I want Mike Hailwood's 6-cylinder 250cc 1966 GP winner...
The 10,500 RPM redline on the early 325cc engines (that's "350" to the rest of us) was due to a couple of tuning items. The cam had 5 degrees more duration than the later ones, with about .010" less lift: but Honda immediately discontinued these in 1969 when the newer engines came out. These later ones can be identified by the larger oil pump (body = 19 mm diameter instead of 16mm) and the countershaft diameter is larger: also there is some extra washers between gears 2 and 3 on the c'shaft that made it shift better when hot. The 2 & 3 gears are a little narrower, too.
The early engines had larger vacuum lift passages in the carbs to more quickly lift the rubber-diaphragmed pistons. This caused the diaphragms to implode with tiny little holes that made them work erratically afterward. Later revisions caused slower lift, but incomplete lift, necessitating the lower redline because the engine would not get proper mixture at WOT. The spark advancers were modified to advance sooner so the throttle response would not suffer too much.
So, if you're an RPM junkie, do this:
1. Get a 1970-72 model engine. If you have a 1968-69, get the oil pump from the later ones. You'll need the flow.
2. Get (if you can) a 1968-69 cam or find one with 5-7 degrees more duration.
3. Get better valve springs and lightweight retainers.
4. Get the spark advancer from the later 350 engines (1970 or later).
5. Get the carbs from the 1968-69 engine and new diaphragms. Set the float level up 2mm from stock.
6. Set static advance 2 degrees more than stock marks, run premium gas.
7. Keep an eye on your plugs, go colder if they turn white.
8. Use CB pipes. The CL pipes cost 3 HP above 7000 RPM. I've never seen "headers" work on the 350.
9. Install a 1-tooth smaller front sprocket or use the stock CL rear sprocket (it had 4 extra teeth).
10. If you have $$, switch the whole drive chain system to a 520 chain & sprockets.
Go get 11,000 RPM. Have a ball! My brother's went 112 MPH after these changes, up from the stock 98 MPH. Quarter mile times dropped .55 second, too, though I don't remember the ET numbers (just the drop).
