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Old 06-25-2012, 06:50 PM   #1
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Altitude Compensating Carburetors

I've been told I need an Altitude Compensating Carburetor for my old 400 CID Chevy engine because of the lack of power going over the top of some of the passes on highway 90 between Seattle and eastern Montana.

Does anyone have any comment about the effectiveness of these units and are they a worthwhile purchase to make?
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Old 06-25-2012, 07:06 PM   #2
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Have you tried having the distributor recurved. It was amazing the power that it gained. Unfortunately I have not been through any mountains.
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Old 06-25-2012, 07:24 PM   #3
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Quote:
Originally Posted by BA-in-Mich
Have you tried having the distributor recurved. It was amazing the power that it gained. Unfortunately I have not been through any mountains.
Where would you get this done? Approximately how much should it cost? How do you know if it's really been done? Not all shops are honest around these parts.
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Old 06-25-2012, 07:32 PM   #4
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Put a after market fuel injector on it. I've never heard of an Altitude adjusting carb. Maybe I've been out of it to long.

Only thing it could do is lean it out as you go up. What you really need is turbo charger or supercharger.
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Old 06-25-2012, 07:41 PM   #5
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I took the distributor to the guy to check out his operation. He also rebuilds the distributor testing machines. When I took it in we went over the distributor, he pointed out the bottom bearing came was a little sloppy. I gave him the numbers I was wanting to achieve ( given to me by Jim Elliott who has since passed ) The unit looked like it just came out of the factory. He told me all that was needed was a lighter spring and the weights stayed the same. I noticed the springs weren't the garaged door springs that Jim E. talked about. Total cost was $125.00 including shipping back. With this and a local guy reworking the carb. this MH has never had the power that it has now + better gas mileage.
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Old 06-25-2012, 07:41 PM   #6
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I've been told I need an Altitude Compensating Carburetor for my old 400 CID Chevy engine because of the lack of power going over the top of some of the passes on highway 90 between Seattle and eastern Montana.

Does anyone have any comment about the effectiveness of these units and are they a worthwhile purchase to make?
None of the passes on I-90 are very high or very steep. Snoqualmie & 4th of July~ 3000', Lookout ~4700', Homestake in MT is highest at 6400' (but its the downgrade going east that is tricky- reduce speed!)

We've been over passes going through Colorado and NM this year that were over 8000 ft with our old 34' Southwind w/Chebby 454 - got a little slow in places, but didn't overheat and we made it over them all.

With a Class C, I'd think you'd do fine. A carb upgrade or a distibutor recurve is not a bad idea, but not solely for a trip over the mountains.
JMHO
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Old 06-26-2012, 08:42 AM   #7
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Altitude compensating carburetors were common for a very short time and rare when they were sold. Your chances of locating one are slim at best. Many people in my crowd have converted the Olds 455 to TBI and it does a better job, but you are talking ~2K$ for the project.

Even altitude mixture compensation won't always get it. Nothing without a turbocharger does well at 10kMSL. We have a well tuned engine at it was doing just fine at 6kMSL on the Blue Ridge Parkway last time we were there.

Suggestions along with Senior Chief, Make sure it is all tuned to spec (at least) if you can find someone to do a distributor curve, that is good. If you have doubts about the carburetor, find a shop that still knows about them (Good Luck There) and have them go through it. It may need alcohol proof parts installed.

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Old 06-26-2012, 09:04 AM   #8
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The only place I have ever seen altitude compensating carburetors is on airplanes.

I have been up and down those hills many times with no problems in a 454 chevy.

Wow 1977 model. If it hasen't been rebuilt it is probably tired. And doesn't that thing have a 3 speed auto? Not much gear to play with.

If you gotta try something a good set of headers would probably do more than anything.
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Old 06-26-2012, 02:37 PM   #9
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Your old 400 small-block Chevy is a great engine that should have enough power to do Snoqualmie Pass OK in your small Class C.

I agree that getting the carb & distributor serviced is a proper first step. The next is the exhaust. When you are pushing the engine hard, does the exhaust sound like a healthy V-8 or like a jet engine taking off? If it sounds more like a swooshing jet your exhaust is severely restricted - either by design or deterioration of the muffler or catalytic converter causing restriction and reduced power. Headers are nice but get the exhaust system right too.
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Old 06-26-2012, 08:03 PM   #10
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Most times your motorhome runs fine correct ?
You are just concerned about a few high mountain passes that you occasionally travel over. Like the others have said a well tuned engine and carburettor will suffice. Of course you could mess around with different carb, intake manifolds, cam shafts and headers to see if there is an improvement.
Or just for kicks try a tank of premium the next time you travel that pass you are concerned about.
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Old 06-27-2012, 01:34 AM   #11
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It's a smog emissions loaded down low compression engine.The heads don't flow worth nothing.I worked on I would guess thousands of them while I worked as a engine tester at the Chevy Engine plant in Tonn,NY.All depends on how much depth $$$$$$ you want to put into it.Carb/dizzy rework will get you close to what Chevy claims it should be.Compression dictates much about cam choices and heads control flow rates.Again that could get expensive.Good thing is if you wanted to build for what it should be,you have a base in C.I.'s to build for torque,which is what you want.Cooling cast into the block with Siamese cylinders has to be mag'ed between the bores and the bore thickness needs to be sonic checked for thickness and shift before any internal work can be done which must be done with deck plates.The bores on these do move around alot.Remember a engine is just a air pump and the amount of gain from exhaust mods is relevant to the restriction of the smog heads,in part,your working with.Same holds true for intake,carb,cam.The heads restriction is like a funnel your trying to get around and those are 76cc chamber heads which a very low compression heads.It's because of that I am refraining from suggesting a EFI to replace the carb which then it wouldn't matter how high you where driving through.

So as everyone suggested carb/dizzy might be your cheapest.Just watch the timing on the dizzy.Don't go too wild.400's heads/head gaskets are not heat friendly.And before you do that,do a leakdown test and compression test to see where your at with this engine.Let us know how the compression tests works out and if you have a low cylinder.Don't lose sight of your working with a engine that is how many yrs old??.And you never said how many miles are on it.
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Old 06-27-2012, 01:48 AM   #12
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I almost forgot.Altitude carbs have to have a consideration for the"possibility" of a increase flow rate to feed the engine(intake,heads, cam,intake valves/exhaust valve/exhaust).Your low compression stock engine almost doesn't.
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Old 06-27-2012, 06:27 AM   #13
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Put a after market fuel injector on it. I've never heard of an Altitude adjusting carb. Maybe I've been out of it to long.

Only thing it could do is lean it out as you go up. What you really need is turbo charger or supercharger.
A distributor re-curve will impact timing and may gain you a few HP, perhaps at the expense of higher octane fuel. It will not compensate for leaner density air at altitude.

I do agree that an aftermarket fuel injection (not fuel injector) kit will solve the issue as long as it includes an 02-sensor for close-loop feedback. In my experience, going to fuel injection - even the simply throttle body systems will buy you some significant MPG improvements. Holly makes one of these systems.

Leaning the carb out will help if it's running rich at high altitude. As mentioned above, regaining the kind of power you have at low altitudes requires forced induction.
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Old 06-27-2012, 06:35 AM   #14
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As said do a search here on Jim's recurve I did it helped a lot mine was under $20 from wrecking yard my main gain was mpg
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