High Altitude Carb Adjustments?

[GrillMaster]

Do you think I will have to make any adjustments? Running a Holley 750DP rich on a 351C with a 4 speed trans. Going from sea level to big bear (8000 ft?). Havent taken her in the mountains yet.

Kirk

 

[CDHUNTER]

typical recommendation is to drop a jet size per 2000ft change. So be prepared to do so your your going to run extremely rich. Good luck and have fun.

 

[broncnaz]

Usually not worth the effort for short trips. If your currently running rich at sea level its going to be bad up in the mountains. You may want to consider getting a smaller carb ie 600 or 650 and jetting it as lean as possible at sea level it would probably help or you could swap it on when you hit the mountains. There are adjustable metering blocks availible that are useful to those that frequent the mountains quite a bit saves time and gaskets.
You can also bump up the timing a little once up there it will help.

 

[GrillMaster]

You just take off the float bowl and change the jet? What's my chance of running ok without change? can you buy holley jets at a local auto parts store? This is my first carb car. everyone changes them roadside when they make a trip to the mountains?

 

[CDHUNTER]

I wouldn't change them unless you are experiencing problems. You'll run rich which isn't nearly as dangerous as running extremely lean. I don't go near mountains but I would expect people don't change them every 2000 ft difference. They probably drive in to their destination, change them, enjoy their weekend or what have you, change them back and drive out. If your not going to be driving around considerably while there I wouldn't bother unless it gets to behaving poorly.

You can find jets available around. Hopefully you can find just the ones you need instead of having to buy the big kit.

 

[GrillMaster]

Help me out as I am not very good with engines/timing. To advance my timing for higher elevation I just rotate the distributor a couple degrees (about 1/16th of an inch?) clockwise?

 

[oleguy74]

right direction,but use a timming liteto see exatly what adv you have.then mark dist housing and block for feauture ref.

 

[broncnaz]

Usually I just set mine by ear/feel when up in the mountains a timing light can help but is not required but you may want to mark your current setting so you can turn it back. If you are running rich right now I'd lean it out now and it wont be so bad at elevation yes you just remove the float bowl to change the jets usually only speed shops will have jets although I have seen them at some parts stores might want to fiqure out what you have currently and order some jets I'd order jets in increments of 2 jet sizes smaller than your current jets maybe 3 different sets to cover you. stepping down 1 jet size is not noticeable enough.
I drive from 2400 ft up to 10,000 ft and while down on power slightly its not worth swapping jets for a weekend but my carb is tuned pretty good for 2400 ft.

 

[DirtDonk]

What kind of ignition do you have Kirk? An option for Duraspark setups is the MSD Timing-Control module. It just replaces the stock type module and has an extra wire harness that runs up to the dash with a dial on it that gives you about 10° of adjustment if I remember.
If you decide to go that way, I should still have one here somewhere you could try.

Just how rich are you running now? Powerful-rich, or crappy-rich?
If crappy-rich, you could be hating life at 8000 ft!
With my fairly stock 302, I leaned out the mixture for altitude quickly by swapping in a PCV valve from a mid-seventies 302 Granada.
Not very scientific, but it acted like a semi-controlled vacuum leak and can help if you're running overly rich at altitude.
It's a band-aid for sure. But hey, if it helps...

Paul

 

[GrillMaster]

just slightly rich right now. Leaving today at 3! Didnt get the jets but bringing timing light. I have a 6AL MSD. Timing control module sounds like a good idea! Can I use that with my set up?

Where do you put the PCV valve? Is that just a one way valve from valve cover to intake manafold?

 

[DirtDonk]

If you're using your MSD 6 box in addition to a stock Duraspark box, then yes, you just replace the Duraspark box with this one.
If you're only running the MSD box, then I think they make a Timing Control box by itself (no integrated ignition module) that would hook up to your 6AL.

