How to set up a winning Penton 250 motor

by Kent Knudson

Originally printed in the 2002 issue #15 of Still….Keeping Track

We asked Kent to write this article to help our members who are building or trying to maintain a Penton motorcycle for racing. The object is to find out what works and share the information.

Vintage racing is very competitive and if you want to win, you need all the help that you can get. Since Kent seems to have all the correct setups on his bikes, it makes sense to publish what he is doing.

The following outlines the various components, specs, and sources used to build our 250 engine that Kevin Brown won the AHRMA Sportsman 250 Expert and 40+ Expert National Championships with. Our engines are prepared by our friend, James Giddings of Giddings Machine Racing, who learned the art of building KTM engines while working in his father's Penton/KTM dealer-ship. I would also like to thank Barry Higgins for sharing his knowledge of these engines with us.

CASES - Check the cases closely for cracks and have them welded if necessary. Our current engine was cracked around the kickstarter stop bolt (on the bottom of the motor), the rear motor mount, and the ignition cover threads when we first bought it.

Don't worry too much about corrosion. Bead blast, thoroughly rinse and inspect corroded areas. I fill the pinholes that go all the way through with JB Weld or Devcon. Otherwise, warm the engine up and change the oil regularly and the corrosion shouldn't get any worse.

GEARS - If you are having trouble with missed shifts, inspect your gears closely. KTM's use undercut dogs for positive gear engagement and even the slightest rounding on the tips of these dogs will prevent full throttle upshifts.

In motocross, where the start is extremely important, a missed shift can be the difference between a holeshot and a back-of-the-pack start.

CLUTCH - Early 250 clutches used 8 steel discs, 7 copper clutch plates, and 1 spacer ring at the bottom. By mid- 1974, Penton was recommending the removal of the bottom steel disc and spacer. The use of a thicker pressure plate (part no. 54-32-006-100) was required to compensate for the loss in clutch stack height. We use the latter setup in conjunction with 1. 7mm wire diameter clutch springs (part no. 51-32- 001-140) that were used in the 400 engine. I would also recommend adjusting the spring pressure with a dial indicator to ensure even clutch disengagement.

For more information about KTM clutches refer to the Penton Service Data and Info binder which contains a section on General Service Info. General Service Info No. 10, dated July 25, 1974 is called "The KTM Clutch Story".

TRANSMISSION OIL - The aforementioned KTM Clutch Story, as well as the 1978 KTM manual, suggests the use of ATF in the transmission. We use one quart of Mobil 1 synthetic ATF.

CYLINDER - We use a stock, unported 1974 cylinder. Although many people swear by the Carl Cranke porting specs, we've found them to be unnecessary. The stock porting provides a very smooth, linear powerband that is still strong enough for Kevin to run with the big-bore bikes in the 40+ class.

DECK HEIGHT - Deck height is set at .030" - .035".

CONNECTING ROD - For our rebuilds, we use new connecting rods from EuroRods.

PISTON - We use original Mahle pistons with .003" clearance. Due to lack of availability of original 2-ring 250 pistons, we are currently using the later style 1-ring piston.

INTAKE - We are currently using an original GEM 250/400 Penton reed kit. Although this kit appears to be fairly small and restrictive, it helps contribute to the strong, smooth powerband that we were seeking. Just make sure you use the 250/400 kit, as I think they made a smaller 175 kit as well. The installation also requires drilling holes in the intake side of the piston skirt. The specs and procedure are outlined by GEM in their installation instructions. Replace-ment reeds can be purchased from Boyesen.

CARBURETOR - The stock 36mm Bing (2/36/102) works great. We use a #160 main jet, #283 needle jet, and a #35 pilot jet. We use a #1 (281) jet needle with the clip in the middle position. This setup has proven to be very versatile for us and has not required any changes in the last 3 years of competition. Although this should provide a good ballpark setting, any variance in engine specs, altitudes, or fuels may require different jetting.

FUEL - This seems to be a hot topic on the POG message board. My intent here is not to add any more fuel (sorry) to the fire, but we use the much maligned A VGAS. A VGAS lO0LL (100 octane low lead aviation gas) mixed 32: 1 with Yamalube R works extremely well for us; we used the same plug for a whole season. IT is very consistent regardless of where you buy it and it is reported to be comparable to (R+M)/2 103-104 octane (AV AGAS uses a different rating system). The knock (oops!) against AVGAS is that it is formulated for use in low-rpm, light load, steady throttle applications. The other options are, of course, pump gas, race gas, or a "blend" of the two. Pump gas can be very inconsistent and may contain up to 10% ethanol. Race gas is typically very high octane and should only be used in very high compression engines (although I've recently noticed several manufacturers offering race gas in the 100-105 octane range). In our case, we've used AVGAS since 1982 with excellent results.

AIR FILTER - Twin Air #154200

IGNITION - As long as you don't need a lighting coil, I would strongly recommend the PVL ignition, which is an internal rotor design. PVL also offers 4 different flywheel weights to help configure the power delivery of your engine. On the 250 engine we opted to not use any additional flywheel weight and the engine is still very smooth and tractable.

TIMING - Ignition timing is set between 2.5mm and 2.6mm BTDC.

SPARK PLUG - NGK B8HVX gapped at .024". The VX is NGK's top-of-theline plug for this application and features a fine wire platinum center electrode and a tapered ground electrode for maximum efficiency.

EXHAUST PIPE - Stock 1974-75 enduro exhaust.

KICKSTARTER LEVER - Newer style lever supplied by Alan Buehner

SHIFTER - Original Hi-Point folding shift lever.

SPROCKETS - We use a 14 tooth 520 PBI Pro steel front sprocket (#93-14) and a 57 tooth 520 PBI aluminum rear sprocket (#5257-57).

BEARINGS and SEALS - We start each season with all new bearings and seals, which sounds like overkill but it's a lot cheaper than the cost of attending another race (transportation, hotel, entry fees) due to breakage. All clearances are set using the factory manual.

GASKETS - Complete gasket sets are supplied by Alan Buehner.

PAINT - For the silver engines we use PJl silver barrel paint #17BRLS. The blue K1M oval is hand painted with Martin-Senour Acrylic Enamel - GM fleet blue 99L 11540.