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blueosprey90 · Dec 30th, 2018, 14:54

You may have several problems. I run H-6 carburetors on my MGA and I think these are very similar to the HS-6 except for the connection between the float bowl and the main carb body.

1) symptom is that the carbs die at idle: I believe this is caused by your carburetors being unbalanced; and at idle, the engine is running off of just one carburetor.

When you tune your carburetors, they need to be balanced. De-link the carburetors at the throttle shaft so you can tune them individually. My car has a W shaped spring linkage that is loosened. Balance the air flow into the carburetors with a Unisys device, heater hose to you ear or by other means. Basically the adjustment is made at the throttle set screw on each carburetor. Once balanced, the carbs can be re-linked at the throttle.

2) symptom is that the car is running too rich: The car is running too rich! Setting the jets at 12 flats (two full turns of the adjusting nut) is an OK starting point, but I don't find it very accurate. I follow a different procedure that you might use if you have a vernier caliper. I remove both dashpots and pistons from the carb body. If you look straight down into the carb bodies. you will see the top of the jet inside of the jet bearing. The distance between the top of the jet and the top of the jet bearing (these look like two brass circles, one inside the other) need to be exactly the same on both carburetors. The shorter the distance, the more lean the carburetor is running. I usually set my MGA at .08" (2.05mm), but depending on humidity and temperature, I run anywhere from .07" to .10" (2.55mm). I've found that it usually takes 1 to 2 flats to make a .01" adjustment and that the carbs do not adjust exactly the same.

Note: a) While you have the pistons out, identify and record each needle. They should both be the same. Record the needle type as you may want to change needles based on needle profile and the type of driving you do.

Note: b) Once you make both of the adjustments described above, you can fine tune by removing the dashpot damper caps and inserting a straw or piece of wire (equal lengths and bent to point at each other - i.e., "diving rods") in each. As you rev the engine, they should rise the exact same amount. If they don't, you need to fine tune. If I recall correctly, it is the air flow (balance) adjustment that you will revisit.

Note: c) When I set my bridge at .08", I am shooting for a lean setting. A lean setting makes the engine run hotter. I have exhaust gas temperature sensors and can monitor my exhaust gas temperatures. If I am running over 1350 degrees at wide open throttle, I adjust the carbs maybe one or two flats richer to bring down the temperature. Since you won't have these sensors, you need to learn to "read" your sparkplugs. A whitish, light tan color is what you want to see. A whitish grey color is too lean. A dark tan or black is too rich.

3) Symptom is that the car won't start when hot. This sounds like vapor lock. Do you have ethanol in your gasoline? What happens is that the heat from the engine causes the fuel to vaporize in the carburetor. At higher altitudes, I imagine that fuel vaporizes at an even lower temperature than at sea level. A heat shield, heat wrap on the manifold or duct work to bring cool air into the engine bay are potential solutions.

4) Tuning the carburetors is only one step. Make sure your timing is properly set and make sure that your valve lash is also properly set.

Just for reference, this is my journal description of my effort to tune my new PV444 running H-4 carbs. You'll see that it was multistep and somewhat of a trial and error method:

August 15, 2017: Partial tune. I got the car warmed up with the idea of checking valve lash. I hoped that I could bump the engine with a remote start button, but was unable to make the connections. So to turn the engine, I had to heave the car forward and back while in 4th gear.

Valves don’t move all that much. All of the exhaust valves were too tight and I adjusted to 0.18”. The intake valves were a bit loose, but I mostly left them alone. The intake valve lash spec is 0.16”. (Note: these were the B-16 spec, not the B-16B spec.)

Plugs looked very sooty, with possible exception of #4 Cleaned those up.

I took the dashpots off the carbs and measured the distance between the jet and the top of the bridge. The rear carburetor was set at .135”, the front one at about .13”. I reset the jets on both carburetors to .10”. One flat seemed good for about .01”. I may lean them out further by one flat to bring them to .09”. We’ll see how it runs for now.

The pistons had carbon soot on the engine side, evidencing that there had been backfires into the carbs as I had suspected.

I tried to balance the carbs. At idle, it seemed as if the engine was running on just the front carb. Because I didn’t want to take off the air filters, I balanced using those “divining wires“ that go into the top on the piston when you take out the damper caps. The engine really wanted to run on the front carb, but eventually I think I got them balanced.

… Seemed to run better, smoother. …

August 20-29, 2017: Engine Tune up. 67,660 miles. New spark plugs NKG 6HS. Pulled the distributor and did a proper job on the points. Filed down both faces until smooth and reset points gap to .018”. I think the flywheel is off by 180 degrees. (Actually it was the distributor drive gear) With that assumption, I set the static timing at 6 degrees before TDC with rotor pointing to 2:00 rather than 8:00. A shot of starting fluid into the carbs, and it fired right up.

I let it warm up and reset the valve lash to 0.020" for both intake and exhaust. Car seemed to idle on the fast side. Using a dial back timing light, the advance at idle was about 28 degrees. Too far advanced.

I removed the air filters from the carburetors. Engine seemed to be running off of the front carburetor only. Attempted to synchronize the carbs using a Uni-Syn device. Very difficult to get them balanced, but eventually I got close. Leaned out the carbs one flat, so jet should now be below the bridge .09” rather than .10”.

Set the timing at 20° before TDC using a dial back timing light at “fast idle” I’d guess I was at least at 1,500 to 2,000 rpm. And at idle, it read 5° before TDC which I thought was a bit low. AARGH! I need a better system! Checking the manual after the fact, I see that I should have set the advance at 23° to 25° before TDC. Revisited and corrected.

September 3, 2017: I revisited the timing and reset to 24° before TDC by timing light.

