1970 911t starting issues

[dpetteng]

So, a little background.

I purchased my 1970 911t when I was living in the states and shipped it back to the UK in 2016. It is a real project but I loved it because it was genuine one owner from new. I saw the car running in the states.

So at the end of last year we pulled the car out of storage and replaced all the rubber components, fuel tank, oil tank, all service parts, brakes etc etc.

The car run very poorly and when we checked, 2 cylinders were down on compression.

We stripped the top end down and the cylinders in questions had some issues.

We sent of the cylinders to EBS in the states for boring out to 85mm and they supplied some JE 85mm 9.5:1 pistons. As this was happening we checked the bottom end, cleaned and rebuilt.

We built up the engine and also fitted a new fuel pump. The car is running its original Zenith carbs that were rebuilt when I was in the states.

So, the car is a real pig to start, like it takes ages and coaxing into life. Once it is running it runs perfectly!

Any ideas what my starting issue could be?

[Trackrash]

The first thing I would check is the operation of the accelerator pumps. Before starting, look down the carburetor and work the throttle to see if you have fuel squirting in.

Then use a timing light to check the advance and operation of the distributor.

You probably should go up a size in the idle jets, since you modified the motor. Those Zeniths were tuned specifically for the stock 2,2 T motor to meet smog regulations in the states, and were tuned very lean to the point that those motors would hardly run with the air cleaner removed.

Good luck and let us know. BTW, you are using the throttle judiciously when trying to start, correct?

[Trackrash]

I'm sure you are aware, but regrinding your cams to E specs and some mods to your carbs will REALLY wake up that motor, now that you have some performance pistons.

[dpetteng]

what would you class as "judiciously"? LOL

So the accelerator jets all seem to be squirting when I pump the throttle.


You gave me advice on the rebuild before and I have plans for cam upgrades. The changes to pistons was to get her running and then can do performance enhancements in the future.

Will look at the jetting once I have her starting properly!

[dpetteng]

What idle jets should I have as standard and what should I change to please?

[Trackrash]

As far as idle jets go you will have to experiment. IIRC they had 45s originally. I would go up to 50 or 55. Lots of variables. Aircleaner, mufflers, displacement, compression ratio will all contribute to the carbs needs.

[dpetteng]

Thanks Gordon

[ed mayo]

Do you pump the throttle while cranking ,,,,or before cranking?

[dpetteng]

generally before and then pump it as I am trying to get it to fire

[ed mayo]

That's what I thought. Try this method; before cranking give throttle two full pumps, then hold halfway down while cranking,,,but do not pump while cranking. Depending on each engine and temperature conditions you may have to stop cranking if a no start on first attempt and repeat process. I see many people do this who didn't grow up with carburetors, pumping furiously while cranking,,,,,pump first, then cranking with throttle part way open,,,,sometimes even fully open,,,,you have to determine what works best for your engine.

[dpetteng]

Ok, so lots of good news. The car starts and runs. Mainly down to accelerator pumps over or under volume pumping.

So she starts, runs ok, but not perfect. I think the running will be down to higher compression pistons and old jetting. So any suggestions what jets I should buy to make up for the 85mm 9.5:1 pistons?

[ed mayo]

I have virtually the same engine, but I only went for a little over 8 to one compression. All I have done is to go up two sizes from stock on idle jets. From memory I think that is a 57.

[dpetteng]

so a dumb question, how many jets (both idle and main) do zeniths have? I am assuming 6 of each?

[dpetteng]

Does anyone know what the standard main jet sizes were on a 2.2 911t?

[Trackrash]

Here is what my Haynes has listed. I can't be 100% sure if it is correct.

[dpetteng]

Originally Posted by Trackrash

Here is what my Haynes has listed. I can't be 100% sure if it is correct.

You Sir, are a legend, thanks

[dpetteng]

So, is the idle metering jet the same as an idle jet? Was going to try 55's to account for the increased compression and slight cc increase. Thoughts? It gets expensive buying lots of jets so I want to get it right first time LOL