Hello everyone I recently bought this motor and after rigging it up on my boat and running it i started checking the plugs and noticed that the #3 plug was very clean. After reading some of the other threads I learned about the water in the cylinder problems. Well i put new plugs in and left the #3 cylinder with an old plug from the #1 cylinder to see if it checked out like they say. after running it at 5000 RPM for about 20 minutes got the boat home and all the plugs match in color. when i pull the #3 plug wire off i cannot tell a difference in the motor as when i pull the #2 off. When i was running in the canal i noticed a little sputtering in the engine while getting on plane after that it smoothed out and it seems like i can feel that 1 plug kicking on and off while running. could this be a power pack problem,trigger switch etc. ? I checked the compression and the cylinder's are within 5 pounds of each other. Sorry to be so long winded but any feedback would be greatly appreciated.
I removed the starter and the bracket it is connectd to and found a yellow and red wire coming from the powerpack that isnt connected to anything. I also couldnt find anything it would connect to anyone know what it could be?
yes it may be time for a cleaning. do the fuel pump while your in there.are you sure the yellow/red wire goes to the pack? yam calls it a CDI, or does it go to the starter solinoid? also be aware that the CDI may have 2 wires, both yellow, hanging out, to bypass the CDI.time to buy a sevice manual
i have a service manual but it covers to many model's amd motors. After reading your reply and asking a few more question's from other mechanic's . they were leaning towards the same senario the jet. So today i raced home itchin to see what i could find and maybe didnt clean good enough the first 2 times. well i opend u #3 carb remove the smaller jet and seemed like the opening inside was to small. I sparyed some carb cleaner through it and didnt see a difference so i pulled the old bread bag tie trick and scraped around trying to clean a little better and wa la there was something stuck in there. But it took a couple of trys i would get the hole open spray some cleaner thru it and it would be colggged agian. but now i am sure it is clear of any debree. i started it up and it seems a little smoother while reving it up but i forgot to adjust the idle air mixture screw because i was kinda in a hurry to go eat with the family. I know "hurry" is a big no no when workin on these things but i did remember i forgot. so ill make the adjustments tomorrow and fill you in on the outcome. And agian thanks for leading me in the right direction.
opps i forgot. the mechanic says that wire might be for some kind of accesorie like alarm buzzer or something he wasnt to sure. and yes i guess you can call it the CDIi am used to "powerpack" with my merc's.
I have done a compression check about 1 hour after running the motor and decarbonizing the motor and the #1 cylinder 120lb #2 cylinder 112lb #3 is 105 Other mechanics tell me it should be fine. And 1 told me about being no more that 18% difference between cylinders. Any thoughts?Also is there anything i can without digging into the power head for a rebuild?
helo have a leak down test done. that will verify the sealing capeabilities. most 3 cyl yamahas have a large variation in compression from top to bottom. so in my opinion a leak down is the only method to condem or verify a cyl sealing capeability.
Thanks rodbolt. if it is possible would i be able to perform a leakdown test with my compression tester?and i do not understand the cylinder sealing part unless it is just another mechanic sayin.
hello someplace DonS posted a leak down test and what it was. basically its a two gauge set up with a known constant air supply. by introducing air at a known pressure then monitoring how much air is flowing at the known pressure we can figgue leakage. so you bring the piston to TDC. you supply the air in the spark plug hole. in a perfect cyl when you apply 100 psi(just for reference) then the second gauge will show 0 leakage.now we will add ring end gaps. still a perfect cyl but now we are leaking 2-4% of all the air applied. now we will add a bit of wear and we are up to 5-8%. most manufactures allow 15% leakage of the cyl. I tear it down at 12% on a two stroke. so with no published data on the yam 3cyl for compression numbers and a note about the fact that yamaha varies the compression top to bottom but wont say how much I leak down test them if there is a doubt.the tester is only about 250 dollars
well I am on my second one but they do last a long time. they dont bounce well though so call a dealer in your area and ask how much to do a leak down test.
Balzy on www.screamandfly.com sells a large guage tester for around $100.00. I have one and it really works well. It comes ready to use. The big guages are nice for old eyes too.
Hello RodBoltI just spoke to a local "marine mechanic" and asked him about the leakdown test . And he says that he doesnt do that at his shop. And I also told him about my compression on all of my cylinders he says i am ok with what i have and shouldnt have to do a leak down test. I also asked him about the feul pump and he says that they are nortiorus for wearing out or going bad.I am going to buy a new complete pump this weekend coming up. complete pump just bolt it on and go is only $40 so i am going for.Correct me if i am wrong but a worn out feul pump would cause a weak idle and an inconsistant WOT?
