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Discussion Starter #1
Well, I've now installed my 155cc big bore kit, as purchased from the Faddybike Germany website - http://osc.faddybike-deutschland.de/product_info.php?cPath=131&products_id=1113&osCsid=57aad731e782adfd4cd6de16028781ba

For fitting instructions, Snakeibf's thread is excellent - it's what I used! Thanks mate! http://www.hondagrom.net/forums/13-grom-performance/756-yuminashi-150cc-light-bore-kit-installation-2-0-a.html

155 gasket 1.JPG 155 packing.JPG Well first impressions were of a well packed kit made from genuine parts. All looking good so far. Unfortunately that's about as far as it went. Closer inspection revealed:

1 - The head gasket was VERY POOR. As you can see, when located over the bore, none of the holes line up. When located on the holes, the bore doesn't line up. And the gasket never matched the shape of the cam chain slot. Also, as can be seen, they cocked up and took a chunk out of the side when cutting the bolt holes presumeably. The cutting was also of poor quality too. This was a faddy bike own part, so I would have expected better. As it was, it could not be used at all. A new one would need to be made.

2 - The bore of the cylinder had a lovely polished finish. Looks pretty, but won't hold oil, and the engine would have died very quickly indeed. This would need honing to give the required 'brushed' style finish that can hold oil.

3 - The piston was a tight fit in the bore. It was a smooth fit, but tight, and needed some pressure to get it to move. When hot this would cause the piston to seize, as a) the piston gets hotter than the cylinder, and b) the aluminium piston expands twice as much as the steel liner for a given temperature rise. This would need honing out to give the right clearanve. The piston should just drop though (might need a tiny tap or poke), but should not be rattly.

I discovered these faults on Friday night, and I really want to fit this kit this weekend. I ran a friend who restored classic british bikes, and who luckily was able to make copper gaskets. I had to make a 2x scale template of the required gasket in 40 thou (1mm) thick plasticard (card make from plastic...), and he would then use that to make a 1x scale gasket in 25 thou (0.6mm ish) copper. This process uses a special pantograph milling machine, so it was VERY lucky I knew the right guy.

Making the template involved taping a piece of thick paper over the new barrel, and rubbing my thumb over all the edges. This left a neat and accurate impression in the paper that I then drew over. This was photocopied up to 2x scale (that 2x long and 2x wide, not A5 paper to A4 paper). This 2x scale image was then cut out, and used to outline the image onto the plasticard which I then cut out with scissors and finished off with a sanding attachment on a dremel to tidy the edges. This took about 1.5 hours all in, plus the trip to the library for the photocopier.

When I visited Ken on saturday afternoon, he mounted the plasticard onto a thicker piece of plastic using solvent. He used double sided tape to attach the copper sheet to another piece of plastic. Both of these were then mounted on the miller in the right places. The miller works by using a probe on one arm to trace the outline of the template. The arm moves, via linkages, the milling head at exaclty half scale distances, to cut the copper sheet. working 2 to 1 means any errors in the template are halved. The gasket did require a little sanding before fitting, but I had erred on the side of caution with the template. I can sand gasket off, but I couldn't add it back on. I still have the template, so in future I can get Ken to run off a batch of 58mm bore gasket for a very reasonable price. Certainly a damn sight cheaper than any where else in the UK.

It was Ken who confirmed my thoughts about the piston and barrel, and who recommended I have it honed. It now being after 4pm on a saturday, I rushed out and just managed to find a shop open where I could buy a honing tool and some liquid gasket. http://www.millfieldautoparts.co.uk/ This place is a gold mine, and is even open until 5.30 on Sundays!

On sunday, I began to work on the bike. As I took the existing 125 barrel and piston apart, I noted how it fitted together. How the piston fitted in the bore, what the ring gaps were like etc. I also check how the rings were assembled on the piston. I also measure the two pistons to check things like deck hieght and skirt length.

I honed the cylinder out to get the required fit to the piston. This took over an hour. I would hone (with copious amounts of wd40) for about 5 mins, then stop, wipe down to remove the grit, thoroughly degrease, oil up, and test the piston. Then reat the process. It took me a good 6-8 cycles to get the right fit. Bloody messy!

