Ignition timing advance (1 Viewer)

[ducktapeguy]

Finally had some time to install an FJ60 dizzy in my 75, but before I did I wanted to baseline the stock vac advance one so I can compare the two.

This is the ignition advance curve from my original FJ40 distributor. I didn't notice until after swapping dizzys that my vac advance mechanism has leak in it. Looks like it still advances, but it won't hold vacuum. Not sure how much the vac leak affected my readings, I may try and rerun it again at a later date. Dotted line is the ported vacuum contribution to the advance

Here's the curve from an FJ60 large cap dizzy. Dashed lines are vacuum readings and correspond to the scale on the right. A couple things I noticed was the mech vac advance doesn't come on until after 1000 rpm, unlike the stock FJ40 which starts immediately off idle. Also, I wanted to see the difference between ported and manifold vac, it looks like the only difference is at low rpm, above 1300 rpm they pretty much follow the same curve. This agrees with what I've read on the web, but wanted to see it for myself. Using a hand held vacuum tester, it appears the advance mechanism doesn't start until about 1-3 inches of vac, and maxes out by 9-11 inches (or around 1200 rpm).

Just curious If this seems right. In terms of actual on road performance, there's a slight difference, but I haven't put on enough miles yet to verify. There is however a noticeable improvement in starting with the FJ60 dizzy. The engine runs smoother and quieter, I can idle down <500 rpm, and starts much quicker both cold and hot. I thought old dizzy was working ok, but I didn't really have anything to compare it to. I did have some problems with a slightly jumpy timing mark, which I attributed to either worn points or worn bushings, but the new one is rock solid.

 

[FJ40Jim]

Stock 1969-1976 USA dissy is vacuum retard, not advance. Post a pic of the old dissy so it can be identified.

79-87 dissy is dual vac advance.

<science fair judge> If the data above is collected by free-revving the engine, the vac advance data is not very useful, since it is a function of throttle position and load, neither of which are on the graph.
Add vac reading scale on top horizontal axis, then plot both vac response curve & mech advance curve on same sheet.
Consider plotting both stages of dual stage vac advancer. Good luck at state finals!
</science fair judge>

 

[ducktapeguy]

Hi Jim,

I was hoping you'd chime in.

I'm fairly sure it's a vac advance dizzy because, well it advances. But it took me a while to realize it because I always assumed it was stock vacuum retard dizzy, I didn't realize it wasn't stock until I really started reading this forum. That explains why the PO didn't have the advance port connected, along with half the vacuum hoses. Also, the advance diaphragm was bad when I first got it and the old carb didn't have ported vacuum.

I know static vacuum readings aren't really applicable for doing any type of tuning, but that wasn't my intention. What I was really looking for is to create a baseline for comparison between my old FJ40 dizzy and the FJ60 one I just installed. It would be great to be able to measure all these parameters while on a dynamometer, but unfortunately I don't have that capability. Really the vacuum measurements were an afterthought, I just thought since I was going to be doing a timing curve it would be interesting to see what was happening with the ported and manifold vac. I had plotted vacuum readings vs advance but didn't think the data was all that interesting.

I guess what I'm really wondering is whether the advance curves look correct. 40 years of tinkering by who knows how many previous owners has taught me not to assume anything is completely stock on my FJ40. For all I know the FJ60 dizzy could have been recurved at some point, and the FJ40 dizzy could have been tampered with. A few other people have posted similar data, but nothing really seems to match up. I'm guessing there's probably multiple variations of the dizzy along with changes to the engine that I really don't know what a "correct" curve should look like. Forgot to mention that all of these curves are with a base timing set to 7 degrees.

Does it look like i could benefit from a recurve? As far as on road performance, the only difference I notice between the two dizzy is when shifting into third gear. With the FJ60 dizzy it seems as if there's a very slight flat spot when accelerating after the shift, but it could just be my imagination. I've been doing a lot of overdue maintenance in the last 2-3 months, so I didn't drive it a whole lot and it's hard to make a comparison.

 

[White Stripe]

I have I think close to 60 degrees advance on mine in the upper rpm range. The compression on the 2f is really low, I have noticed a big difference. Its like a whole new motor. And gas mileage is much better.

 

[FJ40Jim]

The pictured dissy is non-USA 75-80 2F w/ vac advance. It is a better match for the 75 2F, because it has enough mechanical advance.
The 81-later dissy has less advance because of the higher CR and fast burn chamber.

If the 1975 points + ignitor ignition doesn't bother you (I see nothing wrong with it), then run that dissy.