17 thousandths of an inch?
 

Hi good people,

Thoughts on this welcome...

Ok - an xt350 rebored up a bit to ???cc (bloke told me when i got it), 60-odd thousand miles.

Lumpy running, farts and bangs - especially when dropping off the revs - tends to then stall and stay stalled for a few sweaty, cursing minutes until she'll boot up and pull off sweetly enough (for a while (maybe 5 minutes and/or until i lay off the revs again))

After a few months of fiddling and finding that timing, electrics, carburation all hunky-dory i got to my limit (tether-wise and practical mechanical!) and took it to mechanic.

The exhaust heats up too much and, after a boot or two, he thought compression was down, but it seems that this is sporadic...

A-ha! Sticky exhaust valve-y type problem, you'd think, but he says no... he reckons that the [a] the cylinder is an oval shape by 17 thou and [b] its a touch too big anyway (on the phone just now he said something about 40 somethings - but I didn't quite catch the units - whereas the upper limit should be 35(?)

He reckons it can't be ground round (because its been ground out to max already) and so it'd need a new barrel (either ground to fit my piston or plus an original piston)

===================================

Sorry for maybe too much detail, but I'm a bit of learner on bike mechanic- stuff...anyway, how does that sound?

If compression is the problem, how does this explanation fit with it being sporadic?

Why does it play up and die when the load on the engine is reducing?

17 thou doesn't seem like much to me (or is it?)

Is it not poss to just get slightly larger piston rings?

Worth the rebuild...?...or, bearing in mind the mileage, is the bike a dog and it's time to take it whimpering round the back of the shed??
(interesting thread elsewhere re. the 200k+ mileages btw).

Hmm - well, cheeers for reading people.

C u on the road one day (he says with crosssed fingers)

;-)

Pev


ps.
I got this bike to learn some basic mechanic-ing before i go to the Americas and buy one out there...you can therefore imgine my pleasure at finding myself landed with a lemon from day 3!

teehee
;-)

hasta pronto...

sounds like more than just the barrel - but it's definitely got to be sorted first.

And a mechanic would call 17 thou on an out-of-round barrel "big enough to drive a truck through"

So yes, .017" is a LOT! It should be around .001 - .004 or so CLEARANCE - and ROUND.

So yes, replace the cylinder or possibly reline it. Start with a new standard size piston and rings, then you'll have a chance to bore it out again - if you use the old piston - the NEW cylinder will have to be bored out to the LAST oversize to use... doesn't make a lot of sense does it?

Once that's sorted, see how it goes from there.

good luck!

Grant

You say the bike sounded ok at higher revs?

There's an old saying among tuners; It runs best just before it blows up.

A friend's CB350 twin engine died this last summer. When we took it apart we found the barrels out of round between .006" and .010" and one of the pistons was melted. The plug on the melted side was ash white (with molten aluminum lumps). We found air leaks which presumably caused lean condition, overheat, engine death.

Unfortunately, this friend is not particularly mechanically savvy so she had no clue of a problem until she lost power on the freeway.

Did the XT350 smoke out the exhaust? If the cylinder was really worn or accidently overbored that much it should be making lots of mechanical noise and smoking enough to kill all the mosquitoes in the area. Check the compression with a compression gauge and do a leakdown test (that is if the motor is still together) if your mechanico doesn't know what those tests are perhaps you should talk to other mechanics. Feel free to email me about problems with US type yamaha's.

Two things...
Firstly - 17 thou is a bit slack, but it doesn't explain all your symptoms.
Second may help, more of a story really.
Don't know if the same on the 350, but a few years ago a friend bought an XT250 from me, but he could never get it to run right after a rebore. Turned out that he was colourblind and had reconnected the ignition trigger wires the wrong way round, which resulted in it running on full advance all the time, red hot exhaust pipe and lousy running at low revs. It wasn't intermittent like your problem though, but it did mean I got to buy the bike back for less than I'd sold it to him for! maybe worth checking teh advance curve?
good luck

If you do not:
- Have the bike smoking a lot
- Have the bike using lots of oil
- Have very intensely black engine oil
I would not think there is something wrong with the barrel / piston. Btw, if you replace the barrel, also replace the piston. The old piston in the new sleeve makes just for a waste of $$.

