gx200 rpm loss after a few seconds

[BigDuke6]

governor is removed, 6 degree key installed, valve lash is .001 intake, .002 exhaust. tried 2 different, new carbs, 3 different spark plugs. plug color is good. This is a new motor, ran 5 cycles, 10 mins on, 20 mins off then installed on cart. We ran 10 laps, let cool 5 times. It worked fine. Now on cart it goes great for 100 feet then loses rpm. removed it from cart, runs wide open for a few seconds then can hear the rpms drop, allot. Any ideas what to check next?

 

[Bbqjoe]

Fuel delivery problem is my first guess.

 

[BigDuke6]

flow from tank is good, and from bowl. There is no sputter, back fire or skip, it just decellerats. Tried 2 carbs 2 as it was my first thought.

 

[Bbqjoe]

Fuel problem is my second guess.

 

[mckutzy]

A guess, since you changed carbs a few times.
Insulator or gasket damaged...

 

[Bbqjoe]

Insulator or gasket damaged...

(Fuel problem)


Of course we don't know if the engine comes to a stop yet.

 

[itsid]

magneto ?

remove the magneto from the engine and inspect it closely for cracks as well as the spark plug wire and boot.

reinstall, reset flywheel distance to specs, try again (shroud still removed)
observing the magneto closely, check the spark plug wire for hotspots
(that spot marks where the core might be broken)
if it is evenly warmed up and not hot, check carb for air leakages.

if it got really hot though or unevenly hot maybe even bulges slightly, replace it;
chances are it's delaminating or leaking and heating up not able to keep up with the rpms for too long.

'sid

 

[Bbqjoe]

magneto ?

remove the magneto from the engine and inspect it closely for cracks as well as the spark plug wire and boot.

reinstall, reset flywheel distance to specs, try again (shroud still removed)
observing the magneto closely, check the spark plug wire for hotspots
(that spot marks where the core might be broken)
if it is evenly warmed up and not hot, check carb for air leakages.

if it got really hot though or unevenly hot maybe even bulges slightly, replace it;
chances are it's delaminating or leaking and heating up not able to keep up with the rpms for too long.

'sid

If the magneto or the plug wire was intermittent, don't you think there'd be some backfiring, if not some sputtering?

 

[Bubba_Z]

100' under load....or a few seconds free running and loss of power/RPM after break in. .001-.002 sounds tight. are you sure after heating the motor, the valves are not closing all the way?...anyway, call Jody at ARC, those guys are masters of WTF.... I have never seen any motor builders that are willing to help like ARC. There customer support is second to none. And if it is happening to you...it has happened to them. It takes a day or two for them to get back to you but....once on the phone you get the no bull **** answer. I agree with sid and Bbq, look at the areas they are talking about....... but you are looking for a ghost in the machine, My suggestion is take it to guys that do this every day..........

 

[itsid]

If the magneto or the plug wire was intermittent, don't you think there'd be some backfiring, if not some sputtering?

Not at all..
misfire is a timing issue,
for that you need a spark while a valve is open.
and while a defective coil could cause a spark delay (rather a prematurely collapsing field) and as such cause backfire theoretically..
it has to fail in a very specific way to do so.

What I have in mind is the actual coil being just barely cracked, so when cold it's acting normal and working fine,
and as soon as it reached the "critical temperature" the crack opens up, creates a short with an adjacent winding (making it a much smaller coil effectively)
the coil no longer fully charges and the weaker field collapses into a much weaker spark
(maybe even skipping every other rotation or so);
causing the engine to loose power.
basically the temperature controlled mechanical version of a rev limiting coil

And yes, that scenario is nearly impossible to reproduce if you'd try;

the most likely coil failure is it just no longer works at all.

But since I've seen a lot of temperature failures in electrical connections lately (generally they belong to electronics rather)
I wouldn't want to rule them out any longer.
Especially since I had a somehow comparable failing coil in my 914 once..
it failed to spark on one cylinder when getting hot and one cylinder only.
changed plugs, wires, boots, distributor cap and finger first.. no difference. got a new coil.. and it's been fixed.
(haven't seen such thing before or since)

Anyways, long story short...

it's a thermal issue of sorts, either fuel or compression or spark,
you named one, I added the second
and now we need someone kicking in asking to check the cylinder head gasket and bolts to complete the list

'sid

 

[bob58o]

Did you check the cylinder head gasket and head bolts? If not fuel or spark, it could be a compression issue.

LOL

 

[itsid]

thanks bob!
task complete.

Duke, now you know all the things you should look into,
start with whatever you feel most comfortable with and work your way through.
One of these will at the very least point you towards the correct solutin for your issue.

'sid

 

[BigDuke6]

Thanks as I have a few spare motors, ima swap magnetos, after testing and comparing resistances. May as well swap cams and fly wheel too. I pulled it out tonight and will start checking tomorrow. I figured magneto to Sid, as unlikely as it seams. Could that damage spark plugs?

 

[KartFab]

Maybe emptying bowl faster than fuel can fill it? Vacuum in tank? If engine is on an angle, shouldn't you get a pulse pump and an external tank?

 

[BigDuke6]

Magneto was bad, swapped to a different one and.... same issue. Drained oil to remove side plate and there is metal, lots of it. Crank had obvious heat build up, purple where connecting rod meets it on outer edge of lobe. Cold the rord has maybe a 1/16" play up and down, left and right in any position. Zero play, output shaft to fly wheel. I figure a bad crank casting ate the connecting rod. The heat may have damaged coil. I hate racing.

 

[KartFab]

Was your connecting rod stock or billet?

 

[BigDuke6]

stock, per our club's rules.

 

[KartFab]

Sorry to hear that. You should talk to your club and tell them that a stock connecting rod has poor oiling capabilities at high rpm versus the billet, and a $60 billet rod that is stock length provides absolutely no performance benefit. It only adds safety and longevity to the engine so it wont wear down and throw the rod.