What is this bore doing?

[Ostsideend]

Hi Guys,

i do have a small sized 2 stroke outboard engine (Yamaha Malta 6L5 with 3 HP) for my dingi. By fun i did a bet against a buddy of mine who will have the faster Yamaha Malta next spring time. So i am tuning this small piece. (This is another story )

My question hoped to be answered:
There is a bore which goes directly from the compression area out into the exhaust channel.
I cannot immagine what this is used for. It seems that it is stupid to drill a hole from the compression area directly out into the exhaust but there is probably a reason for doing it and i am too stupid to understand.



The small drill above the exhaust channel goes directly into the exhaust...



From the backside...

What does that bore do?

Best regards
Oliver

 

[Silvertip]

Re: What is this bore doing?

It is probably a compression release that essentially releases compression for easier pull-starts. When the engine is running the hole is small enough that it doesn't result in significant power loss. One could of course tap that hole, insert a threaded plug from the back side and pickup some small amount of lost power. Whether it is enough to notice is questionable. Just don't go so deep that the plug enters the cylinder bore and snags a ring.

 

[Ostsideend]

Re: What is this bore doing?

Hi Silvertip,
thanks for your answer. I also thought about this (like the small 50ccm motrized bicycles called "Mofa" in germany in the old days. They had a decompression leaver which ended in a small valve in the head for easier starts)

But why does it ends into the exhaust? It would be easy to do a shortcut back into the intake.

I guess i will plug the bore and reduce the cylinder head volume down to 1:9.5 compression ratio, open up the width of the channels and do a little bit modified pre ignition time.

Thanks again!

 

[Silvertip]

Re: What is this bore doing?

Into the exhaust is the logical place for it. If it went into the intake spent exhaust gas would dillute the intake charge more than it already gets during transfer cycle.

 

[Silvertip]

Re: What is this bore doing?

Unless you are an experienced two-stroke tuner, be very careful with how you modify intake, exhaust and transfer porting as it may hurt performance rather than improving it. As for ignition timing, that is linked to throttle opening so again, too much timing advance can hurt performance and possibly destroy the engine if carried to extremes. Stuffing the crankcase is another performance option but not always possible to do on some engines.

 

[Silvertip]

Re: What is this bore doing?

Drilling that port into the exhaust is the logical place for it. If it were drilled into the intake ports or crankcase, crankcase vacuum/pressure would be lost and the engine wouldn't run. Besides, why would you want more exhaust gas in the intake charge. Unless you are an experienced two-stroke tuner, be very careful with how you modify intake, exhaust and transfer porting as it may hurt performance rather than improving it. As for ignition timing, that is linked to throttle opening so again, too much timing advance can hurt performance and possibly destroy the engine if carried to extremes. Stuffing the crankcase is another performance option but not always possible to do on some engines.

 

[Faztbullet]

Re: What is this bore doing?

Silvertip is corect it is a idle relief hole to drop compression. The Johnson/Evinrude 90* engines have them and plugging it is worth 10 psi in compression, Mercury's have a milled slot instead of hole.

 

[Silvertip]

Re: What is this bore doing?

Double posts are what happens when one is interrupted by phone calls that really anger a person. Darned sales people trying to make a living!!!!!!!!!!!!!!

 

[Ostsideend]

Re: What is this bore doing?

Thank you guys!

Better a double posting than nothing

I will be careful in porting, timing and open the width. If it goes worth i already bought a second Malta. If this fails again i'll get another one from internet.

I'll test it in a water basin with a force gauge clamped between motor and clamping fixture to see the result before spring time.

force detector


detector clamped between motor and "swivelling unit"



force during idle (max with the original motor was 380N peak during acceleration)

 

[Faztbullet]

Re: What is this bore doing?

If ya want to beat him this is what I would do:
A) Plug idle relief port
B) Clean up any casting flashing on intakeside of engine
C) Google Chris Carson Reeds or TDR reeds and see if they can make you a set of lightweight fiber reeds(quicker throttle response)
D) Install a next size larger carb if possible, we found out racing years ago that the slight increase in the front put more to the prop without exhaust or port mod's(stock engines)
E) If it has a shiftable lower unit, grind the reverse gear teeth off so gear oil is not spun against itself

 

[robert graham]

Re: What is this bore doing?

Maybe you could just plug that little relief port by forcing some JB Weld down into it, then if it doesn't suit you for any reason you could just drill it back out easily without screwing anything up! You might also get some 100 octane or model airplane fuel to run in it for a few more RPM's, just long enough to pass your buddy in the water! Maybe try running motor without the airbox for a bit more free breathing. Port modification could and would work but knowing exactly where to remove the metal in the ports is critical and not reversible. We used to screw with the ports on racing motorcycles and easily change the torque/power curve, but motors/throttles became more like on/off switches, not very usable power, but strong! Good Luck!

