...cont'd<P>Replacement "depression chamber" on its way, seems the old one couldn't hold a vacuum beyond 2"...<P>Takes 2 weeks to get here, so won't know for sure til then.<P>New "PCV" (?) valve, it was fouled and not working OK. <P>A lot of the carb systems, etc, were checked REALLY carefully today, and that was all that showed up...<P>sigh............<P>Don

I'm not gonna go back to read and see if you've done the test, but what's the compression?

fasteddy,<P>The mechanic ran a "computer compression check". Apparently the diagnostic computer cuts out the cylinder sparks in turn, and measures the change in RPM (?). If the cylinder won't hold pressure, then there's no effect on the RPM, and the computer flags it as low.<BR>I think that's how it works...<P>All cylinders with good compression.<P>On the round trip to the Rubicon, I travelled more than 3,000 miles in a week and half (I think), and didn't even burn a liter of oil. I'm assuming the engine's good. Rebuilt 2 years ago, including cylinders bored, and new larger pistons.<P>But in the space of 3 days this week, it suddenly decided burned a liter...<P>Frustrated...<P>Don

1. Vacuum lines cracked or intermittently leaking (see at mani sensor below carb) discovered during depression C replacement?<P>2. leaking/out of round throttle shafts. When the carb warms up and expands possible leaks when vacuum is max? spray carb cleaner on outside, does idle change?<P>3. Timing, out of round dist shaft / float at high RPM (worn advance springs ?-if any)<P>4. Gaskets? Head/Manifold/CARB same test as #2<P>5. clogged air tube/heat riser opening at rpm after warmup<P>????? Arrrgh, where is the bicycle<BR>my 2c

Those are all good ones to check. I don't trust computer "compression" checks. What happens if all cylinders are low? The way the pressure jumps up with each compression stroke can give a clue to what's wrong, and you can test for rings or valves by adding tsp of oil to each cyl. and retesting compression. An improvement means rings, none means valves/head gasket/cracked head. A better test is a cylinder leakage test, which tests the cylinder's leakage rate after the application of regulated pressure. The sound of escaping air will often lead you to the leak (from the radiator, crankcase, exhaust pipe, carb, etc.). Have they scoped the ignition?<BR>The oil consumption is puzzling. Is it smoking from the tailpipe, and is the inside of the pipe very sooty black, or oily black. What does your undersheild look like? Any drips or puddles? Are the plugs sooty, white, or normal light tan? Are the electrodes pitted or smooth? Do the white central insulators have tiny shiny metal flecks on them (sign of detonation). Are all the plugs the same? One very white one is usually a head gasket water leak. Does the engine spin smmothly when cranking, or with a weak spot?<p>[ 06 December 2001: Message edited by: fasteddy ]

...all good comments.<P>The behavior is inconsistant, to say the least, and that's the maddening part of it.<BR>Typically, it runs VERY smoothly, and the problem kicks in about 50% of the time when under load. The engine was test driven with an in-line vacuum gauge. No vacuum problem.<BR>BUT the depression chamber (secondary) operates on it's own system, from the venturi on the carb. It's a more subtle vacuum than the rest of the system, and harder to check during a test drive, but the depression chamber won't hold a vacuum, so it's being replaced, and hopefully the problem will disappear. If not, then the investigations continue...<BR>The problem (power-loss, feels like a stuck carb valve) occurs with a very cold engine and any other temp, as well.<BR>All the air ducting controls work, and the problem is not related to any specific RPM, it can hit between 1800-4000 rpm. Most noticible trying to climb a hill in a high gear. But not ALWAYS. I climbed one hill on the freeway in overdrive without any difficulty, caught up to the vehicle in front, eased off the throttle, changed lanes, and then eased back on the throttle, and THEN it lost power! Wierd.<BR>I should pull the plugs and actually take a look inside. I have a compression tester (or used to...), but I somehow doubt there's suddenly a problem there. I lived with bad cylinder scoring, low compression, bad head gaskets, etc, and this looks like none of them. Mostly because the exhaust is clear, no oil smell, and was pollution tested last week with below average hydrocarbons. I'll check the tailpipe for oily soot, etc.<BR>There is NO oil leaking outside the engine, went over it and cleaned it thoroughly this evening, and ran it to look for any sign of oil. none.<BR>I have noticed that my oil pressure is sometimes showing quite low even though the oil level is good. And along with it comes the lifter chatter. Oil pump may be a problem. But again, it's infrequent.<BR>Changed the oil and filter 10 days ago (10-30 Castrol GTX), and the oil level didn't move in 5 days. Then sometime during the next 3 days, 1 liter disappeared. No blue smoke, no spillage. No oil on the ground under it, and it's always in the same parking spot. Even had some newspapers under it to check...<P>If I had to explain it, I'd say that the when the problem occurs with the power-loss, if you push the throttle, there will either be a backfiring, or a sudden loss of power to 1 or more cylinders. Then you pull over and rev the engine for a few minutes to smooth it out again, and all's well. I have this delusion that all of a sudden a lot of oil got sent into the carb at that point, and that's why the power loss to several cylinders (and the recently soiled pcv valve). But HOW can that much oil get burned through a carb that quickly without any obvious sign? Admitedly at night I can't esee the exhaust...<BR>Running the engine with the oil cap of produces a throbbing air movement at the valve cover opening, but if you cover the hole with your hand, you can feel a slight negative pressure. I gather that's ok. A positive pressure would be a problem.<BR>Other idea: could I have bloked oil passages, and the oil is all pumped on top of the head, and getting forced into the carb? Or into the cylinders?<BR>Having needed a lot of helicoils on the spark plugs in the past, I'm reluctant to pull the spark plugs unless it's urgent. And since it runs and idles beautifully under no load, I presume the plugs are good...<BR>The mechanic that's going though it has the Mitsu book, and read through a lot of it last week, and checked everything he could find against Mitsu's published numbers, and SO FAR only the Depression Chamber stands out...\<P>Bicycle time? It's a mix of rain/snow/wind here now. Even walking's ugly, and the busses are full...