120 HP mercruiser 153 ci 2.5 L issues

[tannergallant123]

Hi guys,

I have a 1972 mercruiser 120 Hp 2.5 L boat I decided to restore a couple years ago. The engine got fully rebuilt. Couple things I’ve replaced when it wouldn’t run.

-a new replacement carburator from Amazon (2 barrel) with electric choke
-new mechanical fuel pump
-in-line fuel filter before the carb
-changed oil and oil filter
-distributor had so much play in the shaft that the rotor was hitting the distributor cap so I replaced it with an original one with no play from nla marine
-new cap, plugs, points and gapped correctly
-new starter
-new ignition coil

It’s just been an ongoing thing where one thing breaks down after another and I’m at the point now where I feel like everything is replaced and it still doesn’t run good. It only seems to start up and idle nice when it is too far advanced (turns over rough but when it idles it idles good but seems high). If I try and back off the advance it doesn’t seem to run nice which is why I have it advanced. The issue I’m having now is it will run good (around 30 mph) but when I go to idle it tends to bog out and then it’s hard to start even if it wants to start at all. I thought maybe water got in the gas so I put some sea foam and gas line antifreeze in and the issue doesn’t seem to be fixed. When I got rid of the automatic choke because the new carb has electric there is now a threaded male fitting that doesn’t have anything attached to it. Does this need to be plugged? Do I have a vacuum leak possibly? Running out of thoughts. Also my mechanical fuel pump has 3 ports (one for in one for out and one for a return). I blocked off the return end with a bolt in the tubing. Does anybody have any input on where I should go from here or things to check to get it running? Any help would be awesome.

Thanks

 

[tannergallant123]

Also I’m not sure what to do with the house coming out of the valve cover. It leaks a tiny bit of oil so I don’t want to hook it up to the carburator

 

[ScottinAZ]

I would start by getting rid of the chinesium carburetor. 99% are a poorly made copy of the original and do not function well. Find a good rebuilt original.

the port you have blocked on the pump is the overflow that in the event of a ruptured diaphragm allows the fuel to go into the carburetor throat and not accumulate in your bilge causing a fire hazard. The fitting on the carb for this is part of it being a marine carb, and as its missing, is a HUGE fire hazard. As it sits, in the event of a blown diaphragm you will dump the gas into your engine and thin your oil.

 

[ScottinAZ]

Also I’m not sure what to do with the house coming out of the valve cover. It leaks a tiny bit of oil so I don’t want to hook it up to the carburator

that hose is the breather, and goes to the air cleaner.

 

[tannergallant123]

Okay thank you for the input. So I need to run a fuel line from the fuel pump I have blocked off to the carburator or back to the gas tank? I guess my next point of attack is to buy a rebuilt original carb and see if that fixes the issues I’m having? For the hose breather is it okay the way I way it. The air cleaner doesn’t seem to have anything I can connect the hose to.

 

[ScottinAZ]

there should be a fitting on the air horn of the carb (or nearby) that the line you have blocked off should run to. A good functioning carburetor is the next step.

as for the breather, venting to air is "ok" but not ideal. Your air cleaner (actually just a spark arrestor) should have a provision for one. I will try to get a pic of my slightly newer (1980) 2.5 a bit later this morning, I have to head to work.....

 

[tannergallant123]

That would be awesome thanks a lot.

 

[ScottinAZ]

The first is the fitting where the hose from the pump should go. The second is how the breather attaches to the spark arrestor. Yours may be slightly different, or you could be missing parts. Being older either is possible

 

[tannergallant123]

Ok thanks a lot, when I’m home from work tonight I’ll take a look and send pictures

 

[nola mike]

1. Did you set the dwell, then set timing?
2. Did you check gas for water? If it's significant, the Seafoam and dry gas won't do anything.
3. Did you adjust the mixture on the carb?

 

[tannergallant123]

I did drain the fuel tank 2 months ago and put all new fuel in. And yes set the timing and dwell. In order for it to run I have to advance it to the point where it sounds clunky when starting. And yes we have adjusted the mixtures on the carbs. Spark plugs are quite black so thought there was a potential vaccuum leak?

 

[ScottinAZ]

I did drain the fuel tank 2 months ago and put all new fuel in. And yes set the timing and dwell. In order for it to run I have to advance it to the point where it sounds clunky when starting. And yes we have adjusted the mixtures on the carbs. Spark plugs are quite black so thought there was a potential vaccuum leak?

Black and sooty is excessive fuel. A vacuum leak will have it run lean, which will give whitish tips to the plugs

 

[tannergallant123]

Okay now what would cause excessive fuel? Maybe just the new carburator I bought off Amazon is junk?

 

[tannergallant123]

https://www.amazon.ca/Carburetor-Rochester-Mercruiser-Electric-No-3310-864940A01/dp/B09SYWFHQ4/ref=asc_df_B09SYWFHQ4/?tag=googlemobshop-20&linkCode=df0&hvadid=579292723119&hvpos=&hvnetw=g&hvrand=12052996749058655686&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9000988&hvtargid=pla-1646532910014&psc=1

 

[ScottinAZ]

Okay now what would cause excessive fuel? Maybe just the new carburator I bought off Amazon is junk?

Very likely. Porous castings aren’t unheard of, along with poor workmanship (sealing) the Amazon repops aren’t the best, but the price is good. A reworked oem carb is your best bet

 

[tannergallant123]

Now I don’t know too much about this stuff. If the carb is giving too much fuel if I put smaller jets in would that work? Also would would it cause it to operate poorly after running good for a 5 minute run.

 

[ScottinAZ]

the issue with the Amazon (and other sourced) knockoff carbs is that they are usually poor workmanship and materials. You can try to go with smaller jetting, but that wont help if the castings are leaking, and the gasket surfaces arent flat and true (and therefore dont seal) There can also be issues where they are different enough from the original that you cannot get parts to fit them, so if your needle and seat arent sealing, you may just be out of luck.

I looked at these to replace mine when I was having issue with the accellerator pump, and the general consensus was, get a factory carb, and get that up to snuff. Very few of these carbs are working well out of the box, and frankly the problems you are stating you are having lead me to believe that your carb is the root of the problem. Over/under fueling at different RPM's, black plugs and generally running like dogcrap are all signs of a bad or badly adjusted carburetor.

 

[tannergallant123]

Okay thanks for your help. I took off the old carb and got this new one because of all the other issues including the distributor but now that it is in good shape maybe I’ll go back to the old carb with automatic choke.

 

[ScottinAZ]

with a good rebuild kit (mercrusier preferably) and a little time and knowledge (some of the instructions arent that clear) you will have the better carb. Main thing, take pictures, and get it CLEAN inside. any crap in the passages will cause issues. Time and patience are your freinds.

 

[matt167]

I put Chinese carbs on lawnmowers, where if they quit working, you might have to walk to the garage. They usually work good enough though not perfect and it’s easier than rebuilding a varnished and seized original. I wouldn’t think of putting one on any kind of vehicle you take a distance. Land or sea