787 RFI low power

[Pedec]

Could just need top end like most 2 stroke.

 

[mattrourke]

Yeah could be but since I don't know any of the history behind it, I may just go for the full deal and know that my bases are covered. We will see what I find out.

 

[Pedec]

My many years of running 2 stroke dirt bikes a easy fix was a top end for a bad running bike with low compression. If you know how many hours where on 3d total would help. I know mine has 179 hours after dealer put it on computer so bottom end should be good for awhile yet I would think on mine but you never know with age also. Seals go.

 

[Pedec]

If it was a 4 stroke it would be very expense. I learned that also.

 

[mattrourke]

@Pedec @Poormanracing @mikidymac Well I got a scope and in cylinder 2 I found what would appear to be a hole in the top of the piston. I guess I can stop chasing all the fuel and electrical problems. Strange that my compression tester was still giving me 135 but it is a cheap tester so who knows. Pictures attached for people to confirm. If this happens, I'm guessing a full rebuild and not just a top end is prefered since those pieces may have gotten in to all sorts of things.

 

[mikidymac]

That piston is already on the 3rd overbore at 0.75 and the maximum you can go is 1.00 so those cylinders are on their last leg.

Pull the head at this point and take a good look.

If you had one cylinder at 150 and one at 135, that 15 psi difference isn't normal and was the clue something mechanical was wrong.

 

[mattrourke]

That piston is already on the 3rd overbore at 0.75 and the maximum you can go is 1.00 so those cylinders are on their last leg.

Pull the head at this point and take a good look.

If you had one cylinder at 150 and one at 135, that 15 psi difference isn't normal and was the clue something mechanical was wrong.

Originally when I tested the compression it was 135 on both so I thought I was good. My other ski had 150 on both. I will pull the head and go from there.

 

[mikidymac]

Originally when I tested the compression it was 135 on both so I thought I was good. My other ski had 150 on both. I will pull the head and go from there.

Ok, guess I was confused with the different compression numbers.

It is possible that is a chunk of carbon and not a hole, a hole should show zero compression.

 

[Pedec]

I agree that might be carbon. Might just need some new rings and give it a try. If cylinder looks ok. Remember this is a 2 stroke simply works some times.

 

[mattrourke]

Ok, guess I was confused with the different compression numbers.

It is possible that is a chunk of carbon and not a hole, a hole should show zero compression.

My thoughts too when I saw it. I will pull the head and see what is what to be certain.

 

[Pedec]

I know not your problem anymore probably but I found a pic of your exhaust pipe and what can happen if froze. Not sure if freezing happens where you live.

 

[mattrourke]

Ok, guess I was confused with the different compression numbers.

It is possible that is a chunk of carbon and not a hole, a hole should show zero compression.

Well thank you for staying calm and rational. Was not a hole. I posted pictures of what I found. The little bit of water in the one cylinder spilled in from the cover, it was not already in the cylinder. I think I was just hopeful to find the problem once and for all. All I could find was this small crack in the top cover... Would that cause any issues?

 

[mikidymac]

Honestly the cylinder walls don’t look great.

 

[mattrourke]

Honestly the cylinder walls don’t look great.

Ok that's ok. I don't know really what to look for so I'm counting on you guys to provide the verdict. I guess it circles back to whether or not that is likely to be my low power issue given all the other things I've eliminated? If I try to squeeze one more top end rebuild out of these cylinders, can it be done with the engine still in place or is it coming out to complete this?

 

[mikidymac]

Just cylinders the engine can stay in but I still strongly recommend checking squish and leak down of that engine.

 

[Pedec]

That crack how would it affect engine? Just trying to figure that out. If you run your finger on cylinder wall with finger nail is there any rough spots especially around ports especially exhaust port. Till you get cylinders off and look at bottom of piston for wear or rocking hard to tell. Not terrible like some I have seen.

 

[Pedec]

Dirtbike/sled comparison. It was not chewing itself apart.

 

[mattrourke]

That crack how would it affect engine? Just trying to figure that out. If you run your finger on cylinder wall with finger nail is there any rough spots especially around ports especially exhaust port. Till you get cylinders off and look at bottom of piston for wear or rocking hard to tell. Not terrible like some I have seen.

I don't see how it would either but just a defect that I observed. Only if it could affect the water jacket at all.

 

[Pedec]

If you feel any thing rough with finger nail cylinder will need to be done. Very crude way of testing.

 

[mattrourke]

Just cylinders the engine can stay in but I still strongly recommend checking squish and leak down of that engine.

Do a squish test the way it is right now? I've never done one before but I read some instructions and will give it a go. Will also see what I need to do a leak down test.

Thanks!

 

[Pedec]

What does dome of head look like. Inside where spark plug sits.

 

[Pedec]

Hard to tell in pics can you see much cross hatching or line in cylinder walls still. Those line up and down are no big deal as long as there are no edges. My Ktm 20 250xc- w 2 stroke had those at 10 hours and now at 140 hours still the same.

 

[mattrourke]

Hard to tell in pics can you see much cross hatching or line in cylinder walls still. Those line up and down are no big deal as long as there are no edges. My Ktm 20 250xc- w 2 stroke had those at 10 hours and now at 140 hours still the same.

I could see a decent amount of cross thatching. Will try and get a better picture.

 

[Pedec]

Once you get cylinder off take ring off piston and put in cylinder and measure ring gap. How much distance ring gap is with feeler gauge. Push ring down in cylinder a little past top of travel most wear at that point in cylinder. I use top of piston to push ring back down in cylinder to make sure it is square in cylinder from the top.

 

[Pedec]

Be interesting to see if once you get piston out if you can see more blow by on piston with lower compression. Blow by is dark colouring under piston rings. Or scuffing on bottom of piston from ring being worn and piston rocking. How many hours will you put on 3d in a year. If less than 30 hours a quick
Fix might work but if put 100 hours a year on might need more work.