1966 L79 with A.I.R. optimum tune up specs

[Robert B.]

The owners manual for my '66 L79 with it's original K19 system calls for dwell of 28 - 32 degrees, timing of 10 degrees BTDC with vacuum advance disconnected and plugged. I'm running the correct AC44 plugs. Are these the best settings for good idle quality and smooth acceleration, or should I make some changes in timing? The vacuum advance is a 236 (repro from LIC).

Would appreciate any advice -

Bob

[Timothy B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Robert,

I think those tune up specs are fine for your engine. The AC45 plug would probably be better but if you just have the heat range 44 plug it will be fine.

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

L-79 with K-19 has the same OE spark advance map as without K-19.

The centrifugal is very lazy - 30 @ 5100, so lighter springs will improve low to mid range engine performance. Verify that the repro 236 16 VAC meets spec - start at 4", 16 deg. @ 8". Many do not. If so replace with a current B26.

Complete OE spark advance map specs are in the CSM and AMA specs. Search for a thread started by me in Dec '12, L-79 distributor blueprint overhaul. You should also download the pdf my 2012 San Diego tuning seminar Powerpoint slides. Search my name San Diego Corvette.

Idle behavior with a L-79 cam is 750 @ 14-15" (with a slight lope) with about 25 degrees total idle advance (The sum of initial and full vacuum advance) and the idle mixture screws about one turn out from the seat with a properly functioning OE carb.

Total WOT advance should be in the range of 36-40 as high as possible in that range as long as it doesn't detonate. So with 30 centrifugal, initial should not exceed 10, and if it runs there detonation free it's okay.

Initial can be tricky to set because centrifugal may start below the speed that you can get the engine to idle stably, while you check and adjust timing. I prefer setting total WOT a few hundred revs above the point of centrifugal advance. With lighter springs that get the centrifugal all in by 3500 you can set it at 4000.

Total WOT advance MUST be set a few hundred revs above the speed of max. centrifugal. Check the specs and then test with a timing light to see if the centrifugal is OE or modified.

If your L-79 is set up per the Dec. '12 thread I started, it should be a very sweet running L-79.

Duke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Duke, good to hear from you. I don't drive this car very much as I am getting it ready for flight judging at our regional in April. I am trying to get better idle quality, and it seems like I am always tweaking idle mixture or idle speed. Eric Jackson restored the carb about a year ago, and I asked him to check every air and fuel passage to make sure it was to spec, so I don't think the carb is the problem here. And I have also rebuilt the distributor - although it was in good condition, I cleaned, checked and lubed it - and it is the correct distributor for the engine. Your comment about initial timing being tricky caught my attention - would you recommend setting timing at 3500 or 4000 rpm? And if so what should I look for in terms of total advance?

Bob

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I answered your questions in my previous post, but I'll add unequivocally that you must set total WOT advance several hundred revs above the point of max. centrifugal. If your centrifugal is OE that means about 5500 revs. With lighter springs it should be no more than about 3500, so you can set at 4000.

As I previously stated, you need to use vacuum pump to check if the repro 236 16 VAC meets specs. There's a good change it isn't even close. Just because if has the "right number" doesn't mean it meets OE spec, and several have reported this problem with repro 236 16s A VAC that meets the Two-Inch Rule is essential to having a stable idle.

If your engine's idle behavior is not as I specified with the mixture screws out about one turn from the seat, the VAC may not meet specs or it may not have a real L-79 cam.

There's nothing more I can say until you run the checks and tests I recommend and post the data.

Duke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Duke, are you referring to your December 16th, 2016 post "How to fine tune spark advance with dwell angle changes"?

Also, I forgot to mention this car is a factory A/C car running R12 - would you recommend any changes in tuning considering the additional load from the compressor?

Bob

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

No, revisit post #3.

With AC you should set idle with the compressor engaged as low as possible commensurate with acceptable idle quality and stability. I don't believe the engine has an "idle up" solenoid, so with the compressor not engaged idle speed will likely be 100-200 revs higher.

Report idle speed/vacuum under both conditions as this information is absolutely necessary to select the correct VAC, but you need to test the currently installed VAC to determine if it meets 236 16 specs. If it doens't you are just chasing your tail trying to get the engine to idle properly whether the AC compressor is engaged or not.

