altitude: how much is it worth to mess with carb - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Gregmaz · 02-19-2013, 04:52 AM

I live at 5000 ft & just bought a remanufactured uremco carb for my 250. The thing purrs right off the bat but it is tune for sea level.
Is it worth it to crack it open in order to put new jets or even mess with its tuning: how much would that cost & when would I get the money back? 5 to 10% fuel economy? Protecting the barings from crude?
I have dealt with a few carbs in my motorcycle days this is why I ponder about tweaking a blue print one.
I see 3 adjustments screws on this Rochester, idle richness & speed, & this top screw mounted on a spring which could be the main richness.
BTW do you guys have a choke cable braket attached to the carb? Can t remember if the other one had one & I forgot to take it off or else. No schematic available.
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Wrenchbender Ret · 02-19-2013, 07:03 PM

If it runs good I would leave it alone. You can drive it a bit & read the plugs. If there is black soot on the insulater then it is running rich. Slowly turn the idle mixture in till the engine starts to slow down. Then back it out enough to speed back up, about 1/2 turn. The screw on top is a bowl vent. It should open a little when the throttle is closed.

**TBONE1964** · 02-19-2013, 07:15 PM

http://www.hotrod.com/techarticles/e...g/viewall.html

Here is a link to a good article about the subject. It does get deep and there is not a simple answer.

I wonder what they did in that area when there were nothing but carbs. Did GM and others have specific carb set ups for high elevations?

**FirstOwner69** · 02-19-2013, 07:23 PM

Way back when, we had a 65 Impala SS with a 327 and 4-barrel. We lived in Southern California and made a trip to the high elevations of Yosemite . The car ran like crap there, so I tweaked the mixture screws and solved the problem.

But, I don't know anything about the type of carb you have.

68gmsee · 02-19-2013, 08:49 PM

Yeah, what kind of carb is it? Can you post more pics? From the pic you posted it doesn't look like any rochester single barrels I'm familiar with.

re. screw on top. I believe it's the idle vent screw described below.

On some applications. the external vents are replaced
by an idle vent valve. The vent valve is located directly over the
pump plunger. When the throttle valve is closed, the
top of the pump plunger opens the vent valve and
allows fuel vapars to vent from the float bowl during
hot idle operation. The vent valve automatically
closes after the throttle valve has moved from the
idle position into the off-idle range.

**Steeveedee** · 02-19-2013, 09:54 PM

Quote:

Originally Posted by TBONE1964

http://www.hotrod.com/techarticles/e...g/viewall.html

Here is a link to a good article about the subject. It does get deep and there is not a simple answer.

I wonder what they did in that area when there were nothing but carbs. Did GM and others have specific carb set ups for high elevations?

Yes. There were high altitude carburetors. You'll find smaller jets and idle bleed ports in them. The reason for this is because the lower atmospheric pressure combined with a rich jet setting will cause a rich condition, as has been mentioned. High altitude and California are synonymous for carburetion, btw, meaning that, generally, a carb from California will perform reasonably well in the mountains. That's mostly due to smog tuning.

pritch · 02-19-2013, 10:24 PM

That looks like a Rochester Model B, which was replaced by the Monojets in what-68 or so?

There nothing simpler than chaging the jet on it. Remove the 4 shiny screws and lift off the top (taking care to not tear the gasket) and the main jet is right there. Jets are availble in all sizes, for about 10 bucks. I'll be going through that same issue soon. I live above 5000' and rarely get under 4000'. But up to 7 or 8 thousand is common.

Longhorn 70 · 02-19-2013, 10:53 PM

I lived in Phoenix most of my life. I moved to Reno for a short time and had to go to Truckee Cal. MANY times. The thing that made the most difference for me then was advancing the timing a bit when I got to altitude. When I came back down, retard it a bit and I was good again. I lft the distributor just snug enough to turn it a few degrees.

Gregmaz · 02-20-2013, 12:37 PM

That's a 67 Rochester uremco rebuild, looks like the core I gave back, pix coming.
NAPA was the only place for this particular rebuild kit but they couldn't figure out which one, so I spent the big bux at summit.

Guess I have to search for jets about 5 sizes under, hope they can figure this one out this time.

Reno is were I live, it is now a combination a smog and altitude, right now cold but "hot august nights".

