Dauntless 225 burnt valve?

My 68 dauntless started missing on #5. It backfires through the exhaust at idle. The compression is around 80# indicating a burnt valve probably from ethanol. My question is, would it be best to do a valve job on both heads, just the left head or could I replace both with later 231 heads with hardened valves and seats?

 

I'd pull the valve cover and check for other possibilities: broken valve spring, worn out/flat lifter etc.

 

Both of which was said above..............and the later even fired 231 heads will not work on your 225.

 

X3. And what leads to the conclusion of ethanol fuel causing the issue?

 

I have two older small engines with burnt valves that several mechanics have attributed to ethanol. As this engine was built before ethanol was introduced, they said it is very likely the cause.

 

I too would like to understand the mechanism by which ethanol could "burn" a valve. In recent years small engine mechanics seem to blame it for everything they can't fix.

On the other hand, I am personally seeing stuck valves - which I never used to. Possibly due to the reduction or absence of traditional additives in the oil and gas, such as lead and zinc???
Anybody else feel this way?

 

I am not a chemist, so I really don't know. However, I have started adding diesel fuel to the gas I use in small engines (1 gal diesel to 10 gal's gas) and have had no more problems with small engines. Should have been doing that with my jeep!

 

Pretty much I do. Since ethanol burns cooler (less btu's) it seems unlikely that's the cause of burned valves.

 

Won't have any problems with mosquitos putting out that kind of a smokescreen. 10:1 gas/oil mixture? Is that really what you meant to write?

 

x2.
And back in the day of pre-mix for motorcycles I read that too much oil could even make the engine run too lean (hot) because of the amount of gasoline it displaced .

 

The only ill effect is a slight diesel exhaust smell, not as much as a diesel engine. I started doing that with a 49 ford F-3 with the flat head 6. In the 8 years I drove it on unleaded gas never had any valve problems. Sold it and they quit adding diesel and about a year later blew the engine. Not saying the diesel additive prevented it, but sure didn't hurt anything.