Proper Valve springs for 296 cu.in. flathead with Howard cam M12

[restoross]

Have 53 Merc 8ba 296 cu.in. 3 Holley 94's, dual point Mallory,Edelbrock heads and intake, Stainless steel valves, Howard M12 cam. Has a miss starting at 3400-3500 RPM. Vacuum at idle (850 rpm) 15 in/hg with less than .5 in fluctuation, max steady to 18 in/hg at less than 3400 RPM above that the vacuum gauge goes nuts and sounds like a miss. have subbed carbs, ignition etc. This problem showed up after Cam change and new lifters but with valve springs used with SU1A grind cam that was in the engine when I acquired it, my gut feeling is valve float. My research has come up with using Isky 185G single springs or Isky 4005 dual springs (can dual springs even be used on a flatty)?? What say you Flathead gurus for springs that will work???

M12 specs are intake duration 274, exhaust 277, 53 degree overlap 112 centerline, .360 lift both intake/exhaust.

[KiWinUS]

What seat tension do your springs have? Very important information that is needed to help you.
Yes you can get dual springs for a flathead. You do not need them. Race only if even then.

[restoross]

I will have to get that from the machinist this week as he had the equipment to test the spring set up when we assembled the block.

[Pete]

An M12 is not much different than an SU1A.
Isky 186 springs or zephyr's will do fine.
Set them at 75lb. on the seat. Check for .060 minimum coil gap at full lift.

Definitely no double springs, even for the wildest roller cam you can imagine in a flathead Ford.

You might get 94's driveable but a 296 needs WAY more air to use the other parts you have in the engine. Try 48's or modern 97's.
The carb size is definitely NOT causing your miss problem though.

[restoross]

Thanks for your input Pete I guess I'll start looking for some 48's.

[tubman]

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I'd like to try one of these with a couple of Rochester 2G's some time.

[restoross]

Originally Posted by Pete

An M12 is not much different than an SU1A.
Isky 186 springs or zephyr's will do fine.
Set them at 75lb. on the seat. Check for .060 minimum coil gap at full lift.

Definitely no double springs, even for the wildest roller cam you can imagine in a flathead Ford.

You might get 94's driveable but a 296 needs WAY more air to use the other parts you have in the engine. Try 48's or modern 97's.
The carb size is definitely NOT causing your miss problem though.

Pete,
When I looked up Isky part number 186 it came up as a cam not springs, did you mean 185??

[Ol' Ron]

When running the stock car with a 400jr cam I used the isky 185 springs, I also used the SBC valves (1.5) which are longer than stock. this puts the adjusters closer to stock. Testing spring pressure at max lift it came out around 65lbs. With 5.42 gears it probably turned clost to 5k. Single 2GC carb. Probably missed something here, BUT??
Gramps
PS Ran 50 lbs on the L-100. On a side note. Not many 296 engines are built today, 294 yes????
Gramps

[Bored&Stroked]

Isky 185-G springs - and I agree with Pete on the max spring pressure on the seat. You must ensure that you know the installed height to get to 75 lbs. The double spring setup that Isky used to sell was a complete waste of time - they actually perform WORSE than the 185-G springs.

I have a big custom roller cam and my seat pressure is 100 lbs on the seat. With 1.72" intake valves (heavier than 1.6" valves), I've never floated them - even at 6200 RPM.

[Pete]

Originally Posted by restoross

Pete,
When I looked up Isky part number 186 it came up as a cam not springs, did you mean 185??

Yep, typo. Sorry.

[Flathead]

I ran Isky 185 springs on a Potvin 425 eliminator. It regularly turned over 7000 rpm at the drags never a problem. I agree that those double springs aren't worth the bother.

[flathead4rd]

Flathead; I have a Potvin cam in my 286. It is stamped 415 but I can't find any information on that number, Potvin cam. Any ideas as to lift (i'm assuming 415) but what about the duration? It runs good.

[Flathead]

Never heard of that one. I would guess that it may be similar to the 425. Others here may have an answer. I was very happy with the 425, idle was good and it made power on a 296 with four 97's.

[Pete]

The actual cam lift of a 425 is .416.

One thing for sure, it is NOT an original Potvin.

[restoross]

Originally Posted by Flathead

I ran Isky 185 springs on a Potvin 425 eliminator. It regularly turned over 7000 rpm at the drags never a problem. I agree that those double springs aren't worth the bother.

Thanks for your input.