The Ford Barn

Southtowns27 · 07-03-2025, 12:57 PM

I have an 8CM in my truck with about 500 miles on it since being built. It's a stock rotating assembly, bored .060" over, with a Schneider S100 cam, Isky springs, and adjustable lifters. Offy heads and intake. I clayed the engine when it was built so I know the clearances are good. It won't run up past about 3500 RPM. I can ease it up to that speed, but if I try to make it run faster it drops a few cylinders and actually slows down. I've also noticed that if I whack the throttle it also seems to drop a cylinder while it's winding up. It'll cruise down the road at 55mph no problem though. At high RPM vacuum is about 22 inches and fluctuates maybe an inch. Compression is about 125psi and they're all within 10psi of each other. Things I have tried so far: Different plugs, plug wires, different timing, condenser, swapped the Mallory dual point distributor for the stock one, it had Speedway "97's" on it, swapped those with new real Stromberg 97's (2psi fuel pressure, .043 jets, 69 power valves), reset the valve lash, different plug gaps, disconnected all the vehicle electrical and ran a jumper wire right to the coil, different coil, unhooked the wiper vacuum, unhooked the PCV system.
NO CHANGE.
What am I missing here???????

Randy in ca · 07-03-2025, 01:36 PM

Lack of fuel? Is there still 2 psi when it starts to stumble? Electric pump?

petehoovie · 07-03-2025, 02:12 PM

Quote:

Originally Posted by Southtowns27

I have an 8CM in my truck with about 500 miles on it since being built. It's a stock rotating assembly, bored .060" over, with a Schneider S100 cam, Isky springs, and adjustable lifters. Offy heads and intake. I clayed the engine when it was built so I know the clearances are good. It won't run up past about 3500 RPM. I can ease it up to that speed, but if I try to make it run faster it drops a few cylinders and actually slows down. I've also noticed that if I whack the throttle it also seems to drop a cylinder while it's winding up. It'll cruise down the road at 55mph no problem though. At high RPM vacuum is about 22 inches and fluctuates maybe an inch. Compression is about 125psi and they're all within 10psi of each other. Things I have tried so far: Different plugs, plug wires, different timing, condenser, swapped the Mallory dual point distributor for the stock one, it had Speedway "97's" on it, swapped those with new real Stromberg 97's (2psi fuel pressure, .043 jets, 69 power valves), reset the valve lash, different plug gaps, disconnected all the vehicle electrical and ran a jumper wire right to the coil, different coil, unhooked the wiper vacuum, unhooked the PCV system.
NO CHANGE.
What am I missing here???????

ford38v8 · 07-03-2025, 02:21 PM

THat's a beautiful engine, I bet you spend lots of time admiring that honey!
Your description of what you've tried sounds like you have a fuel issue, and from the picture, I see several possible causes:

Your high vacuum together with the small air filters suggest an air restriction. Try a temporary removal of the air filters for effect.

It takes only a very small air leak to lower your bowl level to the point of lack of fuel on acceleration. The fuel pump's glass bowl should have a cork gasket soaked in oil before careful installation. Also, check the tightness of those worm gear clamps.

The inline filter is redundant with the glass bowl, and may be clogged anyway.

Your rubber flex line may be collapsed internally. I would change out that line to a Shewman fuel resistant line, advertised in the V8 Times, without check valve.

What is not shown in the picture is your tank to firewall fuel line. Original lines being copper coated steel, they rust, flake debris, and develop pinhole leaks. Not cheap, but a Cupronickle fuel line is a great solution to many issues.

Ggmac · 07-03-2025, 02:42 PM

Blocked exhaust ? Scratch that , you had good vacuum. What gearing and how heavy is the car ?

john in illinois · 07-03-2025, 02:46 PM

Sponsored Links (Register now to hide all advertisements)

In my opinion .043 jets are too small . I have a flathead 2 carb engine in my roadster. I put an A/F gauge on it and found stock .045 to.046 worked best.
also it is impotant to check float level very carefully. that has a lot rot do with fuel mixture.
Another variable is air filters. restrictive filters can make jetting difficult.

