On Tuning

Dear Mr. Motorhead,

With Spring just around the corner, I’m thinking I should adjust and tune-up my engines.  Is this a difficult job or can I do it myself?
Not So Sure

Dear Not So Sure,
It really is quite simple. I’ve reprinted the primer on this procedure as explained by
BSLOL Member Steve Merjanian. Go to it!!!

Adjusting Chris Craft (Hercules) 4 & 6 cylinder engines is a simple and logical procedure. I shall assume there is fresh oil in the engine, the shaft is aligned, the transmission is adjusted, the propeller is in good shape, the fuel pump is functional and the fuel line and fuel sediment bowl are clean.

Click Here for Chris Craft Engine Specifications!

The greatest initial concern should be valve adjustment which assures adequate valve heat dissipation and engine breathing. Consult the engine chart on page 17 to determine the intake and exhaust valve gap adjustments for your engine.With the engine cold, remove the valve covers which are underneath the exhaust manifold and behind the carburetor.The smaller blocks (ie: A,B,H & K types) use 7/16″ nuts for the valve tappets while the larger blocks (ie: L,M & W types) use 1/2″ nuts.Use a long thin section open end wrench which is specially made for adjusting valves.This valve wrench will fit the lifter while conventional open end wrenches (7/16″ & 1/2″) will fit the jamb nut and tappet.The lifter (two flats) is on the bottom of the assembly, next is the jamb nut (hexagonal nut) and the tappet is the (hexagonal bolt) on top.Between the tappet bolt face and the valve stem is the gap to be adjusted. The first valve at either end of the engine is an exhaust.The next two are intakes, the next two are exhausts, etc., etc.Have a friend turn over the engine by hand (a socket wrench on a flange coupling or flywheel bolt works fine) while you watch the intake and exhaust valves for a selected cylinder go up and down. Turn the engine an additional 90 degrees once both valves are down and seated.
Use a feeler gauge to measure the exhaust and intake gaps. They will probably be tight.Put the thin section valve wrench on the lifter (bottom, two flats) and use another wrench to loosen the jamb nut (hexagonal nut). You can now turn the tappet (hexagonal bolt) to adjust the gap. If you tighten the jamb nut just enough to let the tappet turn, you can snug up the jamb nut without moving the tappet out of adjustment. Repeat this procedure for the remaining valves, doing a cylinder at a time.

The standard Kettering ignition system consists of a coil, condenser (capacitor), distributor, ballast resistor (12 volts only), ignition switch, spark plugs, high voltage ignition wires, low voltage primary wires and a battery. The battery should be fully charged. Check the spark plug wires for cracks, frays and tight connections at the distributor cap, spark plugs and coil. Use Champion UJ6 or J8J plugs for cast iron heads and H-10 or H- 10J plugs for aluminum heads. The newer designations for the UJ6 & J8J are J6C (normal running) & J8C (hotter plug for low speed operation). All spark plugs are gapped to 0.028 inches and the points are adjusted to 0.022 inches after being filed clean. To adjust the point gap, first remove the distributor cap and rotor. Crank the engine with the starter in short bursts until the points are wide open (point rubbing block on peak of cam). Adjust the point gap by loosening the jamb nut and turning stationary point until a 0.022″ feeler gauge just passes through the point gap.Reassemble the distributor rotor and cap. To check for spark, remove the coil wire from the center of the distributor cap and position this loose wire about 1/32 inch from a head bolt. Crank the engine starter with the ignition ON and look for a white spark at this 1/32 inch gap. If there is no spark, make sure the points are clean and try again. If again there is no spark, change the condenser (capacitor) and try again. WARNING: TO AVOID HIGH VOLTAGE SHOCKS, DON’T HOLD ONTO THE COIL WIRE WHEN CRANKING THE ENGINE. Another common problem is a short in the low voltage wire which goes from the negative side of the coil to the distributor. It usually shorts out where the wire passes through the distributor body because the insulating material has broken down. Replacement parts are hard to find, but carefully jury-rigged electricians tape will work. Some additional preventive maintenance includes filing the rotor tip clean, cleaning the inside of the distributor cap of any carbon tracks left by the rotor, checking the spark advance weights (below the distributor point plate) for free movement, lubricating the advance weights with a dry spray (e.g.) LPS, WD40, CRC, etc.) and lubricating the distributor shaft felt wick (under the rotor) with light machine oil. Ignition coils usually fail slowly and will usually give a red spark at the above cited 1/32 inch head bolt/coil wire gap, instead of a good white spark. Both oil filled and epoxy coils can be used with good results. Please refer to the following chart when selecting an ignition coil. For example, a six volt coil will eventually overheat and reduce its output when used with an eight volt battery.


