And here we have---

[Frank Acampora]

For those of you who read my last post on using the wrong plugs, I was looking for some parts down in the garage and came across this: Here we have a classic example of detonation damage due to a lean running cylinder. Note how much more severe it is than the simple melting of a hole from using the wrong plug.



Although severe, the damage is confined to the hot exhaust side of the piston, melting the piston and ring lands and breaking the rings. Also note that the piston crown is not damaged except for the broken rings banging against it, and the cool bypass side is almost untouched.

Mess with your carbs without knowing what you are doing, set them too lean, and this is what you can expect. Way better to run a bit too rich, smoke a bit more, and replace fouled plugs often rather than replacing pistons.

 

[BobList]

Re: And here we have---

when mine got leaned out due to a fuel screen clogged solid.. that's what the piston looked like. Melt-o-Rama

 

[my1stboat1]

Re: And here we have---

so on an 85 hp force... 1988 model with a serial num of 856x8a The idle mixture screws should be set to what... Using 89 octane fuel only with penzoil full synthetic 2 stroke oil mixed at exactly 50to1 cause mine are at 1 1/8 turn out from a lightly seated position and however my boat runs fantastic it does smoke, and seems my high end rpm could be alittle higher... What do you think FRANK

 

[Frank Acampora]

Re: And here we have---

There is no absolutely correct setting. You set the needles to wherever the engine needs them with this warning: It is possible to set low speed needles leaner than 3/4 turns out and get a great idle yet do damage at or near full throttle. If your engine likes it and runs well a 1 1/8, leave it there.

After setting idle mixture, the acid test is full throttle acceleration. If the engine accelerates smoothly with no problems, no change is needed.

Otherwise: Do this in stages, moving needles only 1/8 turn or less. If the engine sags then picks up and accelerates well, needles are set too lean and need to be screwed out a bit
If the engine stumbles then clears itself out and continues to accelerate, it is too rich. Screw the needles in.

Of course as I have said before: Never set the low speed needles to less than 3/4 turn out, no matter how the engine runs.

 

[RZR2007]

Re: And here we have---

There is no absolutely correct setting. You set the needles to wherever the engine needs them with this warning: It is possible to set low speed needles leaner than 3/4 turns out and get a great idle yet do damage at or near full throttle. If your engine likes it and runs well a 1 1/8, leave it there.

After setting idle mixture, the acid test is full throttle acceleration. If the engine accelerates smoothly with no problems, no change is needed.

Otherwise: Do this in stages, moving needles only 1/8 turn or less. If the engine sags then picks up and accelerates well, needles are set too lean and need to be screwed out a bit
If the engine stumbles then clears itself out and continues to accelerate, it is too rich. Screw the needles in.

Of course as I have said before: Never set the low speed needles to less than 3/4 turn out, no matter how the engine runs.

Great info, Thanks!

 

[fucawi]

Re: And here we have---

Frank we all love you and would not be without your help ...BUT " Detonation damage due to lean running " Damage due to lean running yes but not detonation damage ...Detonation is the explosion of the end gasses caused by slightly advanced ignition ..the pressure rises too early and the remaining charge explodes causing a pinging sound ....Now a weak mixture is slower to burn which is equivalent to retarded ignition ....so not detonation damage ...Weak mixture we all love in our cars but they are built with hardened valves/seats knock sensors and run 20% max output most of the time ...Outboards run 100% for long periods ...dont go lean like frank says.

The hole in the piston picture is for sure pre ignition ..early burning of the charge without explosion followed by overheat of piston and melting a hole....nothing to do with weak mixture .

I did note on a look through a 35/45 manual that it can have two alternative ignitions std and CD
and the plugs specified L4J or UL18V no other changes to the engine. So it would seem that these two plugs are interchangable or the L4J should be ok in a CD setup.

I also note that the idle jet operates up to 2000 rpm so weak mix above this cannot be blamed on the idle jet setting ....if the main jet is weak then should we not be blaming the float level for the weak mix or indeed insufficient fuel pump output??

Frank I have met thousands of people who confuse detonation and pre ignition and as I said before we must be careful to use the right terms or we continue to spread misinformation to the next generation ....

