Guidance Please? 1967 L71 Engine Reinstall & Run-In

[Timothy B.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

Rich,

Your work looks very nice, one thing to remember is final adjustment of the emulsion screws after vacuum advance is connected and engine warmed very well. In other words with everything hooked up.

I don't know how warm the temps are in Fl now but this mod will help in the summer heat.

Also, if you feel like playing you can remove the advance weights in the distributor and power time the engine to 38* WOT timing and then see where the inital timing falls. By removing the springs the engine does not need to be revved high to move the weights out. Set your dial back to 38* and time the engine recheck inital then plug in the vacuum advance and do the A/F screw adjustment etc..

Don't forget to reinstall the springs.

[Duke W.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

You're following the correct path, so you'll get there. If you still get some off idle stumble with full time vacuum advance and the idle mixture adjusted to optimum with the vacuum gage, try richening the idle mixture screws in 1/8th turns increments until the stumble goes away.

The nominal specs for the VACs are as follows.

B20/B26 (VC-1765) 0 @ 6", 16 @12" (equivalent to the OE MS 355-16)

OE MS 201-15: 0 @ 8", 15.5 @ 15" (The closest replacement to this is B22 - VC-1802)

MS 163-16: 0 @ 9", 16 @ 18" (Current replacements are stamped B1)

It sounds to me like whoever is "reproducing" the 201-15 bought the B1 unstamped from Standard Motor Products and then stamped them MS 201-15, but the actual function doesn't meet the 201 spec. This is common with "reproduction" parts where the people who offer them have no understanding or appreciation for the functional specifications. The proper starting point for a 201 reproduction is the B22, not the B1.

Your B26 is a little on the aggressive side, but okay and easily meets the Two-Inch Rule to keep it locked at full advance at idle.

The current replacement for the 163 is marked B1 and IMO is a boat anchor. I don't recommend it for any OE combination. For base engines with manual transmissions, I recommend the B22, and B20/26 for automatics.

The problem started years ago when GM consolidated part numbers and substituted the 163 for all 201 applications, and the aftermarket followed suit. This was a bad decision on GM's part.

The L-71 OE centrifugal is 0 @ 900, 30 @3800 (max), which is pretty aggressive for a big block, but don't change it. Experiment with how much initial you can run, and run as much as possible - up to 12 degrees - as long as there is no detonation .

You can easily check the centrifugal curve against specs with your dial-back light and the VAC disconnected. It should start at 900, so you must set initial below 900, and use the dial-back light to verify when the centrifugal starts. Initial must be set a couple of hundred revs below this speed. Or you can dial in max desired max WOT advance above the speed where the centrifugal is all in and let the initial fall wherever it may.

With a little more work you should have highly optimized spark advance and fuel maps. Don't switch back to ported until you have optimized the full time vacuum advance setup. Once it's dialed in - you'll never go back!

Duke

[Stephen B.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

Rich,

Put the car in forth gear and roll the rear tires a bit until the damper mark aligns with the timing cover mark approx TDC. Remove the distributor cap and look to see if the rotor is aligned to #1 on the cap. You may need to jump the distributor one tooth clockwise.

After you are satisfied and the oil pump drive is engaged, repeat the TDC check to be sure it's correct. Put the vacuum advance in the middle of it's travel and it should run..

On a small block sometimes the aftermarket cams require the lower distributor gear be turned 180* but it's a last resort to getting the vacuum can and tach cable to good position in that tight area. The correct orientation is with the dimple on the lower gear aligned to the rotor tip as you know.

You can also tee into the choke pull off if it's easier as that port is full manifold vacuum. Remember about the vacuum can specs mentioned in a earlier post because this will affect idle. If the 201 vacuum can takes 15" to pull to the stop and the engine makes 13-14 the idle will be unstable.

Tim
You touched on it briefly,but you can be 1/2 tooth off if you've removed the distributor gear and replaced it 180 degrees off.If my memory serves me theres an odd number of teeth on this gear thus causing the problem.Is that correct or is my Alzheimer's acting up.
Stephen Barrett(21558) 59,66,71,73

[Timothy B.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

Stephen,

Many times if using a non GM camshaft it's necessary to turn the lower distributor gear 180* so after setting initial timing the vacuum advance control is approx in the middle of the space between the coil bracket and intake manifold. Doing this moves the timing about 13.5* and many times it's all that's needed.

The OEM cams do not require this, on my 63 with the timing set at 8*BTDC the vacuum can is approx 1/8" from the coil bracket and the tach cable fits nice.

On my 67 after installing the breakerless SE I found it better to rotate the lower gear to put the vacuum can in the middle because it was hard against the coil bracket. The camshaft may have been changed at one time in previous life.

