1988 kx5 motor

[boost165]

Just after a bit of info about the 88 kx5 engine I have got this engine for my 2000 kx 250 hybrid I now that the 88 motor has only the kips in the cylinder but now kips power valve in the exhaust port like the 89-onwords  models and the intake port has now bridge for less piston wear I assume, The power valves are in really good condition witch I have read are a hard part to find, what I wanna now is if  this year engine any good with it not have the exhaust kips.is there much power loss ? Any advice on this year motor would be grate

[dave916]

88 motor is fairly similiar to later 89/on motors {crank/transmission/ port timing 

top of exhaust port is about 2mm lower than 89on but overall is got more area and alot cleaner port roof
the  89on centre kipps is pretty crude and may and also has issues with cracks in exhaust bridges

main differance on the 88 motor are
differant top end/ larger and heavier more water a kips capacity no centre kipps,
 differant intake area  and top reeds run closer to piston than 89on{not good}
bigger crankcase volume {differant cases  with no centre kipps hole ]  holes in 88crank could be plug for less volume
lighter flywheel /slight differant timing 2degree

current  building a 88 will post pictures when i get off my ass in gear

[boost165]

Thanks for the info I did wonder about the two big holes in the crank but as the bottom end is all done and built with new rod and new gear box from an 87 model witch didn't fit but does now . What a Ballack that was so it can stay like that lol is the power around the same as later models would you now? Many tanks

[dave916]

Have not drove them back to back but if both motor were set up the same they should be pretty similiar , 88 should be the faster of the earlier motor / probably slight more of a hit compared to 89on [lighter flywheel and no centre kipps}
Not sure how case volume would effect performance?
With the top end u probably need to kipps bolt mod and mod the inlet for reeds your using/ radvalve wont work well without reed spacer and  filling rear inlet voids

88 std exhaust port


86  std smaller ex port

[boost165]

I have done the kips bolt mod ready for when the top end goes on I have already got a boyesen reed block  came at rite price so is this not the best one to run on this year ? What sort of spec engine are you building or hoping on building if you don't mind me asking 

[dave916]

spec of motor i am building will be stock[port timing}  [look for smooth power rather than more top end}
there will be a few little differance to normal builds
running roller bearing on flywheel side/ shimed on clutch side to centre crank  /balanced crank /epoxy cases
top end will epoxy inlet {dercon f} to reduce boyese port size and flaws /going to run a boyesen radvalve with 10mm spacer
top reeds of radvalve will almost hit piston /complete block boost port/ damage or jam reeds on casting flaws of boyesen ports

unfinished work i am currently doing


stock top cating flaws is where your radvalve reeds will jam


reduced boyesn port size

[motopunk]

you could use all gears of the 87 motor , but you need the 88-2004 shafts...

i have an 86 and 88 engine and love to ride both . the 88 has a smoother power delivery and handles better on hardpack . also know the 89 and 99 motors from testrides on borrowed bikes ...  the 88 is very close to the 89 and up motors in performance. i think its the cylinder and slower second gear , that also was changed in 89 to faster second gear...

[boost165]

Love the work your doing I'm going to have to tidy my inlet up to stop reeds jamming that's  a good tip thanks  the complete gearbox from an 87 will fit with with some changes I had to cut the input shaft down on the left hand side wear it sits in the bush and has a needle roller bearing so I had to change it to a normal bearing and also the inner clutch hub is different only 6 splines unlike the 88 onwards I couldn't find a 87 clutch hub. parts are really hard to find in the uk but I did find on eBay a kx 250 hub witch had 6 splines apparently it was from an 86 model 250 I don't now but I took a chance on it and it fitted happy days ,, thanks for the info guys it all helps

[dave916]

Love the work your doing I'm going to have to tidy my inlet up to stop reeds jamming that's  a good tip thanks  the complete gearbox from an 87 will fit with with some changes I had to cut the input shaft down on the left hand side wear it sits in the bush and has a needle roller bearing so I had to change it to a normal bearing and also the inner clutch hub is different only 6 splines unlike the 88 onwards I couldn't find a 87 clutch hub. parts are really hard to find in the uk but I did find on eBay a kx 250 hub witch had 6 splines apparently it was from an 86 model 250 I don't now but I took a chance on it and it fitted happy days ,, thanks for the info guys it all helps

A Reed spacer is the way  to go/will need less work on casting flaw area
the reasons i used epoxy is to fill the huge rear void{for reed screws} dont need it with reed spacer
the rear void {reed screw area] and the boyesn ports are far bigger in  88 than later years
Dont go mad grinding casting flaws{boyesen ports} as the cylinder is prone to cracking in that area[like kr1 cylinders}

[boost165]

Oki I will just smooth of wear the reed petals hit this is all much needed info

[dave916]

where u top rad valve reed could get damaged or jam


blocking boost port etc


massive void in rear of intake needs to be filled when  using spacer and radvalve as reeds dont have stopper plate
epoxy also help with the big case volume on 88
88 on right 18mm void compare to about 11mm on 89on left pic

[dave916]

your 88 vs 90 cylinder
both stock
88

90

[boost165]

I must have a diffrent rad valve because it is same length as the standard kx one I can see wear you are coming from with yours but just put mine in and comes now wear near the piston must be made slight different or of kx 250 or kdx As I  it was secondhand don't real now mmmmm interesting but it's defo a boysen rad valve. is there some way of posting pics with out using photo bucket so you could see ?

[dave916]

i have got a kx250 and 500 both are the same maybe the made a shorter one for 86/88
dont think there is anyway of posting pic unless  u use photobucket or similiar hosting site

even my stock reed cage is running  blocking the boost port {only about 5mm differance to the rad valve}
maybe all my cages are post89

it is the top reed your checking as my pic are upsidedown