Ignition troubles

[Ghinmi]

Hi guys,

Now that I've got my 401 broken in I decided to give it a little hell. Problem is that the ignition isn't cooperating. When I accelerate hard from a dead stop, it starts cutting cylinders and has a hard time gathering speed. It feels like a rev limiter but the engine isn't even turning fast yet. If I'm not taking off hard it doesn't do it. If I'm already moving along and floor it, it doesn't do it. Since I have a few spare HEI units from my chebbys, I tried a different ignition module but it didn't help. Next, I unhooked the vacuum advance (and capped the open line). Aside from crappy idle, it helped a lot. I did get it to cut cylinders once from a dead stop but not nearly as bad as before. Any ideas what could be wrong with my vacuum advance that would be doing this to me?

Thanks,
Greg

[Dusty]

CHECK THE FIRING ORDER AND ALSO CHECK THE COIL

[AMX69PHATTY]

Distributor Phasing - rotor button pointing directly at a post when it fires.

Mechanical advance moves the "trigger", star wheel, interuptor, etc.

Vacuum advance moves the "switch", HAL pickup, points, etc.

And the Cap is keyed to the cast housing, which is rotated for initial advance.

Take an old distrib cap and cut a wide slot out of the side
that goes 2/3 of the way around the cap.
With this cap iinstalled on the dizzy you can see inside to determine
if the rotor button points to a post when both mech & vac are engaged.

just a WAG :-|

[Ghinmi]

Thanks for your help guys! I spent all week checking out the ignition but it turned out to be a carb problem. When I initially set the float levels I didn't have a pressure gauge on the line. Before I drove it, I put a pressure gauge on and found out I wasn't getting much fuel pressure. Fixed the pressure problem but didn't think about how it would affect my float levels. They were WAY high. Readjusted it and no more popping or misfiring. However, it still doesn't pull like it should until the rpm's get up there. Here's a video of what it does. Just seems like it should pull better than that. http://www.youtube.com/watch?v=PpH9QoVVrAk

Here's the specs:
1976 Wagoneer
3.54 gears and 27" tires
Th400/quadratrac
Stock converter (may be part of the problem?)
AMC 401
0.030 over KB 354's
Balanced rotating assembly
502 heads gasket matched, polished exhaust, polished chambers, etc.
Air gap with 670 street avenger
Edlebrock headers
Comp Cams Magnum 270 (224*@0.05 and .480 lift, 110* LSA)

Is something not matching in my setup or do I just have more tuning to do?

[jeepsr4ever]

Sounds awesome!! Hard to tell by the steering wheel but what rpm did you hit?

[Ghinmi]

Shifted a little over 5500. I need to mount the tach where you can see it better. I've just got an el cheapo sunpro for the time being and haven't found a good spot to mount it yet. I'm going to redo the dash with aftermarket guages (and a real tach) over the winter. I'm also going to build a 2wd th400 at the same time. The current one doesn't shift when or how I want it to anyways.

I still don't think it pulls like it should at low rpms. It launches (and spins the tires) but then bogs some. Should I switch primary jets? If so, higher or lower? Or do I just need a higher stall converter.

[jeepsr4ever]

Larger!

[AMX69PHATTY]

What distributor and what mechanical advance curve ?
Full mechanical advance by 2500-3500 may help low end power.

[Ghinmi]

It's a Pro-form HEI unit. Stock curve whatever that is. I didn't have a tach when I timed it. I went with 36* at full advance. Does that sound about right? I'll recurve it (if needed) this weekend and see how it goes.

Also, any guesses as to how much horsepower this thing should make once I've finally got a good tune in it?

[AMX69PHATTY]

36* at full advance sounds about right, but this does not include vacuum advance.
Disconnect vacuum advance when setting & checking timing.
Most stock mech adv curves are pretty slow and may allow
more advance than is desired.
Mr. Gasket Delco Dizzy kits include a bushing to limit total mech adv.
Understanding is ~12-8* initial and the dizzy set up for ~24-28* total mechanical
adding up to ~36*total in by or before ~3000 works pretty well.
Then whatever vac adv there is goes on top of that.

