centauro cylinder psi

[BRIAN]

I recently took my centauro to a shop to have the fi sorted out. The tech checked the bike over and came up with some issues to deal with. One thing he brought up was the big difference in cylinder psi. He said the right was 175 and the left 225. He suggested that the right could have a bent rod preveting a full pumpup or the left had carbon build up causing over pump.

What should the psi be and does the tech's explanation make sense?

[pete roper]

Throttle wide open one assumes?

175 sounds about right, maybe just a trigle low, depends on lots of things. Right is too high. Before you go ripping the head off i'd check the valve timing. My guess is that the cam on the high side is running advanced.

Pete

[BRIAN]

Thanx for the reply.

The tech said that the left was 225 possibly because of carbon build up because the fi was not working as it should. I've had carbon build up in cylinders before but never to the extent that it would cause the psi to jump like this. Does that make any sense?

[pete roper]

I think carbon build up is highly unlikely. Carbon will form from oil deposits burning onto the piston crown or combustion chamber surface. With Nicasil bores if the rings loose their tension or go beyond service limits the oil that slips past tends to wash the squishband clean rather than build up deposits. The 'Old' Hi-Cam has valve guide oil seals but any oil coming down the inlet guides would tend to form deposits on the backs of the inlet valves rather than the pistons/combustion chambers.

I have't seen a badly coked up engine for many, many years and never one that runs FI. as I said. Check the valve timing on the high side. Betcha its advanced. The 'Old' Hi-Cam has vernier wheels for the belt sprockets. The cam needn't be out a full belt tooth, just timed wrong. Get your bloke to stick a degree wheel on it and check the timing.

Pete

[BRIAN]

I will pass this on to the tech tomorrow. Can you explain to me how timing being off would affect psi? I thought that psi is measured with all valves closed and the piston moving thru tdc. How would the timing being off raise the psi.

thanx again for your time.

[draidt]

I get Pete's line of thought, If the cam timing is off and the valves closing sooner then they should on the compression stroke there would be a bigger volume of air that gets compressed equaling a higher then normal compression.

[BRIAN]

got it..thanx again

[pete roper]

Pretty much. advancing lobe centres will, especially at low RPM, enhance cylinder fill and boost VE. That means that there will be a more complete cylinder fill as the piston passes BDC and the inlet valve closes earlier so less gas is forced back out of the cylinder as the piston accends on the compression stroke meaning a higher pressure will be reached at TDC compression.

Pete

[BRIAN]

got it..valve timing not ignition timing..