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Discussion in 'Your 250cc Projects' started by Mike Green, May 9, 2020.
!5 Lb of boost won't sneak past a good valve grind
True - under static conditions. The basic problem with a 180 degree twin as Mike has is the dynamic pulsations in both inlet and exhaust tracts. A boost gauge may show a steady 15lb - but I'll guarantee that at some point in the range, there are pressure spikes higher than that.
The last blow- up showed - IMO - that valves had been hanging open and touching the pistons.
I saw this when I rebuilt a turbo'd 1400 Kawasaki in a sports car. Witness marks on the exhaust side of the pistons.
Asking around the local guys who had race turbo experience brought out an interesting - and not unique - case of the mystery Subaru blowups. Only after they put pressure transducers everywhere they could did they discover pressure spikes in the exhaust which were blowing the exhaust valves open. Poor pipe layout apparently.
The Kawasaki I simply changed the exhaust cam from the aftermarket high lift one to a stocker - on a wider lobe center.
No loss of HP - but it's still together.
Watching Mike's progress with this has been like old times. I've built several blown motors. Roadracing and speedway. The same problems show up - and usually the same solutions too.
I've managed to get a bit of work done on the FZR between painting windows and working on my son's CB125T(150cc). After measuring the installed height of all the valve springs I got some new spring base washers made from 4140 and then nitrided. I have the head together with the cams in and have measured all the valve clearances with the smallest shims I have installed. All 4 inlets are EXACTLY the same clearance and need to have larger shims installed. The exhausts have the next size up shims installed and they too are very near to exactly the same clearance. I could leave them as they are but will go one shim larger on both valves on one cylinder. I suspect that NZ Cylinders set the installed height of the valves after working on the seats. Nice work if they did.
I have reassessed the way the cams are fitted in this new head and decided that it was nowhere near good enough. The original cam locating discs, which originally fitted into machined slots on the far left of the head, were fitted into roughly ground out slots on the far right of my cylinder head. Doing this resulted in the cam lobes not being central on the tappets. What I have done is machined the slots slightly oversized to true them both up in relation to the cams. I have then machined the disc off each cam and made a boss on the end of each cam onto which I will press a new disc which when fitted in the slot will locate the cam lobes centrally over the tappets. It was very interesting machining the disc off the end of the cams just how hard they were. Hardness varied with the depth and also how far over toward the cam bearing I machined. I used a ceramic bit in the end to try and get the best finish possible. It's not bad but still not as good as the ground finish I achieved originally I am impressed again with the engineering of these engines. Being just a little bit rough I leveled the head in the mill by clocking off the engine mounting bracket points on the end of the head. I then cut both slots exactly the same height. The machine marks on each cam cap are essentially exactly the same so even the castings are precise.
Nice work! What was the original idea in dropping 2 cylinders? And do you know what power you’re making at the flywheel?
It was to make a 125cc engine for a class of small bike racing. Just before it dropped valves it made 32hp at the wheel. I'm aiming for a bit more once back together, hopefully 40hp
Mike 32 is pretty good out of a 125 4 stroke. 40 will be awesome.