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Rough running 350

Hello I have a merc 350 I got it out for the first time and it ran rough only 3700 rpm's started to pop out of the intake, idles fine starts fine. Rebuilt carb, new rotor and cap, new plugs, marina checked it out great compression said it was bad gas put a cleaner in and said it would be good. Its the same, i can only think it is the timing, which I don't how that would change over the winter or it is the fuel pump. please help I need it
 

mishawakajim

Well-known member
Bummer of a way to start off the season. Just a thought. When was the last time you changed the Fuel Filter? Another thing I always do on my first start up is I put a can of Seafoam in the fuel tank. For $6.99 to $10.99 a can it's one of the best fuel system cleaners I ever used. Best of luck. Jim:thumb:
 
Thank you for your help, yes I have put sea foam in the tank and the marina put in a cleaner. I took out the filter and cleaned it.
 

jumbofordman

Well-known member
Make sure the fuel line is unobstructed from the tank to the carb. We had a 454 that would only spin up to about 3200 rpm and were pulling out our hair trying to figure it out. Finally we pulled the tube from the tank and there was a bunch of crap stuck in the screen at the very top. It was apparently flowing enough gas to get to 3200 rpm and that was it. I've encountered car that behave like this, too. So whenever you have something that runs fine at lower rpms and hits a brick wall at a certain rpm, I always thoroughly investigate the fuel delivery system.
 
I did question myself on that, I have 2 wires crossed on each side of the cap, and I'm wondering if I put them back right. I'm trying to figure this out, how do I know my order, and I'm taking my pickup tube out this morning to see how that looks thanks a lot for the info
 

1954bmw

Active member
Not sure about boat motas, but back in the day, small block Chevy's had the firing order cast into the front of the intake manifold. You should be able to see the timing marks on the front pulley, align those, pull the cap to make sure you are on number one plug and go from there. If the firing order isn't on the manifold, google it, good luck.
 
Thank you pickup tube is clean now that isn't aligning up straight like it did before, this boat is getting the best of me::brickwall:
 

CheckmateFever

Well-known member
I'm gonna say firing order as well! I have a Merc 350 in my Enchanter, i had the exact same issue once years ago, turned out 2 wires were crossed, i heard its a fairly common mistake. Boat idled just fine and would come on plane ok but the more throttle the rougher it would run. Sounds like that's what you have going on! Good Luck!
 

Jimway

Well-known member
Small or big block-18436572. #1 on cap should be at 5 o'clock when looking at cap from front of engine. firing order will continue clockwise from there. 1357 cylinders on port (left) side of engine from front to back, 2468 on starboard (right) side. Like Beamer said, line up timing marks at crank pulley. Pop distributor cap. If rotor is pointing to #1 smile and take sip of available adult beverage. If rotor is pointing at #6, rotate crank another turn and line up marks again, verify order and you are done with that part. Is there a Quadrajet by chance, in the center of the intake manifold?
 
You guys awesome, :cheers: No Holley carb everything on it is built up to the land and sea extension on the lower unit. I will check this out today thank you again
 
I did question myself on that, I have 2 wires crossed on each side of the cap, and I'm wondering if I put them back right. I'm trying to figure this out, how do I know my order, and I'm taking my pickup tube out this morning to see how that looks thanks a lot for the info

If you look at the front of the engine #1 spark plug (for a small block chevy) is the first plug on your right. The right bank is numbered 1,3,5,7 and the left bank is numbered 2,4,6,8. Follow the wire connected to #1 spark plug to the cap and mark it. The firing order for a small block chevy is 1,8,4,3,6,5,7,2 and it should be in your manual. Then go in a clockwise direction from #1 wire, the next post clockwise from #1 wire should go to #8 cylinder. The next post clockwise from #8 cylinder should go to #4 cylinder and so on.
None if this will work if #1 is in the wrong place.
 

Jimway

Well-known member
Just a thought here, a low fuel level in the secondaries on a Holley could cause some grief. I'm going to guess HEI ignition. Mechanical fuel pump? Vacuum secondaries? Another thought is that hooking up a vacuum advance to straight manifold vacuum will cause timing to retard as engine comes under load. Some Holleys have the vacuum advance port and straight manifold vacuum port close together. Q-jets can be tuned, believe it or not.
 

Jimway

Well-known member
Typically no but you will find some here and there. Trying to cover all the bases with the questions. You never know what your going to find or who did what. Seen plenty of rigs with standard starters, water pumps and alternators in there instead of marineized components. Some boats will benefit from vacuum advance, think big engine (no replacement for displacement) and light load. In a jet drive as an example, you can save maybe a gallon of fuel per hour. You might not use a lot of advance and have it start coming off around 8 inches of manifold vacuum. If the power valve is set lower than that then the advance is gone by that time.
 
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