Points gap verse dwell

[edwjmcgrath]

Welcome to mud, rookie. Many people here, including pinhead and me, have found that if they set the points with the gap spec ON FJ40's, the dwell is way off (but it runs well). And if they set it using the dwell spec, the point gap is dangerously tight. This is not a general vehicle question, but FJ40 specific.
Pinhead did this for a living. I've worked on point ignition vehicles for 45 years and neither of us have ever encountered this kind of discrepancy. It should be a simple matter of geometry, or as you say, "a basic timing issue".

Where else would you look to explain this consistent discrepancy? Would you run your points at .007-.005" gap if that's what setting them to the dwell spec said?

 

[Pin_Head]

Maybe the Toyota dwell spec is in crankshaft degrees, which are 2X that of the distributor? That would explain the discrepancy.

 

[Weber Sarge]

I've wondered about that - crankshaft degrees is how some industrial point ignitions are set up , maybe that is the answer . I'd be curious to see if anyone has rebuilt a distributor with a new cam gear as well as a new cam shaft to eliminate any wear play in the teeth and see if that makes a difference in gap/dwell specs .
Sarge

 

[coonassjohn]

Well The answer should be forthcoming with so many experienced people. I myself, although a rookie,
Am an aviation maintenance engineer with over 45 years experience on just about every piston and turbine engine ignition system. This is the engineer and troubleshooter in me asking if anyone has considered asking Toyota what may cause this, or our egos standing in the way??

CAJohn

 

[Pin_Head]

Toyota publishes both a gap spec (.016) and an angular dwell spec for the same engine and distributor. I could not get the engine to run and meet the dwell spec, so I assumed that there was a problem with my meter and just set the gap. When I started hanging out here much later, I found out that a lot of people had the same experience.

 

[Pighead]

Ditto.

 

[zags]

Maybe the Toyota dwell spec is in crankshaft degrees, which are 2X that of the distributor? That would explain the discrepancy.

I dug out an old tune-up spec book I had and looked at other straight 6 engine specs. Dwell varied from 34 to 45 degrees. and point gap from .016" (GM, used points) to .027" (Ford 300) so I would say the Toyota spec. is correct, since all of these ignitions worked exactly the same back then.

I will be running points in mine when I get it running, so I will be anxious to blow the dust off of the ol' dwell meter and check it out.
Is the OP sure that the point contacts are square and don't have arc damage on them that might throw off his measurement?

Could worn bushings be changing the gap with the engine running as opposed to when you are setting it with the engine off?

If those numbers are correct, (which is a first for me) I would break the engine in at the correct dwell like Mark stated, then change the points and put the gap down to .016" and run it. Any less and your points won't last very long.

 

[Ggodard]

Dwell is how long the points stay closed. The parts that affect this in the system are the lobes of the distributor shaft and the phenolic (or whatever it is made of) contact bush on the points themselves. I haven't gotten around to fully setting my 11/73 yet (I set my point gap within spec, but dwell is at 55.8* IIRC). The options are either new points and hopefully the bush is longer and takes it up, or a new distributor shaft (these usually come wrapped in a new distributor, which seems to be NLA).

I believe another option for proper rectification is electronic points. I've had poor luck with these in the past, and I've had good luck. The brand is important. I want to say Pertronix was the one that did me well enough. The problem is that electronic points can fail in weird and inconsistent ways...

Hi all... I love this site. It's so super helpful. All of you. Thank you!
I thought I could reach a decision without having to post, but since I live in Guatemala, I really want to be sure before I order parts from the States.
I have a 1974 FJ40 (3 gear) Lots of problems with my points all of a sudden. I had new ones put in -but still fizzle pop as I drive along. Loosing power etc. Friends here that rebuild Rovers recommended that I go with Pertronix and call it a day-
I have heard otherwise though here in these forums. What do you recommend? Help! Suggestions? I need to DRIVE!
I hope that's enough info-
My ride:

 

[Pin_Head]

I wouldn't presume that it is an ignition problem. Ignition problems usually show up at high RPM although they can also be random if they are due to an intermittent issue, like a bad connection. This would act like shutting off the key switch momentarily. With fuel problems, you will have more like bucking and spitting back through the carb.

Points ignitions are very reliable and a major attraction is that they are easy and cheap to fix. Pertronix is also reliable, but you will need to buy new parts if something goes wrong with it.

 

[Ggodard]

Thanks Pin_Head!
I put a new fuel pump (original Toyota) thinking that would fix the problem.
It did for about a day, and then the same thing happened.
I decided to change the points (severely worn out) and it's been loosing power upon acceleration ever since....

 

[Pin_Head]

it sounds more like a fuel problem. Typically due to dirt and rust in the system. Check for clogged fuel tank outlet, lines, filter and junk in the carb.

 

[Ggodard]

Got junk in my trunk... I sprayed and sprayed carb cleaner - rust in the tank is definitely something I have to contend with.
Maybe I'll go take the carb to be cleaned. I'm sure it needs it. Thanks for your help.
I'm still wondering if it would be a viable and worthwhile upgrade to get the Pertronix though.
Thanks again!

 

[Pin_Head]

Fooling with the ignition won't do anything for your fuel problem, but it will empty your wallet.

 

[Ggodard]

hahahaha Thank you again... !!!

 

[Pin_Head]

Misfiring at wide open throttle (like going up hill in high gear) can be a symptom of ignition problems, but loss of power under normal acceleration sounds more like a fuel problem.

 

[melvinjo]

As some have already said: A .018 to .020 gap, you are good to go, If your dwell doesnt come around to the recommended setting, but your engine runs OK, stop where you are at, don't worry about the dwell. I have adjusted points on so many dif make engines and sizes over the years,one thing I can is that the dwell settings only came into spec maybe an average of 2 out of 10 times. Focus on points gap and timing first, and if the engine runs well, disregard dwell

 

[edwjmcgrath]

Here's a cheap recommendation.
Go buy a clear inline fuel filter. I'll bet that your problem goes away for a while. When it comes back, I'll be that if you look at your clear filter it's full of junk.
Sounds like lack of fuel to me. You can buy a 50 fuel filters for the price of the pertronix.

 

[sggoat]

gotta agree w/Edwjmcgrath--even with a new tank, it still gets trash in the filter. The glass ones are super easy to see when you get the trash coming through; just replace the filter and clean the old one--ready for the next load to come through.
There is also a small possibility your secondary may be restricted(Pin_Head has the test for that, too--paperclip)

 

[edwjmcgrath]

I was thinking clear inline disposable filter, but either way, I'll bet it's fuel.

 

[Ggodard]

I have spares in the tool box.... they cost .8 cents here. I'll change that first.
Like I said.. you guys are fantastic.... Gracias amigos!