Another timing curve question

[DrCharles]

You may have a better idea when you open it up. With a small pry bar push weights out until you see the heavy spring take up some slack. Is right at the begining, at the near the limit or something in between?

Thanks for the suggestion. I took it out and test-reassembled with just the heavy loop spring, since I can't get the superlight spring until tomorrow. Using the FBO limiter plate as a guide, it seems to hit the loop and the plate more or less simultaneously on the 14 degree slot. I think that will probably work

So if I set the initial to 16 for easy starting, it will idle at 30, and slowly advance on the heavy spring thereafter. I will still have to modify the longest slot on the FBO plate (18), since 16+18 would only allow 34 max. which at high RPM would be less due to box delay. Will have to determine just how much less by actual timing light reading. I need a limit at roughly 20 degrees...

The other option is to eliminate the limiter plate, and shorten the distributor slots instead (stock range will allow too much total advance when starting at 16). I think I'll file on the limiter plate instead

 

[Mattax]

Vizard led me down the rabbit hole. Fortunately I found some other crazies that eventually helped me find my way out.
(That rabbit hole included a $400 bill from a well known company to rework my carb, purchase of an LM1, auxbox and I then a step by step effort 'til I finally could see my way clear.

I do have a wideband - shows about 13.5-13.8 at idle currently. Not sure how reliable it is at idle with lots of overlap, since it's only measuring the oxygen in the exhaust.

I think with a hot cam like that, anything from 11 to high 13s could be what it will end up liking.

I agree that 4-corner idle would be a big help, but by the time I finish modifying (or replacing) the metering blocks and the main body, I might as well buy the carb I should have in the first place! But this one cost me something like $60, not $600, so I'm willing to experiment and learn a bit.

It just needs a rear block and you have to have guts to drill the idle port. I didn't have the guts. For 'poor mans four corner' its possible to cheat and not have to make the tricky angle. But for an engine with low vacuum at idle, true four corner would be better.

Does the increased advance at idle reduce the torque (at idle) significantly? If it's cleaning it up and probably creating more vacuum, isn't that an indication that it's running more efficiently?

It's cleaning up because there is almost zero load so the engine can be run leaner than when there's a load. However the additional vacuum is also pulling harder on the idle and transfer ports. The typical 4180 was probably set up assuming vacuum at idle in the 11 to 14" range. When your engine is making 8"Hg its still not drawing as much fuel as Holley intended to put in the ballpark.

So yes your engine is probably running more efficiently - but only for a no load situation. If it was running richer it probably would have a bit more power. And being richer it should burn quicker, so less timing needed. Power at idle speeds is very relative. What might your engine be making to overcome friction at 1000 rpm? 6 Hp? An increase of just 2 Hp would be alot. So when I wrote efficient I meant its ability to produce torque and power. I'm sure you've driven vehicles where you can just slip the clutch and put it into first at 600 rpm with hardly any blip of the throttle. In this case, we not that's not going to happen.

Back to the carb.
If I was in your situation, my course of action would be to do a few experiments with richening idle with the distributor setup as it is. That way you know the idle timing is prety much whatever you set it at, baseline being 22.
I would not use toothpicks - except maybe if I wanted to test the extreme case of no airbleed. Even then I'd use something else.

One check would definately be to reduce the primary idle air bleeds. Get some wire between .016 and .030. Stick it in one airbleed and run it up over the choke tower if its still there and down under the air cleaner gasket. Then do the other one. Another way is to make both out of one wire. Harder to make but also harder to lose.
If that seems promising, then see what effect closing the secondary and opening primary has. As well as just doing one or the other.
Two goals here. To observe the effects of more fuel for a given situation and whether changing any one or combination it will run strong enough that you can reduce the idle rpm a little. If your at 1100-1200 now, getting it to a stable 1100 or 1050 would be big deal.

Without going 4 corner idle, idle can be made richer by reducing the primary idle air bleeds permanently. But if reducing them alot (say they are .074 now and your reduce them to the equivalent .66 - that's alot) then increase the primary idle feed restrictions instead.

