TheSamba.com :: View topic - Idles Fine, Sputters on Acceleration, Stalls When Given Gas

Sat Jan 24, 2009 9:28 am

Alright,
I recently purchased a new bus (My 2nd, although my first running). It is a 1978 Automatic Transmission. Upon my purchase we swapped the improperly installed Type 1 for my 1973 1.8 Type 4 engine. Here comes the problem... The bus idles fine, perfect actually, but upon initial acceleration the bus sputters and stalls. If I let it get past the sputter by letting the bus start moving at idle speed, and then slowly accelerate it accelerates flawlessly. I have cleaned the carb completely out, and checked all of vacuum lines I could spot. I'm assuming it's timing as the fuel filter is brand new, all new spark plugs, wires, distributor cap, and just about everything else that seems like it could be the problem has been looked over. I am new to these VWs and still learning my way as I go, so I'm looking for some advice. I am running a tank of 91 Octane with a bottle of Octane booster through at the moment, and it did seem to clear some of the roughness of the problem up, but the problem persists... Does it sound like the timing needs to be advanced? I'm at a loss for what to do, and I need this bus to run as it's my daily driver now!

Updates Below:

Alright, Here's what we did last night and tonight.
We got the timing to about 7.5 BTDC,
The Accelerator Pump is functioning, and is shooting a stream of gas into the carb upon acceleration,
Found out the Butterfly Valves were sticking open upon deceleration causing too much air to enter the engine,
Lubed up the butterfly valves,
Installed a stiffer throttle spring (Helped out a lot, the original was pretty weak),
Double Checked to make sure all of the mounting bolts on the carb were tight,
backed the fuel/air mixture screw out 2.5 turns.

Also, The main thing that makes me think I know what the culprit is was this:

I pulled the main vacuum line the runs from the Carb to the Distributor and it idled perfectly and stayed running. Before that (Due to the cold I believe) it would not idle and would not stay running.

I have read that the weber progressive carbs are known for not putting out enough vacuum for a vacuum advanced distributor to work correctly.
Do you think my problems would be solved if I were to switch to a 009 Mechanically Advanced Distributor?

That would remove one point of failure.

It was about 20-25 degrees tonight and we were working out in the cold so we weren't able to do a lot.

Where should I go from here?

Sat Jan 24, 2009 9:30 am

Your distributor, is it vaccuum adv or mech adv?

Sat Jan 24, 2009 9:36 am

Oops, sorry I didn't mention that.

It's Vacuum adv.

Sat Jan 24, 2009 9:41 am

The carb = a progressive Weber or Holley? Do you see a squirt of gas down into the throat of the carb as you open the throttle? If you don't your engine will sputter badly or just plain die as you try to get it to rev.

Sat Jan 24, 2009 9:42 am

Well, if you have checked for vac leaks then see if the advancing mechanisms moves freely. Also, might want to re-check the timing if that distributor works as it should.

Sat Jan 24, 2009 9:44 am

The carb is a Weber Progressive...Geez I'm leaving out all of the details!

Wildthings, What's the best way to check this?

Sat Jan 24, 2009 10:05 am

Remove the air cleaner and look down the throat of the carb while you work the throttle. It should shoot a stream of gas into the primary throat. It is easier and safer to do this with the engine off.

Sat Jan 24, 2009 10:15 am

Alright, So here's my list of things to do when I get home from work.

Continue Checking For Vac Leaks,
Check If the advancing mechanism moves freely,
Double Check the Timing (What's a good place to start?)
Double Check the Fuel Pump/Accelerator Pump,
Check if there is a squirt of gas shooting down the throat of the carb as the throttle is opened up.

If There is not a stream of gas, what do I do to fix this?

Sat Jan 24, 2009 10:18 am

When a carb butterfly is opened it creates a lean condition. Weber typically deals with this by adding an enrichment tube and a series of holes in the throttle area, plus some form of pump that squirts fuel through a nozzle aimed at the venturi. Generally if the enrichment holes are not properly aligned and the butterfly set accurately, they will cause a flat spot as the throttle is opened. If the acceleration pump fails then the engine will typically die or gasp when the throttle is opened quickly. The easiest test is to test the pump. With the car warm and turned off, look down the carb. If you can't see clearly, get a small mirror. When you open the throttle, you should see a pencil lead size stream of fuel squirt down the throttle body. If you see none than this is your problem.

Sometimes the weights in a distributor stick as the grease gets old. Also vacuum cans wear out. You can easily test the vacuum advance and weights by holding a timing light on the pulley and open the throttle. The timing should jump.

Sat Jan 24, 2009 11:24 am

I'll check that as soon as I get home. It sounds like this could very easily be the problem.

If this does turn out to be the problem, what is the fix?

Sat Jan 24, 2009 3:06 pm

If it is the accelerator pump on the carb, one would need to look to see if the linkage is all there and moving the pump arm. If the arm moves then it is likley a worn out pump diaphram that comes in the kit. There is also a small ball bearing that sits in a small well that is part of the assembly on most Weber carbs. When the pump recharges the ball comes off its seat and allows fuel to pass. When the pump diaphram moves the ball seats which allows the fuel to be forced out the small nozzle. Sometimes when people rebuild the carbs they forget to put that ball back in.

As to the timing, it might be best to have a shop look at the distributor because while it is not complex, the failure to do things right may render it in operable.

Mon Jan 26, 2009 1:01 am

Updated...

Mon Jan 26, 2009 3:24 pm

Lots of folks will tell you not to replace a vac distributor with a mech adv one but I am not one of them.

I always replaced the vac distributors with the mech type AND put in a pointless set up in them as well. Ive always had EXCELLENT results. I hate points and condensors. They are an antiquated technology that are really waaaay past their time. My old 79 w/2.0L FI ran much better (power/mpg) after the conversion. Im getting another 79 2.0L FI next month and its on the top of the list of things to do.