Timing Question
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From: McKinney Texas
Timing Question
I performed a tune up (plugs, points, wires, etc.) on my '65 327/350. I set the dwell to 30 using a dwell meter and am getting about 18 steady inches of vacuum (bounces a little between 17-18 due to the cam). I set the idle at 750 rpm and the engine runs smooth. Haven't touched the idle mixture.
When I put a timing light on it the timing mark is completely out of the range of the tab in about the 12 o'clock position. If I try to dial it back to get it into the range of the timing tab (8 degrees) the car misfires and stalls. I am a novice mechanic and have little experience tuning old cars. Any advice? Or if it runs fine should I leave well enough alone?
When I put a timing light on it the timing mark is completely out of the range of the tab in about the 12 o'clock position. If I try to dial it back to get it into the range of the timing tab (8 degrees) the car misfires and stalls. I am a novice mechanic and have little experience tuning old cars. Any advice? Or if it runs fine should I leave well enough alone?
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You have to disconnect and plug the vacuum advance signal line to check initial timing.
You also have to make sure that you check initial timing at a speed low enough that the centrifugal advance has not started, which will usually require revs to be under 700.
Verify where the centrifugal starts by slowly bringing up the revs to see where the advance begins to increase. You should check and set initial timing at least 200 revs below this point.
Eighteen inches of manifold vacuum (vacuum can connected) @ 750 is high for L-79; 14-15 is more typical.
Duke
You also have to make sure that you check initial timing at a speed low enough that the centrifugal advance has not started, which will usually require revs to be under 700.
Verify where the centrifugal starts by slowly bringing up the revs to see where the advance begins to increase. You should check and set initial timing at least 200 revs below this point.
Eighteen inches of manifold vacuum (vacuum can connected) @ 750 is high for L-79; 14-15 is more typical.
Duke
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From: McKinney Texas
Thanks Duke. The vacuum advance was disconnected and plugged when I hit it with the timing light. I'll try backing off the idle a bit and check the centrifugal advance as you describe. With respect to vacuum, I thought 15-20 inches was normal.
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In normal configuration with the vacuum can connected, the following are the typical vacuum @ idle RPM.
300 HP cam - 18"@500
L-79 cam - 14"@750
Duntov and LT-1 cams - 12"@900
30-30 cam - 10"@900
At a constant idle speed the more valve overlap, the less manifold vacuum, and high overlap cams need higher idle speed to achieve acceptable idle quality.
You should verify that the piston is actually at TDC when the balancer notch is at zero on the timing tab. Your timing tab should be spot welded to the timing chain cover. In 1969 Chevrolet switched to a timing tab that is bolted to the cover with a couple of the cover screws and the notch on the balancer moved about 10 degrees. Pre '69 balancer/tab cannot with mixed with '69-up balancer/tab.
Duke
300 HP cam - 18"@500
L-79 cam - 14"@750
Duntov and LT-1 cams - 12"@900
30-30 cam - 10"@900
At a constant idle speed the more valve overlap, the less manifold vacuum, and high overlap cams need higher idle speed to achieve acceptable idle quality.
You should verify that the piston is actually at TDC when the balancer notch is at zero on the timing tab. Your timing tab should be spot welded to the timing chain cover. In 1969 Chevrolet switched to a timing tab that is bolted to the cover with a couple of the cover screws and the notch on the balancer moved about 10 degrees. Pre '69 balancer/tab cannot with mixed with '69-up balancer/tab.
Duke
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From: McKinney Texas
The block is stamped HT, so I assume it's an L-79. Can't explain the high vacuum. I tried dropping the idle and by rotating the distributor counter-clockwise got the timing to back off a little but still does not fall back down into the range of the timing tab.
I will check to make sure that #1 is at TDC when the mark is at zero and go from there. I'm beginning to learn that it's never easy with these things. Incidentally, the timing tab is spot welded on and is marked 0 with numerous hash marks through A. How many degrees does each hash represent?
I will check to make sure that #1 is at TDC when the mark is at zero and go from there. I'm beginning to learn that it's never easy with these things. Incidentally, the timing tab is spot welded on and is marked 0 with numerous hash marks through A. How many degrees does each hash represent?
