1972 120 mercruiser idles rough unless I give it throttle

[Lewis72]

Hi all, In April I removed my great grandfathers boat from his garage in hopes of getting it running again. It had been put in storage since 1981 when he passed away(19 foot Renken, 1st and only owner). The first thing we did was do a compression test and the highest reading we got was 53 psi. After an new head gasket, valve job, rebuilt carb, all new gaskets on every thing (except oil pan), starter, spark plugs, points & condenser, new hoses, belt, fuel pump, fuel lines, fuel tank, ect we were able to get it to run again. She has an even 130 psi of compression on all 4 cylinders, she's been set to what the mercruiser service manuel said believe it was 8 degrees btdc and the points were gapped. When i get my grandfather to pump the gas, the accelerator pump pumps gas into the carb so i know thats working. What i was going to ask was when i got the fuel pump i got a pump for a 2.5L 153cid chevy 2 since it was much lower in price. I later found out that the fuel pressure for my engine is suppose to be 6 psi. The pump i got is rated at 3.5-4.5 psi. Could this cause my idle problem or am I looking for the wrong thing? Im sure I'm going to get yelled at since the marine pump has a safety line (which my original didn't have) that routes gas to the carb in case of a diaphragm rupture but i didn't know this until later. Sorry this is so long just wanted to give all the info i had.
Thanks a lot!!!

 

[CharlieB]

Re: 1972 120 mercruiser idles rough unless I give it throttle

Yes, you will get yelled at for not using an approved fuel pump. When it leaks you can destroy the boat, killing everyone on board.

With that out of the way, no, the low fuel pressure will not have an effect on idle. What it will do is allow the carb to run low on fuel at WOT, leaning the mixture and melting the pistons.

With new points and plugs your most likely cause of a poor idle is a dirty carb from a combination of evaporated fuel residue and corrosion. Order a carb kit with new gaskets and clean the carb completely.

 

[Lewis72]

Re: 1972 120 mercruiser idles rough unless I give it throttle

I just rebuilt the carb though and soaked it in Macs Cleaner from Napa for 24 hours then blew it out with compressed air. Should I just try the rebuild again. I cleaned the flame arrestor in hopes of helping it breath but even if I take that off it still chokes out in idle. I've set the choke too. Could there be vacuum leaks somewhere? The carb kit I bought came with a diffrent mounting gasket like this..

The one I took off looked like this...

Would this have anything to do with my idle issue?
Thanks again!!!

 

[fishrdan]

Re: 1972 120 mercruiser idles rough unless I give it throttle

Can't see the pics, need to re-post them. To see if the carb base gasket is leaking, spray some carb cleaner around the carb base and see if the idle changes.

I just rebuilt the carb.............

Did you adjust the idle mixture and idle speed screws, you'll probably need to play with them a bit to get it to idle well, mixture-speed-mixture-speed-mixture-etc.. Their final adjustment should be with the engine warmed up, boat in the water, drive in gear.

The fuel pump isn't causing your problem (unless it's just a bad pump) as I ran one of those pumps for several years on my 140HP,,,,,,,, before I knew better and changed it over to a "marine" fuel pump with the sight tube connection. My 72 Mercruiser fuel pump didn't have the sight tube connection either, but it's a safety thing that needs to be upgraded. The correct $100 fuel pump is better than a burned/blown up boat, injured passengers and trips to the hospital.

http://www.azcentral.com/news/articles/20100414boat-explodes-arizona-man-hurt.html

Five injured in Lake Mead boat explosion | Las Vegas Review-Journal

You mention setting the point gap, but did you adjust dwell, with a dwell meter? If not, get a dwell meter and set the dwell to mercruiser's specs, then re-adjust the timing if needed.

Did you replace the cap, rotor and plug wires while working on the ignition?

 

[CharlieB]

Re: 1972 120 mercruiser idles rough unless I give it throttle

When rebuilding any carb you must pay close attention to the float height. Adjustment must be spot on AND you have to check to ensure the needle/seat seals off completely to prevent any flooding. A simple test is to invert the carb and blow into the fuel inlet, you should not be able to push any air in until you turn the carb upright..

Check the throttle shaft fit to the base plate, it should be snug, no side play. If there is side play the shaft and plate are worn and will allow air to suck in, leaning the idle mixture. Either have a machine shop refit the baseplate and shaft or replace them.

 

[Lewis72]

Re: 1972 120 mercruiser idles rough unless I give it throttle

This is the gasket that I took off it...
http://www.carburetor-parts.com/assets/images/g252.jpg
This is what came in the kit and what is on it now... (just the flange gasket in this picture)
http://quadrajetparts.com/images/products/rochester-2bbl--2g/rebuild-kits/Q4030K-w600l.gif
I've messed with the idle mixture screws while on muffs in the drive way and i just find it runs best at the stock spec. which is 1 1/4 turns out from all the way in.
The rotor was replaced but the plug wires weren't and neither was the distributor cap( They showed no signs of corrosion or cracks in the insulation so I just left them alone).
I'm going to buy a new fuel pump, dwell meter, and test for vacuum leaks with the method above and will then report back with my results.

Last question, will the Airtex fuel pump (Part#: 60032) be ok to use? I crossed referenced it on their website and it has the safety line conection? Plus I can pick it up at Oreiley's for 97 bucks instead of a Sierra for like 160.
Thanks again to all who've contributed!!!

 

[CharlieB]

Re: 1972 120 mercruiser idles rough unless I give it throttle

You said when you remove the flame arrester that it still 'chokes out', do you mean the motor acts like it floods and dies? Is any fuel dripping from the boosters? A poor sealing needle/seat or float set too high will flood the motor at an idle yet seem to run well with the throttle opened.

The airtex 'appears' correct but if it does NOT have that vent fitting for a diaphragm leak to route to the carb it will not be considered safe as the factory 86234A05.
Here is one New Marine Carter Style Fuel Pump Replaces Mercury 86234A05 Sierra 18 7278 | eBay

 

[Volphin]

Re: 1972 120 mercruiser idles rough unless I give it throttle

I run a Holley 712-801-1. Great little marine unit.

 

[fishrdan]

Re: 1972 120 mercruiser idles rough unless I give it throttle

My 140's idle mixture screws are close to the stock spec, but idle improved by playing with them. Without looking at the carb rebuild box, (where I wrote down the idle screw positions) I think my idle screws are set at something like 1 1/8 turns out and 1 3/8 on the other. It many need a "little" tweaking.

Last question, will the Airtex fuel pump (Part#: 60032) be ok to use?

Plus I can pick it up at Oreiley's for 97 bucks

That's the right pump, good price too as my Oreiley's sells it for $123...

I agree with Charlie, check to make sure the carb isn't leaking fuel, due to a improperly set float or leaky needle/seat. Fuel pumps that put out too much pressure can cause this too, pushing past the needle seat. You would need to put a low pressure fuel pressure gauge between the pump and carb, to test the fuel pressure.