Dewll angle plus CDI

bertdejut · 12:43 PM

Although having an Austin Healey (1954) for a while, I now got a 911 2.4E (1972). And that's a different ball game in terms of more complex matters like MFI, CDI etc. Anyway, so I'm a newcomer and I'd love to learn as much as possible about the 911s and try to get the engine running properly.

Engine has been restored in 1999 so that should all be fine, but when I bought the car it was running ok when cold, but got worse between 2000-3000rpm when hot. I read an article on the bulletin board about the vacuum and after replacement it was a lot better.
However, after pooring some injection cleaner in the petrol, the engine started running slightly worse (maybe because injectors were cleaned and mixture got richer??).
Anyway, I had a look at the points gap, but they didn't seem to open. I replaced the points and after that the idle speed went up from around 700 to around 1150 rpm. When pulling up, I still got some hesitations in the engine.
I got myself a Gunson gas tester, air flow analyser and a timing light. Tried to measure the dwell angle, but everything remained on 60 degrees. On this bulletin board I saw a few articles about it and I'm a bit confused now. Some people say that dwell angle can't be measured on my model (CDI) and should be skipped, while others seem to be able to get a reading (?!?!!?).

My intention is to get the timing right with the timing light, then get the air flows right for each cylinder and if the idle speed is still too high reduce the air intake. Then measure the CO level and then maybe adjusting to richer/leaner mixture by changing the fuel levels.

Is this the best way of getting the set up right??

Any help would be greatly appreciated.

Thanks!!

'Dutch' Bert

Bill Verburg · Bill Verburg's Garage

I never had trouble measureing the dwell on my '72. If for some reason you can't just use feeler gauges and set the point gap to .4mm on a Marelli or .35mm on a Bosch dist.

It is easier to set point gap with the dist out of the car in a bench vise.

Tim Walsh · Tim Walsh's Garage

hmm.. I've never been a fan of setting point gap and dwell. When my car is back on the road (2.4E spec engine) I'm planning on replacing the ponits with an ignitor.

I remember using the gunston to check the dwell on david clarke's car and we had something kinda strange go on but I think it was something simple. Hopefully he'll chime in here with definitive information.

JSDSKI · 05:05 PM

Point gap is .014-.016. Magnecors and Petronix ignitor really smoothed out the running in my car and, like Tim, I'm happy to leave point adjustment in the 20th century. MSD is next up. (All recommended by Steve W @ Rennsport). With an MFI car, you probably want to research those upgrades here on the board.

BEFORE you start to play seriously with your MFI system, read "Check, Measure, and Adjust" from the tech articles on Pelican. Follow the check list 1-10. Don't do 4,2,7, and then 9 just because they seem to match your car's symptoms - ask me how I know

Oh and congratulations on your acquisition and welcome aboard!!

bertdejut · 09-27-2003, 05:48 AM

Thanks a lot guys!!
I've got the check list and work from there.

Final question on the dwell angle. I get the impression (also in Hays manual) that a dwell meter needs two connections (one to the distributor terminal 1 and one earth connection). Both my Gunson gas tester and supastrobe have only got one lead, which needs to be connected to the terminal. So no earth lead.

I also read somewhere that a dodgy coil can (obviously) result in no dwell reading (mine remains on 59-60 degrees whether connected or not). I'll put a new coil in the car even though the car is not running badly.

Early_S_Man · 09-27-2003, 01:21 PM

Bert,

Assuming no serious wear on the distributor cam or points' rubbing block ... if you set the point gap to 0.014" when installed, then your dwell angle will be correct! The current through the points in a CDI-system is only 140 mA, or 0.14 Amps, so any erosion occurs very, very slowly. The trigger signal [where the dwell measurement is made -- distributor points at the Black/Violet wire to ground] for the CDI-unit through the 100 Ohm pull-up resistor inside the CDI box to the pulse-shaping circuit, and has nothing to do with the coil. The coil condition will never affect the dwell angle reading.