Engine rebuilds

red_eye_righley · Post by **red_eye_righley** » 02:43 am Jul 22 2009

Hi guys,

I still havent sourced an engine, but figured while I look I would try and decide the best route for all rebuilding that I will be doing. I would like to re-build the engine myself. I am sure I will find info on how to do a KDX motor (I have actually only helped a guy do a KTM 2 hunny, but have built many car engines and hope that I will figure it out pretty easily), anyway - I am not asking abt a how to for rebuilding an engine but rather what mods can be done while I am doing this (if there is already a good post somewhere that shows the progress of building a KDX engine etc I wudn mind it being linked here

) . Now, I would like to send my head and carb to RB designs as I have heard so many good things about it (or I might just get some on fleabay and get them sent to him and then he can send em over here). Also, I might be interested in a 225 big bore, but will probably leave it as a 200 at first. Regarding other mods - anything that can be done? Maybe cleaning the ports out, matching cases, "stuffing cases" or anything else? Anyone ever done anything like this? I have heard good things about guys doing this on their motors, or doing some "blueprinting". Any thoughts would be greatly appreciated and will add to my understanding of these motors greatly. Thanks guys!

Post by **fuzzy** » 09:25 am Jul 22 2009

Most leave them stock. The KDX motor doesn't take to a lot of typical 2T hop-up stuff, plus most of that is made to enhance top-end power (while sacrificing low). Head is good to do (more compression, better shape). Porting I'd leave alone if you're going to a 225 as it will be worked on then. For that matter I'm not so sure if you wont need a completely stock head for the 225 also.

Top end is a piece of cake.

Bottom end much more elaborate. I would just send the whole thing to Eric Gorr for a rebuild and 225 if you're going that route.

Lots of good vids on youtube from rockymountainmc of 2-stroke case split/bottom end job.

pietman · Post by **pietman** » 12:59 pm Jul 22 2009

Hi righley, you're a Saffa right?

You might get lucky on Junkmail. I've seen a few KDX engines, and stripped bikes on there in the past.

There's currently a '97 KDX200 on there for R6500. You can take the engine, carb, CDI, etc out & sell the rest for maybe a couple of grand.

http://search.junkmail.co.za/view/Johan ... 631-200929

Here's another: "KDX 200 : 2000. Stripped but complete R5000"

http://search.junkmail.co.za/view/Johan ... 209-200930

red_eye_righley · Post by **red_eye_righley** » 12:40 am Jul 23 2009

Hey fuzz: My understanding isnt great, but from my ubderstanding

if the ports are just polished (and not changed otherwise) and the cases stuffed and cases matched will improve the "flow characteristics" and the engine will run more efficiently (ie: it wil run better everywhere). I am not arguing, would just like to know if I am completely on the wrong train of thought here or not. I would love to send a motor to RB and get them to do the whole hog for me, but money is an object and transporting an entire engine over here from the states is expensive (I'm in South Africa-sorry, didnt mention that). And yes, I am after bottom end, not top.

Hi piet, yes - Im a saffer, where you based? I have those two links saved to my favourites, thanks. I think the first one looks better as it can be started and I can atleast see if the gearbox is ok etc etc. The other one seems good, just worry about it being in pieces and then it comes time to rebuild the motor and I find a horror story in there (but I spose if it turns over and can go through the gears it must be ok at least - any thoughts guys?)....

canyncarvr · Post by **canyncarvr** » 02:10 am Jul 23 2009

Re: '..if the ports are just polished...'

I'm not sure where that idea ever came from......but if 'polished' means to make everything real smooth, you're barkin' up the wrong tree. On intake ports..what do you expect to gain from a smooth surface? That's something you dont want.

Many an 'expert' has the opinion that they know the secrets of the porting universe. 99% are hacks that don't know squat.

Re: 'stuffing'

An honest question: What do you hope to gain..and how to you hope to research, design, and implement a change in crankcase volume?

Re: 'Any thoughts would be greatly appreciated'

Leave it alone.

