wrote in message
oups.com...
IIRC, the A75 had a slightly different carb venturi diameter and a
larger main jet in addition to the larger piston pin.

Dan


The secret to getting 75 or 80 horses out of the A-65 is not in changes to
the carb but basically revving the engine higher. The A-65 gets 65 horses
at 2300 rpm, which is red-line. The A-75 gets 75 horses at 2600 rpm and
the A-80, 80 horses at 2700 rpm.

The NAS3A1 Stromberg carb uses a #49 main jet for all three engines and a
1.25" venturi. Larger jets and venturis are for the C-series engines.

I just rebuilt an A75 and did nothing to alter the carburetor but rebuild
the needle valve, although some people do rejet for experimental use. But
if you alter the carb it's not legal for a certified A-65, A-75, or A-80.

A few other differences: timing is staggered on the higher HP A-series
engines, and the cap end of each rod is drilled .063" to make an oil squirt
hole to spray oil on the opposing cylinder walls and pistons. The higher
horse engines have smaller diameter piston pins, but you have to make sure
you use the correct wall thickness because there is an AD against the
original pins. The exhaust valves have stellite faces on the higher horse
engines because of higher speeds and temperatures. And, btw, you can buy
that exhaust valve described by the initiator of this thread from Lycon for
around $100, and it is not sodium-filled. None of them are.

Pistons for the A-65, A-75, and A-80 have some variations. The A-75
pistons are same as A-65's except for a cooling relief cast inside the
hollow piston. They are 3-ringed and compression ratio is 6.3 to 1. A-80
pistons are 5-ringed and 7.5 atmospheres compression.

To the guy who started the thread, all valve dimensions are the same for
intake valves. And all dimensions for exhaust valves are the same.

I use the cheap valves because they are a better value and are quite durable
for the A-65. But I don't burn 100LL except as a lubricant for the seats.
Anyone who uses 100LL in any of the A-series engines (other that 1 part in
10 ratio, avgas to mogas) is asking for trouble, like sticking valves,
fouled plugs, crud, and accelerated wear. The compression ratio of these
engines is just too low for avgas, unless you can get 80 octane avgas.

If an exhaust valve starts to leak when the engine is low-time, I just
remove it, clean it, lap it back in with some fine lapping compound and
check for concentricity and guide clearance. It it's not burnt it will
work. Sometimes just exchanging the rocker arm or making sure dry
lifter-to-pushrod clearance is within limits cures the problem.

These are wonderful little engines and most problems with them are not
simple but not all that complicated either.

And, btw, if you go to 75 or 80 HP you have to re-prop. Check your tcds.

You can verify most of the above information by checking the official engine
overhaul manual. Buy a complete reproduction of the original manual, not
that shabby thing sold by Jim Irwin. It's like him--- more than a few
pages short.

Wicks has a manual they'll sell you.

Oh, and if you need a carburetor overhaul kit, don't buy that from Irwin
either unless you want to get shorted on crucial parts like those washers
used to set the float level. Fresno Airparts in Fresno, CA sells Stromberg
parts. So does Wicks.