LOW SPEED STUMBLE

I had the carbs adjusted and synched during engine rebuild about 1500 miles (but 5 years) ago. Lately an old problem is reappearing.

On taking off from idle the car will stumble and stop even with WOT. 2-3 seconds later it will "clear it's throat" and take off with tires spinning. Feels like old Porsche turbo lag. Very disconcerting on uphill starts or turns into traffic. I keep waiting to get T-boned by a car coming around a blind corner near my house while I'm sitting in the middle of the road waiting for the carb to catch it's breath.

If I run it up to 2500-3000 or so, and slip or drop the clutch I avoid this problem, but I hate doing that. I assume that at those RPM's the main jets are flowing. So it appears to be a low RPM problem. I've read all the posts on adjusting the Webers but before I start messing around with something I've never done before I'd like to know if there is a specific part of the carbs I should be looking at. No sense in doing a full carb adjustment and synch, as it runs beautifully once it gets going.

Ignition is all new, dizzy, rotor, wires, BPR7EIX plugs, optical pickup (Crane XR-700 which I have been using for 10 years with zero problems), coil is good. So it's probably not the ignition.

Any advice on the low speed stumble greatly appreciated.

Champ: Thanks for the help last week. Beautiful engine.

 

I would say its a carb issue, check idle jets & progression port circuit, these are still in operation at this speed. Revving the engine to pull away or flooring the throttle is dumping huge amounts of fuel through the main jets and masking the problem.

Does it pop or spit through the carb or exhaust especially on idle.

I would say your carbs need a retune, there is no shortcut I am afraid.

 

No popping or spitting ever. looks like I'll need to get them overhauled. Thanks

 

Sounds like an accelerator pump problem to me or somewhat stopped up carb discharge tubes on all three carbs. there is also a check valve in the screw that holds the discharge assembly down and the little ball in it gets stuck and it can cause that problem. I owned and operated a Porsche repair shop for 32 years and would see this problem on cars that didn't get driven very much. It happens even more in cars that have fuel with ethanol in it. I had the exact same problem with my Dino as a result of not driving it enough and couldn't get ethanol free fuel then. I should have known better. In my opinion the carbs won't need an overhaul, just the tops taken off each one and take off the the screw holding the discharge tube assembly and with a fine wire strand clean it, blow it out, check the hold down screw with the check valve in it to make sure it's not stuck and put it back together. If you are real careful you won;t even need new carb gaskets.It took me about 4 hours to do all three. It won't change any mixture adjustments at all. I did mine with out taking the carbs off the engine, way faster, dose not change any linkage adjustments.I would do it for you if you lived in Florida close to me, I like working on those cars. Good Luck

 

is there anything that can poured in there to free up or dissolve unwanted matter?

Crazy it as it may seem, once upon at time, I had a little stumble issue. with the cover off, placed the flat of my hand over the carb so it would get no air, and revved it. I though I would create sucking effect that would clear any loose gunge. repeated it a few times on all carbs. problem gone, never to return.
Who knows.......

 

I added Techron fuel system cleaner about a tank ago. Made a big difference in my 360 and STi. I guess the injectors were fouled a bit. Don't know how much of a difference it will make on a carb car.

You are correct-It just doesn't get driven much what with the kids and the errands and the other cars to drive. Only put on 1500 miles since 2008 when it went through a year long mechanical and cosmetic restoration.

It seems to be getting a tiny bit better after the Techron treatment, but it's still not great. Once it gets moving everything is great.

Guess I'll just have to drive it more. I'm going to try that trick with my hand to create some extra vacuum and see what happens. Thanks