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Join Date: Feb 2002
Location: NH
Posts: 84
Loose Valve Seat – how do I know for Sure

A few weeks ago my engine developed a rattle at low RPM. It just didn’t sound right. I reset the valve lash and that helped a little. 2 Drivers Ed days later the noise is a lot worse.

At any load(throttle position), and below 4000rpm, the car rattles and bucks. Hit 4000 and it smoothes out and will run to 6000 no sweat. In this range, its smooth but power is a little low. My first thought is that a valve seat is loose. It could be moving around at low speed and not at high speed. It sounds like #4 is the source of the rattle and #4 spark plug is carbon fouled. But, #4 compression is strong. If it were a loose seat, why would the plug become carbon fouled and and why would cylinder still have high compression. Could this just be a carb falling apart?

Any thoughts before I pull it out?

Engine
1.5 year old 2270cc (78.4 X 96)
2.0 heads with 38x 44 valves from Headflow Masters.
12 mm spark plugs, port &polish ect.
44IDF Webbers
Webcam 163/86B

Testing:
Cylinder 1
Spark plug clean
Compression strong (150)
Leakdown good, only ring leakage
Valve lash good

Cylinder 2
Spark plug clean
Compression weak (140)
Leakdown bad, Lots of intake leakage, some on the rings
Valve lash good

Cylinder 3
Spark plug clean
Compression a little weak (145)
Leakdown bad, some intake leakage, some on the rings
Valve lash good

Cylinder 4
Spark plug black with carbon
Very strong Compression (160)
Leakdown good, only ring leakage
Valve lash, exhaust up to .010
Old 08-22-2003, 12:52 PM
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Fuel fouling would be black and sooty and usually dry, or obviously fuel-soaked. Oil fouling is usually kinda tarry and sticky and nasty. From the description, I would say you need to address some carb or ignition problems on #4. And very definitely keep an eye on the exhaust valve clearance!!

Sounds like #2 and #3 have some problems, though 140 is not a bad compression reading. The weakest cylinder is about 13% less than the strongest, which is not great but is not awful.

In my experience so far (3 times), valve seats that are popping out of the head will give you a zero (or very low) compression reading. They get banged up and distorted when slamming in and out of the recess in the head and having the valve close on them. This distorts them to the point where they won't seal in relatively short order...

I'd suggest checking the ignition and carbs thoroughly, and if you still have issues then consider popping the heads off. HFM has a good rep, they should stand behind their work if that actually proves to be the problem.

--DD
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Old 08-22-2003, 01:04 PM
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From the leakage you seem to have I'd pull both heads. Number's 2 & 3 seem to have intake valves or seats that are getting cocked and that is accounting for the leakage. Number 4 could have a seat coing down on the exhaust since the lash has gone up and compression is higher than the rest. This can happen when an exhaust valve and/or seat is moving and the valve is sealing well but not opening so no pressure goes out, just up. It can be seen since the one with a seat pulling will have a different pump-up rate than the others. You did not mention how many cranks you put on each cylinder? Good luck
Old 08-22-2003, 07:46 PM
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Old 08-22-2003, 07:46 PM
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