250 HEI Timing, vacuum advance question

[felm0001]

Long time reader first time poster,

I don't know too much about timing and ignition, but I used shifty's thread and installed this HEI from summitracing in my 1967 c10 with the stock 4.1l L6 and the old rochester 1bbl.

the truck starts and idles at 550rpm nice and strong.

Under load I and getting pop, bang, rocks and coffee can style ignition + black smoke. I set the vac can wide open at 9deg, then closed it a quarter turn at the time (that took an hour or two). There is definitely a spot there where ignition gets a bit smoother under load (probably around 7-7.5 deg adv out of the can), but still no joy.

I might have the initial timing at 4deg advance, unless each hash is 2deg and the 4 on the harmonic balancer is really an A... then I would be at 8deg initial advance. I set the timing by ear, then put a light to it.

according to summit the specs for this distributor are:
16 degrees of mechanical advance
and an adjustable 5-9 degrees of vacuum advance

so without the vacuum advance I think I am at(16 + 4 or 16 + 8) 20deg or 24deg total advance

with in the last 1-2 months: plugs are new, wires are new, valves are new, pressure check was around 150 on all cyls.

I feel like this is a timing issue, but if the vacuum advance is only adding advance, and it runs fine without the vacuum advance plugged in (I took her around the block), shouldn't the measly 5-9 deg advance only help underload? is there another issue I am overlooking that could cause this poor ignition?

[PanelDeland]

WELCOME!!!

Did you disconnect the vac advance when you timed it?It needs to be disconnected and plugged.
Did you check for full 12 volts at the dist.Even though you followed "shifty"'s very good instructions,you may have corrosion reducing voltage.
Did you use dielectric grease and tighten the firewall plug well?

My manual shows TDC at idle on a manual trans and * BTDC with an auto.

These are just the first things I thought of off the top of my head.

[felm0001]

Thank you for the reply,

Yea, I disconnected and plugged the vac advance when I timed.

Yep, I rewired the whole engine harness with 12 gauge. I am currently still picking the tar crap out of the bulkhead connector. (I have a duck tape bulkhead connector in there now)

The firewall plug is tight. I did not use any dielectric grease though. The battery is new as of Jan this year and the posts look clean enough, could the issue be from the draw on the distributor if it fires fine with the vac advance disconnected?

I have the Haynes GMC truck manual and he has 4deg advanced for the 1967 250 manual on points. Not saying he is right, but that is what I have been going off of.

When I set the timing initially I turn the distributor until she ran smooth, then I set it to the closest mark on the balancer, which was i guess is 8deg of advanced.

But I am down for anything so, I will see if I can get it running smooth at TDC at idle and then see if that sets the vac up for a bit more success.

[geezer#99]

Try 12-14 degrees advance. My 250 ran like that.

[69gmc292]

Did you check to see if TDC on the balance is true TDC? I have seen balances spin and that will throw your timing off. Also is your engine stock or has it been rebuilt? In my 292 I run about 8-10 degrees of static timing. I also put an HEI curve kit in it and I am running the lightest springs so my mechanical advance comes in faster.

[felm0001]

geezer#99, ill give it a try.

69gmc292, yea the balance should be set, I used the napkin in1cyl spark plug hole method. Lined up at 0 just fine.

As far add the engine goes I'm not too sure, I had the head off and put in new valves, push rods, and rocker arms 2 months ago. Compression in the cylinders were consistanly 150 and the tranny and clutch worked so i stopped there for now.

I am going to unbolt the distributor tomorrow and play with it a bit based on your alls' recommendations. Would it be better to be as retarded as possible at idle to make room for the vac advance?
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[geezer#99]

Won't hurt it. I ran mine with 14 initial plus 10 from the vacuum at idle.

[felm0001]

Ok here is the results from today,

I tried 0 initial advance and that didn't work out. My theory is that more advance is needed to solve the misfire issue.

I went up to 12-14 and it ran smooth, but when it throttled up there was black smoke and unfired fuel out the pipe. I reduced the vac advance and it only made it worse. I found that more initial advance fixed the misfire on the throttle up. (please correct me if I am wrong, those are observations not mechanical facts)

So now I have teased the initial timing set way advanced off the balance scale, but there is no misfire and she runs strong at 500 rpm. I have the vac can wide open @ 9deg advance.

**She throttles up fine, but now on the way down there is misfire and black smoke out the pipe. I tired more initial advance and everything got worse, I tried less initial advance and same result. I tired backing off the vac advance and it was worse. Teased the fuel mixture screw on the carb and no joy.

any ideas?

[geezer#99]

Time to look at the carb. Likely flooding!!
Get it idling then look in the carb and look for excess fuel.

[felm0001]

going out to try that now.

[felm0001]

carb was a bit wet... so I fiddled a bit with the mixture and found that the pop and bang was happening once I was up trying to maintain high rpm, throttling up was fine, but at high rpm I think it is getting too much advance. I tried adjusting the distributor while at 1000-1500 rpm to see if I could get it running smoothly and I could, but once I brought it back down I had retarded the initial timing pretty far. something might have high fived the valve cover... ran fine after I set the timing back to 8deg adv so I am going to ignore that for now...

And everything still runs smooth with the vacuum can disconnected.

I am connecting the vac advance up to the carb where the old points distributor vac advance was connected. Maybe there is something up with my vacuum source?

[geezer#99]

THe vacuum source on the carb is manifold vacuum.
Better fix the wet carb problem. A rebuild might be in order.

[felm0001]

I just rebuilt the carb in may. I have another rebuild kit so I can do it again, but I am going to see if it still floods without the vacuum advance connected tomorrow.

[70STOVEBOLT]

From everything I've read, inline sixes don't like a lot of initial advance. That is an A on the timing tab not a 4, stands for "Advance". If you connect a timing light and watch as you turn the distributor counter clockwise you will see the mark move toward the A. I don't know the range of my vac advance, but it is the stock distributor with a Pertronix Ignitor.

[felm0001]

Thank you all for the responses.

70stevebolt: yea I kept my old distributor, I may upgrade it with the pertronix and plug it back in. Depending on how well this upgrade runs.

geezer: I have the stock intake and exhaust manifold in there right now, I am thinking of upgrading and getting a quad intake and just buy a new carb. Keep this one for my kids to play with.

So I took the truck down to a "classic restoration shop" we have around here. I have been there once before and wasn't too pleased, but the main man is wiser than me, so I figured he could help.

For 25 bucks he told me that the newer HEIs have too much mech advance built in so the vac is probably too much. He set the initial to about 14-15deg and capped my vac line. I took her home and she ran ok, had some popping and putting while idling at the intersection, so I set my timing around 8deg initial advance and am keeping the vac line plugged for now. Runs strong and I took her on the hwy at about 70mph for about 10 minutes. I have the cheapest 85 +10% ethanol gas in there so I assume 14deg + some higher octane stuff would run better.

Anyway, I guess the answer is, 16deg mech + 5-9 vac advance is too much advance for my 250.