Reading abt the symptoms, I think it is either carburetion or ignition.
You say the carb is OK, are you sure there is enough fuel flowing to the bowl? Easy check. If not, check the filter on top of the float needle seat.
Are the two inlet rubbers between carb and cil head intact? Check with ether or something.
On the XT6 there is a mixture enriching device on the L/H side of the carb. It enriches the mix when closing throttle to avoid banging and f*rthing. Check this too.

If all of that does not work, start on the ignition.

Auke

Hi everyone,

First off, thanks for all the replies - top!


You've given me plenty to think- and chat-over with el mechanico...I'm going to see him and it tomorrow - I'll keep you posted.


For the record:
- it doesn't visibly burn oil
- I don't know how the cylinder is - I'll find out tomorrow
- the compression tests fine with a guage (but, like I said, the mechanic seemed to think that this was only a temporary thing due to oil around the sloppy piston holding the compression for a while.... shrug?)
- i don't know what a leak-down test is, but I'll mention it to him
- I'll check the fuel floating in the bowl and also the filter on the float needle seat


Thanks again!

Pev

Re-reading your post, methinks this mechanic must be a pretty special guy being able to measure the piston play and cilinder roundness with head and cilinder installed. Did he measure exhaust temperature with a thermometer and I would be interested to know what figure it should be. You sure you got the right establishment for the job?

Leakdown test; of course this is something of an indication.
But, following is also true: the only indication of piston wear, within limits, is oil consumption, and not engine ' behaviour' as to torque or output. Within reasonable limits, that is.
So if you don't see the telltale smoke from the exhaust, look for the problem somewhere else.
Exchange CDI's, sparkplugs, check carbs, wiring, etc.

Good luck!
Auke

Is there a phrase book they use?

...He did indeed say, unprompted, that "you could drive a bus through it" ! It made me smile (through the pain)

;-)

Well, he took the cil head off to measure the diameters and there is burnt black deposits on opposite sides where the gap is.

He reckons there are 4 over-sizes you can re-bore to, and mine is already on the last one - so I'm thinking that I need a 'new' barrel and piston or maybe a new engine (if that works out easier to find) - sigh!.

Q: If swapping and engine is, say, 50 out of 100 on the difficulty scale for a bloke with reasonable gumption and a few standard tools - sockets etc. but no fancy stuff eg. valve spring compressors etc - how many out of 100 could it be to fit a 'new' barrel and piston....?

Hmm - so tomorrow (being Monday) I'll be on the phone to see what I can find.

And, why the hell not, while I'm at it?!... I live in Manchester, UK and if anyone's near and happens to have a good engine in the corner of the shed, feel free to let me know!!

Cheers for your attention and advice

Pev

Ok, that explains it.
Changing cilinder and piston is 80 / 100. But, if the rest of your engine is ok now, why replace it with something you do not know the condition of. Changing cilinder and piston should not take more than 3 - 4 hours in a good wordkshop, just consider that option.

Auke

just to say thanks, everyone, for your help and replies. Most kind to this fresh newbie.

In the end, the piston and cylinder were replaced and the valve seats re-ground... and th'Wasp seems to be fine now (which is not quite the case with my bank balance).

Just gotta run her in now... potter potter potter...

:-)

Bugger, just read the last post after writing this!!! So most of it is pointless! But d'you still fancy a beer?


I have done a bit of work on bike engines in my time. I'm no expert but I've stripped and sucessfully rebuilt a few singles before. If you want to do the job and have a manual, a bit of good advice, some local knowledge of where to get the bits you need, and the tools for the job, you should find it no more that a steady weekends work. Luckily for you your last post said you live in Manchester in the UK...As do I!

So, if you get the good advice from here, you buy a manual, I can tell you who to see locally for advice and bits, and lend you any special tools you might need if you dont have any specialist kit, then you should do OK!

Where have you taken the bike to get the current diagnosis?

Oh, and d'you fancy coming for a beer with a few of us on the 13th over in Hollingworth?

Just drop me mail with your phone number and I'll give you a call!

All the best,

Dave

[This message has been edited by davidlomax (edited 03 March 2006).]