Gregmaz · 02-20-2013, 12:43 PM

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Gregmaz · 02-20-2013, 01:05 PM

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Gregmaz · 02-20-2013, 04:37 PM

Can t post pix anymore for some reason
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Gregmaz · 02-20-2013, 04:44 PM

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Trying for the last time
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68gmsee · 02-20-2013, 07:59 PM

Yeah, that is a Rochester B. I've had several monojets but not a model B. I like those early models. Info here http://thefirstgensite.com/library/6...67S6BP4_16.pdf

Gregmaz · 02-21-2013, 12:59 PM

Thx for the manual.
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Gregmaz · 02-28-2013, 02:12 PM

let it be known that no smaller size jets are available for this type carburetor.

Summit doesn't know of any, Uremco did their research and so did Delco to no avail.

Wrenchbender Ret · 02-28-2013, 04:31 PM

How small do you want to go. I may have a .052 or so around.

Gregmaz · 04-07-2013, 09:15 AM

Thx, I live at 5000 ft but dunno what size is in. Are you sure it woild work in this rochester B 1barrel? If so I ll open it up & check the numbers. Should b 2 sizes per 1000ft, so 10 sizes down I presume.
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pritch · 04-07-2013, 12:59 PM

Quote:

Originally Posted by Gregmaz

let it be known that no smaller size jets are available for this type carburetor.

Summit doesn't know of any, Uremco did their research and so did Delco to no avail.

I disagree. This guy sold me some just a couple months ago. The price has even gone down, they used to be 9 bucks each.
http://www.carburetor-parts.com/Roch...Jets_p_97.html

Wrenchbender Ret · 04-07-2013, 02:09 PM

I think you are making too big a deal out of it. Read the plugs. If they have a coat of black carbon on them you are running rich. About 2 sizes smaller would work. If they are white with blisters you are too lean. Pure white or a slight brown is normal. If its running good leave it alone.

02-19-2013, 04:52 AM	#1
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	altitude: how much is it worth to mess with carb I live at 5000 ft & just bought a remanufactured uremco carb for my 250. The thing purrs right off the bat but it is tune for sea level. Is it worth it to crack it open in order to put new jets or even mess with its tuning: how much would that cost & when would I get the money back? 5 to 10% fuel economy? Protecting the barings from crude? I have dealt with a few carbs in my motorcycle days this is why I ponder about tweaking a blue print one. I see 3 adjustments screws on this Rochester, idle richness & speed, & this top screw mounted on a spring which could be the main richness. BTW do you guys have a choke cable braket attached to the carb? Can t remember if the other one had one & I forgot to take it off or else. No schematic available. Posted via Mobile Device Posted via Mobile Device Attached Images __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-19-2013, 07:03 PM	#2
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: altitude: how much is it worth to mess with carb If it runs good I would leave it alone. You can drive it a bit & read the plugs. If there is black soot on the insulater then it is running rich. Slowly turn the idle mixture in till the engine starts to slow down. Then back it out enough to speed back up, about 1/2 turn. The screw on top is a bowl vent. It should open a little when the throttle is closed.

02-19-2013, 07:15 PM	#3
TBONE1964 Cluster King Join Date: Nov 2012 Location: Junction City, OR Posts: 5,263	Re: altitude: how much is it worth to mess with carb http://www.hotrod.com/techarticles/e...g/viewall.html Here is a link to a good article about the subject. It does get deep and there is not a simple answer. I wonder what they did in that area when there were nothing but carbs. Did GM and others have specific carb set ups for high elevations?

02-19-2013, 07:23 PM	#4
FirstOwner69 Senior Member Join Date: May 2010 Location: Wisconsin & Arizona Posts: 4,852	Re: altitude: how much is it worth to mess with carb Way back when, we had a 65 Impala SS with a 327 and 4-barrel. We lived in Southern California and made a trip to the high elevations of Yosemite . The car ran like crap there, so I tweaked the mixture screws and solved the problem. But, I don't know anything about the type of carb you have.

