1972 Chevrolet Service Manual valve adjustment

[Gary S.]

Quick question - will the 72 CSM valve adjustment procedure work with the Hinkley/Williams valve adjustment numbers. That is, set timing tab to TDC on #1 do appropriate 4 valves, rotate the engine 180 degrees and do another set of valves, rotate 180 etc. I am thinking about another valve adjustment to see if i messed something up.

I ask because I measured my vacuum after a bad day driving locally. I am getting 10.5 to 11" with engine warmed at 900 rpm, new points/condenser (set at 30), new distributor cap & rotor, timing set as described here https://www.forums.ncrs.org/showthre...chisler+timing, coil and condenser are original. I checked all of my vacuum lines by disconnecting the line coming off of large port on the back of the intake manifold and blocking it - no change. At that point the only vacuum being drawn was from the full time vacuum port at the base of the carb. I even plugged the line going to the distributor to see if that was leaking. with all vacuum disconnected and plugged I am still getting 10.5-11". I have a low speed, low rpm (15-20 mph constant speed, second gear) stuttering for lack of a better word. Perhaps even "hesitation" at steady throttle. Plus, if I get off the gas while descending a hill, this stuttering/bucking/whatever word I care to call it gets worse.

I went over the carb, the intake manifold and any potential leak spot with an unlit propane torch trying to see if rpm would increase. Nothing. Rock steady at 900 rpm.

I am out of ideas so that led me to the idea that perhaps one or more of the valves is misadjusted.

So, anyone have any ideas? This is driving me crazy. A search here suggested that the idle transition circuit wasn't seeing enough vacuum thus causing my low speed/low rpm issues. So I need to figure out how to get the vacuum back to where it was at 15 +/- a while back. Fwiw, at WOT the car is very impressive.
Gary

[Duke W.]

Re: 1972 Chevrolet Service Manual valve adjustment

Is the engine a real LT-1 with a real LT-1 cam? What are the clearance set at now?

I'm not familiar with the '72 CSM valve adjustment crank indexing scheme, but are you ABSOLUTELY, POSITIVELY certain that it is applicable to the LT-1 mechanical lifter cam?

Earlier service manual say you can adjust 8 named valves at TDC #1 and the remaining 8 at TDC#6, BUT THIS ONLY APPLIES TO HYDRAULIC LIFTER CAMS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The Hinckley-Williams method will work for ANY mechanical lifter cam, but requires specific valves to be adjusted at 8 crank indexing positions, not 4 or 2.

Duke

[Joe L.]

Re: 1972 Chevrolet Service Manual valve adjustment

Is the engine a real LT-1 with a real LT-1 cam? What are the clearance set at now?

I'm not familiar with the '72 CSM valve adjustment crank indexing scheme, but are you ABSOLUTELY, POSITIVELY certain that it is applicable to the LT-1 mechanical lifter cam?

Earlier service manual say you can adjust 8 named valves at TDC #1 and the remaining 8 at TDC#6, BUT THIS ONLY APPLIES TO HYDRAULIC LIFTER CAMS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The Hinckley-Williams method will work for ANY mechanical lifter cam, but requires specific valves to be adjusted at 8 crank indexing positions, not 4 or 2.

Duke

Duke-----


There are specific valve adjustment procedures in the factory service manual for hydraulic lifter applications and mechanical lifter applications.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

Duke & Joe,
thank you for the reply. In the spring of 2019 I followed the H-W method as opposed to the CSM procedure and I used your numbers of .020 for intake and .026 for exhausts. This was a basic tuneup and not chasing any specific demons. Now, due to the low vacuum and the, assumed, low speed/low rpm drivability issues I am redoing this.

Yes, this is a real LT-1 with all original components, around 47000 miles and has been judged twice both times as a Top Flight, not that this guarantees anything, of course. Judging, appearance, discussions back to owner number 2 who bought the five year old car with 5000 miles from a local doctor confirm the unmolested condition.

Am I certain that the CSM procedure is applicable to the LT-1 cam? No. It is, however reprint of the CSM but I have no way of knowing if their procedure applies to a mild solid cam versus a 396 cam or LT-1 cam. I am now just the low speed issues and trying to figure out if a valve adjustment error on my part can affect vacuum which, in turn, affects low speed/rpm issues.

At this point I am at the limit of my knowledge and looking for an alternative point of view. Thus the questions.

Photo is canted but I will fix that after a quick trip out.

Gary

[Duke W.]

Re: 1972 Chevrolet Service Manual valve adjustment

The tighter valve clearances I recommend increase overlap somewhat, and that has a greater negative affect on the low compression LT-1s than the '70 high compression version. Low compression and high overlap is NOT a good combination, but GM had to lower the LT-1 CR to comply with the Fourteenth Floor edict that all '71 engines had to operate on 91 RON unleaded gasoline.

