Reasons why 500's ping, Now 2-stroke porting idea's ;-)

[John Orchard]

The hose on the pilot jet thing was a common "fix" for the big flat spot bog that Honda XR's had when landing from jumps or going over a long stretch of rough ground. Never seen in on a CR till now though.
K

Yeh Mick I remember making all sorts of baffle plates to cure a lean 'bog' after jumps or big hits back in the day, I'd never considered airation from vibration though.

[Lozza]

Single cylinder engine balancing is a subject all on its own. Ideally you shift vibes to a rpm range that's never used.  Default factor is 60% reciprocating over rotating masses. High revving kart engines can be as low as 55% and low revving mxers can be as high as 67%. Main thing is to sdd mass to the light side of the crank rather than try to take it from the heavy side. With the CR I would expect the balance would need to be raised rubber mounting at the head stay helps a lot to. Has anyone actualy checked the balance factor with the 500?

[matcho mick]

on that vibration bit,bitd engine vibes on Bathurst's conrod straight bounced the fuel off the tank bottom,causing stavation,clever guys lengthened the tap internal pickup spigots,

[sa63]

 Crank balancing according to rev range is one thing, the crank halves being trued is another. Never a bad idea to get the crank checked for how true it is if you pull the motor apart.
I found 1/4" hydraulic tubing has a good interference fit in the pilot jet holder, soput in the extension in my PE38 today

[mick25]

Interesting stuff!  I knew there was another reason, besides being fast, that I have you as my factory rider. 

The CR500RE has a different crank balancing than the CR480, maybe that is why it might vibrate more?  This makes me wonder about the infamous YZ490 and it's pinging problems.  I think vibration is almost always mentioned in the YZ490 tests.

there is a yz 490 in number 46 of the vmx mag
The yz 490 featured has had some head work and a few other big mods to stop the pinging , maybe a good read.

[John Orchard]

Ok I'm going testing out The Farm this week, I've rubber mounted the airbox, raised the float level from the 16mm float height to 15mm, I've also totally removed the tin baffle plates, they were sure to vibrate and make the fuel airate with any rpm on.

Still keeping the rubber fuel hose pilot jet extension, with 160 main jet, 50 pilot jet, DGG needle with B8ES plug.

[John Orchard]

Ok went testing ....

I didn't find any further improvement over my previous pilot jet extension, maybe a touch worse?  With the airbox rubber mounted, it appeared to cause the carb to vibrate around a bit more?  I tried pulling the carb body hard against the tension of the rubber joints with a big cable-tie to the frame, it seemed to reduce the vibration going through the carb but didn't reduce the high rpm/low throttle lean ping. I ended up putting the tin fuel bowl baffle plates back in and kept the higher float-level.

In an effort to establish other area's that could be causing the leanness; I did notice plenty of smoke coming from the pipe join to the barrel, while this is not an odd thing as many bikes leak at the header joint, I have to wonder if a big bore is more likely to take-in some fresh air with a negative pulse travelling past the leak?  I'll put an 'o' ring in for the next test.

My next area of concern is with the carburetor 'pilot air bleed', from my understanding & experience on how the air-bleeds work in a carb, the effects of bleeding air into a fuel circuit (pilot or main circuit), the higher the rpm, the more air is bled into the circuit.

So, with the pilot jet down from the stock 68 to a 50, the bike still started cold without need for the choke, so the pilot jet is still too big, or there could be some other reason for the super-low rpm (kickstart speed) richness?

As the rpm's increase, while still with a slight throttle opening (still on the pilot circuit) the affects of the pilot air-bleed (air-screw) is leaning-out the mixture too much?  The mixture screw was set at the stock 1.5 turns.  So I figure for the next test I'll try going down to a 40 or 45 pilot jet with the air-screw turned right in, maybe around 1/4 turn?  I wouldn't be surprised if I have to go even smaller with the pilot jet, maybe a 30 or 35 and seat the air-screw all the way home? But we'll see.

I have found in the past that the 'pilot air bleed' can have an affect mixtures during larger throttle openings, found by accident one time with an RM125B, that with the air-screw seated closed I got a high rpm/full throttle rich miss-fire, so with that in mind, if I end up seating the CR's air-screw home I may have to compensate further up the rpm/throttle opening settings?

