picko990

why would an engine change tune significantly mid-race? I'm referring to becoming richer, not leaner. the motor is not new so it was not the old break-in completion. it has about 8 litres on it. there are no obvious leak, it held the tune. i had to pit a couple of times to get it leaned out. probably went about 4 hours on the clock. it became so rich i could barely do the jumps and it is a small track!

it seems to still have a stable tuned once leaned out again but i just thought it very strange!

any thoughts?

ncdirtracer

iTrader: (4)

if your hsn is easy to turn you might check the O-rings in it. could be turning from vibrations

wingracer

iTrader: (24)

If it was just a one time thing, it's probably just one of those things that happens sometimes. A bit of trash in the carb or tank, something plugging the pressure nipple, cap not fully seated, who knows. If it does it all the time, most likely a tank issue.

dan_vector

iTrader: (27)

Is it cold where you are? Sometimes the motors take a long time to warm up to operating temp and once this happens they tend to richen up a little bit. Once leaned out you say its ok after that. What do you do when you bring the car in at the end of the run? Do you richen it back up prior to the next run or does it start off lean then get better? Obviously you don't lean it out 4 hrs each run without first richening? Could also be an air leak on the carb but thats unlikely given that the tune holds after being leaned out. Just a bit more detail needed from you mate regarding what you are doing with the engine at either side of the run - also what engine is it and what plug are you running?

picko990

Thanks for the imput. I don't do anything between runs. I usually temp it when I bring it in. It is an arrow gtr with as works st3 plug, which is like the p3. It seem that it is random and it is not cold. I'm in the tropics! It happened during the final and the leaning by the pitman was not complete but it definitely improved the engine so not really sure where it is going yet. But it is weird. Now that I think about it it started after the first fuel stop and by the next fuel stop it got progressively richer. Then we started leaning for the last 10 minutes.

Maybe I'll try again And then open it up if it does it again.


Edit.... I will check the looseness of the needle.
Thanks

houston

iTrader: (96)

Sounds like your engine "dropped its nuts" so to speak , what I call it when the engine finally gets past its breaking in point , some engines it happens just under a gallon , some it takes up to 2 gallons and some it never happens sooooo thats my best guess

Is this a re occurring problem?

Hoese37

iTrader: (5)

How many minutes in did it change? It may have just fully heat soaked in. Happens here when we run outside in the cold.

patrickgarcia

I own a arrow engine and the needles are very easy to turn. A drop of loctite should help, if that's the problem. Another thing that crossed my mind... If the engine is getting rich on the middle of the race, did you check the air filter? Sometimes it's so dirty that the engine can't breathe.

Frank L

iTrader: (148)

Check the:
Tank
Front bearing
Carb boot
Pinch bolt
Back plate

I had this happen last race and I had a small tear in the carb boot.
Is the engine wet anywhere?

Jaz240

iTrader: (52)

Oh snap! The pinch bolt is not tuned

wingracer

iTrader: (24)

It needs to be re-tuned every time you replace the muffler bearings.