high altitude carb jetting

Hey guys! I could use some advice on carburetor adjustment and
jetting. I just moved from ~1300ft to ~7200 ft. I towed by 1965 stock
1200 Bug behind my uhaul truck, and when i unloaded it up here, it
would barely run, and kept stalling. So i adjusted the idling mixture
and now it works fairly well. Only problem is its soooo sluggish, and
it wont go over 50mph on the highway. I used to easily cruise at 65. i
know it's due to the altitude causing the carb to run too rich, and re-
jetting the carb is my only choice. but rather than screwing up a
mostly-working carb, im going to use a Brosol H30/31 i bought years
ago but never used.

What would be the best jet sizes for a H31/31 on a 1200cc motor at
7200ft? Anyone have any ideas? I've checked the usual sites but can't
find anything that addresses BOTH altitude and an H30/31 size.

On Oct 10, 5:36*pm, wrote:
> Hey guys! I could use some advice on carburetor adjustment and
> jetting. I just moved from ~1300ft to ~7200 ft. I towed by 1965 stock
> 1200 Bug behind my uhaul truck, and when i unloaded it up here, it
> would barely run, and kept stalling. So i adjusted the idling mixture
> and now it works fairly well. Only problem is its soooo sluggish, and
> it wont go over 50mph on the highway. I used to easily cruise at 65. i
> know it's due to the altitude causing the carb to run too rich, and re-
> jetting the carb is my only choice. but rather than screwing up a
> mostly-working carb, im going to use a Brosol H30/31 i bought years
> ago but never used.
>
> What would be the best jet sizes for a H31/31 on a 1200cc motor at
> 7200ft? Anyone have any ideas? I've checked the usual sites but can't
> find anything that addresses BOTH altitude and an H30/31 size.

-----------------------------------------------------------------------------------------

Dear Tom,

I don't have the answer to your question but it appears no one else
does either. But maybe some general information will be of use to
you.

At 7200 feet above sea level a cubic foot of air doesn't have as much
oxygen as a cubic foot of air at 1300 feet. That means you've been
running TO RICH. The perfect blend of fuel and and air has one pound
of fuel for every 14.7 pounds of OXYGEN. Not 'air' but OXYGEN...
which makes up only about 20% of that cubic foot of air. The rest is
mostly nitrogen.

Notice that the ratio is based on the WEIGHT of the fuel and oxygen.
(Or 'mass' if you want to get picky.) At sea level, with a stock 1600
engine, the perfect fuel/air charge means a drop of gasoline about
1/16th of an inch in diameter, with the perfect charge of oxygen being
about three tuna-fish-cans of air. The point here is that it's VERY
easy to over-jet the thing (and most guys do).

Clearly, you need a smaller jet, along with the proper emulsion tube.
Mexico City, which is a bit above 5000 feet, has thousands of VW bugs
zipping around on their de-rated low-compression 1600cc engines. The
jets they use would be a good starting point for your car.

Most Solex jets have the same thread. You may be able to find a
suitable jet in a carb used on a vehicle other than a VW.

If you have a set of wire-gauge drills you can determine the diameter
of your present jet. Then you want to find something that is SMALLER.

Right now all we know is that you're giving it too much gas. Try to
tune it for the minimum amount.

When running rich (or lean!) any other problems, such as improperly
adjusted valves, will be amplified, so make sure your valves are
properly adjusted.

I'm sorry I can't be of more help.

-Bob Hoover

PS -- If you have a set of wire-gauge drills (ie, smaller than #80)
you can make a smaller jet by soldering your existing jet closed then
drilling it out using a smaller drill. But don't try this unless you
have a spare jet on hand. If you screw something up it won't prevent
your car from running. The trick for drilling a clean hole with a
small drill is to spin the drill at high speed. You can use the SFM
formulat (surface feet per minute) to calculate the speed required for
any diameter of drill bit but the odds are you don't have anything
that can spin that fast --it's something like 12,000 rpm for a #80...
and even more for the smaller drills.

PPS -- I know a fellow -- a pretty good engine-builder -- who
'rejeted his carb without soldering. He cleaned up the jet with a
whisp of steel wool spun on a drill-bit using one of those 12v
'dremel' tools. When it was bright metal clear throu he used JB Weld
(!!) to seal it up. But he didn't drill it out. He coated the drill
with wax and ran it through the JB Weld then let it cure over night.
He had to use pliers to free the drill but it left a nice clean
hole. Then he use a 3/16" bit to chamfer the JB Weld.

