Mercruiser 120 poor starting

ESGWheel

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Aug 29, 2015
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588
I did test 2 this time. I can try test 1 tomorrow.
Please make sure you ground out the wire from the coil to the distributor when doing test 1 or even better remove the + wire to the coil.

There was more than one option re the gasket in that area so i used the one that was similar to what has been there before.
Matching the gaskets was the right think to do.

More: Also please get a can of Starter Fluid when you can, not sure if available in your area but it’s a can of compressed ether. Here is what I am talking about link. This will assist in follow on testing and is better then trying to dribble gas down the carb.
 

Scott06

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Apr 20, 2014
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6,777
I haven’t read all the posts, but…
has anyone suggested the idle control valve could be the problem? I used to have a boat with a Merc I-O that was having starting/idling issues. Replaced the ICV, about a $40 part and all was good.
If you are talking about an IAC or idle air control valve there isn’t one on this carbed engine
 

Guffer

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Thanks for all your help.

I did test 2 this time. I can try test 1 tomorrow.

Re the float - when I rebuilt the carb I put the float back in as it was. There was more than one option re the gasket in that area so i used the one that was similar to what has been there before. In the pic - the gasket I used is like the one on the left rather than the one on the right.View attachment 407451
Ok, did test 1 and there was plenty of fuel coming out of metal fuel line leading to the carb….
 

nola mike

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5,443
You may or may not have 2 problems, which the parts cannon may or may not have helped. I'd start with spark first. The points system is pretty simple. Ignition on, there should be 12v at the coil pos. Coil neg will be either 12v or 8ish depending on whether the points are open or closed. Disconnect all wires except the points from the neg side. To check the coil you can repeatedly ground a jumper from the neg side and should get a spark from the coil tower (with everything else disconnected from neg) Yes? Good. Reconnect points. Check volts at neg when points are open. 12v means no internal grounding of the points (good). Voltage drop when closed means points are grounding/making contact (good). At this point should have spark from coil tower when cranking. If so then we move on to plugs/cap/rotor
 

ESGWheel

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Messages
588
Great! You are getting gas to the carb!

But looks like the carb is not filling up. The fuel level seems too low. See my blown-up picture of your picture. Note the Accelerator Pump bowl gets fed with the slot down the side of the bowl. I would expect the level to be somewhere around the line I drew.

So now it becomes a matter of why so low.

The float could be too heavy but that would cause it to overfill and flood out. So, I going to discount the float itself as the issue.

The float adjustment could be an issue. When you rebuilt the carb, the kit should have a carboard measuring tool or some why to know the adjustment is correct. This involves holding the top of the carb upside down with the float attached and measuring between the top (or bottom) of the float and the bottom of the carb as indicated in the kit instructions. See this link for some pictures and how to.
  • Did you do this?
Another potential issue is the needle and seat being clogged or somehow not letting fuel in. You should be able to remove the entire needle & seat and pull out the needle and inspect the whole arrangement. Needle should move smoothly in and out of the seat and when you use your mouth to blow into the seat (where the gas would come in) it should unseat the needle and allow air flow. Keep your finger lightly over the needle and work it back and forth a few times (i.e. needle gets blowen out just a bit against your finger and you use your finger to push back in).

Another potential issue is a built-in filter in the carb that is clogged up. I think you checked this already, but best to double check.

Let us know.

Acc Pump Bowl.png
 

Guffer

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You may or may not have 2 problems, which the parts cannon may or may not have helped. I'd start with spark first. The points system is pretty simple. Ignition on, there should be 12v at the coil pos. Coil neg will be either 12v or 8ish depending on whether the points are open or closed. Disconnect all wires except the points from the neg side. To check the coil you can repeatedly ground a jumper from the neg side and should get a spark from the coil tower (with everything else disconnected from neg) Yes? Good. Reconnect points. Check volts at neg when points are open. 12v means no internal grounding of the points (good). Voltage drop when closed means points are grounding/making contact (good). At this point should have spark from coil tower when cranking. If so then we move on to plugs/cap/rotor
Ignition on, there should be 12v at the coil pos. YES
Coil neg will be either 12v or 8ish depending on whether the points are open or closed. YES 12v
Disconnect all wires except the points from the neg side. DONE
To check the coil you can repeatedly ground a jumper from the neg side and should get a spark from the coil tower (with everything else disconnected from neg) Yes? DIDNT FOLLOW. I PUT A SPARK PLUG ON CENTRE LEAD AND GOT SPARK

Good. Reconnect points. POINTS WERE ONE WIRE I LEFT ON NEG SIDE

Check volts at neg when points are open. 12v means no internal grounding of the points (good). GOT THAT
Voltage drop when closed means points are grounding/making contact (good). YES, HAVE VOLTAGE DROP

At this point should have spark from coil tower when cranking. If so then we move on to plugs/cap/rotor
 

Guffer

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Joined
Nov 1, 2024
Messages
30
Great! You are getting gas to the carb!

