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258 / 4.0 Head / 260H Cam
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tloftus View Drop Down
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    Posted: Jun/25/2020 at 9:40pm
Finally getting the AMX buttoned up and ready for a drive.  Having some issue under load though with the motor stumbling and dying.  

First, about the car -- 258 / 4.0 Head / Offy Dual Plenum intake / HEI distributor (coil in cap), Autolite 985 plugs, 8mm wires, fuel regulator set to 3 psi (Same issues at 4psi), Edelbrock 1405 (500cfm).  Comp Cam 260H camshaft, SR4, 2.53 AMC15 rear end.  All emissions removed with the exception of a valve cover breather on the front and a hose on the back of the valve cover to the base of the carb (3/8 PVC port) although the elbow is not a PCV valve it's got the small metering hole in it.

Unplugged vac advance and plugged line.  Set initial timing to 12 degrees.  Adjusted idle mix screws for max RPM (830).  Set idle speed screw back to 800.  Manifold vacuum 11.5", ported vacuum 0".  Plugged the vac advance back in, idle went up (sorry didn't write this number down).  Set idle speed screw back to 800.  With vac advance, timing went to 26 degrees, manifold vacuum 13", ported vacuum 0".

So it idles well, and i can hit the throttle and it revs great in neutral.  So I decide to take it for a spin around the block.  I (wisely) went up the hill of my street, started out okay, then got weak, and died when it went under any kind of load.  Rolled back down the hill to my driveway, wouldn't start back up.  I figure I flooded it trying to get it to restart, so I let it cool down.  20 minutes later, starts up fine.

Pull it into the driveway, chalk the wheels and perform the same test... when it gets under any load it wants to choke out and die.  Then when it does it's a beast to get it started again.  Let it cool down and it will be fine again.

Double checked plug wires / firing order, pulled the cap and rotated to #1 TDC and rotor is pointing at the #1 wire.  1-5-3-6-2-4

Engine freshly rebuilt (has really only been run to break in the cam), carb is new, distributor is new, heck everything under the hood is new.  So I feel like it's probably something I'm doing wrong.  Admittedly this is not something I'm very familiar with, so I'm coming to the minds here for assistance!  I'm frustrated to the point of paying someone to get it right but I'd really like to learn vs just letting someone else do it for me.

Thanks in advance!

Tom
1966 AMC Rambler American Rogue - 232 I6
http://theamcforum.com/forum/1966-amc-american-rogue_topic20995.html

1978 AMC AMX - 258 I6
http://theamcforum.com/forum/1978-amc-amx_topic62333.html
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Heavy 488 View Drop Down
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Is your advance connected to manifold?
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tloftus View Drop Down
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Originally posted by Heavy 488 Heavy 488 wrote:

Is your advance connected to manifold?

Correct -- the distributor vacuum advance is connected to the manifold vacuum source on the carb.  Nothing else in between.  Ported source on the carb is capped off.
1966 AMC Rambler American Rogue - 232 I6
http://theamcforum.com/forum/1966-amc-american-rogue_topic20995.html

1978 AMC AMX - 258 I6
http://theamcforum.com/forum/1978-amc-amx_topic62333.html
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Heavy 488 View Drop Down
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May I ask why?
When the engine needs timing, the manifold vacuum drops and takes it away.
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tloftus View Drop Down
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Originally posted by Heavy 488 Heavy 488 wrote:

May I ask why?
When the engine needs timing, the manifold vacuum drops and takes it away.

Absolutely -- because that's what the Edelbrock manual said.  Manifold vacuum for non-emissions controlled engines.  I certainly don't know if that's right -- but my understanding is that once the throttle plates open ported and manifold vacuum are essentially the same and by that point the mechanical advance in the distributor should be taking over anyway.  Again, though, I could be entirely wrong -- it seems that everyone on the internet has a different opinion ported vs manifold.  I can flip them around and give it another go tomorrow.