Maybe it is something you already know...but THISmight give you an idea or two.

pietman · Post by **pietman** » 03:35 am Jul 23 2009

red_eye_righley wrote:Hi piet, yes - Im a saffer, where you based? I have those two links saved to my favourites, thanks. I think the first one looks better as it can be started and I can atleast see if the gearbox is ok etc etc. The other one seems good, just worry about it being in pieces and then it comes time to rebuild the motor and I find a horror story in there (but I spose if it turns over and can go through the gears it must be ok at least - any thoughts guys?)....

I'm in Centurion, and you? I'm also really keen on building a KDX hybrid, but it's not financially possible at the moment.

red_eye_righley · Post by **red_eye_righley** » 03:41 am Jul 23 2009

Jks jks jks, thanks for that info, I have seen it and had a skim over but should really read it thoroughly - will get on it. I am not interested in drastic design changes, I have heard (probably from one of those experts online) that there are some relatively minor changes that can be made to a 2 smoke engine to improve efficiency/performance (and most of that was through paying more attention to detail that does not happen in the factory as things are mass produced). I will do some more reading, any other info that you may have would be appreciated - although there is plenty of reading just from the one link.
Thanks

Wobbly · Post by **Wobbly** » 04:08 am Jul 23 2009

I would have thought that cleaning things up would be a good step. Have the porting on the barrel matches to the crank. Have the port entries cleaned up. You can polish the exhaust port, but really just cleaning it up would give you the best result. Skim the head a bit if you like, but keep in mind the grade of fuel you run. You can work out your squish with just a but of solder taped to the piston crown.

If you have the motor apart then it's worth doing that, as you're really just applying some extra quality assurance to the way the motor was meant to be :)

KarlP · Post by **KarlP** » 07:39 am Jul 23 2009

When I did my hybrid I put much more emphasis on the chassis and suspension (CR250 chassis). I put a completely stock 220 motor in it at first.

I also had a KDX220 with the head and carb done by RBD at the time.
That motor had significantly better performance than the stock 220 motor in the hybrid.

When the hybrid with the stock motor was all sorted out chassis wise I swapped motors with the KDX and sold the KDX as a nearly stock bike.

I did not want the hybrid build to get hung up by motor modifications. I felt that the stock KDX motor was plenty of motor for the stock KDX chassis and what holds the KDX back IS the chassis. The slightly hopped up KDX motor in a more effective chassis was icing on the cake.

canyncarvr · Post by **canyncarvr** » 04:17 pm Jul 23 2009

Re: '...I would have thought that cleaning things up would be a good step.'

Nothing wrong with a 'blueprinting' approach..port matching and the like. I didn't mean to imply there was. It's things like 'polishing' and 'stuffing' that I would wonder about the 'why' of.

Re: '...there are some relatively minor changes that can be made to a 2 smoke engine to improve efficiency/performance..'

What changes...and to what engine?

I'd be willing to bet'cha that there has been no flow testing done on KDX cases to the end of eeking out the last fraction of a pony, let alone woods (real) testing. Without knowledge gained from testing of that sort, 'ya never know what you're gonna get.

Admittedly, I'm biased. I've seen, heard of, and dealt with disasters of my own from 'experts' that knew how to make 'relatively minor changes' that were promised to be 'wonderful'.

red_eye_righley · Post by **red_eye_righley** » 12:25 am Jul 24 2009

OK then, those are the answers I have been looking for
Just trying to get a better idea for any possible improvements (not that I think its really necessary). I will look into the blueprinting side of things and perhaps go that route, if any in terms of improvements. Thanks for the help, although I feel like I have been beaten down with a big stick

KarlP · Post by **KarlP** » 09:50 am Jul 24 2009

"Thanks for the help, although I feel like I have been beaten down with a big stick"

Ahhh- Don't feel bad. I think it's as much a cultural and communication thing as a fact that trying to get much more out of the KDX motor is a risky business. If you want a BUNCH more performance than the KDX motor try any of the big four 250 2 stroke motors. 'course, that wouldn't be a hybrid, would it?

The key to the hybrid is actually low, friendly power in a really good chassis.

You could, just for fun, put a KX 250 motor in a YZ 250 frame......

How's that for a whupping

canyncarvr · Post by **canyncarvr** » 11:19 am Jul 24 2009

Re: 'Thanks for the help, although I feel like I have been beaten down with a big stick'

Nah. If that ever happens...you'll know it.

