Mercruiser 120 poor starting

[Gorr001]

Hi there, I recently had head worked on to improve compression on cylinder 4. On putting it back together it, it’s starting but petering out after 30 seconds.

It’s had new spark plugs, fresh oil and filter, fuel pump filter changed. Fuel is only a few months old with stabiliser.

I’ve not touched the carburettor as it was running fine before it was winterised.

Any thoughts on what i should look at would be great. Thx!

 

[Bondo]

Hi there, I recently had head worked on to improve compression on cylinder 4. On putting it back together it, it’s starting but petering out after 30 seconds.

It’s had new spark plugs, fresh oil and filter, fuel pump filter changed. Fuel is only a few months old with stabiliser.

I’ve not touched the carburettor as it was running fine before it was winterised.

Any thoughts on what i should look at would be great. Thx!

Ayuh,..... Welcome Aboard,...... It sounds like it's running outa gas, so start with the carb,....

 

[Scott06]

Hi there, I recently had head worked on to improve compression on cylinder 4. On putting it back together it, it’s starting but petering out after 30 seconds.

It’s had new spark plugs, fresh oil and filter, fuel pump filter changed. Fuel is only a few months old with stabiliser.

I’ve not touched the carburettor as it was running fine before it was winterised.

Any thoughts on what i should look at would be great. Thx!

Would seem based on this it is not getting fuel is fuel getting to the carb. If it is dig into the carb. I recently put a 4 bbl edlebrock back on my 65 pontiac. It ran perfectly when I removed it two years ago. I inverted the carb to get the gas emptied out of the bowls, put it back in the box on the shelf. stab the throttle to set the choke it would run for a few seconds then die , stab the gas repeat.. took the car apart and cleaned it works perfectly now.…

 

[airshot]

Need to start with basics and trouble shoot....compression test results ? Spark plug strength test..? Fresh fuel, not months old stuff, filters clean? Carb clean..? Actual factory rebuild kit for the carb, not just spraying some carb cleaner here and there. All guesses without actual numbers and tests.

 

[Gorr001]

Many thanks for all the responses. I will go again tomorrow and check everything, filters and plugs are new so should be good.

I have a 2nd carb i have just rebuilt it after a professional clean up. I might try to see if i can get my hands on a float pin (or is there an alternative?) and see if this makes a difference. That’s the only thing i appear to be missing.

Fuel tank is old (may be 40 years) so expect it has is rust/ gunk and that may be the problem/part of the problem. Trying to work out if it’s worth cleaning it or put in a new plastic one.

Currently there is the fuel pump filter and an inline Sedimenter but no water separating fuel filter. If i go plastic tank then maybe i can install rubber house and extra filter before fuel pump.

Thanks again

 

[airshot]

Your on the right track, new plugs does not mean they are firing hot....a nice blue spark that jumps an 1/8" or greater gap is needed. Check cap and rotor for cracks or carbon tracking. Steel fuel tanks that old are a definite problem and need changed out. Water separating filter is a must !!
Compression test is needed or your wasting your time.

 

[ESGWheel]

Post checking if good compression and spark etc. from above, and still have the 30 sec. run then stall, check this: post stalling, take spark arrester off so can see down the carb throat. Then pump open the throttle fully in a rapid motion while looking down the throat. Should see a stream of gas from the carb’s accelerator pump. No gas? Bowl is empty. Now with the coil wire removed crank the engine for a bit and then redo that accelerator pump test. If you get a good squirt, then the issue is intermit / poor fuel delivery. And if no squirt, fuel delivery issue.

I believe this engine has a mechanical pump that is subject to contamination and age. An inline fuel pressure gauge T’ed off between the pump and carb will tell you what is going on: if pressure is good while cranking then and subsequent short run period then it’s the carb. Else it’s the pump or upstream components (anti-siphon valve as an example).

One option is to cobble together an electrical fuel pump sucking from a nice fresh can of gas. Make sure to pull the inlet hose fm the boat tank to the fuel pump and plug it up! If it runs nice, its fuel delivery.