Merc 3.7 170 High RPM Hesitation

SeaRayClyde37

Seaman Apprentice
Joined
Jul 26, 2014
Messages
33
THE FAMOUS 460. I have an 86 or 87 Mercruiser 3.7 170 in a Sea Ray. Last summer It ran awesome until the starter burned up. Then the Fuel Pump decided not to work. Since I bought it I have rebuilt the carb, replaced the starter, and replaced the fuel pump with an electric fuel pump. My first issue was the Starter. It smoked so I replaced it. Next the mechanical fuel pump went out. Put an electric one instead, along with rebuilding the carb while I was at it and replacing fuel filters. It doesn't have a syphon valve at the tank pickup. Then soon after points went out. Replaced them. It runs great except for at higher rpms say around 3000. It starts to stumble...the first thing I'm going to do is replace the plugs. After the carb build and before the points it was backfiring a lot and running like crap. Maybe I fouled one before points and timing was set. Any advice after plugs??? Maybe too low of PSI on fuel pump? Guess I should double check that...
 

Bondo

Moderator
Staff member
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Apr 17, 2002
Messages
70,464
Ayuh,.... It's the Infamous 470,.... which uses a few Ford 460 parts,....

It Sounds like a fuel delivery issue,.... It's goin' Lean at higher rpms,...
 

mr 88

Commander
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Nov 3, 2010
Messages
2,115
I would replace the points with a Petronix conversion kit which run around 100.00 . That will eliminate your points from being the problem and improve your starting etc. The plugs are probably not the problem and as mentioned double check the carb. I would start out with replacing the points and take it from there.
 

SeaRayClyde37

Seaman Apprentice
Joined
Jul 26, 2014
Messages
33
Ok I think I'm going to get the 1146a and 40611 flamethrower coil. I'll tie in before the resistor wire for 12v. Should the timing be at 4 TDC like the books says because a lot of people are saying 10 now.
 

SeaRayClyde37

Seaman Apprentice
Joined
Jul 26, 2014
Messages
33
Also noticed it seems to idle high fine. It just leans out in gear at high rpms. Maybe venturi clog huh. Changed the plugs before the lake yesterday nothing changed. I also use a bottle of Seafoam with every tank fill. Just doesn't make sense because I just rebuilt the carb and I performed surgery on that thing made sure everything was completely dust free...
 
Last edited:

salty3rd

Petty Officer 2nd Class
Joined
Jul 13, 2010
Messages
109
I would check all the hoses/fittings between the suction side of the fuel pump and the fuel tank. I have had the same issue and found a bad gasket on the inline fuel/water separator. If you still have problems after you upgrade to the Petronix and check the fuel lines, I would try running off a pony tank to see if their is a fuel tank issue. Good Luck
 

SeaRayClyde37

Seaman Apprentice
Joined
Jul 26, 2014
Messages
33
so I found out what was causing the high rpm hesitation. #4 wasn't even firing. It was the rotor in the cap. The tip is made of two pieces and a rivet holds them together. The rivet popped out so the top tip was floating around. Replaced the rotor along with the new pertronix kit. Everything seems good. For Now. lol
 

SeaRayClyde37

Seaman Apprentice
Joined
Jul 26, 2014
Messages
33
I clipped my timing light to every spark plug wire to make sure I had them and #4 wasn't firing. I thought it was missing a little. duh
 
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