Slightly hotter cam for X250, 80-82. Pointless or not?

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arcticXS

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As mentioned in another post, I am working on a XT250 (air cooled, 2 valve , sohc, ca 1980-82)
I have started some work on the head, and now I wonder if a slightly hotter cam is worthwhile or not.
Megacycle have a cam with 246/ 245 duration and .415/ .395 lift. Lobe centers 106/ 106.5 This camhaft also requires other springs.
The stock cam specs are 232/ 233 and .380/ .382 lift. Lobe centers 108/108

Not having messed much with 4 stroke singles, I wonder if getting this cam will give any meaningful power increase.
The only other real change, apart from lightly cleaning up the ports and a fresh 3 angle valve job, is a slightly larger diameter header and a Supertrapp muffler. I may also look for a slightly larger carb later.

My plan for this bike is just short trips, light commuting, maybe some winter riding on snow covered roads, possibly also some light exploring on trails and unpaved roads.

All and any input is most welcome :)
 
When I had my XT500, I tried the old tried and true method of low restriction exhaust, removed the air box and hung a big K+N filter on it, and bumped up the main jet, pilot jet, and raised the needle a step or two. The increase in power was really noticeable, the bike would wheelie with ease in the first two gears.
I had it in the dealership once and the service manager was asking me what I had done to it because he was impressed with the power gain and he told me at the time that it put out more power than one he had just ridden with a hotter cam. :shrug:
 
Mailman said:
When I had my XT500, I tried the old tried and true method of low restriction exhaust, removed the air box and hung a big K+N filter on it, and bumped up the main jet, pilot jet, and raised the needle a step or two. The increase in power was really noticeable, the bike would wheelie with ease in the first two gears.
I had it in the dealership once and the service manager was asking me what I had done to it because he was impressed with the power gain and he told me at the time that it put out more power than one he had just ridden with a hotter cam. :shrug:
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It's kind of "all of the above" answer. The cam alone isn't going to do much if you don't increase flow and fuel. Just doing any "one" of the mentioned improvements isn't going to make that big of an difference.
 
You will see a bump in power but I suspect the duration numbers will move that power up the rpm band which I think would be lots of fun, but not in snow. When you have a lumpy cam and more duration you tend to lose low rpm vacuum. That causes the lumpy idle you find in cammed V8s because they dont draw much air (vacuum) and end up with mixture issues at idle, hence the lumpy idle. Keeping it on the cam is fun but may overwelm the bike in low traction snow conditions. Go for it is what I say. YRMV
 
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Having built a race engine or two (4 and 8 cylinder automotive), I can tell you that you will feel a noticeable difference. As Kevin said, the higher duration will move the power band up the RPM range, the increased lift boosts power everywhere in the range and broader lobe centers smooths out the powerband somewhat. Lots of knowledge available on the Web for inquisitive minds.
Remember the three C's of performance; cam, carb and compression. Add an E for exhaust and you'll really feel the difference. In my experience, compression has the most dramatic seat-of-the-pants impact. The Yamaha XT250 of the era had 9.2:1 compression, which is not super aggressive. Maybe aim for 10:1?
 
You need to calculate dynamic compression ratio based on the octane rating of fuel you plan to run. http://wallaceracing.com/dynamic-cr.php. I'm thinking 8.0-8.5 DCR for 93 octane. It's all about when the intake valve closes. Do your own math. It's been a while since I used these calculations when building my Mopar slant six.
 
Mailman said:
When I had my XT500, I tried the old tried and true method of low restriction exhaust, removed the air box and hung a big K+N filter on it, and bumped up the main jet, pilot jet, and raised the needle a step or two. The increase in power was really noticeable, the bike would wheelie with ease in the first two gears.
I had it in the dealership once and the service manager was asking me what I had done to it because he was impressed with the power gain and he told me at the time that it put out more power than one he had just ridden with a hotter cam. :shrug:
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MM,
Good to hear about your XT500 !
Unfortunately, the XT250 monoshock severely limits space for air filter options and modifications. So it looks like a near stock air box is the only option.. And the stock filter is just a flat foam filter maybe 3x5" or less. But I will give the setup a good hard look anyway
 
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arcticXS said:
MM,
Good to hear about your XT500 !
Unfortunately, the XT250 monoshock severely limits air filter options and modifications. So it looks like a near stock air box is the only option.. And the stock filter is just a flat foam filter maybe 3x5" or less. But I will give the setup a good hard look anyway
Click to expand...
One thing's for sure, if Grizld1 were still with us, he'd have a good time helping you.
 
