C 2 66 K19 Air Carb - Ported Vacuum?

[Mark P.]

I need to bring this up again since I am discussing (arguing) with a local mechanic on the subject of vacuum. I recently put an original Holley 3606 carb back on my big block and I had some recommendations from a past post from both Duke and Mark Gorney saying that I should convert to full time vacuum advance. I'm not the sharpest knife in the drawer when it comes to carburetion and vacuum, so please be patient with my explanation. It appears that I have been connected directly to full vacuum and it has been running either advanced or waaaay advanced all of the time. No wonder it was pinging like crazy at certain rpms.

Anyway my local mechanic for GM says it should have ported vacuum, not full time vacuum, so I am a bit confused. Since all ports off of the carb seem to offer full time vacuum, how do I access ported vacuum and what are the drawbacks from going with ported vac as I was origianlly recoimmended from this post?

Bottom line is, I need to find a good vacuum source that won't advance my timing so dramatically all of the time. Any help or advice is very appreciated.

Thanks for your input,
Mark

[Duke W.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

As usually it would help if you specified which of the five available Corvette engines you have and transmission type as this effected some ignition configurations like K-19 and PG.

K-19 had ported vacuum because it was part of the emission control strategy.

Non K-19 engines have full time vacuum advance as did most non-emission controlled engines of the era.

Set the INTITIAL TIMING to spec at less than 700 RPM with the vacuum advance control signal line disconnected and plugged. When you reconnect the VAC about 16 degrees should be added to timing placing the TOTAL IDLE TIMING in the mid 20 degree range (except PG, see below.)

One must also have a properly matched vacuum can to the engine's manifold vacuum characteristics, so you should state the ID number on the VAC and also the OE centrifugal and VAC specifications from your CSM or AMA specs along with a statement of whether you have tested them and whether or not they are in conformance with OE specs.

When you open the throttle to accelerate - even partially - manifold vacuum and vacuum advance go to zero or are greatly reduced, so they are not likely the cause of your detonation problem, however, if you give a more thorough description of the conditions under which the engine detonates someone might be able to help you correct it.

For K-19 cars, I recommend converting from ported to full time vacuum advance. If it a PG and K-19 car the distributor must be modified as PG is has 40 degrees of centrifugal and if you set the initial at greater than the OE 4 deg. AFTER TDC spec the engine will heavily detonate.

Duke

[Mark P.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

Sorry for the lack of information. It is a 427 390 HP car with manual trnasmission. The car no longer has the K19 pump or tubing and the manifold has been plugged. The detonation occurs around 1500 to 1900 RPM.

I'll have to check on the VAC numbers and specs, but before I recently purchased the correct carburetor for the car, I did not have the detonation problem, so the Edelbrock carburetor that was on the engine before must have been providing the appropriate vacuum at the time.

My limited knowledge on the subject makes me assume that since it worked before, it's a configuration problem with the new carburetor and knowing that it is missing the K19 components tells me that there's a good chance that has something to do with it. So, knowing that it's missing those components, how can I get the advance the correct vacuum to work properly (if in fact that's the problem?

[Clem Z.]

the correct carb is most likely calibrated leaner

than the other carb since it is for smog control

[Mark P.]

Re: the correct carb is most likely calibrated lea

Actually, I thought the car has really been running rich since the conversion and wanted to go leaner than it currently is. I haven't torn it down to see what it is jetted at yet. I was told it was adjusted appropriately for this altitude (4500 ft), though.

Mark

[Mark P.]

Re: the correct carb is most likely calibrated lea

Clem,

FYI, I wanted to let you know I replaced the timing chain that you advised me on for the 66 427 and it was as you diagnosed....loose as a goose. Looks to be the original chain on the car. Anyway, it is running smooth and quiet now.

Thanks again for the help,
Mark

[Duke W.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

First things first!

Does the detonation at 1500 to 1900 occur at part throttle (light to moderate acceleration) or full throttle? I need a more through description of the operating conditions under which detonation occurs.

What is the initial timing?

Does the centrifugal curve, particularly below 2500 conform to OE spec in the CSM and AMA specs, and what are those specs.

