Part-throttle Stutter?

[opelbits]

Recently, my car has developed a slight part-throttle stutter. What happens is that when you gently (but not too gently) push on the accelerator, the engine starts to stutter, like a cylinder drops out. Push down really gently and it doesn't happen. Push down quickly, it doesn't happen. Sounds like a part throttle lean miss, right? Well, it doesn't do it right away. I can warm up the car for however long I want to and it runs and drives fine, until I drive it 5-10 miles. Then it will start the stutter. The longer I drive it, the worse it gets. It got so bad one day that I couldn't accelerate on the highway at all and barely made it home.

So far, in trying to track this down, I have retimed it, checked vacuum advance, replaced the Pertronix (always have a spare), swapped from a 32/36 (throttle shaft play and slightly warped base-what do you expect after 17 years?) to a virtually new 38 DGAS (very nearly correctly jetted according to info on this site), and the result is always the same, a stutter with a slightly cracked throttle that starts after 5-10 miles of driving and gets progressively worse.

It really sucks to have the nice, satisfying song of the DGAS change to the putter of a motorboat.

Any ideas? At this point, since I get it with two different carbs, one new and one old, I am suspecting my idle mixture/speed adjustments. Or maybe float levels (fuel heats up and level in bowl drops), but on both carbs?

[Anonymous D]

Most problems are related to a vacuum leak. Have you tested vacuum connections (with carburetor cleaner spray) to make sure they're airtight? Vacuum leaks will affect driveability after warmup, when the choke opens up (and can interrupt the distributor advance curve), which could lead to your situation.
Besides that, I'd suspect a possible vapor lock. Make sure the fuel line isn't contacting any warm surfaces, and install a see-through fuel filter just before the carb (so you can visually identify if that's the problem).

[opelwasp]

Did the emulsion tubes get changed to F66? This is a classic symptom of the original F50.

[opelbits]

Anon-

No vacuum leaks. After the motor is hot and the choke is off I can drive for awhile before there are any signs of the problem. Sometimes ten minutes, sometimes all day (but usually ten minutes or so).

I don't think it's a vapor lock, but it might be possible. I have two of the clear filters, one between the tank and pump (electric rotary mounted near the tank) and one up near the motor. The fuel line is rubber and dips down and across the front crossmember, then up to the carb to avoid the hot area around the stock over the motor position. I try to maintain no contact or proximity with any heater hose, but it may have shifter. I will look at that.

I also have been using the phenolic spacer at the carb, although when I last swapped carbs (Sunday), I took everything out and used one new gasket to make sure there isn't a vacuum leak in the gaskets/spacer.

All the efforts to reduce heat transference to the fuel line really paid off. I have had no problems with fuel temperature, even on 90+ degree days. At least twice now, including this past Sunday, the stutter was present in cold rain/snow with temps in the mid 30's.

[BQS4]

just out of curiosity, if you have a weber, have you checked the accel pump diaphram?

[opelbits]

Opelwasp-

The 38 has F66's. The stutter originally appeared with the old 32/36. two very different carbs, the 32/36 is very old and the 38 is near new. So I am inclined to look at what is common between the two. That means something other than the carb itself, or my adjusting of the two carbs.

[markandson]

I would say to look for something heat related. Try checking the compression when cold and then after the problem starts, could be something in the valves or rings.

[opelbits]

BQ-

Both carbs have good or new accelerator pumps.

Markandson-
I will check that tomorrow morning. Sounds like an avenue to pursue, even if I don't like the sound of it..

[GTJIM]

Check the outlet from the tank for obstruction - especially if you haven't already removed the Fuel Filter 'Sock' from inside the tank.
Check the venting system to the fuel tank - make sure all the lines and the carbon filter are working properly.
Sounds like classic case of fuel starvation - maybe caused by vacuum in fuel tank .....

[opelbits]

Did the compression checks. Nice even 150, cold or hot.

Opened the mixture screws a hair (1/8 turn) with the idea that it might be leaning out. Not much change. Put them back and gave the speed screw a 1/16 or so further in, thinking maybe I don't have the throttle blade positioned right in relation to the idling/progression holes.

I drove it today. Didn't stutter much, but it's still inconclusive. I did an oil change today, it was about time for one, so if my oil was diluted with gas that may slightly change how it runs.

I'll put some more miles on it tomorrow, but I need to get some higher octane in it. I put 87 in it the other day and today it was pinging slightly. For some reason, this motor has never liked 87.

[opelbits]

Jim-

I have my doubts about fuel starvation. If it stutters and I give it a little more gas, it stops stuttering. Obviously the accelerator pump has an effect, but once past the stutter point I can run as long and as hard as I want, well past any effect the accel. pump has. But that is usually. Sometimes, but only after I have been driving for 15 minutes or more on the highway, when it acts up, any throttle at all and it stutters and giving it more gas just makes it worse.

