93 Nighthawk 750 needs valve job

[Lucky30Eight]

Hey all, first time posting.

I have a 93 Nighthawk 750 with 20,500 miles that I noticed is low on power. It is also a little difficult to start (takes a small blip of throttle), and the idle can be uneven. No smoke from exhaust that I have noticed. I have owned the bike for a few years and it was running really well up until maybe a month ago. I cleaned and re-jetted the carburetors for correct air fuel mix at my elevation less than 1 year ago, and the spark plugs are a nice tan with no oil or carbon fouling. I checked the easy stuff, air filter, spark plugs, and coils already.

I just did a compression test and here are the results:

(Clymer manual states standard compression is 142-199psi)

Cylinder 1: 75psi -- Wet compression 65psi

Cylinder 2: 105psi

Cylinder 3: 110psi

Cylinder 4: 110psi

With Cylinder 1 being so low compression I tried a wet compression test and the psi dropped to 65psi. I am not that mechanically capable but I believe that means that it is likely not piston rings, but an issue with valves. I would also guess that the valve cover gasket needs replaced as well, since compression is low across the board.

I am unsure of what to do at this point. I have contacted all of the motorcycle mechanics in my area (Farmington NM) and no one is willing to work on the bike. My question is, does anyone know of a mechanic I could ship the engine to? Or should I attempt to do the work myself? I have never torn down an engine before and I am having a hard time finding the information on how to go about doing the work.

Any help is appreciated. Thanks

 

[brettp]

Valves could very well be the issue. You can do it yourself. Watch some YouTube vids to gain some familiarity with the process and tools (if applicable).

 

[Biebs]

How are you doing the compression test?? Throttle wide open?? At 20,000 miles you should not have valve problems depending on how the bike was treated. Maybe a leak down test putting compressed air in the cylinder to check for leaks. Remember these bike have hydraulic lifters doing a compression test with no oil pressure will not let valves open completely

 

[Lucky30Eight]

How are you doing the compression test?? Throttle wide open?? At 20,000 miles you should not have valve problems depending on how the bike was treated. Maybe a leak down test putting compressed air in the cylinder to check for leaks. Remember these bike have hydraulic lifters doing a compression test with no oil pressure will not let valves open completely

Throttle held wide open with a tester that screws into the spark plug hole.

I feel like 20k is very low to be having issues as well, but the previous owner definitely neglected the bike. I replaced the exhaust when I got it because the system was rusted out, and what I could see through the exhaust exit from the head was stark white, leading me to think it had run very hot and lean before I got it.

Doing a leakdown with compressed air is not something I am familiar with. Can you give me some instruction or link to something that does please?

 

[Biebs]

Link to motor trend article on leak down test.

https://www.motortrend.com/how-to/cylinder-leak-down-test/

You can just put pressure into cylinder at compression stroke - both valves closed- and listen for air.
How much psi I’m not sure someone else might have better info on psi.

 

[brettp]

20k is low for a valve job need, but it is even less for needing rings. One of your valves could be stuck. Could be from sitting or could be from a lack of oil and why the bike was laid up in the first place. Valves are easy to check. I would do that.

 

[Lucky30Eight]

20k is low for a valve job need, but it is even less for needing rings. One of your valves could be stuck. Could be from sitting or could be from a lack of oil and why the bike was laid up in the first place. Valves are easy to check. I would do that.

I hadn't considered a valve might be stuck. I believe I can simply remove the valve cover and check the valves from there correct? If a valve is stuck, would I be able to get it unstuck from there as well? Or would that require taking the head off of the block?

 

[brettp]

Yes, you can check the valves. Maybe, depending on why it is stuck.

 

[Biebs]

If you put air pressure into the spark plug holes you can hear if a valve is stuck open.

Seems you are interested in a tear down… have at it!!!

Then someone can buy your parts bike/ unfinished project.


From your first post:
With Cylinder 1 being so low compression I tried a wet compression test and the psi dropped to 65psi. I am not that mechanically capable but I believe that means that it is likely not piston rings, but an issue with valves. I would also guess that the valve cover gasket needs replaced as well, since compression is low across the board.