83 XC500 dyno results

The opportunity came along to stick my 1983 XC500 on a dyno. It's not running at all right so I knew the results would be bad, but I thought I'd share anyway (and hang my head in shame!)

It's a bit of a hotchpotch bike: 1987 bottom end, 500 air cooled head (don't know if it's off an '82.5, '83 or '84) with a Mikuni VM44 roundslide.

The jetting is stock for 82.5 44mm carb as far as I know, although we raised the clip and took off the airbox lid as it was running so poorly prior to that). Also the bike is running on 32:1 premix (which will lean up the fuel mix?) and with a WR silencer (which will richen it up?). The carb is new. The engine is new. It has MZ-B ignition fitted which has been advanced (or retarded?, I get confused) as suggested by the service bulletins. The silencer is repacked. Jetting is as follows:

• Air screw: 1.5 out
• Pilot (idle) jet: 35
• Throttle slide: 2.5
• Needle: 7DH3
• Clip: top
• Needle jet: BB-5
• Main jet: 400

Dyno chart is attached. It kind of ran OK until about 2400 rpm, then went all over the place until about 3100 rpm then went well until 4000 rpm which were the max revs.

What surprised me was

• Huge torque (approx 45 ft/Ib)
• Low max revs (4000 rpm)
• Low max power (approx 22 bhp)

This is the starting point so hopefully will improve with jetting changes. I'm trying to smooth out the power and keep the noise down, so it's a pity about the airbox lid restricting the flow too much.

I will keep you posted.

If anyone has comparatives of max revs and power output for these engines, or any other views, I'd appreciate hearing them.

Cheers
White Husky

max 22bhp and max RPM 4,000 ?

:cripple:Tumbleweed blowing across the room?

Come on guys - I expected to be ridiculed over these figures. Surely these old 500s made more than 22bhp and more than 4,000RPM?

Didn't the old air cooled Honda CR480 do about 50bhp and 6 or 7k RPM?

I will be re-jetting but surely that won't get me out of this pickle. Does anyone have a clue as to what I should be looking out for? Compression maybe? Internal engine seals?

All help and advice appreciated.

Cheers
Lucien

I would check compression first, then do a leak down test and see what you find out from those.

Regardless of how it is running there seems to be something screwy with the attached file's graph. I see two different torque values for the same rpm, horsepower is basically torque times rpm. Is this post a trick question?

My 500 makes a lot of torque at rpm levels below where you ran yours so something seems amis at the low end as well.

An '04 wr250 says 42 hp on the certificate of origion not sure what that means on a dyno (rear wheel I presume)

I have wheel dyno'd alot of semi trucks and the tires can be a factor toward an erronious report. Was there a possibility of slippage? Were was the engine rpm measured? Possibility of clutch slippage? Were were they measuring the air fuel ratio? Just some thoughts............Scott

How does it feel when you actually ride the bike ?

I got my XC up and running last night for the first time since the rebuild:
(Woessner, rod, MZB).

Initial impression:
very smooth on the bottom end, perfect for short-shifting
good transition to midrange - which feels OK
top end has a nice hook, but needs some space to stretch her legs - perfect for the longer straights

Once run in, I do hope tp get to a dyno too - but on the whole - there is a LOT more than my other 250's.

Yep - thanks for that Tommie. That's the first thing I'm gonna check on Friday when I look at the bike again.

Hi Fran. Yes that odd line doubling back on itself is the first thing I asked about. Apparently it was caused by the bike spluttering. Doesn't make too much sense to me either. I didn't know HP = Torque x RPM / 5252 so I googled it. Found this interesting idiots guide to power and how on a motorbike it's the torque that the rider feels. All makes sense when you read it, but had not occurred to me before. Here's the link: http://www.38large.com/dyno/dyno_explanation.html

Good to know. I will mention that when we're tuning it.

Great info Steve, thanks. I was hoping someone would have some HP figures somewhere. I told my dyno friend I was hoping for 50 or 60hp at 6 or 7 thousand RPM but he laughed at me (in the nicest possible way).

Thanks Scott. It was dynod at the rear wheel and it's on knobblies so that's not going to help things, but conditions were dry. Clutch seemed OK. Engine RPMs were measured by a sensor clamped to the HT lead. Air/fuel ratio was measured by a probe up the exhaust.

I've only ridden the bike once, a run-in ride of 15 miles or so. It was with a shorter 250CR silencer AND a db-snorkel fitted (to kill the noise) so I'm guessing that'd richen it up a bit. There was no real 'hit' or 'pipe' so at the time it felt much smoother than I was expecting, but in hindsight that may be just because the bike never revved enough to be 'on the pipe'. The db-snorkel is supposed to feel a bit like adding a flywheel weight, so I figured that was what I was experiencing. It ran well but I do recall it not 'taking off' as one would expect when the revs climbed, ie running out of torque.

Am I reading the Air/Fuel ration chart correctly? I figure that below 12.5 (12.5 parts air to 1 part fuel) the bike is TOO RICH. Is that right? That's why we raised the needle clip (to lean the mix).