 

[Mattax]

Thanks for the suggestion. I took it out and test-reassembled with just the heavy loop spring, since I can't get the superlight spring until tomorrow. Using the FBO limiter plate as a guide, it seems to hit the loop and the plate more or less simultaneously on the 14 degree slot. I think that will probably work

So if I set the initial to 16 for easy starting, it will idle at 30, and slowly advance on the heavy spring thereafter. I will still have to modify the longest slot on the FBO plate (18), since 16+18 would only allow 34 max. which at high RPM would be less due to box delay. Will have to determine just how much less by actual timing light reading. I need a limit at roughly 20 degrees...

The other option is to eliminate the limiter plate, and shorten the distributor slots instead (stock range will allow too much total advance when starting at 16). I think I'll file on the limiter plate instead

You know the spring perches can be rotated?
That might get the long spring in action another degree or two earlier.

Stock slots might work if you can get it to idle decently at the 22 or something like that.
Whats the stock plate stamped?

 

[DrCharles]

You know the spring perches can be rotated?
That might get the long spring in action another degree or two earlier.

Stock slots might work if you can get it to idle decently at the 22 or something like that.
Whats the stock plate stamped?

I have not taken the entire mechanism apart past the weights and springs, but the spring posts on the plate end are round. Are they mounted eccentrically? I'd assumed they were permanently riveted or welded in place...

The T-shaped piece with the slots has "L13" stamped on the flat part and what looks vaguely like "13R" on the underside. Is that 13 dist degrees i.e. 26 crank degrees?

 

[Mattax]

Stock slots might work if you can get it to idle decently at the 22 or something like that.

Maybe it would look something like this.

In which case it probably be fine with vacuum advance.

 

[Mattax]

The T-shaped piece with the slots has "L13" stamped on the flat part and what looks vaguely like "13R" on the underside. Is that 13 dist degrees i.e. 26 crank degrees?

Yes. You got it.

 

[DrCharles]

Here's a snap of the mechanism. From what I can see of the underside of the spring post (through the screw hole), it looks permanently affixed.

 

[DrCharles]

OK, with 26 deg in the distributor, (14 taken up by the superlight spring and the loop), I'd need to start at 10-12 initial, then idle at 24-26, max at 36-38 less box retard. Sound doable?

 

[Mattax]

I have not taken the entire mechanism apart past the weights and springs, but the spring posts on the plate end are round. Are they mounted eccentrically? I'd assumed they were permanently riveted or welded in place...

Now we're getting into SSSS.

The real funny part is that this is common knowledge in the slant six crowd. Go figure.

Taking the shaft out is a bit of a pain. It's easy to crack the nylon collar when driving the roll pin out or in.
You can turn a screw driver into a adjuster tool.

Of course its easier file the fit with the distributor disassembled.
Anyway that's an option.
There's no guarentee it isn't already maxed out in the direction want.

 

[DrCharles]

Interesting! Thanks for the secret sauce. But I think I'll leave well enough alone and try it as-is for now. Old nylon parts (especially at engine temps) do indeed get very brittle. Good to know though. Will report back here tomorrow or so

 

[Mattax]

Sound doable?

Sure does. And its possible that the engine may not see that full 26* in slots due the electronics maybe mechanical slack or whatever. Maybe 25 or 24. Doesn't matter. It only really matters if when locking out or hitting the advance stops at 2000 rpm and then expecting it to be the same at 6000 or 8000 rpm.

 

[Mattax]

With the tool, then collar doesn't need to come out.
It's possible you could turn the perches with pliers - I didn't want to do that.

 

[DrCharles]

With the tool, then collar doesn't need to come out.
It's possible you could turn the perches with pliers - I didn't want to do that.

I learn something new every day! I never knew those were adjustable in the first place I carefully used a pair of small dikes with the sharp edges in the pin slots, and was able to get another 2 degrees out of the loop. Now it's right about 12 before it takes up the load.
So first attempt will be with the limiter plate at 18. Initial 18, idle 30, max 36. Will probably tighten the vac can screw a little too (or disconnect the line for initial test). Thanks again.

 

[DrCharles]

I measured the slots on the stock piece and they are .445 which corresponds well to the table that's floating around here and elsewhere (attached): 13 distributor degrees, or 26 crank degrees.