:-| Guess 325-350 HP

[Ghinmi]

Yep, I disabled the vacuum advance when I timed it to 36*. IIRC, the base timing ended up at 8* so I'm good there. Just need to make sure it's coming in at the right time. Is that guess at the wheels or flywheel?

[1980_Cj7]

Understanding is ~12-8* initial and the dizzy set up for ~24-28* total mechanical
adding up to ~36*total in by or before ~3000 works pretty well.
Then whatever vac adv there is goes on top of that.



I used to think the same thing until I read that writeup someone posted not too long ago, and reminded me that the vacuum, and vacuum advance, goes away when you have your foot in it. Guess it still comes into play with light throttle at a steady cruise though.

[AMX69PHATTY]

Guess is at the flywheel.
Yeah, vac advance only comes into play when there is vacuum.
Was just saying it is not included in the 36* total or setting initial.

The Holley 670 Street Avenger may be jetted a little lean and
the Accelerator Pump Squirter may be too small.
Do you know anyone with a Holley Double Pumper you can try on it ?
I'd try at least a 750 if not larger double pumper just to see what it does for comparision.

[Ghinmi]

So the 670 is too small? That's what Holley recommended for my engine. My cousin has a 750 laying around that I could probably try out. I couldn't find any place locally that could even get a recurve kit or a set of jets, so I can't do much to it until I get them in the mail.

Just for fun, I bought one of those accelerometer deals that measure 0-60 and quarter mile times. It claims to be accurate within 100 milliseconds. I tested it on my other car (stock '86 Monte SS) and it spit out times really close (but a bit slower) than factory specs.

Yesterday I ran mostly 14.6-14.8 depending on how I launched with a best of 14.57 @ 96.3 mph and 0-60 of 6.23 on the same run. Today I had a couple friends with me (extra 350 pounds or so) and ran a best of 14.8x @ 94.x IIRC and 6.5 0-60. The speed seems kind of high for the time, especially for something with the aerodynamics of my Jeep. Figuring it weighs 4500 with just me, plugging the numbers into the online calculators I end up with 288 whp from the time and 314 whp from the mph. I don't know what drivetrain losses are in my setup (running in 4wd). Not saying I take any of these numbers as gospel or anything but it's fun to tinker around with.

I think it's got a lot more in it once I figure out that low end problem.

Comments?

[AMX69PHATTY]

Saying the 670 is "small" depends on the "working" rpm range.
I have a Holley 850cfm double pumper on a 367, but 3k stall & 3.73's. Is the 750 he has a vacuum secondary or a double pumper ? like list 0-3310 (vac), or 0-4779 (dp)
Just guessing at the jetting and squirters, cause supposedly with the edelbrock carbs, the electric choke ones are set up much leaner than the manual choke models, and don't know if holley may do the same thing with the avenger's. Usually if there's a stumble when throttle is quickly opened, it points to accelerator pump adjustment, cam, or squirter(s).

It may just be the dizzy mech advance curve, they are usually very slow coming in stock. That would go along with it being better as rpm's come up. Low end was greatly improved on mine once the dizzy mech adv was set to be all in by ~3000 rpm. Some have it come in even sooner, which is ok as long as it doesn't cause detonation.

[ironman_gq]

size is relative. most quadrajets are between 700-800cfm i believe and GM used them on everything from 305's up. I say its how well the secondaries are set. it also makes a big difference that the quadrajet is spreadbore but a squarebore will work pretty well if tuned properly to work with your engine. Use one of the online CFM calculators to help size your carb.

[Ghinmi]

So I finally got back home to work on it. I found the problem, but I don't know how to fix it. The mechanical advance is coming all in at 3000 rpm but it's not advancing far enough. With 8* base timing, I'm not even getting close to 36* total no matter how high I rev it. I tried lighter springs but it does not advance any further. Does anyone know what in the world could be limiting my mechanical advance like this?

[AMX69PHATTY]

Is it a Delco Dizzy ?

If so, weights and springs are right under the rotor button.
Make sure they are clean, rust free, free moving.

Under the advance weight plate on the bottom
is a pin riding in an oversized arc shaped slot.
The pin in this slot determines total mech adv capability.
The Mr. Gasket re-curve kit comes with a small brass bushing
that is made to be pressed over that pin
which increases it's diameter, and reduces total mech adv capability.
Might there be one of those on there ?
Are the weights hitting the inside of the rotor button
maybe, and limitibg total mech advance ?

