1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

[Kenneth M.]

I purchased a frame-off restoration with a rebuilt original motor, just this last year the motor bucks and sputters when I'm going through the gears and when I really get on it in 1st, 2nd & 3rd gear it just sputters & bucks.

My question is - is the original factory installed 327ci/340hp a solid lifter? if so, what should the cam lash be? (should the cam be a 30/30?). I've had the valve covers off and I know the motor has solid lifters, I want to set the lifters to the correct spec's.

Also, the original carb should be a AFB 3461, there is no tag attached so I have to remove to check it out. Secondly, the distributor seems to be in question,regarding the vacuum advance relative to the carb, I can't seem to get enough advance on the distributor to allow the motor to run at it's top performance.

Am I making sense? I know there are a lot of questions, any suggestions, I would appreciate any help.

[Ed S.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Kenneth, I am sure many will respond with specific "how to's" but for starters you need the GM 1963 Corvette spec book - click on the link and download, and you need a copy of the 1963 shop manual, available through the NCRS store. "Popping and sputtering" could be caused by a number of things, but it sounds like an ignition issue (unless it is popping back through the carb).

https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Corvette/1963-Chevrolet-Corvette.pdf

[Richard G.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

*My question is - is the original factory installed 327ci/340hp a solid lifter?
Yes the 340HP is a solid lifter motor.

*if so, what should the cam lash be? .012 intake .018 exhaust.
Also, the original carb should be a AFB 3461, Carter aluminum Type AFB four-barrel with the numbers on the carb base stamped into the front side.

*Secondly, the distributor seems to be in question,regarding the vacuum advance relative to the carb, I can't seem to get enough advance on the distributor to allow the motor to run at it's top performance.
The motor should run ok even without the vacuum canister connected. Remember the earlier Corvettes didn't even have a vacuum advance. At WOT it does nothing.

Does the 340hp have the radio shielding if so is it in place? So many issues can be traced back to defective wires.
Posting a picture would help, we would know how stock the motor is. So many 340hp motors have been messed with over the years it is difficult to know where to start. I pulled my hair out once only to find out the new AC Delco spark plug set had a defective wire. I would consider paying for a scope of the ignitions system, it can check a lot of parameters very quickly. At this point it could be so many things you will have to start at ground zero. Confirm the harmonic balancer and timing tab is correct and go from there.
Rick

[Duke W.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

If an original spec "Duntov" cam was installed (The 30-30 was not OE in '63) I recommend cold valve lash of .010/.016. Search the Web for Hinckley-Williams Corvette valve adjustment paper. When you had the valve covers off you should have measured and recorded clearances. Get the paper and be sure to use the recommended indexing scheme to ensure that your measurements are accurate.

Do you have any documentation on the engine rebuild? Even if it's a "numbers matching" restoration, that doesn't mean all the internal parts are OE equivalent. If you can determine the brand/part number of the installed cam, it can probably be identified.

The OE 201 15 VAC does not pass the Two-Inch Rule, and should be replaced with a current B28, which is available in the Airtex brand, 4V1031. The ID data is stamped on the VAC bracket, and you may have to loosen the dist. cap to see them. Anything other than a GM/Delco 236 16 or a B28 will not work properly with any OE mechanical lifter cam - idle instability and stalling will result.

Check distributor shaft end play, dwell and dwell variation and report the results.

As stated you need a 1963 Corvette Shop Manual, and also download the 1963 Corvette Vehicle information kit from the GM Heritage Web site.

Many currently available "restored" AFBs were assembled from a basket of parts, so what you have may not be calibrated properly for the engine configuration... whatever it is.

It's not uncommon for "restored" cars to run poorly because they may be assembled from a bucket of parts and all the bugs haven't been worked out. There are plenty of Top Flight restored cars that have only been driven on and off trailers. If the owners ever take them out for a real drive, they might be surprised on how poorly the car drives, so you may have a lot of work ahead of you to get the car dialed in.

Duke

[Gene M.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Ken, i suggest you obtain information of all the parts the engine builder used in the rebuild. If not all solid lifter original configuration the fixes would be a bit different. Such as pistons, camshaft, and distributor if not original configuration the entire characteristics would differ. Insure the engine is a “matched set”.

[Kenneth M.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Richard,

Thank you so much for your reply.

I have adjusted the cam lash to your recommended setting

Took the carb off and verified it was a AFB3461 - noticed that the mechanical butterflys were not working properly and adjusted the linkage

I'm down to the distributor now (I think this may be the problem), I'm taking it to my mechanic friend and have him check it on his machine, it may be a simple adjustment or replace the distributor.

