Won't Idle

[Tom Scully]

1984 Yamaha 40ELN.<br />The engine will not idle.<br />Spark OK.<br />Fuel OK.<br />Compression is 60-65 lbs. on all three cylinders.<br /><br />When run at fast idle the motor intermittently "squeaks" or "chirps" accompanied by a reduction in RPM. Also there is a puff of smoke that comes from the bottom of the powerhead around the area of the shift linkage.<br />I suspect the crankshaft oil seal and or the gasket at the base of the power head is bad.<br /><br />Question 1: What is the minimum compression for this motor to run properly? 65 sounds too low to me.<br />Question 2: Can the lower crankshaft oil seal and O-ring be replaced without the need for special tools?<br /><br />Any other theroy's are welcome.<br /><br />Thanks,<br />Tom.

 

[Jammu]

Re: Won't Idle

From the Yamaha service data: 726 KPa or 7,4 kg/cm2.<br /><br />You don't need any special tools for the job.

 

[Tom Scully]

Re: Won't Idle

Thanks!

 

[classic 24]

Re: Won't Idle

compression sounds a bit low. is your gauge accurate? the squeaks or chirps sounds like the carbs are set too lean.

 

[Tom Scully]

Re: Won't Idle

I thought the same thing about the guage so I checked it on another motor. Also I checked the 40 with another guage, same readings within a couple of pounds.<br />I adjusted the carbs, no effect.<br />The thing that worries me is the puff of smoke coming from the bottom of the power head when it "chirps".

 

[rodbolt]

Re: Won't Idle

that chirp is a backfire. flame in the crankcase. its possible its blown a seal or the intake gasket or power head mounting gasket. its usually caused by dirty carbs but occasionally can be caused by worn piston skirts/rings.<br /> do a leakdown test to elimiate ring sealing problems. much more accurate than a compression test.

 

[Ray Neudecker]

Re: Won't Idle

It does sound like the crankshaft seals may be leaking and allowing the engine to suck air. Not an uncommon problem on those models.

 

[Tom Scully]

Re: Won't Idle

Thanks rodbolt!<br /><br />I will do a search to find out more about a leakdown test.<br /><br />Just for info: 726 KPA = 105.3PSI

 

[Tom Scully]

Re: Won't Idle

Thanks Ray!

 

[bigbrownbuku]

Re: Won't Idle

the puff is likely the cause of your lean running problem. if air is coming out its certainly being sucked in. find the source of the puff.<br />60 psi is far too low.

 

[Tom Scully]

Re: Won't Idle

Update:<br />The outer seal of the lower crankshaft was blown out.<br />Questions: <br />Can I do a proper leakdown test on a cool engine or does it have to be warmed up first?<br />Do you think the backfiring is a symptom of the blown seal or is the blown seal a result of the backfiring?<br />Thanks,

 

[Tom Scully]

Re: Won't Idle

btt

 

[jesse699]

Re: Won't Idle

Originally posted by Tom Scully:<br /> btt

 

[jesse699]

Re: Won't Idle

Originally posted by Tom Scully:<br /> btt

same 40 hp same trouble what did u find out ?

 

[jesse699]

Re: Won't Idle

Originally posted by Ray Neudecker:<br /> It does sound like the crankshaft seals may be leaking and allowing the engine to suck air. Not an uncommon problem on those models.

 

[jesse699]

Re: Won't Idle

how do i do a leak down test?

 

[Tom Scully]

Re: Won't Idle

The lower crankshaft seal was blown.<br /><br />Basically a leakdown test requires filling a cylinder with pressurized air and seeing how well it holds pressure. This tests the sealing ability of the piston rings. You will need a special tester.<br />Do a search, there are a few good ones already posted.