L-71 Timing

[Donald B.]

Gentlemen: Good Morning/afternoon. I Recently Fired My L-71 That Was Professionally Rebuilt To Factory Specifications Including The 143 Cam That I Bought Almost 20 Years Ago. The Carburetors Were Restored By Mr. Koontz (sp) And The Factory Ti Distributor Was Restored By Dave Fiedler. The Vacuum Can Is A Repro 201. Long Story Short The Engine Wouldn't Idle Very Well. I Went To The Archives And Found A Wealth Of Information Dealing With The Subject That Everyone Here Is No Doubt Already Familiar With. My Plan Was To Order The Recommended Aftermarket Vacuum Advance That I Soon Found Was Not Available. I Then Checked And Found That There Was No Vacuum At The Factory Port Allegedly Designed To Advance The Timing. The Vacuum Did Appear At Higher Rpm But Was Not Present At An Acceptable Idle Speed Of Approx. 900 Rpm's. When The Timing Was Advanced To Around 16 Degrees The Motor Idled Nicely And Runs Like A Watch. So, Since I Don't Want To Lose Points For Re-running The Vacuum Advance Line To A Decent Vacuum Source As Outlined In Previous Discussions, I Decided To Remove The Distributor To Have It Re Curved To Provide More Initial Advance At Lower Rpm. I Assume This Will Be Accomplished By Modifying The Centrifugal Advance Mechanism. True Or Not True And Is This A Valid Method For Improving Low Speed Engine Performance/idle Without Rerouting Vacuum Lines And Or Losing Judging Points? As Always, Thank You For Helping.
Don

[Joe R.]

Re: L-71 Timing

Don,

I think you would be better served by converting from ported to full time vacuum advance. It can be accomplished one of two ways. The first way is to tap into a full time vacuum source on the intake manifold. It is easy to do and reversible, but would cost you some points on the judging field. The second way is to drill a hole in the carb's throttle plate. This way is undetectable by a judge. A sharp carb man can do it for you. Jerry Luck has volunteered to do mine. As you have probably gathered by now, I'm going with option #2 on my '66 L72.

Check this thread out:

https://www.forums.ncrs.org/showthre...unnin&uid=8568

Joe

[Timothy B.]

Re: L-71 Timing

Plus, If you modify the throttle plate for full manifold vacuum and (you never will) decide to go back to ported that can be accomplished with JB weld to seal the modification.

[Ken A.]

Re: L-71 Timing

Drilling holes in throttle plates is a BAD idea. It does not correct the basic problems you have. There are other improvements you can make to the center carb that will correct a host of problems- such as rejetting, enrich the idle circuit, larger pump discharge & cam change. You are on the right track in converting your distributor to full mechanical advance. You want a total of 36-38 degrees at 3400 rpm with 10-12 degrees at under 800 rpm, which is where your engine should idle. Also make sure your plugs are AC 45xl or equiv. These engines were very poor running and poor performance in stock condition, but a little tuning will give a real "475 HP"

[Doug M.]

Re: L-71 Timing

Donald, Standard Motor Products, VC181 (stamped B26) is the same specs and performs as NAPA VC1765(stamped B20). Tested myself with mighty-vac.and have it on my car. And for bonus points it says made in USA!
Doug MCClure

[John H.]

Re: L-71 Timing

I Decided To Remove The Distributor To Have It Re Curved To Provide More Initial Advance At Lower Rpm. I Assume This Will Be Accomplished By Modifying The Centrifugal Advance Mechanism. True Or Not True And Is This A Valid Method For Improving Low Speed Engine Performance/idle Without Rerouting Vacuum Lines And Or Losing Judging Points? As Always, Thank You For Helping.
Don

Don -

That's not a good idea - the vacuum (engine load-sensing) and centrifugal (engine rpm-sensing) advance systems complement each other, but are lousy substitutes for each other. Teeing the vacuum advance line into the rubber hose that runs from the front of the center carburetor baseplate to the choke pull-off diaphragm to provide full manifold vacuum to the distributor takes two minutes at most, and reversing it takes even less time. Don't screw up your distributor by trying to make it do two jobs at once.

What makes your post capitalize the first letter of each word? It makes your post very difficult to read.

[Duke W.]

Re: L-71 Timing

The first thing you should do is tell us the exact year and then quote the centrigal curve specs from the CSM. Each year L-71 was different.

As previously stated a handy guy can convert the center carb to full time vacuum advance by plugging one hole and drilling another, none of which will be visible from the outside.

For about two bucks worth of vacuum tubing and a tee, you can make a new harness to tap full vacuum from the choke vacuum break hose, and you can swap back in forth in a couple of minutes if the car is judged.

The correct VAC has full advance at 12" and is stamped B20 or B26. If you search those codes you will find threads with all the manufacturers and parts numbers, and they are all the exact same part manufactured by Standard Motor Products, including what comes in a "Delco" box.

Duke

[Timothy B.]

Re: L-71 Timing

Donald,

If you are thinking about changing the inital timing to 16-18* the centrifugal advance has to be altered. I am not sure if there will be cranking problems when the engine is hot.. I agree it's not a good idea and not necessary with a vacuum advance distributor.

To see how your engine will IDLE with full time manifold vacuum advance try a test and set your idle timing at 27*. With the vacuum advance plugged, set idle RPM at 800+- and adjust A/F emulsion screws to best idle. This is equal to a 15* vacuum advance and approx 12* inital timing.. Don't leave it set like this, just try it for your idle test because this setting will affect total timing.

If everything is stock I think the emulsion screws will be no more than 1 1/2 turns out and engine will be smoother and run cooler. You may be able to unplug the choke pull off vacuum line to see if the engine speeds up and gets rougher. If it speeds up and gets smoother the idle circuit is lean and vise versa..

Remember this is just a test but will give a indication where you want to go with this..