Grrr! High-speed miss - SOLVED!

[twsnagel]

d***, I thought I almost had my spree done. But in some unseasonably warm weather, I took it out.

It had a higher speed miss.

It starts, and idles beautifully, but as soon as I give it gas under load (up a hill) or as soon as it hits about 20mph, it dies.

As I am accellerating, it sort of goes up in "hitches". It's like it hits its stride for a couple of seconds, and accelerates well, then the tone of the engine goes down and it struggles. These are nuances, not major changes in tone but I think maybe they're the key to what's wrong.

I did a plug chop and it looked normal. I have cleaned the carb 3 times. (I know the old refrain on this board - and trust me, my carb is squeaky clean). I checked that my bystarter is working and properly sealed to the carb. My airbox is not modded. I have a strong spark.

I don't know whether it's dying for want of spark, want of gas, or want of air. I guess if it's starving or bogging, I could adjust the throttle slide needle.

Any ideas?

[Farfignugin]

Seems almost like it's running lean. I'm not too sure, but start there. Check fuel flow, or other related things.

[twsnagel]

Fuel flow is fine. If I suck on the vacuum line (carefully) a stream of fuel comes out of the fuel line.

Bought some new plugs today, hope that will work.

If they plug was faulty, it would fail at higher speeds, right?

[noiseguy]

Sounds like lean to me too. Check your main jet size, for presence / installation of air filter, and vacuum leaks. Spraying starter fluid around the manifold will find vacuum leaks (the engine will increase RPM if you find a spot.)

Only other thing is your plug cap. Make sure the plug cap is tight to the plug wire (they unscrew on bikes, unlike cars.) If it won't tighten, unscrew the cap, trim 1/4" off the wire, use a nail to open a small gap in the copper core, and screw it back together.

[hemlockz]

my theory: When it missed you blew a seal and are loosing compression, possibly also sucking in extra air causing it lean out once intake pressure builds up as you try and rev it up. Check the ring seal with a compression guage, but more than likely it is a crankcase seal that is leaking. In my case it was the reed gasket.

[twsnagel]

Will try a new plug tonight. I'll make sure the plug cap is good. I think it is, since I replaced the CDI/Plug wire unit in an effort to troubleshoot. No change.

My compression is slightly high, 120psi, so compression is not an issue.

Air box is tight, filter is slightly dirty, but doesn't warrant a cleaning yet.

If it's leaning out, I'm going to adjust the throttle needle to give a richer mixture.

[twsnagel]

Just read your post again, hemlockz.

I sure hope it's not the cylinder gasket, I really don't want to tear that down again.

I'll check the reed valve, I have a spare.

Also, I'll give the cylinder bolts a little tweak just to be sure.

[hemlockz]

My leak was between the underside of the reed and crankcase. The paper gasket was leaking on the side closest to the cylinder. Its because I reused the old paper gasket when I took off the reed for cleaning. You can only loose crankcase compression from the bottom reed gasket, or the cylinder gasket as long as your reeds are working.

[MySpree]

dont over tweak the bolts. these old bolts seem to have a tendancy to seem tight, but you can still turn them over fairly easily, and all there doing is stretching and getting ready to break. i had 2 top end bolts break, and they dont just spin of and break, what they do is they stay for about a minuet+ and then they shoot out like a bullet out of a gun, shot up and took out the lights in my garage. its best to jst buy new bolts if you have a leak there. i have also found great success with a product called seladore, a gasket sealant that just sprays out, sticks like crazy. so far its the only one i can find that seals things, and is impervious to gas. to make sure your leak is fixed, or to even see if you have a leak, put the engine at an idle, and spray the outside of the intake, were the intake meets the crankcase, and were the outside of the carb being careful not to get it in the carb through the air filter intake, if the engines RPM s rise, they you got your self a leak.

[Kikwear]

If it is a fuel problem, make sure your float is at the correct height.

For diagnostic purposes only, run the bike with the airbox completely removed. If the problem is amplified, you know it is an air issue and most likely is running lean.

[twsnagel]

It was the spark plug. Runs like a watch with a new one! Thanks everyone!

[noiseguy]

The easiest stuff is always the first things to check...

[twsnagel]

Funny thing is - I did check it! It must have had a cracked core or something because it looked great, and sparked strong when it was out of the head.