Technical Carburetor problem - 351 Cleveland

On some distributors the breaker plate does not pivot on the same center point as the distributor shaft.

 

Any Ford (Autolite) distributor I've seen doing this had a worn out breaker plate. Replacements readily available.

 

Johnny, the points plate pivots on a single pin in these, instead of rotating like we Ford fans wish it would. So the point gap varies from the static setting when it does. I about fell over when I saw it happen on my dwell meter last summer. With some creative fine tuning on the actuator arm, and limiting vacuum advance as much as one can stand, it can be minimized somewhat. Just a really screwy design.

 

This needs some explaining. What I mean is. How can dwell change be good?

 

It is not good. It sucks.

 

Thanks. It's an answer but

 

I have to run through the blizzard to grab one and take some pics. Give me a few minutes and I will.

 

I believe I found it all the while staying warm.

Your right, it sucks.

 

Well hell, after that midnite run I'm gonna post mine too. The pivot point is at 6 oclock right under the cam. A worn plate will aggravate the issue, but a new one will not eliminate it.

 

The acceptable limit is 6° of dwell variation when distributor is put through the paces on a distributor machine, at least the spec in the manual.

More likely these days are faults in the distributor shaft - end play, most distributors are going to be loosey goosey by now with well worn bushings. It's easy to check (if the advance mechanism or distributor cam is removed) by wiggling the distributor shaft end, basically no play is allowed. Put the point block arm on a lobe and watch. The point gap changes, it's toast.

This can be seen with an ignition scope too, the bushings wear in an egg shape pattern, and the display will shimmy around like an out of balance washing machine.

 

I'm just asking. All distributors no matter brand? If yes then Autolite would have to fall under the same criteria? Autolite has done a damn good job of making an egg round therefore post #28 is 9 degrees beyond limit. Thanks everyone for explaining.

 

If the dwell changes its because the point gap changed .right ??

 

New breaker plates seem surprisingly available, at least for some model distributors. And for the most part they don't seem horribly spendy.

https://www.bing.com/search?q=distr...-25&sk=&cvid=08974ECE5A674DF99847F88C68FC365C

The plates that pivot concentric with the distributor shaft usually rotated on a separate bearing or at least a bushing to keep their movement smooth and centered in the housing. And when they moved it didn't affect the dwell unless they were badly worn.

The plates that pivoted on an offset pin also moved the points further from or nearer to the center-line of the cam, which affected dwell even when new. This made it more critical to accurately set the point gap, or dwell, to allow for this small amount of built in "drift". And of course changes in dwell also causes changes in ignition timing.

But of course this all saved the manufacturers about 8 cents per distributor, so there's that...

 

Correct. And as points wear the gap decreases, dwell increases and the ignition timing retards.

https://www.jalopyjournal.com/forum/threads/hot-rod-technical-library-basic-ignition-systems.983424/

Scroll a bit further than half way down this linked page to get a clearer visualization of how this all ties together.

 

Ford/Autolite, yeah. From '64 manual. I don't know about the others. Did Ford want the breaker plate dwell to change?

Now if the distributor cam itself starts to wobble, the dwell/timing starts to move, it's not going to run as well as it might. I think this is one reason why points get such a reputation today, they were always kind of a pita I suppose but most original distributors are very worn out by now. With a sloppy distributor achieving a consistent dwell is impossible.

If running points it's especially important that the distributor & bush are tight. Y Block distributors are getting harder to find, so I rebuilt an extra I had laying around, what I found was if the bushing is toast, most likely so is the distributor shaft itself. It's made of steel, but it doesn't take much. Replacement of the bushing alone isn't enough.

The slop changes the dwell/timing enough that it is erratic, and typically needs detuning from the stock ignition advance curve or the engine will knock/ping way before it should.

 

It's funny how far we have gone into distributors on a carb thread. The experience I wrote about earlier with the 15 degree variation was on a bone stock original 34,000 mile 428 distributor, from 1966. No appreciable wear on that one, it was as tight as the one I pictured, and that one was a brand new replacement with 4500 miles on it. The variation could be minimized with some tinkering with actuator arm length, and limiting the total vacuum advance. If the point gap is too tight initially, it will get smaller as the vacuum advance kicks in, and will buck like a bronco, my actual experience there.

 

@Truck64 Anything that affect dwell will affect timing and quality of spark. So yes, a worn unit isn't helping matter's.

 

It's easy to think one thing and it be another (not saying that's the case here. HRP will have the final say). I had a situation in Ventura when suddenly my 56 didn't want to stay running. I first assumed carb. Turned out the Mallory ate it's self to death internally.

 

"Ninety per cent of carburetor problems are ignition related."

 

I was thinkin' of that quote while reading all the posts above! And it's true! Far more running issues I've encountered in 60 years of fiddlin' with cars have been ignition issues. Even when the owner and I first suspected fueling causes. Barring dirt and water, there's really not much to go wrong with most carbs. Maybe sticking power piston (Carter) or blown power valve (Holley), not much else just 'breaks'. At least IME.

 

went from the carb to ignition?

 

Probly carbon on the valves

 

I would help with carb. , but I’m not sure what type of carb he is using . Either I missed the post or it was never posted . Probably my mind is in a misfire mode . Sorry

 

Holley. HRP

 

"Could be float related, it's a Holley carburetor,." Hey now! Look at a Motorcraft lol. How did the bore look where the needle and seat assembly go. If pitted or deposits from evaporation are present. The o-ring will never seal properly.

 

Ok a couple of questions , do you have bigger than stock cam ? Have noticed if your car sits for 2 or 3 days , is the fuel bowl empty ( have to crank for a bit to get fuel in the bowl ) ? I think we can figure this out .

 

Does anybody remember the idle pots on the throttle arm from the 70's?

 

Dash Pot , I did 3 weeks on the farm for have pot in my car , I guess the Copper , didn’t understand it was a Dash Pot on my carb !

 

I tend to agree with the guys who think HRP's problem is probably not the carb at all. I had an OT car 351C with a huge cam in it. (oh yeah, the days of 4 - 6 MPG around town) When the breaker plate in the distributor decided it was going to start floating all over the place the car would sometimes run great and sometimes it would run terrible. BTW - stalling while sitting idling at a light was also a problem due to my messed up distributor. And yes, when I tried to set dwell it would bounce all over the friggin place!

 

Is the check valve in the power brake booster?