Merc 20Hp -69 Increase output for racing

[Chinewalker]

Re: Merc 20Hp -69 Increase output for racing

In #192 - very little of your exhaust goes out that way. Most of it goes right down the leg of the motor. The area you are referring to is where it goes out at idle - called an exhaust relief. At speed, it really doesn't do much.

In #193 - you can polish that a bit. Take some of the sharp edges off the inside area. Don't mess with where it meets the reeds as that could affect the seal area. If they are worn a bit where the reeds hit them, you can resurface the reed block, either on a mill or even just using a glass plate and sandpaper, using a Figure 8 motion...

#194 - not sure I'd machine anything there - maybe just clean up the intake passage a bit, but otherwise better left alone...

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

Hi
Thank you for important info.
Work has started.

Im planning to install an small blower from pc (12v) for the careburator otherwise i will not do any adjustments.

" propeller casing " outlet of exhaust gases duct will be increased abit.
Also exhaust gas duct " exhaust relief " i have drilled the pipe abit.

When the engine is then hopefully running.
What shall be a correct rpm and timing of ignition ?

 

[Chinewalker]

Re: Merc 20Hp -69 Increase output for racing

Correct RPM? No idea - the changes you're making may change what is now "correct". As for timing, Set the points to open where the ram rider aligns with the crankshaft keyway. Done. Since timing changes when you advance the throttle, you can read the plugs to see where your max advance should be. You want your plugs running a light chocolate. If they're white, you're running too high on the timing (or too lean on the carb - same symptom).

Not sure a blower will gain anything other than complexity...

 

[Faztbullet]

Re: Merc 20Hp -69 Increase output for racing

Not sure a blower will gain anything other than complexity...

I agree......if anything it will more likely restrict the incoming air flow as its a constant rpm fan and very little CFM if any will be attained at WOT

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

Hi
I was planning to charge this engine abit.

I know from other engines (diesel) thats scavenging air pressure is important but it s hard to do it for a engine with careburator.

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

Okey i will try to adjust the needle valve so i get chocolate colour on plugs.

( If i close more fuel less air ) Correct ?

Im planning to run the engine first with 4% of oil with high speed oil.

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

what do you mean with CFM and WOT ?

 

[jbjennings]

Re: Merc 20Hp -69 Increase output for racing

Cubic Feet per Minute and Wide Open Throttle?:redface:

 

[Faztbullet]

Re: Merc 20Hp -69 Increase output for racing

The needle valve only controls the idle and lower part of mid range pull over circuits it will do nothing to fuel mixture at wide open. That carb has a fixed size main fuel jet and only way to richen or lean is to increase/decrease jet size orifice.

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

Hi
Do you think i should increase the diam of the jet ?

 

[Chinewalker]

Re: Merc 20Hp -69 Increase output for racing

May not have to change anything. You're still running a stock lower unit with a stock prop, so other limitations may negate any significant gains in RPM or speed. Run it - hard - with what you have now. Just a quick blast around the bay. Read the plugs. Go from there. Dark chocolate to black, go leaner (smaller) on the jet. Light chocolate to white, go richer (larger) on the jet. Simply making the jet larger may only succeed in making the motor run rich and perform worse....

I would get an assortment of jets - a dealer may have them. And try them. Don't go hog wild - up or down one size may be all that is needed. Change the jet, go for a blast, read the plugs.

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

HI
I will do so....

Im not searching for new type of pistons so i could get higher combustion pressure since the engine only have cooling water cover no cyl head to machine off....

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing

Hi
Im now planning to weld some material inside the combustion space to get higher combustion for an other power block.
Lets see how it will end ....
Still waiting for some spare for my first powerblock.

One of my friend told me he came up to 7200 rpm with one of his engine block but he also managed to destroy several blocks.

How many cm3 does an 25SS and Merc 20 hp have ?
Do you have pic of 25ss ?

