First thing I would check is the oil pressure, then make sure the gasket for
the upper tensioner isn't blocking the hole the oil come through. Are the
chains new when the engine was rebuilt?
1. The tensioner spring only provides part of the pressure, oil pressure
increases this to about 25 pounds pressure on the tensioner with full oil
pressure, less at idle
2. Wouldn't a new tight engine have more oil pressure? Either way, 20 psi
should work.
3. I've run mine with open headers, if the chain rattled, I didn't hear it,
but it ran fine.
I would check float level, condition of the distributor, especially the
advance springs as far as the idle, and for the chain, I'd bet either the
upper tensioner is NG, or the gasket is on wrong or the oil feed hole is
plugged up. Lack of oil pressure could be a whole bigger problem. What is
the gauge reading at idle? Could go to heavier oil too.
Verify the timing isn't swinging up and down with a timing light.
I think you have 2 separate problems, Idle and loose chain tension. People
have shimmed out the tensioners with good result, opinions vary if this is a
good idea,but I've never heard of it causing a failure. I added a much
stronger spring on the inside of the tensioner.
All,
How much do people's U20 chains slop around at low idle? Here are my
symptoms: at 700 rpm's, even with a warm engine, the idle is swinging from
660 to 740 and there's a chain noise caused by the chain slopping around
against the housing. (At 1000 and higher there's no problem, no backfiring
on slowdown, or anything else timing related -- at 80 mph drivers, and local
law enforcement, have complained about a little lead foot disorder but that
fades on the other side of 100 mph)
The first thing we did was looked for vacume leaks -- the carb is freshly
rebuilt and bushing are in good shape -- a quarter can of carb cleaner and
no leaks were found.
Still confused, we pulled the valve cover off and ran the engine with the
valve cover off -- messy, but a useful diagnostic. When we looked in, it
looked like the timing chain was slopping all around and the tensioners
weren't doing such a good job.
The engine is freshly rebuilt. The tensioners have plenty of material left
on their feet. All of the oil passages were freshly cleaned when we did the
rebuild. We've got three theories:
1) The tensioner springs are shot, if so, does anyone have the specs for new
ones?
2) Because the engine is so new there's just not enough oil pressure to push
out the tensioners are low idle.
3) We put on really loose exhaust (big pipe, fancy muffler, straight shot
bent under, not over, the back axle) and the lack of back pressure is
compromising the engine's overall tuning.
Any ideas?
Nathaniel
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