[Stephen B.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

My first encounter with this was as a result of reinstalling the gear 180 off thus the position of the vacuum advance being off.I learned from that experiance of putting something together basacwards.I think there are others out there who didn't know this.Don't you just love this site ?
Stephen Barrett (21558) 59,66,71,73

[Richard M.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

If anyone has a 67 AIM, could you check this for me?...........

UPC L36 Sheet A16 shows View A with a clip 10.00 off the CL, then "something" down from the corner along the radius edge. All I see on my copy is 2 odd looking characters, like "&&". Does anyone have a dimension?

I found this thread that has some info, but not about this clip.
https://www.forums.ncrs.org/showthre...=6981#poststop

There was evidence of repairs to this firewall years ago when it looks like maybe a motor mount broke or ??, and I don't see any hole remaining. It's glassed over. I need to hold the + battery cable away from the manifold as shown on the AIM.

Thanks,
Rich

edit...I see the picture in the above thread shows it just under the lip if the heater box reinforcement, but the View A shows it much further down than that, along the outside corner which seems several inches down. The phot in the thread appears to show a pop-rivet, cannot be original rivet right??

[John H.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

If anyone has a 67 AIM, could you check this for me?...........

UPC L36 Sheet A16 shows View A with a clip 10.00 off the CL, then "something" down from the corner along the radius edge. All I see on my copy is 2 odd looking characters, like "&&". Does anyone have a dimension?

I found this thread that has some info, but not about this clip.
https://www.forums.ncrs.org/showthre...=6981#poststop

There was evidence of repairs to this firewall years ago when it looks like maybe a motor mount broke or ??, and I don't see any hole remaining. It's glassed over. I need to hold the + battery cable away from the manifold as shown on the AIM.

Thanks,
Rich

edit...I see the picture in the above thread shows it just under the lip if the heater box reinforcement, but the View A shows it much further down than that, along the outside corner which seems several inches down. The phot in the thread appears to show a pop-rivet, cannot be original rivet right??

Rich -

That dimension is .88" (7/8"), and you'll find a lot of variation in the location - I think they "eyeballed" most of them; yes, it's a pop-rivet as shown in the photo.

[Richard M.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

Ok John thanks for clearing it up. I guess my copy is missing the "dot". Now to drill and get Rosy the back-up riveter to help me.

[Richard M.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

A photo of the clip in question after I installed it below. Orange on the black is just the droplight reflection....
I believe a standard rivet was used originally. I could not get either a pop rivet or a standard rivet in that space. I used a #10x1/2" Phillips screw and just mashed some dum-dum in the cross-recess and shot it with some blackout. Looks pretty typical if I say so myself.

We decided to not modify the center carburetor to provide direct manifold vacuum to the vac advance can. I instead tee'd off of the main vacuum secondary port source hose and ran that connection to the can. I used a block-off plug on the ported inlet on the carb.

So.....we ended up doing a body-on restoration on this car..... Went through the whole car. It is being picked up today by the enclosed car transporter heading to it's new owner in Norway, who BTW has joined the NCRS after we first spoke. He had the opportunity to spend 2 weeks here in January, also during the NCRS Winter Regional. He met many of you there and is very excited about his Corvette coming his way. He and I also have become friends and I look forward to our continued friendship. It's really great to meet people from around the world who share the same passion that we all have for these great cars. It has been a great experience at all levels and also a very fine big-block learning experience as well.

I appreciate all of your help and guidance that you've helped me with.

Thanks guys.
Rich
p.s. I'll get some final photos up here later after the car gets picked up.

[Duke W.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

We decided to not modify the center carburetor to provide direct manifold vacuum to the vac advance can. I instead tee'd off of the main vacuum secondary port source hose and ran that connection to the can. I used a block-off plug on the ported inlet on the carb.

It's not clear to me what you did here. Are you talking about teeing in the VAC to the end carburetors actuation vacuum control? That uses center carb venturi vacuum to open the end carbs, which will yield full vacuum advance at high air flows, which could cause a catastrophic detoantion problem.

The easiest way to implement full time vacuum advance is to tee into the choke vacuum break signal line. This always provides full manifold vacuum to the chock vacuum break, but the modification is visually detectable.

Modifying the internal passages in the center carb allows full time vacuum advance, but the modification is not visually detectable.

Duke

[Richard M.]

Re: Guidance Please? 1967 L71 Engine Reinstall & Run-In

Actually I did tie into the choke vacuum break. I got it wrong in my description. Sorry for the confusion.

Hard to see it but the 3rd photo shows the hose with the white tracer from the chrome pipe going into a tee in the choke vacuum circuit.

Rich