I've been experimenting with my Y block the last few days, total timing (minus vacuum advance) tops out at 38° on the damper. I have the vacuum advance adjusted almost fully clockwise, so it pulls in early. Seems to run good, very smooth and quiet at cruise and no ping or predetonation.

Today I connected distributor to manifold source vacuum. Running about 12° or so of initial timing. At idle it's showing about 35° BTDC. When throttle is goosed, it immediately jumps even more. BUT, remember the engine during this testing is in neutral, with absolutely no load on it. In real life, when throttle is depressed the vacuum goes away completely for a second.

The only downside is the (surprise!) idle RPM is a little uneven. That's why ported or spark source of vacuum was invented, to allow for a steady idle. It goes way farther back than emissions control. Some manufacturers have always used ported, some used manifold. When emissions concerns came to the forefront, ported manifold was used to reduce certain pollutants. Retarding ignition timing is one way to do that, at the expense of fuel economy. I'm a little unclear on the strategy, but I think the idea was to increase HC to bring NOX down. The fuel mileage went down (and performance) and the excess fuel burned in the catalyst. Some smog era engines automatically defaulted to manifold vacuum, switching from the ported source, in the event of overheating. So it's well known that idle temperatures will be reduced with lots of ignition advance. It's important to understand that the total mechanical advance won't change with this arrangement. Vacuum advance is load dependent, but the mechanical advance in the distributor is RPM only. They complement each other to provide maximum ignition advance at all times, under all conditions, as far as practical short of any ping or engine knock.

 

Took it for a short test drive, engine idle is nice and smooth and seems to really likes part throttle cruise. Just seems to float along. Lots of advance should keep engine temps a lot lower in hot weather and improve mileage or range a bit more. Should probably re-adjust carburetor idle mixture, but will keep it hooked up like this for a while.

A lot of this stuf is experimenting to see what works in your particular application.

 

When you changed from ported to manifold, you'd have had to back out the screw to drop the rpms, and that's all.

The smoothness of the idle should not have changed !

Your tinkering with the idle mixture should sort it out.

You're on the right track with your experiments.

 

Yes this intake has the egr port on the back right side of the spacer. This was an original source of vacuum leak. I saw the PO tried sealing it with silicon but it didn't work. When I found that leak I decided to experiment with a different spacer. I have a few laying around off of 302s and 400s. I made a gasket to block off that egr port and used the different spacer without egr hookup on the back. That is the one the leaked in the corners of the two front legs because the angle is just slightly different. I will have to go back to the old spacer and I special ordered the exact gasket.
Have also been looking for the egr delete plate but no luck for an FE so far. May have to make one.
spent last 2 day's going around with Advance Auto. Luckily the distributor is under warranty and the replacement is on the way. I hope to install it this weekend and run a compression test and keep hunting leaks. I will also confirm damper position but I am near certain it's accurate as I checked it only a few months ago.
I appreciate all the help and I will update as soon as I get the dizzy in and new gasket installed under the original spacer. Looks like the egr port is easy place to leak. Such small clearances.

 

I had read about the "old school" guys timing with only a vacuum gauge and a tach, so I thought I'd give it a try. I got the vacuum up really good by rotating the distributor. As the RPMs increased, I'd back off the curb idle screw and go at it again.

When I got the maximum vacuum at a reasonable idle speed, I threw the timing light on it. It's a digital timing light, with the up/down buttons to determine advance. I found the timing was off the chart (damper lines), and used the up/down buttons to determine that I had set it to about 40 degrees BTDC. Vacuum advance disconnected and plugged. Manifold vacuum.

As you explain, the engine was under no load, as the transmission was in park. The engine started and ran fine, and seemed to make a lot of power. I was on flat terrain, just running around the house.

I didn't experience any pre-ignition, but I'd also read the 351Ms, and their heads, may not display audible pinging. I knew it wasn't right, so I set it back to factory. Somewhere around 12, if I recall correctly.

The trucks starts quickly, runs cool and the plugs look good. I haven't messed with it since...other than having the transmission rebuilt.

 

So if I understand correctly, the base distributor timing was indicated at 40° BTDC?? And it started OK, i.e. no starter kickback, and no pinging? Something is definitely screwy there! I use a basic no-frills analog timing light from Fire River or something like that. Runs on two D cells. You say you "got the vacuum up really good", what did the vacuum gauge top out at?

So let's come at this from another direction. Let's say you set the base timing (with a light) to 6° BTDC, that's probably OEM correct? Or even 12°; vac advance disconnected and plugged, and keeping the idle RPM at factory spec - say 550 or 600 - what is the manifold vacuum reading on the gauge then?

 

Yep, the timing went to about 40 BTDC and it ran fine. The highest vacuum was 18+ Then I set it back to 12-14 BTDC

The vacuum at 550-600 was in the 12-14 range

I say range because I was still playing with it, cuz the idle speed (550) was too low

The spec is 12 BTDC, IIRC, and that's what I set it back to

All three trucks are the same. I can rotate the distributor until I can't turn it any more, and the vacuum will go up a bit. They start and run fine. They've got straight-up double roller timing chains and new dampers.

With the ignition timing set at spec, ive adjusted the idle mixture to max vacuum-about 17. I do this in Park, and keep the idle speed below 800. When I put it in drive, idle speed is at spec-650. The vacuum drops to about 15.

