Re: 1988 XR-4 modifications
Whoa there cowboy. First understand what you have before you start throwing money at it.
1. The reed setup is in the main engine block and crankcase pair. Mating a foreign lower half to your engine can be done but is quite involved and requires a complete tear down. What you gain probably won't be worth it on a 23 year old motor.
2. If speed is what you want, the smaller LU is desirable. If it is in good shape and especially the clearances are set up right and you run good lube in it, it should stand up. It isn't quite a tough as a 4 1/2 inch case which is why when they get neglected they put the pinion out the side and folks just put on a bigger, cheaper big one.
3. Your engine is quite strong right out of the box, usually about 162 HP, and is detuned a bit for that. Bigger carbs might give you a marginal boost at WOT, but the flow capabilities of the vertical reeds are the limiting factor. Forget it.
4. Minor mods that help a bit.
a. Chit can the advance module and set your WOT timing to 23?.
b. Have a shop specializing in marine engines cut your heads to increase compression from about 120 to about 145 PSI.
c. Run premium fuel in it if you increase compression. (Mandatory if you do b, worthless otherwise except to eliminate ethanol)
d. Set up for 6000 - 6100 rpm at WOT light load.
This'll give you about 180 for a coupla hundred bucks. If you want more, trade it in for the right engine.
BTW, all the "minor" stuff, carbs just right, fuel pump in good shape, fuel lines good, wiring in good shape, etc. all add up.
A set of Chris Carson reeds will improve idle and be easier for the engine to digest if broken than the original steel reeds. Won't do much on the top end, though. Well worth the C note if you're going through the carbs anyway. Probably not worth paying somebody to install them.
