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Backfiring 50

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patd59

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"Hello, I have a question. My

"Hello, I have a question. My friend has a 1985 200MCM with a 305 motor, 2bbl carb. We replaced the head gaskets recently, and after re-assembling everything, and setting the timing, the motor has a backfire through the carb. Its minor at idle, but under load it gets worse. I've replaced the plugs and wires, cap and rotor, coil, fuel and oil filters, and rebuit the carb, yet still have the same problem. I know that the plug wires are not crossed, and aside from the backfire, the motor actually runs pretty good, and idles smoothly at 600 RPMs. I have the timing set at 6 degrees btdc. I'm starting to get stumped. Can you guys help me? I also want to say that I have a lot of experience with motors in cars, but none whatsoever with boats. I would really appreciate any help. Thanks in advance."
 
Try 8 deg. BTDC timing. Plugs

Try 8 deg. BTDC timing. Plugs should be AC MR44T or Champion RBL8 gapped @ 0.035".

Burned exhaust valve? Check compression.
 
"are you absolutely sure you h

"are you absolutely sure you have the correct firing order with the wires going to the proper place, guessing you have 2 wires on wrong.

the order is 1,8,4,3,6,5,7,2 with 1 at the front(pulley) Port side of the engine and 3,5,7 towards aft on the port side, leaving 2,4,6,8 on starboard. .. right ??

and if you have 1 engine looking down on the distributor the rotor rotates clockwise."
 
"First off, thank you for all

"First off, thank you for all of your help so far, and i'm sorry I haven't had a chance to log on until now.

Doctur- I adjusted the valves by tightening the rocker arm nut until the was no more slack in the pushrod, and then backed off a full turn. It was per spec in the shop manual. I adjusted the valves on my pontiac 400 the same way with no problems.

Guy- I am pretty sure the plugs are NGK's(I'll double check later on), and I know I gapped them at .035. They looked exactly the same as the old ones in size. I did a compression check after the head gaskets were replaced, and they all came out at between 65-70. I tried moving the timing around between 4 and 12 degress BTDC, and still got backfires.

Brian- I am 100% positive that the wires are in the correct places, I even re-checked them this afternoon twice to be sure. I've had that happen to me enough to be extra sure.

I'll also add that the boat ran just fine before. The reason we changed the head gaskets was because my buddy ran the boat withut water for about 2 minutes, and we got milkshake in the crankcase. But we also found that the port side exhaust manifold was leaking water into the numbers 3 and 5 cylinders( I found water on the plugs), so we replaced all the gaskets in it, and changed the oil again afterwords, and it stopped that. I also pulled all the plugs this afternoon to make sure they weren't obstructed by anything, and they all checked out. Do you think this could be a carb issue?"
 
"A backfire as you descibe is

"A backfire as you descibe is an exhaust valve, exhaust lifter or exhaust cam lobe.

Motor will run pefect until you give throttle under load.

I had this exact issue once and two cam lobes were round not egg shaped any longer and when it happened it was the difference between monday it ran fine then tuesday it started to back fire.......

pull the valve covers and watch the rocker arms move.

also many like to adjust the rocker arms when running, loosen till clacking and tighten till the clacking stops and then tighten 1/2 to 3/4 turn for each."
 
""I did a compression chec

""I did a compression check after the head gaskets were replaced, and they all came out at between 65-70."

A little low?"
 
"60-70 is almost a dead engine

"60-70 is almost a dead engine, your description is slightly confusing,static adj. is tightening rocker nut untill you can`t spin the push rod with your fingers,then 3/4 of a turn more.
redo the compression test with carb wide open and a good battery,and all the plugs out."
 
60 - 70 ..................valv

60 - 70 ..................valves are stuck open

adjustment is wrong as suggested.....
 
"I think i understand you but

"I think i understand you but am not sure, if you just took the slack out of the pushrod/rocker that would leave the valves not opening enough, you need to turn the nut in.

BUT If you tightened completely then backed off 1 turn, Meaning to me you bottomed the lifter then backed out 1 turn ?

This will leave the valves open, possibly destroying the lifters and camshaft !!!

that would cause backfire as the intake valve is still open, and confirm your low compression.

You "slowly" tighten the rocker nut while turning the push rod(with your fingers), till it lightly stops turning, then tightin about 3/4-1 turn IN.

You get the rocker and pushrod so there is no up/down slack, then tighten 3/4-1 but mostly they like 3/4 turn.

When you know you have it right and if the engine is fresh and properly rebuilt you should have from 140-160psi depending on the configuration.

Lower than 100 is really not usable and a waste.

good luck"
 
"Thanks for all the input, I r

"Thanks for all the input, I really appreciate it.

Guy, Ghost and BT- I obviously did the compresion check wrong, and i'm not being clear. The 65-70 reading was for a single compression stroke, if that makes sense(which it probably doesn't). I will redo it on Monday or Tuesday and get back to you, and I now officially feel like a moron.

Brain- I did't tighten the nut all the way down, my decription form earlier was not clear. What I did was how you described, tighten until pushrod just stops moving, checking it with my fingers, then tightening a full turn. I dont know why I said I backed off a full turn, I think i'm starting to lose my mind!

Well like I said I'll redo the compression test this week(the right way) and i'll give yall the numbers. I apologize for not being clear. This motor is old, and while I'm not sure exactly how many hours it has, I know its up there. Keep up all the info, yall have been very helpful."
 
"many beleive 1 full turn is t

"many beleive 1 full turn is to tight, try 3/4

the best way to do a compression test is to disconnect 1 side of the coil, remove the spark plugs and turn the engine with the starter watching the needle till it stops moving up."
 
"OK cool. I will back off of t

"OK cool. I will back off of the lifters 1/4 turn, and I'll do the compression test sometime this week. I'll let you guys know the results. Thanks for all the help so far!"
 
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