More . The interior cooling is from the air and fuel vapor passing into the chamber via the inlet manifold of course . Sixty years ago I learned this lesson on my Matchless twin after polishing the y manifold like chrome . Ran awful , bowl got really hot percolating the gas . Tumbled to the cause in ten seconds , a dumb kid . Went home and fashioned my first heat block out if an asbestos shingle , popped it in between the flanges and all was rosy . Dumb but not stupid . These heat blocks are readable available now . Sid .