85 Big Red 250es hard starting - Rebuild time?

[Epic69]

I have an 85 big red 250es been hard starting for years, in winter I don't bother trying to start. In summer needs choke and will start. Now I have invested in a compression tester. Did a compression test today cold and dry and topped out at 80 psi. Spark plug is dark black. So next steps here? Should I proceed and pull head off? Or would you recommend checking valves and re-adjust and maybe that might help? I have not tried pouring some oil down spark plug hole and try compression test again.

So far I have done a carb kit on it this last fall.. didn't seem to make much difference so now looking at possible top end rebuild....

Thoughts?
Thanks!

[MrConcdid]

Check the valves. .005 then retest. Throttle wide open choke off for best results. Does it smoke?
If compressions stays low after adjusting valves and with throttle and choke open then yes. I'm not sure on the 85 but I know on the 84 and earlier you have to pull the engine. No biggie about 6 bolts.
MrC.

[ps2fixer]

Yea adjust the valves and retest, or do the oil trick to see if the compression increases (just takes enough to get a coating on the rings). I'm pretty sure adjusting the valves can be done for any atc in frame, just isn't super easy to reach both sides always. Since it's shaft drive the drive shaft can be a bit of a pain to deal with. I think pulling the rear axle, then diff and sliding the drive shaft back is the easiest route to pull the engine if needed, service manual might have a better procedure though.

Compression specs from Honda is 164-192 psi for the 250es.

I have those specs and more on my page:

https://atvmanual.com/honda/atc250es/1985-specs

@MrConcdid

ATC250ES is Kick + Electric start.

[Epic69]

Check the valves. .005 then retest. Throttle wide open choke off for best results. Does it smoke?
If compressions stays low after adjusting valves and with throttle and choke open then yes. I'm not sure on the 85 but I know on the 84 and earlier you have to pull the engine. No biggie about 6 bolts.
MrC.

When it does start I do not see smoke. I tested the compression with no carb attached. I will try the oil in cylinder and see if it goes up. If it does then I can assume valves are sealing good and issue is the rings? but when I see no smoke while running that really leads me to think its a valve issue correct?

[Epic69]

When it does start I do not see smoke. I tested the compression with no carb attached. I will try the oil in cylinder and see if it goes up. If it does then I can assume valves are sealing good and issue is the rings? but when I see no smoke while running that really leads me to think its a valve issue correct?

Ok quick update, poured some oil down spark plug hole, retested compression. went up to over 165 now. So that definitely means I need new rings correct?

[MrConcdid]

I don't use the oil test, cause I am not sure what it means other than the oil helped raise compression.

Hard starting can be a lot of things.
Weak spark, blockage in carb, timing off, valves leaking, and of course worn rings.
I always follow this list
Check spark make sure it good
Buy a new ngk plug
Clean fresh gas from clean new tank
New fuel filter
Check set valves
Clean and set carb to factory specs. Jets and float, adjustment screws also get new gasket for carb to intake.
Then start it.
Since your doesn't smoke, I think it's either carb or spark.
That's just me. No smoke no rebuild

MrC.

[ps2fixer]

The oil test tells if it's a top end leak (head gasket, valves), or if it's the rings. The rings are a small enough gap the oil film restricts the air enough to increase the pressure. Normally the pressure doesn't jump right to spec like that lol, but that suggests the top end is quite healthy assuming too much oil wasn't used (less air volume also makes for higher compression).

Based on the data I have, sounds like pull the top end, new rings, hone the cylinder (aka resurface it), and put it together with new gaskets. Proper fix is to actually inspect all the top end parts per the service manual and replace anything out of spec. Doesn't hurt to check everything, a sloppy bearing right now would be a problem in the future.

[Epic69]

The oil test tells if it's a top end leak (head gasket, valves), or if it's the rings. The rings are a small enough gap the oil film restricts the air enough to increase the pressure. Normally the pressure doesn't jump right to spec like that lol, but that suggests the top end is quite healthy assuming too much oil wasn't used (less air volume also makes for higher compression).

Based on the data I have, sounds like pull the top end, new rings, hone the cylinder (aka resurface it), and put it together with new gaskets. Proper fix is to actually inspect all the top end parts per the service manual and replace anything out of spec. Doesn't hurt to check everything, a sloppy bearing right now would be a problem in the future.

Ok pulling it apart today, got the cap nuts off, now looking at the head bolts (star pattern) what size are those? Looks my T40 fits pretty good but not sure. And I worry about those, usually have issues with stripping out the centers... what is course of action if they do get stripped?

[Epic69]

Ok pulling it apart today, got the cap nuts off, now looking at the head bolts (star pattern) what size are those? Looks my T40 fits pretty good but not sure. And I worry about those, usually have issues with stripping out the centers... what is course of action if they do get stripped?

Ok little more research and found out those 3 head cylinder bolts are 6mm allen wrench. So i'll find one of those... I did try the T40 on 2 of the bolts and now not sure of the condition of the bolts. Thanks for the support!

[ps2fixer]

Yea if they are a hex shape, they are allen head, if they are a star shape, they are torx. If you destroy one, I suspect a lot of time with a small die grinder to grind the head off would work, remove the head, the vice grips to extract the bolt. Also an "easy out" could work. Might be possible to drill the head off, but careful to keep centered and not go too far.

Service manual says they are torqued 16-20 ft-lb so shouldn't be too hard to break loose. Head cap nuts are 25-33 ft-lb

[Epic69]

Yea if they are a hex shape, they are allen head, if they are a star shape, they are torx. If you destroy one, I suspect a lot of time with a small die grinder to grind the head off would work, remove the head, the vice grips to extract the bolt. Also an "easy out" could work. Might be possible to drill the head off, but careful to keep centered and not go too far.

