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Ford 9N, 2N & 8N Discussion Forum
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Tach- Dwell Meter

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Jim, Dundee, IL

05-25-2004 07:26:09




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I found my old Tach/dwell meter after missplaceing it for about 30 years. I tried to put it on the 8N and it tried to kill the engine. Also is "dwell" the same as "point air gape". No batteries are used in the meter but 2 capacitors are and were subject to freezing temps. The meter did try(Needle moved a lot) to indicate, but it wasn't logical. Any Ideas? Jim




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Dell (WA)

05-25-2004 09:11:20




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 Re: Tach- Dwell Meter in reply to Jim, Dundee, IL, 05-25-2004 07:26:09  
Jim..... ...sounds like your old oil-filled capacitors have sprung internal leaks. Iff'n yer a good enuff electronics geek, you should beable to figger out replacements.

Airgap dwell is a new one for me. Early Ford specs used to be "Percent Dwell", amount of time the points were closed with respect to the ignition cycle. 90* dwell is close to 100% for a 4 cylinder engine but gitts complicated with 6 and 8 cylinder engines.

Most common is degrees dwell, amount of rotational degrees the points are closed. Most of the time, the dwell is about 2/3's the camshaft rotation. Thus most 4 cylinder engines have a dwell of about 60* outta 90* camshaft rotation. 6 cyls will be about 45* outta 60* rotation. 8 cyls will be about 30* outta 45* rotation.

N.B.....the more degrees dwell, the smaller the points gap and HOTTER the sparkie. BUT TOO SMALL a points gap and ya burns yer points; 'cuz the coil primary electrons will "jump the gap" inspite of the condenser. I always set my points for minimum dwell, (max gap) so I don't haffta be constantly re-adjusting the points gap as the rubbing block wears down closing the gap..... ....Dell

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gahorn

05-25-2004 10:50:18




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 Re: Re: Tach- Dwell Meter in reply to Dell (WA), 05-25-2004 09:11:20  
Dell, I believe "airgap" is a reference to magneto tune-ups and is not applicable to distributors. (Also called "e-gap".)



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Bob

05-25-2004 09:52:18




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 Re: Re: Tach- Dwell Meter in reply to Dell (WA), 05-25-2004 09:11:20  
The FO-4 manual lists the dwell for the front mount distributor at 35-38 degrees. I suppose this was checked on a distributor machine, as it would be hard to access the connection between the points and the coil with the front-mount distributor on the engine, and running.

There is no spec given for the side mount. I have been intending to do some research on specs for other Ford four cylinders of the era to see what other Ford distributors are set to.

I have a distributor machine, and one of these days I'm gonna set aside a little time, and chuck up a front mount and a side mount distributor with the points set to spec, and verify what the actual dwell is!

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gahorn

05-25-2004 10:29:41




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 Re: Re: Re: Tach- Dwell Meter in reply to Bob, 05-25-2004 09:52:18  
At some point in it's past, someone had substituted an automotive "can" coil on my 9N and drilled a small hole in the upper-side of my dist cap to provide a high-voltage lead into the rotor circuit. (This Einstein used common 10 ga. house-wire.) When I restored the system to it's correct coil, I simply used some high-temp RTV (silicone sealant) to plug the small hole. I now realize it would make a great access hole for a small stud to alligator-clip onto and could provide access to the points/condenser circuit for a dwell meter. Just thought I'd share the idea.

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Dell (WA)

05-25-2004 10:13:16




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 Re: Re: Re: Tach- Dwell Meter in reply to Bob, 05-25-2004 09:52:18  
Bob..... ..given what we know about frontmount coils and their "sensitivity" to insulative tar meltdown due to increased heat-up for longer time closing of the points. (smaller gap) It would be very difficult to decrease the frontmount 0.015" points gap to reach what I consider to be typical dwell for 4-cylinder engines. (and I've checked alotta engine tune-up specs)

Given the rare and "non-standardized" earlydays use of points dwell, could your front-mount dwell specs be 35-38 deg OPEN? instead of the more commonly used points dwell-time closed? That'd be 52-55 deg "closed" dwell time, somewhat inkeeping with the common 4-cylinder engine tune-up specs nowdays, and more inkeeping with the 0.015 points gap. Justa thought.

I don't have access to a distributor machine, I urge you toodoo yer empirical testing as soon as you can..... ..respectfull, Dell the pointless

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