First off, WELCOME!
Glad to hear from another Jeeper!
We'll try to give you answers and information that will make you spend your money a little 'Smarter',
And we will try and help you out with sources and explanations of Jeeps and Jeep related problems.
Please understand I'm not ripping on you.
I know you purchased this vehicle and this is what you were told...
What I'm going to do is try and clear up some things so you know what you have and what you are talking about so when you ask for information, or need to order parts, you have the correct information.
This happens all the time (I ran a shop for over 20 years and it's REAL common) and I put another 10 years in with companies like MSD, Holley, Blower Drive Service, ect. before I opened my own 'Hot Rod' engine shop, so I've seen this happen time and time again...
Originally Posted by Pugnacious
Hello all ... I am new to this forum and jeep ownership. I bought a 78 CJ5, 304 smallblock AMC V8...
No such thing as 'Small Block' vs. 'Big Block' when talking about AMC engines...
They are all the same external dimensions and where one comes out, another will drop right back in.
AMC engines have 'First Generation', which is 290, 343, 390, and 'Second Generation', 304, 360, 401.
...w/ performance high-range cam,
'High Range' cam is a mistake at high altitudes, and a High RPM camshaft is a mistake in low rpm trail jeeps, and low mid-range RPM street jeeps.
Higher RPM cams don't work well, and aren't fuel efficient at low and mid range RPM's, and most High RPM range cams don't start working well (Efficiently) until you reach about 6,000 RPM, which most Street/Trail Jeepers aren't going to see unless they miss a shift or break drive line parts in a hard pull!
Edelbrock Street Performer Intake,
Vic Edelbrock doesn't make a 'Street Performer' intake.
They make two or three versions of 'Performer
The standard 'Performer' intake is a very good, Dual Plane
intake manifold, and will work well with most 300 to 360 CID engines up to about 5,000 RPM depending on engine displacement.
The larger the engine, the sooner it's going to run out of 'Intake'...
The next step up is the 'Performer RPM
' intake, which has slightly larger intake runners and a larger plenum and will allow the engine to make power up to about 6,500 RPM, which is about the limit of mostly 'Stock' engines.
These are horrible for 'Trail' jeeps since they don't have very good idle and just 'Off Idle' characteristics.
There is a 'Performer Air-Gap
' which again, works great on mid range engines up to about 6,000 to 6,500 RPM depending on engine displacement, but they don't work worth a darn on low RPM 'Trail' Jeep engines.
Holley 4 barrel 600 CFM carb (re-jetted for the higher altitude,
A 304 CID engine in a Jeep is a 4 stroke engine, so you automatically have to divide by two, since only 4 of the 8 cylinders will charge (have an 'Intake Stroke') on each revolution...
304÷2=152 Cubic Inches Of Displacement (CID)
Consider even the most well equipped engine will only run about 90% Volumetric Efficiency (VE).
And your engine is using stock 304 heads and running at high altitued through an air cleaner, figure it (liberally!) at 85% VE...
152x.85% VE=129.2 CID per Revolution.
Now, figure your RPM limit on cast iron rods and cast iron crankshaft at 5,500 RPM...
That's 5,500RPM x 129.2 = 710,600 CID per Minute.
Now, to bring that into perspective, 1,728 Cubic Inches in one Cubic Foot.
710,600 ÷ 1,728 CID = 411.226 Cubic Feet per Minute (CFM)
Since your engine only requires 412 CFM to operate under optimum conditions at 5,500 RPM, why buy a 600 CFM Carb?...
Heddman Headers &
Flowmaster true dual exhaust.
Headers and full duals without a cross over is a VERY good way to have Right/Left fuel management issues.
Do your headers have long tubes?
The longer the tube, the more Torque that cylinder will produce.
If you have the 'Shorties', you might have well stayed with the stock exhaust manifolds...
It is running too rich. Many people are telling me the 600CFM carb is too large for the 304.
Not at altitudes of 4 or 5 thousand feet...
'Air' is much thinner, meaning there are less molecules of oxygen in the 'Air' at that altitude.
That means you will need more 'Air' for the same amount of Oxygen at a lower altitude...
So in this case, a 600 CFM shouldn't be too big for proper tuning.
Some things you didn't tell us,
1. Did YOU
do the 'Tuning' on the carb, or did some guy/goofball/kid at the 'hot rod' shop that couldn't keep a job at McDonalds do the 'Tune'?
2. Is the carb a 'Double Pumper'?
Or is it a Vacuum Secondary?
3. What is the 'List Number' on the front of the choke air horn?
Assuming your did you own 'Tune',
4. What were the vacuum readings when you achieved the highest steady vacuum for your idle mixture?
5. What is your 'Part Throttle Cruise' vacuum reading?
6. What Jet Size & Power Valve are you running?
7. If you didn't do your own tune, do you need directions on how to tune your carb so it works better?
If so, take the 'List' number on the choke air horn to the parts store, and get some float bowl gaskets and metering block gaskets, and find/beg/borrow/buy a vacuum gauge, and I'll tell you how to tune your carb.
Has anyone had success using this carb, or should I drop down to the 390CFM carb?
390 would be too small by far at your altitude, stick with the 600 and tune it.
Assuming the floats are correct and the jets are correct or close (about 4-5000 ft)... can this 600 be made to work?