1988 Mercruiser 5.7 timing issue.

dbohnett

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Jul 14, 2022
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Just installed this engine. Coil, cap, rotor, plugs, wires all replaced. All plug wires connected to correct cylinders. Did not move the distributor. Starts OK, idles a little rough. Checked the timing and it is set to about 33 degrees {approx} BTDC at 750 RPM, warm engine. Try to rotate dist. to achieve 8 degrees and engine runs terrible and backfires. Any ideas?
 

dubs283

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What firing order are you using?

Checking timing at #1 cylinder?
 

dubs283

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That's correct for a LH rotation sbc. Is this a sterndrive or inboard engine?

Did the engine run before install? Did you replace the long block?
 

dbohnett

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Yes the engine ran before install. Did not touch the distributor before I checked the timing. I replaced a leaking oil pan gasket while it was out of the boat and must have taken out a bolt that I didn't need to because the oil pump came loose. Reinstalled pump with no issues. Good oil pressure, 50 psi at idle and close to 60 at higher rpms. Not sure what "replace the long block" means. Replaced the previous 305 with the 350. All mounting was the same.
 

dubs283

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Gonna need a timeline here starting with the upgrade long block from 305 to 350

Impossible to not touch a distributor with this procedure
 

dbohnett

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This is a different engine. Removed the 305 and installed a totally complete 350. I did not remove the distributor, only tried to adjust the timing. Also, this is a sterndrive. Thanks
 

dbohnett

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This a 1988 engine, thunderbolt IV ignition. Thought grounding was only used on newer systems.
 

dave8311

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Something is wrong with the plug wires orientation or the timing light, or the wrong balancer or pointer. I don't think a sbc will run at 33 degrees.

The easiest way to check this is to get or make a piston stop. Take out all the spark plugs, put the stop in number 1 cylinder. Rotate the motor by hand until you hit the stop. Make a mark on the balancer where the 0 timing mark on the pointer is. Then rotate the motor the other direction, make another mark. Measure the distance between the two marks, and make a new mark halfway between the two marks. That is an accurate TDC. If it doesn't match with your balancer, then either the balancer or pointer is wrong.
 
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dbohnett

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Thanks... Plug wires are correct. Will make a tool and check for true tdc. Will post the results.
 

stresspoint

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Sep 19, 2022
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my bad , i was unaware tb4 had no base timing which could have accounted for an issue i had previously .
i would guess that maybe the harmonic balancer may have slipped the rubber and you are getting a bad read.
? can you get the motor to run correctly in another positing .
 

Scott Danforth

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Verify TDC with a piston stop. The damper ring could have slipped
 

dbohnett

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Did the piston stop test. Both measurements equal distance from factory damper groove so #1 cylinder is hitting tdc correctly I assume? Engine runs fine from idle to about 3000 rpms where dist. is currently set but will stall out and backfire when trying to go wot. Probably a different issue.
 

dbohnett

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Also, I pulled the exhaust manifold to make the piston stop test easier and the #1 cylinder port was wet and dark black. Did not smell like gas. 3, 5 and 7 all dry, looked normal. No water in manifold.
 

Scott Danforth

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Did the piston stop test. Both measurements equal distance from factory damper groove so #1 cylinder is hitting tdc correctly I assume? Engine runs fine from idle to about 3000 rpms where dist. is currently set but will stall out and backfire when trying to go wot. Probably a different issue.
what is your timing at 3000 RPM. should be 20 + base timing or 22 + base timing or 24+ base timing (depending on which module)

if that is working, then its simply fuel related. or more importanly..... Lack of fuel. test the fuel pressure.
 

tank1949

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Apr 4, 2013
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Just installed this engine. Coil, cap, rotor, plugs, wires all replaced. All plug wires connected to correct cylinders. Did not move the distributor. Starts OK, idles a little rough. Checked the timing and it is set to about 33 degrees {approx} BTDC at 750 RPM, warm engine. Try to rotate dist. to achieve 8 degrees and engine runs terrible and backfires. Any ideas?
Recheck plug firing order. MC have a motor anchor process that makes it different to see motor vibrate, if a plug wire is dead or wrong on plug. Recheck firing order and examine plugs were not fouled or electrode damaged when installed.
 

ROY WILLIAMS

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Aug 8, 2022
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Also, I pulled the exhaust manifold to make the piston stop test easier and the #1 cylinder port was wet and dark black. Did not smell like gas. 3, 5 and 7 all dry, looked normal. No water in manifold.
the plug wires of that resistance of the OHMS ..
did all those plug wires of that resistance ...
the cylinders of that compression ..pressure gauge ..
 
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