Triumphrat

moharrow · 10-31-2008, 06:48 PM

my 750 bonneville was smoking and generally lacking in power so I decided to tear down the top end and see if I could resolve the problem. everything looked fine until I checked the exhaust valve pushrods. they were both bent. does anyone know what would cause this?

TBS · 11-01-2008, 06:42 AM

Was it running fine before. If it is a newly built engine I would suspect incorrect valve timing as it sounds as if the pistons have kissed the valves, thus causing the bent pushrods, however I have come across some extremely poorly manufactured aftermarket pushrods that bent due to the fact that they didn't seat properly inside the cup which was drilled leaving a point, rather than milled flat and slowly worked their way down to the bottom (leaving a swage mark around the pushrod) thus opening up the tappet clearance and then popped off the rocker arm.

Mastiff-man · 11-01-2008, 08:42 AM

I agree TBS, there is some missing information...Bent pushrods shouldn't cause somking to begain with, plus why only the exhuast?

Smoking could only come from rings, valve guilds, or bent valves.

GrandPaulZ · 11-01-2008, 10:35 AM

2 separate issues.

Could have been bent from the day they were incorrctly installed, just not bad enough to keep the bike from running. However, properly adjusting the vales would result in more and more bend to a point where the bike wouldn't run.

Snakepit · 11-01-2008, 01:21 PM

Also if new exhaust valves were installed and not of oem quality. Bargain valves will sometimes hang up in the guides when heated.

moharrow · 11-01-2008, 04:56 PM

it had begun loosing power about a month ago and the smoking began at about the same time and got progressivly worse. it was still running when I tore it down just not a lot of power

I am replacing the valves, valve guides, springs and the pushrods I just want to make sure this issue does not come back to haunt me

GrandPaulZ · 11-01-2008, 07:04 PM

Either get a good micrometer and calipers, or have your machinist check everything for tolerance.

El Tejon · 11-04-2008, 04:49 PM

Not usual when some ham fisted shadetree does a valve job. The push rods are not aligned with the rocker arms and get trapped twixt the rocker arm and the rocker box. Kick the engine over hard and you have bent pushrods. You should see some scoring damage on the inside of the rocker box where the rocker arm ends and the pushrods fit.

moharrow · 11-07-2008, 09:58 AM

I did not find any scoring on the inside of the rocker box but the top of the right hand pushrod was split and both pushrods showed abrasion marks at the point of the bend,

also I rebuilt this engine about 5 years ago and it has run fine up to the time it started to smoke
I am waiting on the head to get finished before I can procede further, the shop thinks one of my exhaust valves was sticking