Flathead tuning
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5th Gear
Joined: Jan 2021
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Flathead tuning
Hey all. I need some help. I have a 1953 8ba rebuilt 30 over with 4” crank Max 1 cam Offy aluminum heads and Speedway three duece intake with 3 super 97 carbs and progressive linkage. Timing is 6 degrees initiall and 32 total all in by 2900 rpm. When I hook up the vacuum gauge the needle is erratic at idle (600 rpm) if I increase the rpm to say 900 the gauge evens out around 18. If I increase idle to 750 the needle waves between 14 and 18. There seems to be a slight miss sound at the tail pipes. Sorry about the long explanation but I don’t know what to do next?
Fleet Owner
Joined: Jul 2004
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From: NM
32° is way more advance than a flathead needs, 22° - 24° total at 2000 - 2200 RPM is usually best (mechanical + initial, vacuum disconnected).
I assume you have some kind of electronic ignition with both mechanical and vacuum advance? What vacuum advance source are you using, manifold vacuum? If you have an adjustable vacuum advance diaphragm, shoot for around 10° max vacuum advance.
I assume you have some kind of electronic ignition with both mechanical and vacuum advance? What vacuum advance source are you using, manifold vacuum? If you have an adjustable vacuum advance diaphragm, shoot for around 10° max vacuum advance.
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5th Gear
Joined: Jan 2021
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Yes. Pertronix distributor with there coil. No vacuum advance hooked up. I will have to set up the max advance yet but just trying to get this running a little better. It starts good and drives down the street and back okay. Just can’t get a steady vacuum reading. Vacuum is from manifold under back carb
Fleet Mechanic
Joined: Sep 2010
Posts: 1,834
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From: Mason City, Iowa
How aggressive of a cam are you running? With a larger cam you may need more initial timing, with less throttle, and maybe a somewhat leaner setting of your idle mixture screws. If you use more initial timing you will be using less mechanical advance to get to your "all in" timing. Such as (just an example here) if you get the smoothest idle at let's say 10 degees initial, and your max advance should be 22, then you will need only 12 degrees of mechanical advance to get to an "all in" timing of 2000 RPM. So with a aftermarket distributor like a Pertronix, you will need to play with your advance springs and advance limiters to give you your timing "curve". This does not include using manifold vacuum though. Once that is connected then the distributor advance changes and you may need to go back again and compensate for that.
Another thing to kind of keep in mind is the crank damper. Sometimes the outside rim slips, and the timing marks are no longer accurate. It might be a good idea to see if the 0 TDC mark is truly showing TDC.
Another thing to kind of keep in mind is the crank damper. Sometimes the outside rim slips, and the timing marks are no longer accurate. It might be a good idea to see if the 0 TDC mark is truly showing TDC.
Mountain Pass
Joined: Sep 2022
Posts: 114
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From: Fort Collins, CO
Yep, what hooler1 says about camshaft. Big, long duration cams have a lumpy idle. The overlap of both intake and exhaust valves being open at low rpm cause this along with poor vacuum. Air / fuel mixture takes time to flow, so big cams account for this by opening valves sooner and closing later to get more mixture in the cylinder at high rpm where max horsepower can be developed. Listen to cars at the drag strip. New cars with variable valve timing are trying to get best of both low and high rpm power. So, see if you have the lift and duration specs for your cam shaft.
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5th Gear
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So I did a little more checking on my flathead. Linkage was not working properly. Now both front and rear cars are closing all the way. I disconnected linkage so just dealing with main carb for now.
The miss or popping is from cylinder #4. If I pull cyl 4 plug wire the miss or popping goes away? There is good spark at the plug. Any ideas?
The miss or popping is from cylinder #4. If I pull cyl 4 plug wire the miss or popping goes away? There is good spark at the plug. Any ideas?
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Fleet Mechanic
Joined: Sep 2010
Posts: 1,834
Likes: 467
From: Mason City, Iowa
So I did a little more checking on my flathead. Linkage was not working properly. Now both front and rear cars are closing all the way. I disconnected linkage so just dealing with main carb for now.
The miss or popping is from cylinder #4. If I pull cyl 4 plug wire the miss or popping goes away? There is good spark at the plug. Any ideas?
The miss or popping is from cylinder #4. If I pull cyl 4 plug wire the miss or popping goes away? There is good spark at the plug. Any ideas?
The only other thing maybe a damaged spark plug or a bad wire. Might be interesting to try a different wire and plug.
https://myflatheadford.com/routing-s...ires-in-looms/
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Fleet Owner
Joined: Jul 2004
Posts: 27,297
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From: NM
Look at the plug end of #4 wire, inside the boot. I had one where the terminal was just touching the core of the wire, it had been crimped on but came out while putting the boot on/off the plug. If you have suppression type wires that can happen pretty easily. At least it was easy to fix, too. The end at the cap can do that too. You might also check the resistance of the wire over its full length, could be a break in it.