 

[Faztbullet]

Re: Merc 20Hp -69 Increase output for racing

Basically the same engine ,the 25ss has different midsection and speedmaster 1:1 gear lower unit and turns 6000rpm. If its a 25XS it a totally different engine. As for welding your wasting your time and ruining a spare block. The problem with the inline engines is the reed block only flows a certain amount of air and thats it, even if it had a head you could shave you could not increase the air flow to see any improvement from the increased compression. Increasing jet size richens mixture which will hurt you if you dont increase rpms.

 

[ingin689]

Re: Merc 20Hp -69 Increase output for racing

No way to machine the "cylinder cover" as it is part of the block. The cover you see is not a cylinder head - it is a water jacket cover.

Regardless, anything you do to a bone stock motor with the full gearshift leg will likely not be noticed in actual improved performance due to lack of propeller selection. Personally, I'd spend my time on set-up and making sure you're getting all 20 original ponies. There are a few props that are available - including a rare brass prop by Michigan.

If you really want to wake up the 20, put it on a direct drive lower unit from a Merc KE7 and build up a "Rat Racer".

we are working on the same exact motor. i completely understand what you are saying about making the motor run %100 efficient and starting with the bottom end and props. is this KE7 lower end a direct bolt on? what needs to be done to make this lower unit work? what props can you get for it? whats the differance in the KE7 and the KF7? this 69 merc200 is going on a 1984 10ft cougar cub.

 

[Chinewalker]

Re: Merc 20Hp -69 Increase output for racing

Having not done the conversion myself, I can't speak from experience on whether the Merc 200 powerhead is a direct bolt-up. I am pretty sure the rear-most bolt holes won't align perfectly and you may have to modify the bolt holes. You will also need to use an adapter palate, as the water will not align. Most guys model one after the plate between the powerhead and lower unit of the standard Merc 200, using studs in the plate to accommodate the different placements.

A KE7 lower unit has a smooth propshaft with pin drive, while the KF7 lower has a splined shaft. The smooth propshaft is desirable, as you can then use a variety of racing props from the old Merc racing motors. Only thing you'd have to do is make a new spot on the prop for the drive pin as it's in a different place than the racing units.

 

[Merc69]

Re: Merc 20Hp -69 Increase output for racing



Power heard
There is not much to do.... I machined the fresh and exhaust ducts. I was thinking if its wise to grind even more to grind off the tree "holes" perhaps the piston rings dont like it.

I tried a Original Quick Silver prop. Its too big for this V-type boat. Pitch 9 1/2 17. You can see on the pic.

So...KE7 lower unit will fit my machine.
Do you have any ?

 

[zul8tr]

what were the results of the mods you did?

To answer a post about the differences of the Merc 200 and Merc 25SS block they are essentially the same except for different exhaust port timing. I have measured several and find the 25SS has higher exhaust ports in the 0.030" range for improved high rpm performance (Note APBA stock outboard inspection manual puts a 0.025" +/- on the intake and exhaust port timing, wow! no wonder there were fast blocks and slow blocks ). I raced a 1973 25SS Merc in hydro class back in the 1973 - 1980 in Region 5 APBA and it was set up to run in the 7100 rpm range. Used stock 0.061" fixed jet in the KC16A carb and changed the plugs to Champ projecting L61Y instead of the L78V surface gap plugs, ran the timing at 39 - 40 degrees, set with an inductive light. Still have the engine and hydro and run it at AOMCI meets here in Orlando Fla., runs as good as when new.

Also if the Merc 200 has the same top bearing oilier system as the Merc 25ss check the lower 90 degree elbow that has a one way check valve that can stick and prevent oil to the top roller and ball bearing.

These 2 cylinder engines are inherently not balanced and at high rpms they tend to loosen fasteners, keep a check on these, also the 3 piece flex plate flywheel tends to crack at the inner bolt circle so to check tap test the plate for dull sound different, replace as needed.

There are several other things that need attention on the 25SS that I found when racing it and fixed them.

 

[GA_Boater]

Please - Old topic, over two years since it's had activity.

 

[zul8tr]

Please - Old topic, over two years since it's had activity.

So what, I think information always helpful