WR500 on the dyno

pellam · May 8, 2011

Just for grins I put my recently freshened Yamaha WR500 on a dyno. 46 hp and 40lbs/ft. Not bad for a near 20 year old bike with 30 year old technology.

I think it would've topped 50hp except it would only run cleanly to 6200rpm. After that it showed horribly rich and the graph looked like a sine wave as it dropped.

The graph looks very good up to that rpm. A/F ratio shows slightly rich but steady up to 62k rpm, torque is flat and steady with hp building nicely.

I tried various jetting changes and plug gaps. No difference.

I'm wondering if it's a spark issue. Weak coil or plug lead? Timing? Or maybe it's just the stock pipe and porting won't flow over 6k rpm? For some reason it's not burning the fuel at that speed.

Ah well, more fiddling in between rides.

scottiedawg66 · May 8, 2011

what was the best air to fuel ratio you could achieve?

check the plug gap you could be blowing your spark out. try a smaller gap and see if it helps.

pellam · May 8, 2011

I closed the gap down to 24. Double checked the manual and it calls for 20-24. So I will close it to the low end.

The mixture is 12:1 till 4.5k rpm. Then rises to 13.5 from that point till 6k. Then it drops 11.5 at 7k.

Power builds from 25 hp at 4k rpm to 45 at 6k. From there it signs off as it goes horribly rich.

scottiedawg66 · May 9, 2011

funky AF line no doubt...im no jetting or power valve guru, but it its too bad that lean spot shows up in the middle of the rpm range. otherwise you could just drop your main jet size...

have you tried different settings on the clip? i cant imagine its easy to find different needles for that bike?

pellam · May 9, 2011

I dropped the main one size. No change. That's why I think it may be electrical.

Weak coil?

scottiedawg66 · May 9, 2011

does it break up over 6200 or just not make any more power?

pellam · May 9, 2011

It breaks up. Sounds similar to being loaded up. Runs clean to that point then just won't go.

Multiple runs and same result.

I shut the dyno cooling fan off to let it get closer to real world temp and that helped. Bumped from 43 to 46. Ran it again next day this time with a good warm up and got the same results.

I put a NGK B8EG in for this. I usually run the B8ES. I don't know if that would make a difference.

I'm leaning towards weak spark. A weak spark wouldn't have been as noticable with the old worn out, low compression motor. But a tight higher compression could be blowing out the spark at high rpm. Bike starts easily tho.

The real sad thing is that the engine builder, Russ Fletcher, passed away soon after doing the work. He was my go-to guy for advice.

dirt bike dave · May 9, 2011

Thanks for the interesting info. It's always fun to see the results when someone puts a bike on the dyno.

What size carb is on the bike now?

What are your goals for this machine? Sounds like cracking 50 hp would be nice, but are you willing to give up low end or spend lots of money?

I can't help but think a 'powerjet' carb like on the '97 - '99 CR250 would help, as there is a solenoid on those carbs that leans out the bike at high rpm. But it's all integrated with the black box and I imagine it would be a nightmare to fit something like that onto your WR.

pellam · May 9, 2011

The carb is the stock Mikuni VM38SS. 50 pilot,Q8 needle,3.5 slide 440 main.

My goal is a well running woods bike. Which it is. I don't want to lose the low and mid range.

For internet bragging I'd like to break 50 hp. I've been offered a Keihin FCR39. Which I may get. I'm going to check the coil with a dynamic tester. Close the plug gap. There are limits to how much I'll spend on it tho. At some point parts availability will relegate it to a garage piece.

It would be interesting to see what could've been if the factories had supported open class 2 strokes. Imagine what a FI and a fast ECU with 3d mapping could accomplish.

whenfoxforks-ruled · May 9, 2011

Isn't 6200 about redline? And I do not know how trying to jet the fcr 4 stroke carb will be?

helio lucas · May 10, 2011

clean you carb very well and the air filter as well. while you are at the carb, inspect the reeds for any crack, wear or leakage.
inpect the exhaust sistem, both expansion chamber and silencer for any signs of blocking. the packing in silencer has to be in good condition as those big bikes tend to be very sensitive.

sorry if you already check these, but trying to go on the simple, cheap, quick first :cool: :cool:

jsantapau · May 10, 2011

pellam said:
The carb is the stock Mikuni VM38SS. 50 pilot,Q8 needle,3.5 slide 440 main.My goal is a well running woods bike. Which it is. I don't want to lose the low and mid range.

For internet bragging I'd like to break 50 hp. I've been offered a Keihin FCR39. Which I may get. I'm going to check the coil with a dynamic tester. Close the plug gap. There are limits to how much I'll spend on it tho. At some point parts availability will relegate it to a garage piece.

It would be interesting to see what could've been if the factories had supported open class 2 strokes. Imagine what a FI and a fast ECU with 3d mapping could accomplish.

didnt the yz 490 have a 40mm carb?

dirt bike dave · May 10, 2011

jsantapau said:
didnt the yz 490 have a 40mm carb?

Doing a little digging on the web, and it looks like the YZ490 first got a 40mm carb around '84 or '85, and the early 490's had a 38mm.

