912 Engine Trouble - Ignition? Fuel?

[andyh2000]

Hi All,

I got a '67 912 about 6 months ago and have been playing around with it ever since.

I am having trouble with the engine intermittently not revving past 4000 rpm or the engine stutters like not all the power is there. I should say, in neutral it revs fine, but under load it develops this stutter / unable to rev.

Sometimes I start it up and it runs fine all day, I'll start it up the next day and the problem will be back.

This initially seemed to me to be an ignition problem and I went about replacing all the ignition components.

The set up is 050 distributor, pertronix Ignitor II, flamethrower II coil, new wires, plugs. Timing is 35 degrees at full advance.

Problem is still happening though.

Any ideas what could be going on?

I'm going to start investigating the resistance in the wires going to the coil - you can see I'm getting desperate!


Could this potentially be a problem with the fuel pump?

I can take video and post it if that will help.

Please share your thoughts!


Andrew

Andrew,

Please give some more info regarding the engine setup, carbs and so on. Was the car stored for a longer period befoe you started to drive it?

I think the 050 dizzy is not the best choice for a 912 engine. A 031 or 022 will do a better job with their advance curve adapted to the 912 engine.

You say it sometimes runs fine and the next day the problem returns. Did you check the advance mechanism of the distributor. If the advance doesn't work properly, like sticking somewhere in its movement, the power under load can be lost.

Otto

[andyh2000]

Thanks for the reply Otto -

The car wasn't stored for long before I got it, the guy who had it before me drove it probably once a week. Don't know if he ever experienced the problem.

I got the car with the 050 - I don't know much about it, when it was installed etc.

The carbs are weber 40 idf, I can't remember the jet sizes. Other than that to my knowledge everything else is stock.

The advance may make sense though . . . this only happened once, I was driving along with the problem occurring and then with the throttle on it was like someone flipped a switch and it started pulling properly.

Any other thoughts on things to check out?

Andrew,

I assume that you did a proper valve job (0.1 mm inlet, 0.15 mm oulet), have clean air cleaners and so on. In other words, you have done the basis maintenance.

Regarding the Webers I can say that in general these IDFs are sold - and jetted - for VW and not for our 356/912 engines. Tuning the Webers is a delicate job. For example changing from a 125 main jet to a 120 gives a change in performance - not as extreme as your engine does. There is no standard for the jetting of the carbs. It depends on your engine, the altitude you live/drive, season and some minor things. A jetting like 50 idle, 120 main and a 200 air correction at sea level is more or less average. The emulsion tube should be a F7. From there on you can fine tune the carbs.

Not to forget are the correct synchronization of the carbs and a well adjusted linkage mechanism.

For a basic tuning of the carbs, look here:

IDF Adjustment Controls

It gives a simple adjustment without the need of expensive special equipment.

Just a short anecdote: The engine of one of my 912 ran fine. After winter storage it started running super rich. Checked and cleaned, everything, readjusted the carbs and almost gave up. Left it for a week and started searching again. Within an hour I found that that a gasket between the base plate of the air cleaner and the carb was deformed and covered the air inlet of the idle circuit. New gasket and a carb adjustment and the engine ran as before.

Otto

[andyh2000]

Otto - thanks again for the great input.

Yes I adjusted the valves and added air cleaners, as the guy who had it before me was running without them. I may want to check the valves again - they seem to be quite noisy, is that normal?

I also repaced the gaskets between the carbs and the intake manifolds and the gasket between the air cleaners and the carbs.

I do want to get the carbs tuned, but my thinking went that I should be able to get the engine running consistently before tuning the carbs. Thoughts?

I will take a look at the distributor and a few other things this weekend and let you know what I find. If I run into the problem again I will take video and post.

[WFBowen]

In addition to the above:
Double check all of the wire connectors to coil, and inside the dist. for any rubbing or interference with the advance plate.

Like Otto, here's my anecdote: I had a 69 911S with a nagging, intermittent miss. After going through virtually everything with no solution, I had the car idling while talking with some friends. The sun was going down and in the dark there it was: #2cyl plug connector arcing to the engine case. The plastic insulator/connector had a tiny hairline crack which I hadn't noticed. Sometimes they're cheap fixes, sometimes not.
Bill.

Bill, Andrew,

Indeed an electric problem could be the cause of your problem. Check the points Bill mentioned and also look at the ground terminal right from the voltage regulator.

With a cold engine the valves can be heard, but the noise should disappear as soon as the engine runs a few minutes. Popping and backfiring often indicate wrong valve settings.

The IDF adjustment link in my previous post contains a step-by-step instruction for the initial settings, not for a fine tuning of the carbs. A correct setting of the idle mixture and air bypass screws is very important and determines the performance of the engine significantly.

Sorry, I'm sometimes not precise enough with choosing my words. English is not my native language.

We'll follow you during the weekend for any help - note the time difference with Europe please...

Otto

[69 Soft Targa]

You might check the screen in the gas tank. It can become clogged with silt and choke off fuel flow just when you need it. Just to mess with you the sloshing off the fuel may clear the screen enough to make the problem intermittent.