If you are not absolutely positively sure that the procedure you refer to is for mechanical lifters (print is too small for me to read) then use the H-W indexing method, which guarantees that the lifter is on the base circle for each valve you adjust assuming the procedure is executed properly.

Another issue that can affect low speed/load operation and idle quality is the spark advance map. Emission controlled maps were set up for emissions compliance, not performance/fuel economy. I recommend the OE ported vacuum advance be converted to full time and this must be accompanied by installing a 8" B28 VAC. Then install light springs in the centrifugal advance and use a dial back timing light to set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation. A more thorough discussion is in my tuning seminar. The perfect spark advance map for the LT-1 is the same as OE for the 365/375 HP 327 from 1965.

My recommendation is to make the above spark advance map modifications before you readjust the valves. They will improve idle quality, low speed torque, and fuel economy. Normal idle behavior is 12" @ 900, but that's better than the 30-30 cam engines that are only 10" @ 900 due to the 30-30 cam's higher overlap. In case you are not aware the LT-1 inlet lobe is the same as the L-78 through LS6 lobe (same on both sides) which is slightly shorter than the 30-30 lobe (same one on both sides), and the exhaust lobe is the 30-30 lobe advanced four degrees.

My current recommendation for LT-1 clearance is .016/.023". The .020/.026" was based on dial indicator diagrams I took back in the seventies. The latest recommendations for all cams (Sept 2008 revision of the paper) are based on actual data from the GM drawings that allowed me to pick out the exact top of the clearance ramp along with my measured low lift rocker ratio of 1.37:1. At full lift the rocker ratio increases to 1.44:1.

If you have not already done so modify the spark advance map as recommended above and if you are still not satisfied with the low speed performance and idle quality, readjust the valves to larger clearance. My recommendations are based on ensuring that the valve is lifted off and returned to the seat at no more than clearance ramp velocity, and yes. it does decrease low end torque and idle quality, but it's a lot easier on the entire valve train and valve seats.

From this and the other thread you referenced it's not clear to me what you did to the spark advance map. Also, when using the total WOT advance method to set timing it MUST be done with the engine a few hundred revs above the point of maximum advance. Typical internet advice is to set it at 3000. OE centrifugal curves max out from 2350 to 5100, so on and engine with max centrifugal at over 5000 if you set total WOT advance at 3000 it will be overadvanced above 3000.

In order to use the total WOT method you MUST measure the RPM that full centrifugal occurs, then set the timing in the 36-40 range a few hundred revs above the speed that max centrifugal occurs.

Duke

[Joe L.]

Re: 1972 Chevrolet Service Manual valve adjustment

Duke & Joe,
thank you for the reply. In the spring of 2019 I followed the H-W method as opposed to the CSM procedure and I used your numbers of .020 for intake and .026 for exhausts. This was a basic tuneup and not chasing any specific demons. Now, due to the low vacuum and the, assumed, low speed/low rpm drivability issues I am redoing this.

Yes, this is a real LT-1 with all original components, around 47000 miles and has been judged twice both times as a Top Flight, not that this guarantees anything, of course. Judging, appearance, discussions back to owner number 2 who bought the five year old car with 5000 miles from a local doctor confirm the unmolested condition.

Am I certain that the CSM procedure is applicable to the LT-1 cam? No. It is, however reprint of the CSM but I have no way of knowing if their procedure applies to a mild solid cam versus a 396 cam or LT-1 cam. I am now just the low speed issues and trying to figure out if a valve adjustment error on my part can affect vacuum which, in turn, affects low speed/rpm issues.

At this point I am at the limit of my knowledge and looking for an alternative point of view. Thus the questions.

Photo is canted but I will fix that after a quick trip out.

Gary

Gary------


For 1970-72 there were only 2 mechanical lifter camshafts among ALL Chevrolet engines. These were the LT-1 cam and the SHP big block cam. For 1970 and 1972 the latter was not used for any Corvette application.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

Thank you once again, Duke. Last year I mentioned my ignition setup and here is a cut and paste:

In 2004 the original distributor and carb were rebuilt at Camaro Hi-Performance, by Mr. Jerry MacNeish, whom some of you probably know. The specs on the distributor (1112101) from the report are:
1. Dwell 30-32
2. Centrifugal advance curve total degrees 26*
3. Total advance in at 3000 rpm
4. Set total timing at 36* vacuum disconnected (and plugged) at 3000 rpm
Testing performed on Sun #404.

In addition, per your suggestion, I have the B-28 vacuum as well as full time ported vacuum.

I will re-read your paper again and give the valve adjustment another after I ensure that the timing is set up per MacNeish's paperwork.

Gary

[Terry M.]

Re: 1972 Chevrolet Service Manual valve adjustment

Duke & Joe,
thank you for the reply. In the spring of 2019 I followed the H-W method as opposed to the CSM procedure and I used your numbers of .020 for intake and .026 for exhausts. This was a basic tuneup and not chasing any specific demons. Now, due to the low vacuum and the, assumed, low speed/low rpm drivability issues I am redoing this.