Thanks much ray i just finished cleaning the carbs for the 3 rd time so i know they are not the prob. so hopefully this weekend will tell all fingers are crossed
Ok so far i have.1- cleaned the carbs 2 times2- replace the feul pump today3- put the correct sparkplugs. from b8hs10 to b7hs10 4- performed the compression,114,110,105 and still the same problem motor feels like 1 cylinder is cutting in and out while driving at 5100 rpm i found 2 green wires that lead to the oil sensor that go to the top of the oil tank that look like the rubber melted. i do beleave that this electrical should be seperate from ignition electrical but what would be the possibility of these 2 wires touching and grounding each other outsending an electrical surge causing my problem to occur? oil injection seems to work fine.so i guess this weekend i will be pulling apart all electrical leads and cleaning to ensure good grounds,positive connections etc.is there any type of contact goop i could put on the conectors to help out?
hello now it will be time to use a test wheel or a water test. use a CD-77 peak reading meter or its eqivalent. test the coil primary voltage at the speed range the trouble occurs. if its ign you will see a fluctuation on the meter. I prefer analog meters for this test cause of the faster reaction time. also make index marks for cyl 2 and 3 and check timing for cross fire at the trouble speed. when the light is on #1 1 sould show and when on #2 2 should show. your coil primary voltage should hold steady between175 and 215 volts above 1000 rpm. I still thinks its fuel . have you verified the tach is correct? if its off you may be on the rev limiter. buy some Dielectric grease from any parts store. use it on all connectors after cleaning.
it is not a digital tach i went with the needle type.it seems like it is happening at idle also and i know it is the #3 cylinder because like this thread began i cant tell a difference when i pull the plug wire off.A mechanic said that it is possible that the cdi shuts down a cylinder at idle speed. which i cant see why the heck the motor would shut down a cylinder at idle and flooding out that cylinder.if it was a coil it is either good or bad. or would it kick on and off?if it were the cdi i think it would affect all cylinders?if it were the pulser sensor that too should affect all cylinders shouldnt it?my clymer manual refers to a flat spot but says "when throttle is opend" which tells me while throtling up. and what is a cylinder that is galling?
hellonope that model does not have the CDI that kills a cyl at any speed unless its on the rev limiter. its rare a yamaha coil fails. usually its a poor ground and yes then it can come and go. the CDI can fail and only affect one cyl. the 91 40 has 3 pulser coils. a failure of one will affect that cyl. a "galling" cyl means a piston is sticking. your compression numbers dont indicate it. to test the CDI charge coils and the pulser coil outputs as well as the CDI output you MUST uses a peak reading multimeter. no other meter will work. there are some adapters to use on some digital meters called a DVA adapter. I dont own one of those but I do own 3 cd-77 meters and the merc one. I dont like digital for CDI testing. the resistance test is iffy but heres some numbers.#1 cyl white red to black 196 ohms w/b to black is #2 196 Ohms and W/G to Black is #3 196 ohms.
thanks rodbolt would the black on the ohm test be any good groundor the black that is with the w/g? And all i have now is a small digi. ohm meter for now and i guess i will pull all the electricl apart this weekend and clean everything up real good.before i got the motor i was kept on the intracoastal in ft. lauderdale and the ppl had money and didnt really care about saltwater maint. so hopefully the cleaning will do.Thanks agian for your help
I IGNORANTLY broke the bracket that holds the staters,pulsers,and alt. by trying to pry off the flywheel. I have to wait untill that comes in next week This weekend i will pull apart the hole electrical system clean all contacts. And I guess I will soak the carb at the same time.there is a broken ground wire that got pinched off behind the starter the wire is doubled 1 wire goes to one of the trim relays and the one that is pinched is in the same eye ring but cant find its partner
1991 40 hp yamaha #3 cylinder
I recently bought this motor and after rigging it up on my boat and running it i started checking the plugs and noticed that the #3 plug was very clean. After reading some of the other threads I learned about the water in the cylinder problems. Well i put new plugs in and left the #3 cylinder with an old plug from the #1 cylinder to see if it checked out like they say. after running it at 5000 RPM for about 20 minutes got the boat home and all the plugs match in color. when i pull the #3 plug wire off i cannot tell a difference in the motor as when i pull the #2 off. When i was running in the canal i noticed a little sputtering in the engine while getting on plane after that it smoothed out and it seems like i can feel that 1 plug kicking on and off while running. could this be a power pack problem,trigger switch etc. ? I checked the compression and the cylinder's are within 5 pounds of each other. Sorry to be so long winded but any feedback would be greatly appreciated. 
This weekend i will pull apart the hole electrical system clean all contacts. And I guess I will soak the carb at the same time.
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