Hi fellow trials guys and girls.
I picked up this bike last fall and I've tried nearly everything I can think of to cure this. Here's what the bike has done since the day I brought it home. When you hold the throttle steady at part throttle, the bike leans out, revs out of control and detonates horribly. For example, if I try to hold a steady speed at part throttle in 3rd or 4th gear it will do it. Wheelies are nearly impossible because of this problem.
The engine has great primary compression and will start on first or second kick.
Here's a list of what I've tried.
several spark plug styles and heat ranges.
Jetting. I've tried jet sizes and needle positions till the engine would choke on all the fuel. Still does it.
Fuel,, yes, even with race fuel it leans out at part throttle and deto's.
reeds,, brand new carbon fiber. still does it.
crank seals.. you guessed it, brand spankin new.
muffler, cut apart and repacked.. same thing.
I've had the cylinder and piston off. All well within tolerance and cleaned.
screen in delorto, clean.
screen in tank, clean.
fuel tank is vented properly.
float level raised as far as possible and still leans out at part throttle.
spark timing. yup, monkeyed with that too.
Slide cutaway, temporarily altered to increase velocity at low and mid range.
The squish band is .040 at its narrowest point. Thinking of narrowing that to .030.
:banghead:
I'm stuck with this bike for a while so I hope someone out there has an answer.

A pluged pilot jet will make a mess out of mid range. My friends Montesa was doing the same thing. I pulled the pilot jet and sure enough it was blocked. Flushed it out and all was well. Sounds like you might have a bigger demon to find. Have you posted this on RYP site? Ryan has allways been very prompt with replies.
I would pull the pilot jet one more time just to make sure you have no blockage.

On the carburetor I've went as far as removing the gallery plugs to insure all circuits are open and clean. The jets were checked with a plug gauge to unsure the orifice is the proper size. I've tried several main an pilot jet sizes. As well as needle jet sizes. Needle taper has also been experimented with. Being a seasoned machinist, I'm able to create most of these very small parts.
I haven't been to RYP. Probably because of a $300 piston. I do realize that business has to make a profit. But I feel there's a point where you're being held over a barrel, because you have no other choice.

I had a problem similar to this on an XR250 one time. It would only run with the choke on. I ultimately tracked it down to a bad o-ring between the intake manifold and cylinder head; a vacuum leak. Your problem seems to be in the transition zone between pilot and main system so this could be it.

A good way to check for a vacuum leak is get some starting fluid and squirt it around the joints downstream of the carb. If the bike suddenly runs better for a second, it means your sucking in burnable fuel instead of air. I fixed the XR by replacing the o-ring with cork gasket material and never had another problem.

An over-advanced ignition-spark will also cause detonation and loss of power. I know the old vacuum and mechanical advance systems could suffer from this at times and I would assume that electronic systems can too.

-John

To post the question on the RYP forum costs nothing but your time.
I know what you mean about parts, but what ya gonna do.
You should get a prompt reply, the last thing Ryan would want is a post of a bike that runs poorly.
Put everything back to stock setting and think simple.
Did the bike ever run proper? Did the problem get worse slowly or just happen?

John, I've done the carb cleaner test. And a leak down test.

Boom Boom, Yes the bike has done this since the day I got it home. The previous owner didn't mention any problems with the engine. I got a smoken deal on the bike so I'm not bothering him about how it runs.

its the black box

Don't overlook the vent to the fuel bowl on the carb. I don't know about your specific bike but they are usually vented through a piece of tubing that hangs donw and frequently gets either pinched or clogged. I had a spider put a web in one once and it drove me nuts. I will never forget to check the small things first.

FESTER said:

I agree. You've done everything you can do to the carb. The black box controls ignition timing. Sounds like it is way too advanced. Are there any timing marks on a Sherco that can be used with a timing light?

Timing light?!? You know how long it's been since I had to use a timing light? Man, I must've been driving crap for about 3/4 of my adult life! And condensers, and points, and that worthless vacuum advance that never really worked quite right. Not to mention EGR valves. Thank god for computers.

-John

Carburetor vents are both open.

It does lean out fairly quickly. For example, if I ride a wheelie over a small log and try to continue the wheelie, it'll be leaning out by the time I've cleared the log.

Black box, hmmm. It does start very nice and idles super. I have retarded the timing to the point where it's a turd but I haven't watched it with a timing light. I'll do that tonight.

John, No timing marks. Timing is set by measuring piston distance from TDC. And then there's holes in both the flywheel and stator plate in which you line up with a pin.

While you're here, what do other Sherco owners use to hide the huge lump on their forehead? Ok,, I'm just being funny. A fellow I ride with has an 04 Sherco 2.9. It runs fine and there's no lump on his forehead.

I know you say the float is set proper, but the wheelie over the log comment just sounds like running our of fuel. Let your bike idle, pick up the front end (front wheel up on the pick up bed), let idle. If the idle picks up then engine dies, your float is sticking and not opening or your tangs are not letting it open proper. Keep thinking simple.
If your bud has the same carb, swap carbs, see what that does.
Start off by setting everything you changed back to spec.

His bike is quite nice and has never had a wrench on it. I'd feel pretty bad if something went wrong. Plus he's quite a ways away.

I have rode the front wheel up a pole and locked it at quite a steep angle. It idled normal.

I did buy another carburetor. Not a Delorto but a two cycle carb from a similar cc engine. The butt head keeps giving me excuses why he hasn't shipped it yet. So that's starting to tick me off. I've checked and he's a legit business. But apparently once he has your money he'll ship your stuff at his leisure.

I'm going to mark TDC on the flywheel and watch it with a timing light.