But looks like the carb is not filling up. The fuel level seems too low. See my blown-up picture of your picture. Note the Accelerator Pump bowl gets fed with the slot down the side of the bowl. I would expect the level to be somewhere around the line I drew.

So now it becomes a matter of why so low.

The float could be too heavy but that would cause it to overfill and flood out. So, I going to discount the float itself as the issue.

The float adjustment could be an issue. When you rebuilt the carb, the kit should have a carboard measuring tool or some why to know the adjustment is correct. This involves holding the top of the carb upside down with the float attached and measuring between the top (or bottom) of the float and the bottom of the carb as indicated in the kit instructions. See this link for some pictures and how to.
  • Did you do this?
Another potential issue is the needle and seat being clogged or somehow not letting fuel in. You should be able to remove the entire needle & seat and pull out the needle and inspect the whole arrangement. Needle should move smoothly in and out of the seat and when you use your mouth to blow into the seat (where the gas would come in) it should unseat the needle and allow air flow. Keep your finger lightly over the needle and work it back and forth a few times (i.e. needle gets blowen out just a bit against your finger and you use your finger to push back in).

Another potential issue is a built-in filter in the carb that is clogged up. I think you checked this already, but best to double check.

Let us know.

View attachment 407465
Thanks for all the help. Much appreciated. Have checked the filter a few times and it’s fine. The needle moves freely but I will re look at this again and check it moves when air is blown in. Re the float adjustment. No i put it back as it was….partly this was because I didn’t find my carb listed but looking again I see a Mercury (Kiekhaefer) Marine 4 cylinder 153 (which maybe the one) and a couple of options - I can pick one!

There is a Power Piston Assembly that seemed to be loose when I took carb apart. Is this meant to lock in somehow? Might this be an issue? #33 in this photo.image.jpg
 

Guffer

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Messages
30
Ignition on, there should be 12v at the coil pos. YES
Coil neg will be either 12v or 8ish depending on whether the points are open or closed. YES 12v
Disconnect all wires except the points from the neg side. DONE
To check the coil you can repeatedly ground a jumper from the neg side and should get a spark from the coil tower (with everything else disconnected from neg) Yes? DIDNT FOLLOW. I PUT A SPARK PLUG ON CENTRE LEAD AND GOT SPARK

Good. Reconnect points. POINTS WERE ONE WIRE I LEFT ON NEG SIDE

Check volts at neg when points are open. 12v means no internal grounding of the points (good). GOT THAT
Voltage drop when closed means points are grounding/making contact (good). YES, HAVE VOLTAGE DROP

At this point should have spark from coil tower when cranking. If so then we move on to plugs/cap/rotor
Re plugs. These are new and I’ve gapped them.
I put a new cap on. Is it possible put this on the wrong way (I took screws out before realising they are different sizes.) Does it matter?
The rotor looks in good health. The one that came with the new Quicksilver distributor cap seems too tight to fit, so haven’t used it.

I will continue on with the fuel issue and then come back to the spark issue, once I know there is fuel to ignite.

Many thanks
 

ESGWheel

Chief Petty Officer
Joined
Aug 29, 2015
Messages
588
There is a Power Piston Assembly that seemed to be loose when I took carb apart. Is this meant to lock in somehow? Might this be an issue? #33 in this photo.
Boy I am exercising my memory cells now. The Power Piston No. 33 in the picture is meant to be ‘floating’ and not locked in. It’s a device that will move up and down in response to differential air pressure vs. the spring pressure. It’s used to enrich the fuel mixture under load. So, it does not get locked in.
 

nola mike

Vice Admiral
Joined
Apr 22, 2009
Messages
5,443
Great! You are getting gas to the carb!

But looks like the carb is not filling up. The fuel level seems too low. See my blown-up picture of your picture. Note the Accelerator Pump bowl gets fed with the slot down the side of the bowl. I would expect the level to be somewhere around the line I drew.

So now it becomes a matter of why so low.

The float could be too heavy but that would cause it to overfill and flood out. So, I going to discount the float itself as the issue.

The float adjustment could be an issue. When you rebuilt the carb, the kit should have a carboard measuring tool or some why to know the adjustment is correct. This involves holding the top of the carb upside down with the float attached and measuring between the top (or bottom) of the float and the bottom of the carb as indicated in the kit instructions. See this link for some pictures and how to.
  • Did you do this?
Another potential issue is the needle and seat being clogged or somehow not letting fuel in. You should be able to remove the entire needle & seat and pull out the needle and inspect the whole arrangement. Needle should move smoothly in and out of the seat and when you use your mouth to blow into the seat (where the gas would come in) it should unseat the needle and allow air flow. Keep your finger lightly over the needle and work it back and forth a few times (i.e. needle gets blowen out just a bit against your finger and you use your finger to push back in).