02-19-2013, 08:49 PM	#5
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: altitude: how much is it worth to mess with carb Yeah, what kind of carb is it? Can you post more pics? From the pic you posted it doesn't look like any rochester single barrels I'm familiar with. re. screw on top. I believe it's the idle vent screw described below. On some applications. the external vents are replaced by an idle vent valve. The vent valve is located directly over the pump plunger. When the throttle valve is closed, the top of the pump plunger opens the vent valve and allows fuel vapars to vent from the float bowl during hot idle operation. The vent valve automatically closes after the throttle valve has moved from the idle position into the off-idle range. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

02-19-2013, 10:24 PM	#7
pritch Registered User Join Date: Mar 2004 Location: Hoytsville, Utah Posts: 3,365	Re: altitude: how much is it worth to mess with carb That looks like a Rochester Model B, which was replaced by the Monojets in what-68 or so? There nothing simpler than chaging the jet on it. Remove the 4 shiny screws and lift off the top (taking care to not tear the gasket) and the main jet is right there. Jets are availble in all sizes, for about 10 bucks. I'll be going through that same issue soon. I live above 5000' and rarely get under 4000'. But up to 7 or 8 thousand is common. __________________ '68 Panel Project Boogie Chillin' '68 C-10 Ol' Green

02-19-2013, 10:53 PM	#8
Longhorn 70 Registered User Join Date: Dec 2012 Location: Chandler AZ Posts: 1,149	Re: altitude: how much is it worth to mess with carb I lived in Phoenix most of my life. I moved to Reno for a short time and had to go to Truckee Cal. MANY times. The thing that made the most difference for me then was advancing the timing a bit when I got to altitude. When I came back down, retard it a bit and I was good again. I lft the distributor just snug enough to turn it a few degrees. __________________ 1970 Longhorn, Front Disc, 350/4 bolt, 882 heads, HEI, Edelbrock, 700R4, HO-52/4.11. 1996 Corvette, Collector Roadster, LT4, 396, 450RWHP, 6sp, 4.11/Dana44/posi 5 point roll bar And let us not grow weary of doing good, for in due season we will reap, if we do not give up. Gal 6:9

02-20-2013, 12:37 PM	#9
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb That's a 67 Rochester uremco rebuild, looks like the core I gave back, pix coming. NAPA was the only place for this particular rebuild kit but they couldn't figure out which one, so I spent the big bux at summit. Guess I have to search for jets about 5 sizes under, hope they can figure this one out this time. Reno is were I live, it is now a combination a smog and altitude, right now cold but "hot august nights". __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-20-2013, 12:43 PM	#10
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Posted via Mobile Device Posted via Mobile Device Posted via Mobile Device __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-20-2013, 01:05 PM	#11
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Posted via Mobile Device Posted via Mobile Device __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-20-2013, 04:37 PM	#12
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Can t post pix anymore for some reason Posted via Mobile Device __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-20-2013, 04:44 PM	#13
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Posted via Mobile Device Trying for the last time Posted via Mobile Device Attached Images __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-20-2013, 07:59 PM	#14
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: altitude: how much is it worth to mess with carb Yeah, that is a Rochester B. I've had several monojets but not a model B. I like those early models. Info here http://thefirstgensite.com/library/6...67S6BP4_16.pdf __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

02-21-2013, 12:59 PM	#15
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Thx for the manual. Posted via Mobile Device __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

02-28-2013, 02:12 PM	#16
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb let it be known that no smaller size jets are available for this type carburetor. Summit doesn't know of any, Uremco did their research and so did Delco to no avail. __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living



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02-28-2013, 04:31 PM	#17
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: altitude: how much is it worth to mess with carb How small do you want to go. I may have a .052 or so around.

04-07-2013, 09:15 AM	#18
Gregmaz Registered User Join Date: Jan 2013 Location: Reno, nv Posts: 32	Re: altitude: how much is it worth to mess with carb Thx, I live at 5000 ft but dunno what size is in. Are you sure it woild work in this rochester B 1barrel? If so I ll open it up & check the numbers. Should b 2 sizes per 1000ft, so 10 sizes down I presume. Posted via Mobile Device __________________ 67 C10 fleetside 250 Rochester B, limited slip "still in its juice" I drive for a living

04-07-2013, 02:09 PM	#20
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: altitude: how much is it worth to mess with carb I think you are making too big a deal out of it. Read the plugs. If they have a coat of black carbon on them you are running rich. About 2 sizes smaller would work. If they are white with blisters you are too lean. Pure white or a slight brown is normal. If its running good leave it alone.

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