I have a 1965 L75 that was rebuilt about 8000 miles ago. It has a Crane HMV-278-2 cam with 278 degrees duration on the intake & 290 degrees on the exhaust. Essentially it is one cam hotter than a 350hp cam and it is still a hydraulic. The car runs very strong, but has had a surge only around 1500rpm. The car has not been driven much at all over the years & lately I have been driving it more & have noticed this surging and have convinced myself that I may have a problem. The idle is pretty lumpy and I always thought it was due to the cam, but now the engine surge off idle has me concerned. I decided that I have had it with this surge and I started trouble shooting the engine. When I measured engine manifold vacuum I was getting around 15 inches of mercury, but the vacuum gauge needle was in a dither, it was vibrating at a high frequency thru a swing of 4-5 inches of mercury. I also measured the ported vacuum and it was around 7-10 inches. I don't think this dither is normal, it should be closer to rock steady. I also checked for vacuum leaks in the power brake boost unit, in the distributor advance can, under the carb base, and the intake manifold seal with none found. I also checked the advance: centrifugal, distributor can, and overall advance with my timing light. They all seemed OK. I intend to run cylinder compression tests next to see if I have a low one. Sorry for being so long winded, but I feel my surge is somehow associated with this vacuum gage needle vibration and is somehow causing this surge as the carburetor leaves idle. I also substituted another near new carburetor that I had on the shelf and I still had the surge. Does anyone have experience with an unsteady vacuum?
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Re: L75 Vacuum Reading
Ported vacuum? On a '65?
If it doesn't already have a SHP 8" VAC that's the first thing you need to install. The OE VAC is not suited to a high overlap cam. If the carb is the original AFB the VAC signal line should show full manifold vacuum. If not...
Duke -
Re: L75 Vacuum Reading
Harold,
Wow, that's a big cam,; it sure ain't an L75 anymore! More duration on both intake and exhaust than both the first and second design "off road" (140 and 754) GM cams. What are the rest of the specs on this cam? What is the lift, lobe separation, etc? What is your idle speed, because with all that duration, I don't expect 15 in-hg idle vacuum.......at least not below 1200 RPM. If you still have your original Holley 2818 carb installed, or facsimile thereof, then there will be NO provision for ported vacuum!
If the vac gauge flutters rapidly, then this is an indication of a leaky valve. If the gauge slowly wanders back and forth, thru the range that you indicate, then that indicates a possible vacuum leak. If you've got a vac leak, then you might be experiencing a "surge" due to a lean mixture, due to a vacuum leak.
Give us the answers to these questions first. After that, you should carefully go thru your distributor's advance characteristics...both vacuum and centrifugal.
JoeComment
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Re: L75 Vacuum Reading
He said it's a '65 L-75, so the original carb would be an AFB.
Like most "cam specs" offered up, the durations he quoted don't mean jack sh... because they don't include a lobe or valve lift context, nor is any indexing/lobe separation info offered.
No context, no details, no dice!
DukeComment
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My response to your...
...e-mail got bounced.
DukeComment
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Re: L75 Vacuum Reading
Correctamundo about the Carter.......I was thinking L79. The 300 horse job has the aluminum four barrel......still, no ported available.
Those durations that he listed were probably at J104D. Again, I was thinking @ .050 lift. With that in mind, it looks more like the L79 cam, as he said.
Thus, 15 in-hg @ about 700-800 sounds about right.
JoeComment
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See My Response to Duke.........
........for correction. You should have AFB, and the cam specs are probably closer to those of the L79. Found the specs for your camshaft:
Durations @ .050 lift are slightly longer than L79 cam, and lifts are higher also. Sounds like a nice street cam!
JoeComment
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It's SAE J604d...
...which is valve timing at .006" valve lift, which is .004" above the base circle on a hydraulic lifter cam or .004" above the tops of the clearance ramps on a mechanical lifter cam, assuming the true rocker ratio is 1.5:1. It's not, but who sweats the details...
DukeComment
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Re: L75 Vacuum Reading
I took my distributor in today to have it recurved to match my Crane 278 camshaft specs. They also replaced my vacuum can which was a 238-24 (I assume the original can). The replacement can has less travel. The surge I was experiencing is now history & the car runs so strong that I can spin the rear wheels from a roll on in 1st gear just like in the old days with leaded fuel. Thank you Joe & thank you Duke for the help.Comment
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Re: L75 Vacuum Reading
In '64 the 250/300 HP engines had a 24 degree VAC, and I think it carried over to '65. In '66 they went back to a 16 degree VAC.
A 24 degree VAC is too much, even for the slow OE centrifugal curve. A sixteen degree VAC seems to work best on all SB and BB engines, as long at it's max advance vacuum is properly matched to idle vacuum.
DukeComment
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