1973 Mercury 40 4hp: Compression, Spark, Fuel - No Start

[PJG001]

Hello. Title says the jist of it. I recently acquired a '73 Mercury 4hp. Guy I got it from claims he shot some ether in it and it fired though I bought it knowing it wouldn't run and needed carb work. Non-salt water environment. After a rebuild I definitely am not getting any firing. *this is where I note that I am very green and this is new to me but I'm pretty mechanically inclined* Well I've:

• rebuilt the entire water pump
• replaced the fuel and vac lines
• replaced the fuel pump/gaskets
• cleaned the carb and replaced the gaskets
• tested for compression and regularly get high 80s-low 90s
• tested and confirmed spark with existing Champion L78V spark plug (perma gap)
• replaced spark plug with equivalent NGK BUHW-2 (perma gap) and confirmed spark
• tested out an old spark plug I had: Champion J19LM and confirmed spark
• sprayed starting fluid into carb and/or plug hole on several occasions to no avail
• took the flywheel off and gave a visual inspection (though no idea what I'm visually inspecting) and everything looks fine, including the key.

I'm using a drill on the flywheel nut to turn it. Even tried with the lid on. For the life of me it will not start; no sputter, smoke, kick, or anything. Nada. Nuthin. Every little thing I've found online I've tried and nothing is working. I've read that maybe there is an air leak in the cylinder somewhere but with 87-93 lb compression readings one would think that's enough to get a sputter or something to happen. I'm also wondering if even though I'm getting spark, it's not enough spark. I've even read somewhere that a spark on the outside while grounded doesn't mean I'm getting spark while it's screwed in - but I've tested 3 different spark plugs.

Looking for any insights, tips, tricks, and troubleshooting. Thank you in advance.

-pjg

 

[racerone]

Does spark jump a gap of at least 5/16" ( 8mm ) , yes or no?

 

[PJG001]

Does spark jump a gap of at least 5/16" ( 8mm ) , yes or no?

I’m leaning toward ‘no’ with the caveat that with the perma-gap plugs I can’t really tell. I “tested” by placing the plug, in the boot, against the cylinder block and turning the flywheel to visually see a spark. I don’t have one of those spanning test devices.

 

[racerone]

Glue 2 pieces of wire on a piece of cardboard / plywood.----Any well insulated wire with 5/16" gap.-----Stick one end in the plug boot.----The other end grounded on the block.

 

[PJG001]

I live a few miles from a parts place so I just picked up a cheap ignition spark tester. Longest I could get the spark to span was about 1/8" - so does that mean too low of spark to get combustion?

 

[racerone]

Perhaps not enough voltage produced to jump the gap in the sparkplug when air is compressed to 100 PSI.-----A simple fact that many folks can not comprehend !

 

[PJG001]

Well I guess that's the next place to start troubleshooting! I'm about to take off for the weekend and will be away from the motor but I'll still have access to reading this forum and completing research. Would love any insight on diagnosing/repairing or straight up replacing the ignition coil. Thank you racerone and anyone else that may chime in - all input is greatly appreciated.

 

[sunk ship]

Hello. Title says the jist of it. I recently acquired a '73 Mercury 4hp. Guy I got it from claims he shot some ether in it and it fired though I bought it knowing it wouldn't run and needed carb work. Non-salt water environment. After a rebuild I definitely am not getting any firing. *this is where I note that I am very green and this is new to me but I'm pretty mechanically inclined* Well I've:

• rebuilt the entire water pump
• replaced the fuel and vac lines
• replaced the fuel pump/gaskets
• cleaned the carb and replaced the gaskets
• tested for compression and regularly get high 80s-low 90s
• tested and confirmed spark with existing Champion L78V spark plug (perma gap)
• replaced spark plug with equivalent NGK BUHW-2 (perma gap) and confirmed spark
• tested out an old spark plug I had: Champion J19LM and confirmed spark
• sprayed starting fluid into carb and/or plug hole on several occasions to no avail
• took the flywheel off and gave a visual inspection (though no idea what I'm visually inspecting) and everything looks fine, including the key.