I am surprised that an engine that is close to stock would require such a drastic change from the stock fuel curve?  I did find many years ago when racing a Honda VTR1000 Firestorm, that once I opened up the airbox, I had to go from the stock 175 main jets to 300 (1.75mm to 3.00mm), this gave perfect power at 10,000 rpm but a very bad rich stumble at 6,000 rpm (at WOT), so I went back to the 175 mains but completely blocked-off the main air-bleeds ... perfect all the way through the rev range.

Stay tuned, will test again next week when the new jets arrive.

[80-85 husky]

lots of good work happening there John keep at it

[ARN]

hi john

I recently did the squish on a 77 yz400 ,as it ran ok before this, the amount removed from the dome of head was the same as removed to get a 1mm squish. but it pinged real bad ,so I read up about it and checked comp ratio .it ended up being 22 to 1
,Shane ended up fixing for us he had to remove 18cc extra from what I had done, now its a rocket.

cheers arno

[Lozza]

I would still be looking at the cylinder head

[John Orchard]

But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.

I'll down-size the pilot and screw in the air-screw and see  :-)

[FourstrokeForever]

But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.

I'll down-size the pilot and screw in the air-screw and see  :-)

Personally, I think the bike isn't getting enough fuel at low rpm John. I'd try a fatter pilot jet and perhaps remove some of the slide cut away if it ends up with a bit of a bog down low.... Just a thought......

[sleepy]

But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.

I'll down-size the pilot and screw in the air-screw and see  :-)

In my experience (spent 10 years driving a dyno) the idle - pilot circuit responds a differently to air correction compared to main. The air screw is called the idle air screw and has it's greatest effect at idle. I would go back to the stock pilot or slightly bigger.
Would you descibe the ping as only roll on-off throttle at high or on pipe RPM? If it is try a lower or richer slide cutaway to get it on the main curcuit with a little less throttle opening.

[John Orchard]

Thanks for the input guy's, spent another day at The Farm today; I think I've nearly worn-out the screws holding that carb in & together  lol.

Just about perfect now, waiting on a needle coming from Keihin in Japan to finish it off.  It has all been carburation related and is what I thought it was ... running lean at part throttle.

Extending the pilot pick-up got me 50% there but a change in needle was also required, I've posted before that "where you shift power to, you must shift fuel to", the reed & combustion changes boosted low-down so I needed more fuel at part throttle.

What had me confused was how early the straight section of the needle came-in, in this case it comes into play at idle but with more dominance once the throttle slide starts to lift; I guess a 500 has such a strong intake pulse that it draws from the needle jet immediately?

So I've gone down on the pilot, from 68 to 50, stayed with the same air-screw setting (1.5 turns) and gone to a needle with a smaller 2.645 straight section (DEE) on the leanest clip, going to the leanest clip cleaned up the blubbering half-throttle, stock needle is an N28 (DGJ / R1468N) with a 2.685 straight section, my DEE replica is a sanded-down needle, when I get the factory DEE will do a final confirmation test.

:-D

[OverTheHill]

i haven't read it all but had strange things happen on an old ktm125 think about "86 & did a crank for a guy, off down the paddock & went through a blubbery stage, not idle or just above but about half revs maybe about where it realy wants to get on the boil, & although it was carburation symptoms the cause was 'reeds', someone sugested that & i poo poo'd it but learn't something that day. By memory were fibre reeds & must've lost their tension because new ones damn well fixed it.
With your CR500, if it has a round slide carb [suppose it's keihin but i'm only familiar with mikuni] i think they have a progression hole at an angle coming off the pilot, just in front of the pilot outlet & fed by pilot & think you can increase the richness [bit like less cutaway on the slide] by increasing the jet size then i suppose back the pilot air screw out a bit to keep idle ok [if it was ok before]. Does seem strange if you have to go too far from std though!!, don't suppose it's been ported [over ported] . Had to lengthen the header by heaps on an IT400 motor i bought for vmx & someone had got in there & given it 192 degrease on the exhaust port & "went" like a roadracer but no good in the dirt--not knowing this, i was trying to carburate the flat spot out [almost blubbery], was a pain till i went looking for other causes.
Anyway--don't talk carburetors to me----had to fix a honda quad starter motor the other day for someone not far up the road & could see an "85 XJ600 Yamy in the shed--do you want it for nothing, it's been under a hedge since 2003. raced home & got the trailer, had to knock the calipers off so as to be able to wheel it. I "will" get it going, carbs [4 of the buggers] are the worst mess i've ever seen [& i've said that before] but realy this time. Touched a battery [screwdriver trick] onto the starter & instantly spat a pint of dirty water all over the shed. Anyway, sorry, gettin off track again.--where was i.