Do I need a CDI in 69 911T upgrade
 

I have added a new engine to my 1969 911T and stayed with carbs. I have a 2.2L 1970 block with 2.7L S engine P&C and crank. The heads are from a 2.4L.

My engine runs well but always has exhaust popping noise. So the fuel is not being fully burned. Do I need a CDI in 69 911T upgrade?

My 1969 911T does not have a CDI. See photo of my engine compartment electrical side.
http://forums.pelicanparts.com/uploa...1380658518.jpg


I pulled a photo from this forum of a 1969 911E electrical compartment with components IDed.
http://forums.pelicanparts.com/uploa...1380658656.jpg

Thanks,
Steve

I can't say whether you "need" one or not, but I will say that it probably will run better with one. I had a 2.4 E cammed motor with Webers and I swapped the CDI for an MSD. Worked great.

Roger

No .guarantee that your popping will be eliminated, carbs are well known for that, especially when decelerating.
Your engine will run better overall with the CDI that's a no-brainer. Plugs will not foul as easily for certain.

CDI allows you to run a colder plug, with a wider gap, and to have the plugs last a LOT longer. Make sure that your plug wires are good, and the resistance of your plug connectors is correct. If you still have points, be sure that your dwell angle and ignition timing are OK for your engine configuration.

Your popping noise can be any number of things, including a simple problem like crud in an idle fuel jet, all the way to difficult, like experimenting with jetting, venturi size, etc. Sort the ignition first and see how the car runs. Like others wrote, no guarantees, you just have to go a step at a time.

You may want to consider a Pertronix (or similar product) as well, unless you have a deep affection for points!

Roger

Does MSD and Pertronix systems eliminate points?

I do want to eliminate my points


Steve

The Pertronix does. This can be used with either a CDI or MSD.

Roger

The popping you hear is most likely because at light loads or decel the distributor curve is not optimal for these light loads. The dist curve is really dialed in for WOT loads but at light loads you have high intake vacuum and here the air fuel mixture is very thin, meaning the fuel particles are not tightly packed as they would be at WOT. Because of the loose packed mixture the flame front speed is greatly reduced and the burn rate in the cylinder take much longer. What is happening is that ignition timing is not optimal for these light load conditions which require a lot more ignition advance but the distributor can deliver this and the end result is the mixture is still burning when the exhaust valve opens.

The EFI/DME cars have fully tuned ignition timing that can provide more ignition advance for light loads but the older cars can't do this.

I don't think you'll be able to eliminate the popping without a EFI system. Also open exhaust systems tend to increase the popping you hear.

As Sal states, the popping may not go away. My 2.4 always popped on decel, even with a Pertronix and MSD. I used a O2 tester to dial in the idle mixture, etc. Car ran great, idled great, but still popped on decel. Again as Sal states, muffler choice may impact this as well. I had M&K race mufflers, which are basically straight pipes. I'm sure this contributed to the popping (at least to the volume!).

Roger

Sal,

Thanks for the clear and understanding explanation.

I'm working to get the engine running right. It is good now but perfect (or what can be achieved) is better.

My big concern was increasing the displacement by 25% and keeping carbs and original spark. Were they compatible?


Thanks,
Steve

The displacement change by itself won't impact your ignition performance. However, you will need to understand what pistons you have and what the CR is, to make sure you get the correct plugs (heat range). Do you have Zeniths or Webers? If you have Webers, you will have no problem getting the right sized venturis and jets for whatever configuration you want. The Zeniths may be a bit more tricky to find stuff for, other than a stock set up.

Do some searching on this forum regarding venturi or jet sizing. Ton of info.

Good luck!

Roger

Roger,

Thanks for the input. My carbs are Webers. I have a good book and researched the proper setting. The Webers are centered for 2.5 displacement.

idle 65
main 140
venturies 32


Thanks
Steve

Air jet and emulsion tube number?

Peter Z

The Air jets are 185
The emulsion tube is F1

Does this sound correct for a 2.5L?

Peter Z

Correction Air jets are 210 old were 185

I think that you would be better off with 185s! That said, I would drop the airs down to 160, and drop the idle fuel jets to 60. Is there a chance that you can find a set of F3 emulsion tubes?

Never make changes with out reading the spark plugs! What plugs, make & number, are you using?

Peter,

I'm running NGK plugs BP6ES

In the past I have saw F3 emulsion tubes on Ebay. What does the change in emulsion tubes change in the function of the carbs? How the air-gas mix (emulsifies)? Book is vague on this.

Steve

0.65 idle jets are way too big. somewhere between 0.55 and 0.60 for that size engine, unless it's full race. F3s rock.

My understanding is that these help to mix the fuel with air, i.e., suspending it in the air.

Rather than Ebay, you may want to buy a set from some place like PMO (I don't think our host sells them). They aren't that expensive ($60 a set). I have a set of F3s I can sell you too ($30). But again, they aren't that expensive to begin with.

Good luck.
Roger