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Original Distributor ?

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sail

sail

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On my 66 TR4A the distributor is Lucas part #40795, also says 25D on it. In looking at a spare for the boot, what are the significant differences in the older D4's ie 40480. How about 40842 abd 40850 which are for low compression and 83mm engines? If you say they are curved differently I won't really know what you mean. Thanks
 
If you're looking for a "get me back home" kind of spare, most any 25D should work. Performance won't necessarily be optimal, since the advance curves will be different (basically meaning that the distributor is "tuned" for your specific engine), but it'll do. Example: years ago, to get a buddy's MGA 1500 running after his original distributor seized and sheared off the drive, I put in a distributor from an early Triumph Herald 1200 engine. Worked fine!
 
:iagree:

Actually, even the 'correct' distributor is probably not optimum because our fuel is considerably different these days than it was in 1966; and the factory had to accomodate fuels of the day all over the world.

Having the curve(s) be non-optimum means your fuel mileage won't be quite as good as it could be; and maximum power across the rpm range may be somewhat less than it could be. But we're talking at most 20%, more likely 5-10%, so as Andy says, almost anything will get you home. The only caveat is that if you can hear the engine knocking (aka 'ping' or preignition), you need to stop and retard the timing a little.
 
40795 = 25D4 Distributor. Used from 1964-1971 on Morgan +4 and TR4A
40480 = DM2 Distributor. Used on 1956-1961, TR3 and Morgan +4

The table I have does not list applications for the 40842 & 40850
 
Thanks for the info. I know better what to look for. Just missed it on ebay, was signed in, bid and the darn site wanted me to log back in, so too late. Thats ok, plenty to do with what I have without dinkin around with spares. There will be another.
BTW Moss has 40842 as low compression 4 & 4A. 40850 4 w/optional 83mm engine.
 
40842 is listed for later, "low compression" TR4.
40850 is listed for TR4 with the special order 83mm pistons & liners.
 
Oops, I see Richard answered the question while I was distracted. Sorry for the duplicate.
 
[/quote]
No, but the table does show the advance curves of these distributors that can be used as a comparison.
[/quote]

Thats interesting, I will have to study it a bit.
 
I find that it helps to plot the numbers out, rather than trying to make sense of the corner points in my head. Eg,
 
sail said:
If you did that in 10 minutes I will just smash my computer now.
Click to expand...

I don't think Randall just did that, as there are only two of the distributors being talked about on that chart. Of course, I could be wrong...it wouldn't be the first time. :jester:
 
No, I didn't do that chart just now; although I don't recall it taking a lot longer than 10 minutes.

My job involves a lot of plotting of data, so I'm fairly adept at it. Plus, I get these little windows of time (like now) when I have nothing to do but wait for the bits to go 'round.
 
Here's the update, using Chichak's numbers.

The max advance for 40850 seems suspicious to me, as a TR4 with 83mm liners is essentially identical to a TR3 but 40698 has over twice the maximum advance. But I'll let someone else verify the advance numbers.
 
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