I would have been hugely grateful to anyone that beat me with a big stick had it resulted in me not ruining my last RD with someone's 'expert' port tuning.

Post by **fuzzy** » 01:47 pm Jul 24 2009

Case stuffing, rod knifing, ceramic case bearings, honed/ground fitting, etc is all generally high dollar stuff that is generally the absolute last things done when building an "all out" high $ engine. The types of things that could turn a let's say CR125 spec shifter kart motor from a $1000 motor to a $5000 motor. The $5000 one will make like 2 more hp than the $1000 one that just had the ports/head done, but in some sports there are people with the money to burn. WHy so much? The people who do it have to RUIN lots of stuff to get the 'hot ticket.' Some of these mods do revolve around extreme high RPM reliability....Not something anyone ever needs to worry about unless you're trying to turn the KDX into a shifter kart. These motors also run VERY narrow-band pipes to make all of their power above what a stock engine poops out at. THey have their PV's disabled/filled/etc. These prices will include a mangeto conversion as well....Another non-woods friendly item, but big time spark at 12k RPM.

Best bang for the buck lies in your head and carb. If you're after a 225 the job includes head cut, porting, and replating...as they are all necessary, and you can specify fuel rating and power delivery preference at that time.

kawagumby · Post by **kawagumby** » 10:02 am Jul 25 2009

It's probably been said here already, but the trick to performance is just good tuning - jetting, timing, pipes and reeds (the RB mods are great). (The real trick to going fast is practice and physical conditioning anyway) As far as an engine rebuild goes, you can do it yourself IMO, if you have some mechanical experience and aptitude. Look carefully at transmission bearings, especially the small ones - they seem to fail the most, and they are easy to replace. Cleaning up casting flaws anywhere in the fuel-mixture-exhaust flow-line is a good idea, but porting and polishing never netted me any worthwhile gain on any bike I've owned. I've read some opinions that polishing can actually make things less swirly and hurt performance....

red_eye_righley · Post by **red_eye_righley** » 01:00 am Jul 27 2009

re: "cultural and communication thing", yes - us chaps with our pet giraffes over here really need to brush up on our people speak

Jokes. Yeah, was just pulling the micky - I really am grateful and from what all you great chaps have said it seems best to:
1. Buy a friggin KDX and KX
2. Do the hybrid conversion
3. While the frame is getting coated:
a) Full rebuild of motor and tranny (include transmission bearings and components that need it as well as clutch).
b) Get ordering of a carb and head from RB designs.
c) Maybe clean up the casting flaws off the barrel.
d) Ride it as a 200 and be happy (maybe change to a 220 later on - not too sure yet)...

All I was a saying is that you chaps gave me good info with CONVICTION, nothing wrong with that!

Thanks again!

canyncarvr · Post by **canyncarvr** » 01:17 am Jul 27 2009

Re: '..you chaps gave me good info with CONVICTION..'

So...you got beaten down with a big CONVICTION stick?

I guess I use (try to) English as a second language. Problem is...I have nothing different to use as a first language.

I'm not sure where giraffes are commonly kept as pets. 1600 Pennsylvania Ave? I DO know that is a place where words mean things...just not at all what you could ever hope to imagine they could be twisted up enough TO mean...and about the time you get a line on it...everything changes.

Re: 'I've read some opinions that polishing can actually make things less swirly and hurt performance....'

Yep. Not only less swirly...but sticky, too.

Anyway, it seems I might be chapped now, so I best go get something for it. A pint perhaps....

red_eye_righley · Post by **red_eye_righley** » 12:26 am Jul 28 2009

re: I'm not sure where giraffes are commonly kept as pets. 1600 Pennsylvania Ave? We're in Seffrica - but no, we dont really keep pet Giraffes.

re: "that is a place where words mean things...just not at all what you could ever hope to imagine they could be twisted up enough TO mean...and about the time you get a line on it...everything changes. "
Sorry, confusing text, not really sure what that means - English is my second written language and 3rd spoken one. Anyways, thanks for the help.

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Engine rebuilds

Engine rebuilds