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Merc backfiring please help

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rob smith

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"hello. i have a 1975 mercruis

"hello. i have a 1975 mercruiser 888. it's a ford 302 and i had it rebuilt a year ago or so. the engine runs fine in the driveway and in the water in nuetral but it backfires at 1500 rpm only when under load. i've replaced the ignition, plugs, wires and coil. any ideas? thanks for looking."
 
Did it backfire before the reb

Did it backfire before the rebuild or just after the new parts were installed?
It sounds like either a timing problem or two plug wires that are crossed or on the wrong plugs. The Fords usually have a designated plug wire routing to eliminate cross firing. Double and triple check your plug wires to make sure that they are connected to the right plugs and have solid contacts. Sounds simple but a common problem. Is the distributor advance working properly.
Water in the fuel will also cause a backfire. Will it go over 1500 RPM or stop there?
 
"hello cal. thanks for the rep

"hello cal. thanks for the reply. it was a restoration that took four years to complete. the problem started after the rebuild. the entire ignition system is a new msd. new distributor, wires, plugs and coil. it has a mild cam in it and a dual plane intake with an edelbrock 600 carb. the carb and intake were from the old engine. the engine was built by a local racing shop in town with a great reputation. it will only backfire under load and at 1500 rpm. it will not excelerate through that rpm range. i can place it in nuetral and it revs up fine but when placed in gear it backfires. the base timing is 20 degrees and the advance brings it up to 32. it is working as i've double checked the timing and firing order. i've checked for vacume leaks, changed the water fuel seperating filter, cleaned the carb and reset the float level, installed a fuel pressue gauge to ensure proper fuel delivery and checked all eight cylinders for compression with a compression gauge. evrything seems within the normal parameters. this has me stumped. again, thanks for the reply, rob"
 
The firing order for the stand

The firing order for the standard 5.0L is:
1-5-4-2-6-3-7-8.

The firing order for the HO engine which is the PCM Engine. is:
1-3-7-2-6-5-4-8. The rotor inside the distributor rotates counterclockwise on both engines.
 
"Rob, if the firing order that

"Rob, if the firing order that Robert states is what you have set up the only other ideas I have would be a cracked dist cap, leaking intake manifold, maybe a valve floating or just try advancing the timing a notch or two. These situations might be possible with the rebuild.
When I rebuilt my AMC 327 I had to re torque the heads, reset valves and generaly check every nut and bold several times after a running/heat up until it finally settled down.
Cal"
 
the cam i have fires 1-3-7-2-6

the cam i have fires 1-3-7-2-6-5-4-8. i'm thinking i might have bad gas.
 
"hello cal. the whole distribu

"hello cal. the whole distributor is new. the compression test rules out a valv hanging open or bad valve timing. i was also thinking my harmonic balancer might be slipped and my timing, therefore, off. i've had the intake machined flat as it had a slight twist to it. i took it off this afternoon and it appears to have been sealed up perfectly. ( good seal around the intake ports and water jackets). i'm also going to try attaching a seperate ground from the battery to the block to make sure the distributor is getting a good ground."
 
"thanks for the help guys!<[im

"thanks for the help guys!
thumbs_up.gif
"
 
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