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Discussion Starter #1
The Demon 650, four corner idle mix carb was supposed to be a perfect match to the Air Gap intake. It was delivered (the carb) the same week that Grant announced they were going **** up, citing all manner of production issues, among other things.
My new build just would not run right. Spent the last three weeks trying to figure out why. Cylinders 3 and 4 were stone cold. I was sure it was electrical, but everything checked out, after exhaustive troubleshooting.
Today I noticed that the right rear idle mix screw made no difference whether it was fully closed, or fully open. The sight windows show fuel.....uneven front to back, but fuel nonetheless. I can see fuel dripping in to the primary venturis when it's running.
I decided to pull the head on that side and check the valves for any obvious issues, as it was popping back through the carb under load. The image below is exactly what I saw when I pulled the head. 3 and 4 have never fired a single stroke! Compression is 150 in each. Plugs are firing.
I am now of the conclusion that the Demon is the problem.

Question: How does a four corner idle mix carb work? Does each idle screw control the feed to just TWO cylinders? If yes, what would cause a carb like this to not send fuel to two cylinders? Is it possible for these two cylinders to have NEVER seen any fuel at all?

No further questions, yoronner.
 

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Its a carb....it doesn't matter how many idle screws are in by nature of the beast it is impossible for no gas to have gotten into the cylinders.

I think you need to continue looking at looking elsewhere. Where your lifters opening the valves?
 

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I'd say it should be unlikely the carb would cause that issue. If you look at the runner layout of the manifold, the right side plenum feeds cylinders 1, 4, 6, 7 and the left side plenum feeds cylinders 2, 3, 5, 8. With cylinders 3 & 4 not getting fuel but other cylinders fed by the same common plenums are getting fuel, I'd think you could rule out the carb as the problem (though it sounds like it may have unrelated issues).

Either cylinders 3 & 4 are not building compression to suck in the air/fuel, or the intake valves aren't opening or the plugs aren't firing.
 

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Discussion Starter #4 (Edited)
Yes the valves are functioning properly......to the best of my knowledge, but it is popping through the carb when I put it in gear and run it down the driveway.
There was no "AH HA!" moment when I dissambled the head.
An inline spark tester says 3 and 4 are firing normally, but when I actually pull those plugs and short them, the spark does not look strong at all.
New coil....same thing. New plugs....same. Closed the gap to .030 ....no change. New plug wires....nada. Rebuilt the stock dizzy. Pulled the firewall feed harness and brightened up all the connections....this did seem to make a slight improvement.

Ironically, if your heard it running, it doesn't sound bad at all!
Nothing exiting through the exhaust, but the exhaust on that side is barely warm, and with not nearly the same force as the driver's side.

I am positive that I am not 180 degrees off.
If it were two chambers that were in sequence in the firing order I could begin to understand it......but 3 and 4?

After three weeks of this, I am ready to hang myself!
 

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Were 3 & 4 making compression before pulling the head? Have you looked in the intake manifold to make sure something wasn't stuck in the runners?
 

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Discussion Starter #6
"Were 3 & 4 making compression before pulling the head?"


Um, Wilit....not to be a smart a$$, but how would I check the compression AFTER removing the heads?

I have not actually looked into the runners.....I'll do that.

In a previous thread I mentioned that this head fell off the block, during the mock-up, and crashed to the concrete floor. At that time I pulled it all apart and checked every valve for possibility of being bent by working them up and down (and rotating) in the head/guides and feeling for any resistance......I decided to go ahead and put it back together.
The wear marks (roller mark) on #4 intake look a bit uneven.....dead center, but heavy on one side......all the others look fine. I spent hours aligning the valve geometry during the build. I am feeling a slight roughness in now as I work the valves as I did before. I'm going to order a new set of valves and guides for this side, just to be on the safe side.

As I said above, the spark looks weak to me, on these two bores, but I was out in broad daylight too. The coil spark is strong. New cap too.

Thanks for your comments,

Tom
 

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"Were 3 & 4 making compression before pulling the head?"


Um, Wilit....not to be a smart a$$, but how would I check the compression AFTER removing the heads?
If the head gasket didn't get mangled when you pulled the head off, you could reassemble it and get a reading. Obviously it's not going to be 100% accurate, but any measurement that is close to cylinders 1 & 2 should rule out mechanical problems.

I have not actually looked into the runners.....I'll do that.
Had an aquaintance that had a dead cylinder. Couldn't figure it out for a long time. Ended up pulling the engine apart to find a shop towel jammed in one of the runners. Ran great after that. :)


In a previous thread I mentioned that this head fell off the block, during the mock-up, and crashed to the concrete floor. At that time I pulled it all apart and checked every valve for possibility of being bent by working them up and down (and rotating) in the head/guides and feeling for any resistance......I decided to go ahead and put it back together.
The wear marks (roller mark) on #4 intake look a bit uneven.....dead center, but heavy on one side......all the others look fine. I spent hours aligning the valve geometry during the build. I am feeling a slight roughness in now as I work the valves as I did before. I'm going to order a new set of valves and guides for this side, just to be on the safe side.

As I said above, the spark looks weak to me, on these two bores, but I was out in broad daylight too. The coil spark is strong. New cap too.

Thanks for your comments,

Tom
You didn't mention all of the troubleshooting you did in the first post, but did you test the resistance of the plug wires for 3 & 4 with a multimeter? What distributor are you running? Also, how's the inside of the cap look? Does it look like the rotor is making good contact with the 3 & 4 terminals?
 

