94 vfr750 troubles

Just got a new to me 1994 vfr750 with 78,000 miles. It was having some issues starting but would start and run on what sounded like two or three cylinders. I thought it was probably a carb issue so I pulled the carbs to take a look and they are all very clean, I then did a compression test with the engine cold the number one cylinder was 70 psi number two 85 psi number three 80 psi and number four cylinder was 90 psi. Does this engine need a rebuild? What should my next course of action be with it. Should I put the carbs back on and test warm?

 

All the spark plugs where in good wish it was something simple but I think my compression is just too low

 

The compression spec is 200 psi so your numbers are well off. Are you using the compression tester correctly or does it have a fault?

 

Something about those numbers does seem incredibly low. If it's not burning oil, my guess is that something could be amiss with the test equipment, or the method. The hard starting does give ya something to consider.

I will make an assumption (bad idea already, lol) that the compression test is being conducted with the engine completely cooled down. Doing it with the engine hot, not only causes a burn risk, the compression numbers will be inflated by approximately 5%. Pull ALL the plugs first. Make sure the throttle is held wide open when cranking the engine...might want to disable the ignition system before you do it, all it takes is one spark to completely ruin an already trying day. As you have already pulled and reinstalled the carbs, you prolly know all this, but I'll say it anyway. Trust, but verify.

If you do all of those things, and still getting those kinds of numbers, you might wanna check to make sure the valves are adjusted correctly, if they check out, I would proceed to a leak down test to determine where you are losing the pressure. I seriously doubt both head gaskets are leaking, so the next most common things are the valves, valve seats, or the piston rings, or some combination of all three. A serious boil over can do some nasty crap to an engine. A leak down test will help isolate the problem area.

 

The compression test was done with a compl

The compression test was done with the engine completely cold and the carbs off I just don't think the compression will come up that much even with the engine warmed up. The valves are all within spec, so I'll do a leak down test and post up the results

 

Excellent. I figured you were competent on the mech side. People without a lot of mech experience typically don't pull their carbs Best of luck with your investigation.

 

I recently bought myself a cheap compression tester and tested my ST1100, and came up with values around 140psi, engine warmed up and throttles wide open, valves all in spec. Obviously, that is pretty low as the spec is 199psi. For comparison I also tested one cylinder on my VTR1000 and got a similar number. From that I concluded that either both bikes have serious engine issues, or my compression tester is not very accurate.

 

Have you checked the valve clearances?

 

Yes valve clearances are all within spec

 

I put a little oil in the cylinders and rechecked the compression with about a tablespoon of oil in the cylinders compression went to a range of 120-140 psi. I think it may just be a carb issue I am going to go over the carbs again reinstall them and see what happens doing a compression test with the engine warmed up

 

Interested to see what you come up with. I hope it all works out, and it's not something too hard to deal with.

 

Put it back together today and warmed up the engine only tested compression on cylinders 1 and 3 both were around 95 psi guess the engine is just getting tired after 25 years, looking at used engines on eBay but will probably just ride it for the next season as long as it keeps running fine then next winter see about doing pistons, rings and a valve job.

 

Sorry to hear that, Vroom.