Mercruiser 120 Distributor Cap and Rotor Rubbing Issue

[Ryan's Rally]

A few weeks ago, I posted about my boat not starting. It?s a 1978 Mercruiser 120. I did a full tune up, new starter, fuel pump, coil, and pertronix electronic ignition kit. I was getting inconsistent spark issues, so I tried taking the pertronix kit out and installing a new set of points. When I did that, I noticed that the keyway on the rotor was stripped out. I replaced that, and the boat fired right up. I took it out on the lake to test it out and set the timing, idle speed, and carb mixture screws. After about ten minutes, it started running rough and then died and would not re-start. I got it back to the dock and took it home. I took the distributor cap off and noticed that the rotor had been rubbing on the contacts of the distributor cap, and the rotor was worn. I installed another rotor, and it did the same thing! I've now replaced the rotor 3 times and the cap twice. I also noticed that the shaft on the distributor has some up & down play, as well as rotational play. I?m trying to decide if I need a whole new distributor or not. Would a bad distributor cause the rotor to rub against the cap and ruin it? Is it normal for the distributor shaft to have some play in it? If not, can I repair it or do I need a whole new distributor? I'm at a point where I really don't want to keep putting money into it, but I also don't want to give up on it if it just needs a new distributor.

 

[Bondo]

Is it normal for the distributor shaft to have some play in it?

Ayuh,.... The rotational play is the advance mechanism,...
A tiny bit of up, 'n down is Ok,....

Does the shaft move, when pushed side to side,....

The shaft rides in bushin's that can wear, causin' side to side play, which would allow the rotor to hit the cap terminals,....

 

[thumpar]

From the wear it looks like the rotor is riding too high. Where are you getting the parts from?

 

[Ryan's Rally]

I will try pushing the shaft side to side and report back. As for the parts, I used the Sierra 18-5268 tune up kit from Amazon. The rotor is 18-5418 and the cap is 18-5385.

 

[Bondo]

Ayuh,.... Where is it hittin' the posts,....

In the picture, I kinda see it hittin' only one,....

 

[stonyloam]

Yeah, looks like the rotor and cap are not matched correctly, and the rotor is riding too high for the cap. What I would do is : get a new rotor and grab my Dremmel tool and grind off the offending parts of the cap, or get OEM parts. The latter being way more expensive I wold try the former first, but that is the farmer in me. I don't think side to side play is your problem.

 

[Grub54891]

Some of them kits are really tough to get the rotor down completely. I've had a couple that I thought would snap when they went down so hard. But they did go.

 

[Ryan's Rally]

So, I didn't get a whole lot of time to mess with the distributor this weekend. I checked the shaft for side to side play and the shaft seemed tight. I'm concerned with the amount of up and down play though. I didn't measure the amount, but it seemed like more than just a little. I'm considering just ordering a new distributor. I'm thinking of going with the Sierra 18-5315-1 distributor. Is that the correct distributor for my engine? Mercruiser 120 2.5L inline 4 (1978 I think). I have never replaced a distributor before, but I do have the manual. Is their any tips or tricks to installing a new one? Is it as simple as lining up timing mark w/ rotor pointing towards #1 on cap to find TDC. Unbolt clamp and remove old distributor and replace w/ new one. Re-install clamp and set points w/ dwell meter. Set timing at 8* BTDC. Thanks again for any and all advice!

 

[stonyloam]

That is pretty much it, double check that the timing mark on the harmonic balancer lines up with the 0 mark on the pointer and you will be at exact TDC.

 

[thumpar]

If you are going to replace the distro you might as well go with the Voyager EST kit. The cost will be the same and you will then have electronic ignition.