The "correct carb" for K-19 may only have a ported VAC signal line, so you may be back to ported vacuum advance and will have to convert to full time vacuum advance. Most Holleys have an external choke vacuum break hose, which can be used as a source of full time vacuum by teeing into it.

It's easy to check if the VAC signal port on the carb. has vacuum at idle, or you can check total idle timing with the VAC connected and it should show signifantlyl more timing than when disconnect and plugged. Or, if it has ported advance and you disconnnect the VAC signal line there should be no change in engine idle speed or character. If full time vacuum advance disconnecting the line will increase idle speed, but the engine will run rough because you have created a vacuum leak.

Still need the vacuum advance control ID number.

Though it may sound counterintuitive, ported vacuum advance can actually increase detonation tendency. It heats up the combustion chamber boundaries because the total idle timing is retarded from what is ideal and more of the fuel's energy is converted to waste heat rather than work on the pistons.

With the above information, I can probably offer some suggestions, but I need to understand the current spark advance map and VAC configuration.

Duke

[Clem Z.]

you are welcome *NM*

[Mark P.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

I'll give you some information, then I'll have to do some research for the rest.
It was detonating loudly under minimal load to medium load, less at full throttle, but tought to get it to do it unless driving it. It disappears completely after 2000 rpm both under load or not.

It definitely has vacuum at idle as the rpms increase notably upon connecting to vacuum, but it runs smoothly at idle and driving it, only at lower rpms when disconnected from the distributor, even while driving it. While it was disconnected from the distributor, I did not plug the vacuum line from the carb. It's almost like it prefers not having the distributor connected to vacuum at all.

I'll check on the rest.

Thanks,
Mark

[Clem Z.]

the power valve could be opening too late

and causing a lean condition till it opens try a #75 or #85 power valve to see if coming on the rich stop sooner helps with the problem.

[Duke W.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

The VAC and centrifugal calibration may be incompatible, which is why characterizing their behavior (including VAC ID no.), initial timing, and comparing to OE specs is the first order of business.

Duke

[Mark P.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

Duke,

It appears that that the vacuum advance on the car has a number MS 236 16 number on it, and though I'm unfamiliar with the unit, it could very well be a non-original distributor and vacuum unit. Could you identify it? Under the circumstances, I imagine you'd suggest I get an original distributor and vacuum unit and go from there? I don't suppose that there is a workaround on this setup until the time that I get the right parts on the car... As far as the timing goes, it was changed before I got to it, so I don't have a baseline. That may be a moot point, based on the non-original equipment finding.

Sorry to tire you with my lack of knowledge on this subject. I would also need to know if anyone out there has any suggestions on where to get an original distributor. Any recommendations are appreciated.

[Duke W.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

That's an original Delco 1116236 that was used on '64-'67 L-76,79,84. It is the most aggressive vacuum can available (0@4", 16@8")and is suited to any other engine that idles at 12" or less. Do not through it away as there are many small block owners who would like it. Replacement VACs with the same specs have different IDs because they are no longer made by Delco, including those sold by Delco.

It is too aggressive for L-36 and may be the cause, or partial cause of your low rev detonation.

All BBs with full time advance should be okay with a 12", but it would help if you would characterize the idle as follows: - xx" @ idle speed @ xx total idle timing to be sure.

Until you get a timing light, measure the initial timing, and characterize the centrifugal curve you will just bump around in the dark.

Also, what is the number on the distributor ID band, if it has one.

Duke

[Mark P.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

Thanks for the response, Duke. I'll check the info out you suggested. Unfortunately, no ID band left on the distributor. Looks like it's time to hunt for a distributor.

I'll get back on the specs.

Thanks again.
Mark

[Duke W.]

Re: C 2 66 K19 Air Carb - Ported Vacuum?

You don't need a new distributor. All that's necessary is to recalibrate the current one back to something resembling the OE spark advance map, which will be a new VAC, maybe different centrifugal springs and maybe a little work on the cam slot to alter the total centrifugal - maybe 20 bucks worth of parts, but to get there we need to know the starting point - where everything is now.

All the tach drive single point distributors are essentially the same with the exception of a handfull of parts that determined the spark advance map.

I think you can buy bands that have the correct data to pass judging scrutiny if you desire.

Duke