burning valves

96 gm 350 carbed pulled heads had a valve job done 2 burnt intake valves . new impeller installed ran engine for 1 hr again 2 burnt intake valves during teardown we found waterintake hose on transom was missing the hose clamp but hose was in transom intake fitting . did I lose vaccum? water temp gauge never showed hot could this be the problem? Don S what are your thoughts?

Re: burning valves

What kind of RPMS you turning at WOT, and how old is your gas?

Is the total timing advance dead nuts on?

Re: burning valves

Too high combustion chamber temps can happen without any overall engine overheating.

Per other thread: "Running lean, too much compression, too much timing advance, poor fuel, detonation, over propped... "

http://forums.iboats.com/search.php?searchid=8144779

Your problem can be within your 383 build up, which is anything but a stock 350. You have had a number of posts about timing your engine over the years, also about increasing prop pitch.

Re: burning valves

max rpm 4800
timing 30 deg max
fuel reg 87 oct 3 weeks old timing issues years ago was a bad ign module replaced it problems went away heads started to turn blue after 1 hr of running after2nd valve job

Re: burning valves

What type of spark plugs?

Re: burning valves

Your 383 may not run well on 87 octane.

What cylinders were the valves burnt on? How did the rest look?


Please provide info on your 383:

- Pistons: Flat-top or if recessed/domed, provide cc
- Heads: What and any additional work performed, chamber cc
- Cam: Type and specs
- Intake: Type, brand, model
- Carb: Brand, model, and tuning
- Exhaust: stock thru prop or hull? other?
- Ignition: Assuming T5, but what model was it originaly, L, LX, MAG

Re: burning valves

plugs ngk
cyl burnt 3/4
pistons flat top
intake stock 350 4 bbl
carb weber 4 bbl
exhaust stock prop
ign t/5 lx

Re: burning valves

jwkoehn said:

plugs ngk
cyl burnt 3/4
pistons flat top
intake stock 350 4 bbl
carb weber 4 bbl
exhaust stock prop
ign t/5 lx

Click to expand...

Cam stock?

Heads stock?

You probably have way too much compression for one. Not sure if it is causing your problem, but it certainly is not helping. Your stock heads have 64cc chambers and require recessed pistons to keep compression ratio down. Just by going to flat-tops can create a problem in a 350. Add to that the long stroke and overbore and you are probably in the high 10:1 area, approaching 11:1. Add to that a heavy boat, and could be a recipe for your problem. If your block has been decked or the heads shaved, would only add to the compression problem.

You can try to run 93 octane, but your T5 ignition does not allow for adjusting to try to compensate.

If you haven't retuned the weber for the extra displacement, may be causing it to run lean. Boat calibrations run on the rich side to help cool the intake valves. Your stock weber has 3 step rods with special jets. Finding tuning parts can be an issue. You might be able to buy the 350 Mag tuning parts (which are richer than LX), but still might not be good enough for your 383.

Re: burning valves

cam is stock heads are stock. we are putting in a thicker head gasket
to reduce comp ratio average compression was 175-180

Re: burning valves

jwkoehn said:

cam is stock heads are stock. we are putting in a thicker head gasket
to reduce comp ratio average compression was 175-180

Click to expand...

The stock mercruiser head gasket is think. Unlikely to be able to go more than another 0.010 thicker. You need a much larger change than what a head gasket can do. Going to 76cc heads still leaves you higher compression than the stock 350. A custom cam might help reduce the dynamic compression.

A compression test does not measure compression ratio. You want to be down around 9.0:1 with non-vortec heads.

PS: I ran some numbers in a compression calculator:

Enter Cylinder Bore Size: 4.030"
Enter Piston Stroke Length: 3.75"
Enter Head Gasket Bore Diameter: 4.040"
Enter Compressed Head Gasket Thickness: 0.041"
Enter Combustion Chamber Volume In CCs: 64
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-'): 0
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : 0.026

CR= 11.04:1

Going to a thick 0.061 head gasket only gets you to 10.53:1

76cc heads and 0.061 head gasket gets you down to a manageable 9.3:1


The 11:1 compression is probably the root of your starter kick-back issue, which you had to set the timing just right to turn over the engine.