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henkie22222
xs 360 owner
According to "4 stroke performance tuning" by A. Graham Bell (look it up) you should raise the compression ratio on a Yamaha xs 400 from 9,2:1 to 10,2:1 and you should fit a free flow exhaust and intake to enhance performance. So you can probably mill down a lot
jayel
#9 Guru 78 SR500 80 XS11
heck of a difference in milling a 2-stroke and a four-stroke, milling changes the installed height of the camshaft with respect to the cam drive, which in turn retards valve timing and ignition timing (if the cams also drive a distributor or crankshaft/camshaft position sensor).I had the head milled down 15 thousandths on my razz and it made a big improvement. Don't think I would go that far with the 400 but with all the engines I got why not have 5 or 10 taken off
In these with a chain drive cam, assuming valve clearances were reset (obviously) I don't see how timing would be effected....but I'm willing to admit I have not studied the issue in great detail. The only issue i see is making sure the cam chain tensioner has enough take-up for the chain.
I know they do this with the xs650's all the time so why not
The stock compression ratio's for these bikes are (u.s. models) 9.2:1 for the 77, 9.4:1 for the 78 2e and all others 9.3:1 the amount you would take off would be very small.
The stock compression ratio's for these bikes are (u.s. models) 9.2:1 for the 77, 9.4:1 for the 78 2e and all others 9.3:1 the amount you would take off would be very small.lhaolpa
XS400 Junkie
Unless you can get some good gas,more compression won't give much return from the milling and resultant compression boost.Chain probs would be something to consider as well. lha
henkie22222
xs 360 owner
That is not completly correct i believe. The only problem you will get with a higher compression ratio is that you get spontanious detonation wich will completly screw up your pistons, the only way to get around this is by using a gas with higher octane ratings. I dont know if these are easy to get in the US, but they are more expensive.
Do mind that i am a two stroke guy, so not everything might apply to your xs
I think Jayel is right. Your valve timing AND your ignition timing would be slightly affected unless you re-degree the sprocket on the cam. Valve clearances shouldn't be affected at all.
probably need a vernier cam sprocket and dial it in using a degree disk, brit bike style!
jayel
#9 Guru 78 SR500 80 XS11
XSjohn (650 forum) was a fan of lowering the compression, thicker head gasket to get around lower octane fuels, had to re-degree the cam sprocket to get the cam timing back where it needed to be, it would change the ignition timing on a points model 650 because the points/advance ran off the OHC, Electronic ignitions that run off the crank may not be affected
XSLeo
XS400 Junkie
XSChris, you missed the XS650 compression ratio by one. Most were 8.4 to 8.5 -1. The first few years they tried an 8.7-1 but had issues with running so they shaved off the top of the piston to drop CR to 8.4-1 they then redesigned the pistons to the 8.5-1.
How when you bore them out to the 750 cc engine then the compression comes up to about 9.1 with Mike's kit. More so with home built 750's. 10-1, even 11-1 depending on the pistons.
The higher compression is fine if you have access to racing fuel, pump gas won't cut it.
Leo
How when you bore them out to the 750 cc engine then the compression comes up to about 9.1 with Mike's kit. More so with home built 750's. 10-1, even 11-1 depending on the pistons.
The higher compression is fine if you have access to racing fuel, pump gas won't cut it.
Leo
XSLeo
XS400 Junkie
Just reread your post. Misread it the first time. But what I said still applies.
Leo
Leo