I shall return.:thumbsup:

shercos are great bikes,let me know if its the box,just a hunch,swap it with your buddies bike,you shouldnt of had to do all that stuff.whos this dealer you speak of grasshoppa?

Ok, I grabbed the degree wheel and marked the flywheel with five degree increments. The bike idled at 15 degree's BTDC and when revved the timing advanced about 3 degree's. The timing stayed rock steady.

Fester, I'm going to give him one more day. If it doesn't show up, I'll call him. If I get another lame excuse, I'll unleash the hounds. He's not a Sherco or trials bike dealer or poop would have hit the fan already.

I did notice that it tends to get worse as the engine warms up. With an infra red temp gun I get a reading on the head and cylinder of 160 degree's. I don't think that's over heating. And when I had the piston out there were no signs of heat under the dome. Piston wash looked normal for a low rpm engine. The rings were sealing perfectly with no carbon buildup on the sides of the piston.

I did run the engine without the radiator cap just to watch for flow. That little pump really moves the coolant.

I'm stumped but I aint giving up.

Thanks for your suggestions. :thumbsup:

Just asking but, what kind of fuel and fuel-oil mix are you running? The GasGas bikes knock and run like crap on pump gas. I always run 100 octane aviation fuel in mine mixed at 80:1. I've heard that Shercos aren't as sensitive to fuel as the GasGas bikes but it is a thought.

What's the condition of the air filter?

Also, does it behave the same under load as it does unloaded? I've seen cases where (usually cars) idle fine at various speeds but give out under load. Manifold vacuum can help you deduce problems like this.

John

15 degrees advance sounds high to me but I've never had the need to see what a proper running 2-stroke Trials bike ran at. Most other engines run at about 5 degrees advance at idle and increase to somewhere near 30 degrees at full advance.

When I added the Kehin Carb Kit to my 04 300 Pro, I made a mistake when installing and in the process of discovering my mistake, I messed with the float level. My bike knocked like the description in this thread. Elevated Revs with no load at less than 1/3 throttle made a detonation sounding racket that was unnerving.

Changes in jetting up and down one size as well as adjusting slide needle elevation seemed to have little to no affect.

After hearing how sensitive the Kehins were with float levels, I made a series of adjustments to the float level toward rich or a higher level of fuel in the float bowl. Now it's much better with only a little ping now and then when snapping the throttle closed.

I too mix 3 galons of 100/130 octane low lead AvGas to 2 gallons of unleaded premium. Straight 100/130 AvGas burns too slow. And the other octane rating AvGas is too low at 80/87. Most General Aviation Airports seem to carry only the 100/130 octane fuel now. Don't use the 80/87 octane AvGas!

Keep in mind that the super high octane level prevents the fuel from igniting without a spark and assures that the fuel will "burn" and not explode. This fuel is meant for engines that almost NEVER see any revs above 2,900 RPMs. It also has some exotic additives and properties that help stabilize the fuel for long periods of storage and other additives that retard the growth of nasties in the fuel. And, the fuel has properties that make it more suitable at higher elevations and lower temperatures... something to do about vapor pressure... all qualities that are important for aircraft, but NOT for ground based high rev engines. So use AvGas carefully. I have heard that in high revving engines, AvGas is still burning when leaving the exhaust ports or exhaust valves and can damage the valves with prolonged use at high revs.

I am still waiting to hear from a fuel chemist or scientist the exact description of what is in the fuel and why and how it can affect jetting and engine performance in NON-Aircraft use. We had one such person on Thumper Talk, but he only gave us a few reasons to be concerned and never came back for a more detailed reason to use or not use AvGas.

And if anyone knows the full story about AvGas properties or knows someone that does know, I would sure like to hear their take on using it in our Trials Bikes as opposed to other high compression engines. My guess is that the lower Revs we run are compatible with the intended use of AvGas.

CRM114,
I have blended race fuel with pump premium. When an engine deto's on that mixture there's problems. I haven't tried av gas. My premix is 80:1. I do clean and lightly oil the air filter often. By lightly I mean the foam filter oil gets squeezed into the filter to coat all the pores without excess.

It leans out both under load and freewheeling under part throttle.

Now you've got me trying to think of a way to measure crank case pulse. Imagine a pair of sneakers and a hand full of nuts and bolts in a clothes dryer.

2ply,
my timing light testing was more to see if the timing got way out of hand then to determine the actual timing. 15 degrees BTDC at idle doesn't seem out of hand to me. I do know that if I retard the spark the engine is a total pooch and will still lean out and deto.

--
The carburetor will be here tomorrow. I have my doubts it'll change the engines behavior.

take a peek at your spark arrestor screen and see if its clogged?

FESTER said:

Hi Fester, There's no spark arrestor. I've cut open and repacked the muffler. There is perforated pipe that runs the length of the muffler. There's steel wool packing in the first half and fiber packing in the second half. The steel wool was in tacked but the fiber needed packing real bad.

The test carburetor arrived. And of course a part was missing.

OK,, this carburetor yielded the same results. Identical to the Delorto.
I was thinking maybe too much back pressure so I ran it without the muffler. And guess what, it's spunky as hell but still leans out and deto's. And man is that baby LOUD.
Here's a recap,
several styles and heat range's of plugs
new main bearings and seals
check for erratic spark
carb vents open
both fuel screens clean
tried jetting, needle and needle jet till I was blue in the face.
tried float settings till it was raised as far as it could go.
temporarily changed cut away to increase low and midrange venturi velocity.
tried another carburetor.
cut appart and repacked muffler.
head pipe scrubbed clean as a whistle.
installed carbon fiber reeds.
leak down check.
I'm probably forgetting some.


remember, the engine has run like this since I got it. All of the above has been done to try eliminate the engines ill manors.