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Old 03-25-2013, 09:06 AM
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destroyed plugs and valves

Hye guys. I have just encounterd a problem with a 383 that has me baffeled...Friend of mine at works has this

early 90's roller block with stock cam
6.0' rods
D shaped KB Hyper. pistons (-12cc dish)
64cc heads off of an earlier model 350 with new valves, studs, springs, etc
Initial timing at 8 not sure of total
Stock TBI with stock distributer and sensors
Factory roller cam


This engine has been driven for about 4,000 to 5,000 miles. It is in a Surburban, and the truck was just lifted about 15" with 44" Super Swampers added. It also got a gear change to a 5.30.

Truck started loosing power, until it finally sounded like it was running on 3 or 4 cylinders. Pulled plugs and 5 of them were completly melted away. I mean nothing left except the porclen, and the threads were galled as well. the other 3 plugs looked great. ( no brown specs on proclen, strap changed colors right at the 90degree bend, showing good timing, etc.) We pulled one head, and at first sight every thing looked great. Pistons looked brand new, with no obvious signs of premature wear or detonation. After looking farther into it, we noticed the intake valves, on the "bad" cylinders, were sunk into the head/seat. These cylinders also had little to no cranking compression before the dissambly. The good ones were consistant165-170psi. Would detonation/ preignition cause the intake valves to "recess" like this? Seems wierd everything looks ok except plugs and intake valves. Gonna try to post pics for him.[IMG]bad IV 001.png[/IMG]

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Old 03-25-2013, 11:10 AM
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trying to post picures... here goes
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Old 03-25-2013, 11:16 AM
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The pic on the left shows one of he worst intake valves.
The middle one we tried to give the picture some prespective by adding a little paint to the valve, and the last one is the same valves in the head before removing. Any ideas? Everyting else looks good. Is this just really bad detonation/preignition.

The motor was run on 87 octane
it has Iriduim plugs ( which I believe were too hot)
According to my friend he had base timing at 8 degrees with stock TBI and distruibter. Had a felpro pn 1004 gaskets, and block is standard height. I would have thought to see damage to piston and such with the plug completly gone, and the intake valves looking like this. No piston material on plugs and pistons look great.

chevyguy13
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Old 03-25-2013, 12:02 PM
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The valve on the left in pic #1 actually looks stretched. The exhaust valves also do not look like they are seated correctly. My guess is too high of a spring pressure or incorrect springs for a roller cam and/or possibly cheap valves as well. Just a guess though. Usually too hot of a plug will start to damage pistons before it shows up on the valves.
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Old 03-25-2013, 01:18 PM
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You may not have enough gear to move that big heavy truck with huge tires, which in turn was lugging the engine and causing detonation and/or preignition. Could also have had not enough timing or been running lean.
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Old 03-25-2013, 01:25 PM
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thanks guys!

S10 Racer, The valve is streached. its actually longer. lol What would cause this? Heat? How does the valve streach and change shape like that?

Chevyguy13
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Old 03-25-2013, 02:34 PM
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Quote:
Originally Posted by chevyguy13 View Post
thanks guys!

S10 Racer, The valve is streached. its actually longer. lol What would cause this? Heat? How does the valve streach and change shape like that?

Chevyguy13
Here's a pretty good read on the subject.
Tuliped Intake Valves Shortly After Rebuild On Gasoline Engines
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Old 03-25-2013, 02:46 PM
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Makes sense. Thanks a lot. Ive just never seen or heard of this before.

Chevyguy13
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Old 03-25-2013, 03:47 PM
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i see it quite a bit on throttle body 350's. Usually from an intake/lean mixture air leak.
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Old 03-26-2013, 09:55 AM
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Quote:
Originally Posted by chevyguy13 View Post
Hye guys. I have just encounterd a problem with a 383 that has me baffeled...Friend of mine at works has this

early 90's roller block with stock cam
6.0' rods
D shaped KB Hyper. pistons (-12cc dish)
64cc heads off of an earlier model 350 with new valves, studs, springs, etc
Initial timing at 8 not sure of total
Stock TBI with stock distributer and sensors
Factory roller cam


This engine has been driven for about 4,000 to 5,000 miles. It is in a Surburban, and the truck was just lifted about 15" with 44" Super Swampers added. It also got a gear change to a 5.30.

Truck started loosing power, until it finally sounded like it was running on 3 or 4 cylinders. Pulled plugs and 5 of them were completly melted away. I mean nothing left except the porclen, and the threads were galled as well. the other 3 plugs looked great. ( no brown specs on proclen, strap changed colors right at the 90degree bend, showing good timing, etc.) We pulled one head, and at first sight every thing looked great. Pistons looked brand new, with no obvious signs of premature wear or detonation. After looking farther into it, we noticed the intake valves, on the "bad" cylinders, were sunk into the head/seat. These cylinders also had little to no cranking compression before the dissambly. The good ones were consistant165-170psi. Would detonation/ preignition cause the intake valves to "recess" like this? Seems wierd everything looks ok except plugs and intake valves. Gonna try to post pics for him.[IMG]bad IV 001.png[/IMG]
Stock TBI will not work on a 383 unless you put a custom chip into the computer to instruct it that it is now feeding 38 more cubic inches. TBI does not measure air flow into the engine, it does a computation based on preprogrammed relationships between throttle position, manifold vacuum, and RPM. From this and other moderating data it computes a number that is then fitted to a preexisting map position as best it can. Since this process has no way of understanding there is 38 more cubic inches to feed, the injection will constantly under-feed the fuel requirement which results in the engine running lean all the time.

Contact this guy <<< TBI CHIPS >>>

Bogie
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Old 03-26-2013, 10:19 AM
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I was curious when I noted it being a factory roller cam and block, was this TBI setup and computer a factory match for what would have been the 350 engine and am guessing it never came out of a truck ?

Also that 8 degrees initial, I was a little shocked at that because it seemed typical that the trucks through those years with TBI were set at 0 degrees with the timing wire disconnected. Not to say that it can't be advanced a little depending on elevation and fuel richened up a little and so forth. I had bought a chip very lately ... well a whole MEMCAL being that mine is a 1995 truck and the instructions called for making sure to reset the timing at 0 if it had been tweaked previously as the chip maker is pushing the limits and its all based on them knowing where the engine timing is initialized at.

Definitely a heads up for myself too though as I had installed a different cam and then ordered the new programming after because I felt it was under fueling and can see where driving it around some and too lean is not a good idea !.
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Old 03-26-2013, 10:54 AM
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Tapered seat/gasket seat plugs

My first thought was how early are the heads? If you put tapered seat plugs in an early gasket seat head, the plugs will fry in very little time. But in order to use the TBI manifold on early heads, some of the bolt holes have to be redrilled. Don't know if thats the case here. I did run into the wrong plug problem on a 350 w/camel hump heads years ago.

Not disputing any other posts about the ECM fuel curve, I do know of a GM dealer that replaced a '91 TBI 305 under warranty with a 350 crate motor without doing anything to the ECM. It was a friends truck and was driven about 100k miles without a problem. Thats a 45 cubic inch difference. 383-350 is only 33 cubic inches. ?????????

Nolan
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