Mercruiser 120 poor starting

[Guffer]

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.

 

[Guffer]

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.

 

[Guffer]

Many thanks All, compression is 150 across the 4 cylinders and there is good spark on the plugs. It ran for a little longer yesterday then died. I concluded it was the carb and will clean/ rebuild this. In taking the carb of i found it was dry, as was the pump and sedimentor….

I had opened the inspection hatch on the 80 gallon petrol tank and used a camera to see inside and it looked surprisingly clean. However when i checked through a small opening next to pick up there appears to be either dirt/ corrosion at the bottom of the pick up. Assume i now need to empty the tank and clean it or replace the pick up and suck up the dirt? I haven’t found a new pickup for a metal petrol tank. Any thoughts on how to manufacture this and how best to clean the metal pick-up would be great. Many thanks!

 

[ESGWheel]

Guffer, if I may, I am going to suggest a different approach but will address a possible ‘cleaning’ method at the end for your pickup tube.

As you are aware, troubleshooting is a process of elimination. If not this, then go after that based on a logical sequence and tests along the way. While I am not there and only can read what you post, it seems to me the logical sequence is being missed. Let me explain.

You suspected Carb issue but only to find out was dry. While you subsequently inspected (but not tested) the upstream components, you found some crud around the pickup tube and assume that is the issue. While it may be the root cause, the logical sequence of continuing to backtrack and test from the carb was not fully done. For example: while the fuel pump visually inspected OK, does it really work? There are check valves in that pump that may be faulty and would not know with a visual only. So, to help avoid frustration, suggest continuing to follow the issue upstream one step at a time. Here is what I mean > upstream from your carb are the following:

• Note I am not sure if all these exist but outlining them regardless.
• Fine Mesh Filter in the carb body at the inlet of the gas line > if plugged it can starve the carb
• The Fuel Line to the separator > while not likely, a rubber line can separate and collapse internally blocking flow
• The Separator > your visual of this validates its good, but could always bypass it to be sure
• Fuel Pump > internal check values that can fail, diaphragm can be ripped
• More Fuel Line
• Anti-Siphon Valve > this can get stuck closed precluding sucking fuel
• The Tank Pick Up Tube > as you have already seen, some gunk on the end of it can plug it up.

Here is a suggestion that could be easy to do now you have access to the tank > apply a new rubber line to the inlet of the pump and drop the other end in the tank and try to start / run. Also suggest having some way to keep the hose in place in the tank access port and the fumes down. It may take more cranking to get out all the air of the line & fill the carb. If it runs (and stays running) then you know fuel pump is good and all the upstream components and it’s something still downstream like the Anti-Siphon Valve or that pickup tube.

To determine which, assuming you can, replace the Anti-Siphon Valve with an appropriate adaptor and retest or simply replace it, they are not that expensive.

Finally, if still need or want to get rid of the junk at the end of the pickup tube (assuming it cannot be removed), with the Anti-Siphon Valve removed and an appropriate adaptor in its place and a hose connected to it, use very low air pressure (3 to 5 PSI) to ‘blast’ away the debris. You may be able to suck up thru the access port any large chunks floating around with something like a turkey baster.

 

[Bondo]

I had opened the inspection hatch on the 80 gallon petrol tank and used a camera to see inside and it looked surprisingly clean. However when i checked through a small opening next to pick up there appears to be either dirt/ corrosion at the bottom of the pick up. Assume i now need to empty the tank and clean it or replace the pick up and suck up the dirt? I haven’t found a new pickup for a metal petrol tank. Any thoughts on how to manufacture this and how best to clean the metal pick-up would be great.

Ayuh,.... Doesn't the pickup tube unscrew from the tank,..??
If so, remove it, 'n clean it,.....
If there's a screen on the end of it, remove it, then make sure the fuel goes through a quality filter, before the fuel pump/ carb,....
No reason to remove the tank 'n clean it, if it's as clean as you say,....

