1974 F Distributor Vacumm Advance/Retard? (1 Viewer)

[mikeob1]

So today I got up before the sun and started in the garage while it was a balmy 90. I pulled my distrib off my 74. Gave it a good looking at and ready my trusty FSM. The points looked good, except they were gapped at .009 inch. I changed that to .016 inch based on FSM. I also checked the condensor and it was in spec at .022 micro-farads. Cap and rotor were fine and I added some new ignition wires from Napa. Nice wires, except the coil line was too short, so I used my old one.

I put in six new plugs (NGK-BPR5ES-11) gapped to .028 inch.

I put the dizzy back in and fired up the F engine and set the timing so the BB is dead on the pointer at 7* with the vacuum line DISCONNECTED FROM THE DIZZY. My tach was reading about 900-1000 RPM. I tightened down the clamp and double checked the timing. Same deal...7* pefect.

Now...I connect the vacuum line and check the advance. Low and behold, we are at TDC. 0*. The other end of the vacuum line is connected to the port on the intake manifold.

Which brings me to my question... What purpose does this vacuum retard serve?

 

[grant5127]

the '74 dizzy is retard... under load manifold vacuum is lost thus advancing the timing.... its just the opposite of ported vacuum advance systems.... timing should be set @ 600/650 RPMs.....

 

[Pin_Head]

The vacuum retard is primarily to reduce NOx levels on emission controlled engines under high speed cruising conditions. It also serves to reduce the reduce pinging or knock under load, by retarding the timing.

The spec on plug gap is .039 I believe and 1000 RPM is too high to set the timing as the centrifugal advance may have kicked in as Grant pointed out. Idle RPM is 650 max I believe.

 

[mikeob1]

Thanks guys. I think I understand correctly...

So, when i'm idling or in a steady cruise, and my intake manifold vacuum reads a fairly steady numer...let's say 16 inches of Hg....and then I stomp on the gas....manifold pressure drops to 3-5 iches for a second, which causes the vacuum retard at the distributor to cancel it's effect, thereby advancing the timing 7* or more.

Why does advancing the ignition reduce emissions? I'm guessing that maybe the initial dump of fuel air mix on peddle stomp creates a situation where the spark needs to happen earlier in the power-stroke to ignite the fuel-air mix a little earlier in order to burn it all up instead of sending it out the exhaust. Not sure if I'm correct here, but just guessing.

Also, thank you for the point on idle speed to set timing. I understand the centrifugal advance may have kicked in at 1000rpm. I wish I could get the engine to idle down to 650 rpm. I can't seem to get it below 800, and then it runs rough and dies. The carb was bought re-built from SOR back in 2000. The truck sat in storage for awhile.

Any ideas for getting the truck to idle slower? Maybe the idle ciruit is clogged (not enough fuel) or brake booster leak, or maybe PCV valve?

One more thing...according to FSM (grey F book, page 2-3), plug gap is 0.7~0.8mm (0.028 ~ 0.031"). I picked 0.028 only because it was the number on the left...no reason behind it...just cuz, I guess.

 

[Pin_Head]

I think I understand correctly...

So, when i'm idling or in a steady cruise, and my intake manifold vacuum reads a fairly steady numer...let's say 16 inches of Hg....and then I stomp on the gas....manifold pressure drops to 3-5 iches for a second, which causes the vacuum retard at the distributor to cancel it's effect, thereby advancing the timing 7* or more.

That is the way a strictly mechanical vacuum advance works, retarding the timing as air velocity in the primary bore drops. The retard distributor does the same type of thing, but under electronic control using vacuum switching valves.

Why does advancing the ignition reduce emissions?

I believe it is retarding the ignition that lowers the combustion temperature.

Any ideas for getting the truck to idle slower? Maybe the idle ciruit is clogged (not enough fuel) or brake booster leak, or maybe PCV valve?

If you can't get it to idle by adjusting it according to the FSM, some things to consider are:
1) Is the idle fuel cut off solenoid working? Is it getting power? does it make a click when you turn on the key?
2) Do you have a manifold vacuum leak?


My FSM says the plug gap in the US is .039 and .031 non-US.

