'74 50hp rude missing at idle and wot

[jedimaster]

Ok, so I have a '74 evinrude.
Compresion is 135 and 133
Spark seems to be ok as both plug spark equally as blue and bright when plugs are removed and engine is cranked(I know thats not the best way to check spark but at least there appears to be strong spark while not under compresion)

Ran it through a seafoam cleaning and no difference.

Was very difficult to get a measurement of the timing as I couldn't get it to idle under 1,000 but the timing was going up and down as would be expected with the revs.


Any ideas on what I should do?
How can I narrow this down a bit perhaps to a given cylinder?
Both plugs are getting fuel as both are wet.

My initial feeling is its a timing issue or coil/spark related, perhaps bad spark plug wires?

Spark plugs both appear to be in good condition

 

[HighTrim]

Re: '74 50hp rude missing at idle and wot

First what type of plugs are you using? Are they torqued to spec?

Spitting back, sounding like a mild backfire, indicates a carb slow speed restricted jet..... something causing a lean carburetor scenario.

While the motor is backfiring, remove the face plate on the carbs and stick 2 fingers in the throats, 1 carb at a time. You are choking it of sorts. If the RPMs increase slightly, and the coughing stops, you have found the trouble carb.

If you can't find anything wrong with the fuel system, consider a possible leaky lower crankshaft seal, admitting air to the lower cylinder and resulting in a lean sneeze of the lower.

Now Backfiring out of the exhaust port is caused by having the ignition system fire a particular cylinder when the exhaust port is open ( ie piston down). This would sound like a shotgun going off..... is this the condition that you're experiencing?

This is usually due to a sheared flywheel key, a flywheel with melted (moved) center sensor magnets if they exist, timer base leads routed incorrectly to the powerpack, or having the leads from the powerpack to the coils reversed to mention a few. This is much louder than the mild backfire previously mentioned.

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Its not backfiring, as in bang as you describe, no booms or flames out the carb as I woudl interpret as a backfire. Its just mising.
It seemed like it was just one cylinder missing, so I pulled one wire and it stalled, then I put it back and pulled the other and got the same result.

Seems very odd. I'll try choking the carbs one at a time tomorow and report back.

I was also recomended to pull the fly wheel and check to see if there is any rust of corrosion that built up on the pickups. I'll try that as well tonight.

Plugs are UL77V

As for tourque I didn't know they needed to be tourqued, What shoudl they be tourqued to? I had them installed to bottom out then 1/4 turn. Whats the tourque spec for them?

 

[larky]

Re: '74 50hp rude missing at idle and wot

20 ft/lbs I believe on the spark plugs

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Checked the spark plug torque and in the book it says 17 - 20 rechecked the plugs and they are fine.

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Anyone know what size the nut is on the flywheel? I went to take it off but my largest socket wouldn't fit, 1 1/4".

Also I was thinking I should just replace the Coils, and rebuild the carbs.
Do you figure this is a waste of money without further diagnosis?

I mean its got compresion and is getting fuel, so how quirky can this be to solve?

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Tried to get the flywheel off but its seized on, don't wanna break it with my jaw puller will have to get a different puller. Changed the coils, as after further inspection the lower coil had a crack in it. Still missing.

As a recap now.

Compression 135
Both plugs are wet but not totally soaked
Vent cap off, primer bulb firm(new fuel line, bulb and fitting)
Sea Foam went through
New coils

Still waiting to get spark plugs

MIsses at WOT and at idle and through out the rev range

Unable to check timming as I can't get it to idle.


Any thoughts?

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Ok, so I took the plate, off and I think we are getting somewhere.
If I cover up the top carb throat it starts to stall. If I cover up the bottom throat the rpm's pick up.
So this would mean something is wrong with the bottom carb? I will take it off now. What should I be looking for with that carb?

 

[HighTrim]

Re: '74 50hp rude missing at idle and wot

If you are taking them off, clean and rebuild them. The rebuild kits are cheap. Do you have an air compressor? That would be best, but if not, get one of those aerosol cans of compressed air to blow out all the passages after you soak and clean. Pay special attention to the jets, because as I mentioned earlier, that is where I believe your troubles lie.

http://www.meegsonline.com/tmd/TMDsVideoChronicle2.wmv

 

[jedimaster]

Re: '74 50hp rude missing at idle and wot

Just finished rebuilding the carbs, the lower one had not been done in a while it had a different carb kit than the upper so I think at some point somone did the upper but not the bottom. Anyway I did both and now its running great.

Thanks for the help. btw the timing is now spot on.
Had to adjust the mixture screws 1 1/2 ont he top and 2 1/4 on the bottom. Is that fairly normal?

 

[HighTrim]

Re: '74 50hp rude missing at idle and wot

So glad you got it.

Joes advice in the link below will help.

http://forums.iboats.com/showthread.php?t=167352