Distributors 101… NASA AIX AMC Javelin update

Ok, after the epic fail at Buttonwillow it was time to dig into the Javelin and figure out wtf was going on with the timing. I had the car dyno tuned before the race to make sure the engine was running good. And it did run good on the dyno. So what happened? Well, the guys tuning it actually had mentioned that they thought the advance was sticking or something (but they didn’t say “hey, your distributor is junk, change it). They pulled the cap, changed springs and I guess got it to behave well enough that it did a few pulls without sticking… Once I got on track and started beating on it though it just got worse and worse.

The only positive in all this is I’m learning a lot more about distributors. I pulled the distributor (it’s a Pertronix Flame Thrower) and immediately saw that the gear was chewed up 🙁 When the motor was first disassembled after I blew it up, Rob discovered that the cam gear was really knife edged. but, he said the distributor gear looked ok… unfortunately I didn’t think to look too closely at the dist. gear before I put the distributor back in the car. So I can’t tell if all this wear is just from the little bit of running I did at Buttonwillow or if it was already worn some. Either way the gear is toast now.

I ordered an MSD distributor (because after talking to people about my problem everyone said “run an MSD”). Jeff also said I should just remove the mechanical advance and lock out the timing. So, my first distributor lesson was removing the vacuum advance, removing the mechanical advance springs and weights and removing the gear so I could rotate the shaft to lock the timing. I also installed a bronze gear on the MSD.

After all this monkeying around with the MSD I felt like I knew a lot more about what I was looking at. So I went back to the Pertronix and took a closer look at the mechanical advance mechanism. I immediately saw the problems. It was floppy and one edge of the center part that the weights rest against was worn. I could see how the weights were slipping OVER the center plate… so that explains why the timing was all over the map. I also realized that there were washers/shims missing between the distributor housing and the gear on the pertronix. There’s a good 1/8″ of up/down movement. The MSD doesn’t have this up/down play. After I got the motor rebuilt the Pertronix distributor was not installed, I put it in. When installing it I noticed there WAS a VERY thin shim on the shaft between the housing and the gear. It was so thin it actually tore while I was installing the dist. so I just pulled it out. I’m pretty sure the dist. is not supposed to have as much up/down play as the Pertronix did but I’m a little confused because even if the shim had not been damaged it wouldn’t have gotten rid of most of the play… my theory at this point is that the up/down play the Pertronix had caused the gear wear AND the advance issues. Like I said, just a theory. I did some research on distributor gear wear and found a lot of posts from AMC/Jeep people experiencing a similar issue. They talk about “cam walk” and there are a couple of options for preventing this. One is a plate that bolts to the engine block behind the timing gear and the other is a “cam button” that rides on the end of the cam gear bolt head and fills the space between the bolt head and the timing cover. None of this is supposed to be necessary if you are running a flat tappet cam (which I am).

So that’s the latest. The MSD is going in with a locked out advance and I’ll try to set the timing around 34*. Assuming I can get the car running well again I will try to get to a Sears Point test day and depending on how that goes decide whether it’s worth trying to run the last race of the year Nov. 10/11.

1974 Javelin-road race, American Iron, Drive Train, Electrical

2 comments


  1. Jonathan Feltis

    Hey Asif, glad to see you got the engine back together. Sorry to hear about the distributor deal.

    On the gear, I have heard a lot of people having bad luck with the MSD gear awhile back, did MSD update the gear? From my reading every body said to use a set of matched stock gears or the aftermarket gears that are matched from bulltear.

    Well I hope that you get the car sorted out, so you can have fun with it soon.

    Jon

  2. R B

    You are probably right about your engine failure being related to you distributor. What type of camshaft is in the car? The type and material will determine what material distributor gear to use. The three most common types of distributor gears are bronze, cast iron and steel. Cast iron are used for hydraulic and solid flat tappet cams. Steel are used with iron hydraulic and solid roller cams. Use a bronze gear if your camshaft is a billet steel hydraulic or solid roller. New style composite gears are available and from what I understand can usually be used with any material or style cam. Also you should always use a new gear with a new cam. I would hate to see you chew another gear or worse yet a camshaft on your new engine.

    If you contact the camshaft maker in your engine they should be able to tell you what you should be using.

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