Are you familiar with how the PCV system works?
You should have a PCV valve plumbed between one valve cover and the carburetor's large vacuum fitting. The other valve cover would have a filtered-air inlet. Either with it's own filtered cap, or a hose plumbed into the air-filter housing to get clean air into the engine.
If you don't, then you could very well be running overly rich just for that reason alone. Depending on how your carb was set up originally.
Oh, and you definitely DO want a PCV system on your engine. No ifs-ands-or-butts in that regard.

If you do have an existing valve, then what I was talking about was simply replacing the existing one for a different one that allows the engine to basically suck more air.
With my 302, it was accomplished by replacing my existing one (not stock to begin with) with one from a mid-seventies 302 in a passenger car.
With your Cleveland however, which one you use would depend on what setup you currently have.
I found mine through accidental experimentation with multiple units. Very likely you could use the same models I did, but I'm only guessing.

Most Ford and GM PCV' valves fall into two sizes of hose barb, and straight or 90° ends. From there they can have a different number (usually 1 or 2) and different rated "pills" which are the actual slugs that are inside and do the opening and closing of the port along with a spring.
I'm currently using an early '70's Corvette 350 version, for normal use, to get the 90° 3/8" hose fitting and the right vacuum signal. The Granada one had the same physical dimensions as the Corvette, but with a different pill rating.

Hope that didn't conuzzle things just before your fun trip (and you ARE going to have fun, no matter what!) but you might want to check to see just what's plumbed into your valve covers. Just for giggles.

Paul

 

[GrillMaster]

not sure which ignition set up I have. I do have a hose from one to carb may have a PCV in the elbow fitting, have to check. Other side has a breather.

 

[sprinksbeme]

On a recent trip to Big Bear I too need a jet change, while searching around town for Holley jets a Hot rodder sugested just creating a small vacuum leak as changing jets for the weekend was a waste of time.

If you have trouble with the carb while there go to the Car Quest auto parts, it is on Big Bear on the east end of town, Bill is a old school parts guy. and knows his holley stuff

Dirtdonk gives good advise, a little more air and she ran good enough not to be annoying.
Marty

 

[DirtDonk]

not sure which ignition set up I have.

No prob. When you get the chance, check it out and see what type of "boxes" you have under the hood and let us know. No emergency for this trip. More important to come back with some good pictures of your smiling face having fun in your Bronco.

I do have a hose from one to carb may have a PCV in the elbow fitting, have to check. Other side has a breather.

If you have a standard elbow fitting, you probably don't have a PCV valve.
Of course, a PCV valve can look like an elbow (since many have that 90° bend I mentioned), but at least they look like a mulit-piece elbow. If yours has one of the all-black plastic elbows stuck right into the valve cover with just a rubber grommet and no oil-filler lid, it's probably not a valve.
Good luck. See if you can snag a PCV, along with some hose, real quick before your trip.
If you don't have time to mess with it before you leave and it runs crappy at the higher altitude, try to find some time around the campfire to mess with it. Good excuse for a couple of trail-buddies to hand you drinks and food and point and laugh and give bad advice while you're stuck under the hood.

Enjoy the trip. And bring back pics!

Paul

 

[GrillMaster]

Was thinking I could just disconnect that hose that runs from the carb to the valve cover. That would create a good vacuum leak and maybe it would help if it turned out to be an issue. Do I need an PVC valve? and WHY?

 

[oleguy74]

To Much Of A Leak.pcv Should Be There.purges Moister And Oil Vapors .just Drive It And See What It Does.

 

[DirtDonk]

Nope. That's too big a leak and it wouldn't run right. And running too lean is not a good thing.

The PCV is important for so many reasons. It's been described in other threads better than I could do it, but basically it helps to keep the inside of the engine clean and free of acids and other things that build up inside an internal combustion engine.
It also keeps pressure down, so if you have excessive blowby or other issues, you won't have oil blowing out the dipstick tube and out all the other places that oil tries to leak from.

Bottom line, whether it's a "smog" item or not, it'll help your engine live a longer and happier life. The PCV system is a good thing.

Paul