 

[FLFJ40]

I am thinking about putting my stock dizzy back in with petronix. I read most of the stuff in the FAQ about dizzys and it seems as clear as mud. I like the way the F engine runs with the H.E.I. and vacuum advance. I think that the weber isn't ported right for the retard system, is this right? I am thinking of buying a later model toyota dizzy and putting the petronix in it. After reading the extensive post on ignitions it seems like the performance results I feel from the H.E.I. may have more to do with the loss of the worn shaft and points than the electronics from the H.E.I. One more thing that I have read that seems counter intuitive to me is that the points type system is more reliable than an electronic system. I had nothing but trouble with the points, they were always slipping. I was once stranded because the points had slipped and I didn't have the feeler gauge to get it right. The electronic ignitions seem to be point and shoot.

 

[Pin_Head]

Vacuum retard distributors aren't going to work will with any strictly mechanical vacuum I know of. They are designed to be run by the smog computer. Most people just leave the vacuum retard disconnected and run them as a mechanical advance distributor. This is fine.

As long as you get a spark at the right time, all ignitions are the same. I learned this the hard and expensive way from my youth in drag racing. I also learned not to trust my subjective feelings about how well my engine ran. Every time I dropped a few hundred on modifying my engine, I was positive that it ran much better, but when I got it out to the track, many times it actually ran worse or no better. Data is more reliable than feelings or opinion.

Reliability means different things to different people. There is no doubt that electronic ignitions have a much longer mean time between failure. However, when it does fail, it usually means replacement of expensive parts. Points ignitions may need work more often, but when something goes wrong, it is often more easily fixed with simple tools. You don't need a gauge to set the gap; you can use a match book cover or eyeball it. The points shouldn't slip either.

 

[FLFJ40]

Yeah, after that event the #6 rod bearing started to knock, probably not helped by the massive backfiring from the slipped points. I never really drove it with that dizzy installed again. Rebuilt the engine and added the H.E.I. I never investigated what was causing the points to slip.

 

[mikeob1]

Thank you, Pin_head. I checked the fuel cut-off solenoid and it is clicking. My vacuum is steady at 17 inches when idling at 950 RPM measured at the intake manifold port. I can get it to idle down as low as 750, but it is very choppy and will die when I'm breaking or decelerating from full power.

A thought came to mind that makes me wonder if this is main jets. When I moved out of AZ, the jets in the bowl were switched from the high altitude to the low altitude.

We are only 1500 above sea-level here in Phoenix, but the air temp is hot, effectively giving us less dense air. I know in flying carb'd small planes, that temp is accounted for.

I'm gonna go pull the carb, and primary main and secondary main jets in the bowl and measure them. I'll post what I find.

FLFJ -> I don't know much about the pertronix. I'm paying my dues learning the ins and outs of my stock gear. Once I can make it run like a top, I am fully expeting to have a Nirvana-like experience and hopefully some dream-time visitors in the form of Ancient Japanese Toyota engineers will come to me in a lucid dream and give me a signal that I am indeed on the enlightened path.

Ok...back to reality. Now if I can just get the thing to idle slower...

 

[Pin_Head]

I would check for manifold vacuum leaks first. It is more likely and easier to spot.

 

[mikeob1]

Thanks for the advice, Pin_head. I pulled carb and verified the Main and Slow jets. The Mains were correct according to FSM: 1.18 mm for primary and 1.8mm for secondary.

The Slows are spec'd 0.5 mm for primary and 0.8 mm for secondary. I didn't have a wire to measure these, but mine were stamped 60 and 80. So it looks like my primary slow is maybe 0.10 mm too big? Dunno if it matters.

I'll take your advice and keep trying to solve the idle problem by looking for a vacuum leak. I'll plug the port for the Brake booster and the PCV connection. The two other ports on the intake will be plugged as well.

When my rig idles for awhile It can maintain a pretty steady 17 inches of merc at 950 RPM and 11* advance. The Vacuum needle bobbles periodically about the width of the guage's needle. I'll keep working it...striving for the 650 RPM idle.

Again..thanks for the advice. I'll post what I find.

 

[Pin_Head]

Since you have the carb off, make sure that the bottom of the intake is not cracked. this is a common source of vacuum leak and it is hard to spot with the carb on. The jets sound normal.

 

[mikeob1]

Hi Pin_head - I already had the carb back on before I saw your last post advising to check for cracks on the bottom of the intake. What I did do is plug the vacuum ports on the intake, The PCV port on the heat riser, and the brake booster port on the right arm of the intake.

Still have a rough, high idle (1100 RPM (fluctuating) 15" Hg at intake, timing = 10* advance). Also little pops coming from the tail pipe .

I'm still working the path of vacuum leak, and sprayed a little carb cleaner around the heat riser and got no boost in rpm. I don't want to spary that stuff around too much...no need for a wild fire.