I know -- it sounds sorta kooky. But according to him it ran just
fine.

ALSO
I believe the distributor might need advancing......add maybe 8-10 more BTDC
to it in the high altitude area only.
Not sure of this but one of my non-VWs came from Colorado in a high altitude
area and it had a different sticker for the tune up specs. I think the idle
timing was supposed to be set at 18 degrees BTDC in HA areas instead of 8
degrees BTDC at sea level.
Double check this though and let us know what you do and what works!!! ;-)
Good luck with it!
--
later,
(One out of many daves)


"Bob Hoover" > wrote in message
...
On Oct 10, 5:36 pm, wrote:
> Hey guys! I could use some advice on carburetor adjustment and
> jetting. I just moved from ~1300ft to ~7200 ft. I towed by 1965 stock
> 1200 Bug behind my uhaul truck, and when i unloaded it up here, it
> would barely run, and kept stalling. So i adjusted the idling mixture
> and now it works fairly well. Only problem is its soooo sluggish, and
> it wont go over 50mph on the highway. I used to easily cruise at 65. i
> know it's due to the altitude causing the carb to run too rich, and re-
> jetting the carb is my only choice. but rather than screwing up a
> mostly-working carb, im going to use a Brosol H30/31 i bought years
> ago but never used.
>
> What would be the best jet sizes for a H31/31 on a 1200cc motor at
> 7200ft? Anyone have any ideas? I've checked the usual sites but can't
> find anything that addresses BOTH altitude and an H30/31 size.

-----------------------------------------------------------------------------------------

Dear Tom,

I don't have the answer to your question but it appears no one else
does either. But maybe some general information will be of use to
you.

At 7200 feet above sea level a cubic foot of air doesn't have as much
oxygen as a cubic foot of air at 1300 feet. That means you've been
running TO RICH. The perfect blend of fuel and and air has one pound
of fuel for every 14.7 pounds of OXYGEN. Not 'air' but OXYGEN...
which makes up only about 20% of that cubic foot of air. The rest is
mostly nitrogen.

Notice that the ratio is based on the WEIGHT of the fuel and oxygen.
(Or 'mass' if you want to get picky.) At sea level, with a stock 1600
engine, the perfect fuel/air charge means a drop of gasoline about
1/16th of an inch in diameter, with the perfect charge of oxygen being
about three tuna-fish-cans of air. The point here is that it's VERY
easy to over-jet the thing (and most guys do).

Clearly, you need a smaller jet, along with the proper emulsion tube.
Mexico City, which is a bit above 5000 feet, has thousands of VW bugs
zipping around on their de-rated low-compression 1600cc engines. The
jets they use would be a good starting point for your car.

Most Solex jets have the same thread. You may be able to find a
suitable jet in a carb used on a vehicle other than a VW.

If you have a set of wire-gauge drills you can determine the diameter
of your present jet. Then you want to find something that is SMALLER.

Right now all we know is that you're giving it too much gas. Try to
tune it for the minimum amount.

When running rich (or lean!) any other problems, such as improperly
adjusted valves, will be amplified, so make sure your valves are
properly adjusted.

I'm sorry I can't be of more help.

-Bob Hoover

PS -- If you have a set of wire-gauge drills (ie, smaller than #80)
you can make a smaller jet by soldering your existing jet closed then
drilling it out using a smaller drill. But don't try this unless you
have a spare jet on hand. If you screw something up it won't prevent
your car from running. The trick for drilling a clean hole with a
small drill is to spin the drill at high speed. You can use the SFM
formulat (surface feet per minute) to calculate the speed required for
any diameter of drill bit but the odds are you don't have anything
that can spin that fast --it's something like 12,000 rpm for a #80...
and even more for the smaller drills.

PPS -- I know a fellow -- a pretty good engine-builder -- who
'rejeted his carb without soldering. He cleaned up the jet with a
whisp of steel wool spun on a drill-bit using one of those 12v
'dremel' tools. When it was bright metal clear throu he used JB Weld
(!!) to seal it up. But he didn't drill it out. He coated the drill
with wax and ran it through the JB Weld then let it cure over night.
He had to use pliers to free the drill but it left a nice clean
hole. Then he use a 3/16" bit to chamfer the JB Weld.

I know -- it sounds sorta kooky. But according to him it ran just
fine.