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Stumbling in 1st Gear

rstockbridge

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I have a 400 4 speed that I just installed a new carb on. The car runs fair when it's cold, but once it warms up it stumbles when taking off from a stop. I have the drivers side vacuum port plugged, the passenger side port goes to the distributor, and the PCV port goes to the brake booster as I do not have a PCV valve. The port in the back is plugged. Anyone got any ideas what my issue could be?
 
A little more info please, what kind and size carb do you have? What intake? What is your timing set at? What other engine modifications do you have (what cam ,headers, headwork, Heads (Stock or aftermarket?)) Everything can play a part in the overall tuning.
The More info the easier it is to help you out! :)
 
A little more info please, what kind and size carb do you have? What intake? What is your timing set at? What other engine modifications do you have (what cam ,headers, headwork, Heads (Stock or aftermarket?)) Everything can play a part in the overall tuning.
The More info the easier it is to help you out! :)
750 CFM electric choke carb, aluminum intake, mild cam, headers, stock heads, 6AL box, MSD ready to run distributor. I'm not sure what the timing is set at
 
Sounds like a vacuum leak or a plugged carb. Choke on its enriching the mix and helps hide the lean condition. Choke open now it will be real sensitive to the lean mix. Lightly hold a rag over the air horn and see as you gragually choke it if it wants to smooth out.
 
Sounds like a vacuum leak or a plugged carb. Choke on its enriching the mix and helps hide the lean condition. Choke open now it will be real sensitive to the lean mix. Lightly hold a rag over the air horn and see as you gragually choke it if it wants to smooth out.
I'm curious if it would resolve the issue if I added a PCV valve and moved the brake booster line to the outlet on the back of the carb.
 
I'm curious if it would resolve the issue if I added a PCV valve and moved the brake booster line to the outlet on the back of the carb.
Adding the pcv will do nothing. Booster will do nothing. Only thing with the booster is if it has a leaking diaphragm it is a vacuum leak.
 
Sorry, ^^^^ this is wrong.

As a veeeery rough guide, a V8 inhales about 20 cfm at idle. A carb designed to use PCV gets about 3 cfm from the PCV.
By blocking the PCV, t/blades have to be open further to get idle air.

People's idea of what constitutes a 'mild' cam................varies.
It is likely you have too much transfer slot exposed in the pri bores, which can give an off idle flat spot. Try hooking up the PCV. There is no valid reason not to use it. Make sure it is not vibrating, which would mean your cam is not making enough vac to operate it.
 
Here are the cam specs:

Cam Style:
Hydraulic flat tappet
Camshaft Use:
Street/Strip
Basic Operating RPM Range:
2,000-5,800
Intake Duration at 050 inch Lift:
227
Exhaust Duration at 050 inch Lift:
241
Duration at 050 inch Lift:
227 int./241 exh.
Advertised Intake Duration:
279
Advertised Exhaust Duration:
296
Advertised Duration:
279 int./296 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.486 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.473 in.
Valve Lift with Factory Rocker Arm Ratio:
0.486 int./0.473 exh.
Lobe Separation (degrees):
107
 
We need all of the info. Maybe a Holley or Edelbrock carb, maybe a single plane or dual plane intake manifold. It all works together. Hook everything up correctly and it has a better chance of working. The best place to start.
 
We need all of the info. Maybe a Holley or Edelbrock carb, maybe a single plane or dual plane intake manifold. It all works together. Hook everything up correctly and it has a better chance of working. The best place to start.
Sorry for the lack of info... still a learning process for me. I have a Edelbrock 750, and dual plane aluminum intake
 
Ignore the ignorance of post #8.

From the specs, you have one of those Bootlegger/Muthathumpa/Whiplash/Bumrush cams which as a very poor choice for a street car. They were designed to impress the drive-in crowd with the rowdy exh.

