Oil on spark plugs xj750 seca

Hey everyone,


I’ve used this group a lot, and it’s been a great help—thank you!


I recently bought a 1983 Yamaha XJ750 Seca and later picked up a 1984 Yamaha XJ750 Seca as a donor bike. Neither of them ran when I got them, but I managed to get both running.


Following advice from this group, I cleaned the carbs and bench-synchronized them. Before syncing them on a running bike, I decided to check and adjust the valve clearances.


While working on the donor bike, I noticed that there was a lot more oil under the cylinder head compared to the other bike, which seemed odd. Any idea what could be causing this?


This led me to check the spark plugs on both bikes:

• Donor bike: Spark plugs were dry.

• Other bike: Two of the spark plugs were wet and black.

• All four cylinders get hot while running.


What should I do next?


I’ve read on this group that I should perform a compression test, which I plan to do.


If the compression is low, what are my options? From what I understand, the possible causes are:

1. Stuck piston rings (the bike has been sitting for years).

2. Worn piston rings.

3. Leaking valves – but I don’t fully understand what actually seals the valves properly.


How do I determine the exact issue after the compression test?


And if compression turns out to be good (120+ psi), what should I check next?


I’m a young guy and still learning a lot, which is why this group has been such a huge help. Any advice would be greatly appreciated!

 

Here's a graphic that shows how the valves seal against the head. A compression test is a good sanity check to start with..

Besides potential low compression, The cylinders that have black/oil coated plugs are likely running rich, possibly either jetted differently than the others, or the idle screw settings are way off. etc. If you install fresh plugs and run the engine, do they turn black right away? Also possible jetting/mixture is rich across all cylinders but the ones with clean plugs could have intake air leaks making those cylinders run lean?

A lot of variables to consider... There are others here who can give really good guidance on tune/mixture/etc..

 

Hey, thanks for your response!


I just performed a compression test, and here are the results for each cylinder:

• Cylinder 1: 120 psi

• Cylinder 2: 130 psi

• Cylinder 3: 140 psi

• Cylinder 4: 125 psi


I also checked the valve clearances, and here’s what I found:

• Cylinder 1: Intake (0.00-0.05) - Fail | Exhaust (0.10-0.15) - Fail

• Cylinder 2: Intake (0.05-0.10) - Fail | Exhaust (0.10-0.15) - Fail

• Cylinder 3: Intake (0.15-0.20) - Fail | Exhaust (0.15-0.20) - Pass

• Cylinder 4: Intake (0.00-0.05) - Fail | Exhaust (0.15-0.20) - Pass


The spark plugs were slightly damp and black, but I don’t think it was oil after all. The engine is struggling to run properly—maybe the low compression is the issue.


Also, the new spark plugs are turning black quickly. Could this be caused by the low compression? I’m thinking that if the combustion isn’t optimal, it might not be burning all the fuel, and some of it could be depositing on the spark plugs.


Hopefully, adjusting the valves will help.

 

If I am reading your numbers correctly, #1 and #4 have practically zero clearance on the intakes. Could account for the lower compression on those two. I'd start with the valves and get them all in spec then check compression again.

BTW: In order to adjust the valves, you'll need a tool like this.
https://www.amazon.com/Pit-Posse-PP2854-Yamaha-Motorcycle/dp/B004N08MSK

Motion Pro makes a similar one. I think I have the MP version.

 

Or adjust them with the zip tie method.

https://www.xjbikes.com/forums/thre...valve-adjustment-with-pics-parts-i-ii.116006/