Carb hard start & stumble
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
Carb hard start & stumble
On a stock 350 w/Holley 650 double pumper that has just been professionally rebuilt, the car is hard to start, then stumbles slightly at acceleration when warm. Timing is correct and the mixture seems good (definitely not rich), and there are no apparent vacuum leaks. Hard acceleration from cruising speed is no problem at all.
I am new to carb diagnosis and after reading similar posts on the forum, I am a bit confused as to what the problem might be. I'd like to do the work myself so I'll learn how the system works for future problems. Any suggestions or ideas for a fix?
I am new to carb diagnosis and after reading similar posts on the forum, I am a bit confused as to what the problem might be. I'd like to do the work myself so I'll learn how the system works for future problems. Any suggestions or ideas for a fix?
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
? valve timing retarded?
Is it possible the camshaft was installed with alotta retard in valve timing? Typically, retarded valve timing improves mid/upper rpm performance while lower range suffers.
*edit* maybe you meant only the carb was built ... and not the motor ... if so, ignore me.
*edit* maybe you meant only the carb was built ... and not the motor ... if so, ignore me.
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
valve timing retarded?
Good point, Jackson. Only the carb was rebuilt. Engine is completely stock. the carb was rebuilt to: 1) ensure that it worked properly, and 2) solve a rich condition, and 3) stop fuel leaking from both bowls. If anything (post cab rebuild), I'd say the engine may be running lean, but I cannot see where that would make the car be hard to start, run rough until at temp, then stumble upon acceleration.
I will recheck the timing. Open to any other suggestions.
I will recheck the timing. Open to any other suggestions.
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
valve timing retarded?
Additional information - I discovered this morning that the problem may be in the manual choke. It seems to be sticking open when the engine is cold. If I hold the choke closed during startup, it starts right up and runs much better. I wonder if the choke spring loses tension over time to cause the choke to stick open, or if I have the choke linkage connected to the wrong place, i.e. push vs. pull position on the carb? As the engine warms up, the choke appears to work properly.
Even so, if the car is at cruise temp, I wouldn't think the choke being loose would cause a stumble when accelerating from a traffic light.
Even so, if the car is at cruise temp, I wouldn't think the choke being loose would cause a stumble when accelerating from a traffic light.
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
The intake is a stock GM, with a single opening that leads into the manifold caviy rather than the four hole or two double wide opening configurations that I have seen before. The carb is (I think) called a spread bore, and has about a half inch spacer between the carb and manifold.
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Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Is the choke manual (sounds like it by your description) but if its elecrtic the heater may be shot or it isnt getting the 12 Volts that it needs to operate.
In any case, it sounds like you figured out the cold start problem.
Can you drive the car to the carb rebuilder and have him diagnose it?
You stated one of the reasons for the rebuild was to cure a rich condition
Stumbling can be a sign of a lean condition so perhaps its set up too lean
I know nothing about Holleys so maybe a Holley guy will jump in and give you some advice
In any case, it sounds like you figured out the cold start problem.
Can you drive the car to the carb rebuilder and have him diagnose it?
You stated one of the reasons for the rebuild was to cure a rich condition
Stumbling can be a sign of a lean condition so perhaps its set up too lean
I know nothing about Holleys so maybe a Holley guy will jump in and give you some advice
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
The choke is actually a divorce spring setup instead of a vacuum choke like I first thought. The vacuum device I spoke of earlier appears to be for the secondaries. To hold the choke in the closed position, I used a magnet on the outside of the carb body and stuck it directly on the choke linkage. The engine fires right up, and runs fine. Then when the engine is warm, I take the magnet off so I can drive the car.
A friend told me that the spring can, over time, lose its tension and needs to be replaced. He also advised me that if I want to go for reliability, I should consider getting a Rochester Q-jet type replacement. I don't know the model number off hand, but I am sure I've seen it elsewhwere in the CF. My next steps are 1) replace the divorce springat the intake, 2) adjust the fuel misture slighty to keep the engine from running too lean, and 3) If necessary, take the car to the carb shop to see if they can do anything. Thanks for all your help in educating me on carburetors.
A friend told me that the spring can, over time, lose its tension and needs to be replaced. He also advised me that if I want to go for reliability, I should consider getting a Rochester Q-jet type replacement. I don't know the model number off hand, but I am sure I've seen it elsewhwere in the CF. My next steps are 1) replace the divorce springat the intake, 2) adjust the fuel misture slighty to keep the engine from running too lean, and 3) If necessary, take the car to the carb shop to see if they can do anything. Thanks for all your help in educating me on carburetors.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
The choke thermostat should be ok but you can buy a new one if you want, you live in a warm climate so as long as the choke opens all the way and STAYS open your fine.
Actually many people in warm states dont even use a choke. You just need to baby the pedal for a minute or so till its warmed up.
If you just had the Holley rebuilt I would certainly ask the rebuilder what changes he made and ask him to take a look at it
If its a Holley spread bore than Im not sure what intake you have but it doesnt sound sock unless the 72's came with Holley carbs as an option.
Can you post a pic of the intake with the carb removed?
I hope somebody will pop in soon and clear up the Carb/Intake mystery
Thats about the best I can do for now.......... sorry
Actually many people in warm states dont even use a choke. You just need to baby the pedal for a minute or so till its warmed up.
If you just had the Holley rebuilt I would certainly ask the rebuilder what changes he made and ask him to take a look at it
If its a Holley spread bore than Im not sure what intake you have but it doesnt sound sock unless the 72's came with Holley carbs as an option.
Can you post a pic of the intake with the carb removed?
