L-71 vacuum reading

[Robert D.]

What is the narmal manifold vacum reading for an L-71 motor. Stock rebuild the most i can get is about 11 inches at 800 rpm its steady and consistent. Does this sound correct
Thanx
Bob

[Duke W.]

Re: L-71 vacum reading

Manifold vacuum is normally specified in inches of mercury, and typical L-71 readings should be 14-15" @ 900. Lower idle speed will reduce vacuum, but if you're seeing 11" @ 800 you may have an aftermarket camshaft with higher overlap than the OE cam.

Look at your rebuild documentation for the brand/part number of the installed camshaft and post the data. Maybe the rebuild was not "stock".

Duke

[Robert D.]

Re: L-71 vacum reading

Sorry im getting 11 inches the cam is the stock crane L-71 cam 11" @ 800 ill try it again at 900

[Mark E.]

Re: L-71 vacum reading

Stating the obvious here, but also check ignition timing at idle and carb's idle mixture. Each affects vacuum.

[Richard M.]

Re: L-71 vacum reading

Bob, Is the engine setup and are you testing manifold vacuum with it's vacuum advance configured for stock ported vacuum?

If it is being tested this way, based on idle speed(if throttle plates are above the transfer slots in the bores), the ported "port" may be active and affecting your manifold reading.

The last L71 I restored, using full manifold vacuum for the non-stock VC1765 vacuum advance can, yielded apx 15" Hg.

Rich

[Duke W.]

Re: L-71 vacum reading

Stating the obvious here, but also check ignition timing at idle and carb's idle mixture. Each affects vacuum.

True. I believe the recommended initial timing is only 4 degrees, and with the OE ported vacuum advance that's the total idle advance. Adding full time vacuum advance with a B26 VAC brings it up to 20, and if you advance the initial to 8 that gets total idle advance in the ballpark of the range for most efficient idling, and with 30 centrifugal total WOT advance is 38.

The improvement in idle quality, the torque torque curve, throttle response, less tendency to run hot in traffic, and better fuel economy is noticeable.

Duke

[Robert D.]

Re: L-71 vacum reading

Ok so that means disconnect the vacum advance plug the port then check manifold vacum correct? Duke isnt the initial timing 8 degrees? the distributor is setup to have 28 degrees total advance at 3900 rpm so total advance is the 8 - 10 degrees initial plus the 28 correct?
Thanx guys
Bob

[Duke W.]

Re: L-71 vacum reading

According to the AMA specs, maximum centrifugal is 30 @ 3800, and the initial timing spec is 5, but I recommend at least 8.

In order to check manifold vacuum you have to tee the vacuum gage into a hose that has full manifold vacuum. If your engine still has the OE ported vacuum advance, the VAC signal line will not work. Use the choke vacuum break hose.

When setting total WOT advance with a dial back timing light the VAC signal like should be disconnected and plugged with golf tee.

Duke

[Robert D.]

Re: L-71 vacum reading

I was using the pcv port that should be full manifold vacum right or am i mistaken? also i rechecked the valve lash the lash on the valves were a little tight so loosing them got me to between 11 and 12 inches of vacum. again the cam is the crane blueprinted cam for the l-71. had the timing between 7-12 degrees initial but evertime i changed it and set the idle to 900 the vacum went back to 11 -12 inches. the needle is pretty steady at idle bounces very little between 11 and 12 at idle which i expect for the cam profile. if i increase the rpm to 1000 from 900 the vacum goes up to 15. also if i raise the rpm the vacum needle is steady as a rock.

[Richard M.]

Re: L-71 vacum reading

I've never used the PCV port. Easy way to verify is to take identical readings at the choke pull off with a Tee, or the manifold fitting with a Tee.

Does Crane show different lash specs than stock on the Cam Card? Last one I used had different specs.

Also, one other thing I came across on the last L71 diagnosis, was that the tolerance on the throttle plates to the bores on the two secondary Holley carburetors was all over the place. By design, you inevitably loose some vacuum there. You can prove it by covering your hand over the air horn when running and observe your vacuum readings. Don't be surprised if you stall the engine.

I was chasing a ghost in this video. It was determined to be normal. The stumble problem found was the primary float level too high and accelerator pump issues.

https://youtu.be/OLpWAC0DBpk

[Duke W.]

Re: L-71 vacum reading

...what Richard said about the vacuum gage connection. Are you using the car tach or an accurate test tach?

Vacuum increases with idle speed, but what you are reporting seems like a lot.

Some years ago the specs in the Crane catalog for what they claimed was a L-71 cam were a lot closer to the L-88 cam than the OE L-71 cam.

Scan and post the cam card or type in the specs.

Duke

[Robert D.]

Re: L-71 vacum reading

Duke the cam specs are as follows Crane cam #3863143
power range 3000-6400
lift @ .050 242/242
lobe sep 114 degrees
gross lift 520/520
valve lash is int .24 exh.28

[Robert D.]

Re: L-71 vacum reading

Rich
i will measure from the choke port and check it the lash specs are in the above post i hit send by accident might by losing it through the secondaries ill check
thanx guys for your help
Bob

[Duke W.]

Re: L-71 vacum reading

The 242 is .050" lifter rise duration, not lift, and what you provided matches OE spec, so this cam should be a duplicate of the OE cam.

What method are you using to set lash?

Duke

[Gene M.]

Re: L-71 vacum reading

Duke the cam specs are as follows Crane cam #3863143
power range 3000-6400
lift @ .050 242/242
lobe sep 114 degrees
gross lift 520/520
valve lash is int .24 exh.28

A modern profile could put the power range more in the 1500 to 5000 RPM range which would be more fun on the street. A power range of 3000 to 6800 RPM is more for the track. But don't take the engine apart when everything is all done and complete.