 

[Frank Acampora]

Re: And here we have---

There we go again! I was wondering how long it would take! You do seem to have the knowledge but seem to not apply it correctly to 2 cycle outboard Force engines. Call the terms whatever you want, I am giving practical advice to prevent engine damage which is PROVEN to happen.

Idle mixture or low speed circuit in these carbs is behind the butterfly and is ALWAYS at manifold vacuum so therefore always delivers fuel--even at full throttle. A venturi will get progressively richer as airflow through it increases. So: The high speed jet LIMITS fuel delivery at full throttle to maintain correct air/fuel mix and is sized to compensate for the extra fuel delivered by the low speed circuit. THUS: A too lean mix at low speed will also result in a too lean mix at full throttle, resulting in piston and cylinder damage. You can hear the detonation (or whatever you wish to call it) occuring and it sounds like a loud rattle. But, the damage happens so quickly, only several seconds, that by the time you react and back off the throttle, the piston looks like the one in the photo.

Typically, this damage will only occur to one or two pistons (in three and four cylinder Force engines) because the cylinders do not charge evenly and usually, one will run slightly richer than another. So, the leanest running one will be carried by the others and be preferentially destroyed.

 

[fucawi]

Re: And here we have---

Dont usually do Carbs but your logic is wrong .....First the volume of fuel that can pass the idle jet is minimal compared to the the main jet and the manifold vacuum is maximum at idle and is used to pull fuel from the float chamber through the idle jet ..manifold vacuum reduces as the throttle is opened this cuts off the idle mixture and the main jet takes over using Venuri effect. Venturi effect does not exist at idle as there is insufficient air flow but as air flow increases the suction created by the ventui draws fuel via the main jet . ...thats why one is called low speed circuit and the other high speed ........

I agree that weak mixture is bad on a two stroke but your explanations and confusion between detonation (pinging ) and pre ignition worry me ....Detonation is over advanced timing not weak mixture ..

 

[BobList]

Re: And here we have---

Pre-ignition means that the fuel is igniting BEFORE the prescribed time of the spark, when the piston is coming up, fast. This is caused by a hot spot in the cylinder, face of piston, etc. Carbon deposits glowing red can do this.. pre-ignition can take out a cylinder in a few revolutions. It can raise combustion pressures in the cyl. many times it's threshold. This one is like smacking the top of the piston with a hammer as it's coming up. Not good.

Detonation is when the fuel fires at the correct time the spark fires, but somewhere else in the cylnder, an explosion is started nearly the same time ( hot spot again).. the flame front from a spark plug should travel across the cylinder, smoothly.. when detonation occurs, it's like having 2 spark plugs on opposite sides of the cylinder.. they both fire, flame fronts collide ( like two ocean waves hitting each other head on), and bang bang bang... this generally doesn't kill a piston, exept over a looong period of time..if at all. Heavy spark knock is a symptom of this. This one is like having 2 explosions in the cylinder at the same time, creating knock. But it's not slamming downward on the piston on the upstroke...the multiple explosions are slamming each other.

 

[fucawi]

Re: And here we have---

Boblist you have an excellent understanding but the detonation is not glowing particles its a pressure rise causing the " second ignition" like how diesel fuel ignites.....Thanks for your comments

 

[BobList]

Re: And here we have---

yes, it's actually a pocket of hot fuel mix somewhere else in the cylinder.. I was trying to keep it just a bit simpler.. but it's like a diesel explosion elswhere at the same time.. now... about pre-ignition induced detonation... hoo boy! heh heh.

Anyway, an electrode plug vs. a flat plug.. even if the spark timing is perfect.. the electrode plug can be as much as 1/4" DEEPER into the cylinder when the spark occurs... I wonder if this can effect the flame path it's SUPPOSED to take or not.. interesting.. but my brain hurts now...

I'm a car guy..2 strokes are new to me.. they are like oil burning industrial furnaces belching smoke as far as I'm concerned, but boy do they run great when everything is just right..

Bob

 

[jason32038]

Re: And here we have---

Frank my piston looked like the piston in the first 3 pics you posted except that it didnt have all the dings on the top of it. Before my piston blew it wasnt firing on the cylinder due to a loose wire for most of the summer. I fixed the wire and she screamed for 5 minutes but when I came to shore it was sputtering like it was pre detonating. My question is...will excessive build up of oil and or carbon on the piston cause this type of damage also? When I checked mixture screws they were at 2 turns out for some reason and the seats looked good and there were no fuel restrictions. Also...do you think running the carbs dry once to check static timing would have caused this severe of damage? Thanks for the input everyone.

