Four corner seizure

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PBR Allstar

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Okay, on my second 4 corner seizure in as many races, I'm pretty sure they were both due to a lean condition and me being too impatient with jetting and making too big of swings with my jetting.

Both happened after 3 laps of running with the engine (Taft/cr250) at running temp of about 130*.

The carb is a PWK, first stick happened on a 172 main and the second on a 185 main. Fuel is pump 91 mixed with redline 2s oil at 6.5oz to the gallon. I'll post up some pics of the dome and the piston tonight, but Im wondering if there is anything else I should be looking at in regards to this type of engine failure?
 
Track is a 1/5 mile.

Both times the piston stuck about 3/4 of the way down the straight away, full throttle, Installed a new pump & lines after the first one, as well as checking the float level.

This last time out the session before it stuck (qualifying) I had a 200 main jet in it and it would not run at all under full throttle, it would bog completely and not even sputter, let off and it would run fine on the low speed circuit, stand on it again and it would just have the BWWAAAAAAAAA air sucking sound. (I always thought this was a lean condition, and an overly rich condition would be more of a gargling as RPM increased)
 
is your tank vented well? If its not too late see how much fuel is in the float bowl. That would either lead to or rule out a fuel delivery problem. Has the engine ran before with this carb and fuel system?
 
Yes, tank is vented with the same size as the feed line. First thing I checked both times was the fuel level in the carb and it was perfect so I think the delivery system is okay. It's a first for just about everything on the engine except the ignition system. The carb came from Gleason.
 
Piston top (underside is clean, no ash).
 

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When we was developing my dual carbs for the 250, one weekend we had really good air, found the low side was lean, not augmenting the hi side enough., it was pig rich on top. but still stuck. we had to open up the low side metering holes..
 
I think I have the low speed pretty good. The air here at sea level in California is really good, I keep notes on density, but to be honest it really doesn't fluctuate much.

I did seal everything up and pressure tested the case and crank seals. All was okay, held 7psi for ten minutes before I released it.
 
Check manifold and carb gaskets, make sure it not leaking there for some reason. I assume its a slide carb check carb out real good. Sounds like it got aa air leak some where. Getting any poping, backfire out of it on deceleration before it stuck?
 
The pictures aren't the best and my eyes are not getting better, but I see detonation on that piston head. I think. See the light spots around the edge, kind of like a really fine sandblasting, that's detonation, if I'm seeing it right, which I can't guarantee.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley)
 
PBR Allstar said:
No, piston was in correctly. I don't think it would run the other way around.
Click to expand...

Should run no problem but you most likely will snag a ring because the ring gap will most likely end up aligning with a port window. Having said that I was able to run a TT port in a KT without the ring catching. Was a fairly narrow port though.
 
Here's a topic that I'd like to throw out there, Talking with some of our 250 engine powered karts the issue of the remote fuel pump has been the center of attention. The theory being made is the length of the pulse tube from the cylinder to the pump, a couple drivers had issues with fuel starvation at high rpm, The cure they found was to shorten the pulse line to under 2". The thought being that rpms over 5000 the pulse becomes a constant low pressure zone with too long a tube length, causing the pump to stop pulsing. I'm not convinced that that happens, how ever the guys that have done the change have been pulsing off the transfer port and not in the intake track. Is it possible the wave reflection from the pipe is causing such a low pressure effect that the transfers become locked in a low pressure condition at high rpm?

I believe the pump should be mounted in a way that the pump sits as high or higher than the fuel bowl. preventing oil from puddling in the pump at a lower level.
 
arc100 said:
Should run no problem but you most likely will snag a ring because the ring gap will most likely end up aligning with a port window. Having said that I was able to run a TT port in a KT without the ring catching. Was a fairly narrow port though.
Click to expand...

Well, yeah, it would probably "run" in that sense of the word. But yeah, the rings would snag and then the running would cease.
 
I'm with Tim on this one, I'm not convinced in this. I have heard it from multiple people though so were gonna give it a shot. Thing is, I ran a trannyless 250 for a season with the pump mounted a foot away with no issues. I've gone on micro racing . com and asked about it and people can't seem to give me a good engineering based answer as to why the pulse line goes to a steady state of vacuum?

The other big thing were going to try is adding an inch of flex (from 13" - 14"). This makes the most sense to me at the moment, even though the seize pattern looks like a cold seize, the engine temp is good and the piston clearance is good, so I have the suspicion, (and I think Tim is with me on this) that were not moving enough heat into the pipe.
 
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