Mercury Mark 40-H

[John (Taylor) Gabrowski]

Pulling the compression up to 190 psi per cylinder is in the realm of even using methanol for fuel successfully. Its going to be interesting how the Merc pistons and rings are going to withstand the operating pressures and stresses and at what BTDC timing and what fuel gas type when you run her. Seems like heavy duty Turner pistons would be called for but they used smaller wrist pins of the 600-4 series of Merc rods so will the larger wrist pins on the later model Merc pistons with their thinner bosses than the Turner's take the pressures without ovaling out or fracturing will be interesting? Ignition timing and gasoline octane are sure going to be the issues.

I just love the Merc/Tillotson KC6 carbs being used. They are high velocity pullers if you ever seen them perform before. I use a set of them on a Mark 55H D-Mod with Turner high dome gasser 2 ring ultralights (170 psi cylinder compressions). Settled on them after trying KA7As then KA19s because out of the 3 carb models the KC6s pulled the best in the high ranges.

Keep on keeping on!

[Mark75H]

Originally Posted by John Taylor

Pulling the compression up to 190 psi per cylinder is in the realm of even using methanol for fuel successfully. Its going to be interesting how the Merc pistons and rings are going to withstand the operating pressures and stresses and at what BTDC timing and what fuel gas type when you run her. Seems like heavy duty Turner pistons would be called for but they used smaller wrist pins of the 600-4 series of Merc rods so will the larger wrist pins on the later model Merc pistons with their thinner bosses than the Turner's take the pressures without ovaling out or fracturing will be interesting?

Been there, done that. 190# is nowhere close to the limit and Turners are not required; I use std Merc pistons and standard rods with much higher compression ... no trouble at all.

[John (Taylor) Gabrowski]

Seems to be some interesting experience with stock Merc pistons you got there Mark75H that is not necessarily shared by others who maybe did not play it as safe as you did on the mixture control and timing fronts. What gasoline did you use and what timing did you restrict yourself to once you had the high speed jets fairly well adjusted?

[Mark75H]

Originally Posted by John Taylor

Seems to be some interesting experience with stock Merc pistons you got there Mark75H that is not necessarily shared by others who maybe did not play it as safe as you did on the mixture control and timing fronts. What gasoline did you use and what timing did you restrict yourself to once you had the high speed jets fairly well adjusted?

Don Muncie would be proud, this motor runs on AV gas, but he would not like the fixed jets I use.

The rest is secret, as the motor is still in use as a competitive racer.

Guess what this doohickey does ...

[John (Taylor) Gabrowski]

That thingy looks like a little knock sensor? or Some new type of on the go electrical high speed jet flow meter I hear some snowmo types are using on their sled carbs to keep tunning on the go easier and faster?

In any case the 40 cube bore 55H stock D-Mod here with the Turner 2 ring high dome ultra-lights with KC6 carbs uses .063 screwed in jet equivilent to 1 & 2/3 turn open on Merc adjustable high speed needles with timing maxed at .235 BTDC on what was premium high test pump gas (as of 2008 must have 10% ethanol). Looks like AV gas here is the only non ethanol stuff left unless you get racing gas from the drag and oval track people now to.

[Jeff Lytle]

That dohickey is a drive drum motor off a furnace humidifier with a twist of course!--high torque, slow rpm...........I see what you're up to Sam, very clever! Set them rich, then lean it up with the humidifier motors.

[Mark75H]

Yes, it is a gear reduction motor, but the ones on humidifiers are usually AC ... this one is rated 18vdc.

I already gave a clue so here it is again ... I'm happy with the performance I'm getting with fixed jets. Clue #2 is that I'm only planning on using it for testing, it only turns 3 rpm and the thread pitch is 28 ... 1/7 of an inch per minute output.

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Additionally ... if you are using factory fixed jets - your pipes are only pulling the factory amount of air into the cylinders. Just like an expansion chamber that can use the same jets as a bike with a muffler ... it ain't doing anything.

[John (Taylor) Gabrowski]

Tech Article:

Using screw in jets that the Tillotoson KC series of carbs came with are okay but for those that like precise high speed needle tunning on most Merc 4 cylinder 30,40 & 44 cube engines with belt driven magneto ignition, the offset of the KC carb jet opening and using a high speed needles and packing gland nuts present some small problems. The common KA7A packing gland nut and even a cut down length high speed needle makes for close quarters for tunning the carbs modified high speed needle with the engine running with a fixed or mobile magneto.

One thing that comes up is the extension of jet needle threads to ensure that the needle enters and bottoms out with its taper against the oversized inner jet orfice screwed into the carb jet bottom to begin with. Careful use of a die to extend those needle shaft inner threads without extending the threads into the packing gland nut preventing an air leak is a must. The rest of the jet needle penetrating the packing gland to where it extends towards the magneto gives you jet needle turn in/out space that is tight but useable.

Another variation fix is to install an appropriate length "steel" "idle" jet needle from another suitable Tillotson carb as donor which still keeps the whole space tight but there is a screwdriver slot to work with some prefer over twisting a small knob.

The last variant is to obtain a suitable jet (cross T) needle, jet needle tension spring, jet needle collar and o-ring packing gland system from a Tillotson HD or HR self pumping carb of early snowmo vintage uses. This system offers the most working space with a small T handle to twist the jet needle back and forth. The spring loading of the jet needle keeps the needle in place once tunned. The collared O-ring packing gland system works exceptionally well and is well proven on the snowmobile apps. With this in place there is no need and dispenses with having a screwdriver, 3/8 and 7/16 inch wrenches at hand.

If this all sounds too complicated and even tricky and your not used to it from your Merc stock outboard engine days (KG4, Mk20H, Mk30H & Mark 55H) then stick with experimenting with fixed screw in type jet slugs.

[Droll-l6]

Originally Posted by Mark75H

Yes, it is a gear reduction motor, but the ones on humidifiers are usually AC ... this one is rated 18vdc.

If I understood your idea right.... do some research on step motors and their possibilities .


Arne Kjetil

[Mark75H]

Step motors require a pulse to move each portion of a rotation, I have neither the time to activate several hundred pulses nor the desire to connect my very simple unit to a computer.

I have experience with step motors at work. There is a very high rate of "false steps" where the controller sends a pulse and the motor attempts to move, but does not. Many of the units I work on have position sensors at each end of travel, and the "false step" error requires driving to what is suspected to be the nearest end and counting back to where the software predicts the correct position is ... wasted time. It gives so much trouble I have a hard time figuring out why they are not using more position sensors such as middle of action, to reduce the time running to the far end and counting back. Some units only have one position sensor and must go to that end regardless of whether it is closer and count from the one known position ... wasted time in the cycle, error and inefficiency in operation.

Until step motors that actually report their true position at all times are cost effective, I have little use for them. A nice idea, but much too expensive for almost any application at this time.