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Mercruiser detonation chevy 350 with 4bbl carb

L

lime4x4

Contributing Member
recently had a power issue wit

recently had a power issue with the engine that was in my boat. so i installed a 1987 truck 350 engine.I replaced the freeze out plugs with brass ones and replaced the head gaskets with marine ones.Compression on the new engine is at the high end 170 to 175.I have really bad detonation now when under load.timing was set to 8 deg before. I'm going to try retarding the timing and c what happens.Is this compression to high for a marine engine?
 
""Is this compression to h

""Is this compression to high for a marine engine?"

No, not at all. How about the cam? If you left the truck cam in the engine, that may be contributing to the detonation issue. Step up the the octane and see if it helps. I hope you are using a marine starter, marine alternator, marine fuel pump, marine carb and have a gasket under the dist. cap.
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"[b]"recently had a power

""recently had a power issue with the engine that was in my boat."

Ayuh,... A Lean condition will cause Detonation...

Was the old motor running Lean as well,..??"
 
"What distributor did you use?

"What distributor did you use? if you used a marine dist, depending on the style you may need to follow certain instructions for the electronic advance to do it properly. What Octane fuel are you using ? is it fresh gas? less than a month old ?

How was the carbon buildup on the pistons and heads before you installed ?"
 
everything is marine.I run a m

everything is marine.I run a marine electric fuel pump.The old engine had poor performance and compression was 95 to 100 on the old motor. The old engine never had detonation. Would octone boost additive help?. Currently i have 75 gallons of 87 octane gas in the boat. Fuel tank is 100 gallons
 
I understand why you are using

I understand why you are using 87 oct.

were the headgaskets you used like thin stamp metal?

You would need a lot of octane booster or some racing gas.

You can try running it on a portable tank of 93 octane.
 
I forgot the camshaft was from

I forgot the camshaft was from the old engine. It was checked out before installing it into the other engine. The head gaskets were bought from my local marina and they were composite head gaskets
 
"i was wondering about the hea

"i was wondering about the head gasket as there are some metal ones, which will raise the comp ratio and cause that.

curious, new lifters with the new cam or was it a roller cam & lifter setup?

Was anything done to the heads? ( but also if they had a light shaving it shouldn't be pinging, though maybe hmmm.. )

How fresh is the gas ?

You are using a marine dist and not a auto one with a Vacuum Advance on it right?

hmm.. need to think a bit."
 
"What Carb,..??
Did you Rebui

"What Carb,..??
Did you Rebuild it,..??
It's runnin' Lean,.... Fix it...."
 
gas only 2 weeks old.Carb is q

gas only 2 weeks old.Carb is quadrajet and was rebuilt last year.Marine dist with thunderbolt IV ignition.No work down to the heads besides new valve stem seals and carbon cleaned off the valves. New lifters
 
"[b]"Carb is quadrajet and

""Carb is quadrajet and was rebuilt last year."

Ayuh,... A year is a Long Time,...

Fatten up the Fuel,+ See if the detonation Stops...."
 
"well... you have to make sure

"well... you have to make sure that the timing is correct first of all. make sure the total advance is something nominal like 28 or 32 or something and let the base timing fall where it will... and you might want to make sure you don't have some monster vacuum leak. you keep talking about 175 compression... this motor you put in... was it a long block or did you use the old heads and if you did, were the piston tops the same ? total compression number doesn't mean anything... the compression RATIO , however if more than 9 or 9.5 to 1 will ping like hell on 87 octane."
 
the engine was out of a 87 che

the engine was out of a 87 chevy pickup truck. It had 3000 miles on it. All i did was swap over over the camshaft and all marine stuff to new motor.Replaced the freeze out plugs with brass ones and replaced the headgaskets with marine ones.
 
even knocking the timing down

even knocking the timing down to 0 deg it still pings just not as bad..If the ratio is higher then 9 to 1 i would really like to know how that happened when using a stock motor
 
"John, just for giggles, what

"John, just for giggles, what was the condition of the exhaust flappers? What condition is the water circulation pump on the front of the engine. What is the engine temp like? Maybe your hearing a pinging like sound but it's not. What type of spark plug are you using. When running the boat out of the water on a garden hose and muffs, does it still do it at idle? Or a quick rev?"
 
"John:

I thought the 80&#39

"John:

I thought the 80's was when the auto industry was still working auto carbs to provide a lean mixture w/a lower comp. ratio. Many drivers had to use higher octane fuel to stop the knocking. W/o a knock sensor to retard the engine timing, your only choice is what octane you put in the tank.