It seems that the Mothership in Iwata had the solution for me all the time 😁. Due to a hamfisted PO, the drive end of the cam was damaged beyond repair. So I ordered a new "22 PS/ full power" cam and rockers from Kedo. When comparing base circle and can measurements of the old and new cam, both have a 32.00 mm base circle, while measuring the cam at max lift gave 38.50 and 40.20 mm respectively. So 1.70 mm more lift.
The back story is that Germany had insurance classes based on horsepower limits. 17, 27 and 50 hp. So selling a 22 hp bike in a class where it would compete with 27 hp bikes made little sense. So the little XT was restricted to 17 hp. And I guess some of them ended up in other markets, like Norway.
On a side note, other Yamahas met the same fate in Germany. The 30 hp RD250, and the 32/33 hp XT/SR 500 were most common in 27 hp version.
 
@arcticXS
Without meaning to spoil your fun i would have investigated a hint more into the megacycle stuff.
Admittedly the KEDO stuff ain't bad but usually they are to my knowledge reground Groszewächter camshaft and in comparison to those the Megacycle or web cam stuff is top level.
In particular considering the fact that often megacycle offers a short duration high lift profile for a lot of engines which if combinef with suitable porting gives a bunch more Tq as well as power.

Kind greetings Christian
 
Hudriwudri said:
@arcticXS
Without meaning to spoil your fun i would have investigated a hint more into the megacycle stuff.
Admittedly the KEDO stuff ain't bad but usually they are to my knowledge reground Groszewächter camshaft and in comparison to those the Megacycle or web cam stuff is top level.
In particular considering the fact that often megacycle offers a short duration high lift profile for a lot of engines which if combinef with suitable porting gives a bunch more Tq as well as power.

Kind greetings Christian
Click to expand...
Christian, thanks for your input! The camshaft I got from Kedo, is a OEM Yamaha camshaft. And was listed as 22 PS/ offen in their catalog. I should have suspected that my XT was a 17 PS version, when I noticed that the rubber carburettor holder/ inlet stub was restricted to somewhere around 23-24 mm. The OEM carb is a 28 mm pumper with a slightly odd bore, V- shaped at the bottom half, I guess for higher air velocity at low throttle settings.
Weirdly enough, the intake port diameter in the head is 32 mm, leading me to think that a TM32 or FCR31 may be the optimal carb for this engine. Alternatively that the port could benefit from some epoxy or welding, bringing it down to 30 or even 28 mm. What's your thoughts around this? Would a 32 mm carb make sense at all?
The head is getting pretty much a full rebuild. New camshaft, new camshaft bearings, new rocker arms, new timing chain, new chain sliders, new camshaft sprocket, new valves, new valves seals. The piston looks ok, and is still the original 75.00 bore. I also got a new SS headpipe from Kedo, a couple mm larger than stock. And have a Supertrapp mufflers I plan on using.
 
@arcticXS
Hi there,
Hum kinda hard to say without knowing the flow characteristics of the head.
I assume if correctly reshaped the head should support a 32mm carburetor for sure, although I think a fcr31 might be kinda expensive, hence I'd look for a Tm pumper or a dell'orto pumper.
The larger exhaust pipe i would try after trying first the original (hopefully single laye) exhaust header with the Supertrapp as you might otherwise loose too much density/heat for good wave propagation.
The intake port i would not downsize yet at that point but rather just work on the SSR area and leave as much intake flange bottle neck area as possible in order to keep gas velocity high.
In case you unthrottle the carb flow thus you will get higher intake flow hence velocity anyways in the intake port ;)

Lotsa fun and kind greetings

Christian

Ps: there is also from time to time some corner on xt500.org where they discuss the smaller models. Unfortunately I have very specific knowledge just on the bigger models (2 as well as 4v).

PPs: just read it now, yes usually they did such v-notches for higher velocity of the carb slide seat.
 
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