[Tru-Craft]

You said sometimes it won't stop stuttering.
Both carbs did the same thing.
Engine under load/heat makes it happen.

I would check electrical stuff:
*Ground strap.
*plugs and wires.
*Distributor Cap! mine was cracked and it drove me nuts. I was trying to adjust my carb and it would stutter randomly, giving me false readings
I found it by accident, opened the hood at night and saw sparks.
Lyle

[opelbits]

Heat and load are definitely factors, but once it starts stuttering, it will do it in neutral sitting still. Not as bad, but it will do it.

Ground straps are good. Plug wires are new (fancy-dancy red Taylor's, replaced after seeingmy own light show!). Tried different (but not new) caps, too, no change.



It's pretty frustrating for me when I get these oddball problems. All the usual answers that worked for other people do not eliminate the problem. Even though in the past my odd ball problems have turned out to be genuinely odd ball (like a carbon locked piston engine...), at some point all logic points to a failure on my part. I don't mind finding out I did something wrong, IF in the process of that realisation it gets fixed!

[MICAH1]

Opelbits,
I read this thread a few times, and it seems like everything has been suggested but the possibility of a worn electric fuel pump. First, how old is the pump and have you checked your fuel pressure @ idle and under load (higher rpm). Don't think it is your carbs since it takes place with either unit. Just a thought...

[opelbits]

My pump is a few years old. I forget exactly, but probably around 4 -5 years.
I haven't checked the pressure, but I'll put it on the list.

Thanks to all for their advice and suggestions.

[West Coast GT]

I think you've eliminated carb problems by duplicating the condition using two different carbs.

As you said, look for common element.

I like the suggestion of a worn fuel pump and/or bad ground. (we're talking aftermarket electric fuel pump aren't we?)

But don't overlook bad fuel. Its been known to happen, though illegal, to sell contaminated fuel; or worse yet, watered-down gasoline. You mentioned you had filled up with 87 octane recently. Was it soon afterwards that your stuttering started?

My recommendation is to burn through as much of your current fuel as you can, without working on the car. This way you won't be introducing more variables to the mix. Then fill up from a different station - stay away from an independent station for a tank or two.

If the condition continues then look to the fuel pump. The pump's motor may have a problem as it warms up. They're relatively inexpensive, so I'd recommend just replacing it anyway.

Keep in mind, the key to isolating a problem is to change one variable at time. That was pretty iffy swapping carbs, but fortunately its worked in your favor.

[opelbits]

The stutter actually started a couple months ago. I've run three different grades of gas thru it and from several different stations (and different companies).

My reason for changing carbs was the throttle shaft in the 32/36 has some wear and play. Wiggling the shaft (without turning it) would result in droplets of gas coming out of the accelerator nozzle. My thinking was that the shaft is moving around due to road vibration, engine movement, and of course, who can keep their foot completely still when when driving (a good bump moves you and your foot around). All of those things combined with a sloppy shaft can cause an inconsistent, rough idle, which I have had for awhile. Not to mention the possibility of a vacuum leak around the shaft. Perhaps things were getting worse. Perhaps the slop in the shaft allows for an inconsistent activation of the acclerator pump arm in that the throttle plate begins to open, but the accelerator pump lags far enough behind in operation to cause a lean miss when trying to slowly acclerate.
Anyway, I figured swapping carbs would be a good idea, if for no other reason than to eliminate a known problem.

I will check the electrical connections and ground on the pump (12v rotary pump). We live near the bottom of a valley and the humidity and dew are crazy. In the summer, as soon as the dew point arrives, it looks like it has just rained because there is so much moisture in the air. So that has been something for me to learn to deal with. Things would be better on the west coast....you don't have a guest house do you?

[West Coast GT]

Well,
Sounds like you have very good reasons for swapping that carb. Although it turned out to not be the source of the problem, your car will certainly be running better because of it.

[markandson]

There is a very tiny orifice in the vacuum line that comes out of the base of the carb, I think its hose might run to the dizzy. I would start looking for vacuum problems now since all else has failed. Check that little orifice, if it is plugged or partially plugged it might cause erratic symptoms. Just another avenue to check.

[opelbits]

I didn't think about the orifice. I cleaned and bead blasted the manifold a few years back and made sure it was open and clear then, but since then?
I have checked the advance mechanism in the distributor and it works just fine, moving when it should and it holds the vacuum, no leak down.
But that is assuming the orifice is still clear. Vacuum can't do anything if it can't get there.

On a related note, I think I need to double check the integrity of the plastic vacuum lines.