The marine specs. for that year are MR43T plugs gapped @0.035" and timing @ 8 deg. BTDC. Today's fuel is not the same. I would set the engine up to those specs. and jump the fuel octane up to 91 and see how it does. Try 5 gallons of fuel. If it works then add an equal amount of 87 octane to dilute the fuel to 89 octane and try that fuel."
 
using mr43t sparkplugs correct

using mr43t sparkplugs correctly gapped exhaust flappers ok water temp stays around 145 deg.I will try running 5 gallons of higher octane fuel. It doesn't do it at idle or when reving the engine with no load.The problem i'm going to run into if i have to run higher octane fuel one is cost 2 we boat at chesapeake bay alot and the marina's down there only have 87 octane fuel
 
"i don't know... somethin

"i don't know... something just doesn't add up here. if that motor as described in a mechanical sense then the compression ratio is no mare than 8:1 which would be fine with 87. the same with the timing spec... if you are saying that you can set the timing at base zero and STILL get big detonation, then that doesn't make any sense either. if it was lean enough to do what you say, the plugs would be china white and you would see obvious evidence of the detonation.

so... it really IS detonating but with no hard evidence of it other than the sound ( huh ? ) or something else that sounds like detonation is happening...

lets face it... we aren't talking rocket science here. this isn't some hi zoot all electronic injected blower motor with a million variables.

it either has the right CR or it doesn't.
it either has enough fuel or it doesn't
it either has acceptable timing or it doesn't
it either has a MONSTER vacuum leak or it diesn't

the only variable left is combustion chamber temp. the only thing that could do that was if you got the wrong head gaskets that blocked off the head coolant passages so the heads getting really really hot or that the bore sizes were wrong and they get into the bores.... rare but it happens.

if i were standing where you are, i would have to seriously think about having the heads off again to see if i screwed something up somehow. you have done all the easy stuff.

forgive me for not knowing your abilties, but this isn't going to be something really dumb like you are looking at the wrong timing marks or the tdc mark isn't really at tdc or you are using one of those digital advance timing lights which is lieing to you or something ?"
 
i've been working on gm en

i've been working on gm engines for over 20 years. I did have a plug pulled it was whitish in color but not bright white. Timing is set correctly. Shows no signs of a vacum leak.What i did shouldn't have changed compression ratio. The old engine never detonated (pinged) it had low compression had to push the engine hard to keep the boat on plane. This engine even when the timing set to 0 the boat stays on plane at 3500 rpms old engine anything below 4000 rpm's the boat would slow down and come off plane.
 
the thing that i noticed is ho

the thing that i noticed is how high the static compression is. A normal chevy 350 has 150 to 160
 
you are correct about that.

you are correct about that. ya know... if you git the cam in way retarded it would certainly pump the static compression numbers. i don't have any idea what it would do to make it ping but i am officially out of guesses.
 
"a plug pulled it was whit

"a plug pulled it was whitish in color but not bright white."

Then you know from experience that the mixture is on the lean side. Try 1/4 turn rich on the mixture and recheck the plug color later.
 
I'm know carb expert here.

I'm know carb expert here.When i started working on gm's carbs were phased out and throttle body and fuel injection was in. But i was always under the impression that the mixture screws only effect the fuel/air mixture while at idle?
 
"thats true enough but on some

"thats true enough but on some it also serves to as signal to the main circuit. having said that, what you describe sounds like it needs about 30 more points of main jets as opposed to a 1/4 turn... but as i said before , this whole deal just doesn't add up for me. way too much 'problem" for nothing to be big and obviously wrong..."
 
I didn't get to try anothe

I didn't get to try another fuel source yet but this is what i found so far As long as i keep the engine below 3500 rpm's no pinging. The pinging starts when the secondary air valves start to open up
 
"Did the engine have an EGR va

"Did the engine have an EGR valve when installed in the truck and if so, is it still installed and functional"
 
"Double check your spark plug

"Double check your spark plug wiring........You may have a crossed wire and that would definatly cause a pinging under load at the rpms you are stating.

The compression and all seems fine to me. Something else is going on here. Did you mess with the timing chain? Are you sure the secondaries on the carb are functioning correctly? (gas flow) maybe float is out of adjustment, Maybe too much fuel in bowl?

Just some other questions guesses."
 
"[b]"I did have a plug pul

""I did have a plug pulled it was whitish in color but not bright white. I didn't get to try another fuel source yet but this is what i found so far As long as i keep the engine below 3500 rpm's no pinging. The pinging starts when the secondary air valves start to open up"

Ayuh,.... You've defined the Problem as Fuel Delivery,....
Yet,...
You don't want to mess with the Carb,..??

You need to fatten the fuel in the upper rpms,...
Until you Do,... This Problem isn't going away..."
 
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