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Old 01-23-2014, 10:14 PM
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strad strad is offline
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replacing distributor 351W -- advice welcome!

Ok. It is an 84 PCM 351 engine with an original Prestolite distributor. The distributor has some rust issues, and the mechanical advance is getting stuck advanced, which makes it difficult to restart the engine, and causes the engine to idle quite high (1100 rpm or so) after a run on the lake. I've figured out that I can remove the cap, reset the advance to base timing by hand, and the engine'll fire right up.

What I've done is purchase another Prestolite distributor. It is not the same part number as the one on the boat (I could not find that one), but rather a part number that would have been available on an Indmar 351 a couple model years newer. It is a brand new part. Not a rebuild.

I think I've got the engine at TDC. And I should add that this engine actually runs quite well with the exception of the advance issue mentioned above, so I'm pretty sure the current base timing is correct. The degree mark is at zero and lined up with the pointer. And the rotor is pointed in the general direction of the number one plug wire. I'm a little afraid of screwing up the next steps lol. my plan is to mark the new distributor with tape to match the old distributor so I know rotor to distributor locating. Not sure how to mark so that base timing is similar though (mark shaft relative to intake manifold??)

And of course I as yet have no idea if the old distributor will come out easily or if it will put up a big fight. I've also got to change over the trigger wheel and reader for the electronic conversion that is on the old distributor. I don't anticipate any problems with that.

I'm most afraid of screwing up the timing somehow. Any pointers welcome. I do have a timing light.
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Last edited by strad; 01-23-2014 at 10:24 PM.
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Old 01-23-2014, 10:53 PM
occva occva is offline
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I replaced mine a couple years ago. I had the same concern about getting the timing close, i didn't want a backfire through the carb damaging my power valve.

I used a sharpie to mark plug wire numbers on the old cap. Then i took a yardstick and placed it across two of the terminals with the wires removed and put a mark where the other end touched the back of the drivers seat. Then remove the cap and did the same with the rotor. Inserted the new distributor with the rotor lined up and the added the cap and adjusted it to the mark on the seat. Replaced the wires and fired her up. A quick check with the timing light and i was maybe a degree out of timing.
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Old 01-24-2014, 12:49 AM
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thatsmrmastercraft thatsmrmastercraft is offline
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This job can be a little intimidating, but once you have done it successfully, it's no big deal. The problem usually come from trying to take shortcuts.

If you mark the distributor location and where the rotor points sufficiently well, installing the new dist should be easy enough. Remember the gears are helical cut so you have to account for that when you are lining up the rotor to drop the dist into place. Take lots of pictures of the whole procedure. You will likely reference them more than you would think.

If it doesn't work out, starting from scratch is the best way to ensure you are doing it right anyway. Using a remote start switch to get the #1 cylinder to the top of the compression stroke is easy. This is where most guys fail and are actually at the top of the exhaust stroke.

I remember at tech school when the instructor made us take the dist out of the engine on a stand and spin the dist and engine so we would have to re-install from scratch. One of the guys was off a bit and got a backfire out the exhaust which blew the exhaust hose from the muffler and threw a 2' diameter fireball across the shop floor. That was good for an afternoon of laughing!
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Old 01-24-2014, 07:13 AM
TRBenj TRBenj is offline
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Everyone likes to overcomplicate this. Unless youre starting from scratch, the position of the engine is unimportant (no need to be at TDC of the compression stroke for #1). The position of the rotor is also unimportant... it can be facing any which way. When reinstalling the new dizzy, the rotor can face a completely different direction... again, totally unimportant.

The only thing you need to do, is before you pull the old dizzy and wires, note where the rotor is pointing relative to the cap. For example, the rotor may be pointing "just before the post for cylinder #5". Then, put it back together the same way. The rotor can be facing a different direction, so long as you install the plug wires and rotate the distributor such that the rotor again faces "just before the post for cylinder #5" then youre golden. Just keep your firing order handy (13726548 for LH) and remember that the rotor always rotates counterclockwise (as viewed from above).

There is no way to keep your base timing the same when pulling the distributor and reinstalling. All you can do is get the new one in a position close enough for the engine to start, and then set it as you normally would with a timing light.
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Old 01-24-2014, 07:27 AM
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CantRepeat CantRepeat is online now
 
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Well, good practices say to have the number one plug wire facing to the front of the motor. Most mechanics in the industry expect it to be at this location and most manuals instruct it that way.

With the engine at TDC on number one you want the rotor to point to the number one cylinder.

With just a little more time and effort it can be done with a little more detail.

A little how to.

http://www.youtube.com/watch?v=5a2X9mSSlQY
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Old 01-24-2014, 08:25 AM
ctjahn ctjahn is offline
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Distrib

Are the caps the same; since they are both prestolites? Then you could just swap the distributer keeping the cap wired until you get it running (limiting variables).

When I did it, I was off by one plug wire which is detectable immediately if you know the symptoms. Moved everything one plug over and we were in business... (thanks to a lot of help from the board)...

Last winter I switched from a pretolite to a Mallory(I think that's what it was). I sent it in to get rebuilt and they sent me a new one. My conversion was a bit more complicated as I had no resistor (Removed when I switched the prior distributor to a pertronix) and different coil. Now its back to a resistor setup and new coil....

Best of luck
Cj
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Old 01-24-2014, 08:56 AM
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strad strad is offline
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Yeah I'm aware of the potential for cap difference. The rotor fits-- figured that out already. The new dizzy has screw mounts on it for fastening the clips on just like the old one. Have not sat down and looked closely enough yet though.
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Old 01-24-2014, 10:00 AM
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CantRepeat CantRepeat is online now
 
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Quote:
Originally Posted by strad View Post
Yeah I'm aware of the potential for cap difference. The rotor fits-- figured that out already. The new dizzy has screw mounts on it for fastening the clips on just like the old one. Have not sat down and looked closely enough yet though.
Well, for air fair and some beer I'd come help you!!
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Old 01-24-2014, 10:44 AM
TRBenj TRBenj is offline
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Quote:
Originally Posted by CantRepeat View Post
Well, good practices say to have the number one plug wire facing to the front of the motor. Most mechanics in the industry expect it to be at this location and most manuals instruct it that way.

With the engine at TDC on number one you want the rotor to point to the number one cylinder.

With just a little more time and effort it can be done with a little more detail.

A little how to.

http://www.youtube.com/watch?v=5a2X9mSSlQY
Every manual I have seen shows the location of post #1 on the cap notionally, but it does not need to be in that spot for any purpose. I have never heard of a mechanic expecting the post to be in any particular position, as its location is completely unimportant.

Yes, the engine needs to be near TDC (actually closer to 10deg BTDC, but 0 is close enough) on the compression stroke of cyl #1 for the engine to run. That doesnt mean you need to put the engine in that position in order to swap a distributor since the engine is currently wired up and running.

If you were starting from scratch, I would agree. Otherwise, it is just overcomplicating the issue.
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Old 01-24-2014, 10:44 AM
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strad strad is offline
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Quote:
Originally Posted by CantRepeat View Post
Well, for air fair and some beer I'd come help you!!

Yup, and after that we'd have to lake test it of course ;-)
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