the_workmaster
Member
Hey guys. I just brought an old 641D back to life with a nice gas engine conversion. The engine seems to have had a very recent rebuild, I was shocked to see how clean the inside of it was, just a film of carbon in the exhaust ports, and shiny gray iron in the intakes, no tar on the valve stems, clean piston domes. New gaskets throughout, new ignition setup, unburned never seize on manifold studs. All pretty big surprises for an engine that was full of acorns, exterior surface rusted like it was parked when I was a little kid. So it runs great!
I was surprised to read a few things while trying to tune my Schebler Marvel carb though (I just rebuilt that, and have experience doing carbs). Before reading, I set the idle and main mixture screws to 2.5 rotations out, a good baseline for many carbs. It started and ran great like that, but low end was 1200 rpm. I adjusted the throttle linkage (alot), and got it to drop down to about 900 rpm. The lowest I've been able to drop it is 600, and it runs awful, threatening to die, but I guess my goal is 4-500, from what I've read. It also likes to run rich. The idle mixture screw is best set between bottomed out, and one turn out (bottom is rich on these carbs), and the main mixture likes to be 2-2.5 turns out, when I read it should be near 1. I guess that all screams vacuum leak, right? But with such a simple intake system, that's hard for me to believe. It all looks like it's making good contact.
The throttle linkage also required dramatic adjustment just to allow the idle speed screw on the throttle arm to make contact with the pin, and I'm not confident that I have the spring tension where it should be related to the (governor??) arm lever at the front of the engine. Is there a baseline for setting all of that?
Maybe my ignition timing is off, messing up everything else? That's a matter of twisting the distributor I assume?
Thanks for any help, Alex
I was surprised to read a few things while trying to tune my Schebler Marvel carb though (I just rebuilt that, and have experience doing carbs). Before reading, I set the idle and main mixture screws to 2.5 rotations out, a good baseline for many carbs. It started and ran great like that, but low end was 1200 rpm. I adjusted the throttle linkage (alot), and got it to drop down to about 900 rpm. The lowest I've been able to drop it is 600, and it runs awful, threatening to die, but I guess my goal is 4-500, from what I've read. It also likes to run rich. The idle mixture screw is best set between bottomed out, and one turn out (bottom is rich on these carbs), and the main mixture likes to be 2-2.5 turns out, when I read it should be near 1. I guess that all screams vacuum leak, right? But with such a simple intake system, that's hard for me to believe. It all looks like it's making good contact.
The throttle linkage also required dramatic adjustment just to allow the idle speed screw on the throttle arm to make contact with the pin, and I'm not confident that I have the spring tension where it should be related to the (governor??) arm lever at the front of the engine. Is there a baseline for setting all of that?
Maybe my ignition timing is off, messing up everything else? That's a matter of twisting the distributor I assume?
Thanks for any help, Alex