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So my 1973 1275 with a Weber DGV has been running like a top until just now. I just returned from a one hour drive on the expressway and now the car is stalling on the low end. When I start off from a stop the car stutters until about 3500 rmp then it smoothes our. The interesting thing is the transition form stuttering and lack of acceleration to normal operation and good acceleration is knife edged. All of a sudden the car starts working perfectly. Any ideas?
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low end stutter
- Thread starter mbaha
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RickB
Yoda
Offline
Mine just started doing the same kind of thing yesterday.
It was idling perfectly until I got home, stopped at the post office and when I got back in it would no longer idle.
Fooled around with the carb until I could make it run (stop dying).
Wish I knew why they suddenly start acting this way.
It was idling perfectly until I got home, stopped at the post office and when I got back in it would no longer idle.
Fooled around with the carb until I could make it run (stop dying).
Wish I knew why they suddenly start acting this way.
blkcorvair
Jedi Knight
Offline
1st think I would look at is fuel and or fuel filter. That sounds like a fuel filter that has begun to clog. When there is a load you are demanding more fuel. Often a fuel filter is only noticed at a load and not by simply revving the engine.
If it was raining or just washed it could be a wet distributor issue although that doesnt normally clear up when revving with no load.
If the car is still equipped with its original cat than take the car for a quick up to temperature spin at night and park it in a real dark spot. A clogged cat will often get so hot they will lightly and even sometimes glow bright red.
Definatally start with the fuel filter tho. Sounds to be exactly what your describing
If it was raining or just washed it could be a wet distributor issue although that doesnt normally clear up when revving with no load.
If the car is still equipped with its original cat than take the car for a quick up to temperature spin at night and park it in a real dark spot. A clogged cat will often get so hot they will lightly and even sometimes glow bright red.
Definatally start with the fuel filter tho. Sounds to be exactly what your describing
sqbsprite
Jedi Trainee
Offline
Here's some good info on the Weber that might help. It could be the accelerator pump.
https://www.theautoist.com/weber_carb.htm
https://www.theautoist.com/weber_carb.htm
Morris
Yoda
Offline
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]So my 1973 1275 with a Weber DGV has been running like a top until just now. I just returned from a one hour drive on the expressway and now the car is stalling on the low end. When I start off from a stop the car stutters until about 3500 rmp then it smoothes our. The interesting thing is the transition form stuttering and lack of acceleration to normal operation and good acceleration is knife edged. All of a sudden the car starts working perfectly. Any ideas? [/QUOTE]
A crack at over explaining this from someone who has almost no DVG experience:
The beauty of the SU style carb is that the piston keeps air flowing fast and close to the jets. As you open the throttle, the fastest part of the airflow stays over the jet allowing the carb to pick up plenty of fuel through out the throttle range. Enter the more coventional DVG. When you open the throttle on any carb, manifold vacuum drops until engine speed and demand catch up. When you open the DVG throttle a rush of air enters the manifold, but with out the piston to keep the air flow close to the jets, the air flow over the jets is disturbed and it takes a moment for the venturi effect to pull enough fuel out of the bowl to keep up with the new amount of air being drawn through the throttle. The solution: the accelerator pump. The accel-pump squirts a stream of gas into the air stream thus enrichening it until the venturi-jets catch up with demmand.
So... if you have an issue with your accel-pump, your mixture is leaning out under acceleration thus causing stuttering. If the car runs fine at top end then this is a more likely explanation than fuel supply. If fuel supply were a problem, you would feel it when cruising at top end.
Ahhhhh.... I feel better now. Back to my boring job.
A crack at over explaining this from someone who has almost no DVG experience:
The beauty of the SU style carb is that the piston keeps air flowing fast and close to the jets. As you open the throttle, the fastest part of the airflow stays over the jet allowing the carb to pick up plenty of fuel through out the throttle range. Enter the more coventional DVG. When you open the throttle on any carb, manifold vacuum drops until engine speed and demand catch up. When you open the DVG throttle a rush of air enters the manifold, but with out the piston to keep the air flow close to the jets, the air flow over the jets is disturbed and it takes a moment for the venturi effect to pull enough fuel out of the bowl to keep up with the new amount of air being drawn through the throttle. The solution: the accelerator pump. The accel-pump squirts a stream of gas into the air stream thus enrichening it until the venturi-jets catch up with demmand.
So... if you have an issue with your accel-pump, your mixture is leaning out under acceleration thus causing stuttering. If the car runs fine at top end then this is a more likely explanation than fuel supply. If fuel supply were a problem, you would feel it when cruising at top end.
Ahhhhh.... I feel better now. Back to my boring job.
racingenglishcars
Darth Vader
Offline
The SU actually does have an enrichenment mechanism, though not well known and not completely effective and not so much needed. It's the damper in the piston.
The SU is called a "constant velocity" carb because at lower engine speeds, the air passes the venturi at nearly the same speed as it does at high speeds. The vacuum difference over the venturi determines the piston height and therefore the venturi opening and therefore the air speed.
The DVG just like most other carbs has that inrush of air upon opening, causing a lean mixture. Accelerator pumps help this.
Of course your problem could also be a marginal ignition system not capable of a spark at higher effective cylinder pressures such as when accelerating from low speeds. Or points that are not quite opening well.
The SU is called a "constant velocity" carb because at lower engine speeds, the air passes the venturi at nearly the same speed as it does at high speeds. The vacuum difference over the venturi determines the piston height and therefore the venturi opening and therefore the air speed.
The DVG just like most other carbs has that inrush of air upon opening, causing a lean mixture. Accelerator pumps help this.
Of course your problem could also be a marginal ignition system not capable of a spark at higher effective cylinder pressures such as when accelerating from low speeds. Or points that are not quite opening well.
blkcorvair
Jedi Knight
Offline
Turns out all my lawn equipment has been breaking down this year. Really frustrating. Acting like they have water in the gas. I began to wonder if it is the ethenol in the gas now. Switched gas stations today and threw some high test in my tractor after completely tearing it down and draining the tank. Ran much better. Now I am experiancing it with my Golf car as well. I swear gasoline just gets worse every year. May be part of your problem
Morris
Yoda
Offline
My experience with air leaks is that they cause a high idle, and they kill your top end power. It certainly does not hurt to check for air leaks, but unless it is a variable air leak, like a bad vacuum circuit that is only open under off-idle acceleration, it is unlikely to be the culprit.
But I am frequently wrong. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
But I am frequently wrong. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
