I recently sent a 250 Trail Blazer cylinder off to a guy on eBay in FL to have it rebuilt. The first hokey thing was he wouldn't take payment through the eBay account. The advert was professional enough, and talked about beveling the ports, making lots of compression, etc.

To make a long story short. The cylinder came back cut 73.5mm (.60 over?) with a new Nakamura piston set.

All the ignition electronics were replaced and coil and trigger coil values look good. Has good fire, and Carb was thoroughly cleaned and a new kit including all needles and jets were installed. Settings are 1 1/2 on pilot and slide set to just beginning to move up. Timing is correct.

I've rebuilt Harley Sportsters and 4 cylinder Hondas and carbs, so this isn't all new to me. My first 2 Cycle, though.

Anyway, thing originally had 30lbs of compression.

Would not start after rebuild. Checked and got only 90lbs on new rebuild. No start, just an occasional kickback and if I tried the pull cord after a few pulls it ripped the cord handle through my fingers. Not doing that again.

Anyway, I check compression again, was down to 80lbs. Tried some 2cycl oil in the cylinder and came up to 90 again.

I took it apart and put sealer on base gasket and replaced the top gasket. Noticed rings sticking a little around the alignment pins, so I filed the grooves until the rings could be push all the way in and they would not stick.

Retorqued everything. 90lbs... then with 2cyl oil.... 90lbs... so tried a little 10w 40..came up to 100lbs. Still no start.

It did try to run once for about 3 seconds, so proves timing was right, I think.

This is all after I ruined a new starter trying to start it. Then back to an old starter, after the rebuild, which works even better than the new one did.

I tried at least twelve times letting start cool for 10 mins, after the 2nd rebuild.

So, I pulled it apart a 3rd time (btw-pipes are clear and gets lots of smoke when trying to start, when it kicks back).
Finally, I found a manual (was working off old Polaris guy posts). I mic'd the piston 10mm above the skirt (about 2.890 in, which is about right for a 73.5mm). Then I measured the cylinder with telescopic set using an old .in micrometer, because I lost my digital. It read consistently 2.93.inches. So I am asking someone to check me on this.

73.5mm = 2.893701 or about 2.894

if max clearance = .006 .in then that would be about: 2.894+.006 = 2.9 inches. So, that would be considered the maximum service limit, right?

The cylinder measures an average of about .003 above that, or 2.893 .in.

Is that right, or am I making a mistake here?

Please let me know if you see anything wrong with my procedure.

I was expecting something like .0015-.002 + 2.894 = about 2896 max.

Still it's .001 above the .0021 piston clearance range.

The end result is, I'm still not getting more than 90lbs without oil, and I can't get it started to get it to seat the rings.

Do you know any old school tricks to increase compression and get it to start up?

I guess I'll call the machinist in the morning and see what he says.

Thanks in advance!

 

Same old problem I've seen a lot of people have sending a cylinder out. Unless specific clearances are asked for,some so called experts will bore way too big.. (2.940 cyl- 2.894 piston =.0046 clearance) .0015-.002 max is what I'd try to have on piston to cylinder clearance. Looks like you're between.004-.005. You might try crank seals to see if it will run first if you haven't replaced them already.If you have ,then an expensive lesson learned. You might try to "work" something out with this guy first before becoming ballistic. Even easier "home" way to measure is just take the rings off and use a set of feeler gauges. Look towards the end of this video. Shows the feeler gauge method.

 

Thanks, I more or less thought that would be your answer. I made a correction, because I read the micrometer wrong, upon checking it, because 2.94 would have been .046, and that could not be right.

But still it was 2.893 and the skirt reads slightly less than 2.890 at 10mm (also used better glasses, this crap is harder than it sounds, and my telescoping tools have to be read with a caliper, after the fact).

Anyway, not prone to ballistics (when complaining to the cook).

So, clearance comes out to .003-.004.

I did put in new seals. The carb seems right. Only floods the crankcase if you try too start too long with the choke on, but not when it's off. No carb overflow from the floats. (Yes, I drained the case a few times).

Can hardly believe the tolerances are that tight.

Thanks again.

Would it make a difference that I don't have hoses on the two vent pipes, right now? I'd think it should try to start, even without the carb mounted, by spraying a little fuel into the intake.