 

[Silvertip]

Re: What is this bore doing?

And all the while your buddy is reading this and doing the same thing so you both end up in the same position you would have been if both of you had done nothing to your engine.

 

[Knightgang]

Re: What is this bore doing?

And all the while your buddy is reading this and doing the same thing so you both end up in the same position you would have been if both of you had done nothing to your engine.

And hope your buddy is not on thehullthruth.com or another forum getting even better advise to hot rod up his motor...




Ooops, I forgot, there is not better boat advice on the net than right here at iboats.....

 

[Ostsideend]

Re: What is this bore doing?

And all the while your buddy is reading this and doing the same thing so you both end up in the same position you would have been if both of you had done nothing to your engine.

The risk is small, i am from Germany

 

[Ostsideend]

Re: What is this bore doing?

And hope your buddy is not on thehullthruth.com or another forum getting even better advise to hot rod up his motor...

Ooops, I forgot, there is not better boat advice on the net than right here at iboats.....

The risk is even smaller, my english is'nt very good, but his does not exist

And you are right, there is no better advice than from you guys here at iboats!
Boat forums in germany are crowded by guys saying: No this is not possible, don't do..... not understanding that sometimes you should do something just for fun. Beating the buddy for a 6 pack of german beer is something for fun.

I'll put a sticker on the motor "powered by iboats" if this little thing will become good.

 

[Ostsideend]

Re: What is this bore doing?

If ya want to beat him this is what I would do:
A) Plug idle relief port
B) Clean up any casting flashing on intakeside of engine
C) Google Chris Carson Reeds or TDR reeds and see if they can make you a set of lightweight fiber reeds(quicker throttle response)
D) Install a next size larger carb if possible, we found out racing years ago that the slight increase in the front put more to the prop without exhaust or port mod's(stock engines)
E) If it has a shiftable lower unit, grind the reverse gear teeth off so gear oil is not spun against itself

Good hints! A) and B) is a must.
I will check C)!!!!
D) I already did check my stuff from the old days. Found an 21mm Delorto carb and two (16 and 18mm) Bing Carbs.
E) No reverse gear, just forward and idle

 

[Ostsideend]

Re: What is this bore doing?

Maybe you could just plug that little relief port by forcing some JB Weld down into it, then if it doesn't suit you for any reason you could just drill it back out easily without screwing anything up! You might also get some 100 octane or model airplane fuel to run in it for a few more RPM's, just long enough to pass your buddy in the water! Maybe try running motor without the airbox for a bit more free breathing. Port modification could and would work but knowing exactly where to remove the metal in the ports is critical and not reversible. We used to screw with the ports on racing motorcycles and easily change the torque/power curve, but motors/throttles became more like on/off switches, not very usable power, but strong! Good Luck!

Presently i am thinking about a small spring loaded valve from the backside operated by a leaver. Bur i will find out if i can still get this thing started without a releive valve.

We aggreed to NO fuel tuning (he has someone in his family beein a chemical professional) We will fill in the same stuff out of the same gas tank.

To slitely open just the width NOT the hight or position of ports should not be critical.

 

[robert graham]

Re: What is this bore doing?

If you open the port width you may snag a piston ring. Port timing and size is probably good like it is, but you could clean and polish the ports for better flow. Are your boats identical, otherwise how will you know who really won? Also you might raise your motor up on the transom to get the foot higher and that will help speed. If you extend the tiller arm with a length of plastic pipe will allow you to move forward in boat and gain speed, for sure. Good Luck!

 

[Ostsideend]

Re: What is this bore doing?

If you open the port width you may snag a piston ring. Port timing and size is probably good like it is, but you could clean and polish the ports for better flow. Are your boats identical, otherwise how will you know who really won? Also you might raise your motor up on the transom to get the foot higher and that will help speed. If you extend the tiller arm with a length of plastic pipe will allow you to move forward in boat and gain speed, for sure. Good Luck!

Yeah, open the width just a little (2mm) I will definitively clean and polish as well as do nice porting.

both are "rubber boats" 2 meter length (dingi or pee taxi boats- having a dog on board) different maker but almost the same.

I can operate the tiller arm with my feet but you are right, i will get one extension for it. Thanks!!

 

[Tacklewasher]

Re: What is this bore doing?

not understanding that sometimes you should do something just for fun. Beating the buddy for a 6 pack of german beer is something for fun.

I have no advice but I just want to say, as a Canadian who understands what beer is all about, I completely understand and agree with your reasoning.