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Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3342
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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40 hp merc 2stroke 2cyl. chasing idle problem. carb cleaned.

Customer Question

40 hp merc 2stroke 2cyl. chasing idle problem. carb cleaned. adjusted per service manual. runs well full throttle. but wont hold idle, erratic, dies, very hard start condition.
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.
Hello my name is ***** ***** am I speaking with?
All Mercurys are cataloged by serial number, can I get that number please.
Next, have you checked compression. And have you checked to see what ignition timing was set to?
Customer: replied 1 year ago.
compression 140 both cyl, timing is factory spec per service manual (timing arm slight play at idle).
Expert:  Jason replied 1 year ago.
Do you have another compression gauge you can try? This engine would not have been 140 psi when brand new. Are you using a harbor freight cheapie type gauge, or a good one, like a snap on/?
Customer: replied 1 year ago.
Advance auto guage, has worked on other engines. main point: both cylinders pump well and hold the same indicated pressure (the actual number less important) I am confident compression good. I get more worried when there is cyl to cyl variation.
Expert:  Jason replied 1 year ago.
Okay.... What is the actual timing reading at idle with a timing light?
Customer: replied 1 year ago.
had it at around 8 btdc on sat. I can re-verify later today - will need to get boat back over water to be able to run her in gear (no test tank available).
Expert:  Jason replied 1 year ago.
Did you have the serial number on the engine? There are so many different versions of the 40 I really need to now which one you have.
Customer: replied 1 year ago.
OT 489787 - working on getting her on a lift.
Expert:  Jason replied 1 year ago.
Got it. 8 Degrees would be correct. Does the engine idle at all?
Customer: replied 1 year ago.
Got it to run just now, hard start, hard time settling with idle still drifting but not as bad as saturay : 8deg BTDC @ 800rpm in gear in water.I'm starting to think I may have a bad ground somewhere on the motor with waxing and waning symptoms like this.Thanks for trying to help, it was worth a shot.
Expert:  Jason replied 1 year ago.
A bad ground wouldn't do it.There are a couple of things to check though. Starting with the plugs, are they in good shape, have you installed a new set of plugs yet?
Customer: replied 1 year ago.
Fuel system is good with fresh fuel, clean filters. Carb is clean and to spec. Timing is good... that leaves electrical/spark (plugs are new, not fouled, and gapped to spec, definately running both cylinders - wire off verified).Will verify pertinent ohmages as I go through the electrical system. it's not hard, just time consuming.Thanks again.
Expert:  Jason replied 1 year ago.
Let me ask you another question, 2 questions actually 1. Where are the idle mixture screws set? 2. Does the engine react when you turn those screws in and out?
Customer: replied 1 year ago.
um... there is only one idle mix screw (only one carb - no carb sync issues to make life complicated): it is set just a touch rich from 1.5 truns ccw from a light seat. Closing it kills the engine like it should. throttle plate rests normally, eveything's clean, very low suspicion of air leak around carb - not acting like its running too lean. Thats some of the first stuff I addressed this weekend.
Expert:  Jason replied 1 year ago.
If you push in on the key to activate the primer is there any reaction from the engine? Does it run better, worse, or is there no difference?
Customer: replied 1 year ago.
as i understand it on these pushing key is "quasi" choke - kills spark to deliver just fuel as a makeshift to get things rich for a cold engine.pushing key in on running engine causes stumble or kills it outright.
Expert:  Jason replied 1 year ago.
Okay good. Next bit. There is a fuel line on the fuel pump that runs between the pump and the engine block. remove the line from the block. Squeeze the primer bulb until firm. Does any fuel come out of that line? Also, how old is the gas?
Customer: replied 1 year ago.
gas brand new, stablized religiously,... wouldn't fuel starvation manifest at higher rpms? and yes fuel tank vent is open.
Checking pump and its diaphragm for splits/holes was lower on my list given how well our motor runs WOT - really pulls strongly and smoothly. will chk..
Expert:  Jason replied 1 year ago.
I didn't say anything about fuel starvation..... If the fuel pump does have a torn diaphram it will drip fuel directly into the crankcase, causing a rich fuel condition. Do the above test and let me know if any fuel comes out.
Customer: replied 1 year ago.
no fuel at vaccuum port.
Expert:  Jason replied 1 year ago.
