1968 40HP Evinrude Big Twin electric start carb problem

[andreipou]

I re-tested it just now using compression test kit.
It is 120 on both cylinders.
Good compression was the reason I re-built this engine in the first place.

 

[newhamburgboating]

I still think it's your low speed needle. Loosen the packing nut so it almost falls out. Then tighten the high speed needle in gently until it snugs. Back it out 1-1/2 turns then tighten your packing nut back down. This is the general rule of thumb for starting position. Once you run the motor and have it fully warmed up you can lean that needle down for a smoother idle.

Also, have you removed the hot air choke on the left side of the carb? The spring in inside should be wound enough that it keeps enough pressure on the carb butterfly to keep it closed when off and cold.

I chased a starting problem for a month only to find out the low speed needle was loose in the carb. After 2 or 3 minutes of running time it would actually spin itself tight closing the low speed jet entirely. Drove my batty

 

[Sirrobin53]

needle and seat?

 

[Tim Frank]

Double check that the float is not upside down. Then pop the welch plug, clean in behind it and reassemble.
Don't be intimidated by R&R the welch plug....it is not difficult, just requires basic care.

 

[Tim Frank]

I still think it's your low speed needle. Loosen the packing nut so it almost falls out. Then tighten the high speed needle in gently until it snugs. Back it out 1-1/2 turns then tighten your packing nut back down.

Doesn't add up. High speed or low speed?

 

[racerone]

The late 1960's models all had a fixed high speed jet.

 

[Tim Frank]

Post 22 suggests loosening the slow speed packing nut and then adjusting the high speed needle....maybe a typo?
I still think the problem will be found in the carb.

 

[andreipou]

I decided to start from scratch:

1. Retested compression - excellent
2. Retested sparks -good
3. New plugs
4. Redone timing

Just got new carb repair kit.
I am going to disassemble it again (including Wech plugs) , soak, clean and rebuilt .

Maybe, it will work...

 

[Tim Frank]

Great systematic approach.
The only danger with the welch plugs is damaging the channel behind if you are too aggressive removing it.
I drill a hole with a small bit, say 1/8" keeping the drill under control so when it breaks through it does not jump ahead and ram int the bottom of the channel.
Just insert a sheet metal screw into the hole far enough to bite well, and use vice grips/pliers to pull the plug.

Be sure to clean the fixed HS orifice.

 

[andreipou]

I rebuilt carb with removing Welch plugs.

BTW: It was dry and clean , no signs of dirt.

The result is the same: it starts, runs a few secs and dies.

The same result with choke on or off, low speed needle 7/8 turn open or 2and 1/2 turn open.

When it starts to die it shoots like running on one cylinder.
I know that vacume cut-off switch turns off low cylinder when engine runs too high on idle.

I put ohm meter on contacts of cut-off switch and tried to run.
I see circuit is closed when I start running, but then it opens it and engine dies.
Can it be that this switch kills spark on low cylinder even under normal condition now?

I disassembled cut-off switch cleaned it and tested it by sucking the air out of it and it functions fine.

Is there a way to bypass cut-off switch just to see if engine runs fine without it?

Or what else can it be?

 

[gm280]

I rebuilt carb with removing Welch plugs.

BTW: It was dry and clean , no signs of dirt.

The result is the same: it starts, runs a few secs and dies.

The same result with choke on or off, low speed needle 7/8 turn open or 2and 1/2 turn open.

When it starts to die it shoots like running on one cylinder.
I know that vacume cut-off switch turns off low cylinder when engine runs too high on idle.

I put ohm meter on contacts of cut-off switch and tried to run.
I see circuit is closed when I start running, but then it opens it and engine dies.
Can it be that this switch kills spark on low cylinder even under normal condition now?

I disassembled cut-off switch cleaned it and tested it by sucking the air out of it and it functions fine.

Is there a way to bypass cut-off switch just to see if engine runs fine without it?

Or what else can it be?

