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Chasing a misfire on 351 4V

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RyKR View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RyKR Quote  Post ReplyReply Direct Link To This Post Topic: Chasing a misfire on 351 4V
    Posted: 05-April-2020 at 3:27PM
I have a 351 4v in my 73 Montego. Other than the obvious Torker intake, headers, and 750 holley, I have no idea what is up with the engine. There is a constant misfire just off idle and at sustained RPMs. If I am giving it hell it might miss once or twice between 1500rpms and 5500rpms, but at a constant RPM it will consistently misfire. I'm guessing it is mechanical because I am running out of other ideas. I'll list what I've done and why.

Distributor. I swapped out the old MSD 5 and factory single points distributor for an HEI style. I've checked to make sure that it maintains a steady voltage. I get between 10 and 12 volts to the coil with the engine running throughout the RPM range. I also gapped the plugs to .055 and the initial timing is approx. 20 btdc.

All of the plugs were black and covered with ashy soot. This led me to the following work on the carburetor.

Carburetor. Had an 830 carburetor on it, took that off and put a 833310-1 750 with 4160 jet conversion kit. I ran the idle air fuel screws all of the way in and the engine didn't so much as stumble. From what I've read, this indicates a blown power valve. I put in a 6.5 power valve (vacuum at idle was approx. 15). Runs about 6psi of fuel pressure.

I ran a cranking compression test and the cylinders ranged from 146-155. Vacuum at idle was approx. 15 and steady. The needle didn't jump much, just slowly hovered right around 15. There is no excessive noise from the valve train and none of the valves seem to stick.

I sprayed carb cleaner around the vacuum hoses, manifold, and carburetor in search for a leak and didn't find anything obvious, the engine didn't surge or bog.

I'm thinking about pulling the cam and lifters. Should I just go ahead and pull the heads and see what's going on?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 4:52AM
it's harder to diagnose a pile of parts, steady vac indicates no issue with cam & lifters or valve sealing. non-responsive idle mix screws on a carb that has been running and the screws were responsive at one time may indicate a bad PV but if it's a new install and they were never effective it could just be that the primary throttle blades are open too much at idle exposing the transition slot. to get the primary blades closed more all you need to do is increase the secondary idle slightly with the adjustment screw and decrease the primary idle. most carbs need to come up off the intake to access the sec idle screw

gap at .035" and see what it does

did you select plugs designed for .055" gap or just open  ... what plug?

if that doesn't cure it,

do you have a temp gun? if not use a spray bottle, as the engine is coming up to temp spray water on each exhaust and compare evap-off. you're looking for a port that doesn't get hot with the rest of them, this will identify a single cylinder as the miss'r

ever heard of 'induction misfire'? plug wires 5 & 6 are not friends. 5 goes direct to the plug in front of the cap. use a longer wire for 6 and route it around the back of the cap, and behind 7 & 8 then loop forward to the plug. if 5 & 6 run parallel 6 will induce voltage into 5 and fire the intake charge before it's time. then 5 makes no power when it is time

or the 'dark spark test' ?

you need an area with NO light, as in VERY dark. open the hood, engine running, let your eyes adjust to the darkness and look very carefully at the wires, coil, plug boots, etc...keep out of the fan. you may or may not see some arcing going on. 5-6 induction misfire is not visible as far as i know.

Machoneman adds this suggestion, again with the spray bottle: In the dark, with engine running at idle, shoot water from a spray bottle (you know, a old window cleaner bottle or better, one with the trigger pull handle). Hit every spark plug wire heavily over entire length(coil too) and watch for a blue moon! (sparks and blue glow, that is).

i would do a fine mist first, everything will eventually get soaked




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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 10:14AM
Fuel filter?
revving may get fuel past a "less than ideal" filter, but steady rpm, not so much.
Dark/sooty plugs, may be running rich or ignition issue. When you added HEI, did you bypass ignition resistor wire?
"What we do in full frontal view, is more honest than your cleaned-up mind."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gregaba Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 12:11PM
Did it misfire before you made your change's?
Greg

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 1:30PM
Rock and Randy have good advice. The misting water on the wires works really well, I found a misfire years ago on my old truck with this method very quickly.

If you are running an GM HEI module, make sure it gets at least 12 volts at all times, any less will cause issues.  When running it should be higher than that, like 13.5 -14.5 volts.  Like Randy said, make sure you bypassed the resistor wire.  A bad HEI module will cause all kinds of misfires, but it sounds like it happened before your switch to a HEI.
Vince

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Post Options Post Options   Thanks (0) Thanks(0)   Quote RyKR Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 2:18PM
I appreciate all of the help. This was driving me crazy. I was sure that the cam timing was off or something. I think the misfire is now under control. I reset the gap on the plugs, adjusted the idle air mixture, and the timing. It runs smooth now, it just lacks power. It has a terrible bog off the line and won't even do a burnout now, but it runs smoother. Hopefully that issue will be easier to fix.

