Troubleshooting 1989 Yamaha 50DETO 2 Stroke, Idle/Low rpm rough running

[YAM50DETO]

Hi,

I'm after some advice as I have been trying to troubleshoot an idle/low rpm issue without success for a while now.

I have a 1989 Yamaha 50hp, 3 cylinder, 2 stroke (6H5 L 511690 D) which starts straight away but will cough and die after about 30 seconds as seen in the first video. After the carburetor link adjustment the engine will idle for a bit longer as in the second video and it is similar at low rpm where it seems to be misfiring. Engine seems to run ok at all other throttle positions and will only keeping idling if the remote choke lever is used, which appears to adjust the timing?

As the boat I bought had been in long term storage fuel was the starting point and I have checked the following to date:

1. Drained and replaced the old fuel
2. Fitted new spark plugs (All three were similar and are black/wet, slightly rich?)
3. Cleaned the carbs by hand several times, including soaking in carb cleaner overnight and running the jets through with mono fishing line. Checked the float heights are in spec (15mm +-1mm) and pilot screws 1-3/8 +-1/4 turns
4. Carburetor link adjustment (This did improve the idle time before dying )
5. Checked compression (#1 130psi; #2 135psi; #3 132psi)
6. Charging is ok and battery condition ok
7. Visually inspected the reeds but not removed and look ok
8. Replaced fuel lines and connectors to the engine
9. Checked fuel pump and fuel filter seem to be ok
10. Checked Throttle link adjustment (CDI unit indicator, full retard ATDC 7° Full advanced position BTDC 25°)
11. It appears #3 cylinder is intermittently firing at idle (will fire on only #1 or #2 but not #3 cylinder)
12. Swapped enrichment system from another engine (does not start as well and no change to the idle issue)
13. Swapped #3 coil from another engine
14. Swapped #3 carburetor from another engine
15. Swapped CDI from another engine
16. Coil resistances seem to be ok (Primary ~0.3Ω Secondary 7.8 - 9 kΩ)
17. Resistance of pulser coils seems to be ok (180, 184, 182 Ω)
18. Peak voltage from charger coil, pulser coils and CDI all seem to be ok and above min specs
19. Checked resistance of charger coil seems to be a little low (302Ω) but similar to another one tested & peak voltages are above min

Hoping I've missed something obvious or I need to re-check something but open to any suggestions? I don't currently have a spark gap tester or working timing light...



Thanks

[99yam40]

if the fuel pump has a pin hole in the diaphragm it will leak fuel into #3 crankcase making it too rich
too rich or too lean will cause a cylinder to miss fire.
this is my 1st thought on why #3 is not firing properly

you need to get a hold of a good timing light to see what the timing is doing.
My C40 had the CDI retarding the timing too far, but then you said you tried a different one so not sure 2 different ones would do the same thing

every time you put the carbs back on you need to follow the link and sync procedure in service manual.
if the carbs are not synced properly the cylinders will fight each other

[YAM50DETO]

99yam40

Thanks

I'll try swapping the fuel pumps over this week. If I put a clear pipe on the fuel pump outlet back into the fuel tank would a pin hole show up with air bubbles in the line or is there a way to tell if it is leaking into #3 crankcase?

Yes, looks like I'll have to invest in a timing light at this rate

Yes, link and sync gave me the best improvement to date. Also read about someone else having similar issues and the fix seemed to be around timing and increasing the number of turns on the pilot screws, so will try the pilot screws too this week.

[99yam40]

99yam40

Thanks

I'll try swapping the fuel pumps over this week. If I put a clear pipe on the fuel pump outlet back into the fuel tank would a pin hole show up with air bubbles in the line or is there a way to tell if it is leaking into #3 crankcase?