I hate to bother you again, but would you know the proper advance curve for the RPM's range.

Ken

[Ed S.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Kenneth,
All tune up specs, including distributor curve data, are in the publication I provided in my reply above - at the link. Distributor curve data is on the 25th page of the PDF file - that is not page 25 of the document (printed page numbers are not visible on the PDF version).

Hope this helps.

[Richard G.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Ken's link has a wealth of information in it. It is highly recommended background information with everything from amp draw to correct bulb information.

It is interesting in 1963 Corvettes all HP motors (except FI) had the same distributor with the same advance. 111124 is the tag number.

Let me see if I can get this straight: Information below is straight from the PDF.

Centrifugal advance in crankshaft degrees.
11 deg @ 1600 rpm.
24 deg @ 2400rpm. (Max deg. at RPM)
So only two points in the curve are identified.
The data in the PDF could be interpreted as the centrifugal advance is all in by the defined 24 deg point. It does say Max deg at RPM. However, my understanding is the centrifugal advance maxes out close to 30 deg @ 5000 rpm.


Vacuum advance in crankshaft degrees.
Starts @ 8 deg
15 deg @ 15.5 hg
By most reports 15 inches is the amount of vacuum a 340 HP makes at an idle.
No intermediate vacuum advance point is deified in the PDF.
I am not sure I understand the (Starts @ 8 deg) is specification as it is spelled out in the PDF.
I guess it is typically interpreted as 0 @ 8 deg.

From Duke's 2 inch rule; The vacuum advance control (VAC) should be “locked” at full advance at idle speed/vacuum. So that would require a VAC that is locked in by 13.5 inches.
That would be ok if your motor is producing at least the 15.5 inches of vacuum advance should be all in, when connected to a full vacuum port.
To achieve full VAC at idle it often times requires changing the vacuum can on the 340 hp.
It is my understanding the timing mark often jumps around because the low rpm the centrifugal advance begins and the too high vacuum canister setting of the factory parts.


Initial timing;
The factory is 10 degrees.
I understand some of the difficulty of setting the timing on the 340hp engine is the Centrifugal advance is starting to come in at the recommend idle speed of 700rpm.
One can attempt to get the idle speed down for a short period of time to set the initial advance or use rubber bands to hold the weight closed.
One can see where the factory settings might work ok on the lower hp motors with lots of vacuum and a 500-rpm idle speed. From what I have read the factory distributor setup is not optimized for the 340hp.

Correct me if I am wrong, this is from reading other accounts of the issues and best setup. My understanding of distributer timing is rudimentary compared to others like Duke that have been studying this for years. I thought it might help if a amateur attempted to interpret the information.

I do know my 340hp distributor was missing the felt wick and the ground wire required replacing.
Bushings were in good shape but the axial play was over .060, three times Duke’s recommendations. The plate the points are mounted on pivots. The grease or oil was stiff and required cleaning the goo off the shaft so the VAC could move the plate without sticking. There is a small snap ring that is difficult to see that has to come off to clean this.

I recommend you check all of the above items and then reinstall before bringing it to a shop. It is likely none of these items have been apart in the last 55 years. The distributor is one of the most neglected items on classic cars.
Best of luck.
Rick

[Duke W.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

The distributor is easy to overhaul as long as the shaft bushings are okay and don't need to be replaced. Side play should be virtually undetectable. Search for a thread stated by me in Dec. '12 "L-79 dist. blueprint overhaul"l.

As I believe I've previously stated, the OE 340 HP spark advance map is ill-conceived. If equipped with an OE equivalent Duntov cam or any other OE mechanical lifter cam or aftermarket high overlap cam you need a 8" B28 VAC. Also the OE centrifugal curve is lazy, and the timing tab is not accurate.

The best 340 HP spark advance map is the same as the 365/375 HP engines. All 24 degrees centrifugal is in at 2350 engine RPM so you can easily set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation at, say 2600, or a few hundred revs above where it's all in, and you are done.

I know some say a Duntov cam pulls 15", but mine was always about 12" @ about 850, which is why the 8" VAC is required. Prior to reconfiguring my spark advance map to 365/375 HP specs circa 1965, it had severe idle instability. It would only idle for a couple of minutes before the engine stalled. I had to keep blipping the throttle to keep it running. This is because with the OE 15.5" VAC was not "locked" at full advance at idle speed, so it would lose advance, which caused a further loss of idle speed and further loss of vacuum advance. It was a positive feedback loop that resulted in the engine stalling, and as vacuum fell due to the loss of idle speed the AFB metering rods would start cycling and bang again the cover which cause a rattling noise.

