68 911L running issue
 

Here is some background:
1968 911 L Car was sitting for 18 years
Carbs have been rebuilt, everything surgically clean
Float levels set with gauge
carbs balanced while running
Base start settings taken from Paul Abbott's tech page.
Idle mix adjusted using the "lean best method"
distributor rebuilt by Barry Hershon re-curved to have a total of 24 degrees of advance
New Perma-tune ignition amp
all new ignition parts cap, rotor , wires, plugs
timing set with timing light
Valve lash set properly
Compression 160-175 all cylinders

Here is the problem:
The car idles perfectly. Very little back fire through either the carb or the exhaust. One small pop every once in a while, but by no means regular. The engine will rev fairly freely all the way to 4k plus. The problem occurs when you try to accelerate from a stop. The car will miss-fire through the exhaust and will not rev, just falls flat on its face. I have spent hours trying to re-tune the carbs. To the extent where I have had to stop and set everything to base again because I have gotten so far away.

I am obviously missing something but I fear I can no longer see the forest from the trees or do I have some other problem. Any help would be great.

This may be a bit basic but could it be fuel starvation i.e low tank level. What is your fuel level when you were testing. Seems really odd that this would only happen when moving. Makes me think it is a pump issue?

that's an interesting thought. although it is back firing through the exhaust which indicates rich as far as I know. I will check that.

You didn't mention if your fuel tank has been cleaned, but assuming that is not the problem, it sounds like you have small piece of debris in one of the idle jets. Remove each idle jet and idle mixture screw at each port one at a time. Spray carb cleaner into the mixture screw opening and then in the idle jet hole, alternating back and forth. These two ports are connected so you should see carb spray coming out of the alternate ports. Then used compressed air in the same fashion as you did the carb spray. Then remove the idle jet from its' holder and spray and clean it as well. Now repeat this process 5 more times in the other carb ports. This should take care of the symptom as long as you're not dumping more debris from a dirty fuel tank into the carbs again. You may not ever see the offending piece of debris during this process, so don't be alarmed. Hope this helps.

Fuel tank is cleaned, etched, and sealed with the POR15 gas tank kit. It looked perfect inside. This was done before the carbs were rebuilt and engine was started. Every port on the carbs was cleaned extensively during the rebuild. It is possible I missed something. Wouldn't that effect the idle? The car idles so well, could there be some dirt in the idle circuit and still idle smoothly?

It sounds like an ignition issue to me. I have a 68 coupe and I had issues with acceleration and found it was the distributor advance. Did you set timing at idle and check advance at 6000RPM?

I did not check the timing at 6000 rpm. was uncomfortable revving the engine to that number without load. Obviously you have and everything went OK?

Distributor was completely rebuilt By Barry Hershon

Injection quantity for accelerator pumps.
When idling loosen each idle jet and listen for engine response.
Fuel pressure and flow were not mentioned. Can't assume anything.
Good luck and keep us posted

Whats ignition advance at idle. Your should try 5-7 degrees

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My tank was freshly cleaned and my carbs were freshly rebuilt on my 68. This was my resolution when I experienced the same problem you are having. The most minute piece if debris caused this in my case. Although I had a degraded idle issue as well.

Advance at idle is 6 degrees. That is why Barry set full advance at 24 that way the total would be 30 at full advance.

Keep the ideas coming. Someone will hit on something soon.

Don't use compressed air on an assembled carb...the image below from Performance Oriented website shows the bad float damage that can occur.

What size are your idle jets?

http://forums.pelicanparts.com/uploa...1402095929.jpg

What are engine particulars? A 1968 911L 2.0? If so then idle jets should be 55s.

Sounds like it is lean.

Be sure your auxiliary venturis are installed so the hollow wing is pointing to the emulsion tube supplying the fuel to that cylinder.

Accelerator squirt output may be anemic; check for .7ml output using an insulin syringe with the tip fused closed and a bit of wire to dangle it below each squirter nozzle.

Also, if the squirt amount is deficient then you might want to check that the check valve in the fuel bowl isn't back-flowing upon squirt demand. Pull top cover of the carbs off and look inside the fuel bowl where the valve is and see if there is turbulence in the fuel level during throttle opening. Turbulence indicates a stuck or corroded check valve.

One more thing about accelerator squirt; the squirt will not start until the throttles have been opened a small amount (hysteresis in the gaskets, etc.) but if you open the throttles 20 degrees before the squirt starts then you may need to consider why that is. One possible solution is to install a cam/lever from a idtp carb which looks like a "hatchet" as opposed to the curved cam profile of the OEM version. The cam lever is the flat piece that is mounted with a shoulder screw to the side of the carb next to the accelerator pump body.

Float levels need to be adjusted after engine has run a bit to stabilize and then shut off engine and read fuel level. Meniscus to be between or at upper line on the vial. Remember, 3.5 psi is the specified fuel delivery pressure while the engine is running. I'd recommend plus/minus .5 psi as a tolerance.

FYI: On page 8 of the Edoardo Weber service manual under Idling Jets it states, "Carry out cleaning of jet holders and relative ducts with compressed air"

I think you should posted for sale. Problem solve and you will be really happy with lots of $$$... j/k....some people might not tho...

You have received good advice regarding the accelerator pumps. Thee are 2 issues; volume of squirt and duration. Common assumption is backfire through exhaust is too rich. The bog from off idle that backfires through exhaust is??? Investigate there.

My guess is something in the ignition. You have a very weak spark. It can ignite the mixture when there is no load, but misfires under load. Not an unknown problem on early cars without CD ignitions, and often a bad condensor. Less frequently a bad coil. You mention a Permatune ignition. Is this still dependent on the coil/condensor?
DG (also with '68 911L)

Paul offered some sound advice.

Webers will not respond well to any tuning unless you have a good 3.5 psi of regulated fuel pressure, followed by careful float adjustment. All of the other adjustments & jetting are predicated on this and its the very foundation here!

Naturally, the ignition system must be in top condition rendering a nice fat blue spark so I'd recommend confirming this at the same time as the above.