RPM on OC3 - Fuel & Carb Qs

[oliverchris]

1) When tuning in new carburetor, manual says check that engine idles at (from memory) 3500 rpm.
How do you know the rpm without a rev counter?
2) What octane is recommended for HGs and OC3s with Hercules IXB3s or IXK3s?

[Blake Malkamaki]

I think your engine should idle more like 500 rpm or thereabouts. You could use a clip on tach, but I would just get it to idle as slow as it will without quitting.

I believe those engines were designed to run on 60 to 70 octane gasoline. However, you can't judge today's gasoline by octane ratings alone. I have found that a lot of old tractors don't run goon on 87 octane unleaded in real hot weather. I believe that since most modern automobiles have submersible fuel pumps and vapor lock is not an issue, that most refineries have changed the vapor pressures of their gasolines so they evaporate quicker. Though it's not an issue in a new car, it is an issue if you have gravity or suction type fuel pump. The gasoline evaporates in the fuel line, thus vapor locking.

I have found this to be a major problem in my Farmall Super M when baling hay in hot weather. The higher grade gasoline seems to help solve this. Not sure if it will every time though.

The other problem is there is ethanol in almost all gasoline today. This is not good on old tractors and can loosen gummed up crud in the fuel tank. It can also take the shellac off cork carburetor floats and make them "waterlog" and sink.

Blake

[oliverchris]

Thanks Blake.
You know, the old carburetor I had on there dripped gas a lot, and the engine ran pretty rough. I cannot believe the change with a new reconditioned carb! It now runs beautifully after I had messed about with the mix screw and idling a bit. I am using a TSX403 for an HG. I have sent the old TSX406 off to be rebuilt professionally and then I will put it back on my OC3.