The problem is the cam, not the carb. The carb is just showing up the problems you get with a cam that has a lot of overlap.
I suspect the idle vac is only going to be 10-12", so the stock PCV will be unlikely to work reliably. If it is removed & blocked, pri blades on the carb need to be opened further [ more later on this ] to compensate for the air that the PCV would normally provide. Two choices here, buy an adj PCV [ such as a Wagner ] or gut the stock PCV & braze/weld in a plate with a 7/64" orifice [ this duplicates the flow area of a stock PCV at idle ].

Once the decision is made about the PCV, carb should be removed to check the T slot. At idle, should be about 0.040" of slot showing. I expect it to be much more, which is causing your stumble. In the Edel Owners manual, the section on low vac cams recommends plenty of initial timing &/or using vac adv connected to manifold vacuum to do this.

Very simple first step reqd here: with engine idling, loosen dist clamp & SLOWLY turn dist CW [ this advances timing ]; stop when you get smoothest idle/highest rpm; check what the timing is & report back.

Below is from a Holley carb book. It talks about idle timing of 50* BTDC!! Think about why idle timing isd talked about in a carb book.....& in more than one place.

img307.jpg
 
Hi,
If you don't have a pcv what kind of crank case ventilation do you have?
Without any ventilation you can blow out gaskets and oil out dip stick tube and the motor won't run very good with excess internal pressure.

good luck take care be safe
tim
 
PCV is something that has no negatives, only positives. I guess that is why it is still used to this day, 60+ yrs after it was first used....
 
Ignore the ignorance of post #8.

From the specs, you have one of those Bootlegger/Muthathumpa/Whiplash/Bumrush cams which as a very poor choice for a street car. They were designed to impress the drive-in crowd with the rowdy exh.

The problem is the cam, not the carb. The carb is just showing up the problems you get with a cam that has a lot of overlap.
I suspect the idle vac is only going to be 10-12", so the stock PCV will be unlikely to work reliably. If it is removed & blocked, pri blades on the carb need to be opened further [ more later on this ] to compensate for the air that the PCV would normally provide. Two choices here, buy an adj PCV [ such as a Wagner ] or gut the stock PCV & braze/weld in a plate with a 7/64" orifice [ this duplicates the flow area of a stock PCV at idle ].

Once the decision is made about the PCV, carb should be removed to check the T slot. At idle, should be about 0.040" of slot showing. I expect it to be much more, which is causing your stumble. In the Edel Owners manual, the section on low vac cams recommends plenty of initial timing &/or using vac adv connected to manifold vacuum to do this.

Very simple first step reqd here: with engine idling, loosen dist clamp & SLOWLY turn dist CW [ this advances timing ]; stop when you get smoothest idle/highest rpm; check what the timing is & report back.

Below is from a Holley carb book. It talks about idle timing of 50* BTDC!! Think about why idle timing isd talked about in a carb book.....& in more than one place.

View attachment 1528568
Can I install the adjustable PCV in place of one of the breathers on the valve covers? Or do I need both breathers to stay in place?
 
If it’s got a stumble it’s probably lean, like mentioned need more info. Only one breather required for a PCV. What’s kind of carb? That’s not a very big cam, although it’s kinda funky. Looks like a thumper type grind (don’t care for those).
 
If it’s got a stumble it’s probably lean, like mentioned need more info. Only one breather required for a PCV. What’s kind of carb? That’s not a very big cam, although it’s kinda funky. Looks like a thumper type grind (don’t care for those).
It’s got a Edelbrock 750, and it is in fact a thumper cam
 
Hi,
First thing I would look at would be to see if the accelerator pump is working... With the motor off look in the carb and move the accelerator linkage and see if there is a squirt of fuel on the first movement of the linkage. if no squirt then the accelerator pump is bad.
 
I added the adjustable PCV valve, and the accelerator pump seems to be working fine, but I'm still stumbling in first gear. It's like I'm trying to take off in third gear. Anybody got any other ideas?
 
You need to get your timing set to optimal before messing with carb settings. You never did say what you are running for timing.....
 
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