Two Cr500's Help!

Hi
I had the same issue on a 1983 500XC.
Did all of the above, breather, gas cap, needle & seat larger, drilled the petcock.
Finally fitted a Lectron carb.
All the time I have had an MZB - wondering now....

OK. Life got in the way and I haven't been able to do anything with this project until recently.

I went over the Motoplat carefully and found and fixed a few minor issues but with no improvement in running. I checked fuel flow by pulling the carb away from the inlet and removing the drain plug, then timing how long it took a gallon of gas to flow through. I figure that this will test the entire fuel supply system. It worked out to 13.23 gallons per hour of flow. This would drain the 2.7 gallon tank in about 12 minutes and ought to be enough to get a loaded Peterbuilt down the highway without too much trouble. Did a leak down test, started at 5 1/2 PSI and in 30 minutes it was still a little above 3 PSI, so I think the seals are ok.

So, I bit the bullet and ordered the Electrex stk018. Showed up in just a few days and is a really nicely done piece of kit, it went on without any problems. I installed the whole thing including the kill button that came with the kit so there is no part of the old ignition left on the bike, timed it at 2.4mm and put a new B8ES in the hole. Got it running and warmed up, ran it up through the gears and...…..no change, still falls on its face at full throttle. S.O.B!

What do you set the plug gap too?

.022"

That seems low enough, i have had spark blow out, but was above that by .018.
Is it any gear and just a RPM thing, or does it take a long gear to fall flat?

It will generally not happen in 1st or 2nd, but into third it will just stop running at high throttle, high RPM. Same in 4th, which seems to me like it is a lack of sufficient fuel supply issue. If I hold the throttle wide open in third, it will die, not sputter but die just as if the kill switch was being pushed in, then run briefly and die again. Backing the throttle off to half or so and it will run fine. I've used up my jet supply with 390 being the biggest I currently have. I've ordered some larger ones up to 460 and will see if that makes any difference.

Wasn't a 500CR but I've dealt with two strokes that didn't have enough fuel supply at full power. It's surprising how much fuel an engine needs.

With gas at ~6lb per gallon and your measured flow rate of ~13.23 gallons per hour the max flow you measured is (6 x 13.23) or ~79 lb of fuel per hour.

I have no idea how much HP a 500CR makes... if we say it has 60 HP and if we say it needs to max out to your measured fuel flow to make that power that would make the BSFC 79/60= 1.32 lb per hp-hr

A well tuned high performance two stroke usually has a BSFC in the 0.50 to 0.60 lb/hp-hr range at peak output... So you're well above the required fuel flow... over twice the flow required.

Curious if you had the gas cap installed and tight when running the fuel flow test? The cap venting can significantly reduce the flow.... In my experience the cap vent is often the problem. That's all I can think of.

Strange how it dies right off, I know the feeling of a lean condition. After going W/O through a few gears with my air cooled 430, it was draining the float bowl faster than it could refill. Never died out, just power was falling off so i let off.. Bad venting on my hose on the cap, Ebay deal on the end my hose.

Hows the plug look after a wide open pull? Getting any signs of fueling there, rich or lean?

Check the choke circuit in the carb. I had a similar problem with my little Indian. The choke was bleeding air and leaning it out. Mine was a twist lock style and I added an o ring to the top to stop the leak. I looked at the ignition and carb for a year before I found it. Changed the ignition 4 times. Cleaned the Delorto, 1000 times. Hope you find it. It can make you Crazy. Chris

Pulled the engine apart, decided to start from zero and see if I can eliminate any internal issues. Bore is stock and shows very little wear on the cylinder or piston. Anybody know where I can find a stock, standard bore ring? End gap is pretty big.

OK, so I finally decided that this must be what it seems to be, fuel starvation. As I said in a previous post, after opening up the petcock and carb passages, this thing would flow a lot of gas. But it turns out, not enough. I took a very wise friends advice and put a Pingle "power flow" petcock on it. This requires some tricky machining to make an adaptor from the 1/4" pipe on the Pingle to the 18mm reverse thread of the stock coupler. It will now empty the full tank through the carb in about 6 minutes. All the other things I did had no effect on the dying issue. Changing the petcock solved it and now it will run flat out for as long as I am brave enough to hold the throttle open.

My question now is, why isn't this something that other owners of these big bores have run across? I have two with the exact same issue. I improved the fuel flow through the stock supply system to the point that it is far better than original on both of these bikes and they still suffered from fuel starvation. It wasn't until I more than doubled the flow that the problem was solved. Thanks to everybody that offered advice.

Sounds like you got a big set! I ride as fast as my life insurance will allow. I am so slow. Good job on figuring it out.

Do you have picture(s) of the petcock that was causing the problem. Over the years Husky used two different ones. The early petcocks were restrictive. Glad to hear you got the problem solved!

Marty

I did post that my problem was with my petcock as well, however, re-reading your original post yours is an '83 with a metal tank I assume. I have an 84 with the plastic tank, but the petcock was still the issue. Once replaced with a new acerbic petcock, the issue went away. It's amazing how thirsty the 40mm mikuni is! Glad you solved your problem

I actually have an '83 and an '84, both had the same symptoms. The 83 had the older Karcoma petcock while the 84 has the plastic tank with a different petcock, don't know the make for sure. When I get a chance I'll post pics of the components and also the increase in passage size of the Karcoma after drilling the passages.

You need a MUCH larger main jet. The 390 was way too small.

My '84 came from a sea level location with a 370 main, the 83 came from 5000' and had a 330. Right now the '83 runs about the same with a 370 or 380 at 800' elevation. I should have some time to play with it in the next couple of weeks and I plan to see how big I can go before it starts to blubber. Electrex sent my ignition back with a new control unit and it runs, and seems to start, well now with that ignition installed and timed at 2.0mm BTDC. It will still 'flame out' from time to time, mostly before it's thoroughly warm, so it looks like my problem isn't totally solved.

ive just seized (lightly) my 84 400 after a series of high speed drags and it felt like it was running out of fuel as well. i have a new ktm petcock so i might swap it over. i was told it could have been the plug but im not so sure. its an 8. its a newish top end thats done a fair bit of work so i was confident it would be fine for hard top gear running. grabbed around the transfers. a light hone and a new slug and im away