My FSAE (collegiate racing team) inherited two CBR 600 F4i engines from another university. Unfortunately we have no provenance on them. After putting together a mostly complete harness from ebay, we were able to get our freshly rebuilt engine up and running. It wasn't idling perfect, but it was running. We left it for a week and at the next work meeting. I refused to fire back up again, the more we cranked it the more we had issues with flames coming out both the exhaust and intake manifolds. We have since begun digging back in for a diagnosis.

Our current findings: The engine is properly timed, we checked that as soon as we got the valve cover off. When we went to start checking valve clearances (we expected we would need to make adjustments after having removed all the carbon build up and replaced 3 exhaust valves), we found the intake valves in bank 1-3 were not closing during any point in the combustion cycle. We believe none of them were installed bent, (none showed signs when we carefully spun them in a drill and removed the carbon buildup with an abrasive sponge). The valves are shoved down by the cams but do not return to contact the side or rear of the cam lobe when they should be shut.

Any thoughts or assistance would be most helpful. We are a group of mechanically inclined engineering students, but we are by no means certified motorcycle techs.

 

Hi and welcome to the forum. Freshly rebuilt seems suspect. Take a look at the following diagram for the valves and camshaft layout. The valves always have spring tension (really strong tension) pulling the valves up against the valve seat. On top of the valve stem is a shim, which varies in size (thickness). Then the lifter, which looks like an upside-down cup. The lifter is what the cam lobes press against in order to open the valves. When checking the valve clearances, it's the gap between the lifter and cam lobe (actually the lowest point on the lobe) that you are measuring. If there is no gap, then you can't be sure that the valve is closed tightly, and most likely it is not. If during a rebuild of the heads, you don't keep everything in the same place on the head, meaning every valve, shim, and lifter, then the clearances will most likely be off. It's also not uncommon to have a shim get stuck to the underside of the lifter (inside the cup portion), and you end up with 2 shims on that particular valve. This would certainly keep the valve open all the time. You also wouldn't be able to check any clearance since you would have none.

https://www.partzilla.com/catalog/ho...camshaft-valve

Hope this helps.

 

You are right to be suspect on our rebuilding skills. When we tore the head down we carefully bagged and tagged each valve, valve spring, shim, and lifter bucket to make sure everything went back properly. Everything was reassembled back to its original location with the exception of replacing the 3-4 bent exhaust valves and replacing all of he valve seals. The shims looked to be factory. The cam chain was replaced with a new Honda OEM chain.

The thing that has us scratching our heads is that our most of our valves visually never seem to fully close(I can still see into the combustion chamber) regardless of cam lobe orientation. When the lobe is oriented so that the pointed end of the cam is facing away from the lifter there is a sizable gap (on bank 4 around 50-60 thousands) between the top of the lifter and the bottom of the rear of the cam lobe. It seems to us that the valve should be able to close if it has that much gap, but it isn't doing it.

Tomorrow we going to do another careful disassembly to check for bent valves. Given that we received the head with exhaust valves we fear the worst. If that is the case we are going then have to determine what happened as we believe the engine is still properly timed.

Bank 4, valves not closing despite 50 thousandths clearance.

Showing Cam Position on Bank 2

Bank 2

 

Hey, great photos BTW! Sometimes it's hard to be sure what you're seeing from a photo, but it appears that the intake valve on the left is not closed as much as the one one the right. It will be interesting to find out what is preventing it from closing. I hope you'll post back here and let us know. If there's anything else we can do just let us know.

 

This engine just keeps getting more interesting. On Wednesday we discovered that all 8 intake valves were bent. Furthermore the intake cam had been set up on the timing gear 180 degrees out of phase. So when we originally timed the engine we didn't catch that the cam was incorrectly positioned. But wait......there's more! The students from the university that sold us our engines managed to install the pistons upsided down. According to the service manual the IN on the pistons should be facing the intake. Today we split the case and corrected the piston orientation.

Moving forward we will be double checking the work of the previous school! The crazy thing is that this engine was running (not well) a few weeks ago.

 

OMG, that's amazing that it was running. The intake valves could have hit the pistons since the intake valves are larger and would not fit into the smaller cutouts on top. Sorry to hear. Let us know if there is more we can help you with.