Big problem this time - bike sounds like a sewing machine and no power

**xwhatsit** · 21st October 2006, 22:14

Yeah... engines don't scare me anymore. Arthur C Clarke once said... `Any technology sufficiently advanced is indistiguishable from magic'. Well my engine is not magical anymore. I know stuff about it (ok, maybe not much) and I like that feeling

. I think I'd like to get into this mechanical game. My next bike I think I'll get something a bit older again... something that I can wrap my head around.

I'm seriously considering that XL250 though. If I can get it for under $200 I'll be pleased. Could somebody please look at these photos: http://images.trademe.co.nz/photoser...58554_full.jpg and http://images.trademe.co.nz/photoser...58711_full.jpg and tell me if you think that's a CB250RS motor? It looks damn near identical, except it's missing the rev counter gear and decompressor lever. I wonder...

**Motu** · 21st October 2006, 22:14

Not knowing gets us though where others would give up....ignorance is bliss....

**xwhatsit** · 21st October 2006, 22:18

Did I do something stupid? I mean... I guess it's not smart to run around on a cracked cylinder head. But would you have given up at some point when disassembling the engine?

I don't mean that to come out rude, I sound mad. But I'm interested... what did you mean by that?

**Ixion** · 21st October 2006, 22:25

Well, you might be unwise to embark on a tour of the South Island on it. Cracked head and all. But the proof of the pudding etc, it's together, and running. And as you say apart from the risk of being stranded , a cracked head prolly wont break anything else.

I'm just astonished it actualy ran in that condition says a good deal for Mr Honda's engineering.

Not many bikes would take that sort of treatment.

**xwhatsit** · 21st October 2006, 22:31

Ah yeah... I knew that having an oldish bike might put me in some level of risk right from the beginning so I've got an AA membership if I really get into the shit.

You guys are starting to scare me lol. I knew that fixing it up like that wouldn't be permanent but I wasn't that worried that I was doing something very naughty lol! That said... it must have been running cracked for a while. These things just don't happen overnight. I wonder if that's why the guy sold it to me... he'd only had it 4 months or summat... and the head has obviously been apart before with all the non-standard bolts in the thing.

*Sigh*.

**Ixion** · 21st October 2006, 22:50

Don't fret. I've done worse. So has Mr Motu. I'm just impressed that a modern engine can handle it. Usually these days anything at all not exactly right and they won't run.(It certainly wouldn't if it were water cooled. Go the air cooled engine)

It's back together. And running. That's pretty creditable.You did well . And at worst it dies on you and you have to call the AA for a lift. Anything you get in the meantime while looking for a replacement head is abonus

In theory cracked heads can be reapired by welding or metalstitching . But its not always terribly successful and likely to cost as must as a replacement or more.

**xwhatsit** · 21st October 2006, 23:02

Haha. Not really a modern engine... unless 1978 is modern.

Yeah I don't really want to try and repair the head. Especially as many of the bolt (sockets? Whatever you call where the bolts wind in) are threaded. It's obviously seen some abuse. My borrowed torque wrench and half-guessed torque settings were wasted on it haha.

This is such fun. Going to ask the bank for approval on a loan (what I really mean by that is ask mum if I can borrow $200 off her ^_^) and get that XL250. If that's not my motor in it (according to photos) I'll eat my helmet.

Thanks for all help! Maybe you'll see another thread in a week... `where did this bolt come from? I can't find where it's supposed to go...'

^_~

**Racey Rider** · 22nd October 2006, 07:51

Wow, Good effort!

Surprised to hear the valve seat was still usable. Go the backyard Mechanics! The best way to learn to be sure.
Well done for getting it all back together. And Running!

You say some threads are stuffed. Won't they be in the barrel not the head? (ie. the bolts go down through the head and screw into the barrel.)

**nadroj** · 22nd October 2006, 09:09

I spent friday night with a wrecker chasing a Kawasaki KC100 right hand switch block to no avail but he did take me to 3 sheds in the country where he had heaps of spares ( and I mean heaps). I don't have his phone number but I travel past weekly so can call in & see him if you want me to. Just send me your details and the bike model & parts required & I'll see him next week if you haven't already found what you need. He seems to have heaps of 80's parts, mainly Japa's, including some GSX1100 & Z900 parts but the majority are smaller motors. He also has several race bikes and 2 sidecars (one with a TM400 2 stroke single motor, the other with GSXR1100)

**xwhatsit** · 22nd October 2006, 14:16

Nadroj: Thanks! I've sent you a PM and will be in touch if the trademe XL250 doesn't pull through.

Racey Rider: Yeah valve seat was undamaged, it just became... unseated :P. The threads that are stuffed are the ones that the valve cover screws into. I think. I may have forgotten. If I pick up that XL250 then I've got a cylinder barrel too. When I swap the head out I'll take a look inside the barrel too, check out the pistons and rings etc.

It does run, but it may just be the carb needs tuning in again (due to different airleaks in the engine now or something?) because it idles even worse than before. If I get the XL250 and the carb on it is suitable (I don't think it's standard from the photos I saw) I might take a look at that, because my carb is a bit rooted (the throttle-triggered automatic fuel pump thingie). If not I think I know how to fix it.

I really should be studying for exams, but this is far more interesting lol.

**Motu** · 22nd October 2006, 16:31

Originally Posted by xerxesdaphat

But I'm interested... what did you mean by that?

The exhaust valve seat performs a pretty important function.It is supposed to be an interferance fit,it came out because it was loose,and had been loose for some time.So rattling around in the head it no longer makes good contact with the surounding metal....peening it in tight doesn't mean the seat is actualy contacting the head.Plus the seat face and valve seat will of been damaged,the valve possible very slightly bent.Now there is a very poor heat path from valve to seat to head - if the seat doesn't fall out again soon,it will burn before long.Even mechanics don't fit valve seats,these are done at an engine reconditioning shop....although I have replaced valve seats in Briggs and Stratton engines....

**thehollowmen** · 22nd October 2006, 16:36

Holy (bleep)

I'm amazed that it still ran, although not too surprised.

good luck getting her fixed up.

**JMemonic** · 23rd October 2006, 09:01

Its been said but good job on the effort to fault find and figgure out what is needed to repair the bike.

Personally in your situation I would look at a second hand head as opposed to welding the old on up, get it cleaned up and check it for cracks and faults before you attempt to assemble it on to the bike, lapping in the valves would be a good idea as well that way you can see if there are any defects in the seats.