NC35 Nightmare

[alexrm125]

Hi all!

Bought an NC35 just before Christmas with 'carb issues' - naturally it wasn't carb issues. I am here seeking your collective wisdom and guidance.

So, let me get you all up to date...

I recently bought a spares/repairs NC35. It was sold with 'carb issues' and was only running on 2 cylinders. As sods law dictates - the carbs were clean and fine.

2+4 are the problem cylinders.

Carbs have been cleaned and inspected. Fuel is getting through and no obvious evidence of damage to diaphragms etc

I have changed the HT leads, spark plugs and ignition coils and still no change of state. Spark plugs have all been replaced and have a good spark.The HT leads, suppressor caps and coils have all been replaced and the resistance is in spec.

I have checked the valve clearances and had the cylinder head pressure tested. This found 1 exhaust valve was passing. When the pressure test of the heads was conducted it was an exhaust valve of #4 that was passing.

Initial compression tests showed an average of;
Cylinder 1: 140psi
Cylinder 2: 42psi
Cylinder 3: 110psi
Cylinder 4: 93psi

I am curious as to the consensus - now I am at this stage, is it advisable to put new pistons and rings at it just to eliminate them from the equation or am I heading in the wrong direction? I don't want to box it all back up after lapping the valves in and then still have the same problem and have to strip it all back down thus wasting head gaskets and time. This also begs the question, because the OEM pistons are like hens teeth to find, do cbr400rr or any other pistons fit like for like in the NC35 head?

When running on 2 there are no nasty knocks or anything coming from the engine. If it can be avoided I don't want to have to split the crank. However the lower compression on 2 + 4 was leading me to think maybe a replacement of piston and/or rings or just rings might help?

If new pistons are recommended, what is the process of splitting the engine down that fair? I have the Haynes manual but is very vague and for the most part, useless.

Thank you all in advance!

[Yakama]

That’s sucks but don’t lose heart they are great bikes. You don’t hear of too many people on here having to change pistons unless there is catastrophic damage Id say pull them and check before spending cash trying to track down pistons as you are unlikely to find some new now sadly

[magg]

From the Honda Common Service Manual.

Initial compression test is done after the engine has been run and then allowed to cool. If the compression is low, drop a small amount of clean engine oil into the cylinder and recheck the compression. If compression increases compare to previous reading suspect cylinder or piston ring wear, if compression remains low suspect valve seats or head gasket.

Not sure how that equates to you head leak test results. As Yakama said, pistons should be ok and rings were still available a while back.

[Neosophist]

42psi is a major loss of compression.

they all seem pretty low actually, i thought about 170 was good for a healthy vfr motor.

you need to isolate why you are loosing compression before trying to fix it.

1. valves... are they sealing properly?
2. pistons... no holes?
3. piston rings.. are they installed? worn out?

even with the most worn out piston rings 42 is abysmal... was the bike constantly wheelied perhaps and has starved of oil and caused internal damaged to the bores / pistons?

If you had the heads off to do the valves did you inspect the bores? was there any signs of metal to metal contact? could you rock the piston about in the bore?