Mercury 3.5 hp stops when hot

CAPTAIN FANTASTIC

CAPTAIN FANTASTIC

Well-Known Member
Joined
15 Jul 2009
Messages
3,311
Location
Bristol Channel
Visit site
My one year old Mercury (Tohatsu) 3.5 hp four stroke have been working very well until now. Yesterday started as usual on the second pull and after running for half an hour it stopped suddenly as if it was an electrical cut out. Then after about 20 minutes when it cooled down, it started again on the second pull. It has been doing this for the last two days systematically.

It will run for 20 to 30 minutes without any misfiring. I checked the air vent, there is no blockage there. Unfortunately, i do not have the manual with me, however, it is a very basic engine, so it does not have any complex systems.

Have you had similar problems? any tips please?
 
Be doubly sure the vent is clear or take the cap perhaps.

Check that you have a good flow of fuel to the carb ... check filters. Not sure without checking but I expect there is a filter in the tank outlet/ fuel tap.

Next time it stops check for a spark immediately

No spark probably means defective CDi or HT coil but check other obvious things like HT and stop button

Good spark probably means a fuel flow issue

MY money would be on a defective CDi. Mine ( E'rude) failed totally after one year.
 
Last edited:
VicS said:
Be doubly sure the vent is clear or take the cap perhaps.

Check that you have a good flow of fuel to the carb ... check filters. Not sure without checking but I expect there is a filter in the tank outlet/ fuel tap.

Next time it stops check for a spark immediately

No spark probably means defective CDi or HT coil but check other obvious things like HT and stop button

Good spark probably means a fuel flow issue

MY money would be on a defective CDi. Mine ( E'rude) failed totally after one year.
Click to expand...

thanks VicS; I think you are right; it is likely to be cdi problem. i will test run it and start the process of elimination until i find the fault.
 
Check wiring to the kill switch first, it may be something cheap. Mine had a loose wire at the back, previous owner had installed a kill switch on the end of an extended tiller [brilliant by the way] solder joint was poor and made intermittent contact.
 
I had the same issue

I bought a second hand version, worked great apart from first longer, 30mins jaunt. Changed spark plug (was fouled) and perfect since then. Pretty easy to do so worth a go for you.
 
vyv_cox said:
Does it still turn over when it cuts out? Overheating of the piston can cause seizure in the bore, which releases when it cools down. More of a problem with two-strokes but might be worth checking.
Click to expand...

Turns over without problem; it just cuts out suddenly as if it was an electrical problem. I have test run it in the garden today and it cut out again after half an hour; i tested the plug and there was spark; it started again after 15 minutes on the first pull and run smooth like a clock as usual; must be an electrical fault, either coil/cdi
 
I have a Tohatsu 3.5, same engine, and I know it uses the same spark plug as my Honda lawnmower. The latter developed the identical problem you describe. I eventually found on an internet forum that this was a common problem using this particulat spark plug, cured by a new plug even though the old one appears to work OK when tested.
Try a new plug?
 
CAPTAIN FANTASTIC said:
Turns over without problem; it just cuts out suddenly as if it was an electrical problem. I have test run it in the garden today and it cut out again after half an hour; i tested the plug and there was spark; it started again after 15 minutes on the first pull and run smooth like a clock as usual; must be an electrical fault, either coil/cdi
Click to expand...

If you can good strong blue spark capable of jumping 3/8" from the plug lead its not an electrical/ ignition problem.
The plug perhaps but presumably you have tried a new one ?
 
would not jump more than 1/16th.
Click to expand...
Thats no good!
If that is all you can get from the end of the lead then there is a problem with the ignition system. Everything seems to be rolled into one module which they call the "Igniter" !
 
VicS said:
If you can good strong blue spark capable of jumping 3/8" from the plug lead its not an electrical/ ignition problem.
The plug perhaps but presumably you have tried a new one ?
Click to expand...

How do you do this?I assume you remove the lead from the plug and jam a piece of wire into the lead contact.Then position the wire 3/8 in from the cylinder head.Or do you remove the plug with lead attached and touch it against the cylinder head and observe the spark at plug electrodes.Is there any piece of equipment (aka timer strobe light) to test the spark?
 
Billows said:
How do you do this?I assume you remove the lead from the plug and jam a piece of wire into the lead contact.Then position the wire 3/8 in from the cylinder head.Or do you remove the plug with lead attached and touch it against the cylinder head and observe the spark at plug electrodes.Is there any piece of equipment (aka timer strobe light) to test the spark?
Click to expand...

I pulled the HT lead off the spark plug and put a drill bit into the lead socket; then placed it close to an engine earth point.
 
Billows said:
How do you do this?I assume you remove the lead from the plug and jam a piece of wire into the lead contact.Then position the wire 3/8 in from the cylinder head.Or do you remove the plug with lead attached and touch it against the cylinder head and observe the spark at plug electrodes.Is there any piece of equipment (aka timer strobe light) to test the spark?
Click to expand...

Something like that.

You can buy spark testers or you can make one with bent nails in a piece of (dry) wood. Adjust the gap between the bent nails to whats required . Attach the plug lead to one and earth the other to the engine.
 
Billows said:
How do you do this?I assume you remove the lead from the plug and jam a piece of wire into the lead contact.Then position the wire 3/8 in from the cylinder head.Or do you remove the plug with lead attached and touch it against the cylinder head and observe the spark at plug electrodes.Is there any piece of equipment (aka timer strobe light) to test the spark?
Click to expand...

I have a tool to assist, it basically 2 arms with a pivot point at the top, one arm has contact and a thing that goes in the spark plug lead, the other a contact and earthing point. You can adjust the gap between the contacts and spark jumps between them. Best not done with fuel vapour around and probably illegal now. Strong spark across 10mm gap on my 8HP Mercury.
 
VicS said:
Something like that.

You can buy spark testers or you can make one with bent nails in a piece of (dry) wood. Adjust the gap between the bent nails to whats required . Attach the plug lead to one and earth the other to the engine.
Click to expand...
Bent nails. How heathen.

My home made tester was infinitely adjustable


Sparktester.jpg
 
dustywings said:
Very late to this but am experiencing the same problem with my Mariner 3.5, did you get to the bottom of the problem?
Click to expand...

Yes, I took it to Mercury (under guarantee) and they did a service, they also run it up and down the river Hamble to test it out. I then put it on my inflatable (in West Wales!!) and it cut out when hot again (against 6 knots tide); I was not amused. I took it back to the Mercury dealer in Hamble and he agreed to change the Ignition module; I have since used it a lot, last year and this year without any problems. If you experience similar problems, I suggest that you insist they change the ignition module too.
 
You must log in or register to reply here.

Share this page

Top