Suzuki GN250 Big Bore Kit

[Jacobite]

Public service announcement!

The GN250 big bore kit (out to 300cc) available on ebay does not fit. I got hold of the kit a week ago to discover that the skirt on the barrel is too large a diameter to fit the engine casing.

After having the skirt machined down to fit I also discovered that the deck height on the new barrel is wrong as the new barrel is too long. The original piston sits at .24mm above the deck, the new piston sits at 1.38mm below the deck, a difference of a whopping 1.62mm.

In short if you want to install this kit, expect to spend around another $400 (or whatever 5 hours labour at your local machine shop costs) to get it installed on the bike. I'll post again on Friday arvo when it's back together and let you all know if I have any other issues.

Jacobite

 

[DohcBikes]

The vast majority of big bore kits require case clearancing. Not knowing this was your own fault.

Also, notice i said CASE clearancing, not "barrel skirt"(?) AKA SLEEVE machining. You made the weakest part of your cylinder even weaker.

The deck height issue could either be with the piston or with the jug. Which was it?

 

[Jacobite]

So with the "jug" properly machined the engine went back together with no more issues and fired first go. Have rejetted the carb, seems to have some more power, I'll post again once she's run in properly.

 

[barnett468]

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just how thick is the bottom of the cyl now? . . there is a point at which it can be too thin . . they are larger in od than a stock cylinder for a reason . . as was mentioned, most big bore kits require the cases to be machined.

so just what did you do with the short cylinder? . . using a short cylinder will retard your cam timing and make your timing chain appear to be stretched.

if you did not shorten the cylinder to get the new piston to the top, then your compression is much less than it would have been.

it sounds like the cylinder needed a spacer on the bottom and a custom piston to get the top of the piston where it needed to be.

the compression should also have been calculated for both stock and the big bore kit so you know exactly what you have.

increasing the bore size so you have 50 more cc's will make a noticeable [but not drastic] improvement in low to mid range power if everything else remains the same including compression.

if the compression is the same or more, then you should need to go up on the main jet by at least 1 size and probably no more than 2 sizes, so i would install a new plug and run it for an hour then take a look at it.

 

[Jacobite]

The kit was sold to me as a direct bolt on, no machining required. I assumed it would bolt on after reading a few forum posts that said that it would bolt straight on with no worries and talking to the supplier who said the same thing.

The bottom of the cylinder (the part that sticks out below the alloy heat sink) on the original cylinder sits inside the case with a couple of mm clearance, the big bore cylinder would have dropped straight in if it had been machined properly, the diameter at the top (closest to the alloy heat sink) was the correct diameter, the bottom was .5mm to big, I got it machined so it was straight and the cylinder wall was still .2mm thicker than the original cylinder wall.

As for the deck height being incorrect, I checked the height of the original piston from the top of the gudgeon pin bore to the piston top and the big bore replacement was exactly the same. I had the new big bore cylinder deck height machined to match the original one and everything is working fine.

I have increased the pilot and main jets by one size, the tune seems fine, running on 98 the plug reads nice and coffee coloured after an open throttle run in 4th up my street (quite the decent hill).

I'll post some photos soon.

 

[barnett468]

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ok glad it worked out . . cant imagine how they could unintentionally machined a taper into the bottom of the bore but i guess the chinese can do anything.

so im guessing the new piston is still below the cylinder surface a little . . as long as it runs decent and you're happy thats all than matetrs anyway.
.

 

[Jacobite]

The new piston sits exactly where the old one did, just above the deck. The head gasket provides the clearance so the piston doesn't hit the head. I'll do a comp test soon and let you know where it's at in comparison to factory.