IAT vs IAP distributors

[Darbone85737]

Can anyone tell me the differences in the distributor caps used on each of these style distributors ?  Dimensions and/or distance between contacts?   Can one inadvertently be fit onto the wrong distributor and affect the starting or running of the engine?

 

Why I ask-  I have my new engine all back together and ready to fire up.  Everything is new.  The old distributor cap was tossed and there was a new one in a box in the trunk that I put on. There are no markings on the cap and it was not in a box.   It fit on the IAP distributor and I have static timed the distributor but the engine will not start.  I know the drive is not 180 degrees out. Taking the plugs out and grounding them while turning the engine over will produce spark. 

 

I'm wondering if the cap may be wrong.   If the distributor is timed correctly at the points could there be a difference in spacing of the contacts on the cap that is affecting timing the firing at the plugs and causing the problem.  ??

 

The car is a 1949 P18 with IAP distributor.    

 

Thanks

 

Larry

Edited February 25, 2015 by Darbone85737

[Dave72dt]

If you're getting spark when grounding the plugs, it would mean the cap is at least functional for that distributor, even if incorrect and is not causing the no start issue. Have you done a compression test and has the obvious fuel question been answered. New engine, are you sure the cam is timed correctly to the crankshaft?

[greg g]

There were three at least different dist used. And yes the caps and bodies were different among the types. But if there is spark at the plugs that should indicate things are working. I had a problem with mine with a no start after rebuild and static timing procedure. Everything seemed correct and got spark to the plugs. Further investigation showed that

The plug wires were one cap tower off all the way around. Verify that the rotor definitally aligns with number 1 spark plug lead in the cap. If not redo and give it another try.

It is always best to take the distributer number with you when you get replacement tune up parts.

Just as a point of info, the timing curve varies slightly among the various distributors. I think you would be hard pressed to actually notice a difference from the drivers seat, but it is another difference to be aware of.

Edited February 25, 2015 by greg g

[Darbone85737]

I got the engine to TDC using an wire placed into the hole in the rear of the head near cylinder 6. When that indicated TDC the mark on the crank pulley does align with the pointer on the block and the distributor rotor is at 7 o'clock.   I set the point to where they start to break to fire cylinder # 1 at that point.

 

I checked (and rechecked) the plug wires and assured they were in the proper order, going to the correct plugs.  When grounded to the head each plug will fire in order with a bright white spark.  

 

I reset the valves cold using .010 intake and .012 exhaust

 

Fuel line is not yet hooked up.  I haven't reinstalled the newly cleaned gas tank yet.  I'm just trying to get it to fire using started fluid in the carb . 

 

I have a distributor cap that I ordered using the distributor number coming this morning.  I'm going to compare it to what is on their presently and see if there is a difference and if when installed it resolves the problem.  My thought was that the contact on the cap I have may be indexed differently and causing the spark to the plugs to be out of time

Edited February 26, 2015 by Darbone85737

[Don Coatney]

I got the engine to TDC using an wire placed into the hole in the rear of the head near cylinder 6. When that indicated TDC the mark on the crank pulley does align with the pointer on the block and the distributor rotor is at 7 o'clock.   I set the point to where they start to break to fire cylinder # 1 at that point.

 

 

You are quite possibly 180 degrees out on your distributor. The distributor spins at half the speed of the crankshaft.

[Dave72dt]

Bringing # 6 to TDC does not mean # 1 is on compression TDC.  Since this is a 4 cycle engine and the pistons only go up and down, it must make 2 revolutions to complete the entire process.  As Don said, likely 180 off.  Quick way is to swap the wires in the dist. 180 degrees for test or pull dist and turn it 180.

[suntennis]

Top dead center on the compression stroke for the number one cylinder is where the distributor should be firing for number one cylinder. By removing the plug on number one cylinder and checking for compression as the engine is coming on TDC then seeing the position of the distributor rotor will tell you if everything is setup correctly. Another way to see if you are of 180 degrees would be to install the distributor 180 degrees from where it is which would have the same results as rewiring the cap.

[_shel_ny]

 

 

My thought was that the contact on the cap I have may be indexed differently and causing the spark to the plugs to be out of time

 

I believe that all the caps are indexed the same. I think the the difference in caps may be the height of the cap which could cause poor contact to the center terminal, or too large a gap where the rotor rotates to each individual terminal. Sounds like you have a good spark.

