Sunday, May 17, 2009

Engine Rebuild

Well, the XT started to lose power.
I noticed that the it was burning about 500ml of oil every 1000kms and there was a bit of oil starting to accumulate in the air box.
So I am ( well, Tim is actually) now in the process of a full engine rebuild.









Piston:
I wanted to get a piston that would take it out to about 450cc but after looking around couldn't find anything suitable so just settled on a standard Yamaha piston at 0.5mm oversize. (Now 87.5mm bore)
Piston $198.19
Rings $79.47
Gudgeon Pin $36.95
2 x Circlips $4.20
I did think about using the piston out of an XT600, but after much deliberation decided against that.
The bore was bored out to match the piston by Regal Automotive. ($120)
The piston has exactly the same part number as the SRX 400 piston.

I talked to a guy on the KiwiBiker forums who has tried modifying an XT400.
See this link: http://www.kiwibiker.co.nz/forums/showthread.php?t=7362

Carbies:
I have bought a pair of Kiehin CV carbs off a GPX250 from a wrecker in Upper Hutt. $250. These have a 30mm intake diameter and should be significantly better than the standard YDIS carbs that have an oval shaped intake about 30mm x 18mm. ($260)

Apparently the GPX250 carbs work well on SRX600's and XT600's. I'm hoping that they will be OK on the 400 also but time will tell.
Here is where I found out about it:
the second to last paragraph for Question 3 at this website: http://72.14.235.132/search?q=cache:amUBgpOBxz0J:www.thumperpage.com/articles/srx6faq.html+gpx250+srx&cd=2&hl=en&ct=clnk
They do fit pretty good! Just had to stretch the LHS upsteam rubber manifold to fit and swap cables over to the RHS of the bike. And just had to slightly bend the rear brake fluid reservoir bracket a little bit to give clearance on the throttle cable twist on the carbies.

Reshape Cams:
The camshaft is being modified by Kelford Cams in Christchurch. ($315) http://www.kelford.co.nz/
I am just hoping that they know what they are doing.
My instructions to them are basically: It is a 1992 XT 400 and probably revs to about 9000rpm. Do what you think. [Actually Tim asked to have the lift increased by 0.5mm]
I guess they will just do a standard re-profiling to give a more modern cam timing.
[The 50 thou duration on both inlet and exhaust was increased to 229 degrees, (from what?)]

Head work:
Tim has ground away at the head to get a nice air flow path. He ground the valve seat stems back to the body of the head and will just use a thin section of the old head gasket to increase compression slightly.
Also there was a crack in the top of the rocker cover which was seeping oil. This has been welded up by Regal Automotive (~$40). The crack was due to stress from the top engine mount I guess.

Flywheel:
Tim thought the flywheel was pretty excessively heavy so he machined it down to what you see in the pix. Pity I didn't get a "before" shot for comparison.
Exhaust Header Pipes:
Tim ground out the weld bulge in the inside of the header pipes where they join onto the head for slightly better flow.

Misc:
New bearings throughout engine. Cheap.
Cam chain $80.23 + gst
4 x valve stem seals $32.50 + gst
Oil filter ~$15.00 + gst
Gaskets:
Base gasket $32.08 + gst
Cam chain tensioner gasket $ 7.47 + gst
Crankcase gasket $24.33 + gst
Another crankcase gasket $20.13 + gst
Oil Seals:
Oil seal 25 x 40 x 5 $9.44 + gst
Another seal $8.21 + gst

Piston, rings etc + seals and gaskets came to about $610 all up incl gst.
Rebore and welding $160
Cam Mods $315
Oil and new Plug $70

Total: $1155-ish plus I'll have to sort Tim out.
I also tightened the spokes on the front wheel as they were all a bit loose.
I broke 3 spokes in the process so will need to get 3 from the wreckers and fit them.
I am thinking that if it used to have about 20 - 25 horsepower, then after this work it may get to about 30 hp after I get the jeting and needles sorted in the carbs. Well maybe.
More to come once it is all back together.