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TR4/4A TR4A with TR3 distributor and Vacuum Advance

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carpecursusII

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OK, long story short I've been chasing a dieseling, poor gas mileage, high idle problem for 2 years now. After having switched out the ZS 175 CDs twice and resetting everything I found out yesterday that my PCV valve was assembled improperly and took 20 seconds to fix that. Suddenly the car will idle at 600 rpm like a champ. I enjoyed the much improved idle and my dieseling problem has been substantially reduced. I have seafoamed the car to help get rid of the heavy carbon deposits and I'm sure that this is a partial cause of the dieseling. Any more thoughts here?

I took note of my distributor and vacuum advance today and saw that they are both TR3 units, (DM 40480, 4-11-7). My question is how much would changing to the proper 25D4 40795 distributor with 2-6-3 vacuum advance help me, if at all?

I am running 91 pure gasoline and RL87YC plugs. The carbs are at stock settings .100 jets with 2H needles, no gross jets. I have run 100LL aviation gas in the car and it does not diesel at all. Plug gaps and static timing have been checked. Point gap was checked about a year ago when they were new.
 
I recommend that you call Jeff at Advance Distributors and discuss with him. He is very knowledgeable and helpful. His number is 612-804-5543.Regards,Bob
 
carpecursusII said:
...I took note of my distributor and vacuum advance today and saw that they are both TR3 units, (DM 40480, 4-11-7). My question is how much would changing to the proper 25D4 40795 distributor with 2-6-3 vacuum advance help me, if at all?...
Click to expand...

I think it will help but the big change will be in the centrifugal advance which presents a very different curve, particularly when advance is 'all in' at higher RPMS:

Lucas_Distributor_Advance_Curves_zps03074f01.gif


Lest I sound like a dizzy expert -- I am not. But these things are not rocket science. Were it me (and it once was) I would source the correct unit and carry the other as a spare.
 
My experience with Dieseling is that it was caused bydeposits on the plugs. Once I had some oil rings not seat and it left oil andcarbon deposits that dieseled quit a bit. Another time the problem was weakspark that left deposits. I switched to metal plug wires and that helped. I would suggest keeping an eye on the plugsand setting the fuel mix a little lean along with the correct dwell and timing.Sometimes these old distributors have a lot of play and timing moves aroundjust driving down the road, so you might consider getting a new one. I noticedthat BPNW had them on sale and I would consider that.
 
I always had trouble with run-on on my TR4A with strombergs and the stock dizzy. EFI and Megasquirt cured all that, but also added a high level of tuning with a learning curve.
 
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