67 L-71 Idle Woes

[Bill Becker]

My 67 435 has been back together now for about a year and a half. Now that it's been judged a few times, I'm finally starting to drive it. My problem lies in when the motor has had a chance to get hot, it won't idle worth a hoot. In fact when I come up to a stop sign and push in the clutch it just dies. I'll have to let it sit for a few seconds then try the starter and it'll finally start, but not very convincingly. She runs great when cold. Float levels are all OK. I'm beginning to think the motor doesn't have enough vacuum to pull the power valve closed. I rigged up my trusty mighty vac so that I could read it while driving the car. When motor is cold it reads: 700- 7-8" 800-9", 900-10", 1100-11", 1200-12", 1500-15, 2000-20". When motor is hot, it won't idle below 1000. Cam is Crane's version of #143. Timing is set at 5 degrees. Should I have more vacuum? Any and all insights would be welcom at this point.

Thanks,
Bill Becker

[Jay Hastings]

Re: 67 L-71 Idle Woes

Bill-

check ALL your intake manifold bolts and all the carb bolts. that alum. manifold likes to loosen up. I had to do this many times

Jay

[Everett Ogilvie]

Re: 67 L-71 Idle Woes

I had similar hot issues on a previous tri-power. Generally when they run good cold and bad hot it means you are RICH. My carbs had been kitted but they were still weeping fuel which made it rich at hot idle. It would load up and you could barely keep it running - it would clean up a bit when rev'd, but it would load up again at idle. When I shut it off (hot) I could hear fuel dripping into the intake from all 3 carbs (float levels were good). The fuel weeping is usually caused by warped carb bodies/leaking gaskets - I believe it is called "Holley-itis".

[G B.]

Re: 67 L-71 Idle Woes

Bill -

I think Everett is right. Every original Holley I've encountered has had a warped gasket surface on the main body facing the metering block. They can be machined or filed flat again. I use a special jig fixture to hold the main body while I resurface with a large pneumatic file. Most machine shops can't do this resurfacing on their milling machines because they have no way to hold the Holley body. Do not let them use a belt sander for this job.

The idle vacuum figures you quoted seem low to me. I was expecting you to say about 10 - 12" at 800 rpm. Maybe it's your gauge. Or maybe it's the Crane cam. The stock power valve on a #3660 Holley opens at 6.5". Could be someone installed a higher rated valve though.

[Bill Becker]

Re: 67 L-71 Idle Woes

I can't imagine the carbs being warped as I had the full bore, drop yer pants when ya get the bill restoration job done by Jerry Luck. I really don't want to dive into them again. If I'm only getting 7-8" at 800, what about going to a 45 power valve? If I do have a vacuum leak, shouldn't the vacuum gauge be jumpy?

Bill

[G B.]

Advice you don't want to hear

I now have faith in your carburetors too. I believe Jerry Luck routinely machines the main body gasket surfaces. I wouldn't expect warpage on any carburetor he has prepared. And for the record, I think his labor charges are quite reasonable for the work he performs.

You can certainly try a 4.5" power valve and see if that cures the problem. Personally, I would go after the root cause of the low vacuum myself. I never cared much for those paper thin base gaskets GM used below the two barrel Holleys. But, more than anything else, I'm suspicious of your cam profile. Did you get the "blueprint" type reproduction cam or the "sorta similar" Crane hot rod version? If you've already sealed off all the possible vacuum leak sources and you've still got low vacuum at idle, I'd sure look hard at the cam.

I got into a tail chasing episode with a 454 a few months ago. A shop in Houston had installed a supposedly stock 390hp hydraulic cam, but the engine only had 13" vacuum at idle. To get an 800 rpm idle speed, I had to crack the throttle plates open more than normal. This exposed most of the idle transition slot in the primary venturis, requiring me to turn in the idle mix screws all the way in to lean the idle mix. This caused the car to stumble when the throttle was cracked open quickly... Well, I'm sure you get the picture. I finally convinced the owner to pull the cam, and guess what? It was a "mild" hot rod hydraulic with as much lift and duration as an L-88 cam.