.
But the 18 degree holes on the FBO plate measure out to more like 20-21 degrees by that chart, so I decided to leave it in place. I got the superlight Mr. Gasket 925B springs from O'Reilly (the package looks like it's been on the shelf for 30 years). One superlight, one OEM heavy spring with long loop. Put it back together and then I realized I'd forgotten to put the dist on #1 TDC when I pulled it. My favorite quick way to find the compression stroke TDC without pulling a valve cover is a compression tester and a timing light - point at the gauge and make sure it flashes at the same time the needle kicks. Turns out my 50/50 guess was correct (for once in my life)

Now it starts easily cold or hot, idles at 28, and very slowly advances to about 33 deg by 4000 rpm. Up to the mid-to-high 40's with the ported vacuum connected (didn't adjust the can). It could probably use a couple more degrees of initial but I'm being conservative given the crappy pump gas. Not sure where it stopped advancing since I get nervous lying on an engine revving that fast ;) May need to open the FBO slots a bit more, get a weaker heavy spring, or just have more confidence in my short block and rev it to 5000 or so while holding the timing light! Doubt I can find a spec for the Jacobs ignition retard (slew rate) anyway.

I took it for a short run and it's definitely better off-idle and low-speed cruise. So far so good ;)

 

[DrCharles]

Carb is driving me nuts, although I did manage to do some tuning.
I opened up the mixture screws to 1-3/4 turns each, juggled the primary and secondary idle speed screws, and finally found a combination of settings that would idle around 13:1 on the wideband and minimize the lean spike when just barely touching the gas. It still likes 1150-1200 with the fans off, since it drops close to 100 rpm when the fans kick in and 1000 is right on the edge of spiraling in and dying... I also discovered that the primary accelerator pump linkage was too loose, and taking the slack up further improved the response.

Then the wideband decided to foul again while disconnecting everything for a test run. So I put it in the garage and installed the 8.5 PV. Will play with it again tomorrow. I need to get under there and pull & clean the O2 sensor, I suppose.

Incidentally the PVCR's are right around .064 and I was surprised to find that the primary jets are only 70's. Makes for a nice lean cruise at 14.5-15.5:1... Will see how it drives with the 8.5 PV (cruise vac = 15) when I get some more good weather. Should I expect to enlarge or reduce the PVCRs with this jetting? A #70 is .073. I remember reading that the PVCR should be half the diameter of the mains (for 25% of the flow), which means it's way too big (.073/2=.0365)...

 

[Mattax]

I'd leave the PVCRs until you've hit drag strip (or dyno). Then if the jetting for best mph is different than best interstate cruising, change the PVCRs.

Be interested to know how big the primary idle restrictions are.

I spent the late afternoon modifying a 1" combo wrench to better fit Holley inlets. Something that I've been meaning to do for a long time.

 

[DrCharles]

It might be a long time before I get to a strip (nearest one is over 90 miles away!). But I can watch the wideband while hitting the gas hard enough to open the PV without getting into the secondaries. Or just disconnect and wire the secondaries to the idle stop. If it's going lower than 12.5:1, wouldn't that imply that the PVCRs need to be reduced anyway?

If I take the bowl & block off again, I could measure the idle restrictor too ;)

 

[Mattax]

It might be a long time before I get to a strip (nearest one is over 90 miles away!). But I can watch the wideband while hitting the gas hard enough to open the PV without getting into the secondaries. Or just disconnect and wire the secondaries to the idle stop. If it's going lower than 12.5:1, wouldn't that imply that the PVCRs need to be reduced anyway?

If I take the bowl & block off again, I could measure the idle restrictor too ;)

See what others say. Often the AFR in the begining or right after the gear shift isn't representative of the long pull.
I know I'm not the only one to see that. In fact it recently came up as a question on Speedtalk.

let me get a couple screen shots from my logs, or maybe even some others.
edit: IRFAN screen capture is dead. I'll have to download a new version or something.

The big thing is that comparitive runs are needed before deciding 12.5:1 is the best. It could be 13:1 that runs best. Only way to know is the strip or a dyno.
As far as highway, I know from having it happen that if the primary jets are too lean, the engine will rev and then die, and if you can catch it it will continue surging until you slow down to a speed transition slots provide enough fuel you can drive it. On my car that was jut around 55 - 60 mph in 3rd.

The second thing - and where the wideband can be really useful - is to see if the AFR changes through the rpm range under heavy load/WOT. For example, if when shifting into 3rd it starts at 12.8:1 at 4000 rpm but then starts to run richer or leaner to the top, it probably needs a slight adjustment of the main air bleed.

