Topic: daily rider is acting funny

[Geeto67]

ok I think I need to define two terms before I unfurl this story.

Hard Backfire = The traditional backfire, loud like a .44 being fired off, straight out the back of the exhaust

Soft backfire = popping noises in the the pipes, not a hard backfire bu a couple of them in rapid succession. more of a puff sound. Not detonation.

Ok so I have been all over the electrical system of my 1975 cb750 and I don't think it is the problem. The problems are:

1) the bike is way down on power.
2) one cylinder feels intermittent
3) at 1/4 throttle I am getting soft backfires though the exhaust when hot
4) at 1/3 throttle or upon deceleration and then transfer to acceleration I am getting a hard backfire out the #1 pipe.
5) plugs are a dark tan (not the usual tan you would see on a plug chop test)
6) occasionally I am getting some detonation at 3/4 throttle.
7) there is an oil weep from the valve cover and the head gasket.

I have checked the points, coils, plugs, all of it, it all checks out fine. I am leaning to this being a carb problem but I am not 100% sure. I was hoping my carbs are not gummed up or clogged, and since I ride the bike almost everyday they really shouldn't be. Then haven't been cleaned since 2004 and they were still running good until a couple of months ago.

any ideas?

[TwoTired]

Ideas?
Fuel contamination?
Jet vibrated loose?
Sticking float pivot?
Sticking exhaust valve?
Compression check?
Sticking carb slide?

Cheers,

[mlinder]

My number 1 float valve sticks sometimes. Causes same issue.
Check for that, and replace your plugs.
Does it do this on brand new plugs?

[Geeto67]

valves were recently adjusted (twice, once by me and then again by a professional friend), but the thought of a winged motor (sticking exhaust valve, crack in the head, etc) scares me. I was hoping to keep this bike on her legs till first snowfall and keep the new ducati a garage queen but it is not looking so good for me right now.

At WFO the bike actually makes less power than with the throttle 3/4 open, which is weird. Also when accelerating she sometimes surges, but the backfires which are obviously overfueling backfires would lead me to believe that she is not lean surging.

Guess I hafta take the carbs apart.

[sparty]

It really sounds like you have carb problems.  When I had my old engine and old carbs I had similar issues.  I sometimes ran on 3 cylinders until 3/4 throttle, then the 4th would kick in.  Carbs are real tricky sometimes, especially early 70s carbs - the slides wear and can hang up a bit.