There are a few easy checks you can do before attacking the cam. I'd try a different ignition coil. They're very heat sensitive when they're old and about to fail. I'd also get a new load of racing gas to make sure the fuel isn't perculating in the bowls at 180 degrees. Lastly, I'd make sure the exhaust is unobstructed so there isn't excessive heat crossing over inside the intake manifold.

[Duke W.]

SHP Big block idle problems

Nothwithstanding the Holley carburetor issues, I believe that SHP BB idle problems can be traced to a poor spark advance map that is not suited to the engines' operating characteristics. Chevrolet had a history of not getting this right IMO.

My 1963 L-76 has a history of idle problems. It had poor idle stability. It would load up and start chugging and sometimes even stall. It took me three years to figure out the problem. I was 19 at the time in 1966.

The OEM vacuum can started at 8" and provided full vacuum advance at about 16". Trouble was, the engine would only pull 10-12" at idle and it would vary. This would cause the vacuum can plunger to "dither" - varying the advance, and as idle speed dropped, so would vacuum, so the more advance was lost, which would cause more loss of idle speed. It was a positive feedback loop from, which there was no recovery, and it was worst in hot weather.

In early 1964, Chevrolet released new distributors for SHP/FI engines with a vacuum can that started at 4" and provided full vacuum advance of 16* @8". Once Once I "upgraded" my distributor to the new SHP SB specs my idle problems away. Because the new vacuum can remained pulled to the limit at idle, idle igntion advance was constant, and about 28* with 12* initial timing. The idle became stable, and idle quality improved - no more problems!

As I continued my education in internal combustion engine research I came to realize the cause of the problem. The requirement for stable total idle advance is intuitive, but how does one determine the optimum total idle advance? Experimentation is how, but why do SHP engines seem to want so much idle advance- as much as 30 degrees? The answer is that high overlap cams cause a lot of exhaust gas dilution at idle and low speed operation. During the overlap period the exhaust port is at atmospheric or a bit higher pressure, but the inlet port is below atmosheric, so exhaust gas migrates up the inlet port diluting the fresh charge. Exhaust gas diluted mixtures have slow flame propagation speed and need more spark advance.

From my Corvette News collection I extracted the following data for SHP big blocks from '65 to '67. In order are the vacuum advance start vacuum, max vacuum advance at vacuum, max centrfugal at RPM, and initial timing. All advance specifications are in cranshaft degrees, not distrubutor degrees.

L-78: 0 @ 8"; 15 @ 15.5"; 28@3800; 10

L-72: 0 @ 6"; 15 @ 12"; 30@5000, 8 (8-14 range)

L-71: not avail; 15 @ 15.5"; 30@3800, 5

From the above you can see that they kept changing the spark map even though all these engines were basically the same, Assuming these engine have idle vacuum about the same as SHP smallblocks - 10-14", the vacuum cans are not properly matched to the idle characteristics. Why did they get this wrong after finally figuring it out and correcting the vacuum advance map on SHP small blocks? I don't know. It's a mystery.

I believe you can considerably improve the idle quality of SHP BBs by installed the SHP SB vacuum can that was used on L-76/79/84 engines from early 1964 up that provides full vacuum advance - 16*@8". Use the L-72 initial advance range to achieve total WOT advance (initial plus full centrifugal) in the range of 38 to 42 degrees. This initial timing plus the full 16 degrees vacuum advance will yield total idle timing in the mid to high twenties, which is what the engine needs. The resulting higher idle vacuum will keep the vacuum can pulled to the limit and keep the power valve from opening.

Take the recommended idle speeds with a grain of salt. Chevrolet's recommendations are too low, and I recommend you use my rule of thumb to set "idle speed at the lowest speed commensurate with acceptable idle stability and quality", and that will probably be about 900 RPM.

In addition to improving idle quality, the 8" vacuum can will reduce high operating temperature because the engine is idling more efficiently and rejecting less heat to the cooling jacket. I believe that a lot of BB idle and low speed hot running probems can be traced back to errors in the spark advance map.

Duke

[Chas Kingston]

[Jim T.]

Re: 67 L-71 Idle Woes

I would check the initial timing. I don't have 427 tuning experience, but when I put my ZZ1 GM crate engine in my 70 I increased the advance to 12 degrees BTDC to give better idle quality and I do use a 4.5 power valve in my Holley double pumper.