 

550 is pretty slow, some engines don't like to idle at that RPM, but somewhere around there, maybe 600 or so, is where the vacuum gauge is accurate.

12" to 14" hG on the gauge is low - for a stock engine anyway. The numbers that are considered acceptable depends a lot on the cam, those numbers may be "good" if a hot cam is installed - Elevation above sea level also has a correction factor as well, about 1" per 1000. A lot of tuners recommend connecting distributor to manifold vacuum when a performance cam is installed, because of the overlap. Bringing in lots of advance will get it to idle better.

 

I think the 460s have stock cams. I'm pretty sure the 79 is stock, and the 77 might be RV. They're both pretty smooth, although the 77 is not as smooth and has a bit more oomph

The 351m has a Summit RV cam

All the trucks have Performer manifolds. Two have Holley 600s and the 77 has an Edelbrock 750. Vacuum secondaries all around

The trucks will idle (in drive) at 550 but really like 650. 600 is a bit low. Two of the trucks have AC, and that drops them down about 75 rpm.

So I set the curb idle to 650 and they tolerate 575 at stop lights. The AC compressor in the brown truck likes higher RPMs, so if it's running I double-foot it at red lights to keep the RPMs up to 650

When I shift to park, the RPMs raise to about 800. There is no problem with shutoff, startup or overheating. The idle mixture screws are about 1-1/2 turns out (on the Holleys) and I haven't touched the new Edelbrock. The manifold vacuum is a hair over 17 and the altitude is about 200'

The vacuum advance canisters are all good, and the distributors are on ported vacuum. When I rev the engine (with a timing light hooked up) I can see the advance come in. I've not graphed it yet, because:

• I'm doing it by myself
• I'm leery of working over the fan at those RPMs

The plugs show a tad rich, but pretty good. It's also possible they show rich because I just putt down the street, pull into the driveway, and let everything cool down before i pull them-no hotlap shutoff

Anyway, they run good, have good power and get the mileage I expect. I know that they could be tuned a little tighter, but I'm saving my money for a wideband O2 sensor and gauge...

 

They are real interesting to use, those widebands.

Ok, what I was getting at earlier, in your situation (trying to keep our OP edumacated here and try to stay on topic, sort of)

Your engine wasn't TRULY at 40° BTDC distributor base timing. I doubt it would even turn over? There's an indication error, damper problem, timing light, operator headspace??

 

Well I swapped back to ported on the vacuum advance. I'm starting to get it tuned nice this year with the valve lash and carb setup and advance and ignition etc all sorted out. It now idles smooth down at around 550 RPM but when connected to manifold vacuum it's not quite as steady. That's the purpose of using ported so it makes sense.

 

Sorry for the delay. Had some home repairs that have dominated my time. Today I finally had some time to work on the truck. New distributor installed and mechanical advance working properly. That also cured the late shifting and bad acceleration.

I also ran a compression test with these results: 1 = 180, 2 = 170, 3 = 150, 4 = 150, 5 = 180, 6 = 180, 7 = 150, 8 = 150. Thoughts?

Also still having issues with timing and vacuum. If I loose the mixture screws 3 turns and push initial timing to near 20btdc the truck runs great with steady 19 on the back gauge. But it idles way to fast. Any time I get the idle down near where it would be, the vacuum drops and flutters again between 15 and 17. I also made extra sure the egr is sealed at the manifold and on the back of the spacer plate. Maybe I don't have a vacuum leak and the truck is so tired it can't pull the correct vacuum at the proper idle?? I am at a loss.

 

When you get the idle down where it should be, what happens when you adjust the idle mixture screws in? Most carbs are bench setup 1.5 or 2 turns out so it will start and run, and then turned in (clockwise) to lean out the mixture as lean as possible consistent with a smooth idle. 3 or 4 turns out is going to be pig rich AFR and may tend to foul plugs or "load up" at idle.

Usually the RPM will increase some as the idle mixture screws are turned in and then as they are continued in the clockwise direction there will be a definite point where the engine will start to stumble as it becomes too lean. Just shy of that is "best lean drop".

Make sure the linkage on the carb isn't binding somehow; and when the pedal is floored by an assistant (engine off) the butterflies are perfectly vertical, and consequently the butterflies are also free to return all the way, should be able to throttle it down to a slow idle if you want. The T-slot should just barely be covered, to make sure only the idle circuit is in play at idle.

 

Tedster thank you for all of your help on this. Went back out today. With the truck warmed up and idling around 900 in park, the engine has a slight bit of surge to it and shakes a bit. Turning in the mixing screws didn't do much until a little under 1 turn out at which point the truck stumbled. I did not get any increase in rmp when screwing them inward.

The linkage all moves freely and the butterflies opened properly. Not sure what the t-slot is but I probably should. Do you feel like it's definitely a fuel delivery issue and most like carb issue at this point? Fuel lines are all new and new filter so I suppose that leaves the carb or manifold.

The carb is a clean a recently rebuilt MC 2150. I opened up again last weekend to make sure it's all clean inside. I have several of these carbs laying around. Should I just try rebuilding one of those? If I had the money I would just put on a new aluminum intake and carb.

 

Idle is too high, causing an artificially high vacuum reading.


I find when I have the idle speed at 650ish in drive, that works out to about 800 in park. with a C6