John

ford38v8 · 07-03-2025, 03:01 PM

Quote:

Originally Posted by john in illinois

In my opinion .043 jets are too small . I have a flathead 2 carb engine in my roadster. I put an A/F gauge on it and found stock .045 to.046 worked best.
also it is impotant to check float level very carefully. that has a lot rot do with fuel mixture.
Another variable is air filters. restrictive filters can make jetting difficult.

John

John, I missed that. Good call on the jets.

Tim Ayers · 07-03-2025, 03:14 PM

Quote:

Originally Posted by john in illinois

In my opinion .043 jets are too small . I have a flathead 2 carb engine in my roadster. I put an A/F gauge on it and found stock .045 to.046 worked best.
also it is impotant to check float level very carefully. that has a lot rot do with fuel mixture.
Another variable is air filters. restrictive filters can make jetting difficult.

John

John,

I agree on both your floats and jets comments. OP- Have you checked the plugs to see if you can tell if it is leaned out?

My 292 c.i. engine with an Edelbrock Slingshot with two 97's. I had to mess with the jetting and I settled back on .046 jets and it runs strong. I tried as big as .048 and as small as .043. With .046's it runs the best.

OP- take the air filters off and see if it changes anything. If that improves it, I'd swap the Mallory back it as well.

Kurt in NJ · 07-03-2025, 04:47 PM

In the picture it looks to have adjustable main jets.

KiWinUS · 07-03-2025, 05:40 PM

Quote:

Originally Posted by Kurt in NJ

In the picture it looks to have adjustable main jets.

Great catch. For the trained professionals only.

beast4765 · 07-03-2025, 08:49 PM

I have had a problem in the past where the fuel line was too small. Also the electric fuel pump did not have enough volume to keep up

A bones · 07-03-2025, 09:19 PM

Quote:

Originally Posted by ford38v8

THat's a beautiful engine, I bet you spend lots of time admiring that honey!
Your description of what you've tried sounds like you have a fuel issue, and from the picture, I see several possible causes:

Your high vacuum together with the small air filters suggest an air restriction. Try a temporary removal of the air filters for effect.

It takes only a very small air leak to lower your bowl level to the point of lack of fuel on acceleration. The fuel pump's glass bowl should have a cork gasket soaked in oil before careful installation. Also, check the tightness of those worm gear clamps.

The inline filter is redundant with the glass bowl, and may be clogged anyway.

Your rubber flex line may be collapsed internally. I would change out that line to a Shewman fuel resistant line, advertised in the V8 Times, without check valve.

What is not shown in the picture is your tank to firewall fuel line. Original lines being copper coated steel, they rust, flake debris, and develop pinhole leaks. Not cheap, but a Cupronickle fuel line is a great solution to many issues.

Gold right here

Flathead Fever · 07-04-2025, 01:43 AM

I agree with A bones, try running it without the air filters and see what it does. I have a 324 olds engine in the '28 Mitchel Muffler Pickup. About 25-years ago when I rebuilt the engine. I adjusted the idle air fuel with the air cleaners off. When I sat the air cleaners on the carbs the idle dropped. I thought that was strange at just idle. They were two of those small flathead 97 style air filters I had laving here. I swapped them out to a different design and then then the engine could breathe. Pretty bad when air cleaners won't even flow enough air at idle. That might not be your problem, but it will be easy to check. Also, make sure the throttle valves are opening up all the way. I have not done it yet but I'm going to start installing air fuel ratio gauges on my cars when I build them, that's if I recover from the stroke I had. I used the ones on the smog machine at work to check the engine readings on the dyno, but I was never allowed to bring my personal vehicles into the phone company's garage, too many eyeballs watching us in the garage. It would be really cool to monitor the fuel ratio driving down the road to get everything dialed in. There's lots of videos on those gauges.

tubman · 07-04-2025, 05:05 AM

How does that engine breathe? I see it has an early intake manifold with no provision for a road draft tube. It does have the early style fuel pump stand where the air can get into the engine, but how does it get out? Unless provisions have been made for the air to get out some way (such as a PCV valve), it can cause problems. The air flow through the later (8BA,8CM) engines is opposite that of the earlier engines, so swapping intakes late for early is not a simple bolt on exercise.

BTW, I would consider changing out that intake manifold in any event. It's a textbook example of poor fuel distribution, especially if run with a straight linkage.

chap52 · 07-04-2025, 07:26 AM

Great responses here. Interesting read for sure. Thanks... Chap

Tim Ayers · 07-04-2025, 07:43 AM

Quote:

Originally Posted by Kurt in NJ

In the picture it looks to have adjustable main jets.

