Yes, this is an old thread. A final result is better posted here (sez me) that an isolated new post, so here are some facts for y'all.
I was looking for re-curve information to hopefully resolve a bit of rattle (detonation) I've had for years. It happens right around a best-fill cylinder time..just over 3K rpm. I have reduced timing to lessen the noise, but it is sometimes still there if I don't run 'clear' (no ethanol) 92 fuel.
The problem is now gone, although several things happened at the same time so I cannot tell which did what.
1. My brake booster failed. I have not had good brakes for years, have always considered it a cam issue as it started with the install of a Direct Connection 292º purple shaft cam. I simply got used to brake fade after two pedal strokes. A rebuilt booster (all that in another thread) resolved that problem.
Point is, while the booster was still working (I thought), I did NOT think it had anything to do with low manifold vacuum at idle because I had blocked the booster off and NOT seen any change in vacuum readings. I'm not saying the thing has been leaking all these years, but that after it was rebuilt, my 'bit of rattle' at the occurrence of cylinder fill-time went away.
But that wasn't all.
2. My ECU failed at the same time. It was a blue unit from NAPA, supposedly one of their 'better' units I was told. I had been running an MSD-6C for years, then IT quit some years back. That's how I got to an aftermarket ECU.
Here's a bit of a rub: I have a VDO tach that has been bad for quite awhile. 'Bad' meaning the needle jumps all over the place. I got stranded once when the engine died with no spark, resolved that problem when I disconnected the tach. I found no replacement that suited me, so the tach sat on the column for years.
I went through a couple of
ECU bay-specials, one of them boxed as 'Standard' brand that was dead out of the box. Nothing on the ECU identified it as indeed being a 'Standard' brand. When I finally got an ECU that worked, I was only slightly deliriously happy to find that my VDO worked just fine! Evidently, the earlier ECU clamped the negative signal (minus off coil) to the VDO to the extent the tach could not operate.
I had tested the secondary spark (Big Yellow Accel) with a MAC ignition tester and the newly dead ECU did compare just slightly less output-wise than the ECU I ended up using. A btw, but the working ECU is actually marked as a Standard brand.
I did NOT change anything timing-curve wise in my distributor. I'm still running purple/brown springs, ported vacuum, with an 11º vacuum can. The can vacuum is adjusted close to max 'resistance', not starting 'til around 11in. of vacuum. I specifically do NOT say CW or CCW on the adjustment, because I have seen that direction described BOTH WAYS more than once. I can't say that the references are wrong. Maybe not all cans 'adjust' the same way. I DO know that my direction for 'less' and 'more' is opposite from what I've read more than once. SO...YMMV. I am running 12-33º.
The engine idles well at 800rpm (as suggested by Hughes, whose cam I'm running now). There is no hysteresis or 'looping up' of the idle as I've had for decades. As stated, I could PULL the idle down to 800rpm with the clutch, but the engine didn't want to go there on its own.
I've also had run-on problems when turning the ignition off. That's been a problem for years. I figured it maybe had to with running a four-post idle carb, or some other nefarious gremlin. No amount of secondary plate adjustment seemed to matter. After the booster and ECU replacement, the engine might run on once in awhile...but it's usually too hot at those times.
3. I did away with my cool-can. I've had a large copper coil in my fuel line for decades, the can attached to the radiator support on the right side (passenger) of the engine compartment. I don't generally put ice IN it, so the coil is more of a heater than cooler. During Spring fuel change (formulation is changed here every Spring/Fall) to a 'less volatile' mix, I have been stalled on an unusually warm day with the 'winter mix'..and I can hear the fuel boil in the carb. With no reason for it to be there (no ice in it) it might was well be out of the loop....and so it now is.
Engine runs very well. That is to be expected with the change from 8-28º. I no longer intentionally stay away from mechanical secondary opening. It starts perfectly, runs much cooler, and is just generally a lot more fun to drive.
And there is this, maybe #4: The throttle cable was maybe a tad snug. It seemed to have a bit of play, but it is true that I adjusted it to have MORE play during the whole booster/ECU/timing change fuss.
I am running the OEM choke cam on the Holley 750. I have filed on it a bit..as to get a high idle out it, the high idle screw ends up interfering with the actual idle screw on the other side.
Timing matters, and getting it right might be a problem. What may seem to be the result of what is considered a poor timing curve MIGHT BE something completely different.
It ALL matters! Just get it done right....and be happy!