Duke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Here are the results:

I tested the 236 repro vacuum advance and found that is starts at just under 5" and is all in at 9".

Initial timing is set to 10 degrees BTDC. The timing mark is very steady.

The vacuum advance has been connected to ported vacuum which at idle is only 5". At 750 RPM, total advance is only 18 degrees.

So I connected the vacuum advance to full manifold vacuum and got the following measurements:

At 750 RPM, manifold vacuum is 14 - 15", and total advance at the crank is 28 degrees - a bit more than you stated.

I've actually been able to get idle quality about the same with either ported or full vacuum, but it is much harder to get a consistent idle speed with ported. With full vacuum it is easy to get idle speed very consistent. I assume that's because ported vacuum is right at the point where the advance starts.

So, here's the dilemma - for flight judging the 236 advance works, and the advance needs to connect to ported vacuum as it was originally designed.

So, are the readings I got for the 236 close enough, and should I change initial advance to help compensate? Or should I change the springs also? I've ordered the Mr. Gasket spring set, so that's easy to do.

Bob

PS - I found your earlier post - thank you!

[Timothy B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Robert,

I think the vacuum advance spec you posted is fine, one thing to keep in mind regarding idle speed is the throttle blade position/transfer slot exposure. Approx 3/4 to 1 turn on the primary idle speed screw then open the secondary blades to get the desired idle speed, this may help with the ported vacuum. The light centrifugal springs will also help.

By closing down the primary throttle blades as suggested above you will close off the vacuum port for the ported vacuum advance feed so the idle will be constant. Add extra idle air from the secondary side.

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

The VAC is within spec and may have a bit more than 16 degrees advance. These aren't real precision devices. Total idle advance in the mid to high 20s range is about right for a L-79.

It's possible that with ported vacuum the VAC is seeing enough vacuum to pull the rod a bit, and it may be dithering causing variation in total idle advance. This will result in inconsistent idle speed. It's very important that the VAC be locked at full advance at idle for a steady and stable idle with full time vacuum advance. With ported vacuum advance there should not be enough vacuum to pull the rod at idle.

You can test for this by pulling the VAC signal line and plugging it with a golf tee. Then set idle speed and check to see if the timing mark is moving around. Then connect the VAC. If idle speed increases slightly and you see a few more degree advance, then there's enough vacuum at the ported signal line to pull the rod a bit, and that would be the source of the inconsistent idle speed.

Consider the suggestion from Timothy, however, the best solution is to convert to full time advance by plugging one port in the carb and drilling another such that the vacuum advance port is above the throttle blade. This would not be detectable in Flight judging, but the carb. must be removed to make the modification.

Full time vacuum advance will reduce idle EGT, which reduces cooling system load and will result in better fuel economy in typical around town driving.

Lighter centrifugal springs should have no effect on idle behavior, but will improve low to mid range torque. The engine will feel more responsive in typical part throttle acceleration at low to mid revs.

Your idle behavior is spot on for a L-79, and I assume you measured it with the AC compressor not engaged. Can you measure it with the compressor engaged? I'm curious as to idle speed and vacuum drop with the compressor engaged.

If you have an IR gun check exhaust manifold temperatures with full time and ported vacuum advance. The engine should be fully warmed up from normal driving, then measure after idling for a few minutes. With full time advance they should be no more than about 500F. With ported vacuum advance probably close to 900.

Duke

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I was able to run a few tests today as you suggested:

With full vacuum, I set idle at 800 RPM - I was able to set it easily and it did not drift from 800.

Engaged the A/C compressor with fan on max, and the RPM dropped to 600, and remained pretty stable at 600, although it wasn't completely happy at that speed.

I let the engine warm up for 20 minutes at idle until the temperature stabilized (I can't drive it yet as I have to reassemble some items), and measured exhaust manifold temp at the center small indentation -

With full vacuum it measured about 260 degrees.

With ported vacuum it measured 370 degrees.

So then I changed to ported vacuum and attempted to set the idle speed to 800 - it was very difficult to keep it at 800 - it would "dither" from 700 to 950 without touching anything. When I engaged the A/C compressor the RPM pulled down to about 600 and the engine died.