After this I fitted the piston (no rings) on to the rod, and cycled the crank to make sure they didn't clash. I then fitted the barrel, gasket, and head, and finger tightened the head bolts. I hand turned the engine to check for and feeling of clashing. All clear.

I then stripped down and fitted the rings to the pistion, making sure to fit the rings correctly. In brief, one of the thick rings has a tiny chamfer on the inside edge. This ring goes nearest the head, with the chamfer upper most (at least, that's how it came off my original piston!). The other thick ring goes in the middle slot either way up. To fit the oil rings, fit the crinkled ring first. The two thin ring go on, one above and one below. Once you've fitted the rings, make sure none of the gaps line up. This makes it harder for the hot, high pressure gasses to leak past.

I then reassembled the engine. Before fitting the gasket, I smear a thin film of liquid gasket (NOT SILICONE!) to the copper gasket, and allowed it to dry. I then fitted the head, and torqued up the head bolt. I then had my dinner. After tea, I re-torqued the head bolts. This is important, and the gaskets will relax a little. The figure to aim for is 24Nm. After dinner the bolts had dropped to about 20, so definately worth re-tightening.

I then put the cam chain back on and sorted the timing, and them re-attached the intake system and exhaust. I also replaced the original engine oil. I'll give the engine a few hundred to bed in, then change the oil for fresh. I lastly double check I'd reattached an re-tightened every thing. I also fitted the light-bore injector. One last hand crank to check everything. Yup, all fine.

Now the moment of truth...

Bang, bang, pop, pop, rumble, rumble....

Not good at all - it wouldn't run below 3,000rpm, and kept miss-firing etc. Not sound of clashing, but very bad. After trying to warm it, I thought it might need a run. I limped up and down the road, miss firing all the way. It's not 9pm, dark, cold, and I'm working by the light of a phone torch, and I have to have the bike running for work tomorrow. Oh dear. I try swapping the injector back to stock. Limp up and down the road. Oh dear, still as bad. I park the bike up. But before going inside, I check the plug. Maybe I can tell what the fuelling's doing (wet and black = too rich, white and dusty, or even glazed = too lean)...

Well, it certainly told me what the problem was... the electrodes had been flattened together! The slightly domed piston must have smacked into the plug, and buggered it up! I'm actualy amazed it even ran at all! I quickly hunted out a spare plug remove after the 5,000 mile serivce. I compared the hieght lost from being whalloped by the piston, remove all the washers from both plugs. I then fitted two plain steel washers (M12 I think) to the good plug, then added all four of the normal plug washers on top. I refitted the plug and fired up the engine....

It purred along happily! HOORAY!

I took it for a run up and down the road on the stock injector. Fine, but a little weak at the top. I quickly swapped the light bore injector back on and went out again. Much better. I got home and parked up the bike. Phew.

Monday and Tuesday the bike ran fine, covering 160 miles without a hitch. I'm being genlte, not going about 6,000rpm the first day, and 7,000rpm the second. I've not given it a full run up to 9,500rpm yet. I know how much effort went into getting this thing running, and I don't want to cock it up.

Currently, the bike is definately more, erm, competant. Don't forget I'm currently +1 on the front sprocket (so 60mph on my speedo is really 64mph on a stock bike). The bike pulls better, even at the low running in revs, and I don't have to change down so often, if at all. It will now cruise happily at 57-60 even on cold mornings where it used to struggle to do 50. Motorway cruising is much better, feeling like it want to change up yet another gear. I'm getting no sensation of 4th being too much of an over drive, infact just the opposite. Feels more like I've gone back to stock gearing. Fuel economy seems to be the same, if not better. Acceleration is more eager at all revs (though I've not tested the high band as yet).

So all in all:

# Crap kit = very difficult installation. A novice would have a badly fitting head gasket, leading to detonation, and then the pistion would have seize. That's if the squashed spark plug allowed it to run at all!

# Effect on performance = very pleasing, if not 'arm-ripping'. Makes the bike much more useable. Also nice to still have a cylinder marked '125'.

Recommendation:

Big bore is definately the way to go, but make sure you get a good quality kit. I think the Yuminashi ones will be much better one we're sure the piston skirt issue has been resolved. But that issue was nothing compared to the issues with my kit! Also, always do a plasticine check on the dry assembly. Smear a thing layer over the top of the piston (not the sides!), then dry assembly and hand crank. Anything that comes close, even if it doesn't quite hit, will leave a mark! Simple, but effective.

