Piston Color and Condition

[glenwhitt]

Hoping someone can chime in. i posted the question in a different thread and had lots of good info from timmyboy but I have nothing on what the condition/color of the pistons are telling me here. the pictures are from my 787 with 3 hours on the new top end and 23 on the new bottom end. The three hours also had new mikuni 40mm sbni carbs with mains at 147.5 mag 1 turn and 150 pto with 2 turns along with prok arrestors. There was also a carbon fiber rotary valve that was not factory degree but a bump up to make for low end. Everything else was stock. First two hours were great but very hard starting after warmed up. the last little run it started sputtering and running funny (just did not feel right) so i limped back to trailer and took home. Ran compression and i have 5 psi dif in them. being paranoid i decided to tear down and inspect.

One other thing. The pto jug was full of water so the drain at the bottom must not be working. I replaced the hose with clear hose 23 hours ago when i did the entire motor late last year. The hose is clear of any debree and the mag side was drained fine. When i removed the head i noticed water in the pto cyl on the piston. I cleaned asap and used a suction gun in the bottom end looking for water but found nada there. WD everything and wiped cyl walls clean.

If you have some info please chime in. Thanks

 

[seadoosnipe]

Detonation?...

I know you don't want to hear this but from the angle of the picture (and it could be the way the light is hitting it), you have the early signs of detonation.

If this is the case, I think it could be in part by you changing the degree, ever so slightly, on your rotary plate along with the flame arrestor.

The reason I mention the flame arrestor is because of the increased size of your jets. Stock, you should be running 145 MAG and 140 PTO with a pilot of 67.5.

It's not to say that mods will cause this, just that if the system isn't matched out right, it can happen.

I was reading an article on detonation the other day, I'm going to pop it in here in italics so you can take a read. The reason is because in your situation, it seems possible that your change of duration on your rotary valve, may be playing a part in potential problem. I'd recommend you leave the jets in but go back to a stock plate. The water in the PTO piston is probably just from the drain, nothing mechanical that caused it.

Continue to read below. This came from a racing web site in the U.K. I thought it was straight forward and simple.


Sensing when an engine is detonating has become more and more important in recent years. In older cars, the reduction in the lead content of fuel has meant that detonation is more likely to occur - especially when the comp ratio is wound up to give good performance. In factory turbo engines, dramatically increased boost means that detonation is very probable, while in any engine, poor mapping of the ignition advance can easily lead to detonation. But while sensing when detonation is occurring initially looks to be pretty straightforward, it becomes much more complex when the subject is examined in any depth.
What It Is
Engine detonation occurs when the air/fuel mix ignites within the combustion chamber in an uncontrolled manner, instead of by the progressive action of a moving flame front. The terms 'ping' (a light, barely observable detonation) and 'pre-detonation' (detonation caused by the ignition of the charge slightly before the full ignition of the flame front by the spark plug) are also commonly used. 'Knock' is another synonym.
One definition of knock is "an undesirable mode of combustion that originates spontaneously and sporadically in the engine, producing sharp pressure pulses associated with a vibratory movement of the charge and the characteristic sound from which the phenomenon derives its name". If detonation is allowed to go on for more than few seconds, the very sudden pressure changes within the cylinder can damage the engine. In a worse case scenario, pistons, rings and even the head itself can suffer major damage. Obviously, heavy detonation is something to be avoided! Note also that the higher the specific power output of the engine (ie hp per litre), the greater the likelihood of damage if detonation occurs.
In everyday driving, detonation is most likely to be heard when the driver is using a gear too high for the engine speed and load conditions - like climbing a steep hill with the right foot flat to the floor, while in third gear and travelling at 40 km/h. Depending on the engine, detonation can sound like a 'ting, ting' noise, or even a little like coins rattling in a coin tray. However, in some engines, the audible note is much deeper when heard from the cabin. In turbo/blown cars, or cars where the compression ratio has been substantially increased, detonation can occur at high engine speed and high loads, making it very difficult for the driver to hear it over the general noise level that's present at the time.

 

[glenwhitt]

Jet sizes

Good stuff mister snipe and much appreciated. Are you sure on the jet sizes in the MAG and PTO? I have always ran a larger jet in the PTO to get any decent color, on both top ends. Is this my problem? Do I have a massive air leak in the PTO side that is there after two top ends?

During the last run i had a 147.5 in the mag and a 150 in the pto. Tomorrow i will have a 152.5 jet at my door, along with other goodies, to replace the pto 150 jet and move it to the mag carb. So my next run was going to be 150 mag and 152.5 in pto. I also have oem rv on the way.

Did squish gap test tonight on the head and it is at .043 and.044. I am sure i will have a much different reading on the compression gauge next time when i remember to push the throttle forward this time.

I will solder test the clearance on the rv cover tomorrow along with leak and pop off test the carbs. I have a new 100 psi pop off tester on the way tomorrow also.

I tested the water retun lines and fittings from around each cyl also. It appears the jugs are fine but somthing is blocking this in the last 1ft of the path out. When i did the engine last year i notice the line was full of sand and fully cloged, sucked up some dirt and it went through the system, so when i replaced this i but a splice in it so i could clean and replace the section that lies in the bilge. That part is fine now. It is the other small piece that runs up under the muffler. The piece i can not see. I atttached a clean line to it and blew through but nothing. I is not letting any air through. Is this controled by rpm or something? Should I be able to blow through this.

 

[glenwhitt]

Putting back to stock

Done a lot of work tonight, and drank a few beers

Factory jetting is 142.5 on mag and 147.5 on pto. I rejeted carbs to this. I also checked pop off at 23 psi per carb with black spring. Needle holds fine in seat also with 10 psi. Checked fuel pump diaphragm also.

Squish on head was good so I cleaned and used new gaskets and bolted head back on. With carbs off now I have 149 psi in both cyl (last time I checked before tear down I forgot to open the carbs up for air).

Put 159 degree oem rv back in and timed. Right on the mark at 146.5 (+/- 2.5 with play). Bolted rv cover on also with new oring.

Removed oil injector pump and installed block off kit (put caps on rv cover and zip tied before bolting rv cover down also).

I have pipe cleaned and ready to bolt back on with new gaskets and carbs ready to bolt back on with new gaskets (will synch them to the tee also). Then its to the gas station tomorrow to premix some fuel and start this mother. Sunday I plan on lake testing ( and drinking more beers). Will start at 1.25 turns out on low and .5 turn out on the high. I think this over but better rich than lean on the get go so I can adjust. Oh ya... Will have factory air box on this also.

Bottom line, after several thousand dollars in new motor, and then a new top end, and all the mods, I am back to oem and I have a funny feeling this will be the best it has ever ran

I will update tomorrow or Sunday after the lake test.