The 335 series are smoggers so you can get away with a little more advance but you have to take into account the fuel these days too. I assume you're running regular so chances are 36 is going to be about it before you start getting detonation. this would be the TOTAL mechanical advance at 2500 RPM. forget your initial it's added into the total. with your vac advance hooked up you will likely be in the 50's and that's probably fine.

If you feel your mechanical advance will give you 36 then that's unacceptable unless you want to set your initial around zero. I like at least 10 initial in most cases so that means you gave to limit your mechanical back if you want something like that. end of story.

 

He has a 13l slot from his first distributor that he could change out and get 26* and would probably be right at 36*
If he sets the initial with manifold vacuum and provides a 13l slot for centrifugal he should be good provided the carb is idling ok when initial is set with the highest vacuum minus 1-2 on the vacuum gauge.

 

Oops ! Yes, sorry, I meant 'total'.

38-42 is not low by any means, in fact 36* as 440 suggests, is probably a more realistic figure.

Yes, the current 18L's would give 36* mechanical advance, but, they are simply not ideal. (Sh$t is the word I'd use to describe them. LOL )

38-42, or 36, would only be a guide, and not a number to aim for.

Your engine would tell you what number it likes. Period.

So, as an example, if WOT testing generated pinging at 38* total, but not at 36*, then 36* would be the number to use. (Assuming the 'all-in' rpms are also accepted by the engine.)

Next up would be the idle timing.

If your engine likes 18*, as you observed already, then that could be used.

That would leave the curve needing to be set. 36* less 18* = 18*, which would be achieved with two 9L's. (Haha, assuming they even exist.)

And finally, spring changes would be used to get the ideal speed of the curve, and the curve observed and plotted at 500rpm intervals to ensure it is behaving like it should be.

When that's all dialed in, you'd have the maximum 'pep' that you are looking for.

Of course you can't do any of that until the numbers currently being observed can be trusted.

My timing light has the instruction, but I don't know if that's common on all lights.

When you do get the curve dialed in, then try manifold vacuum advance as opposed to ported, observe the difference, and choose which one you prefer. (After the vacuum advance issue has been corrected.)

 

Highest achievable manifold vacuum reading at idle is also going to be close to best initial timing minus until no ping. With the vacuum advance disconnected and vacuum line plugged of course. Then it's just a matter of determing at what RPM you want to be all in. Driving style, fuel type, engine HP, and gear ratio are going to make that decision ultimately.
One thing that hasn't been discussed is choosing the correct distributor gear type for the cam in the motor and setting the correct distributor gear depth. If the OP keeps ending up with a partially sheared gear pin because the distributor gear depth was never set correctly he could keep having these varied timing issues. It has to be ruled out. Also a harmonic balancer if the vulcanized two piece type can have the timing mark piece rotate independently when the rubber degrades and shift the timing marks making them useless . Only way to verify is with a piston stop and manually setting the motor to TDC.

 

Really good information and thoughts on the previous 3 posts. Most if it I know, there’s good summary review on a lot of it, and a couple of points I didn’t know. Cam gear adjustment for example. I knew to swap gears when I replaced the dizzy to use the mated gear but did not know about measuring and setting it. I just pressed it in place on the shaft, locked it in place on the shaft with a roll pin and went from there. Do you think it should be pulled and reset?

I wasn’t able to spend any time to speak of in the shop yesterday. I did spend some time going over things in my head trying to puzzle things out. Two things came to light: 1) my timing light HT clamp does have a directional arrow embossed on it that points to the plug side. Never knew that (or if I did I totally forgot about it). Not sure what difference it’ll make but I’ll be sure it’s positioned correctly going forward, and 2) (and this is a big one): I double-checked the photo of the new dizzy’s advance arm that I took before installation and the arm is set on the 13L end, not the 18L. That makes a ton more sense with the advance readings I’m seeing. Not sure why I had it in my head it was an 18 but was happy to see the 13 since that is in the range of what I was considering changing it to!

So my next move is to go back under the hood and start playing with the advance and see if I can get it to come in using the things under discussion above as guides. There should be time today. I really appreciate everyone’s thoughts and time helping this noob turn book knowledge into practical application!

 

If you have a stock distributor and the correct gear your height will be within spec.

 

So this morning I had an hour. I started at 16* initial and got a nice curve with 32* at 2,000 and 38* at 2,500. No more exhaust backfires starting in the 1,800-2,000 RPM range like there was at one time (old dizzy IIRC). I was feeling good. I didn’t check to see where the last 4* would come in before reconnecting the vac advance. It started stumbling and backfiring through the exhaust with throttle input. Holy crap.

I retarded the dizzy until I could blip the throttle without bogging and measured timing: 12*. Next I ran it through a curve after plugging the vac advance: only 25* at 2,000 and 28* at 2,500. I don’t like how slow it’s coming in. That doesn’t make much difference because the other problem is that with vac advance connected it goes back to stumbling and backfiring with anything much more than just blipping the throttle.

