carburator calibration
August 24th, 2006, 04:51 PM
#1
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Join Date: Jul 2006
Posts: 8
carburator calibration
okay well I started to do a basic tune up on my oldsmobile omega. I put in a new air filter, spark plugs, wire, distributor, rotor, fuel filter, put a new air filter housing unit pcv valve.
now I need to learn how to calibrate a single barrel carburator for a 250 incline six. I am not getting the right fuel to air intake.
also what is ignition timing and valve adjustments? I read to do this also.
P.S. any other suggestion I will take also.
I am changing my gaskets this week and in the next month I will do the vaccum hoses.
now I need to learn how to calibrate a single barrel carburator for a 250 incline six. I am not getting the right fuel to air intake.
also what is ignition timing and valve adjustments? I read to do this also.
P.S. any other suggestion I will take also.
I am changing my gaskets this week and in the next month I will do the vaccum hoses.
August 27th, 2006, 01:13 PM
#2
Registered User
Join Date: Aug 2004
Posts: 127
The engine in your Omega is a Chevy built 250 CID, so tuneup specs can be obtained from either Olds or Chevy data. The ignition timing is 6 degrees before top dead center. Disconnect the vacuum line to the distributor and plug the vacuum leak before checking the timing. Also, be sure the idle speed is low enough so that the centrifugal advance in the distributor is not active. The book calls for an idle speed of no more than 700 rpm.
The valve lash for your hydraulic lifters is set as follows: Tighten the valve lash until all of the play is gone, and then tighten down the adjusters one full additional turn. Always set the lash for valves at a point where they are normally closed. For mild cams like yours, top dead center on the compression stroke for the cylinder you're adjusting is a good place.
As for calibrating your carburetor, how do you know the air/fuel mixture is not right ? Is there some obvious problem with how the engine runs? True, the 1973 carburetors were emissions control carbs, and were generally set to run as lean as possible. Your carburetor is probably a Rochester Monojet, and uses a metering rod inside of a jet to determine air/fuel ratio during cruising and acceleration. During cruising conditions, the manifold vacuum holds the metering rod down. During acceleration, the lack of vacuum allows the metering rod to be pushed up by a spring into the slimmer part of the rod, providing enrichment.
There are three ways to make small enrichments in your carb without changing the jet or metering rod. You can set your float level slightly higher, increase your fuel pressure ( which effectively raises the float level) , or adjust the metering rod upwards a bit by bending the support arm. However, it's better to change the main jet and/or metering rod in SMALL steps and keep track of the engine response to each small change. A small enrichment in the air/fuel ratio during cruising will smooth out the engine , while an enrichment at the lower ( power enrichment) end of the metering rod will provide more fuel during acceleration. You can enlarge your main jet with drill bits , or with reamers. Your engine also ran a retarded ignition in the distributor curve to make the exhaust hotter ( emissions control) and could benefit greatly from a distributor recurving. Essentially, run as much ignition advance as you can without getting into spark knock on the fuel octane you want to run. Getting rid of the exhaust gas recirculation (EGR) is also a common way to improve drivability and performance.
The valve lash for your hydraulic lifters is set as follows: Tighten the valve lash until all of the play is gone, and then tighten down the adjusters one full additional turn. Always set the lash for valves at a point where they are normally closed. For mild cams like yours, top dead center on the compression stroke for the cylinder you're adjusting is a good place.
As for calibrating your carburetor, how do you know the air/fuel mixture is not right ? Is there some obvious problem with how the engine runs? True, the 1973 carburetors were emissions control carbs, and were generally set to run as lean as possible. Your carburetor is probably a Rochester Monojet, and uses a metering rod inside of a jet to determine air/fuel ratio during cruising and acceleration. During cruising conditions, the manifold vacuum holds the metering rod down. During acceleration, the lack of vacuum allows the metering rod to be pushed up by a spring into the slimmer part of the rod, providing enrichment.
There are three ways to make small enrichments in your carb without changing the jet or metering rod. You can set your float level slightly higher, increase your fuel pressure ( which effectively raises the float level) , or adjust the metering rod upwards a bit by bending the support arm. However, it's better to change the main jet and/or metering rod in SMALL steps and keep track of the engine response to each small change. A small enrichment in the air/fuel ratio during cruising will smooth out the engine , while an enrichment at the lower ( power enrichment) end of the metering rod will provide more fuel during acceleration. You can enlarge your main jet with drill bits , or with reamers. Your engine also ran a retarded ignition in the distributor curve to make the exhaust hotter ( emissions control) and could benefit greatly from a distributor recurving. Essentially, run as much ignition advance as you can without getting into spark knock on the fuel octane you want to run. Getting rid of the exhaust gas recirculation (EGR) is also a common way to improve drivability and performance.
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