I'm thinking I need to pull the manifold and inspect for cracks. All other ports are plugged. If it is vacuum leak it is either a cracked manifold or fauly connection to head.

Am I missing anything, or are there any other tricks of the trade to determine the source of vacuum leak?

Thank you for any advice or help.

 

[grant5127]

fluctuating vacuum readings and pops in the exhaust are valve train related problems.....I would do a compression test to start with....

 

[Pin_Head]

It is a little strange that it would go from steady vacuum to fluctuating just by removing and replacing the carb.

You can see the manifold crack by opening the throttle plates and peeking down at the bottom of the manifold with a flashlight. Try using propane gas from a flexible hose to find vacuum leaks.

You can also try removing the idle mixture screw and alternately spraying carb cleaner and compressed air to clear out anything that might be plugging the idle port.

 

[mikeob1]

hey guys...thanks for the advice...i'll give it a shot. I think i had missed a comma in my statement above. It was the rpm that fluctates about 75-100 r's. The manifold pressure was steady at 15" of merc.

But just to let you know how motivated I am to solve this so I can get on the trails...I ran out to the garage in bathrobe and a key-turning assistant and got some compression numbers for ya.

#1 = 136 psi
#2= 135 psi
#3 = 130 psi
#4 = 130 psi
#5 = 131 psi
#6 = 145 psi

I'll peek in there with a flash light and see what I can see tonight. Carb cleaner / + air in the mixture scew hole. I like the idea of propane. I'll give that a go tomorrow.

Thanks again.

 

[mikeob1]

I looked down the both the primary and secondary with my flashlight and the rest of the lights off. I was basically laying on the front of the engine, feet stuck inside the bumper and nose in the carb.

It was pretty silver under the primary, with a little fresh gas from the accel pump on initial opening.

The secondary was also purdy silver everywhere except for a browner stain near the front edge. I didn't see any cracks and inspected as close as possible.

I can pull the carb tomorrow if need be and check out that brown stain.

The intake hasn't been removed in at least 8 years. Does the intake manifold gasket give up after some time?

 

[Pin_Head]

Everything sounds OK so far. If there is no evidence of vacuum leaks, I would see if trying to blow out the idle port helps. It isn't much work to try.

 

[grant5127]

Check to make sure the throttle butterflys are closing all the way.... I've seen a couple in the past that were slightly bent and wouldn't seal off, mainly on the secondary...

 

[mikeob1]

Hi Guys,

She gave me a sign. She's willing to idle pretty smooth at 675 rpm. She went lower, but my digital tach/timing light fell of the fender and lost the bottom half of the digits so I can't give a precise figure. I'm not quite there yet, but I think the problem is idle circuit related. One caveat -- the choke plate was closed almost all the way.

I wish this were a more scientific discovery, but here's how it unfolded. Last night I blasted air through the idle mixture port, as suggested by pin_head.

When I got home from work, I got brave. In a well ventilated driveway, with a cold motor and starter fluid in hand, I swore I was going to get this motor to show me a vacuum leak or loose some eyebrows in the process. I removed the air cleaner assembly and the heat shield over the exahust manifold so I could get a good visual on the better part of the intake. I sprayed and sprayed and sprayed and got no response from the engine.

Now here is where the accidental discovery happened. I pushed the choke in and she died. I went inside to let the starter fluid fumes dissipate. 10 mins later, I came back out, pulled the choke on and started her up.

HOLY CRAP....she is idling low! I can't believe my ears! I put the timing light/tach on and the RPMs were 675 and timing was 6*. For a minute I was baffled. Usually she is running 1200+ RPM with the choke pulled and fast-idle screw holding throttle open.

I inspected the carb linkage and somehow the low-idle screw is in control, and the choke plate is nearly closed. Passing my finger over the open slit alongside the nearly closed choke plate is enough to cause the RPM to dip. I arrived at a near perfect fuel-air configuration on accident.

I let it idle for 10 minutes. I decided to play with the idle mixture screw and see if it had any effect on the engine. I screwed it all the way in and nothing happend. I screwed it way out, to the point of it almost falling out and nothing happend.

Based on these observations, I'm guessing I need to take that carb off again, and be sure that the slow circuit is not blocked. Also, I'll double check the throttle plate, as y'all suggested and be sure this carb is able to give the engine gas with the main throttle open, and the choke plate open.

I'm not there yet....but at least she gave me a sign.