Bottom end on the way to the grave?

[stelladragon]

Went for a blast on the Dommi this afternoon, and due to the roads now finally being dry, was giving the old lad some beans. On hard acceleration 4,000 - 6,000 rpm (especially in the lower gears) there is a knocking sound from the engine. It isn't massively loud, and gets quieter as it approaches 6,000 rpm and I never rev it harder than that. If you open the throttle wide at low revs, the engine is sweet. Other than that it runs superbly well! It's a '92 Dom with 45,000 miles under it's belt. Any ideas .... crank, main bearings?

Cheers all,

Russ.

[catteeclan]

Hard to say without hearing and sometimes only getting it to bits will tell. If it's a deep sounding knock them crank (big end) or piston knock would be first to check.

[stelladragon]

Good call. I've got it in my head that if it were the bottom end, you would be able to hear it at tickover, but it sounds fine. Here's hoping it's piston slap! If you bring the revs steadily to 6K it's ok, it only makes this sound if you give it a fist full of throttle. I'll use the C90 for a while methinks!

Russ.

[catteeclan]

Good call. I've got it in my head that if it were the bottom end, you would be able to hear it at tickover, but it sounds fine. Here's hoping it's piston slap! If you bring the revs steadily to 6K it's ok, it only makes this sound if you give it a fist full of throttle. I'll use the C90 for a while methinks!

Russ.

Sounds just like my 600lm. Good luck.

[IgorVigor]

check your oil level.

A low oil level can cause more engine noise.

[stelladragon]

I've been changing the oil every 1500 miles, and religiously check it after every ride. Maybe it has run low at some point?

[IgorVigor]

Can you either get a recording of the knocking noise, or see if someone on here lives close to you so they can hear it?

Something that I found on a couple of old bikes, is get the engine in top gear, and open the throttle hard from low revs (2~3krpm where ever your engine will pull from). If you get a really harsh knocking noise, big end is probably going. Just dont mistake big end for thudding of the single.

[stelladragon]

Just tried the test above ... 3,000 rpm in top ... open throttle fully ... pulls well and sounds sweet.

Russ.

[IgorVigor]

When the big end has gone on a couple of my bikes, that was the surefire way of telling (was also told to do it by a bike mechanic as well)

Can you compression test?

[stelladragon]

I have a tester, I'll give that a go. A long serving mechanic friend is taking it for a spin later, second opinion will be posted later.

Russ.