The 8" VAC stayed "locked" at full advance at idle and the quicker centrifugal curve yielded noticeably improved low end torque.

This was the first year Chevrolet equipped Duntov-cam engines with a VAC. They really really screwed the pooch on this one, and AFAIK they never wrote a TSB on the problem. They did fix it on the 365/375 HP engines, but '63 owners were left to fend for themselves.

All the details on how to set up a spark advance map for ANY engine configuration are in my 2012 San Diego National Convention Tuning Seminar, which is an easy Web search. Most OE spark advance maps can be improved and some are just plain ill-conceived. The only OE engine that have an optimum spark advance map as designed are the 365/375 HP 327s, and this or something close is optimum for all SHP/FI small blocks.

Also understand that ALL emission controlled engines have spark advance maps that are set up to emission control, not best performance and fuel economy, but it is relatively easy to optimize them by reconfiguring the map to the antecedent non-emission controlled engine.

Duke

[John D.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Duke, The 10 and 16 valve setting you recommend. I am trying to find the correct words as I never insult anyone. But I don't care for that setting on the fuel injection cars. I had three C1 customers in the row that set their valve lash like that and none of them developed enough vacuum.
So I told them to lift the FI unit off (not remove the air meter) and set the valves cold at 12 and 18 or a thousand more. Barry Holmes told me he sets his 097 cam valve lash and 14 and 20 cold.
Anyhow all three customers had '61 or '62 FI's. They reset the valve lash and I got a call from all three car owners that the engine vacuum went up to 15" and 900 RPM, 16" at 900 and about 17" at 900. Before all three had really low vacuum.
Bill B from the Pittsburgh chapter only had 9" at the 10-16 setting.
Thanks for your help and imput. John D

[Duke W.]

Re: 1963 corvette 327ci/340hp cam type / cam backlash / carb & distributor

Back when I set the valves on my 340 HP SWC I used the Chevrolet recommended .008/.018", and the engine generated about 12" @ 850. It was rather torque shy, especially since I was pulling a 3.08 gear through a CR four-speed in hilly Seattle.

Updating the spark advance map to 365/375 HP specs really improved the low end torque and the 8" VAC solved the idle instability problem because it passed the Two-Inch Rule, and you are probably aware that I recommend changing the '63 FI vacuum advance from ported to full time along with an 8" VAC. '63 was the first year that Chevrolet used vacuum advance on Duntov-cam, engines, and they just plain screwed it up, but fortunately they fixed it shortly into the '64 model year and updating to those specs will considerably improve performance, driveability, and fuel economy on '63 models.

I've always said that the OE mechanical lifter cams have too much overlap for a road engine. Increasing valve lash reduces effective overlap for a smoother, higher vacuum idle, and more low end torque, and since most guys rarely rev these engines into the upper range, they may be more driveable, but increasing lash above what I recommend means the lash is not taken up until AFTER the lobe is beyond the top of the constant velocity clearance ramp. This means that lash is taken up at higher than clearance ramp velocity, and the valve is SLAMMED back down on the seat at higher than clearance ramp velocity, both of which increase valve train shock loading and is evidenced by greater valve train noise. This is particularly true for the Duntov cam since peak jerk occurs just beyond the top of the clearance ramps.

I've always recommended that owners who want to retain the Duntov cam have Crain grind it with a 110 deg. inlet POML and a 114 deg. LSA vs. the 108.5/110 OE spec. The former makes the cam essentially a mechanical lifter version of the L-79 cam and L-46/82 cam advanced four degrees. The latter two are the best high performance road cams ever developed by Chevrolet with enough overlap to create a bit of idle lope, but not kill the low end torque, and with proper setup of the mild OE valve springs they will rev to at least 6500 before lifter pump up.

Back in the day the Duntov cam was considered a "racing cam", but it (and other OE mechanical lifter cams) are not a good choice for today when most guys are just cruising around town and need good driveability. For them the best choice would be the modified Duntov as described above, the L-46 cam advance four degrees from OE indexing, the 300 HP cam, or the McCaw Special as long as it's accompanied by the proper head massaging. The latter two will provide a smooth idle (18" @ 500), stump pulling low end torque and the McCaw Special with the requisite head massaging will make about the same top end power as the Duntov, but they will not have the Duntov cam "character".

Once idle vacuum reaches about 15" there is no idle "lope" because flow past the throttle plate is sonic and choked. That's why low overlap cams have smooth idle speeds with no evidence of lope, but some guys prefer the character of a high overlap cam with a lopey idle, even though it reduces low end torque and degrades around town driveability because "real" Corvette engines back in the day idled like that.

Duke