[Bill Becker]

Excellent Discussion

I appreciate the response guys! A lot to think about.

Jerry, The cam that is in the car is Cranes # 3863143 "factory replacement". According to the card it has 520 lift, and 242 duration. The coil is about a three year old Goat Hill look alike where he stuck a new coil for a 67 TI in a #263 can. Don't know what kind of coil I would try anyway. As I have been battling an overheating problem, I previously removed the heat riser with a straight through spacer, and am currently running straight 110 octane race gas. Also, I didn't mean to imply that I thought Mr. Luck was too expensive, just that, with this car I have tried to put correctness first and cost secondary.

While searching through the archives, I had found a post where soemone listed there L-71's vacuum which was 14" at 800, 15" at 1000, 19" at 1500 and 21" at 2000. While mine is similar at 2000 (20"), I'm way off at 800 (9"). I'll have to go over the vacuum connections with a fine tooth comb.

Duke, are you saying that A.) assuming that there is nothing wrong with my carbs, and B.) assuming I do not have any vacuum leaks, that, by changing my vacuum can to one that is 16@ at 8", and combining initial advance for a total in the mid to high twenties (at idle), that this could possibly give it MORE vacuum and close the power valve?

Thanks,
Bill Becker

[Duke W.]

Re: Excellent Discussion

Not "possibly", but DEFINITELY assuming there are no vacuum leaks, or some other mechanical problem like a leaky exhaust valve.

Try the following experiment. Disconnect and plug the vacuum can, loosen the distributor and start increasing the initial timing. As timing increases idle speed and vacuum increases. Increase the initial to about 25 to 30 degrees, then set the idle speed at about 900 and go through the idle speed and mixture adjustment and iterate as required. Then take a vacuum reading.

You should observe better idle quality and stability and better vacuum. If your pleased, you can buy a NAPA/Echlin VC1810 can (the one I recommend on SHP engines with 16* @ 8") and install it with about 12-14 intitial. Just don't forget to set the initial back lower before you drive it. If you are running straight 110 octane gasoline, you should be able to run 42 degrees total WOT timing (initial plus full centrifugal) and a quick advance curve without detonation.

I don't have Crane specs, but I am suspicious of aftermarket OEM clone cams. I think some if not all are ground on tighter lobe centers, which means they have more than OEM overlap, and any increase in overlap is going to loose manifold vacuum and cause idle stability problems, especially with the OEM vacuum can specs, which are not even aggressive enough for the OEM cam.

Duke

[G B.]

Thanks for the feedback

Bill -

I appreciate your responses to my posts. Often on this board the original poster justs drifts off into the night without ever addressing the comments he has received. I get cheesed off when that happens. To me that's like asking for help and then turning your back and walking away when someone tries to get involved in your problem.

The GM L71 cam had valve lifts of .496" intake and .492" exhaust, but you probably already knew that. I don't know the stock cam duration at .050" lift, but I'll bet it's less than your cam too.

You might be able to get more vacuum by increasing the valve lash a bunch. There's nothing wrong with Duke's suggestion to go to full time vacuum advance and a soft distributor canister. I don't think either of these suggestions will get you where you ultimately want to be, however. You've already spend a good deal of time and money to get the car done right. I'd bite the bullet.

[Duke W.]

Re: Thanks for the feedback

I agree with Jerry. It's always good to get the feedback after the owner trys out some suggestions, even if it's weeks or months later (just start another thread and give Rx and results). Then, hopefully they can advise the next poster who complains about the same problem.

Poor idle quality and low idle vacuum is a fairly common complaint along with hot running at low speed.

Duke

[John M.]

Re: SHP Big block idle problems

Duke, Isn't it true that the L71 Tripower engines use venturi vacuum for the vacuum advance and that there is, therefor, little or no vacuum advance at idle?
John

[Duke W.]

Re: SHP Big block idle problems

As far as I know L71 was designed to use full manifold vacuum to signal the vacuum advance, and if it is other than this, I would convert to full manifold vacuum signal.

Using venturi vacuum to signal the vacuum advance would result in full vacuum advance at WOT and high revs, which is DEFINITELY not what you want.

The L71 does use the center carb. venturi vacuum signal to open the end carbs. Perhaps you have the two confused.

Duke