Safety Car
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From: Fountain Hills AZ
You need to check what your total timing is. It's not unusual for an old performance car to have had some mods made to it's advance curve (or for there to be a problem with the advance mechanism). A stock L79 distributor would have 26º centrifugal advance at about 4100 rpm, giving a total advance of 36º with 10º initial. Vacuum advance adds another 16º on top of that at cruise. With the vacuum advance disconnected, rev the engine until you're sure that the distributor is through advancing and set the timing at 36º. If this results in too much or too little initial, recurve the distributor. Many of us have found that a quicker curve than stock provides better low-mid range performance. In my experience, something like 16º initial with 20º centrifugal (for 36º total) is a good recipe.
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Wrong board! That's a '68 (C3) L-79 distributor, and all '68 engines were emission controlled and have very different distributor calibration from pre-emission contolled engines.
All '65-'67 L-79 (except maybe K-19 AIR) have max centrifugal as I stated previously and a Delco "236" vacuum can, 0@4", 16@8"
Duke
All '65-'67 L-79 (except maybe K-19 AIR) have max centrifugal as I stated previously and a Delco "236" vacuum can, 0@4", 16@8"
Duke
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From: Westport CT
Originally Posted by SWCDuke
In normal configuration with the vacuum can connected, the following are the typical vacuum @ idle RPM.
300 HP cam - 18"@500
L-79 cam - 14"@750
Duntov and LT-1 cams - 12"@900
30-30 cam - 10"@900
Duke
300 HP cam - 18"@500
L-79 cam - 14"@750
Duntov and LT-1 cams - 12"@900
30-30 cam - 10"@900
Duke
Drifting
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From: Allen Park Mi
I used to get 15 inches of vaccuum on a 30-30 cammed 302 with 11 to 1 compression in both of my 69 Z28s.
Neither of them felt like a freak of nature either. Just as strong as any other Z i've ever been in. And those had 800CFM carbs.
It would stand to assume that more cubes would even pull more vaccuum.
But I tune my motors to my ears and performance... not stock calibration and they always seem to run better. FWIW.
Unless both of my vaccuum guages are junk, those are accurate numbers.
That was at 13 degrees initial timing and 38 WOT.
Neither of them felt like a freak of nature either. Just as strong as any other Z i've ever been in. And those had 800CFM carbs.
It would stand to assume that more cubes would even pull more vaccuum.
But I tune my motors to my ears and performance... not stock calibration and they always seem to run better. FWIW.
Unless both of my vaccuum guages are junk, those are accurate numbers.
That was at 13 degrees initial timing and 38 WOT.
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From: Westport CT
That's reassuring. I should also point out that the p.o. from 81 to 95 advised that "the previous owner [the guy before him] had the engine blueprinted." Of course, "blueprinting" an engine is an overly used and often ill-used term, but the car has 15,000 miles on it since 1981, so the vestigages of any tightening up work might still be present, if that is relevant to the vac reading issue I asked about.
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Originally Posted by ctjackster
Duke, others - if one was to observe well above 10" @ idle (more like 14") on an [assumed] L76 w/ 30-30 cam, what would that mean? That the 30-30 was swapped out at some point? Is there another possibility?
Duke
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From: Westport CT
Originally Posted by SWCDuke
I would opine that it doesn't have the original type cam.
Duke
Duke
Can anything be confirmed visually, with the valve covers off? (I am willing to be I will measuring durations soon enough . . . .)
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Place a dial indicator on the top of the rocker arm pushrod socket, as close to inline with the pushrod as you can eyeball, and measure total lobe lift (base circle to max lift). The 30-30 lobe (same on inlet and exhaust side) is .323" and you should also notice very long clearance ramps that appear to be about .020" high (but are actually .017" when you look at the cam lobe engineering data.
Base circle runout can be +/- about a thou and there could be a few thou wear on the nose or a few thou less due to the dial indicator being slightly out of line with the pushrod, but if you measure several lobes (both inlet and exhaust) the data should give you a pretty good indication of whether it's a real 30-30 cam or something else.
