07 RS Viking (973cc Carbed) - Hard Starting/Backfiring

[John B]

You might have been testing at TDC overlap which occurs between the exhaust and intake strokes. At this point both exhaust and intake valves are slightly opened.

Top Dead Center refers to the piston being at the very top of its travel in the cylinder. On a four stroke, TDC will occur at the top of the compression stroke and the top of the exhaust stroke.

I did rotate the crank until the piston was at the very top of its stroke, and then I marked it on the chopstick and repeated it for each cylinder.

If I was off a little bit the piston would move on me and I would rotate the crank and redo the test.

Thanks again everyone for the help...

 

[John B]

sounds like time for a valve set or a bore scope look at the intake valves and see if there is carbon on them holding them open.

I do have a borescope, I should be able to take the carb off and get a look at them, I might be able to with the carbs on...

 

[grizztracks]

I did rotate the crank until the piston was at the very top of its stroke, and then I marked it on the chopstick and repeated it for each cylinder.

If I was off a little bit the piston would move on me and I would rotate the crank and redo the test.

Thanks again everyone for the help...

There are two top dead centers. If you believe you were on the compression TDC and not on the overlap TDC and the leak down test failed I'd pull the valve cover and check valve lash.

 

[John B]

There are two top dead centers. If you believe you were on the compression TDC and not on the overlap TDC and the leak down test failed I'd pull the valve cover and check valve lash.

Gotcha, I am an idiot - now I get it... at overlap TDC the valves are open and the leakdown test will fail, but if I crank the engine 1 more revolution, I should be at compression TDC and the valves should be closed, hence 4-stroke.

I double checked, and could see the difference in the results - they were slightly less terrible at compression TDC, but still bad. But the engine is ice cold at this point, so I will have to re-do it tomorrow when I can warm it up again.

I enjoy this work but I am probably pushing the limits of my capabilities at this point, and I don't really have any life lines I can call if I screw something up or need help. I was actually trying to work up the courage to finally replace the timing chain tensioner before I ran into this issue.

 

[YukonMP]

I enjoy this work but I am probably pushing the limits of my capabilities at this point, and I don't really have any life lines I can call if I screw something up or need help. I was actually trying to work up the courage to finally replace the timing chain tensioner before I ran into this issue.

From here it looks like TYers have your back, go for it.

 

[Snorunner]

If you have a burnt or open exhaust valve. It will pull. Vacuum on the intake stroke. A piece of hard stock paper against the exhaust pipe will verify this. Hold it against the tailpipe . It will pull the hard stock paper to the pipe on the intake stroke. Works on automotive engines with burnt valves!

 

[John B]

Alrighty, apologies to everyone for my incompetence with the leak down test. I redid it yesterday, and again today, and it appears to be fine with very little change. One thought I had is that maybe it had jumped time (since I have not replaced the timing chain tensioner) but I would assume it would not pass a leak down test if that was the case? When I redid the compression test I pulled the carb off and dropped my borescope in the intake and looked at the valves, I could see some leaking past the valves, but it still passed the test with only 2-3 psi drop @ 100psi. I also looked at the pistons again with the borescope. Photos are below.

My cheap $20 compression tester has a bad check valve, but it appears to be ~180 psi in all 3 cylinders.

The carb has been off again, I looked everything over and it appears to be fine. When I took the carb off the boots on the engine were pretty wet with gas. When I put the carb back on it started right up, so it sure seems to be flooding itself.

I pulled the connections on the ignitor and rectifier and inspected them, no corrosion and everything looked good.

Once started it runs normal, idles fine, with no issues. I also made a carb sync tool and synced the carbs.

I checked the oil and it was well past the full mark, and smelled of gas. I am not sure how significant of a problem that is, with the issues I am having getting it started and the fact it has barely been used perhaps that would be expected, but I have changed it again (I changed it at the end of the season last year). The waste oil did not look particularly good, there appeared some fine metal flakes in it.

I know it was suggested I pull the valve cover and inspect the valve lash, but that is probably not something I am not comfortable attempting. Thanks again to everyone that chimed in with advice. Unfortunately I might throw in the towel at this point and bring it to my local dealership and see what they have to say.

 

[Snorunner]

Backfiring could be due to not enough voltage to fire the mixture. Also the leaner the mixture the more voltage is required. Just throwing out the hard lessons I learned in my automotive days. Your picture of the top of the piston indicates you don’t have an over rich problem. Otherwise the carbon would be washed off. I am referring to piston wash. Very important to understand if you are running a 2 Stroke. Helps you with jetting. So thinking about your problem. I would go to a furnace parts store and buy a bottle of oil with a long skinny tube on it for lubricating furnace blower wheel bushings dump out the oil and fill with fuel. Remove spark plugs. Squirt a small amount of fuel in each cylinder. If that cures your starting backfire problem. Then you are to lean at startup. And you can put your efforts into carbs, choke circuit, fuel filter, pump and gas tank vent including cap. It takes a rich mixture to start a cold engine.Hope this helps.

 

[John B]

Just to update the thread, I threw in the towel and brought it into my local Yamaha dealer. They determined one of the pickups on the stator was bad, so they replaced the stator. It now appears to start/run like it used to.

I might actually be able to use it this weekend....

 

[Roger]

I had a 2008 rs Venture, carbed, not starting properly, and actually dying on the trail, when I let go of the throttle!
The engine was defective! It had a cracked exhaust valve, so it wouldn't hold enough compression to start it properly.
My dealers first response once they completed a "leak-down-test", "let's see if Yamaha will cover this under warranty!"
My response to that statement was, "if they don't, you can keep the f...ing sled!"
Apparently my engine was the first 4-stroke Yamaha engine to fail, suuuure!
Anyway, it was fixed under warranty, and I traded it two years later, to the same dealer, as I was starting to have starting issues again!

Sure hope this isn't your issue.

Good luck!