After all these posts one thing I don't think has come up is what is compression when the engine warms up.Back firing can happen when the engine warms up and rings start to loose their seal against the cylinder walls.They can back fire,start to bog down and sometimes die. Let em cool down a while then they'll run good until they get hot again.Back firing can be electrical break down related also,but I'd make sure on compression first just to eliminate it as a cause.

 

'Aight. It will be over a week until I can work on it again, though. We never have checked the compression on this thing, so this might surprise us all. May end up that this thing should have not even been functioning to begin with :P

 

Checked her today, she is about 112psi. Our gauge and rig is older than me, though, so I don't trust it's air retention ability, as she bleeds down pretty quick before you release it. Doesn't have a bit in the line that can rotate, so screwing it in was a bloody nightmare, seeing as how far the plug hole is recessed into the cooling fins, you can't get a normal open end wrench in there. My fingers are begging me for mercy already, and I still gotta get the sucker back out.

Also, I have no way to check it once she warms up, because by that point, I will burn half my hand off! Plus, even if it wouldn't, by the time I get the darn thing screwed in, she will have cooled back off. Yeah, took THAT long. I do get her up to operating temp before I start off, though. She runs fine, even then. Only until I try and move her under her own power, at anything above still, does she get unhappy. Plug was nasty. Was brand new one, put it in as I started getting her apart for the carb rebuild. Cleaned it (we have a mini 'sand'-blaster for this, cleaned so many, we just got to this point :P), now it is only dull grey, instead of world eating black (black hole reference there). Still reeks of gasoline. Gonna take the body off sometimes soon and just pressure wash it. Needs it, and bad. The schmoo is real with this one. Also, it builds back up really quickly. Beginning to suspect a leak in the gasket mounting the carb to the engine, or one of the oil hoses running up to there, because it is all black, oily schmoo, and it sure smells like oil.

 

Don't use one of these sand blasters on plugs! These things have gone the same way as the dodo bird..We had one in the early days at the shop and junked it.Just have a new plug handy.112 psi is close to minimum.If you rebuild the carb and it still leaks,you need to find the problem.

 

Eh, it is convenient. Also, I wouldn't bother if the electrode end was straight up fouled. Was still good, I could reveal the proper colors by scratching it. Used it to get the schmoo off that I couldn't be bothered to find the ice pick for.

Oh boy, well, we have rebuilt the carb. Darn. Any tips on finding the problem? Not looking forward to pulling an engine out if at all possible!

Also, what happens to be the magic number I need to shoot for?

Also, I should probably go about fixing the title to be less urgent, because the time for that is long past.

 

Magic number on a new or rebuilt top end is 135-140 psi. Used cylinder 120 psi or more is good. On the carb check close that the float arm is snug side to side and not worn and wobbly. If it is replace the float arm and pin. Doesn't come in kits and a lot of people over look them. Again the float arm is set parallel with the float bowl gasket seating area. Wouldn't hurt to bend down the small float arm tang just a bit if you have to so the fuel can shut off a little earlier and bowl level is a smidgen lower.

 

Yeah, I made sure of that when I did the rebuild. I actually have it a little lower than parallel already. Good to know what I need to go for. Will try a few leak tests to figure out where and what is causing the compression to be so low. I have that sinking feeling that it might be time for some new piston rings. She has easily got over 10k miles on her, it was a matter of time. Yes, really, 10k. We never really babied this thing after we got it working initially.

 

Don't just slip new rings on a worn piston and cylinder bore.Too many people have found that out the hard way.That's why they make up to 2mm over standard bore size. 10,000 miles on a 2 stroke... Yes I agree it's time. Look at the bottom of this section on Wiseco pistons(the best piston around) and see the over sizes available. Also if you're not the original owner,it may have been rebuilt before.Top of the piston will show it size if it has been. Dealer or a good machine shop can measure the cylinder and tell you what over size size is needed.721M07450 - Wiseco Piston for Polaris 300cc 2 stroke. Std size | ATV Parts | MFG Supply

 

No, not the original owner, but we knew the original. They just dumped it on the side of the road next to a sign that said please take me. Common practice down here. We took her. Cylinder was frozen up, and we got it free, put the right oil in it, they were running really thin stuff, and put a new battery In it. A peice of junk they called it. They never did ANY basic maintenance on it. They probably put about 100 miles on based on how they said they used it, and for how long they had it.

Just checked the compression gauge, it is dead, put 100 psi in it, didn't take but a fraction of a second to get back to 0. Tried test multiple times, and no, the bleed was NOT open, I made sure. Also, it only read it at 30 psi. Tried 85, as it was draining the compressor some, and it read as 50. Thing is all wonky.

Next up, I went out and cranked it. Now it is running rich at idle, which it didn't do last time, and I did in fact store it with the fuel switch off. I was starting to pull the carb out, but then I thought, okay, let's see if she is really rich. Pulled the fuel line off, and cranked it. She smoked just as hard right until she starved. Either she is burning a chunk of oil, and maybe getting too much, or the needle jet is too small. Yes, it is at the very top position, to run as lean as possible. Temporarily will rule out low compression until I get a new gauge.
It ran perfectly until the flood happened. I don't feel like suddenly the compression has been reduced when I have not taken apart the engine, or done anything like that, and have been careful to not damage seals at all. Maybe I am vain, maybe I am right, but I absolutely do not want to put any big chunks of cash in this if I do not have to just to keep it functioning. We got it for pretty much free. So in terms of rebuilding it, if it comes down to it, I will. But not unless it just will not function at all. Because I am a hard-***, and also a bit broke at the moment, so I would prefer not to dump a chunk to get this. Tangent aside, we will see.

 

"put 100 psi in it, didn't take but a fraction of a second to get back to 0. Tried test multiple times, and no, the bleed was NOT open, I made sure. Also, it only read it at 30 psi. Tried 85, as it was draining the compressor some"

Not comprehending what you're doing.. Sounds more like a leak down test on a 4stroke.