 

[DrCharles]

I think my primary jets must be close, then. 14.5 to 15.5 AFR and no surging (at least up to 70+ mph cruise, 3600 rpm in 4th). I have yet to feel any surge/buck during steady-state, and light and moderate acceleration through the gears. (I'd thought I had 73 in there, not 70). Secondaries are still 85 (as far as I know!), no PV, but don't think I have even been into the secondaries very much. There is also a 50cc secondary pump.
It's pretty zippy for public streets I can floor it at 3000 rpm but can't stay on it very long because it's really coming up on the cam then!

So I won't do anything to the PVCR for now. I found a book online that says a 4780 originally had .069 PVCR. Next step is to drive it around with the 8.5 PV and see what the AFR is doing under more throttle and higher RPMs (will probably need a passenger to read the gauge).

Edited to add: 70 jets (.073) and .069 PVCRs calculate pretty close to the area of 85 secondary jets (.100). In fact those are the original specs for a 4780-2... Hoping not to have to change too much

 

[Mattax]

We can copy this over to your carb thread.

As best I can tell, transfer slot exposure on the secondary side is typically none or minimal.

Since I was at the garage and brought the camara...
This is 4779-2 base plate.

 

[DrCharles]

OK, there is a .050 difference in position... although that's not so obvious at a glance to the inexperienced carb tweaker

I've been thinking about high rpm retard due to delays in the electronics (often but incompletely referred to as "slew rate"). My engine will briefly and occasionally see up to 7000 rpm. Do I want to allow the slow advance (from the heavy spring) to go higher than 34... perhaps 38 mechanical , since the actual timing will be a few degrees less when it actually fires at high rpm?

(I won't be able to actually measure that unless I want to rev that high in neutral, AND I have no way of knowing how much my Craftsman timing light may be retarding too. So I'd have to measure the 34 at around 4000 rpm and estimate by slot length)...

 

[Mattax]

Plot the slope and see where it might be if it continues as a fairly straight line.
Its tough without knowing either the true initial or the actual maximum advance. I'm not fond of spinning the engine in neutral to 5000 rpm holding the light there, nevermind 6 or 7!
Maybe the best thing to do is get it on a distributor machine before hitting the track. It will have to be one that goes up to 4000 rpm (distributor rpm).

 

[DrCharles]

Thanks... the big problem is that there are just too many unknowns. I agree that the only way to know for sure is to put the distributor on a machine, using my ignition box. YR referred to that in the distributor tuning thread

The rotor tower allows 26 (crank) degrees with unaltered slots, not using the FBO plate, but that is too much, even assuming some trigger delay, unless the initial is 12 degrees. I definitely don't want detonation at 6000+ under full load

There are too many guesses and assumptions. My brain hurts

 

[DrCharles]

Can anyone recommend a good distributor jockey who has a machine and isn't afraid to use it?

I was having a productive discussion during the last two days with one of the few remaining shops I found by Googling, but he took mortal offense when he flatly stated that the spring pins on my dist were riveted in place and not adjustable and I dared to question him. I proudly shared my newfound knowledge that the pins do turn eccentrically (which is "common in the slant six world") I learn by asking questions and playing devil's advocate, not blindly accepting unfamiliar statements as fact, which some people take as criticism of their knowledge or egos. So that one's not going to work.

The other two distributor services I emailed (Sun. night) haven't responded as of Tues. night.
Any thoughts? Or am I just going to do it myself, one R&R and disassembly/reassembly at a time?

 

[yellow rose]

Can anyone recommend a good distributor jockey who has a machine and isn't afraid to use it?

I was having a productive discussion during the last two days with one of the few remaining shops I found by Googling, but he took mortal offense when he flatly stated that the spring pins on my dist were riveted in place and not adjustable and I dared to question him. I proudly shared my newfound knowledge that the pins do turn eccentrically (which is "common in the slant six world") I learn by asking questions and playing devil's advocate, not blindly accepting unfamiliar statements as fact, which some people take as criticism of their knowledge or egos. So that one's not going to work.

The other two distributor services I emailed (Sun. night) haven't responded as of Tues. night.
Any thoughts? Or am I just going to do it myself, one R&R and disassembly/reassembly at a time?

@halifaxhops is closer to you than I am. You can send it to me if you want, but I want whatever box you are going to use, and your timing light if you have one to test.

You'll eat up a bit of change just in shipping.

Don't forget about Mattax. He may be much closer to you.