Possible head gasket leak?

I recently installed an Athena 290 cc kit on my 08 yz250f, took it out for its first real ride yesterday since the install. Everything was going great and then about an hour or so in at about mid rev it sputters a bit. It runs smoothly at low and high rev but in the mid rev range it loses power for a split second then will be fine again then do it again. There is no sign of leaks from gasket itself towards the exterior of the bike but I didn't bother flushing the oil after the install just topped it back off and figured after I broke it in I would dump the oil. This being said the oil is milky but there is no real way of telling whether it is from coolant leaking into it during the install or after. If my coolant level is lower it isn't much lower than after the install. I am just wondering if I should be replacing the gasket or if my problem is elsewhere

You should've changed the oil before starting the bike. Coolant displaces oil in your bearings and is a horrible lubricant. Is the exhaust smoke white? Experiment with different clip positions on your needle in the carburator to get rid of the hesitation. Did you make any jetting changes after installing the Athena?

I know I really should have changed the oil, wasn't even a cost thing just pure laziness and now I am paying for it. No smoke at all from the exhaust idles great, not the easiest starter but never has been. As far as jetting goes went from a 178s main, 40 pilot, #40 leak to a 178s main, 48 pilot, #40 leak, as well as went from the stock needle 5th clip from the top to the JD red needle 5th from the top. But the hesitation isn't a sign of a failed gasket? just a jetting problem? I think all I have read about head gaskets failing on these big bore kits has gotten me a little paranoid that every little hiccup is my gasket failing. Should also add that I added hotcams while I was installing the big bore.

I doubled check the coolant level and it is topped right up above the fins, I am not sure how fast it would leak but I rode for a good 3+ hours and the level doesn't seem to have changed at all.

Thanks a ton for the help Swede you always seem to give great advice no matter how stupid the question.

Edited by Frecky B

Oh I wouldn't say always good advice. But thanks!! Change the oil and keep an eye on it. Some auto parts stores sell a test kit that detects hydrocarbons coming out of your radiator fill but they are kind of expensive especially if only testing one vehicle. A lot of guys had to go leaner on their jetting after installing a big bore kit. It's one of those things that you will have to make changes, ride and compare, make more changes and ride again.

48 PJ??? Why that big of change?

48 PJ??? Why that big of change?

Not sure that is just what the people at JDjetting recommended with the changes I made. Any recommendations? still not too sure how things react when changing jets

Not sure that is just what the people at JDjetting recommended with the changes I made. Any recommendations? still not too sure how things react when changing jets

Bike will either run better or worse, you've got to be the dyno.

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