Volts Type Primary Resistance
Six 6 Volt Coil 1.6 ohms
Eight 12 Volt with Ballast 2.2 ohms
Twelve 12 Volt Coil 3.2 ohms
NOTES: I have not found ballast resistors in any of the 1950’s vintage twelve volt boats. However, look for the ballast resistor if you have a newer twelve volt boat. The primary winding resistance of the coil can be measured between the plus (+) and the minus (-) terminals.

The carburetor atomizes the gasoline and mixes it with the incoming air so the engine can burn it. If the engine or carburetor has been rebuilt, a good approximate adjustment is 1.5 turns open from the fully closed position on both the idle and the high speed jets. The high speed jet is near the bottom of the carburetor. The idle jet is closer to the to the intake manifold and is found inboard of the throttle linkage idle stop screw. If the engine ran before, the carburetor is probably set correctly.

A dry-land start-up should have water going through the engine. Remove the intake hose from the water pump and replace it with a shorter piece of hose that will go in to a 1 to 3 gallon pail inside the boat. Use a garden hose to keep this pail filled while running the engine. The fuel pump has a hand operated lever which should be worked until the carburetor float chamber is filled and the hand lever has a soft feel. The carburetor air horn may have accumulated water and gasoline during storage. Remove the 7/16″ plug on the bottom of the carburetor and drain away this fluid. Replace the plug. The engine is now ready for a dry land start. Run water into the pail for the water pump, close the choke (pull-out choke knob) and crank the engine until it pops. Open the choke (push-in choke knob), open the throttle about two-thirds and continue cranking. The engine should start. Run the engine with water going through it to clear out any storage oil and condensation. Some tuning adjustments can be made on dry land (ie: idle jet and throttle stop) and they are discussed as part of the in-the-water tune-up.

Assuming the above steps regarding spark plugs, rotor, cap, coil and points have been done, we can now adjust the engine. Allow sufficient time for the boat to soak up, which can vary from six hours to many days. A tachometer/dwell meter is very useful for the tune-up procedure. The six (6) cylinder engines should be set for 35 degrees of dwell angle and the four (4) cylinder engines should be set for 55 degrees of dwell angle. If necessary, readjust the points to achieve this dwell angle. Accelerate the engine up and down quickly. If the dwell angle varies more than four (4) degrees for the six (6) cylinder engines or six (6) degrees for the four (4) cylinder engines, change the points. This test indicates a weak point spring. Put the engine in gear and move away from the dock. Have a friend drive the boat. Loosen the clamp on the bottom of the distributor and run the boat at full throttle. Twist the distributor clockwise and counter clockwise until you achieve the maximum RPM. Back off the maximum by 50 RPM by twisting in the clockwise direction and tighten the distributor clamp. The engine is now timed. If the engine loses power during the season, check the dwell angle before changing the timing. While the boat is at full throttle, turn in the high speed jet on the bottom of the carburetor until the RPM drops. Back out the high speed jet to 1/4 turn beyond the maximum RPM. Slow the boat to an idle. Adjust the throttle stop screw to about 300 RPM over the recommended idle RPM from the chart. Turn in the low speed jet (located midway up the carburetor, inboard of the throttle stop screw) until the RPM drops. Back out the low speed jet to 1/4 turn beyond maximum idle RPM. Readjust the throttle stop screw to the recommended idle RPM from the chart. Your Engine Is Now Properly Tuned. !!!!