Engine won’t start

Crannoid · 27 April 2021, 20:22

Hi,

I’ve got a 1980ish 7.5hp Mercury that won’t start.

The engine appears to have been well looked after. The carburettor is clean, new spark plugs that spark and fuel seems to be getting to the plugs. But it’s absolutely dead, dozen of pulls on the starter cord and nothing. Tried easy start, and and half a dozen pulls afterwards a back fire, but that’s it.

Any one got any ideas?

Thanks

Poly · 27 April 2021, 20:40

Quote:

Originally Posted by Crannoid

The engine appears to have been well looked after.

Is this new to you? Have you ever seen it running? Sorry if I'm giving you basics here but to make an engine run you need:

Air - Fuel - Compression - Spark

to run well it will need the right amount of those, but to run at all they need to happen at the right time. You've covered fuel and spark - so its seems like either a compression issue or a timing issue.

Davie · 27 April 2021, 21:31

Swap the plug leads at the coils before you try anything else.

Maximus · 28 April 2021, 06:14

Quote:

Originally Posted by Davie

Swap the plug leads at the coils before you try anything else.

X2!!

Crannoid · 28 April 2021, 07:23

Thanks for the replies

Quote:

Originally Posted by Davie

Swap the plug leads at the coils before you try anything else.

That came to me yesterday, didn't seem to have an impact

I haven't seen the engine run (It was a gift)

As I say the engine looks like it's really been looked after. It's clean and there is new grease in the important places. Seems an odd thing to do if the engine doesn't run.

I will explore the timing and compression. Going to be a learning curve!

The only other thing I have noticed is a there appears two wires coming from the ignition system under the flywheel to a voltage regulator, then from the regulator to a two-pronged plug on the front of the engine casing near the choke. I assume this might be a electrical output rather than an old kill cord. I will upload pictures later today.

Ribtecer · 28 April 2021, 08:08

I had the 9.8 version, that plug at the front is nothing to do with a kill cord.

Has it had a kill cord fitted? If so I'd start looking there. If not is the off button stuck?

I wouldn't mess about with the timing just yet. Rule out all the simple stuff 1st.

As you pull it over get some one to dab a bunched up rag over the carb air intake in a sort of on off type motion. That has worked for me to drag fuel thru.

jeffstevens763@g · 28 April 2021, 10:10

new fuel? all the old flushed out?

Crannoid · 28 April 2021, 10:43

No kill cord. Kill switch seems to click, have been thinking of disconecting it to rule it out. but need a plan to stop the engine! Probabaly muffle the air intake

Yep new fuel has been flushed.

It's a head scratcher

jeffstevens763@g · 28 April 2021, 11:06

just had lawn mower wouldn't start got spark etc. turned out duff spark plug

neil.mccrirrick · 28 April 2021, 11:09

Take a plug out and put your finger over the hole to see if you have at least some compression. Old trick I used on bikes that refused to start is heat the plugs up in your oven then pop them in. A hot plug will fire more easily than a cold one

Matt · 28 April 2021, 11:40

Pull the plugs, spin it over a bit to dry the cylinders out, then put a squirt of 50:1 into the cylinders, pop the plugs back in and see if it fires.

Quite often the fuel residue in the carbs turns to crap and that's what you first get through.

On my 5, I rebuilt the carb.
On my 2, there was a drain screw I could access on the bottom of the carb. That and a bit of patience until I'd got enough fresh fuel through the pipes, carb & motor for there to be fresh stuff in the right place so the motor would run.

If you see a spark, it'll likely be something simple, so don't embark on major surgery like some people do.

Crannoid · 30 April 2021, 07:40

Compression seems good, as does suction from the carburettor port.

I purchased an adjustable spark tester and can only get a spark of a 2 -3 mm. So i'm thinking it's in the ignition system.

Did a bit of investigating and found this. Looks like a previous repair

Matt · 30 April 2021, 09:32

Even if it ran, that's certainly not something you'd want on a motor you're going to want to use.

Ribtecer · 30 April 2021, 12:21

^^^ I agree with that 100%

Ebay is your answer.

People break engines all the time, just keep looking for the part you want.

Limecc · 30 April 2021, 13:52

Quote:

Originally Posted by Crannoid

Compression seems good, as does suction from the carburettor port.

I purchased an adjustable spark tester and can only get a spark of a 2 -3 mm. So i'm thinking it's in the ignition system.

Did a bit of investigating and found this. Looks like a previous repair

Until you find a replacement part on ebay it may be possible to do a better repair. A sharp soldering iron bit can remove the potting compound, solder on a decent wire then encapsulate with Araldite.

Crannoid · 02 May 2021, 15:36

I found this video

https://www.youtube.com/watch?v=MDbv...mmeN9&index=19

and will check the whole ignition system.

I might look at a repair to the cable. Don't think i can make it any worse!

Matt · 03 May 2021, 09:22

As a bit of fun, worth a go.

However, I've never managed to effect a "proper" repair I'd trust - the issue is that bit of corrosion you see actually goes in deeper, so even with removing a bit of resin with a dremel, blade, etc, you're unlikely to get a a good solderable bit of wire. I suppose if you are super keen you could dremel all the way to the board and resolder new wires entirely - my patience never lasts that long though.