1984 CR500 combustion characteristics?

[nephron]

Eric or Rich, or anyone else, I'm just curious about the 1984 CR500's actual combustion physics and/or other factors that made this thing fry pistons every month and ping like hell. What exactly was it about that bike? Anything that serious/consistent has to be a major design flaw. I'm wondering what did they do, or how did they fix it for the following year?

I put it in this forum, because I'm sure the explanation is quite complex. Was it a chamber, quench/squish, dynamic compression, or ignition timing issue?

Just curious.

:think

 

[David Trustrum]

Well Eric or Rich may add some subtlety but the design flaw was to make an inefficient cylinder size. 125cc seems to be pretty close to optimum for real world engines. Gas flow control & combustion are pretty haphazard in such a large cylinder.

There are some things you can do to help things along such as a twin sparkplug conversion or machining the squish band to lower the MSV in the part of the engine that will tend to detonate & try to condense combustion into a favourable area. Anti detonation inserts. Best go see someone who has built a whole bunch of desert racing motors.

 

[EricGorr]

A quick and easy mod to the head would include adding a blend angle between the squishband and the combustion bowl. This serves to reduce the maximum squish velocity (MSV) which is the primary cause of the pre-ignition and detonation problems.
Regarding the machine work, find a machinist with a lathe and face-plate, use long bolts and spacer pins to fasten the head to the plate. Set the tool angle to 30 degrees. Start the tool bit 8mm from the outside edge of the squishband and make a cut until it blends into the bowl.
David's suggestion of 2 spark plugs is a good one, for best results the plugs have to be positioned from left to right of the bore at 45 degree angles, check to see if theres enough room with the pipe. A good coil to use is one from most Yamaha snowmobiles. Its one coil with 2 leads and is already set-up top work with CDI systems.
Good luck:)

 

[Mig]

I just aquired an 84 CR500 and I am thinking about road racing / Motarding it. I have heard that some individuals are putting in 2 head gaskets to avoid the pinging problem. I would like to run pump gas as it was supposed to be an econo ride but I am thinking twice. I haven't yet had it running full blast as the crank just went.

Mabye a simpler solution?

Mig

 

[cujet]

The 1984 CR was air or water cooled? I cannot remember.

In any case, I would suggest that the head design is partially responsible for the problems. Also carb type and setup plays a huge role in durability. Aftermarket heads were available to solve some problems on those bikes. They work very well. One trick that works is to use 50% to 100% avgas. I know, I know, Rich does not like avgas. But it does work well in the CR500 and there are many who swear it runs as well as anything else in a CR500.

Chris

 

[motometal]

regarding using two head gaskets, the reduction in compression generally of course would help reduce pinging, but you're also increasing the squish clearance. I was under the impression that all else being equal, this would lower performance, and (in theory IF compression was the same) increase tendancey to ping?

 

[Halfast816]

The quick fix back in '84 was two head gaskets and a different needle. Still didn't run as well as the '83 480 or the '85 500

 

[Papakeith]

84 was air cooled

 

[84cr500lover]

I dont seem to have these issues.
My head is stock, with 1 gasket.
I run 1 gal of 111 octane to 4 gals of 91 pump gas, I have ridden with just 91 pump gas. I add the high octane for piece of mind.
I do not however run the stock carb, which is the same carb as the cr480.
I had a 38mm TM, but now run a late model cr500 PJ.
My bike is set up for dez and trails, clarke 4 gal tank, new dg pipe and silencer, sparky, 90mm bore.
She hits harder and is more fun(seat of pants) then my water cooled 1990.
I also had Randy Jackson(formerly of FCR suspension) of R2 performance set up the boingers for my weight and riding style. I am a member of cr500rider(dot)com and ride with lots of watercooled 500's.
Let me assure you, the water cooled bikes have no advantage in engine performance other than aftermarket support.

 

[ian melling]

I dont seem to have these issues.
My head is stock, with 1 gasket.
I run 1 gal of 111 octane to 4 gals of 91 pump gas, I have ridden with just 91 pump gas. I add the high octane for piece of mind.
I do not however run the stock carb, which is the same carb as the cr480.
I had a 38mm TM, but now run a late model cr500 PJ.
My bike is set up for dez and trails, clarke 4 gal tank, new dg pipe and silencer, sparky, 90mm bore.
She hits harder and is more fun(seat of pants) then my water cooled 1990.
I also had Randy Jackson(formerly of FCR suspension) of R2 performance set up the boingers for my weight and riding style. I am a member of cr500rider(dot)com and ride with lots of watercooled 500's.
Let me assure you, the water cooled bikes have no advantage in engine performance other than aftermarket support.

Iam about too buy a very restored 84cr500 for vintage racing everyone seems to have issues with them pinging etc its nice too hear from someone who knows them.I have raced a 87 cr500 in the finke desert race australia, put it in the top ten.We used av gas 100 percent get the jetting right and you could hold it flat for ever,providing you had the road ofcourse.

 

[The Redrider]

Eric or Rich, or anyone else, I'm just curious about the 1984 CR500's actual combustion physics and/or other factors that made this thing fry pistons every month and ping like hell. What exactly was it about that bike? Anything that serious/consistent has to be a major design flaw. I'm wondering what did they do, or how did they fix it for the following year?

I put it in this forum, because I'm sure the explanation is quite complex. Was it a chamber, quench/squish, dynamic compression, or ignition timing issue?

Just curious.

:think

Old post, I had an 84 500R and it didn't ping or fry pistons? I raced it and trail rode it for fun, and it ran great on 91 octane and 40-1 maxima 927 oil. I pulled the jug off and inspected it and cleand it up and installed fresh top end parts, but my piston showed regular 2-t wear like my cr-2fiddy. So this seems odd to me. I would never ruin a classic and run super moto or anything like it. Leave the dang thing stock, these 80's machines are going up in value dont ruin it. For petes sake! LOL, I loved that bike, it was a blast!