Okay....I am ready to re-start my 69 after replacing the gas tank this winter. I know it had one or more vacuum leaks last year before I took it apart (I could hear it (them). I had already replaced most of the vacuum lines in the car. The car (350/350 horse) has a pretty big cam in it now (Comp Cams 305, I believe), and a Holley 750, and I foul the plugs way too frequently (rich). Before I start it up this year, and put a higher power ignition in it, I would like to fix any vacuum leaks I have (it is hard to pinpoint leaks with a loud exhaust while the car is running). Is there a way to test my vacuum lines, such as pumping the vacuum system with a certain amount of air pressure, that won't blow out seals, or damage valuable parts?
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Re: 69 Vacuum leaks
Vacuum troubleshooting techniques have been covered several times and there are multiple threads in the archieves + off-the-shelf booklets on vacuum trouble shooting techniques are sold by several Corvette catalog houses (C-Central, Mid-America, Dr. Rebuild) as well as being in the NCRS library at the Cincy Membership Center. Bottom line is: yes it's possible to find/trap isolated leaks with the engine running using pressure meter and working your way down the lines looking for > 2 psi drops across a given component, BUT you first need a clear understanding of vacuum pathways and system operation before you start. Also, you CAN use a vac pump and do the deed with the engine off, but you have to invest in a sufficiently large pump....
On the issue of running rich, my bet is tinkering with the ignition system to get a hotter spark tain't gonna git you significant relief. With a 350 SB and a MONSTER 750 CFM carb, sounds to me like your only prayer is running wide open throttle (WOT) at the dragstrip! Can't see any way in heck you've got enough native engine displacement to warrant that large of a carb.... -
Re: 69 Vacuum leaks
Jack,
I don't want to get into a big thing about CFM of carburetors as that was covered a week or so ago, but while doing some other research I came across a Corvette News article from Feb/Mar 1970 issue that rates the List 4555 Holley on 1970 LT1s as 800 CFM. That stuck with me because I have not seen numbers for that carb before, and as you know I have a passion for that engine. Actually I am sure I had seen it, but forgotten it - senior moments don't you know. :-)
I know those engines run reasonably well (BG) with that carb on the street, but of course that carb has vacuum secondaries, so may be lots different than indicated above.
Maybe CFM does not say it all?
Terry
TerryComment
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Re: 69 Vacuum leaks..sorry long
AC,
Jack is absolutely correct in that you need to address your rich operation before anything else. If you have a vacuum leak that would introduce extra air to the manifold and make for lean operation. Is the carburetor a Holley? If so, a vacuum leak might lower the vacuum enough for the power valves to operate and give you too much gas. Have you or anyone else checked the vacuum with that cam and chosen the proper power valves to go with the cam, or is this a "let's bolt all these good parts on and make it go fast" operation?
Cap ALL vacuum lines at the intake manifold fitting to eliminate any vacuum leaks and then find out if you are still running rich. If so, address that problem. You can (I think) get some advice here from folks who have been down that path and are better at it then me, but the end point may be that you have too much cam or carburetor (Jacks point) or both. Even if both can work together you will have to "tune" them to each other.
Take the time to resolve your tuning problems then attack vacuum leaks by using vacuum source to pull on small parts of the system at a time. Get a diagram of the system (there was one in back issue of The Restorer) and isolate heater/AC from headlamps, from wiper door and draw down on one part at a time to find culprit. Likely will be multiple places, but organized testing will yield best results. Shotgun approaches will waste lots of money & time. I believe there are threads on vacuum diagnosis in the archives for your viewing pleasure, but isolate the vacuum system completely and resolve your operating problems first.
Terry
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Re: '70 LT-1 Carb
Terry -- I was the one who raised the question of the air flow rating of the 4555 carb; Clem responded that the Holley book showed that carb as 780 cfm. Now, I have not taken a calipers to the throttle venturi bore, but that unit sure looks like a small (600-650 cfm) carb. It has the small float bowls, with the side-pivot float (bigger carbs had end-pivot floats with the larger bowls). Further, it is unlike GM to so oversize a carb. I just wonder if there just might be an error in the literature, here.
Maybe I should take an inside calipers to the throttle bore and compare with someother carbs.
Geezers CarsComment
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Re: we don't need no stinking CFM
Geezer, In my post below directly to AC I suggest the numbers may have been "cooked" in what I quoted from. If you and AC want to measure and compare numbers it is fine by me.
I really didn't want to start this whole thing all over again.
I shouldn't even be advising AC because what I know about carburetors would fit on the head of a pin. All I can do is look up the numbers from the old ones - any mods are way out of my league. I am from the school that if the guys building the cars could have done it better - they would have (well most of the time anyway) :-)
Thanks for bringing it up though.
Terry
TerryComment
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Re: '70 LT-1 Carb
geezer, pri venturi 1 3/8, sec 1 7/16 and throttle bores 1 11/16
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Re: 69 Vacuum leaks
Thanks guys. Just some more info on my car....the carb is a Holley double pumper with mechanical secondaries. I didn't choose this carb, it came on the car along with 2 Q-jets that need rebuilding. The car actually runs quite strong up to the redline while the plugs are clean. I do not have a problem with low speed bogs or other typical overcarburation problems, just the plug fouling problem. I just figured it would be best to fix any vacuum leaks before tackling tuning problems.Comment
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Re: '70 LT-1 Carb
geezer is your corvette a 370HP LT-1? holley lists part# 34R 4857A as the pri float bowl for a 4555 and this is a center hung float type.
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Return of 'stinking' CFM....
There's specsmanship in this episode to a high degree. Q-jet used narrow primary bores with MONSTER spread secondaries, so you only saw the REAL rich end with your foot firmly into the power band. Holley was typically a symmetric setup with secondary operation almost equal to 2X primary.
The 'kicker' in the equation is mechanical vs. vac secondary operation AND cutover characteristics. My '65 396 (somewhat larger displacement than a 350) sported a 3124 Holley rated at 780 CFM with mechanical secondary and there was NO automatic transmission option available (complicates life for a mechanical secondary when tranny is deciding what to do on its own affecting intake vaccuum and mixture needs), but this is similar to the LT-1 environment (no AT allowed until the end of the era in '72 accompanied by CR downsizing).
Bottom line is: (1) vac leaks stoned, (2) get carb tweaked and jetted properly, then (2) use engine exhaust analysis to verify IF over-carburetion is an issue. If so, fouled plugs will continue to 'haunt' the owner until the venturi is properly matched to the engine's current displacement characteristics and owner/operation usage silhouette. It's one thing to run WOT sporadically at the drag strip, another for stop/go driving with an occassional 'whoopee', and still another chore to run wheel to wheel where BOTH stress/performance silhouettes have to be alternatively active....Comment
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Re: 69 Vacuum leaks
holley DPs are very rich on idle because they are race carbs. also a warped metering body will cause this richness. if the bodies are flat you need to restrict the idle feed port in the metering body.
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