I'm using a drill on the flywheel nut to turn it. Even tried with the lid on. For the life of me it will not start; no sputter, smoke, kick, or anything. Nada. Nuthin. Every little thing I've found online I've tried and nothing is working. I've read that maybe there is an air leak in the cylinder somewhere but with 87-93 lb compression readings one would think that's enough to get a sputter or something to happen. I'm also wondering if even though I'm getting spark, it's not enough spark. I've even read somewhere that a spark on the outside while grounded doesn't mean I'm getting spark while it's screwed in - but I've tested 3 different spark plugs.

Looking for any insights, tips, tricks, and troubleshooting. Thank you in advance.

-pjg

I only have automobile knowledge. With boats I'm illiterate. But if it was a car engine I would say check timing, fuel pressure if fuel pump was newer electric, and timing belt, or even something weird like broken camshaft. But, like I said, I don't know anything about boats.

 

[racerone]

What ???------There is no timing belt on a single cylinder 4 HP from 1973.------There is no electric fuel pump !-----There is no camshaft either.

 

[sunk ship]

Like I said, I really don't know anything about boats, sincerely, I was just trying to come up with ideas to help.

 

[PJG001]

Following up to this thread - would LOVE some insights from you pros out there that know about the thunderbolt ignition system and what I could test to continue troubleshooting. I have spark, but not enough. As racerone helped me learn, I can definitely not spanning enough gap when NOT under compression so therefore probably not getting any compression when all put together.

I'd hate to move on from this thing when I'm sooooo close to getting it to go! Please help!

 

[JDusza]

Please provide serial number so we can look up configuration.
That said, compression seems a little low. Try putting a little 2 stroke oil into each cylinder and recheck compression. If number increases, rings are wearing out. Check the condition of your head gasket, may be leaking.
Spark jumping 1/8 seems light. Look to jump 3/8 to 7/16. Not sure of your ignition configuration but you'd like to verify coil input or stator output. Do you have points? Check them for corrosion and regap.
Do not use starting fluid in a 2 stroke. Do not.
Bypass fuel delivery system by spraying premix fuel directly into cylinder. Pull the spark plug, spit fuel into cylinder, replace plug, pull away.
CDI Electronics used to have a good, free troubleshooting guide available on line. I think they changed to a subscription service though. Otherwise, look for YouTube video on a 4 hp. You might recognize your hardware.
J
p.s., (and I'll hear about this) throw out the NGKs. Run with Champions. This year alone NGK failed me 4 times to transfer ample coil energy to spark plugs, (2) Mercury 6 hps, Mercury 9.8, Nissan 9.9. Throw out the NGKs.

 

[racerone]

There is no head gasket on this motor !

 

[JDusza]

Input somehow posted twice. Disregard this entry.
No head gasket? Serial number would help get me to the right parts diagrams.
J

 

[PJG001]

Thanks JDusza-
Serial Number 3657668

What currently existing perma-gap champion sparkplug would you suggest?

I'll test with the premix/oil as suggested

 

[JDusza]

First off I'd say replace your condenser. Clean and regap your contact points.
Stick with Champion L78V. They show on the parts list. Expect them to be design intent.
J

 

[racerone]

The breaker points are very durable on these.----A good thing since new is listed at $117.00-----Condenser ( capacitor ) is listed as NLA on these.

 

[PJG001]

Just ordered that spark plug, should have it tomorrow.
Previously I scraped out some crud from the point and attempted to scuff the contacts with some fine sandpaper. It didn't seem to make a difference. What should this be gapped to? I didn't take anything apart, just used a pick to get the crud. Would it help to add photos to this thread?

 

[JDusza]

Condenser:
Unfortunately, a competitive website.
marineengine.com
I see aftermarket cdi, part number 994-4463 (takes the place of Mercury 336-4463), $55 before shipping.
Must be one helluva condenser @ $55.
J

 

[PJG001]

I have a multi meter that I got free/came with a purchase from harbor freight and I'm not totally sure on how to use it. Is there a way I can test my existing condenser to see functionality?