Dec 30th, 2018, 14:54	#7
blueosprey90 Master Member Last Online: Apr 25th, 2024 13:36 Join Date: May 2017 Location: New Milford, Connecticut	You may have several problems. I run H-6 carburetors on my MGA and I think these are very similar to the HS-6 except for the connection between the float bowl and the main carb body. 1) symptom is that the carbs die at idle: I believe this is caused by your carburetors being unbalanced; and at idle, the engine is running off of just one carburetor. When you tune your carburetors, they need to be balanced. De-link the carburetors at the throttle shaft so you can tune them individually. My car has a W shaped spring linkage that is loosened. Balance the air flow into the carburetors with a Unisys device, heater hose to you ear or by other means. Basically the adjustment is made at the throttle set screw on each carburetor. Once balanced, the carbs can be re-linked at the throttle. 2) symptom is that the car is running too rich: The car is running too rich! Setting the jets at 12 flats (two full turns of the adjusting nut) is an OK starting point, but I don't find it very accurate. I follow a different procedure that you might use if you have a vernier caliper. I remove both dashpots and pistons from the carb body. If you look straight down into the carb bodies. you will see the top of the jet inside of the jet bearing. The distance between the top of the jet and the top of the jet bearing (these look like two brass circles, one inside the other) need to be exactly the same on both carburetors. The shorter the distance, the more lean the carburetor is running. I usually set my MGA at .08" (2.05mm), but depending on humidity and temperature, I run anywhere from .07" to .10" (2.55mm). I've found that it usually takes 1 to 2 flats to make a .01" adjustment and that the carbs do not adjust exactly the same. Note: a) While you have the pistons out, identify and record each needle. They should both be the same. Record the needle type as you may want to change needles based on needle profile and the type of driving you do. Note: b) Once you make both of the adjustments described above, you can fine tune by removing the dashpot damper caps and inserting a straw or piece of wire (equal lengths and bent to point at each other - i.e., "diving rods") in each. As you rev the engine, they should rise the exact same amount. If they don't, you need to fine tune. If I recall correctly, it is the air flow (balance) adjustment that you will revisit. Note: c) When I set my bridge at .08", I am shooting for a lean setting. A lean setting makes the engine run hotter. I have exhaust gas temperature sensors and can monitor my exhaust gas temperatures. If I am running over 1350 degrees at wide open throttle, I adjust the carbs maybe one or two flats richer to bring down the temperature. Since you won't have these sensors, you need to learn to "read" your sparkplugs. A whitish, light tan color is what you want to see. A whitish grey color is too lean. A dark tan or black is too rich. 3) Symptom is that the car won't start when hot. This sounds like vapor lock. Do you have ethanol in your gasoline? What happens is that the heat from the engine causes the fuel to vaporize in the carburetor. At higher altitudes, I imagine that fuel vaporizes at an even lower temperature than at sea level. A heat shield, heat wrap on the manifold or duct work to bring cool air into the engine bay are potential solutions. 4) Tuning the carburetors is only one step. Make sure your timing is properly set and make sure that your valve lash is also properly set. Just for reference, this is my journal description of my effort to tune my new PV444 running H-4 carbs. You'll see that it was multistep and somewhat of a trial and error method: August 15, 2017: Partial tune. I got the car warmed up with the idea of checking valve lash. I hoped that I could bump the engine with a remote start button, but was unable to make the connections. So to turn the engine, I had to heave the car forward and back while in 4th gear. Valves don’t move all that much. All of the exhaust valves were too tight and I adjusted to 0.18”. The intake valves were a bit loose, but I mostly left them alone. The intake valve lash spec is 0.16”. (Note: these were the B-16 spec, not the B-16B spec.) Plugs looked very sooty, with possible exception of #4 Cleaned those up. I took the dashpots off the carbs and measured the distance between the jet and the top of the bridge. The rear carburetor was set at .135”, the front one at about .13”. I reset the jets on both carburetors to .10”. One flat seemed good for about .01”. I may lean them out further by one flat to bring them to .09”. We’ll see how it runs for now. The pistons had carbon soot on the engine side, evidencing that there had been backfires into the carbs as I had suspected. I tried to balance the carbs. At idle, it seemed as if the engine was running on just the front carb. Because I didn’t want to take off the air filters, I balanced using those “divining wires“ that go into the top on the piston when you take out the damper caps. The engine really wanted to run on the front carb, but eventually I think I got them balanced. … Seemed to run better, smoother. … August 20-29, 2017: Engine Tune up. 67,660 miles. New spark plugs NKG 6HS. Pulled the distributor and did a proper job on the points. Filed down both faces until smooth and reset points gap to .018”. I think the flywheel is off by 180 degrees. (Actually it was the distributor drive gear) With that assumption, I set the static timing at 6 degrees before TDC with rotor pointing to 2:00 rather than 8:00. A shot of starting fluid into the carbs, and it fired right up. I let it warm up and reset the valve lash to 0.020" for both intake and exhaust. Car seemed to idle on the fast side. Using a dial back timing light, the advance at idle was about 28 degrees. Too far advanced. I removed the air filters from the carburetors. Engine seemed to be running off of the front carburetor only. Attempted to synchronize the carbs using a Uni-Syn device. Very difficult to get them balanced, but eventually I got close. Leaned out the carbs one flat, so jet should now be below the bridge .09” rather than .10”. Set the timing at 20° before TDC using a dial back timing light at “fast idle” I’d guess I was at least at 1,500 to 2,000 rpm. And at idle, it read 5° before TDC which I thought was a bit low. AARGH! I need a better system! Checking the manual after the fact, I see that I should have set the advance at 23° to 25° before TDC. Revisited and corrected. September 3, 2017: I revisited the timing and reset to 24° before TDC by timing light. Last edited by blueosprey90; Dec 30th, 2018 at 15:21.