 

There can be no variation difference in distance from terminal to terminal using a different cap. 360 divided by 6 comes out the same with any cap.

Edited February 26, 2015 by shel_ny

[jeffsunzeri]

There are 2 types of distributor cap: one is about 3 5/8 diameter and the other is about 3 1/8 in diameter. Trying to fit one type of cap on the wrong distributor will be immediately obvious. 

[Dodgeb4ya]

Actually there are basically 3 types of MoPar 6 cylinder flathead distributor caps....

 

IGS distributor up through 1948... large diameter cap with a 1" inside of cap locating notch 

IAP distributor generally used from 1949-1950... also a large diameter cap with a 1/2" external locating notch

And the 3rd type is a smaller diameter distributor cap used on the 1951 and later IAT distributor cars

[Darbone85737]

Thanks to all. I got the new distributor cap, points, rotor and condenser.  They were all the same as what I had in the car, so my brilliant theory was shot down.  LOL

 

Now back to the drawing board.  

 

If I take the plug out of the rear of the head and use a thin stiff rod to move it up to TDC, then look to see the rotor is at the 7 o'clock position and the timing mark on the crank pulley indicates TDC does that assure me I am actually at TDC ?   Or can I still be  180 degrees out ?

 

Thanks again for the  patience and sharing of knowledge.   

[Don Coatney]

   Or can I still be  180 degrees out ?

 

   

 

 

You are quite possibly 180 degrees out on your distributor. The distributor spins at half the speed of the crankshaft.

 

 

Bringing # 6 to TDC does not mean # 1 is on compression TDC.  Since this is a 4 cycle engine and the pistons only go up and down, it must make 2 revolutions to complete the entire process.  As Don said, likely 180 off.  Quick way is to swap the wires in the dist. 180 degrees for test or pull dist and turn it 180.

 

 

Top dead center on the compression stroke for the number one cylinder is where the distributor should be firing for number one cylinder. By removing the plug on number one cylinder and checking for compression as the engine is coming on TDC then seeing the position of the distributor rotor will tell you if everything is setup correctly. Another way to see if you are of 180 degrees would be to install the distributor 180 degrees from where it is which would have the same results as rewiring the cap.

[_shel_ny]

Try putting some gas in the carb rather than starting fluid. Just fill the float chamber if you do not want to hook up a fuel line. I think that you will be less likely to have a fire ball above the carb that way if the timing is incorrect. I don't like starting fluid.

 

 

Some confusion may arise when suggestions are made to pull the distributor and or install it 180 from where it is. You probably understand what is meant there, but just in case. To do that, remove the bolt holding the distributor, raise it enough to allow the rotor to be turned 180 degrees from whatever position it is in, put distributor back in place, and secure. If you already understood that I will go to my corner and unhook my keyboard for a while.

[Darbone85737]

Thanks. Yes, I just pulled the distributor out enough to clear the tang on the drive from the oil pump slot and  rotated  it 180. I must have had it wrong after all      I also pulled the # 1 spark plug and used the thumb over the hole method to make sure I was on the compression stroke.  The rotor is  at 7'oclock and both valves in   # 1 are closed. 

 

When I had the distributor body timed to break at TDC and put the distributor cap back on it didn't seem the rotor is in a position to make the contact with # 1 terminal in the cap.  Looking at it now I'm realizing I had been  "timing" with the point rubbing block on the wrong side of the distributor lobe  LOL 

 

Got back to it and ....  I'm officially a senile dumb ass  LOL  Anyway,   It's alive

 

Thanks to everyone.  I need to learn it's seemingly always the simple things and to stop thinking so much. I haven't set and timed a distributor/points ignition in 53 years.  Hopefully someone down the line will search, read this and be helped.  Thanks again 

 

Larry

[deathbound]

You're not the first and definitely not the last to be 180* out. Good you got it running, on to the next thing.

[Darbone85737]

The (almost) funny part is that I was SO aware of the issue at the start and thought I was making a conscious effort to avoid the problem  LOL 

[DonaldSmith]

My car was wired 180 degrees off when I got it - No. 1 at 1:00 instead of 7:00.  When I replaced plug wires I undid it, and did it right.

[Niel Hoback]

That's one of the nice things about these cars, they don't need a lot of thinking. Over thinking will just make more work, and you wind up doing the easy stuff anyway!