I also didn't check the timing at a higher RPM than idle -- I can do that tomorrow also.
1966 AMC Rambler American Rogue - 232 I6
http://theamcforum.com/forum/1966-amc-american-rogue_topic20995.html

1978 AMC AMX - 258 I6
http://theamcforum.com/forum/1978-amc-amx_topic62333.html
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tomj View Drop Down
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Vacuum spark advance really helps with fuel mileage, but it confounds tuning. You might try plugging the hose, running no vacuum advance initially. THe downside is it will hurt your highway light-cruise mileage. 

"Too many variables" makes diagnosis hard.  YOu want vacc adv, for sure, but later. FOr now, run without.

See if you can separate symptoms and behaviors. For no reason I can articulate, I suspect a carb issue. So... is this a too-big carb? Bad accellerator pump issue?

What if you sloooowly crack the throttle, does it accell OK? Repeat until you are CERTAIN its behaving right.

Then try a "controlled' punch-throttle. cruise 30 mph in 2nd, whatever, press throttle. Does it do the right thing? Stumble? Bog?

I have no reason to think any of those things are the issue, but being methodical helps navigate the jungle.
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Originally posted by tomj tomj wrote:

Vacuum spark advance really helps with fuel mileage, but it confounds tuning. You might try plugging the hose, running no vacuum advance initially. THe downside is it will hurt your highway light-cruise mileage. 

"Too many variables" makes diagnosis hard.  YOu want vacc adv, for sure, but later. FOr now, run without.

See if you can separate symptoms and behaviors. For no reason I can articulate, I suspect a carb issue. So... is this a too-big carb? Bad accellerator pump issue?

What if you sloooowly crack the throttle, does it accell OK? Repeat until you are CERTAIN its behaving right.

Then try a "controlled' punch-throttle. cruise 30 mph in 2nd, whatever, press throttle. Does it do the right thing? Stumble? Bog?

I have no reason to think any of those things are the issue, but being methodical helps navigate the jungle.


Thanks for this Tom.. I will disconnect the vac advance and try some things.  One of my issues is that I live on a hill.  If I pull out of my driveway and go down the hill.. I'd sure like to be sure I can get it back home without a tow-truck!  This makes things like crusing at 30 tough to test without taking that risk (which I may just have to do).

Thanks again,
Tom
1966 AMC Rambler American Rogue - 232 I6
http://theamcforum.com/forum/1966-amc-american-rogue_topic20995.html

1978 AMC AMX - 258 I6
http://theamcforum.com/forum/1978-amc-amx_topic62333.html
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farna View Drop Down
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A 500 cfm 4V carb certainly isn't too big -- especially not when running on just the primaries, which he should be doing during tuning. Is it possible that the "kick-in" spring for the secondaries is to light and they are coming in too soon?? That would drop vacuum...   Plugging the vac advance line should take that out of the equation. Wouldn't hurt to put a strong spring in to prevent secondaries from opening, or disconnect them for now.
Frank Swygert
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Originally posted by farna farna wrote:

A 500 cfm 4V carb certainly isn't too big -- especially not when running on just the primaries, which he should be doing during tuning. Is it possible that the "kick-in" spring for the secondaries is to light and they are coming in too soon?? That would drop vacuum...   Plugging the vac advance line should take that out of the equation. Wouldn't hurt to put a strong spring in to prevent secondaries from opening, or disconnect them for now.

Hi Frank, thanks for the reply -- on the Edelbrock I don't believe they are "vacuum" secondaries in the sense a vacuum line is connected to them.  I believe the plate is weighted that is overcome at higher air flow into the carb.  Could be wrong about that, but in any case I don't believe the secondaries are opening.

I need to do some more diagnosis but the other day I chalked the wheels again and had my dad look down the carb when I put some throttle to it in 1st and put it under a load and there was a lot of fuel loading in and eventually flooding.

I ordered the jet kit for the 1404 in case I need to step them down.

Thanks,
Tom
1966 AMC Rambler American Rogue - 232 I6
http://theamcforum.com/forum/1966-amc-american-rogue_topic20995.html

1978 AMC AMX - 258 I6
http://theamcforum.com/forum/1978-amc-amx_topic62333.html
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Points: 3553 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/29/2020 at 9:01am
No swappable secondary springs  on Edel/ Carter carbs
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