2.2 T missing when hot

[piggdekk]

Hi guys,
I have a very original '71 2.2 911T with CDI, marelli distributor with new cap and rotor, Bosh blue coil, magnecore wires and 40mm rebuilt and rebushed TIN carburators with 32mm venturis.

It has never ran perfectly, but lately it runs very poorly when the engine is very hot. As long as the temperature is 180 F or so it runs fine, but as it gets close to 200-210 like on a hot day on the highway like yesterday it would only run well if I rev it, while if I drop a bit the RPM it start to misfire and the whole car shakes, definitely not running on 6. The idle is quite low too.

When it misfires it looks like it's very lean (backfires through carbs), but I'm leaning (pun intended) to believe this is more of an ignition issue. I do have an MSD box with matching coil that I could install for testing.

Besides the car not starting at all, what are the synthoms of a bad CDI?
The coil measured fine, although there seems to be some disagreement on whether the blu coil is good or not for CDI.
thank You
luca

[Hi_Fi_Guy]

I’ve been through this with a 2.2 T. Here are my thoughts.

Heat can be an issue with these engines. Especially the cylinder and carb barrels closest to the oil cooler. Have you checked the oil cooler to ensure it is clean and free from obstructions? Mine had nearly 50 years of dirt and grime built up on it. A good cleaning will help lower temps. Also, many states now mandate E10 fuels (10 % ethanol). This percolates at lower temps than non ethanol fuel and can cause a carb fire. The fuel delivery systems were not designed for ethanol and it can wreck havoc. Stabil ethanol treatment can help.

The Zenith carbs were a compromise for emissions over performance. Many people like them and they are easy to work on and upgrade but if any worn areas are leaking air it can be a challenge to stop them popping. Especially on decel or hot idle. Likewise some have mentioned air leaks at the intake manifolds so this is something to check for as well. Always smart to verify a rotary fuel pump is in place and delivering correct fuel volume and pressure.

Many will say only the black coil with a CDI ignition which is most likely correct. That said in my experience as long as the ignition timing is correct (you should verify with a timing light) you will most likely find the carbs/intakes the source of the popping.

When in doubt there is a proper process of elimination. Otherwise you may be chasing ghosts and making matters worse.
- Compression: always ensure compression is present. This includes correct valve adjustment.
- ignition: next be sure the spark plugs and gaps are correct, wires, ignition timing, etc.
- carbs: only after you verify the first two should you mess with the carbs. Too many people blame carbs for issues caused by poor valve adjustments or incorrect ignition timing.

Working through the process of elimination will help you diagnose which components are contributing to your popping issues and how to correct them. Just remember many parts are 50 years old and even rebuilding these old parts may not eliminate the issue. You might have to resort to some modern parts and solutions.

Let us know what you find and if you solve the popping issue.

[piggdekk]

thanks Brian,
the car does not oveheat, but the misbehaving becomes more noticeable when the engine is hotter.
- Compression test done, a bit on the low side, but all cylinders are +/- 5 PSI
- Ignition, spark plugs and timing is fine
- Since I put larger venturi's I struggled to get a good idle transition, but I don't think that's the problem. I really need to modify the zenith for air correction jets as that is the main issue with them.

Still, I wonder if this isn't more of an electrical issue since it does get definitely worst when the engine is hot. Hence my question about CDI going bad.
I read somewhere that I should check also the insulation of the wire that goes to the point, that one I have not checked it!
thanks
luca