CR 500 jetting issue

Sorry guys...I can hear you groan..."Not another jetting issue".

I've looked all over the past threads and can't find what I'm after so here goes.

1983 CR 500 racing MX over 50 class. Run 40:1 mix. All races at sea level at max temp 25 Deg C.
Carb is new VM40.
Jetting: Main jet 350
Needle: 7F7
Slide: 2.5
Pilot: 35

The problem. When I'm on a long straight full bore (well as much as my rear end can stand) on three occasions now the engine has stopped and the rear wheel has locked solid as if the engine has seized. I can then re-start and off she goes.

It did to me again yesterday, which is a bit frightening when in top and full bore. It smelled very hot when I got back to the pits. I checked the float level and moved the tangs from 15mm to 18mm and went up to a 360 main jet. I did the next race and she ran sweet but track conditions probably meant not as fast.

Outside of this little issue the carb jetting is spot on.

I was going to go up to a 380 Main jet and try it.

Any one else had similar experience or advice welcomed.

Cheers Steve

why not go richer on the main until it four strokes a bit then back off a size?

The mere fact of back wheel repeatedly locking up at w.o.t. in top gear is of not so minor concern. They do start up and run without fuss after heat seize.
The 500s are prone to airleaks and fuel supply starvation and untill those are sorted you can jet all you want and never get it right.

I believe the team bikes ran synthetic oil at 40 : 1 .
Everything part from the tm400 gets that in my shed .
Is your fuel tap fully clear . And buy a bigger fuel hose and bigger fuEl filter . I had that problem on my 500 . If I held it open through 4 gears it would die in 4th . Get the next size uo hose that you don't have to stretch onto the spigots

"This." I am very stubborn about "this." Without "this" you are just chasing your tail. And, once you do "this," you might be surprised just how much main just an old air-cooled big bore wants when held at WOT.

Of course, all of the other suggestions to make sure the engine is mechanically correct are spot on. Then, make sure you do "this"......

Start here first. Keep going bigger on the main until it blubbers before you do anything.

or you could switch to 20:1 and watch it do virtually nothing to your jetting! buy twice as much oil and it wont save an engine if it isnt jetted or built right.

Good to see this posted. 40/50/60:1 is fine. It's the jetting. Hi

Just to clear up a couple of things.

I use Maxima 927 with 97 Octane pump fuel. The manifolds and reeds have been checked. I forgot to add that the bike does have a reed force reed block. Back in the day we experimented with Bel Ray synthetic oil up to 80:1 mix and didn't seize up...admittedly we were using leaded fuel then.

I tried the stock set up that the tech bulletin states which is:

Main Jet: 360
Pilot: 45
Slide: 2.0
Needle: 7DH3

With this set up it ran like a bag of spanners and smoked badly. With my set up it is very crisp with a hint of blue smoke, so I'm sure I'm not too far away.

I am going to try a 380 Main jet and just keep going up in size as necessary I think.

Thanks All.

This engine is 4hrs since rebuild. Used for asphalt circuit trackdays and flat tracking. Pressure tested for 10mins at 15psi from where carb goes into manifold to exhaust port blanked off.
Fuel supply increased through carb as well.
VM 40
Pilot 35
Needle 7dh3 middle groove.
Emulsion tube AA5
Main 420 and it can go bigger.
Run at 4000ft altitude. It idles in the line-up for 15 mins and then accelerates through all 6 gears without a cough or splutter all the way to 7500rpm where it can sustain.
Fuel is 95 octane and full synthetic at 25:1

I've found the reason....it's seized....

Classic 4 point seize. Lots of oil on the piston and ring, so assume it was just too tight a clearance?

My Wossner in the 390 should be .08mm clearance. I go to .1mm and it's all good. What ignition are you using Steve, and how far BTDC ?

Hello Grouty,

I bored it larger as Andy advised. I run it bang on what the book says for the motoplat that's on it. I'm going to eke out a bit more on the bore me thinks. There was good lube, and the piston crown and plug were perfect. I just think running it full bore up the hill at Farleigh lugging my fat arse was too much for it.. The bore isn't scored, so hopefully it will clean up with a hone and I can put a new piston of the same size in

Maybe worth a tiny bit of retard so a little more heat goes out the exhaust ?

I'll recheck the timing at rebuild and make a decision then mate....got to get it back together for Farleigh or I'll have to break out the silver streak

Man, sorry to see that. Make sure it's a good 0.1mm next time around, start with a 430 main at sea level, and run two tanks of gas before you start "main jet testing." Something like: 35-40 pilot, AA5 (BB0 for desert/sea level), 2.5, 7F7, 430.

Bill, hard to say what happened to yours not knowing the other specifics.

Do you guys all check the ring gaps, too? I've seen them too tight. I don't think that's what happened to Bengt cuz, as he says, it's a 4-point.

Ran my 1978 390cr after a ray snoad [legendary hampshire tuner even had the maico factory asking for advice on noyce's maicos!] first over rebore and it nipped up on the main straight at matchams on the hottest day of the year in 1980 ... he honed it and all good after that ...just bored it a tad tight . Steve i always ran 25.1 with castrol and jetted to suit ... not for the most power but for most engine life and reliabilty !

Going to have to look at my notes for my 84 jetting ... that had a 40mm .... as you know steve the streak had the 44mm .

I have heard that also, but I have never had any problem with Wossner pistons.