91 VFR750 RC36 - top end noises

Hi folks, ive recently acquired a nice old 750 that has been garaged (unused) since 1995. The carbs were gunked up and ive cleaned them, emptyed the old fuel, changed the plugs and filters, changed the oil, put on a new battery. It now starts on the button and idles nicely. The top ends on both front and back were a bit noisey to start with but soon settled down and i was thinking all was well as the bike started warming through. However there was a sudden change of sound and the front geartrain now has a nasty rattle. I switched off and will let it cool and see what its like tomorrow when its cooled off. Any thoughts? The bike isnt smoking.

My thoughts were that fresh oil and a gentle warm up and any odd sounds would go as it settled down but thinking i may have a bigger problem. I dont know the history of the bike to know if it had issues in the past.

Sent from my SM-G928F using Tapatalk

 

pull off the valve covers and have a look-listen while cranking the starter motor over. check valve clearances. you'll need to remove the radiator for best access to front head

 

Heres a link to a video...

Sent from my SM-G928F using Tapatalk

 

That sounds like a rod knock to me. I own two of these and they dont and i repeat do not sound like your bike. Check valves and go from there. Sounds serious

Sent from my SM-G900P using Tapatalk

 

Ok so ive taken off the rocker cover and nothing visually looks wrong. The outlets are set at 0.25mm both sides but the inlets are at 0.15mm and 0.076mm so I wonder if the noise could be due to the low measuring inlet valve which seems the only one out of tolerance, and by a fair bit. Could internal damage cause the change I wonder? I suppose best thing is to shim it and see what happens next

Sent from my SM-G928F using Tapatalk

 

Compression check it first although that sounds an unhappy bunny

 

Ill get my compression tester back end of the week, ill let you know how i get on. Meantime ill do some cosmetic fixes to make me feel better!

Sent from my SM-G928F using Tapatalk

 

The noise seems awful high pitched and fast for a rod knock. It's really hard to get a true representation of any engine sound with an Internet recording. A compression test will tell you a lot. That said I would also want an oil pressure reading. Low oil psi would indicate a bad big end.

 

Ok so I done a compression test, the rear left cylinder has zero compression. I wonder if the crack I heard before the engine noise was the valve spring breaking? Is it possible to check/confirm and replace without taking the head off?

Sent from my SM-G928F using Tapatalk

 

Possibly but let's be clear.

WHY NOT PULL THE HEAD !

Did you confirm it wasn't rings / rod / piston slap / rod slap.

With a noise that bad, you will find the head is coming off anyway. I'd put money on it.

Did you put oil into the cylinders to test the compression? What was the state of the other cylinders?

 

So far... drained the oil, theres no debris in it. Spark plugs look normal. Bike isnt smoking whilst running. Havent done a leakdown test. Other cylinders are running 6-9psi with engine cold and carbs on/bodies closed. The 6psi front cylinder has a tight inlet valve, not sure if thats relevant. Are any special tools required to pull the head (appreciate there is a procedure to follow to ensure timing is maintained)

Sent from my SM-G928F using Tapatalk

 

Sorry just making sure that you said

6 - 9 psi.

I think the specs are 14 bar + / - 2 and also within 10% of each other

 

That's good news on the oil and plugs though..

 

I broke a valve spring collet on my VTR. (No kidding is broke in half.) It caused a bent valve and zero compression on that cylinder. It is possible. There is something hinkey going on with the left rear cylinder. My advice is pull the head and check for damage. You will be glad you did.

 

a proper compression test is made with the throttle held WIDE OPEN !