If the cut off switch shorts to ground to allow the engine to work correctly, then take the wire going to the connection on that cut out switch and short it to ground to verify the engine runs. If the cut out switch opens to allow the engine to run, just disconnect the wire and see. Either way you should be able to see if your problem is around that cut out switch. I think mine shorts out when a high vacuum is present, but I can't say the same for yours... JMHO!

 

[andreipou]

This switch shows no continuity when normal and gives continuity when you suck air out of it.
I have a backup engine - 1971 40HP SKI TWIN - which is basically the same engine , but without electric start and automatic chock.
I took cut-off switch from it and tried with exactly the same result.
So, it is not the switch I think.


What can it be?
.
I tried low speed needle open from 7/8 to 3 full turns with 1/8 turn steps and can not make a difference.
Chocke on and choke off.
On neutral and on gear.
It starts fine and then shuts down
I am going nuts....

 

[Tim Frank]

Be sure to clean the fixed HS orifice.

Did you remove this and clean it?

 

[Tim Frank]

Double check that the float is not upside down. Then pop the welch plug, clean in behind it and reassemble.
Don't be intimidated by R&R the welch plug....it is not difficult, just requires basic care.

Some floats will go in upside down and give similar symptoms.
Not sure if that is possible with our motor, but I managed to do it with one of mine years ago and it took me 3 weeks to twig to what i'd done wrong.

 

[andreipou]

Did you remove this and clean it?

Yes. The hole there is huge.
I do not know what takes to plug it...

 

[andreipou]

Some floats will go in upside down and give similar symptoms.
Not sure if that is possible with our motor, but I managed to do it with one of mine years ago and it took me 3 weeks to twig to what i'd done wrong.

I do not think it is possible with this carb.
Also , I have original repair manual with photos, so do check step by step.

 

[Tim Frank]

I'd still bet the house on the carb not being clean.
Maybe consider taking the carb to shop that has an ultrasonic cleaner.
All it takes is one of those tiny LS channels to be plugged and they can be a $%#@ to get opened back up.
Did you blast with compressed air?

 

[andreipou]

Yes , I have big compressor and I hit it hard.
There are only a few channels in this carb and you can actually see them and I try to blow each and see air coming out on another end.

I am disassembling it again and doing another run today since I am out of ideas.

Just for my education: Besides vacume cut-off switch is there anything else that would cut the spark intentionally?

Because it starts strong and then dies shaking and "shot-firing" like something kills one cyl ...


Besides trying another cut-off switch, I also took two wires that sit on non-grounded terminal of cut-off switch, removed them from switch and joined together.
In my understanding this is the way to bypass cut-off switch completely , so it will not kill low cylinder.
This did not help.

 

[racerone]

Post a picture of the adjusting collar on the thin rod that goes to near the tower shaft.--Post a picture of it at slowest idle setting.-----Often adjusted by folks because it " looks " wrong when it is correct !

 

[andreipou]

I solved the problem!

You will not believe what it was!:mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new::mad-new:

I should share this because nobody deserve to suffer like that.

I disassembled it again , cleaned and took the book and started to look at it evaluating every detail of the carb and analysing how it would work, all holes , passages , etc.
Then I started to put every part together and something struck my eye.
See picture below.
The left gasket is the new one and the area circled with red is for the small hole in the carb body that is the channel that feeds low speed orfices.
The way this gasket made , it only seals gas from leaking outside, but does not seal it from carb bowl !!!!!!!!!!!!
I think what happens is: When you just start, there is not much vacume in carb and gas goes up in this channel , but then it gets some vacume from the head and gas leaks back to the bowl instead of up to the low speed needle.

As soon as I put old gasket (right one on the picture) engine runs as a rocket.

Who would think of that!

Thanks every one for help, but this one you have to see to believe....

I need to contact BRP and tell them not to screw customers like that.
But the good thing - I can assemble\disassemble this engine with my eyes closed 100 times a day now