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Evinrude OMC 70 HP 3Cyl 2S 1984 E70TLCRD too lean ?

C

Cape Horn

Member
Hi, my 70HP shows following behaviour while idling at 1.200RPM :

topCarb: if air intake is fully bunged with cloth while idling, motor continues fully unimpressed without stopping, without changing sound !
midCarb: if air intake is fully bunged with cloth while idling, motor continues for 5 secs, sound goes down accordingly, then stops
topCarb: if air intake is fully bunged with cloth while idling, motor fully stops after 1sec.

Furthermore: if I solely disconnect topCarb from this thin 3Carb throttle rod linkage while idling, overall RPM increases to 2000.

I'd like to guard against some damage. any ideas ?
thanks & regards
Joerg
 
If by "Bunged", you're saying that you actually plug the carburetor throat fully with a rag... that action should result in flooding that particular cylinder with fuel immediately... with no delay.

Remove all spark plugs....

Check the compression. What are the actual psi readings of all cylinders?

With the plugs still removed, with a tester whereas you can set a 7/16" gap for the spark to jump (auto part store), the spark should jump that gap with a strong blue lightning like flame... a real SNAP! Is it?

Remove the intake manifold so that you can visually inspect the reed/leaf valves and the main gasket between the manifold and the crankcase.

Clean and rebuild the carburetors as needed. Manually clean the brass high speed jets that are located horizontally in the bottom center portion of the float chamber, way in back of the drain screw with the blunt end of a piece of single strand steel wire... NOT a wire with a pointed end!
 
joereeves said:
If by "Bunged", you're saying that you actually plug the carburetor throat fully with a rag... that action should result in flooding that particular cylinder with fuel immediately... with no delay.

Remove all spark plugs....

Check the compression. What are the actual psi readings of all cylinders?

With the plugs still removed, with a tester whereas you can set a 7/16" gap for the spark to jump (auto part store), the spark should jump that gap with a strong blue lightning like flame... a real SNAP! Is it?

Remove the intake manifold so that you can visually inspect the reed/leaf valves and the main gasket between the manifold and the crankcase.

Clean and rebuild the carburetors as needed. Manually clean the brass high speed jets that are located horizontally in the bottom center portion of the float chamber, way in back of the drain screw with the blunt end of a piece of single strand steel wire... NOT a wire with a pointed end!
Click to expand...


hello joereeves, thanks for your reply ! Yes, that is what I'm saying, I put a rag in the throat of the topCarb and nothing happend, no dying of the motor as with the other two carbs. Compression is ok with 130+psi on each Cylinder, so is spark electrode, looks fairly dry and brown on each plug. Did the gap test and also Strobo test, ignition seems ok on all three cylinders. Did thorough carb cleaning except welch plug removal. Motor has strong WOT performance but bogging / hesitation from idling up to 2.500 RPMs so no jump start but nothin' too serious.

I just was perplex why this top Carb did not react to air closure by this rag, maybe it sucks air elsewhere... or it does not get any fuel during idling by the idling jet. Furthermore if I disconnect topCarb from the throttle rod linkage linking all 3 carbs, overall RPMs do increase. I think throttlevalve then pushes down to further closure by a millimeter or so. Don't know ...

I'll do further study on the phenomenon, focus main gasket at the block / reeds as you recommend and come back hopefully with more insights. thanks for now Joerg

ps: in post #1 there is an error in 3rd Carb-Line: -> "topCarb: if air intake is fully bunged with cloth while idling, motor fully stops after 1sec" -> bottomCarb !
 
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All carburetor throttle plates MUST BE CLOSED at idle.----In my opinion there can not be a " large air leak " on the top cylinder as you are thinking.------Checked the reed valves ?----I would say you have to revisit carburetor on # 1 cylinder.
 
NOTE: The throttle roller should be approximately 3/8" in diameter. If it appears to be approximately 1/8" in diameter, the outer covering has broken away... in which case replace it.

With the throttle cam roller "NOT" touching the cam.... make sure that at idle "ALL" throttle butterflies are closed.

Make sure that the throttle butterflies just start to open when the scribe mark of the cam is dead center with the throttle roller.
********************
Hesitation of the engine when throttle is applied is due to:

Restricted high speed jets..... Just because you can see thru them doesn't mean that the ID isn't coated!

The throttle butterflies opening too soon..... Opening before the scribe mark of the cam is dead centered with the roller.

The timer-base under the flywheel is sticking..... Check for a smooth flow from retard to full spark advance position.
********************
 
joereeves, racerone ! thanks for your time spent with this topic and your contribution !

I did further testing yesterday. Prior to testing, I disconnected this thin rod linking all 3 Carbs. nutshell ->

all Carbs suck plenty of fuel when covering the air intake by a rag. That at least soothes me from running that motor too lean on any cylinder.
Nevertheless, lower 2 Carbs rag coverage leads to dying motor (too fat), but topCarb coverage does not, even no changes in sound. mmhh, that seems little strange.

While individually pushing each Carbs throttle lever, RPM uptake is not explosive but at least similar within those 3 Carbs, looks ok.

I'll now try to clean again every Carb incl. welch plugs which I havent removed before.

Furthermore, I'm thinking about preventive gasket change on head / block etc. cause the motor is 35 years of age now. What do you think ?
Furthermore I'm not sure whether I should put back the original Thermostat wich I decalcified with new gasket though, pls. see Pic ? What do you think ?
Furthermore I'm not sure whether to bung / plug this little tube hose which was formerly connected to the airbox, pls. see Pic. It sucks air .. What do you think.

I'd be happy to get some feedback since I'm somewhat Newbie ...

best regards
Joerg

IMG_1861.jpgIMG_1862.jpgIMG_1859.JPG
 
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I've re-read what advice I've written and think that just about covers it.

A new thermostat is always welcome.

That hose (red circle) attached to the carburetor face plate filling.... fuel/oil drain to bottom main bearing area.
 
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