'Bout that time again! xr 250 top end rebuild

[MIKETHEBIKE]

Great write up and pics, well worth the read,

[lowflyer]

Brilliant write up

Lots of good tips illustrated along the way, well done


You taking notes Mr Beddows ??

[waffles]

Cheers guys!

[xralanp]

Excellent work! I'm impressed that you have the self control to photograph things as you go along, I do a bit BUT mostly I'm so intent on the work I rush on and on.
Cheers.

[jasonbc]

Some of you may remember that I rebuilt my bodged together xr250 over the summer, during its time with a previous owner it has sustained some serious damage and had been rather badly repaired. I rode it a couple of times and was out with some friends when they pointed out that it was churning out funky coloured smoke from the exhaust when I rolled off the gas. That, coupled with the fact that it was drinking oil like its going out of fashion, all pointed to the need for some investigation. I started this before Christmas and completely forgotten to post up here so I am going to copy across my rebuilt writeup!

27th November 2011

This is how the xr looked after its makeover in the summer....



Compression test was the first task, according to the Haynes manual compression for my bike should be between 180 and 191 psi and the xr was producing somewhere around 80. So here we go again, stripped down and back in the workshop for some engine tinkering.



The first thing to check over was the carb as it may have been tinkered with by the previous owner, the bike is running too rich so checking the settings would have been the logical place to start. Subframe had to be lifted up, wiring free'd up, cable ties cut and bolts out. Sadly I rounded one of the bolts holding the subframe in place so it needed some "persuasion", I need to add some of those onto my shopping list.



Taking the carb apart revealed there was some grit in there but nothing that would suggest the fuelling was set up incorrectly. Arse, time to tackle the engine it is then! I took off the exhaust (this needs repacking anyway) and headers which were starting to discolour where they bolt onto the engine. Another sign that all is not well and its running too hot.



This looked familiar, I got a lesson in how 4 stroke engines work by cranking the engine over and watching what happened. After finding all of the bolts (there are loads by the way, some of them are craftily hidden) off came the top of the engine. The plan was to strip the engine down to check out the barrel and piston.



Camshaft, just to the right of it is the camchain tensioner. The Haynes made it look very easy to remove using a high tech piece of equipment - the Honda opened-up-paperclip. Half an hour of struggling, head scratching and cursing later the executive decision was to leave the f*****g thing in place and carry on removing bits of engine. Clutch cover off with some difficulty as bolts were stuck and the gasket had welded everything together.



Im going to work on the clutch later as the bike has issues going from 1st to neutral when the bike is running.



By this point it was getting on a bit but may as well keep going and delve deeper. I pulled out the oil filter at the bottom of the engine and checked it over. Looked in good nick to me so I gave it a quick shake off and replaced. Time to go for broke and get to the piston. As the bolts came out they appeared to have baked on oil over them and they smelt like burnt chips.





Viola, it looks terrible! The smell was awful and there were chunks of oil sitting ontop of the piston. Turning the engine over so that the piston dropped back down revealed something interesting.



Around the side of the barrel is heavy corrosion and some pitting, pretty sure the engine has been drowned in a puddle at some point. Hopefully this is the cause of the poor compression and terrible oil comsumption!

The to-do list now stands at:
New subframe bolts
Rebore
Oversized piston
New gasket set
Inspect clutch plates
Repack exhaust
Stop fiddling with my bikes!

If you go for a oversized piston don't get a 440 the bikes can't cope with them and it will end in tears over a period of time. You may not need a rebore can't judge by the photo a good honig might be enough. I would definatly go for new rings and in the head decoke the valves and lap them in i would go for new valve seals and there wouldn't be any point really fitting new valves seals without fitting new guides they are less than a tenner each just need them pressed in then cut.

Dunking a puddle won't leave that corrosion, as soon as the water gets in and the bike is running the engine will blow the water out through the exhaust valves and the heat from combustion will get rid of any water vapour.

While you got that engine cover off don't forget to blow out the oil strainer it could be full of sh1t

[waffles]

If you go for a oversized piston don't get a 440 the bikes can't cope with them and it will end in tears over a period of time. You may not need a rebore can't judge by the photo a good honig might be enough. I would definatly go for new rings and in the head decoke the valves and lap them in i would go for new valve seals and there wouldn't be any point really fitting new valves seals without fitting new guides they are less than a tenner each just need them pressed in then cut.

Dunking a puddle won't leave that corrosion, as soon as the water gets in and the bike is running the engine will blow the water out through the exhaust valves and the heat from combustion will get rid of any water vapour.

While you got that engine cover off don't forget to blow out the oil strainer it could be full of sh1t

I have already fitted an oversized piston with rebored barrel along with new rings and done all of the valve guide seals etc with writeup and pictures..... Im pretty sure I mentioned checking the oil strainer also.

[hotbulb]

Another excellent write-up! And as Boboneleg says, you'll have a bike you can be confident in, because you know the work's been done properly.
I'm jealous of your cylinder-head: how did you get it so shiny?

[Lutin]

Top job.

You are clearly in the wrong profession - you could, and probably have, taught a few bods on here what a bike rebuild is all about.

Not sure about polishing the inlet tracts, by the way - exhaust yes but not sure about the inlet. I'll dig about and see if I can find the article I have in mind.

[416cc]

I have already fitted an oversized piston with rebored barrel along with new rings and done all of the valve guide seals etc with writeup and pictures..... Im pretty sure I mentioned checking the oil strainer also.

Mmmmm. Yes. Not sure how one would fit a 440cc kit onto an XR250R bottom end. And the checking of the oil stariner was mentioned earlier in the post.

Methinks someone didn't read the whole post...

Another excellent write-up! And as Boboneleg says, you'll have a bike you can be confident in, because you know the work's been done properly.
I'm jealous of your cylinder-head: how did you get it so shiny?

Cylinder head had lots of elbow grease expended on it. It took longer than 5 minutes...

Top job.

You are clearly in the wrong profession - you could, and probably have, taught a few bods on here what a bike rebuild is all about.

Not sure about polishing the inlet tracts, by the way - exhaust yes but not sure about the inlet. I'll dig about and see if I can find the article I have in mind.

The way that the inlet ports were previously shaped was like partially shutting the door onto the ingoing air/fuel charge. With the knife edge it more cleanly divides the charge. The inlet ports arn't polished to a mirror finish. Just smoothed & then finished with 600 grit. The exhaust ports are similar but finished to a 800 grit.

[Lutin]

The way that the inlet ports were previously shaped was like partially shutting the door onto the ingoing air/fuel charge. With the knife edge it more cleanly divides the charge. The inlet ports arn't polished to a mirror finish. Just smoothed & then finished with 600 grit. The exhaust ports are similar but finished to a 800 grit.

There has been lots of debate about port shape and polishing. This article from Wikipedia "The Port and Polishing myth" is not the one I was thinking of, but does explain why a smooth finish to the inlet port is not always the best option.

I have another article by Peter Williams that explains why the "technique" of opening out the port to be as big as possible is, more often than not, the wrong thing to do. I have it as a PDF and could PM you a copy if you are interested.