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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16213
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a 40 3 cylinder tohatsu,when it is idling it only

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i have a 40 3 cylinder tohatsu,when it is idling it only runs on 2 cylinders, have done all the obvious things ,plugs etc, but what i do notice is that the butterfly's are not in the same position when the throttle is wide open.. should all 3 carbs be wide open at full throttle
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else important you think the Marine Mechanic should know?
Customer: no

G​ood morning, my name is ***** ***** am I speaking with? I read what you wrote. Do you have a model number on the outboard?

Customer: replied 1 month ago.
Hi mate
my name is***** model no is M40D2, It is a 2003 model,, had major work done on it 18 months ago but work has kept me away from using it till now, both of my boys have used the boat over the last 12 months and have said that it runs very rough down low,and was pretty guttless,, have checked compressions and on my gauge all reading around 115 ,have had the engine running today and found that the lower cylinder is not working at idle ( took plug leads off one at a time ) replaced plug but made no difference it was then that i noticed that the throttle arms are all set different and the bottom only appears to get half throttle when opened up ..Thus my question????

Hi Keith. Got it. If taking the plug leads off made no difference then either that cylinder is not sparking or that cylinders carb is not flowing fuel. Start by changing the spark plug first and retesting. If it still does it, you would need to hook up a timing light to that plug wire and see if the timing light flashes or not. Flash means the cylinder is sparking, if there is no flash, that cylinder is not sparking. If there is no spark swap that ignition coil for a coil that is firing and see if the problem follows the coil or not. If it does, the coil is bad. If it does not, the CDI box is bad. If the timing light is flashing from the start then you do have spark, and because of that the problem would be that carb is not flowing fuel into the engine. Does that all make sense. I think that should do it but I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

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Thanks in advance, and good luck

Customer: replied 1 month ago.
Jason it has plenty of spark (i can vouch for that ,it got me,was to lazy to get insulated pliers to start with )... not knowing to much about the set up of the carbs and i would assume that when the work was done on the engine that they would..the only thing that i can think off is that they didn't set them up right and that the lower carb is shutting off completely in the lower throttle range .. Should all 3 carbs be set with the same butterfly openings throughout the throttle range from idle to full ????
Thanks Keith

It only takes a few thousand volts to give you a good zap. It takes 20 thousand+ volts to fire off a spark plug. You getting zapped is different than a plug actually firing, which is why it needs to be tested with a timing light. (Or a KV meter if you want to get really advanced). On the butteryflies. Yes they all should be even, but that would not cause a cylinder not to fire. What causes a cylinder not to fire is a lack of compression, lack of spark, or lack of fuel. You have compression so that is good. The zap test isn't good enough, so spark will need to be rechecked with a timing light in order to tell you if that spark plug is actually firing or not.

Jason and other Boat Specialists are ready to help you
Customer: replied 1 month ago.
Thanks mate yes i do have a timing light ,will put it on tomorrow and check it,,as for the carbs i will equal them out and go from there, my thoughts are that the lower carb is shutting off altogether down low, but wasn't to sure how they should be set up as i have never had to touch this engine before.
many thanks and i will let you know what the outcome is hopefully tomorrow night,
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You are very welcome Keith. Thanks again. -Jason