Holley 600 tune/vacuum

[Deezy]

Well the good news is I hit the engine bay with "Gunk" brand engine degreaser and can now see the markings on the sawtooth indicator.

The bad news is the degreaser cleaned off a small JB-weld or some other filler-type repair off of the thermostat housing and I now have a small green geyser in my engine bay.

I said I wanted "Old Faithful" but I didn't mean literally...

 

[Rusty Nail]

0° is the "peak" of the tallest "v". The biggest point TOWARDS the block. One is clearly bigger than the others.

The half an inch to the right was a great plan, what happened to that deal?

Initial (base) timing requires the vacuum canister be unplugged and the hose closed to prevent a leak.

Seriously carburetor tuning prior to very,very, critical base ignition timing setting is effort wildly wasted, as it will only need adjusted again.

Domino effect

I knew something was stupid about that housing! I'd throw it away...they're cheap.

It MAY be of benefit to put the timing light down and do it by ear. Adjust,drive,adjust,drive until you rid yourself of the tick.
There is no need to get "the cart in front of the horse", as that will take all the fun out of it.


You're headed in the right direction now. Best wishes.

 

[Deezy]

I was gone all weekend so I ordered the parts. Got the new housing in the mail, but it doesn't have a hole to attach the throttle spring! Any idea of a quick fix I can rig up so I don't have to go to the parts store tomorrow?

 

[Deezy]

Sorry for the double post, but I'll be going to the parts store tomorrow... I drilled out a small hole in a washer to use as a hole for the spring to attach to, which isn't easy to do without a vice, but of course the old thermostat housing has one long bolt and one short bolt, and the driver's side bolt hole on the housing is much thicker/deeper (There's a word for that but I can't think of what it is). The new housing they're the same thickness on both sides. So, tomorrow I'll be going to get a new housing and/or matching bolts.

 

[Rusty Nail]

Pull out one of the intake bolts and use it.
Carburetor mounting bolt?
Thermosrat housing bolt?

Put the return spring(s) around the bolt and tighten it back down.
There SHOULD be two springs present. A small diameter inside a larger one is factory equipment.

 

[MikeB]

Yes, make sure there are no vacuum leaks anywhere. Check for cracks in hoses and loose connections at nipples. Also suck on anything a vacuum line is connected to, making sure the devices themselves don't leak. That especially includes the brake vacuum booster diaphragm and its large hose connected to a manifold port, port on back of carb, or a distribution block. It's very common for transmission modulators, vacuum advance canisters, and brake boosters to leak.

That doesn't look like one of Holley's emissions carbs that have extra ports for EGR and evap canister. What is the list number on the carb? It will be on the front of the air horn and start with "0', for example 0-1850-2.

It's pretty much accepted that the only reason for using ported (delayed) vacuum is to drastically limit spark advance at idle. As I recall, it was also used to control the EGR valve, otherwise idle quality would suffer. In those engines, mechanical advance was also limited and didn't fully come in until 3000 RPM or more. The vacuum advance canisters would allow for as much as 24 degrees advance, so at least you had decent spark advance under light loads, like when cruising.

 

[Deezy]

Pull out one of the intake bolts and use it.
Carburetor mounting bolt?
Thermosrat housing bolt?

Put the return spring(s) around the bolt and tighten it back down.
There SHOULD be two springs present. A small diameter inside a larger one is factory equipment.

Ended up finding the appropriate thermostat housing at an autozone. Had the little ear to attach the spring. I'll inspect closer this afternoon when I get off work, but I didn't notice that it had 2 springs; I will confirm.

Yes, make sure there are no vacuum leaks anywhere. Check for cracks in hoses and loose connections at nipples. Also suck on anything a vacuum line is connected to, making sure the devices themselves don't leak. That especially includes the brake vacuum booster diaphragm and its large hose connected to a manifold port, port on back of carb, or a distribution block. It's very common for transmission modulators, vacuum advance canisters, and brake boosters to leak.

That doesn't look like one of Holley's emissions carbs that have extra ports for EGR and evap canister. What is the list number on the carb? It will be on the front of the air horn and start with "0', for example 0-1850-2.

It's pretty much accepted that the only reason for using ported (delayed) vacuum is to drastically limit spark advance at idle. As I recall, it was also used to control the EGR valve, otherwise idle quality would suffer. In those engines, mechanical advance was also limited and didn't fully come in until 3000 RPM or more. The vacuum advance canisters would allow for as much as 24 degrees advance, so at least you had decent spark advance under light loads, like when cruising.

The carb is an 1850-4. I switched the vacuum advance back to manifold, and it works just fine although I still haven't quite gotten the knock to go away under medium throttle.

I did notice what appears to be a vacuum switch, attached to the firewall behind the brake booster/master cylinder area, that I think was where the open ended vacuum line I originally mentioned in my first post may have been connected. Does anybody know what that switch is? I can try to post pictures later today; I'll be leaving town tomorrow until next Friday.

 

[Rusty Nail]

Likely for the windshield warsher.

Inspect inside of the vacuum nipple on the distributor and adjust advance timing with a 1/16 Allen but you have it connected wrong because you fail to see the process through. If it runs better with more base timing, then add more base timing. The midrange ticking is the result of too little advance timing which is adjusted via that shiny canister on the side of the distributor, not switching ports, as you have proven yourself.
Base timing set to obtain the highest possible vacuum at idle. Idle mixture to obtain the highest possible vacuum at idle. Fine adjustments for driveability to suit but if you read back, that's not what happened soooooooo, good luck with your manifold vacuum, I'm sure it'll run great after you void action of the vacuum advance canister.
What could possibly go wrong using the same advance timing off idle and 100 mph WOT?


Also a good idea to check the vacuum modulator at the tail of the transmission, as that hose is commonly compromised and throws everything off.

 

[MikeB]

That 1850 is a very popular non-emissions carb. So I would not try to make any vacuum hookups other than transmission modulator (if so equipped), vacuum advance, and power brake booster. Also, replace the EGR valve with a block-off plate, or at least verify that it isn't leaking. If you have to pass an emissions test, or must have all the devices installed, that's a different story, and you'll need a different carb.

As for knocking, I'd suggest plugging the vacuum advance port then driving to see if the knock is still there. If it is, you'll have to take a look at initial + centrifugal advance, aka "total" advance. If the knock goes away, then your vacuum advance is too high. That's very common with emission era cars that had zero degrees advance at idle, but have been changed to 10-12.

How it worked back then was when you stepped on the gas, ported vacuum would pull against the canister spring and add 20-26 degrees advance to let the engine wake up and accelerate the car. However, when you add that 20-26 degrees to the current 8 degrees initial advance + centrifugal advance, you might end up with 50 degrees or more. Ex: 8 initial + 25 vacuum + 18 centrifugal = 51. Probably should keep that at or below 46-48 depending on compression, gas quality, and coolant temperature. Then again, the centrifugal may not come all the way in until 3000 RPM or more. But with old trucks, you never know who did what to the distributor. It may even be from a different engine.

As for the switch on the firewall, I don't know what it is. Typically vacuum switches are controlled by coolant temperature, and are known as a TVS -- Thermal Vacuum Switch. These are used to enable/disable vacuum to devices like an EGR valve and a vacuum advance canister. I've been working on a 76 Pontiac that probably has 20 ft of vacuum hose, and 3 TVS switches, one of them multi-port, along with two sensors in the air cleaner housing. Lots of fun, especially since some previous owner or mechanic had connected the hoses improperly.

Sorry if this is too long-winded, but I hope it might help in some way.

 

[74 Shortbed]

If the truck came with a 700R it's probably the lockup switch.