Poor starter, variable idle

[metcalfej]

I have a 1977 Merc 700 EL (70HP).
It doesn't want to start unless I put some petrol into both carburettors, once started it runs fine at mid and high rev's, however on approaching idle speeds the rev rate gets variable and eventually it cuts out. The engine sometimes restarts okay when warm.

All three cylinders run at 125PSI when warm and 120PSI cold. The sparks seem a little poor, but I am not familiar with the plug type (no electrode) so I don't know whether this is normal. I am using a 50:1 mixture and has used a carb cleaner additive twice with the last two tanks with no effect. There is a airlevel in the inline fuel filter which I can't get rid of it is about 1/3 of the height.

Any suggestions, I would like to avoid an overhaul of the carburettors and fuel pump, without a strong or definate reason

 

[Barnacle_Bill]

Re: Poor starter, variable idle

You just gave some very good reasons for rebuilding them. I would start with the fuel pump. Its the simplest.

 

[tashasdaddy]

Re: Poor starter, variable idle

if it runs fine at mid & high, i'd go for carbs. low speed jets clogged.

 

[studlymandingo]

Re: Poor starter, variable idle

You said that you put petrol in the carbs...There is your problem, you've gotta use gas man!!!

Just kidding! It does sound like the jets. But you mentioned the possibility of a weak spark; you should purchase a spark gap tester. Set it to 7/16", you should have a bright blue spark continuously at each cylinder. If this is not the case, you could have stator or other ignition related issues.

Good Luck!!! Keep us posted.

By the way...Welcome aboard iboats!

 

[metcalfej]

Re: Poor starter, variable idle

Thanks, okay I will get my gaskets ordered and a spark gap tester.

James

 

[metcalfej]

Re: Poor starter, variable idle

So I bought a spark gap tester today, can I just check that I am not going crazy are you really wanting a 7/16" gap between the electrodes, nearly half an inch seems a long way for the spark to travel when it usually only travels a matter or a couple of mm, anyway I tried it and got the predictable result of no spark on any of the cylinders.

The plugs are ones I have never seen before, ones without a bent electrode Champion L76V, I don't even know how they fire, is it against the roof of the piston?

Can you re advise me on the gap I should be using

Thanks, James

 

[Silvertip]

Re: Poor starter, variable idle

Surface gap plugs have no ground electrode. The fire to the rim of the plug. Hence the entire shell of the plug is the electrode. There is no gap.

 

[studlymandingo]

Re: Poor starter, variable idle

Needs to have a STRONG spark jump 7/16".

 

[metcalfej]

Re: Poor starter, variable idle

An update
I was trying the spark gap tester today, and obviously I am thick as I couldn't get it to work at all. I attached one end to the HT lead and the other via a crocodile clip to the engine (ground) I could'nt get a spark on any of the cylinder at any distance, so I tried the inline glow testers and the strangest thing happened, the engine fired up and ran with all three cylinders attached to the glow testers, however there was no lights in any of the testers and when I put my hand close I got a shock, can anyone explain this and give me a thick persons guide to how to do a spark gap test?

Second; my manuals arrived from the US and I thought that I would try to tune the idle mixture on the two carburettors, (as it will take another week for the gaskets to come) as I still can't idle the engine I had to do this with the engine in forward with 900-1000 rev. As I was doing this I noticed something which I think is very important. The top carb is spitting petrol back out of the air intake, whilst the bottom carb is sucking as you would expect. This resolves above 2,000 revs. Is this the reason why the engine won't idle? What is its cause? I still don't fully understand why there is 3 cylinders and two carbs but all three cylinder still have 120-125psi.

Thanks in advance to anyone with advice

 

[studlymandingo]

Re: Poor starter, variable idle

Do you have a DVA voltmeter? You need to measure the resistance of your stator wires. There should be a description of this in your service manual that will tell you the correct readings between each wire. If the readings do not fall within the ohm range listed in the manual, then you will need to replace the stator. I'm not familiar with glow testers, but it may have provided some needed resistance.

On the spark gap tester, you should connect the clip to ground (a bolt on the block is sufficient) the other end should be connected to the spark plug wire, if you are getting no spark with this connection, then you have some sort of issue.

Does the glow tester replace the spark plug wire?

 

[metcalfej]

Re: Poor starter, variable idle

The glow tester is a tube of red plastic about 3 inches long, embedded inside is a LED, you place one end on the end of the plug and the other on the HT lead, fire up the engine, and on car engines the LED blinks with every spark - helps you to work out which cylinder is mis firing.

I do have a multimeter so will do the testing you advise

What do you think about the carb spitting?

 

[metcalfej]

Re: Poor starter, variable idle

[whisper uid=71785]

I was hoping that you could get me somewhere with the carb spitting issue, any chance, much appreciated James

[/whisper]