I hope somebody will pop in soon and clear up the Carb/Intake mystery
Thats about the best I can do for now.......... sorry
Last edited by Bob Onit; Aug 13, 2006 at 06:00 PM.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Originally Posted by Vette Set
Additional information - I discovered this morning that the problem may be in the manual choke. It seems to be sticking open when the engine is cold. If I hold the choke closed during startup, it starts right up and runs much better. I wonder if the choke spring loses tension over time to cause the choke to stick open, or if I have the choke linkage connected to the wrong place, i.e. push vs. pull position on the carb? As the engine warms up, the choke appears to work properly.
Even so, if the car is at cruise temp, I wouldn't think the choke being loose would cause a stumble when accelerating from a traffic light.
Even so, if the car is at cruise temp, I wouldn't think the choke being loose would cause a stumble when accelerating from a traffic light.
Is it a manual choke or an automatic choke? You keep talking about a spring. Manual chokes don't have springs, that's why they're manual. If it is an automatic, is it hooked up properly? Do you pump the throttle once before you start it? If so, does this cause the choke to snap shut? Have you ever driven a car with a carburetor before? Some of the young guys don't know how to start anything but fuel injection.
As far as the stumbling goes, make sure the primary accelerator pump is adjusted properly. Also, the off idle system may be a little lean. If this thing has replaceable air bleeds, you may want to go a little smaller on the idle.
BigBlockk
Later.....
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by BigBlockk
What is the number on the carburetor?
Is it a manual choke or an automatic choke?
Is it a manual choke or an automatic choke?
I confused manual with spring rather than Spring VS electric
He has a divorced spring auto choke
My fault.....
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Originally Posted by Bob Onit
Sorry BB, that was my mistake
I confused manual with spring rather than Spring VS electric
He has a divorced spring auto choke
My fault.....
I confused manual with spring rather than Spring VS electric
He has a divorced spring auto choke
My fault.....
BigBlockk
Later.....
Pro
Joined: Jun 2004
Posts: 700
Likes: 0
From: Lantana TX
Originally Posted by Vette Set
On a stock 350 w/Holley 650 double pumper that has just been professionally rebuilt, the car is hard to start, then stumbles slightly at acceleration when warm. Timing is correct and the mixture seems good (definitely not rich), and there are no apparent vacuum leaks. Hard acceleration from cruising speed is no problem at all.
I am new to carb diagnosis and after reading similar posts on the forum, I am a bit confused as to what the problem might be. I'd like to do the work myself so I'll learn how the system works for future problems. Any suggestions or ideas for a fix?
I am new to carb diagnosis and after reading similar posts on the forum, I am a bit confused as to what the problem might be. I'd like to do the work myself so I'll learn how the system works for future problems. Any suggestions or ideas for a fix?
Not to kill it, but a 650 double pumper on a stock 72 may be a little to much. A 650 with vac secondaries would be much better at street driving.
Anyway, it seems that you have the choke all set. So to tackle the stumble. First you need to start at the beginning and make sure you timing is OK, while checking the idle screws all of them need to be all the same distance in or out (use a vacuum gauge to adj to max vacuum). Increase the nozzles buy 3 #'s and see if that works or if it increases the response. Increasing mine made almost my entire stumble go away. Next, the cam needs to be swapped to a correct size. Buy the kit (cheap) and try them all to see which one gets rid of it. It took me a few to get to the 4th cam to get rid of my stumble. The cams can deliver more or less fuel until the main circuit kicks in, which will help you rid through the lean spot. This is what you are likely experiencing
I hope that helps
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
Thank you all for the good insight and discussion on my carb problem. Please excuse my ignorance on some of the part names on the carburetor. I very much appreciate the information presented by the CF members.
BigBlockk – I did mention that it was a manual choke... I knew that was wrong after I hit the Submit Reply button. Sorry for the confusion.
To answer your questions, the list number on the top corner of the front bowl is 6210-3, which according to Holley’s web site is a model 4261. The choke is a divorce (or “remote” as referred in some part manuals). There is a photo of a similar setup on a Buick at:
http://www.buickperformance.com/Edelbrockmod1.jpg
I have driven many carbureted vehicles in my 35 years of driving, so I know about the one pump prior to starting. Sometimes it take more, others, not as much. I tried the single pump method, but the choke stays about 75% open. When I force the choke closed by holding it (either by hand or by amagnet as stated in an earlier post), the car starts right up.
The accelerator pump adjustment is my next course of action. Then assuming that works, I can make adjustments to the mixture to correct the stumble. Excellent advice by all, thanks again!
BigBlockk – I did mention that it was a manual choke... I knew that was wrong after I hit the Submit Reply button. Sorry for the confusion.
To answer your questions, the list number on the top corner of the front bowl is 6210-3, which according to Holley’s web site is a model 4261. The choke is a divorce (or “remote” as referred in some part manuals). There is a photo of a similar setup on a Buick at:
http://www.buickperformance.com/Edelbrockmod1.jpg
I have driven many carbureted vehicles in my 35 years of driving, so I know about the one pump prior to starting. Sometimes it take more, others, not as much. I tried the single pump method, but the choke stays about 75% open. When I force the choke closed by holding it (either by hand or by amagnet as stated in an earlier post), the car starts right up.
The accelerator pump adjustment is my next course of action. Then assuming that works, I can make adjustments to the mixture to correct the stumble. Excellent advice by all, thanks again!
Thread Starter
Burning Brakes
Joined: Jul 2006
Posts: 888
Likes: 1
From: North Texas
Here's a photo of my intake and choke side of the carb for what its worth. The Divorce choke spring is shown at the bottom center of the carburetor.
http://i92.photobucket.com/albums/l1...Pchokeside.gif
http://i92.photobucket.com/albums/l1...Pchokeside.gif