Another potential issue is a built-in filter in the carb that is clogged up. I think you checked this already, but best to double check.

Let us know.
He's got fuel in the accel pump well, and that's a good amount in the bowl. If the pump squirts fuel, looks good enough to run to me...
Ignition on, there should be 12v at the coil pos. YES
Coil neg will be either 12v or 8ish depending on whether the points are open or closed. YES 12v
Disconnect all wires except the points from the neg side. DONE
To check the coil you can repeatedly ground a jumper from the neg side and should get a spark from the coil tower (with everything else disconnected from neg) Yes? DIDNT FOLLOW. I PUT A SPARK PLUG ON CENTRE LEAD AND GOT SPARK

Good. Reconnect points. POINTS WERE ONE WIRE I LEFT ON NEG SIDE

Check volts at neg when points are open. 12v means no internal grounding of the points (good). GOT THAT
Voltage drop when closed means points are grounding/making contact (good). YES, HAVE VOLTAGE DROP

At this point should have spark from coil tower when cranking. If so then we move on to plugs/cap/rotor
Sorry for the confusion, but looks like you have spark to the coil, great. Are you getting it at the plugs? If so, great, we're done (probably). If not you have a problem with wires/rotor/cap. At some point if it doesn't work you'll need to assess the strength of the spark.
 

Guffer

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Messages
30
He's got fuel in the accel pump well, and that's a good amount in the bowl. If the pump squirts fuel, looks good enough to run to me...

Sorry for the confusion, but looks like you have spark to the coil, great. Are you getting it at the plugs? If so, great, we're done (probably). If not you have a problem with wires/rotor/cap. At some point if it doesn't work you'll need to assess the strength of the spark.
Thanks very much. Really appreciate the help. Spark wasn’t coming though to the plugs. I will relook at cap and rotor to see if there is anything obviously wrong. Wires are new. I have cleaned and gapped the points, is there anything else that might need cleaning under distributors cap? Will get something to asses the strength of the spark. If this is an issue what might cause it? Cheers
 

ESGWheel

Chief Petty Officer
Joined
Aug 29, 2015
Messages
588
He's got fuel in the accel pump well, and that's a good amount in the bowl. If the pump squirts fuel, looks good enough to run to me...
The test he conducted was to look for a squirt of the accelerator pump post cranking the engine, and he saw none. Hence the next step of validating getting fuel to the carb and then he took it to the next step to pull off the top. It’s been decades since I have been on the inside of a carb so I could be remembering wrong about the level or in my mind’s eye I was thinking what the level is with the float displacement (something I played around with back in the day….).

Regardless, no squirt and he also said that the float was not adjusted when he rebuilt it which all adds up IMO as needing to validate all is good on the fuel end. As you mentioned he has been applying the parts cannon, so I have suggested a singular focus on fuel until proven good since OP started with a ‘fuel issue’.

Bottom line is we are all pulling for the OP and I know with our collective help he will get this running. That is what this forum is all about! :)
 

nola mike

Vice Admiral
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Messages
5,443
The test he conducted was to look for a squirt of the accelerator pump post cranking the engine, and he saw none.
Missed that part. Still, I'd focus first on the certainty of no spark than the who knows of the fuel at this point. But yeah, for sure check the float drop if you have the top off.

OP, if you have a snappy blue spark on a plug at the end of the coil tower wire, that should be enough to fire your plugs (again, ballpark).
That means your problem is with your cap, rotor, or all 4 plug wires/plugs (unlikely). I think you mentioned replacing the cap but not rotor, or vice versa. Just replace both.
 

Guffer

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Messages
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Not making much progress :-(

I have relooked at the carb and adjusted slightly but nothing major.

Ive used an in line spark tester and there is some spark from center of coil but it doesn’t look that strong.
I tried new cap and rotor, different battery but no luck.

Not quite sure what to try next.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Messages
50,313
Not making much progress :-(

I have relooked at the carb and adjusted slightly but nothing major.

Ive used an in line spark tester and there is some spark from center of coil but it doesn’t look that strong.
I tried new cap and rotor, different battery but no luck.

Not quite sure what to try next.
did not see anywhere where you serviced the points

file them, rough gap to .016, set dwell to 32 degrees, then set timing to 4 BTDC
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Messages
50,313
I haven’t read all the posts, but…
has anyone suggested the idle control valve could be the problem? I used to have a boat with a Merc I-O that was having starting/idling issues. Replaced the ICV, about a $40 part and all was good.
there is no IAC on a 3.0 liter carbureted motor
 

Guffer

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Nov 1, 2024
Messages
30
did not see anywhere where you serviced the points

file them, rough gap to .016, set dwell to 32 degrees, then set timing to 4 BTDC
Thanks. I cleaned the points and gapped them but think it was more than .016. Will file, re gap and set dwell. Thought timing should be 8BTDC?
 
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