XR50 2003 50cc Modified

[Classic Honda 50's]

When I first started the bike I was using an as new OEM CRF50 carb and air filter assembly, the engine started with a few kicks idled and run nice, I like to put a rebuilt engine through 3 heat cycles then retension the head but into the third cycle the engine started to run on when backing off the throttle, went looking and found a few good examples of what can make your bike run rough after it has been sitting around for a while.

The Honda OEM air filter element was less than two years old as I had replaced it when I first bought the bike home, surprisingly it was quite fragile and disintegrating:
DSC02561.jpg


The rubber O ring between the cylinder head and inlet manifold was flat so I replaced it along with the O ring in the insulator spacer and the front face of the carburettor:
DSC02565.jpg


Also lubed the throttle cable just in case it was sticking and the engine was running quite well at this stage with the new parts fitted but it felt restricted and needed some more air.

So I replaced the CRF50 carb with an OEM XR/CRF70 carb and inlet manifold only had to make a small adjustment to the throttle tube for cable slack, the 70 carb makes a lot of difference, will get a video of it running, sounds great with the throaty mid-range/top end roar through the small foam pod air filter:
DSC02684.jpg


Stock jetting as it came out the box, have two of these carbs on my bikes it is one of my favourite 50cc engine upgrades:
DSC02687.jpg


The inlet manifold flange that bolts to the cylinder has had 6mm shaved off it to provide clearance to the frame from when the carb was on my J1, this frame seems to have more clearance so I will be able to get a heat spacer between the head and manifold.


The Takegawa rear shock is spot on for me at 90kg, the kids will grow into it :
DSC02678.jpg

[Classic Honda 50's]

Finished ready for the track and trails:
DSC02673.jpg


DSC02683.jpg

[dex]

Bung inch spacer's or a block of shaped aluminium between the motor and peg mounts ,,
Will open up the peg/seat/bars relationship,,,,and make your original sidestand usable again .
Remember to readjust your rear brake ..
With a tall foam set on the seat ,even better .

[Classic Honda 50's]

Time for the last engine mod for this XR50 with the addition of a Kitaco forged 12:1 39mm piston kit -

DSC03533.jpg

This bike is one of my customer request modified bikes, if someone asks a question on how a part works I try it and find out, best way to get upgrades past your wife

Cheers

Jaimie

[Paulie]

Customer requests you say...
can I please request you do a twin cam? I’d like to learn more about them.

[Classic Honda 50's]

Will add that to the list, I am building a 124 for my JZ with a V2 head and Takegawa piston but would like a Kitaco DOHC set-up to see the difference.

Finishing my dyno is next on the to do list for a proper side x side comparison.

[Classic Honda 50's]

It has taken 3 months to get back to this bike but better late than never


The top end on my XR50 was from a CRF50 with 10 hours use, it was so clean I just bolted it on at the time, the carbon on the piston and combustion chamber almost wiped off with the brush of a rag, I have run the XR70 carb on a few Z50's and didn't need to change the jets but this set-up is a tad rich and will need to go down 1 on the main jet, drop the needle a notch and see how it goes:
DSC03849.jpg


- Wiped the combustion chamber with a rag and almost like new again, if I am going to polish anything on my Z50 head it is the combustion chamber and stock piston crown, the carbon don't like to stick as hard to the polished surface and polishing rounds off sharp edges in the combustion chamber to help stop pre-ignition:
DSC03848.jpg


- Touch of thinners and ready to start work, quick figure 8 lap of the gasket face to see if it is straight:
DSC03850.jpg


- Polished the combustion chamber with the Dremel and some Autosol metal polish and cleaned out the ports:
DSC03866.jpg


- Kitaco Heavy Duty valve springs and TB Titanium retainers should work a lot better than stock, the stock steel valve spring retainer weigh 6.528 grams and the Titanium retainer weighs 3.68 grams:
DSC03867.jpg


- Found an old Z50A heat spacer that is a perfect fit for the enlarged CRF50 inlet port:
DSC03874.jpg


- Cylinder head back together again with cam and rockers installed ready to go on the engine:
DSC03876.jpg


- I like to wipe a new or rebored cylinder with a clean white lint free oiled rag until the bore wipes clean:
DSC03891.jpg