Sounds like a 40mm might be a nice choice for the OP's WR500.

FNG · May 13, 2011

I had a similiar issue with cutting out/missing on the big end with a '95 YZ250.

It turned out to be a poor connection between the plug wire and the cap. I took the cap off the wire and cut 1/2" off to get to some shiny wire and screwed the cap back on and my miss was gone.

Rooster · May 14, 2011

"Don't Ask" by Super Hunky comes to mind here.

arnego2 · Jun 18, 2011

I used a different needle once on my YZ250 and had a similar problem. The needle I used was a NECJ and the needle correcting the issue was the stock N3EW. Running with the NECJ and at 75% throttle it started sputtering. I thought it was electric at first.

2strokerfun · Jun 20, 2011

Rooster said:
"Don't Ask" by Super Hunky comes to mind here.

If you were to ask him almost any question about a YZ490, instead of a WR500, I believe his answer would be something close to this: "Carefully unscrew spark plug and lift from cylinder. Replace bike with different model. Install spark plug and place cap on plug. Go ride."

Lorin · Jul 14, 2011

I ran a somewhat modified 91 wr500 for five years and did well with it. Although I wasnt able to get dyno numbers, it certainly changed the whole nature of the bike. Mods:
changed to an fmf pipe and silencer. Had noleen do their head mod (mill and change squish). Bought a pwk 39.5mm carb (as found on the kx500). Milled cylinder base, and bored the cylinder to 518cc. Ran great and the head mod helped the pinging considerably. Ran the bike reliably for years until moving to a newer, lighter 250. The carb made a vast difference and would probably be my second recommendation (after the head mod). I was able to get good tire life by using the IRC M5b in a 140mm width. Life was very good in those days. Took 3rd overall in the C open class back in early 2000's on my ten year old, air-cooled bike.

ws6transam · Jul 22, 2011

Wow, I've been gone from Dirtrider since February, and THIS stuff pops up! Great reading, as I just finished having a go at some trails here in Mid-Michigan on my street-registered WR500. I'm running a similar combo, except it was Eric Gorr who did my head mod and mild porting of the cylinder. I'm using the 38mm carb with similar jetting except I swapped to a Q4 needle jet which cleaned up the midrange quite nicely. Another change that helped measureably was to swap out my old reeds for some FMF reeds. They brought back the top-end power.

Unfortunately I had a little engine mishap this week - left me temporarily stranded in the national forest in 95 degree heat, which I will bring up in a separate thread.

I've been wanting to dyno this WR500 as well. Were you using a street tire on your rear wheel in order to prevent wheel spin, or was this a water break dyno? Is this brake HP or rear-wheel HP?

I suspect you may be onto something with the weak spark. Power is usually at it's peak in a gasoline engine at an AFR of 12.5 - 13 :1, unless it's turbocharged.

pellam · Jul 25, 2011

ws6transam said:
Wow, I've been gone from Dirtrider since February, and THIS stuff pops up! Great reading, as I just finished having a go at some trails here in Mid-Michigan on my street-registered WR500. I'm running a similar combo, except it was Eric Gorr who did my head mod and mild porting of the cylinder. I'm using the 38mm carb with similar jetting except I swapped to a Q4 needle jet which cleaned up the midrange quite nicely. Another change that helped measureably was to swap out my old reeds for some FMF reeds. They brought back the top-end power.

Unfortunately I had a little engine mishap this week - left me temporarily stranded in the national forest in 95 degree heat, which I will bring up in a separate thread.

I've been wanting to dyno this WR500 as well. Were you using a street tire on your rear wheel in order to prevent wheel spin, or was this a water break dyno? Is this brake HP or rear-wheel HP?

I suspect you may be onto something with the weak spark. Power is usually at it's peak in a gasoline engine at an AFR of 12.5 - 13 :1, unless it's turbocharged.

Hey, WS6, I've been thinking about your bike. Could you send me the your wiring schematics? I want to ride my WR to work.

I used a street tire and the numbers are rear wheel corrected. Reeds are new Boysen. I replaced the plug cap and that helped a bunch. I don't have trouble with pinging but would like to get the head done. Trouble is the man who did my engine work passed away. RIP Russ Fletcher.

Eschrader · Jan 22, 2022

Has anyone used an ECU ignition instead of the stock CDI? And were there any improvements with it?

Also these kx 500 carbs, is an aftermarket complete one off ebay (you know,, those pwc $50 ones) good if I but some good jets or jet kit for it?

WR500 on the dyno

pellam

scottiedawg66

pellam

scottiedawg66

pellam

scottiedawg66

pellam

dirt bike dave

Sponsoring Member

pellam

whenfoxforks-ruled

Old MX Racer

helio lucas

jsantapau

dirt bike dave

Sponsoring Member

FNG

Rooster

Today's Tom Sawyer

arnego2

2strokerfun

Lorin

ws6transam

pellam

Eschrader

Similar Topics

WR500 on the dyno

Sponsoring Member

Old MX Racer

Sponsoring Member

Today's Tom Sawyer

Similar Topics

Privacy & Transparency

Privacy & Transparency