Service manual says they are torqued 16-20 ft-lb so shouldn't be too hard to break loose. Head cap nuts are 25-33 ft-lb

Ok got those out successfully. Pulled head and jug off from right side wasn't too hard.
Zip tied cam chain that worked well. Got one chain guide out not sure how to get other one out.

Valves look really full of carbon, cylinder looks nice. Piston and rings look great, could rings look that good and still need replacing? Should I get new cam chain and guides too?

[ps2fixer]

The cleanness of parts don't mean much, inside an engine with bad bearings but a fresh oil change will look brand new yet xD.

You can check the ring wear by putting just the ring in the cylinder, square it up with the piston, and measure the end gap. Top two rings should be 0.5mm (0.020 in) or less. Standard is 0.15-0.3mm (0.006-0.012in). Oil ring spec is 0.2-0.7mm (0.008-0.028in).

Just for the sake of making sure we identified the problem correctly, check your ring gap, careful removing the rings, they are brittle and very easy to break.

The second side of the timing chain tensioner is bolted in on the bottom side under the right side cover.

Also, it looks like the oem/standard piston, I'd check the face of it carefully for any numbers or like STD. If you have a caliper measure and see what the bore size is. Generally it's based in mm. Standard bore is 74mm (2.91in). This will tell you what rings you'll need to order. There's also special tools to measure the cylinder to check if it's out of round, if the bore tapers, etc. It wouldn't hurt to see if you can either get the tools for that, or see if you could pay a machine shop to measure it for you. If it's worn too much, you'll have to have the cylinder bored which is a machine shop process, not a DIY thing. Also before you assemble you'll want to buy a hone or pay a machine shop to hone it. A hone effectively is to resurface the cylinder walls so the new rings can break in, the tool is quite cheap.





Not sure how "correct" of a build you'll want to do. I'd suggest checking the crank and rod out, measure the cylinder to make sure it's good, check out the head for any cracks (don't think these are common for head issues), lap the valves, replace valve end seals, check the guides, rocker arms, etc. Basically all the parts you can access with out a major tear down of the engine. Service manual should have all the specs and how to check the parts.

Just honing the cylinder, slapping new rings in (the correct size), lapping the valves and putting it back together can get you back up and running, but checking the parts out will make sure there's no future problems.

[Epic69]

The cleanness of parts don't mean much, inside an engine with bad bearings but a fresh oil change will look brand new yet xD.

You can check the ring wear by putting just the ring in the cylinder, square it up with the piston, and measure the end gap. Top two rings should be 0.5mm (0.020 in) or less. Standard is 0.15-0.3mm (0.006-0.012in). Oil ring spec is 0.2-0.7mm (0.008-0.028in).

Just for the sake of making sure we identified the problem correctly, check your ring gap, careful removing the rings, they are brittle and very easy to break.

The second side of the timing chain tensioner is bolted in on the bottom side under the right side cover.

Also, it looks like the oem/standard piston, I'd check the face of it carefully for any numbers or like STD. If you have a caliper measure and see what the bore size is. Generally it's based in mm. Standard bore is 74mm (2.91in). This will tell you what rings you'll need to order. There's also special tools to measure the cylinder to check if it's out of round, if the bore tapers, etc. It wouldn't hurt to see if you can either get the tools for that, or see if you could pay a machine shop to measure it for you. If it's worn too much, you'll have to have the cylinder bored which is a machine shop process, not a DIY thing. Also before you assemble you'll want to buy a hone or pay a machine shop to hone it. A hone effectively is to resurface the cylinder walls so the new rings can break in, the tool is quite cheap.





Not sure how "correct" of a build you'll want to do. I'd suggest checking the crank and rod out, measure the cylinder to make sure it's good, check out the head for any cracks (don't think these are common for head issues), lap the valves, replace valve end seals, check the guides, rocker arms, etc. Basically all the parts you can access with out a major tear down of the engine. Service manual should have all the specs and how to check the parts.

Just honing the cylinder, slapping new rings in (the correct size), lapping the valves and putting it back together can get you back up and running, but checking the parts out will make sure there's no future problems.

Yep planning on doing all that for full top end rebuild, and I'll check to make sure the other parts are within spec. I'm in no rush... Brought jug and head into to the local Advanced Auto tonight will have them do the machine work. Honing, lapping valves, etc.. and measure cylinder to see if need standard piston rings yet.

Thanks for all the help here! Very much appreciated!

[ps2fixer]

Neat, I didn't know Advanced Auto did work like that.

For rings, you can buy OEM, they cost a bit, but you know what their quality is like. Off the top of my head I don't know other brands that are known to be good, people that build engines all the time should know good brands though and should be cheaper. Just don't buy random no name rings off ebay.

https://www.partzilla.com/product/ho...9554e588f59ada

[Epic69]

Neat, I didn't know Advanced Auto did work like that.

For rings, you can buy OEM, they cost a bit, but you know what their quality is like. Off the top of my head I don't know other brands that are known to be good, people that build engines all the time should know good brands though and should be cheaper. Just don't buy random no name rings off ebay.

https://www.partzilla.com/product/ho...9554e588f59ada

Ok I have Head and Jug back from Advanced Auto looks good. Cylinder honed, valves all completed with seals, etc. He said good to go for std piston and rings. I ordered rings from Partzilla. Have gasket set coming too. So next is I'm thinking should I replace the cam chain guide and tensioner? cam chain looks great I don't really feel like replacing the chain... I'm going to use exsiting Cam and rockers they look fine too.