If you zero lash the valve and use a second dial indicator on the valve retainer, parallel to the valve axis, you'll generate a data set the will allow computation of rocker ratio behavior through the entire range of valve motion.
Duke
Base circle runout can be +/- about a thou and there could be a few thou wear on the nose or a few thou less due to the dial indicator being slightly out of line with the pushrod, but if you measure several lobes (both inlet and exhaust) the data should give you a pretty good indication of whether it's a real 30-30 cam or something else.
If you zero lash the valve and use a second dial indicator on the valve retainer, parallel to the valve axis, you'll generate a data set the will allow computation of rocker ratio behavior through the entire range of valve motion.
Duke
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From: Mobile AL
30 - 30 cam vacuum
I get 12 - 13 inches of vacuum at an 850 rpm idle speed with the 30 -30 cam in a stock '64-5 Corvette FI 327. The 097 cam in my '63 FI 327 makes 13 - 14 inches at an 800 rpm idle speed.
The vacuum levels produced by these cams are very sensitive to lash settings and idle speed. If you're one of those who sets the lash tight, you're going to kill bottom end vacuum and power. But I guess you think it's worth it for the extra power you feel at 8,000 rpm.
The vacuum levels produced by these cams are very sensitive to lash settings and idle speed. If you're one of those who sets the lash tight, you're going to kill bottom end vacuum and power. But I guess you think it's worth it for the extra power you feel at 8,000 rpm.
Le Mans Master
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From: Westport CT
Thanks Duke for your advice. And thanks Jerry (as always) for your wisdom too. By the way Jerry, I have not yet checked or set the lash on my solid lifter (using the "cold" technique of Duke / John) so for all I know it may be "loose" - would that be a contributing factor?
First thing I'll do is recheck the Vac readings with another Vac gauge - I recently had the opportunity to observe intake manifold vacuum while setting up a new to the engine carb, and I noticed the readings then (I was more focused on obtaining highest manifold vacuum at that point, rather than the numerical value of the reading itself) but my recollection of the values observed, together with your post on idle vac readings for various 327 configurations, led me to ask. Before I freak out I need to confirm the vac readings. If they hold up (as high), then it's on to setting the valve lash, on my list anyway. Then I'll re-check the mani vac at idle and see if then need to wonder if my L76's 30-30 cam has gone missing . . .
First thing I'll do is recheck the Vac readings with another Vac gauge - I recently had the opportunity to observe intake manifold vacuum while setting up a new to the engine carb, and I noticed the readings then (I was more focused on obtaining highest manifold vacuum at that point, rather than the numerical value of the reading itself) but my recollection of the values observed, together with your post on idle vac readings for various 327 configurations, led me to ask. Before I freak out I need to confirm the vac readings. If they hold up (as high), then it's on to setting the valve lash, on my list anyway. Then I'll re-check the mani vac at idle and see if then need to wonder if my L76's 30-30 cam has gone missing . . .
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Originally Posted by jerrybramlett
I get 12 - 13 inches of vacuum at an 850 rpm idle speed with the 30 -30 cam in a stock '64-5 Corvette FI 327. The 097 cam in my '63 FI 327 makes 13 - 14 inches at an 800 rpm idle speed.
The vacuum levels produced by these cams are very sensitive to lash settings and idle speed. If you're one of those who sets the lash tight, you're going to kill bottom end vacuum and power. But I guess you think it's worth it for the extra power you feel at 8,000 rpm.
The vacuum levels produced by these cams are very sensitive to lash settings and idle speed. If you're one of those who sets the lash tight, you're going to kill bottom end vacuum and power. But I guess you think it's worth it for the extra power you feel at 8,000 rpm.
The '63 340 HP was an anomoly, too, because the Duntov cam would not generate enough idle vacuum to pull the vacuum can plunger to the stop, which could cause the diaphragm to "dither" - varying the idle timing, which could lead to idle instability and sometimes even stalling.
GM did not get it right on this first year of vacuum advance on SHP/FI engines. My recommendation is to "upgrade" the 1963 SHP/FI timing map to '64 specs and convert the ported FI vacuum advance to full time.
Duke
