Timing And Idle Tuning

You raise a good point about what I mean by misfire. Its only occurring at higher rpm, not during idle. Heck, its idling smoother now after the rebuild than it ever did before. As the rpms climb up above 1500, the engine will have some sort of ignition issue - I assume its a mis-fire and think for some reason it missed spark. Its possible its pre-igniton and I am mis-interpreting it and not hearing the 'ping'. My ear for this kind of thing is not the best. When it happens, the entire engine has a minor shake or vibration that is timed with it and there is a soft pfft sound out of the exhause. I tried putting my timing light on the different plugs to see if I could tell if one was continually missing but couldn't tell anything (was just looking at the 'flash' of the light and how repeating and consistent it was or if it skipped a firing).

As mentioned above, with the vac can disconnected and ported vac line plugged, ensure the dwell is right about 30, and everything runs great. I have the initial advance at 8 and it idles between 650-700 (although it would probably like a bit more advance). Running up through to about 2500 rpm (using digital tach on my multimeter that I connect to the negative coil terminal), I can watch the timing light show the advance increase from 8 up to to 20 and then a little beyond the scale. All still seems good there... Can run the throttle up slowly or quickly and it runs without any hiccups or 'misfires' etc.

With the 'new' vac can connected, the idle rpm and initial stay the same (which is good). working the throttle, i can see this vac can affecting the timing, but much more subtly than the previous one, and it definitely doesn't cause the dramatic jump in timing advance and has less affect at the higher rpms - seems like its working more like it should (and as Tarry describes above) and the most advance I saw was around 24. So maybe it was just a bad vac can as its running well, but I think it still runs just a touch smoother with no vac can attached at all.

As it seemed to be running well, took it out for a cruise on sat. Ran great around town, but the engine seemed a bit less 'zippy' - or didn't want to accelerate with quite the same juice. The throttle felt just a bit more sluggish (kind of like it ran before the rebuild - which isn't bad at all) but for the last couple weeks since the rebuilt I kind of felt like it would put you in the back of the seat when the pedal was mashed. Additionally, the exhaust smelled noticeably more of fuel - like its running a bit too fuel rich, but the plugs looked okay (grayish). Might just be a 'placebo effect' or even a change to the much warmer weather.

As part of the drive, cruised up to a couple mountain towns (8500-9000') and definitely noticed some issues with being too rich. Obviously, the change in elevation is part of the deal with carbs, but this seemed significantly worse than I've normally experienced. It didn't really want to idle (sputtered out at a couple stop signs) and the plugs were completely covered in carbon soot. Cruised back down to Denver and of course it started to run pretty well again - although the plugs were still pretty ugly when I pulled them this morning. You could see a portion of the electode had started to turn grey again.

Based on the fuel smell, and that I never go below Denver's 5200' and drive up to over 10,000' quite frequently, thinking of dropping down a jet size. I am currently running 0.49 jets (same as always and recommended for 5000'), but I do have a set of 0.48 jets. Additionally, it doesn't really like much choke (if any at all when cold starting) - just needs a bit of throttle for a minute. Just concerned about getting too lean for lower elevations like Denver (my house)...but it won't be much of wheeler if I can't keep it running at idle in the mts.

 

Points and condensor have been changed. New cap and rotor - so far they do not look have to any unusual carbon deposits. I have checked arcing in the dark at night last week - saw nothing (I have had problems with this before so I am keeping an eye on it). But I did decide today to re-route the wires so the ones going to the passenger bank aren't all parallel and right next to one another in case I am getting some sort of issue induction interference issue. That is next on the list to run the engine and check. The wires are pretty new (10k miles) - they check out ok on the ohm meter based on FSM ohm/length spec, but thinking I might replace them anyway.

Will look into hotter plugs after that - any suggestions? Currently running new NGK OEM replacement since rebuild. Ran delco OEM replacement previously. Would new cam require hotter plug? Anything to look for on current plugs to use as indicator for hotter plug. Plugs are relatively cheap so have no problem trying them.

 

James - I always run hotter plugs after rebuilds with that cam spec. I also run hotter plugs due to the HEI and give them a wider gap when I go that direction as well.