Loss of power/head bolts loose (RPMS 12,000+)

Jared LeCuyer /

I believe my jetting is incorrect but that doesn't seem to be the big issue here. I rebuilt the top end of my moto Morini (ad50) engine with the 70cc bbk. When I was tightening the head bolts I snapped a head bolt off. I replaced the head bolt and is snapped again at a very low torque. (Breaking at the end with the nut). I played it safe on the recent head bolt and tightened it semi tight but definitely not to spec. When I accelerate at about 20-25mph the scoot suddenly gets very loud and looses power. The tachometer also jumps to maxed out when this happens. Could this be from a compression leak at higher RPMS? When I back off the throttle to a point it quiets down and gains power. Compression? Main jet? Any ideas would be helpful

Re: Loss of power/head bolts loose (RPMS 12,000+)

You seem to have a problem by allowing leaks .

Either install higher grade or heavier/thicker head bolts/studs .

Then torque the head down as it should be .

Re: Loss of power/head bolts loose (RPMS 12,000+)

look for a scooter forum

Re: Loss of power/head bolts loose (RPMS 12,000+)

Start with using quality studs and bolts and torque to spec

Re: Loss of power/head bolts loose (RPMS 12,000+)

Jared LeCuyer /

Got some new head bolts on the way, I will see how they go! Worried they will snap again. What's the thoughts on using a different style head bolt, one like a regular bolt to screw right in rather than with a nut and threads on the end? (edited)

Re: Loss of power/head bolts loose (RPMS 12,000+)

Pushrod Fifty /

Bad idea. Head studs, nut and bolts are designed to prevent this sort of thing, the studs are supposed to stretch upon torquing so that the force on the head stays the same as the engine heats up and metals expand. The engine case threads arent made for repeat bolt removal, the threads have a tighter clearance called percent of threads. Studs have a special rolled thread unlike a bolt. Maybe you can buy the real studs and nuts that the Chinese copied. Check Treats, they may have.

Re: Loss of power/head bolts loose (RPMS 12,000+)

Jared LeCuyer /

Appreciate the reply. I bought some new replica head studs from parts for scooters. Hopefully that does the trick. Problem last time was the new one I bought snapped easily so trying all four new is next here

Re: Loss of power/head bolts loose (RPMS 12,000+)

Pushrod Fifty /

You are torquing in gradual value steps in a cross pattern and a new gasket? A torque wrench known to be good? Matching the torque units of the wrench to the torque spec- in/lb or ft/lb. Torque with the nuts and studs dry, no oil on them, as that increases the torque by 30%. Make sure the head holes arent buggered up, causing the nut to tighten in the local area without stretching the stud.

Re: Loss of power/head bolts loose (RPMS 12,000+)

Jared LeCuyer /

Yes to all of those except I never checked for oil on them, possibly could've had some on my hands and got it on the studs. As well, what do you mean by buggered up holes? That sounds very likely given I got a new stud and it snapped in the same spot

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