Yes, looks like I'll have to invest in a timing light at this rate

Yes, link and sync gave me the best improvement to date. Also read about someone else having similar issues and the fix seemed to be around timing and increasing the number of turns on the pilot screws, so will try the pilot screws too this week.

unbolt the pump from the block ,leaving the hoses attached,
pump the primer bulb ,
and see if fuel leaks out of the pulse port of the pump

is there a way to borrow a timing light?

some auto part stores loan tools with a deposit.
anyone that works on automotive vehicles and other motors with spark ignition should have a timing light.


this motor have prime start system, is that correct?
if so is the red lever in the normal/auto position?

it seems that the CDI advances the timing during the start and warm up time, but when it drops back to normal idle timing the motor cannot stay running very well.

also there is a diaphragm in the prime start system that could be leaking or the heater in not shutting off the extra fuel once the motor does not need it

[YAM50DETO]

99yam40

Think you may have hit the nail on the head. The fuel pump is leaking so I'll order a new one, retest and post back hopefully by the end of the week.

Thanks for the quick replies.

[99yam40]

is it leaking out of the pulse port?
leaking out of other places without it bolted up does not count

[YAM50DETO]

I could see fuel through the pulse port, which I presume should be dry. It was leaking elsewhere when unbolted from the crankcase so I will retest it later to try and confirm if it is leaking through the diaphragm into the pulse port and not just leaking around it when unbolted.

Thanks

[ausnoelm]

You cant clean carbs by soaking them!

[YAM50DETO]

99yam40

Checked the fuel pump again and it appears to be only slightly leaking through the pulse port and no where else. Will be interesting to see what difference there is when the new fuel pump and gasket arrive.

I'm not sure, it may have an early version of the prime start system on Carb #3? it has the solenoid but rather that the red normal/on/off lever found on later models it has a black lever that is on or off like a choke (screenshot/photo below)

I have not stripped down the diaphragm in the prime start system or checked the heater is working, just swapped it straight with another known good one and if anything it didn't start as easily and was did not seem to idle any better. Might be worth revisiting or swapping all 3 carbs to see if there is any noticeable difference.



Screenshot 2021-06-01 22.37.25.png#3 Carb enrichment system.jpg






ausnoelm Granted that was the first attempt and cannot even remember what home remedy solution I used in the end to soak all the jets and carb bodies in. They have been cleaned with toothbrushes and fishing line run through all the jets a couple of times since. Various methods tried from online guides/videos. Probably not a touch on the ideal which I guess would be an ultrasonic clean. I'm tempted to buy a cheap small one and clean the parts over a few days as I plan on keeping this engine until one of us gives up for good.

[YAM50DETO]

Just a quick update.

Fitted the replacement fuel pump and gasket with no noticeable difference in idling issue.

Took the boat out on the water and it would not seem to throttle without dying. Swapped out the three carbs from another engine and whilst idling was not changed much it was at least running again.

It appears to misfire after the initial enrichment phase up until 2k rpm after that the motor seems to run fine. Intermittently after running for a while I had the same issue from the morning where it would not throttle up from idle without dying. After the first time this happened it took a couple of minutes of running at a couple of knots before the engine started running again and everything above 2k rpm was fine. This happened a couple of time over several hours out on the water.

As a side note the tacho also hadn't been working and came back to life at the end of the day?!

I'm going to look at running spark tests again and then get the timing checked. Will strip and clean the carbs again although I'm not expecting them to be the main issue to this ongoing saga.

Thanks

[YAM50DETO]

Spark test videos below

Tacho was a bad earth solder connection. Rookie mistake not cleaning old wire before soldering...

#3 Cylinder

#2 Cylinder

#1 Cylinder

[99yam40]

to me it does not look like they are sparking all of the time

[YAM50DETO]

Yes the all look about the same to me so not sure if that is the tool or not? Will get a gap tester as I can't find an old one I used to have...

[99yam40]

inductive timing light should work also

[YAM50DETO]

Good point. I take it all induction timing lights work on 2 strokes and vary only according to extra features, type of bulb, rpm measuring, etc?