We'll start by leaning the main jet and work down. I'll report back on Friday hopefully.

Hmmm
What does your compression feel like when you stroke it by hand. You should hear LOTS of sucking..

On revving, there shoud be a definate top end hook on the power delivery. Motor should still reach 6k. The transition from midrange to top end is not violent, but you will definately notice it,

Any kick-back on starting - that could point to advanced timing, which will also hurt overall power.
Sluggish overall and difficult starting could be retarded timing.

I suggest the following:
1. Check timing
2. New plug
3. Check spark when you kick it properly - should be decent.
4. clean carb
5. Clean air filter
6. fresh premix
7. Remove snorkel
8. Remove silencer for now - peer down the centre and make sure nothing blocking

Start
warm up thoroughly
go for a mixed ride and note what it feels like
make sure all the cobwebs blown out before going back for a dyno

Try these,
roll on from low revs, half throttle
roll on from low revs, full throttle
snap open from low revs
roll on from midr
snap open from mid

and lastly - get to the bottom of third gently, then open her up and hold it there and note where she hesitates and where she starts sings. You will need some space for this one.



PS. At 12.5:1 you are running rich, as you 14:1 is ideal, but with these motors rich is safer.

Let us know.

Ridiculed? Heck no! You got one onto a dyno. That's more than I've done.

Did they make more than 22hp and rev higher than 4,000 rpm? Ya, but that was before they had 30 years of "some dude" messing them up. Once you get it sorted, it will again. Hope you do some more runs. Nice thread, thanks.

Mag 250 Dyno runs

Here are the Dyno results on my completely standard 74 Mag 250, we put a road tire on it to run on the dyno.

So there I was sitting in yet another meeting when I rtemembered these:

Check the HT lead for cracks
Check the plug cap for moisture and a good connection
Check the plug top screw-on attachment is tight
NGK B8ES
You could also check the eqarth between motor mounts and the cases - clean and smooth.


there again - all of these would make it difficult to start too.

Dyno Thoughts

Lucien,
I spoke with my "mixed gas mentor" last night and he had some good thoughts. He said the first thing he would be suspicious of is the air/fuel ratio method. He said trying to measure that at the silencer would be very inaccurate in a two cycle. The heat and soundwaves with a expansion chamber would as he feels render a tailpipe method inaccurate. He stated that he always checked exhaust gas tempature at no more than 6" from the piston. Most EGT readings corralate to the A/F ratio, example 14-1 is near or at 1400 deg. F. Stoich mixture is optimal but dangerous in a two cycle. He said he always strived for 1200 to 1250. The cooling capability around the exhaust port in a two cycle would also effect that temp. also but that would be getting very specific. He also said your pre-mix ratio would have a big effect on EGT and power. We run 28 to one Yamalube R. With running that ratio we have to increase the jet sizes to compensate for that oil ratio. More oil=less fuel. I would be concerned also of running a knobbie on the roller having slippage.
Looking at the dyno report on the A/F it posssibly shows at the top RPM it starts to go leaner which may be the cause for not turning more RPM and low power. Seeing it go up at the very end has me suspicious of a fuel starvation issue. One idea I would look at is the fuel petcock screen in the tank, seen them full of "stuff".

Thanks

Thanks for all these great comments / suggestions / ideas / info guys. I'm heading down to the dyno shop now fully armed!

Will let you know what happens.

Cheers
Lucien

Updates? Hoping this great thread won't die!

Me too!! Inspired me to start up my 500 project again.

White Husky
I have ridden a 500 wr 84. I agree with you, it is 22 HP, but at the front wheel !!!

hp

i rode hva from 76 to 84 mostly open class....and the reason i choose them.... reliability, quality kit, ohlins,mikuni,trelleborg....fast.....er ..no.. that was a fallacy.... torque.... er no...none of that either.... turning ... definately not...brakes!! ha ha ...washboard flat out bumps.... yes wfo for miles when the jap bikes were buckin broncos!all my open classers had very little power compared to the big maicos and yams...and i include my 82.5 silver streak and 84 500 in that....when i got an 85 495ktm water cooler i knew what power was....sorry boys thats the way it was and we know it .

Where is fareham? I really have a hard time imagining keeping most any dirt bike wide open for miles unless the wheels are sinking in sand or something like that or on a highway with sprockets so there is enough wind resistance.


Na it isn't that bad. Right before I finally put new or different better used clutch plates in my 1983 xc500 it would slip the clutch when it came onto the power and amazingly also slip the clutch instead of stalling at very low rpm (the throttle has to be twisted somewhat) at the top of a hill when a downshift should have been done.

Hi guys, thanks for your further comments.

Sorry I posted no follow-up. I was waiting until I had something concrete to report, and unfortunately we haven't had it back on the dyno yet.

I have ordered a new tail pipe and 38mm Mikuni carb so I thought I'd wait until they are fitted. I will definitely be following this up!

Kind regards
Lucien