 

If you look at the manual on these old 250s you'll see piston to cylinder clearance when new are between .0014-.0028 max.the bottom line is that if you're still showing 90 psi on a new bore,that's way too low! Should be close to 130 psi if done right and seals replaced with no leaks. This is where a leak down test might reveal something. Even a case leak/gasket leak can cause it not to charge the mixture enough to ignite.

 

Ok,
I'm going to work on putting it back together and sealing it up today. I have a brass brake bleeder made by one of the Chinese companies, but it lets the vacuum loose when I release it, like a bad check valve or something.

Still I think it will work if I hold it steady, long enough to measure the leak-down rate.

The guy on UTube said to put the piston to bottom dead center. I would think it doesn't really matter as long as the intake and exhaust are blocked correctly. Any thoughts?

While I'm at it, I'm going to check the ring gap, for any taper in several places.

I hate to remove the wrist-pin clips to try the feeler clearance method, I guess I can do that by just removing the rings and sliding the cylinder on. A couple gauges at .001 - .002 should be thin enough to bend around the cylinder, after all, that is just about the thickness of a large piece of paper. A double-check is a good idea, anyway.

You know, I even tried shooting a little air into the cylinder with a hair dryer, while starting. I considered warming it up real good with the dryer, just to get everything to expand a little, and then try it. Bet I could get 500 degrees in there in about 10 minutes.

Well...last resort, maybe. Gotta get it going, somehow.

The machinist mentioned in the ad they would bevel ports. Seems like the manual said its not needed on the 250, but does not look like that was done, anyway.

 

7 psi of air will find it's way out if it's leaking no matter where the piston is located. Only way to use the feeler gauge correctly is to remove the rings. Basically you'd have to tear it back down if it comes to that. On every bore job all ports should be lightly beveled mainly to remove any rough edges from boring that could grab a ring. On that I just used a small round file at an angle to work around the ports. Simple to do.

 

I did get the feeler-gauge clearance test done. The results were not good.

I was able to slide gauges down each side of the cylinder for a measurement. I confirmed, at that point, that they reached the length of the skirt, knowing that the pistons are smaller at the top than the bottom, and the measurement is supposed to be made 10 mm up the skirt (Sorry, didn't mean to sound like a bad porno).

Anyway, I was able to measure up to .006 in, with only slight resistance, at the full length of the gauges. I attempted .007 in, but it would start to stick at 3/4 length, about the point where it reached the 10mm area, so didn't try to force it.

So, it looks like the cylinder is bored just past the specified service limit.

I had to replace plastic coolant elbows on my wife's 3800 engine GM car, so didn't get time for setting up the leak-down test, yet. A 45 min job took all day. Someone had used something like epoxy to seal the oil rings, rather than RTV. It took forever to remove broken plastic pieces and sand the ports smooth. Seems, like nothing is simple, anymore. At least there is great info on the GM forums about doing the replacement.

Do you think I should still do the leak down test? Probably would not hurt to have that info if they ask about it, when I contact the machinist.

 

You already know the cylinder needs to have a new piston and bore job with the old one .006 or over. No sense in doing a leak down test as the engine would have to be assembled to do one... Just replace the crank seals as you will have to have a new over size piston,head and base gasket already.Plus this time specify that piston to cylinder clearance after honing should be no more than .002. Sounds like you'll need to have a 74mm piston kit this time. Plenty of them on ebay. 74mm Polaris 250 piston kit | eBay Plus that's the largest over size you can go before you have to have a new sleeve installed or find a used cylinder,so it needs to be done right! http://www.wiseco.com/ProductSearch.aspx

 

Wow! Your next will be your 15,900th post! Kudos!

I just want to say, I'm sure I can speak for many, that we all appreciate your passion and of course your help. I'm sure many young men (and perhaps women) and elders, alike, like me, who only "thinks" they know something about it, have been blessed by your legacy on this board.

My father was a 21 year motor-pool sgt in the Army. My first overhaul was at 12 years old on a 52 Studebaker, in 20 degrees and blowing snow (yeah, yeah, uphill both ways). I also have a degree in electronics, yet even I took a lesson in modern 2 Cycles from your posts.

Don't think your are under appreciated.



Thanks for the info. I didn't even look. I was thinking 73.5 mm was the maximum bore limit, so you have saved me at least some money, there.

I find it hard to charge someone for my poor judgment.

Well, time to contact the machinist.

 

OPT is the man!