My understanding of exhaust backfires is it can be a result of timing retarded timing. Yet if that’s the issue why only when the vac advance (not vac retard) is connected and functioning?

I had a chance encounter with a gas and diesel mechanic yesterday who I was able to talk to for 5 or 10 minutes. He suggested the possibility of the ignition module being the culprit for all the timing problems. Any other thoughts on this possibility?

He also suggested just bagging it and replacing the Duraspark system with MSD. That thought had already occurred to me but I’m having too much (fun?) puzzling this out. It’s become something of a personal challenge.

 

The durashart is a better system than the MSD. but if you do enjoy trouble it might be a challenge.

Backfiring can be from retarded timing but it can also be from weak spark or lean fuel mixture.

What you're saying here doesn't make much sense. a possibility is when your vacuum advance pulls in you have a short somewhere in the distributor that grounds out with movement. or possibly a bad pick up coil. or as I suggested earlier your
ECM or timing light have problems.

I would set the initial to where it runs well then hook the vac advance up to manifold vacuum. it should pull in and speed the idle up . if it doesn't cause any problems then it's not the problem it's either too much timing or one of the other problems discussed.

 

The mechanic you spoke to knows what he is talking about.

I'm not a fan of the DS at all, and the 'fun' that you're having now will soon turn into a headache, followed by an excessive use of the F word.

When your initial was 16, (at 800rpms ?), your timing at 2,000 rpms was 32*.

Therefore, if you simply turn the dizzy so as to retard to 12* initial, with NO other changes, the timing at 2,000 rpms should be 28*. (16-12 = 4, so 32-4 should = 28)

But you got 25*.

Your dizzy has Covid !!

The speed of the advance from 16* to 32* seems to be very quick, and you lost 3* somewhere between 12* and 28*.

The springs, for starters, need a vaccine !

By all means carry on with timing changes, and spring changes, in order to learn, and you might be able to dial in a linear acceptable curve.

Yes, I would normally associate the backfiring with a 'too retarded' timing curve, but the quick advance in the lower rpms suggests that is not the case here.

Yes, it could indeed be a faulty module.

Seeing as you have already considered a 'modern' dizzy, I would avoid MSD because of the limited curve options, and would consider a Pertronix rather. (You can determine if the Ignitor 1, 11, or 111, best suits you.)

If you do go for a new dizzy, ensure you get a matching coil, then ensure that you have no form of resistor supplying power to the coil.

I have a GM HEI type distributor, but I still haven't dialed it in, and I'm still not sure if they have enough choice of springs/limiters to really custom a curve.

What did you do that you don't see 45* at idle anymore ?

Given that the dizzy has Covid, chances are that the vacuum advance has Covid too. That's a really helpful comment huh ?

 

Thanks for the considered responses from everyone. I’ve embedded some more in the quote above.

 

Replacing the distributor on a vehicle that was manufactured in 1974 is hardly throwing parts at a problem it's part of maintaining it and on the ECM anybody that's running a Duraspark ignition knows when traveling include a known good ECM and coil in your kit for good measure so you can eliminate those from your trouble shooting list while sitting on the side of the road in anywhere America.
If your new Distributor cane with a vacuum advance which some do It could very well be out of adjustment.

 

NAPA ignition module ordered and will be here Tuesday.

 

I've been fixated on 'coincidence' and a 'faulty timing light' until now.

Here's some ''thinking out loud'' comments :

I just reread your thread, (yeah, LOL, it is long), and your 'therein lies the answer' comments in post 20 could well be the answer.

Did you ever check if the springs were in fact too loose ?

77 HOS's comment in post 37 is spot on.

Setting the timing mark and then the rotor is always only ever going to be a starting point, like when fitting a new dizzy, but a swing of +/- 30* is extreme, and could be explained by loose springs.

You didn't have problems until you started adjusting the dizzy components, so I assumed that it was simply 'coincidental', but seeing as you've swapped the dizzy, then maybe it's a coincidence that the Module started playing up at the same time. Hmmm.

Well, you'll soon find out when you swap in the new module, assuming the new one is not faulty.

Did you replace parts exactly as per the diagram Redroad supplied in post 21 ?

I'm a bit stumped with the backfiring ONLY when the vac advance is connected.

It wouldn't hurt to check if the hose connecting the canister to the carb is free of holes or damage which could cause a vacuum leak.

It also wouldn't hurt to connect the hose, assuming leak free, to the manifold port and block off the ported port.

I'm not sure that this would be useful, but if you check and see that the timing jumped from 12* to 30*, say, and the rpms increased, with no other changes made, then at least you'd know that the vacuum advance works. (440 suggested this recently.)

With electronic components, they sometimes pass a test, implying that they are fault free, but they are in fact faulty.

Which means buying a new module is the way to go. If it solves the problem, great, and if it doesn't, you have a spare.

 

Responses are in the quote.

 

I didn’t think to check if the advance is adjustable. I’ll do that when I’m back under the hood installing the spark module. Thanks for the reminder!