Off a tooth???

R

Rick D

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I've always questioned the "off a tooth" logic for engines with regular electronic ignition distributors since you can just freely turn the distributor to position the number one plug wire anywhere the distributor rotor electrode indicates the number 1 position is. Then, you time it on your gun and when you get to the 8 degrees (or whatever your number is) your good.

But this weekend I was told by a factory trained and certified merc mechanic that my distributor can in fact be off by a tooth (1997 Merc 5.7L (350) Thunderbolt V) which would NOT show up on the gun as being off but cause the engine to run bad.

How is this possible? If you are off a tooth (or 3 for that matter) from the original factory position, your timing light will reflect this and after rotating the distributor to to bring the timing back in spec, you should be good to go.

In other words, if you are at top dead center on the compression stroke, I should be able to stick the distributor in at any position along the 360 degree spectrum (that the teeth will allow) that I want because wherever that rotor electrode ends up at TDC, that's where I need to rotate my distributor so the cap corresponds with the number 1 plug being at TDC. I understand if the wires aren't long enough or there is some sort of engine interference that prevents a 360 degree rotation which is more common in cars… I'm not referring to that. What I'm talking about is the mechanic implied that being off a tooth would somehow cause my engine to run bad… BUT reflect properly on the timing gun. This does not make sense to me. I just said "I don't think so" and left it at that.

Am I missing something here?
 
You are correct Rick. It doesn't matter where number one starts, as long as it follows the correct firing order and there is enough adjustment to set the timing. Some later model cars have precise distributor indexing that can affect performance and cause driveability issues if not set properly.
 
The only time it makes a difference is if you have a coil or vacuum advance hung off of the distributor. In that case you may hit against something or the plug wires all of a sudden aren't long enough. I have not seen that old style vacuum advance for a long, long time. You'd have to check the manual to see if there is any kind of stationary sensor around there. But I would think that would make it equally difficult for even a Merc tech to set the timing.
 
I knew a kid that dropped the Distributor in. Then wired the Distributor correctly. It worked till he got ready to hook up the Vacuam Advance.
 
In order to be able to set the important 45* cam retard angle by turning the distributor base ( and reading the effect on a MEFI-4 scan tool)on my blue 5.7L MPI's , the distributor gear must be in just the right teeth on the bevel drive gear. Even one off either direction will not allow setting to spec because the dist flat cap has parallel cables coming directly straight out to L & R and they are snug to the heat exchanger bracket and other parts so available rotation arc is teeny. And the oil pump drive slot has to be in moved to perfect position ( generic paint stirrer stick is great for that) first to accept the accept the blade end of the dist shaft after it rotates into the drive gear bevel or else the dist shaft will never fully seat. So before inserting, 2 things have to be perfectly allowing for the exact arc-travel of the meshing beveled gears.

This makes distributor reinstallation rather tedious in spite of best pre-marking, diagramming, etc., efforts. I don't think I've ever gotten it just right the 1st time on those engines and sometimes it's a ping-pong match. But patience prevails. I expect there are a lot of us who have that sigh of relief when all is done with that.
 
These days factory trained mechanic simply means they know how to read a help guide. Nothing more.
 
I think Rick is correct about the tooth off scenario.
Along the same lines, I'm not sure how important it is to accurately set base timing for electronic ignition. The computer determines what the timing should be. How important is base timing and why?
 
ECM/ECU using "based timing" to controls injectors pulse/time (via other sensors input) wrong or unknown based timing will result in wrong injectors pulse/time = drive ability concern like... too much/too little gas and not enough or too much timing advance = lean burn/detonation...
 
A lot may have changed but I used to put it in in proper position then spin the engine and it would drop into final position when the pump slot came around. Don't think I ever tried to align the pump slot first.
 
quote:
Originally posted by Sandy

In order to be able to set the important 45* cam retard angle by turning the distributor base ( and reading the effect on a MEFI-4 scan tool)on my blue 5.7L MPI's , the distributor gear must be in just the right teeth on the bevel drive gear. Even one off either direction will not allow setting to spec because the dist flat cap has parallel cables coming directly straight out to L & R and they are snug to the heat exchanger bracket and other parts so available rotation arc is teeny. And the oil pump drive slot has to be in moved to perfect position ( generic paint stirrer stick is great for that) first to accept the accept the blade end of the dist shaft after it rotates into the drive gear bevel or else the dist shaft will never fully seat. So before inserting, 2 things have to be perfectly allowing for the exact arc-travel of the meshing beveled gears.

This makes distributor reinstallation rather tedious in spite of best pre-marking, diagramming, etc., efforts. I don't think I've ever gotten it just right the 1st time on those engines and sometimes it's a ping-pong match. But patience prevails. I expect there are a lot of us who have that sigh of relief when all is done with that.

Click to expand...


I'm going to have to google a picture of that setup. It's getting so you need a special tool to do anything.
 
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