69 911T lurching when warm

[bobspace]

My 911 runs great when it's cold. Starts up instantly and for the first 25 minutes or so, power delivery is smooth. After that the engine chugs a little under power and pops and backfires under light throttle. But once it gets really warm, the car lurches dramatically when under moderate to heavy throttle.

My mechanic just replaced my ignition, coil, spark plugs, and wires, and installed a pertronix system, so I doubt it's ignition related. (And it did this before the ignition change.)

Also, he doesn't think it's a carb tuning issue. His first thought was carburetor icing (which doesn't hold up because it gets worse the hotter it gets), and his next guess was water in the fuel system somewhere –*either in the fuel filter –*or in the carb float bowls.

What are the most likely causes here? Do you think my mechanic is onto something with his suggestions?

[1QuickS]

I'm thinking OEM idle jet size for that engine is no longer adequate for today's fuel. I think the 47.5's (or whatever they are) should be replaced with 55s.

When cold the mixture is strong but as the engine warms the effective mixture weakens and results in the bucking/popping.

[bobspace]

Also, I'm using 91 octane. Would using a lower octane help?

#ihavenoideawhatimtalkingabout

[Hi_Fi_Guy]

1QuickS is most likely on the right track. Sounds like a lean surge.

First are the valves adjusted correctly? It may seem odd but on every air-cooled motor I have encountered with supposed carb problems this is the first item to check. Misadjusted intake valves can cause all sorts of issues that can seem like carb/fuel issues at first.

Second IF all the ignition timing is correct and ignition components are working at 100% then the fuel system including carburetors are the next area to inspect. Start with the easiest and least expensive areas first.

Fuel tank venting OK?
Fuel tank screen blocked?
Fuel Filters OK?
Fuel pump working properly (you can pump into a container to determine flow rate)

If the fuel delivery is good then the carbs are next
Enough fuel reaching the floats and the floats set correctly (if not they could cause the lean condition by running out of fuel under acceleration)
Acceleration pumps on carbs functioning correctly? The diaphragms can fail.
Are jets and passageways clear of debris?
Idle speed screws set correctly (not open more than prescribed when throttle pedal is not depressed)
Idle jets sets correctly for balanced airflow?
Carbs balanced side to side?

If everything checks out then it may indeed be time to re-jet and rebuild the carbs. Jetting can make a huge difference in running, performance, and economy. Especially as 1QuickS implied due to the lower quality of today's fuels... especially ethanol content.

Remember when adjusting any idle jet screw to work in 1/4 turn or 1/2 turn increments and allow a full 10 seconds for any adjustment to take effect. Otherwise you can get way out of whack in a hurry.

[Hi_Fi_Guy]

One more item to consider.... fuel turning to vapor due to heat. Are all the engine tins in place? There is no heat source affecting the fuel pump, lines, or carbs?

[bobspace]

I got another suggestion by the former owner of the car: retard the spark timing a click or two. Does this sound like the spark is too advanced to anyone else?

[Hi_Fi_Guy]

The trick is to definitely NOT mess with fuel/carbs unless/until you know all the ignition components are 100% correct and functional. This includes timing.

Otherwise you will be chasing your tail.

[Trackrash]

Originally Posted by bobspace

I got another suggestion by the former owner of the car: retard the spark timing a click or two. Does this sound like the spark is too advanced to anyone else?

My first thought.

[bobspace]

Update! The spark plugs we had in there were causing RF interference with my pertronix ignition. We swapped the spark plugs and now it’s 90% better. There’s probably something else that’s still interfering, but this is a huge improvement.

Also my tach was misreading at idle. 1800 instead of 800. Now it’s at about 1000 with the new plugs. Progress!

[steve911T]

Bob, what plugs did you have in the car? Have had similar problems and had to replace fuel pump which was bad but still have a residual stutter around 2500 rpm. Steve