Looking for real information on jetting a 350 cube, mild cam engine with a 600cfm Holley. We're talking about 6,000 to 8,800 feet elevation. Such as Central City (HRHC). I drive from 600 feet elevation & when I reach Denver it feels like someone ties a trailer to the car. The car is pretty sluggish on the hill, I know it could do better when jetted correctly.
No way to make it run at 6K and 600 feet both best you can do is get a trade off. At altitude you can't get enough air into your engine so you have to lean it out to compensate at lower elevations it will be running lean unless you pull the choke. What is going to work in both places neither one well is to jet it about half way up the hill. Something we used to do when crossing a large grade was to pull the air cleaner element. Top the grade then put it back in.
If you're running good at sea level drop your jet size by 3 (600 ft), that will be a good starting place for ya. I must have missed something I understood that you were going back and forth that is why I gave you a trade off. There is another option, you can get an adjusto jet for your Holley. I imagine that it probably runs about 60 buck these days. It makes your main jet externally adjustable and it will make it way easier to get yourself where you want to be. When you hit a decent number for a specific altitude write it down, keep the info in your glove box. Then when you decide that you need to go to the valley or back to the mountain it is just a matter of turning a screw.
Beaner- all good info. Thanks. I would change jets on arrival to the high country, then after 3 or 4 days, change jets again for the return trip. I would think someone from the area would chime in with real time information. Trying to eliminate the experimentation, make it a one time deal. This engine combo should be very common.
If or when you get to that point do two things in your favor. One is buy an actual jet tool ( about 7 to 11 bucks) it will save your jets and the other is even if you are using the blue reusable gaskets, rub them down real good with chap stick to keep them from sticking and getting torn when you drop the bowl. Oh the two stage power valve that @Ricks Garage mentioned is a damned good idea. I never even considered it.
We were in Central City for the Hot Rod Hill Climb....my flathead with 2 Strombergs ran fine at some rpm....but IF I touched the choke, it fell flat. In the old days, my 70 Z/28 liked the Holley to get cut about 2-3 jet sizes as porknbeaner says. Power valve size didn't make much difference. Attention to the choke was imperative....cold temps cause the auto choke to work....and altitude isn't choke friendly. As a rule of thumb for our dirt bikes, if the Mikuni jet was 200 at low altitude, we cut it to 160 at 9000 ft and paid attention to the head temperature on the motor.
And then for more fun add in that youre more than 1k higher at central city than you are in Denver. Even my EFI system on my stroker runs with less power there. less density to the air, everything runs worse.
Another thing to consider is at elevation your ignition timing could possibly use some adjusting. I live at 6000' and always give my cars a bit more advance than what is recommended in books.
I used to do that with my Harley when I was crossing the divide. I actually had to remove the carb cover and advance my spark about another 4 or 5 when I decided that it would be more fun to cross Angel Pass than to use the Eisenhower tunnel.
Took my old cj7 from home in Kansas at 1000' to 13,000+ hard to keep her going even with my re-jetted altitude carb. We took the cj to Colorado 2-3 times a year, I had a second carb I would install prior to leaving home, change it out when we got back. Loved the old CJ, but the 03 Rubicon don't care where ya go.
We took our old bump and bash truck to the top of pikes peak ( the end of the road?) just to say that we did. That old 318 was pretty weezy at that altitude. When I was in Mexico we used to go once a year to this Tlaxcala once a year then from there to this resort that was about another 2K above Tlaxacala. About 13K as I recall. I remember towing people in the group with my old blazer and was only able to do that in low lock. But the upside was that I didn't have to tow them back down.
Thanks, that's where I'm headed. On my past two trips out there I did bump the timing for some help. Thanks all, for the info.
Create a vacuum leak under the carb... i.e. the intake. You would have to play with different size of leaks to find a happy spot, but I think you could use a solenoid similar to a NOX that could be turned on at altitude to allow air in that will lean it out a bit. Several carbs are out there that were sold in high altitude markets that had altitude compensators that did just that. They also had a dual diaphragm distributor that advanced the timing a few degrees at the point where the alt comp valve opened. Some imports had a stand alone valve to do the same thing, but probably wouldn't create enough of a vacuum leak for a V8. Sent from my Moto G Play using The H.A.M.B. mobile app
Not a lot in the Holley manuals on rejetting the 4150/4160 carbs for altitude. Going further back to the 4000 series, Holley suggested a 0.001 smaller jet at 5000 feet, and another 0.001 smaller jet at 10,000 feet on the primaries only, with no change to either the secondary or power systems. I would be surprised if this is enough, but Holley would be conservative. Too rich is much better than too lean. Just food for thought: Carter did a LOT of research on altitude jetting as did Stromberg. Stromberg made a four-barrel, but not so large as a 600. Carter did offer AFB carbs in this size, with step-up rod recommendations for various altitudes. It requires about 2 minutes to change a set of step-up rods from outside the carburetor. While I don't recommend the procedure, to prove a point, I have changed the rods while the engine was RUNNING. Not suggesting you change this carb to a Carter, but food for thought if you have to replace the carb for other reasons, or building something else. We have fabricated LOTS of custom rods for Colorado visitors AND residents. Jon.
I think the AFB (Edelbrock) is a good carb, have used them before. But, this trip is with a Holley. Not a big deal, only takes a few minutes to do a jet change, as long as you don't drop the "O" rings Again, THANKS.
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