 

[Guffer]

Guffer, if I may, I am going to suggest a different approach but will address a possible ‘cleaning’ method at the end for your pickup tube.

As you are aware, troubleshooting is a process of elimination. If not this, then go after that based on a logical sequence and tests along the way. While I am not there and only can read what you post, it seems to me the logical sequence is being missed. Let me explain.

You suspected Carb issue but only to find out was dry. While you subsequently inspected (but not tested) the upstream components, you found some crud around the pickup tube and assume that is the issue. While it may be the root cause, the logical sequence of continuing to backtrack and test from the carb was not fully done. For example: while the fuel pump visually inspected OK, does it really work? There are check valves in that pump that may be faulty and would not know with a visual only. So, to help avoid frustration, suggest continuing to follow the issue upstream one step at a time. Here is what I mean > upstream from your carb are the following:

• Note I am not sure if all these exist but outlining them regardless.
• Fine Mesh Filter in the carb body at the inlet of the gas line > if plugged it can starve the carb
• The Fuel Line to the separator > while not likely, a rubber line can separate and collapse internally blocking flow
• The Separator > your visual of this validates its good, but could always bypass it to be sure
• Fuel Pump > internal check values that can fail, diaphragm can be ripped
• More Fuel Line
• Anti-Siphon Valve > this can get stuck closed precluding sucking fuel
• The Tank Pick Up Tube > as you have already seen, some gunk on the end of it can plug it up.

Here is a suggestion that could be easy to do now you have access to the tank > apply a new rubber line to the inlet of the pump and drop the other end in the tank and try to start / run. Also suggest having some way to keep the hose in place in the tank access port and the fumes down. It may take more cranking to get out all the air of the line & fill the carb. If it runs (and stays running) then you know fuel pump is good and all the upstream components and it’s something still downstream like the Anti-Siphon Valve or that pickup tube.

To determine which, assuming you can, replace the Anti-Siphon Valve with an appropriate adaptor and retest or simply replace it, they are not that expensive.

Finally, if still need or want to get rid of the junk at the end of the pickup tube (assuming it cannot be removed), with the Anti-Siphon Valve removed and an appropriate adaptor in its place and a hose connected to it, use very low air pressure (3 to 5 PSI) to ‘blast’ away the debris. You may be able to suck up thru the access port any large chunks floating around with something like a turkey baster.

Thank you ESG Wheel for your comprehensive reply. This is my first year with an inboard engine so it all a bit new. If there is no fuel in the filter / sedimentor does that not suggest a blockage in the tank or fuel line that goes from tank to filter/ sedimentor?

- Fine mesh valve in carb is clean
- Fuel filter in fuel pump is clean
- Line from tank to sedimentor is metal
- Anti siphon valve - unsure what this is or if i have one but will look at this
- re fuel pump - will look to see other tests / checks i can do

I like the idea if doing a bypass test. This should be easy enough.

Final question re this issue - could it be an airlock that needs to be unnlocked?

Many thanks

 

[ESGWheel]

You’re welcome and before we get going, I am glad you are taking advantage of this forum. There are a lot of great contributors here that are willing to help. The key is your feedback and not giving up

Frankly your issue is, IMO, a simple one > a lack of fuel delivery. More: there are generally 3 basic ingredients for a gas engine to start: gas, spark and compression. While other things matter (example ign timing) these are the basics which is why folks were looking for compression numbers and spark.

Onto your issue. I am suggesting its an easy one to solve because fuel delivery to a carb engine is straight forward and does not involve a lot of sensors or sophisticated electronics.

Let’s review the basics. Look at the first picture which is a basic diagram of the fuel system. The 2nd is a closeup of the fuel pickup with an Anti-Siphon Valve.