At this point you need to double check the compression. These engines run pretty will and still still make good power down to about 90 psi. But when they get below that they will still make good top end power, but they will have a hard time idling. I know it's not going to be 140 psi, but I would need to know what it actually is next. Get another gauge and re-test it, and let me know what it reads.
Customer: replied 1 year ago.
I'll try to borrow one this week.
Expert:  Jason replied 1 year ago.
Sounds good. Let me know how you make out.
Customer: replied 1 year ago.
I just verified my compression tester on our shop compressor, reads dead on 90psi which is what the unit is set to deliver. the 140 number might be real.
Expert:  Jason replied 1 year ago.
If the engine really does have 140 psi of compression it's going to be loaded with carbon, which is another issue to deal with. Really you want to beg/borrow/steal another gauge and try it.
Customer: replied 1 year ago.
only spec i can find is a min of 120. will scope the cylinder next.
Customer: replied 1 year ago.
edges of piston tops look good, no deposits, some deposits at ports and at center of pistons but they are smooth and glossy not caked up like one would expect on a coked-up engine. overall I'd say the bores look better than many I have looked in.
Expert:  Jason replied 1 year ago.
Mercury doesn't stick compression specs in the factory service manuals for 2 stroke engines. You really need to double check that compression.
Customer: replied 1 year ago.
have a lead on another gauge, will pick up today when they close - need to return in AM. Will aslo do a de-carbon treatment, need some "power tune" or techron. I still suspect something 'lectrical.
Expert:  Jason replied 1 year ago.
Do a compression reading before using the power tune, and then do another one after you run the power tune through it (if the power tune makes no difference). Don't bother with the techron, stick with Power Tune.
Customer: replied 1 year ago.
140psi is real, decarb made little disceranble difference even after an overnight soak with powertune sprayed directly into cylinders. Impressive smoke - glad the FD wasn't called. I wound up pulling t-stat just for grins, its fine. When I was picking up a new t-stat gasket I asked about the compression pressures and they confirmed they do see 130's-140's on motors like ours. They suggested electrical problem in the ignition.I'm admitting defeat. I may make a service appt.Thanks again!
Expert:  Jason replied 1 year ago.
I will opt out. If somebody has an idea they will jump in. Good luck!
Customer: replied 1 year ago.
i understand... spent some more time on it this eve. i can get it perfect on a lift, as soon as its run for a time it'll start dying and wont hold idle. had to paddle it in last run. tried running it very rich, high idle, you name it. everytime one runs it its a different set of adjustments to get it to hold idle in gear. i cant find an air leak never mind one big enough to account for that. the compression is a red herring, the motor was pretty much clean. i doubt a local tech could do much better, they get it running once in a tank and call it good. i keep coming back to electrical but can find nothing wrong. i guess i need to try another carb? too nice a motor to junk it. so frustrating that something so simple can pose so difficult a problem. appreciate your efforts though.
Customer: replied 1 year ago.
starting to wonder if I may have bad low speed coils on my stator - assuming this is a two stage stator...?
Expert:  Charles replied 1 year ago.
Hello, my name is***** will be assisting you from here. Your stator is not a two speed, only one set of leads to the CDM's. test the voltage between the green/white and the white/green. It should be over 180 V AC or DVA while cranking. Then do the trigger wires from one color to ground. Each one should be about 1V. If you do not get these voltages, replace that part. If you do, It could be one of the CDM's as they do ground back through each other and one bad one can cause no spark on the other.
Customer: replied 1 year ago.
Long story short. this looks to be solved... strange one: issue looks like was with the capillary tube in the carb. suspect it was causing inconsistent air/fuel ratio through idle circuit. this was a tough one to find! add it to your lists as last resort checks.
Expert:  Charles replied 1 year ago.
You mean the tube in the center of the main tube. I run a small wire through them. Also make sure that the chamber in the bowl that they sit in is really clean. It can clog that small tube again if dirty.
Customer: replied 1 year ago.
i prefer fishing line of varying sizes, the brass tubes are soft. less risk of scoring the passage or changing an orifice size than with wire. blowing out with air helps get small debris out. i recleaned / rebuilt carb with a fresh repair kit, hate redoing recent work i paid for but now i know its right.
Expert:  Charles replied 1 year ago.
Never thought about fishing line. May have to try that.

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