Cost:

£240 for the kit, plus gasket (free, but it should have cost about £20-30), and a honing too (£28). Total: £300.
 

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Woa. WOnder what all the germs do with theirs, i would never expect that from them, i know they dont make it, but still.
 

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That was a lot to read.

So I didn't.

Seems like the hot ticket here in the US might be to wait for Wiseco to produce a quality piston and then just have the stock cylinder bored. No?
 

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Wow that piston looks terrible lol glad you got it all together and running good!

The compression ratio must be outrageous on that thing when you compare the dome on yours to the yuminashi dish, plus it smashed the plug? Lol awesome

stick that thing on a dyno and see how it does
 

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Discussion Starter #8
Morning all. Not, not a good kit for the novice, but would be easy to make it so. A proper head gasket, a properly honed cylinder of the correct fit to the piston, and a couple of spare washers to go on the plug. Sorted. Some instructions would have been nice, but I just followed Snake's method! Just found out what the piston's from - CBR125 '04-'11 (UK). It's also the standard size piston. I've got 4 oversizes to go up through as well (+0.25, +0.5, +0.75, and +1.00). +1.00 would give me 160.4cc. Not sure I could be bother boring the barrel out for just 5cc more! Still, I now have a spare stock barrel. If this was bored up to 58mm, them the genuine Honda piston, rings, pins, and pin clips would cost about £90. You'd need to pay to have the barrel bored and honed, the piston head and skirts machined, and a new gasket made (I've got a template for it if anyone is interested). Still, could probably get it all done for sub £200 if you use your own barrel.
 

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You'd need to pay to have the barrel bored and honed, the piston head and skirts machined, and a new gasket made (I've got a template for it if anyone is interested). Still, could probably get it all done for sub £200 if you use your own barrel.
This was my plan but with a yumi piston
 

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Have you checked the plug again since its been running?

so now your at 200 miles - time to give it they beans! Let us know how it goes
 

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Discussion Starter #12
Not checked the plug again, but at weekend I'll give it a going over. I think I'm a little down on the compression ratio to be honest - top end power isn't that good, but low down torque is much better. My custom gasket was 25 thou copper, quite thin for a normal bike, but the msx stock gasket is almost shim, maybe only 10 thou! Also, I've back the plug out a little due to it clashing with the dome of the piston. I might have lost a bit of squish too due to the change in shape. I think over the weekend I will spend a day testing the clearances. I'll plasticine the piston to check what clearance I've currently got, and try to move the plug back in a little. If needs be I'll dremmel the top the piston to allow for the plug and valves. I'll also try loosing the base gasket, and maybe even the head gasket too. I might just sand out the stock, but if I think I can get the clearance, I'll try no gaskets and just sealant. That should gain me about 1mm in extra compression, so should make a BIG difference. These things were designed for 91 RON octane fuel anyway. 95 ron is standard here, and 97-99 is available as premium, so I've plenty of room to boost compression before I start to suffer detonation.
 

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Snakebifs dyno showed a drop of in HP at the top end. It probably needs some sort of tuning chip and/or performance head of some sort.
I'm told by roman they will be available soon. I'm still waiting for the head, cam, and my yoshimura exhaust. I'll be also ordering the 31mm intake and EFI controller once they are available.
I saw a Honda nice engine that put out 27.5 horsepower this weekend, but the guy probably had $6K wrapped up in the engine. I'm guessing this engine has similar potential.
 

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Snakebifs dyno showed a drop of in HP at the top end. It probably needs some sort of tuning chip and/or performance head of some sort.
I'm sure it being 150 and breathing through stock port work on a stock tb doesn't help either, need some afrs to see what's going on
 

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Discussion Starter #16
I don't think the fuelling is the issue. When I first ran it on the light bore injector is was bogging slightly at larger throttle openings. The FI eventually compensated it runs nicely now. So I think the injector can cope. The stock valve timings or porting could be an issue. If it's being restrictive the inefficiency caused could be knocking the power, but it certainly shouldn't make things worse. Maybe not as good as could be, but certainly not worse. My main suspicion at present is compression ratio. I think the valve reliefs and thicker gasket used has caused this issue. I'll have a tinker over the weekend and report back.
 
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