Yes, this is a real LT-1 with all original components, around 47000 miles and has been judged twice both times as a Top Flight, not that this guarantees anything, of course. Judging, appearance, discussions back to owner number 2 who bought the five year old car with 5000 miles from a local doctor confirm the unmolested condition.

Am I certain that the CSM procedure is applicable to the LT-1 cam? No. It is, however reprint of the CSM but I have no way of knowing if their procedure applies to a mild solid cam versus a 396 cam or LT-1 cam. I am now just the low speed issues and trying to figure out if a valve adjustment error on my part can affect vacuum which, in turn, affects low speed/rpm issues.

At this point I am at the limit of my knowledge and looking for an alternative point of view. Thus the questions.

Photo is canted but I will fix that after a quick trip out.

Gary

Gary,

If this engine has its original distributer and intake manifold -- look for a chisel mark bridging the distributor and intake at the back of thesae components. Line it up and you will have the original timing setting that was made when the engine was hot fired at Flint engine assembly. You will never do any better.

[Mark E.]

Re: 1972 Chevrolet Service Manual valve adjustment

Did the hesitation/stuttering begin suddenly or gradually get worse? If suddenly, was something changed at that time (e.g. spark plugs, wires)?

One or more cylinders that are weak or not firing can cause these symptoms. Use an engine analyzer to check for a weak cylinder, or just disconnect one spark plug cable at a time and note RPM drop.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

Terry, I had forgotten about the chisel mark. I reset timing based on MacNeish's distributor set up. Now I will see how that lines up with the chisel.

Mark, the hesitation or stuttering at low speed, constant throttle has been there for a while (3-4 years?). For the longest time I assumed it was solid lifters and a mild vacuum leak but then it got worse over the summer and I am not able to attribute it to anything yet. Once the engine cools down, I will do a cylinder compression test. Right now it is too hot. Yesterday I pulled and plugged the vacuum off of the back of the manifold to brakes and headlights and the vacuum is still poor

[James G.]

Re: 1972 Chevrolet Service Manual valve adjustment

FIRST THINGS FIRST...
The H&W method of setting the valves requires that you rotate the engine 90 degrees adjust the 2 indicated, rotate 90 degrees - if you rotated 180 and did the next adjustment I can only imagine your vacuum is down.

If, however you actually rotated only 90 degrees and were errant from memory with the 180 degrees remark then proceed ahead...

I have to say that low speed constant stuttering sounds primary jet circuit related.
My contribution would be what are you running for fuel & what do you have for Jets in the carb - Jerry built it correct no doubt, however he probably did not install larger jets.
72 LT1's are jetted much leaner primary and secondary than previous LT1's -
IIRC 70-71 have 72p 76s and 72's have 68p 72s
I would step up the jets up all the way around as running a fuel with any percentage of ethanol leans you out. due to ethanol possessing fewer BTU's per gallon vs gasoline. 10% mix lowers the BTU's roughly 4%.
also if you still have the spacers in your fuel bowls(71 & 72 LT1's) you probably should remove them.
s-l1600.jpg

[Mark E.]

Re: 1972 Chevrolet Service Manual valve adjustment

Once the engine cools down, I will do a cylinder compression test.

Before doing a compression test (lots of work), check cylinder balance as suggested above. It might identify a weak/dead cylinder and narrow your scope.

[Terry M.]

Re: 1972 Chevrolet Service Manual valve adjustment

Terry, I had forgotten about the chisel mark. I reset timing based on MacNeish's distributor set up. Now I will see how that lines up with the chisel.

Mark, the hesitation or stuttering at low speed, constant throttle has been there for a while (3-4 years?). For the longest time I assumed it was solid lifters and a mild vacuum leak but then it got worse over the summer and I am not able to attribute it to anything yet. Once the engine cools down, I will do a cylinder compression test. Right now it is too hot. Yesterday I pulled and plugged the vacuum off of the back of the manifold to brakes and headlights and the vacuum is still poor

With the modifications made to the distributor I doubt the chisel mark will line up, nor will it be helpful in setting the timing.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

Terry,
Here's an interesting point, the car's initial timing based on the engine placard is 4 BTDC. The car best at 10-12 BTDC. When I was playing with the distributor the other day and reset to 4, it ran poorly so I reset it 10-11 and the stake marks line up. I don't recall the exact words I used when I talked to MacNeish but it was something along the lines of "I want a good running street car, no drag racing, no high speed/high rpm runs etc". Just a good running street machine. I didn't know enough (still don't) about optimum advance curves for various applications.

[James G.]

Re: 1972 Chevrolet Service Manual valve adjustment

Honestly what you need to know is if you used a 90 degree rotation on the valves or 180 degree rotation on the valves... if the later, readjust the valves - using a 90 degree rotation after adjusting the 2 noted valves.