- The new Kitaco 39mm Forged 12:1 piston has a single compression ring with WPC+MOS2 processing and is a little bit smaller than the CRF50 stock Honda OEM 39mm piston, with the WPC Treatment Solid lubricants such as Molybdenum Disulfide (MoS2) can be utilized as media in the WPC process to embed MoS2 into the product surface. The MoS2 shot process sprays fine powders of MoS2 at a high speed against aluminum and other low melting-point materials. The process forms a MoS2 layer, which does not include a binder, on the outer layer of the product. This solid lubricating layer has an improved frictional resistance than a MoS2 coating with a binder. The MoS2 shot process is used on pistons’ skirt parts, other automotive components and bearing metal:
DSC03904.jpg


-Parts ready to go back on the bike, with the Kitaco stock head race cam and ported head the engine lost a little down low but it loves to rev and when you keep it on song goes pretty hard for a 50cc engine, I hope that the 12:1 piston will be the last piece of the puzzle to turn this set-up into a flying 50 from idle to WOT:
DSC03882.jpg

[Paulie]

Talking in percents, rather than BHP or KW, what do you reckon the piston upgrade will achieve?

[Classic Honda 50's]

I would expect around 10% - 15% from where it is now or maybe a little more as I have other mods that have reduced performance slightly at lows revs as a trade off for mid range to top end performance but those mods will now be an extra advantage with the increased compression.

A 10% increase would only be around 0.5HP I am hoping that the increased compression will be the last piece of the puzzle to get this set-up going as good as it can from idle to WOT for what it is, Kitaco claim just under 6HP for their 39mm piston, stock head race cam, 20mm carb and intake kit: https://classichonda50saustralia.com...r-pack-50.html

The article below explains increased compression ratios pretty well.

Benefits From Increasing the Compression Ratio

Increasing the compression ratio is one component of many that will increase an engines HP. An increase in an engine's compression ratio will provide more power for less fuel and add some snap when the throttle is opened.

Raising the compression ratio gives the greatest benefits with initial increases. This means that more HP is produced by the first point increase of the compression ratio as compared to the next point of increasing the compression ratio. To illustrate this lets use a stock compression ratio of 9:7.1 if you increase the compression ratio to 10:7.1 there might be a 4 or 5 percent increase in HP. Further increasing the compression ratio to 11:7 might only provide a 2 or 2.5 percent increase.

The reason for the a smaller percentage in the increase in HP with a further increase in the compression ratio is due to the aspect of the cylinder being like an lung, increasing its volumetric efficieny basically means the lung is filling with more air and is breathing out more through the exhaust. It is not enough to just increase intake or exhaust. Both must be made more efficient.

A stock cam will open and close the engine's valves with little or no overlap. This prevents emissions from escaping, but it also limits an engine's breathing efficiency.

In conclusion a moderate increase in compression will use less fuel to produce more power and the extra cylinder pressure and heat generated will increase the gasoline's burning efficiency. But if you really want to maximize the advantage of increasing a engines compression ratio this use of a Hi-performance camshaft is required.


Cheers

Jaimie

[Classic Honda 50's]

Well it was not to be the last piece of the puzzle for this 50cc modified engine, after I had installed the new Kitaco 12:1 piston I was shocked at how easy the engine now started and the bottom end was a lot better but there was no big "WOW" that was a big improvement moment, I had wanted to use the Honda XR70 OEM carb and inlet but the Kitaco 50cc Kit uses a 20mm carb so I bolted on a 20mm carb and matching inlet which did indeed provide the result I was looking for.

With the 20mm carb installed "WTF WOW" I am quite shocked at how good it goes for a 50cc engine, I thought that the 20mm carb would bog the engine to death but it works extremely well, used the standard Kitaco Kit jetting #35 Pilot and a #85 main jet, turn and a half out on the air screw and middle clip position on the needle. Pulls well through the bottom end to mid-range and just wants to keep going. Need to get it on the dyno to see how healthy it is.

Honda OEM XR70 carburettor, intake manifold and heat stops with a TB air filter:
DSC05027 S.jpg


- Changed to a new TB 20mm Keihin PC20 repro carb and intake to suit, as with any new part inspect and clean prior to use:
DSC05054 S.jpg


- Plenty of room with clearance all round:
DSC05058 S.jpg


- Full throttle - surprising how many bikes turn up to the dyno with performance issues just to find that they are only getting 1/2 the 3/4 throttle
DSC05066 S.jpg

Cheers

Jaimie

[z nut]

had any dyno time yet Jaimie ?, im hang'n out to see the results on this

[Classic Honda 50's]

No - I haven't been waiting to get my dyno finished but am still running around 18months behind at present so I need to go and see Nigel to get a few bikes dynoed.

With the 20mm carb it is a different bike and feels so much stronger, would like to see around 6.0 HP as it is supposed to get.