I've seen them, but never held one on my hand. How do they work? Is it like a metering rod-type set up?

34fordy · 07-04-2025, 08:39 AM

Quote:

Originally Posted by Tim Ayers

I've seen them, but never held one on my hand. How do they work? Is it like a metering rod-type set up?

Yes.
My old 1942 Allis Chalmers WC with a Marvel Schebler carburetor had an adjustable main jet. A simple carburetor that worked very well.

slowforty · 07-04-2025, 11:25 AM

What type of ignition resistor do you have. I had a bad one that would fail when it warmed up. Would not let the car run over 60 mph.

Ken/Alabama · 07-05-2025, 06:09 AM

I’m thinking distributor issues.

hueyhoolihan · 07-05-2025, 09:15 AM

i have similar symptoms with my '41 flathead.

and just a year ago had problems, again similar symptomes, with yet another of my vintage cars. and only two years ago with my 2012 four cylinder honds ST1300 motorcyle.

except for my current problem with the '41 (which has yet to be fixed), in all cases the problem was with an ignition wire that was working perfectly until the engine warmed up to a certain temperature and then all hell would break loose!.

on the honda it was an abraded sparkplug wire that was arcing and on the other vintage (1953) car it was the COIL wire that was arcing (it affected all four sparkplugs simultaneously, of course).

with the '41, i've replaced the fuel pump and can see that the sediment bowl is full at all times, and because the carb is brand new (although i've had it apart looking for a problems) i've been concentrating on the ignition, which has been disassembled numerous times too. it has had the points replaced and a new condenser installed. so am thinking coil...

so, currently have a new coil on the way. and if that doesn't fix it, i'll be installing new caps, both inner and outer, and if THAT doesn't fix it i'll be installing all new wires.

i don't like chasing problems by adding parts, but to be as reliable as i want to car to be, they probably needed replacing anyway. not to mention that i'll gain knowledge WRT the engine.

i'll be following this thread closely.

07-03-2025, 01:36 PM	#2
Randy in ca Senior Member Join Date: May 2010 Posts: 1,816	Re: Won't run past 3500 rpm Lack of fuel? Is there still 2 psi when it starts to stumble? Electric pump?

07-03-2025, 02:21 PM	#4
ford38v8 Senior Member Join Date: Jul 2011 Location: SF Bay Area Posts: 7,279	Re: Won't run past 3500 rpm THat's a beautiful engine, I bet you spend lots of time admiring that honey! Your description of what you've tried sounds like you have a fuel issue, and from the picture, I see several possible causes: Your high vacuum together with the small air filters suggest an air restriction. Try a temporary removal of the air filters for effect. It takes only a very small air leak to lower your bowl level to the point of lack of fuel on acceleration. The fuel pump's glass bowl should have a cork gasket soaked in oil before careful installation. Also, check the tightness of those worm gear clamps. The inline filter is redundant with the glass bowl, and may be clogged anyway. Your rubber flex line may be collapsed internally. I would change out that line to a Shewman fuel resistant line, advertised in the V8 Times, without check valve. What is not shown in the picture is your tank to firewall fuel line. Original lines being copper coated steel, they rust, flake debris, and develop pinhole leaks. Not cheap, but a Cupronickle fuel line is a great solution to many issues. __________________ Alan

07-03-2025, 02:42 PM	#5
Ggmac Senior Member Join Date: Jul 2016 Location: Lake worth Florida Posts: 1,371	Re: Won't run past 3500 rpm Blocked exhaust ? Scratch that , you had good vacuum. What gearing and how heavy is the car ?

07-03-2025, 02:46 PM	#6
john in illinois Senior Member Join Date: May 2010 Location: Illinois Posts: 2,220	Re: Won't run past 3500 rpm Sponsored Links (Register now to hide all advertisements) In my opinion .043 jets are too small . I have a flathead 2 carb engine in my roadster. I put an A/F gauge on it and found stock .045 to.046 worked best. also it is impotant to check float level very carefully. that has a lot rot do with fuel mixture. Another variable is air filters. restrictive filters can make jetting difficult. John __________________ Welcome each day Last edited by john in illinois; 07-03-2025 at 02:57 PM.