An Anti-Siphon Valve is a one-way check valve that will allow fuel to flow out but not back into the tank. The reason is that boats will sit weeks before being started again and all the fuel in the line will gravity feed back into the tank. This in turn causes a lot of cranking / delay in getting fuel back up to the engine. In other words, it keeps the entire fuel line full of gas for the next start. This simple ball and spring type valve can easily get stuck or contaminated. Unfortunately, they are not always easy to remove. A combination of age & sealant causes them to be stuck and if you do not support the fuel pickup tube it is screwed into you can easily damage the mounting for the fuel pickup tube, i.e. damage the fuel tank such it that is needs to be replaced. Ouch!

More: your pickup tube should be removable from the tank. Based on your posts I assumed it was not readily accessible or removable which is why I provided the advice of the gentle blowing it clean and why Bondo is wondering why not just pull it out and be done with it….

This is where posted pictures and/or videos are a great help. Can you post some pics of your fuel tank and pickup tube?

Is the fuel line from the separator to the inlet of the fuel pump a rubber hose? If so, the bypass is easy and as suggested get a long enough rubber hose to go from the pump inlet to the fuel tank and see what happens.

As to your questions:
If there is no fuel in the filter / sedimentor does that not suggest a blockage in the tank or fuel line that goes from tank to filter/ sedimentor?

• Not necessarily. If the fuel pump is not working, there will be no fuel anywhere.

Could it be an airlock that needs to be unnlocked?

• Not really. While vapor lock is a real thing it involves different dynamics like a metal fuel line that that gets too hot and the fuel boils. What is possible is to ‘suck air’ due to a hole / leak in the fuel line to the pump and that will seem like an ‘airlock’ but is really just a suction leak.

Here is a fact: your issue is solvable, and I believe you can do it! Boating can be so much fun and frustrating as well. But its only frustrating if you let it. While you can toss money at the problem and have someone else solve the issue, I think those folks are missing out on part of the fun and enjoyment of solving issues and overcoming challenges.

Please pardon my long-winded posts but as the saying goes, I try to teach folks to fish so they can feed themselves….

Keep posting your findings and questions and do not give up!

 

[Guffer]

Super helpful. I have no access to the boat this week but will get back early next week and I’ll work through each step. Many thanks!

 

[Guffer]

Back to the boat today, and put on rebuilt carb. Someone suggested putting fuel directly to carb. Tried this a few times and no better than before.

Pulled out sender unit from tank. No blockage there.

This is leading me to think that next i should be looking at the fuel pump?

 

[Guffer]

Working back from fuel tank to pump, no blockages, including the sedimentor. Will attach a fuel line next time (one of the compression joints needs sealant to make it air tight).

 

[Scott06]

Working back from fuel tank to pump, no blockages, including the sedimentor. Will attach a fuel line next time (one of the compression joints needs sealant to make it air tight).View attachment 406747

Dump some gas down carb bores and it should fire up if not it’s not a fuel issue.

You can take fuel line off carb crank it over and should pump into a jug .

 

[Guffer]

It fires up if i put fuel directly into carb. Not pumping when i take fuel line off carb though. How best to add a temporary line before the fuel pump?
Many thanks.

 

[Bondo]

It fires up if i put fuel directly into carb. Not pumping when i take fuel line off carb though. How best to add a temporary line before the fuel pump?

Ayuh,...... I believe, if you take the copper line off the fuel pump, you can slip a rubber hose onto the steel fitting at the fuel pump body,.....
Have you cleaned the screen/ filter under that cover on the fuel pump,..??

 

[cyclops222]

Who winterized the boat ? You as usual ? Or some other place ?
Did you have any hard freezing winter weather ? What state was the boat stored in ? Really cold winter in N J. Did you refill your gas tank after the engine repair ?

 

[Guffer]

Ayuh,...... I believe, if you take the copper line off the fuel pump, you can slip a rubber hose onto the steel fitting at the fuel pump body,.....
Have you cleaned the screen/ filter under that cover on the fuel pump,..??

Yes, filter in the fuel pump is new. Will see if I can get rubber line on to copper fuel pipe. Many thanks