Is there a vacuum advance can also on the dizzy ?
It moves the switch plate, not the button & Trigger
which is what the mech advance moves.
Can't imagine how vac adv could limit mech adv.

[Ghinmi]

It's a Pro-form HEI unit, so I assume everything is a delco design. I bought it new and nothing inside is rusted that I can see. I checked that the weights didn't hit the rotor button at full swing and that there's no rust, etc. Everything moves freely. I bought the Mr. Gasket kit and I'm not impressed with it. The holes in the center plate are off so it almost fits but doesn't. Plus the weights are of very questionable quality. It didn't come with a bushing to limit advance either. The springs are good though. :razz: As far as I can tell the vacuum advance is working perfectly. If I make it home this weekend I'll check if there is an advance limiter installed under the plate. Thanks for the help, I didn't even know about the limiting sleeve.

Greg

[ironman_gq]

If its aftermarket it may have been recurved at the factory for what they determined is the normal usage of the engine.

[Ghinmi]

Ok so I'm even more confused now. There's no limiter bushing installed. I took apart 2 other HEI units (a chevy and a pontiac) to compare the inner workings. The weights on all three are identical. The chevy unit had an identical center section but the pontiac had something different. The mr. gasket kit was completely different than everything else I have. To clear things up, here's a list of what I have.

3 sets of identical weights
1 set of mr. gasket weights

2 chevy style center sections
1 pontiac center section
1 mr. gasket center section

I cleaned up the holes in the mr. gasket center section and it fits now. I have tried combinations of all the weights and springs and nothing comes close to what it should (26-28* right?). The problem seems to be in the design of the weights and center section.

When I'm test fitting different weights and center sections, I'm comparing them to the plate with the teeth on it. There are 8 teeth evenly spaced, that means 45* between teeth. So my mechanical advance needs to be a little more than half that distance. The weights and springs I have are not coming close. Short of making my own center section (probably a bad idea), what else can I possibly do? I've tried all the weights and center sections I can and I'm getting nowhere. Help!

[ironman_gq]

how much base timing are you running?

[AMX69PHATTY]

The angle of advance seen at the dizzy is HALF what you'll get at the crank using a timing light.

15 degrees mech advance at the dizzy is 30 degrees at the crank,

cause the crank turns twice to the cam and dizzy turning only once.

[ironman_gq]

very good point!! :t:

[Ghinmi]

Ok so I finally got it figured out! :? I tore apart a third spare GM distributor and found a fourth different center piece. Slapped it in and I'm getting 35* total with 13* base timing. Does that sound alright? Thanks again for all your help guys!

[AMX69PHATTY]

yeah that sounds okay, 35* Total & 13* initial.
That would imply 11* mech adv in the dizzy and 22* at the crank

Is that from checking it running and using a dial back timing light ?

All 22* mech advance in by what rpm ?

May want to try as much as up to 40* total advance
but be careful and listen for detonation.

[Ghinmi]

I don't have a dial back light but I do have an accurate way of measuring the timing. When I had the balancer off, I marked 36* on it. Since 36* is 1/10 of the full circle, I measured the circumfrence of the balancer and divided by 10 to determine where to make the mark. IIRC, the measurement was 21.25" and 1/10 of that is 2.125 so that's where 36* ocurrs. When that mark hits the zero mark on the timing cover I know I'm at 36*. I can read the base timing by comparing it to the scale on the timing cover. Hope this makes sense. Probably should invest in a newer timing light anyways.

The mechanical is all in by 3000 exactly. It took me forever to match a set of springs that wouldn't advance at idle and still be all in at 3000 with this weight setup. Still seems like I should be able to get more than 22* advance though. I think it's alright for now until I get the carb dialed in 100%.

Just another random quick question, should I be running a higher stall converter with this setup? I'm currently geared 3.54 but I'm thinking of going with 3.92 or 4.10 when I switch to 2wd this winter. I will be running a 275/60/15 or larger drag radial depending on how much clearance I have when I spring under the rear axle. Should end up a little under 4000 lbs with me in it.