07-03-2025, 04:47 PM	#9
Kurt in NJ Senior Member Join Date: May 2010 Location: on the Littlefield Posts: 6,553	Re: Won't run past 3500 rpm In the picture it looks to have adjustable main jets.

07-03-2025, 08:49 PM	#11
beast4765 Junior Member Join Date: Nov 2010 Location: SC Posts: 3	Re: Won't run past 3500 rpm I have had a problem in the past where the fuel line was too small. Also the electric fuel pump did not have enough volume to keep up

07-04-2025, 01:43 AM	#13
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,358	Re: Won't run past 3500 rpm I agree with A bones, try running it without the air filters and see what it does. I have a 324 olds engine in the '28 Mitchel Muffler Pickup. About 25-years ago when I rebuilt the engine. I adjusted the idle air fuel with the air cleaners off. When I sat the air cleaners on the carbs the idle dropped. I thought that was strange at just idle. They were two of those small flathead 97 style air filters I had laving here. I swapped them out to a different design and then then the engine could breathe. Pretty bad when air cleaners won't even flow enough air at idle. That might not be your problem, but it will be easy to check. Also, make sure the throttle valves are opening up all the way. I have not done it yet but I'm going to start installing air fuel ratio gauges on my cars when I build them, that's if I recover from the stroke I had. I used the ones on the smog machine at work to check the engine readings on the dyno, but I was never allowed to bring my personal vehicles into the phone company's garage, too many eyeballs watching us in the garage. It would be really cool to monitor the fuel ratio driving down the road to get everything dialed in. There's lots of videos on those gauges.

07-04-2025, 05:05 AM	#14
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 11,623	Re: Won't run past 3500 rpm How does that engine breathe? I see it has an early intake manifold with no provision for a road draft tube. It does have the early style fuel pump stand where the air can get into the engine, but how does it get out? Unless provisions have been made for the air to get out some way (such as a PCV valve), it can cause problems. The air flow through the later (8BA,8CM) engines is opposite that of the earlier engines, so swapping intakes late for early is not a simple bolt on exercise. BTW, I would consider changing out that intake manifold in any event. It's a textbook example of poor fuel distribution, especially if run with a straight linkage. Last edited by tubman; 07-04-2025 at 05:12 AM.

07-04-2025, 07:26 AM	#15
chap52 Senior Member Join Date: Jun 2013 Location: Marana Arizona Posts: 1,829	Re: Won't run past 3500 rpm Great responses here. Interesting read for sure. Thanks... Chap

07-04-2025, 11:25 AM	#18
slowforty Senior Member Join Date: May 2010 Location: Tinley Park Ill Posts: 1,175	Re: Won't run past 3500 rpm What type of ignition resistor do you have. I had a bad one that would fail when it warmed up. Would not let the car run over 60 mph.

07-05-2025, 06:09 AM	#19
Ken/Alabama Senior Member Join Date: May 2010 Location: Alabama Posts: 3,394	Re: Won't run past 3500 rpm I’m thinking distributor issues.

07-05-2025, 09:15 AM	#20
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 249	Re: Won't run past 3500 rpm i have similar symptoms with my '41 flathead. and just a year ago had problems, again similar symptomes, with yet another of my vintage cars. and only two years ago with my 2012 four cylinder honds ST1300 motorcyle. except for my current problem with the '41 (which has yet to be fixed), in all cases the problem was with an ignition wire that was working perfectly until the engine warmed up to a certain temperature and then all hell would break loose!. on the honda it was an abraded sparkplug wire that was arcing and on the other vintage (1953) car it was the COIL wire that was arcing (it affected all four sparkplugs simultaneously, of course). with the '41, i've replaced the fuel pump and can see that the sediment bowl is full at all times, and because the carb is brand new (although i've had it apart looking for a problems) i've been concentrating on the ignition, which has been disassembled numerous times too. it has had the points replaced and a new condenser installed. so am thinking coil... so, currently have a new coil on the way. and if that doesn't fix it, i'll be installing new caps, both inner and outer, and if THAT doesn't fix it i'll be installing all new wires. i don't like chasing problems by adding parts, but to be as reliable as i want to car to be, they probably needed replacing anyway. not to mention that i'll gain knowledge WRT the engine. i'll be following this thread closely.