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Discussion Starter #8
The dizzy is the stock unit, with points......18-20 mil gap.
The wires are new, but I will check the resistance.
The burn marks on the new cap are uniform on all cylinders.
The plugs on the two dead cylinders show signs that they are firing.
Compression on the left bank is 168 to 178.
3 & 4 are 145 to 150, 1 & 2 are 157 each.

I was speculating that because 3 & 4 have never reached full temperature that they have not begun to seat, whereas all the others have begun to seat, reflecting the higher numbers.....just a theory. On the other hand, this head took a hard fall, and could not have landed any worse..... a direct hit on all the valve stems.
There's a lot of variables here, that's why it's so difficult to figure out.

I'm even wondering if somehow there's too much preload on 3 & 4, but I did them all the same...........hand tight until the push rods stop wiggling, then another half turn.

I'm still suspect of the carb. All three other idle screws will kill the engine when turned in too much......the right rear screw does nothing at all, either way.
Vacuum is good.....15 in.Hg @ 1200 (the lowest idle I can get under the circumstances).
23 degrees advance at 1200, 44 degrees all in at 2500 or so.
 

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Discussion Starter #10
I always tripple check the wire positions after swapping caps, wires, or whatever......crossing them is easy to do, as we all know.
The motor doesn't stumble like you get with crossed wires or a misfire for any reason. It actually runs fairly well on six cylinders, because it is NOT misfiring.......they're just not firing, if that makes any sense.
The exhaust gas is barely warm on that side, and a LOT weaker than the left bank.

When I pulled that head apart, two of the four valves on 3 & 4 were oily.
I'm going to rebuild this head and go from there. I will likely put the old Edelbrock carb back on it too.
I'll update in a day or two.

Thanks to all for your input,

Tom
 

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When you get it all back together, please update us. I'm really curious if it all works when it gets back together.
 

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Have you had the head checked for warpage? i'd be really suspicious of it after hitting the ground and 2 cylinders having lower compression. Sounds like the valve guides could be bent (ovaled?) causing the valves to stick and not fully seat also a cause for lower compression?
Let us know what you find.
Jon
 

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Wow, don't try try the easy things first I guess! Wouldn't it have been easier to do a carb swap before pulling head? :shrug: Just saying. Not trying to be a dick, I'm sure you're frustrated. If it was still together I would have tried swapping the wires on 3 and 4 to just humor us with less than 5 minutes of your time at no cost. I'd be pissed at myself for missing something so obvious. Sometimes we can get tunnel vision and rule out easy stuff that others see.

Definitely keep us updated.
 

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two things i would have done.

  • With the motor running I would have remove and re installed each spark plug wire and noted how much rpm drop I have from each wire removal. The cylinders with the least drop are you problem cylinders. If each cylinder is good the amount of drop would be the same.

  • Demon carbs are notorious for having junk in the circuits. Not only do I tear apart each new carb and blow with air and carb cleaner through them, but I also take a wire into each hole to check for plugged passage ways. I find junk in them often. and it is not uncommon to see passage ways plugged.
 

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I was thinking like the others...swap # 3 and 4 as they are side by side in the firing order.

If you had them marked i guess you could easily check them now to see if they were swapped...

Good luck with it....John
 

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I see the roller lifters so what firing order does the cam builder call for?

Looks like your using the old order most likely with an HO order cam.

http://www.offroaders.com/tech/V8-engine-firing-order.htm



Also the demon can be tricky to tune buy you need to start with the basics. Download the owners manual from Barry before they disappear. Looks like you have a 750 speed demon?
 

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Are the cam lobes wiped on those cylinders?
 

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Discussion Starter #18
This cam and head were bought used as a "matched set" , originally designed and built for a 331 stroker.....mine is a 347.
The cam card is a bit confusing, as it states the following,

Cubic inch and engine type: 331 SBF 1,5,4,2,6,3,7,8

and the next line down on the card reads,

FIRING ORDER: STD 351W/302 HO Ford 1,3,7,2,6,5,4,8

The cam was built by Camshaft Innovations.

I am using the stock firing order. Both of the above orders are written in pencil on the card......not sure who, or why they listed both orders.
I did not try the HO order.......should I?

I just dropped off the head at my engine shop. I did not read your post about the firing order until I got home just now.
Perhaps I should go back and show him the cam card and get his opinion?
He's 1-1/2 miles away.

It is a hyd. roller that has been previously broken in....no reason to expect a wiped lobe, I would't think.
Yes, I pulled #'s 3 & 4 with the engine running and it made no difference at all.
Yes, it occurred to me to swap the carbs before pulling the head, but I keep thinking about dropping this head, I need to eliminate that as a variable......it will always be in the back of my mind, otherwise. There was oil getting by the guides....it had to be done anyway.

Thanks again all,

Tom
 

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Discussion Starter #19
This may be the DOH! moment that I've been waiting for......

I've been reading a black and white printout of my cam card.
I just went back and found the original copy.....and guess what?

The HO firing order is HIGHLIGHTED IN YELLOW bt the PO, I presume.

We may have found our "demon" after all?

66 6, if this is it, I OWE YOU ......BIGTIME!
 

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Sorry to say almost any aftermarket cam uses the HO/351 firing order. I made the same mistake with my car. And then when I did have it set correctly I was POSITIVE everything was correct but I happen to have the wires off by one position on the dizzy.

Put it back together and use the HO order. She'll probably work just fine.
 
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