V8FastCars@msn.com
Thanks Lars,
Email sent.
DT
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
The following is a list of just a few of the issues that will cause and/or contribute to the problems you describe:
Lars
- Incorrect plug gap
- Bad plug wires
- Incorrect dwell
- Bad points
- Retarded timing
- Vacuum advance control unit stuck
- Sticking centrifugal advance
- Loose secondary airvalve spring tension
- Sticking secondary airvalves
- Ruptured choke pulloff
- Misadjusted or missing secondary airvalve rod
- Low float level
- Incorrect float
- Secondary fuel transfer tubes fallen out of airhorn - laying in float bowl
- Low float level
- Incorrect secondary rod hanger height
- Incorrect secondary rods
- Incorrect primary jetting & rods
- Commercially rebuilt carb with incorrect air bleeds installed
- Accelerator pump rod installed in wrong hole in pump lever
- Bad accel pump
- Choke misadjusted - not opening fully
- Secondary lockout lever not fully retracting
Lars
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Testing for accelerator pump operation is very easy: Remove air cleaner housing/filter; hold choke flap open (over primary venturi) and look down into the primaries; manually actuate the carb throttle linkage to full-open position quickly. If you see a strong squirt of fuel being shot into each of the two primary venturi, the accelerator pump is fine. If you see very weak streams...or none at all...the pump seal probably needs to be replaced.
But, getting the entire carb reviewed, repaired and rebuilt by Lars is a good way to insure its performance.
But, getting the entire carb reviewed, repaired and rebuilt by Lars is a good way to insure its performance.
Melting Slicks
Joined: Jul 2010
Posts: 2,503
Likes: 33
From: McGrady NC
St. Jude Donor '15-'16
He fixed a bunch of problems I had by rebuilding and setting up my carb. Very satisfied with the results! I highly recommend sending it to him AFTER you give your car a proper tune-up. That was he knows what your engine's condition is and can better judge what settings your carb needs.