Greg, the misfire has always been there. That's what started the trip down the rabbit hole. All work has been focused on fixing this issue. I tried to make one change at a time and not change anything without a reason.

Rocky, the carb wasn't new, it was off of prior project. The primary blades are almost closed, with just a very small part of the transition slot showing. I did just increase the gap on the plugs I had.. They were Autolite's. Iridium 25 (I think).

Vince and Randy, the resistor has been removed. The alternator just started making noise on this last trip so it may be on the way out and not giving enough voltage. The wiring on this car is a nightmare. Someone used extension cord wire and bad connectors. I am addressing the wiring as I go along. 


If the misfire comes back, I will do the mist test. I did check the OHMs on the wires and they were below 4500 per foot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 2:55PM
have you checked the firing order? i've seen several ford v-8's with the wrong order actually run decent but seriously lack power for having the wrong order!

remember fuel can act like an ignition issue and vice versa
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 4:03PM
i missed the voltage to the HEI, pretty sure they like to run at charging volts close to 14 ?

a relay off the battery side of the solenoid triggered by the stock coil wire would do it
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rons73 Quote  Post ReplyReply Direct Link To This Post Posted: 06-April-2020 at 6:57PM
You appear to have a couple things stacking up against you here.  

Low voltage to the coil : you stated 10-12v at the coil, this will limit the ability of the coil to overcome the increasing resistance of aging components. Should see +13.5 volts, as has been mentioned. The coil itself could have been failing, thus starting your problems.  You mentioned the wiring is less than optimal. I would start with addressing this voltage supply deficiency.

Plug wire resistance: 4500 ohms/ft is just not helpful. I would be looking for arcing with the water misting. A substandard coil or one with low supply voltage may have issues getting spark to the plug, or the wires have so much resistance that the spark is arcing to an easier path to ground. There are much better wires available, like order of magnitude better, and they are not expensive. I would find a brand you like that offers a wire set with 500 ohms/ft or less.

Iridium plugs:  These have a much higher internal resistance than standard copper core types.  Higher even than your current plug wires.  The resistance decreases with higher plug temperatures, but at the cooler combustion temps at idle, engine missing is not uncommon with marginal systems.  Wider plug gaps just make it worse.  Now I love iridium plugs for my daily drivers, they last forever, but a standard copper style would probably give you better service in this application.

The steps you have taken to fix the miss, setting idle mixture(richer perhaps?), closing plug gap, and setting timing(you didn't say what you set it to but 20 initial seems like a bit much to me), all should have decreased the firing voltage. This may have allowed the system to keep up with demand for the time being, but please let us know how you made out, and what you did to solve the problem.  It may help someone else who ends up with similar issues.

My suggestions are just what I would do, your mileage may vary.  Oh, and check the float level in that 750.  Sometimes if the fuel is high, it can mess with your idle mixture settings.

Good luck!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 07-April-2020 at 10:32AM
i've used extension cord wiring myself, IMO it's suited fine to low voltage auto use having very small strands x 3 inside the outer sheath. auto purists call it a hack, works for me. i run multiple 30 amp circuits from the battery to the trunk to power a permanently mounted tire inflator and amp. circuit breakers very close to the battery for protection, no issues. X cord also inside my radio console using the 110 household plugs. shoddy connectors are always a problem.

no love for iridium plugs here unless spec'd by the OEM.

not sure the HEI coil is up to firing them, maybe as long as everything else is in order?

lacking power and soft low end could well be a retarded cam timing issue, or the cam was poor choice for the static compression ratio in the engine. a 'normal' performance cam in a stock low compression engine will reduce dynamic cylinder pressure resulting in a loss of efficiency and power. if it's the stock cam and the timing chain is totally whipped out causing retarded cam timing you'll have the symptoms you describe also. 1973 introduced a new retarded crankshaft gear that doubled down on earlier retarded camshaft valve timing events, a tired old chain makes it strike 3. if you do decide to go into the front cover get yourself a timing set with 3 key slots and install it using the + / advanced setting to get back to 1972 timing conditions

is that the right carb number? nothing comes up for me searching 833310-1 750
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 07-April-2020 at 12:25PM
HEI also gets finicky if there is a bad ground.
Also needs a good heat sink.
"What we do in full frontal view, is more honest than your cleaned-up mind."
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2005 F-150 STX RCSB 4.6, 3.55 LSD
How the Heck does a REGULAR CAB SHORTBED weigh over 5200 pounds?
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