I'm going to check a few other things, and try changing the springs when I get the kit, but I now understand the value of using full vacuum on these engines.

So then I measured the temperature at the same point on my '67 A/C coupe which has a 300HP engine, which is running running ported vacuum. It measured 540 degrees after the temperature stabilized.

I plan to convert that engine to full vacuum as soon as I get a chance as I drive that car a lot. Would the same vac advance be a good choice for that engine? How about initial timing?

Many thanks for the feedback on this - I'll report additional results soon.

Bob

[Timothy B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

On the 300hp car the B22 vacuum advance will work with full manifold vacuum advance. If the distributor has it's original specs 6*BTDC will be good.

When you connected the ported vacuum on the 350hp car did you adjust the A/F emulsion screws.

[Robert B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Hi Timothy -

Yes, I did adjust the A/F screws - but still could not avoid the "dithering". I think the problem is that the ported vacuum of 5" is right at the point where the vacuum can starts to advance. I'll check that again tomorrow. Maybe I need to get an advance that starts at 4" instead of 5 - at least for judging. For driving, no question that full vacuum provides much better results!

Bob

[Timothy B.]

Re: 1966 L79 with A.I.R. optimum tune up specs

Bob,

The ported vacuum feed hole in the base plate is just above the primary throttle blade, if (at idle) the blade is open to far vacuum will find this port and cause the condition you describe.

I mentioned in a above post about the proper position of the primary throttle blade at idle, you can remove the carburetor and install a 6 or 8-32 set screw 1/2" long in the secondary stop screw location and open the rear blades to adjust hot idle speed with the carburetor bolted on the engine. Set the primary blade speed screw 3/4 turn in and that should keep the ported vacuum feed closed.

I see no reason why you can't get the engine to idle steady no matter which type of vacuum advance you use.

[Duke W.]

Re: 1966 L79 with A.I.R. optimum tune up specs

I was able to run a few tests today as you suggested: With full vacuum, I set idle at 800 RPM - I was able to set it easily and it did not drift from 800. Engaged the A/C compressor with fan on max, and the RPM dropped to 600, and remained pretty stable at 600, although it wasn't completely happy at that speed. I let the engine warm up for 20 minutes at idle until the temperature stabilized (I can't drive it yet as I have to reassemble some items), and measured exhaust manifold temp at the center small indentation - With full vacuum it measured about 260 degrees. With ported vacuum it measured 370 degrees. So then I changed to ported vacuum and attempted to set the idle speed to 800 - it was very difficult to keep it at 800 - it would "dither" from 700 to 950 without touching anything. When I engaged the A/C compressor the RPM pulled down to about 600 and the engine died. I'm going to check a few other things, and try changing the springs when I get the kit, but I now understand the value of using full vacuum on these engines. So then I measured the temperature at the same point on my '67 A/C coupe which has a 300HP engine, which is running running ported vacuum. It measured 540 degrees after the temperature stabilized. I plan to convert that engine to full vacuum as soon as I get a chance as I drive that car a lot. Would the same vac advance be a good choice for that engine? How about initial timing? Many thanks for the feedback on this - I'll report additional results soon. Bob

I'm surprised the L-79 manifold termperatures are so low, but IR guns typically measure a 4:1 cone, so you have to hold it close the the manifold to avoid picking up lower background temperatures, which get averaged in. The '67 base engine has the same centrifugal curve as the '66 L-79, so quickening it up is a good idea. The silver and gold springs in the kit are pretty close. I recall that they both yield max advance at about 3500, but one is a little more aggressive on the low end. Use that set on the L-79 and the other on the 300 HP. The OE 300 HP VAC is 12", and I think the stamp is 355 16. As long as it passes the Two-Inch Rule, engine idling with the compressor engaged, it's okay to use. The current replacement is B22. As with the L-79 set the idle with compressor engaged at the lowest speed that you find acceptable, and figure with the compressor off it will pick up about 200 revs. Since total OE centrifugal is 30, set initial in the range of 6-10 as high as it will tolerate without detonation. I'm interested in knowing the vacuum @ idle speed on both engines with the compressor engaged. Duke