I'm re-reading through the thread to see what I'm missing and what I've learned.
DeuceCoupe, I see your worn 250 engine with compression from 130-155. Mine measured 100 on the worst cylinder and got up to 110 on the best. The leakdown test confirmed #6 is the worst. It was the Harbor Freight tester, so I'm using it as a gauge to see how the cylinders in comparison to each other and not as a definitive test.
Cyl, Compr, Leakdown %loss
1, 105, -14%
2, 105, -15%
3, 108, -17%
4, 110, -18%
5, 105, -26%
6, 100, -40%
mshaw,
For me, I don't think U need HEI yet - all my tests so far have used the old 1960s single-point distrib, no problems yet.
You might need to use a weaker spring in place of the stronger one down below in the distrib - but first need a tach to see what your timing curve is for sure. 30 total is getting close though, if you just try 12 initial for 32 total you'd be basically "there".
As far as tired, I find that tired engines actually run FASTER if anything, just cuz they're "looser". Unless of course the valve or ring seals are giving up. Not sure if you did a compression test next but that for me is the best "tired" indicator.
Here is what I got from the 250cid that ran all the carb "shootouts" a couple years back:
135
155
142
130
145
138
I consider this a "tired" engine, not the greatest results even on compression but it still ran good and gave decent times in the shootout - after all the car ran "high 10s" (0-60mph that is!!!)
I would definitely do these steps before you tear into the engine, that way you will have a before & after. Also find out where the big Powerglide is shifting - mine was shifting about 4500 or so, probably a little lower than the 4bbl wanted but I didn't want to abuse it so left it there - I was looking for improvements and comparisons, not the last tenth by revving the old six higher.
Thanks for all the photos you are posting it makes for some interesting threads!
Integrated j 78, I think you're onto something here, read on...
I don't think running too rich will make it backfire. IMHO it's a vacuum leak somewhere. Valve springs are possible, so is a timing problem . Could there be an internal leak in the carb? Jay
Gbauer, I realize I made the terrible mistake of changing two things at the same time when I put in the HEI. I tried the idle mixture, leaned it up just a bit. An earlier post I'd made it richer. I'll back it off to where it was before, maybe even make it a little richer.
One of the things I'd done before was to really pop the timing up, on the points to 17BTDC. I'm at 9 now, maybe I'll take it to 12 since that seemed OK before. With HEI, i'm nervous about advancing too much.
You guys keep coming with great advice. You're all pointing now to some sort of vacuum leak (or valves). Before the HEI, one of the tests I ran was to block off every vacuum line. Yes, I even ran without power brakes. It backfired like nobody's business due to not having enough timing advance. That's one reason why I've still got the vacuum advance on the lower carburetor line to keep it advanced as much as possible. I will try swapping the lines after I try the other fixes. But notice that I've tested fully removing every vacuum line with no improvement.
If everything else were OK, here's what I think the list of potential leaks, listed from most probable to least:
1) Intake manifold gasket
2) Carburetor base and 1" spacer
3) Carburetor itself (brand new)
I should be able to discover these with carb cleaner, right?
Mark
Backfires only happen from lean conditions. You have a vacuum leak.
Mine was a hard one to find. It was the vac line to the trans. The rubber hose down right at the trans.
Another time it was the grommet on the brake booster. Another time it was the booster itself. Yet another time it was the vac advance canister.
The best way to find it is to disconnect each vacuum hose one at a time and plug it. Eventually you'll find the culprit. If it's not a hose try the old carb cleaner trick.
I had the same problem. Took me months to find it.