 

[tater76]

Re: And here we have---

Ok folks, so boblist's explanation, (no offense Boblist) and what fucawi are trying to get across "from what I gather" is that ignition or the explosion from fuel and air being ignited is happening at the wrong time RIGHT! Read the first two sentences in Boblist's last post. The explanations sound a little too similar to distinguish between the two debated terms, and WILL confuse some people further. I am truly losing my patience with this, as it has turned from helpful into a heated debate. Fucawi, you must understand that ALOT of the folks on here are from the states, thus the UK terminology may confuse and in some cases, cause anger. You and Frank A. have a multitude of knowledge that has likely helped thousands of fellow boating enthusiasts. Melting pistons can be from lean fuel mix, wrong plugs, AND improper timing RIGHT????? This is what we ALL should gather from this. Now, pardon me as I step down from my soapbox, as I have some new spark plugs, octane booster, and timing adjustments to try out

 

[BobList]

Re: And here we have---

Yep.. bottom line is too Lean= bad. Semantics and theory aside, messing with 2 strokes can be way less forgiving than 4 strokes!!

 

[Frank Acampora]

Re: And here we have---

Well, Fucawi, you are the one with the faulty logic! While Tillotsen carbs are simple, the low speed circuit is a bit more complex than it appears. I simplified it a bit. There are generally several metered holes in the carb upper casting. At idle, air bleeds reduce fuel flow from one hole directly behind the butterfly. Off idle, the other holes start to open and this negates the air bleed, increasing fuel to accomodate the engine's needs. The circuit can supply enough fuel to run the engine at cruise speed--much more than minimal. The carbs are also generally vented at the main dip tube to balance fuel delivery by the venturi. Yes, manifold vacuum does decrease at full throttle BUT It is always there and the low speed circuit is always delivering fuel--it does not "cut off" when the Venturi starts to deliver fuel. Take apart a carb and study it.

I repeat: Running the low speed circuit too lean will also lean the whole carb at or near full throttle leading to detonation and piston damage on a multi cylinder, multicarb engine.

Just because you say a lean mix runs cooler doesn't automatically make it so. You have in other posts stated inaccurate "facts." A lean air/fuel ratio on one cylinder runs hotter and will autoignite at the hot exhaust ports causing detonation and damage. THAT is the reason why the damage is confined to that side of the piston.

Jason: The dings in the photo were from shattered rings bouncing around inside the cylinder. It won't necessarily happen every time. Sometimes they just blow out the exhaust ports. I have never seen this type of damage from excessive oil, but that doesn't mean it can't happen. However, the MOST LIKELY cause was lack of fuel on that cylinder. Running carbs dry (checking timing in the driveway?) should not cause this type damage as it occurs only at or near full throttle. AND--When both carbs (on a 4 cylinder engine) run dry, the engine loses power. There are no good cylinders to carry a bad one and therefore very little damage--if any should be done. If, on the other hand, you did run them dry at full throttle it is possible. As I said before, the cylinders do not charge equally and one of a pair usually runs a little leaner than the other. So it is possible to get a lean run at some point on only one cylinder. I myself would never run the carbs dry at full throttle. I am very attuned to my engines and notice immediately any drop in RPM. If I lose any RPM at all, for any reason, for example running low in the tank and not picking up, I immediately back off until I find and correct the cause.

 

[fucawi]

Re: And here we have---

Never said a weak mix burned cooler because it does not ..it burns slower.
Boblist there is not confusion pinging or detonation is pressure induced explosion
pre igniton is glowing particle or glowing plug electrode induced early ignition.

pinging can lead to pre igniton not the other way about ...11 seconds to make a hole ..oops too late...
Replacing standard plugs with extended nose does move the initiation point of the flame front..but I think the inital discussion was about surface discharge replaced with standard nose plugs.

So weak mix is bad ....caused by
low float level
fuel pump with insufficient output/blocked filter etc
wrongly set idle mix too rich or weak bad for the lower speed range ....

Now you all go enjoy your Chryslers