Burning Exhaust Valves

[Noneshere]

2005 LongBo Adventurer , 157QMJ Air Cooled Long Case Engine with a
Stock 57.4mm piston and carb

Burnt another brand new head up in less then 1200 miles.
Runs like a scalded gremlin for 600 miles then come the overheating fallowed by the overly rich low compression.

The kick start I could once stand up on at TDC now rotates with the grasp of my toes and the whoopie cushion like engine starts no longer just barley catch ignition anymore.

Pop the head off and sure enough it wont hold gas upside down on a table without leaking through the valve and draining out the exhaust port again.

Only the exhaust valve , and the valve lip is cratered badly again too.
lapping drops the valve face flush in the head before it smooths down far enough to seal again. Scared thats too thin and may possibly cause the valve head to break slap off in the engine next time.

It's definitely not a lack of valve clearance causing this.

Everything is still totally stock , ...except my muffler .
It's gutted , (12" section chopped out) nothing more then the aluminum sleeve with the ends "stealthly" rivited back on.

I see many post saying a #115 jet is a good application for engines running de-restricted intake and exhaust situations.

I am still connected to the factory forward mounted under seat air box and filter and I live at sea level altitude if that matters to the equation table.

What should I do to balance this mixture again?
Should I just go on to a bigger jet or put a factory muffler back on it?

[Deleted]

I would try something along the lines of a 112 jet, the upgrade is cheap enough, maybe it doesn't make a difference but for the expense I'd be willing to give it a shot if only for lack of better ideas.

[4getful2]

I agree, plus you may want to adjust the exhaust valve gap to .006"-.007", just to help get rid of some heat.

[qwertydude]

You need to rejet, changing out anything in the exhaust or intake means an automatic rejet. There is no one size fits all jet. You need to buy an assortment of jets and do plug chops to figure out the ideal jet size to use. Either that or if you're not willing to properly get the jetting correct, stick with stock everything.

Get the jetting correct and you won't burn valves up.

[rockynv]

As others said you are running too lean from gutting out the exhaust. You need to rejet or go back to oem.

[Fox]

You want a brown spark plug electrode with a black ring at the end of the threads when all is said and done. Black electrode=too much gas. White or light gray=not enough gas.



Have you dropped the needle clip on the carburetor yet?

[h3nry]

If its gutted there may not be enough back pressure, apparently these things need a little bit, same reason its bad to run without a muffler. I'm not a pro on the subject but just giving my thoughts

[Noneshere]

Well sorry for the slow response , I've since learned to lap a set of valves and run another 500miles so far but the low compression is creeping in again day by day. She still hits 60mph but she takes her time.

Good news is Ebay is finally on the way .
I should have enough to play with and build a second engine sitting in the toolbox with this delivery as well.

www1.picturepush.com/photo/a/12275819/img/12275819.jpg

[imnts2]

it really is simple. A lean mixture runs hot as the devil. A rich mizture run cool. Notice an engine putting out max power often puts out a bit of black exhaust smoke cause it is rich. For some reason, yur mixture is leaning out to much.

Cheers
lefty2

[Noneshere]

All I did...was gut the exhaust trying to figure out why I was running so rich with no carb adjustment responding.
So I thought, well maybe the exhaust is choked up and gutted it nicely internally with a reciprocating saw.
Come to find out ( 1 $80 top end rebiuld kit ) later it was low compression causing the rich diagnosis earlier.
Scoot rebuilt and runs like a top for 800-1000 miles , just a little noisier like a coffee can and popping when letting off the throttle.
Soon the kick start and laggy running tells of low compression happening again.
Then the you just get tired of over heat pull offs and split the engine again to find the exhaust valve tan white burnt everytime.
Lap the valves with some paste and a drill and she runs like a top again till they burn out ....or break.
I still got a factory carb hosed to a factory front mount air filter box, all I did was gut the exhaust.

The scoots plenty fast top end for me , I'd just like a little better sling-shot getting there.
I know can't do nothing without curing this lean burn to low compression problem first.

So with some advice here hopefully I can resolve this problem

[motts]

Do you have the plastic covers on the engine that are used to help with cooling?

[woowilly]

restriictive stock muffler causes more of the exhaust gases to be left in the cylinder than a free flowing exhaust. This lowers peak combustion pressure and temperature. A free flowing exhaust requires a slightly richer than ideal mixture to bring 'em back down. Starting with a EPA compliant mixture that's on the verge of too lean with everything stock, changing intake or exhaust to free flowing will put exhaust valve temperature over the limit. The power increase also means more heat is generated. Fix is to richen it up a bit. Normally mixture is set slightly richer than the peak power mixture setting. This sacrifices a percent or two of power in order to make much more drastic difference in temperature. And also reduces NOx emissions. ( on EPA compliant vehicles, the catalytic converter cleans up the unburned hydrocarbons from being slightly rich. ) Add water vapor to the NOx and you get nitric acid, that's good stuff for pitting valves and valve seats and heat accelerates the metal erostion rate. Less valve seating area due to pitting, means less heat transfer from valve to seat, which makes for a even hotter valve.