One cylinder running hot- over heating 951

[JonRusso89]

Well I got my ignition issues squared away on my 98 xpl (swapped in a newer motor, had to switch mag cups for single coil application.) it starts and runs well.. The issue is that the front cylinder is running very hot.. I noticed it after a test ride on the water.. It idles fine but will only go to about 4,000 rpm and lacks power.. By the time I got back to the dock it was starting to overheat, but only the front cylinder.. The paint was starting to turn a little brown.. So my thoughts are that this cylinder is running lean. I don't think its a cooling system problem, because I started on the trailer without the hose for about 25 seconds... the front cylinder climbed to about 150 and the rear only got to about 116. (this was about an hour after the test run, so it was still warm to begin with) If it were a cooling system issue there should be no difference when running with out water. Carbs are clean, all new fuel lines and filter, ect.. I guess i'm going to pull the carbs and start looking for anything suspect. The one question I had before I dig in is if its possible that the timing could be too advanced, making it run hot? There isn't much adjustment on that mag cup, but i turned it as far clockwise (advanced) as it would go. If this were the issue, again, i think both cylinders would run hot. And its not detonating, so I'm leaning towards a lean condition... Any input?

 

[SabrToothSqrl]

So, I take it was right about the Magneto?


oil pump / lines.

the lines can easily get kinked when putting the carbs back on. My guess is you may be starving the front for lube...

 

[JonRusso89]

Yeah it was the magneto.... Running premix though..

 

[SabrToothSqrl]

my guess then would be air/fuel mix. the LS should be 1.5 turns out from closed, the HS closed on a stock 951. how do the spark plugs look?

as far as timing goes, I was not aware that it was possible to adjust the timing on these engines... the MPEM does all that...

 

[JonRusso89]

my guess then would be air/fuel mix. the LS should be 1.5 turns out from closed, the HS closed on a stock 951. how do the spark plugs look?

as far as timing goes, I was not aware that it was possible to adjust the timing on these engines... the MPEM does all that...

The holes on the mag cup are over sized and allow for a few degrees of adjustment.. But I highly doubt that has anything to do with this... I just checked compression 125 in both.. After the comp test i pumped the throttle a couple times and cranked the motor with the plugs still removed.. a mist of fuel was being pushed out the rear cylinder on each stroke.. the front cylinder however had no fuel spraying out what so ever.. The cylinder was definetly firing cause it idles smooth.. i guess its getting just enough fuel.. I'm pulling the carbs.. Perhaps I messed up the routing on the fuel lines for the carb?? Its pretty straight foward but i guess i need to double check..

 

[Kave_Man]

I noticed today on my 951 that both oil pump lines going to the carbs (2 little clear lines painted engine silver) were pinched before they reached the carbs. There is a small oil line bracket that is held in place by the lower left mag carb bolt It did not cut the lines but they were definitely pinched/restricted. Could this be the reason why my top end is fried?

 

[JonRusso89]

So i pulled the carbs apart.. the mag carb, which was the overheating cylinder, wouldn't "pop" when i checked the pop off.. It would get to about 20 psi and slowly start to hiss.. Even when I put a little oil in the seat to keep it from sticking.. no pop off, just slow hissing as the pressure slowly bleeds down. I'm gonna try some other needles/ springs and see what I come up with. The other carb pops off nicely but at around 15 psi. Seems low?? Thoughts? This cylinder seems to have a lot of fuel going to it.. Wet plug and lots of fuel spraying out when I crank it without the plug in..

 

[SabrToothSqrl]

I noticed today on my 951 that both oil pump lines going to the carbs (2 little clear lines painted engine silver) were pinched before they reached the carbs. There is a small oil line bracket that is held in place by the lower left mag carb bolt View attachment 13496 It did not cut the lines but they were definitely pinched/restricted. Could this be the reason why my top end is fried?

The factory lines are fairly flexable and resist pinching, but if they were restricted, it could have caused the engine to run low on oil for a long period. I replaced my lines with new thicker ones that are harder to pinch. Weed Whacker Fuel Line section of your local hardware store. 1/8" inside diameter i think?

 

[riding high]

So i pulled the carbs apart.. the mag carb, which was the overheating cylinder, wouldn't "pop" when i checked the pop off.. It would get to about 20 psi and slowly start to hiss.. Even when I put a little oil in the seat to keep it from sticking.. no pop off, just slow hissing as the pressure slowly bleeds down. I'm gonna try some other needles/ springs and see what I come up with. The other carb pops off nicely but at around 15 psi. Seems low?? Thoughts? This cylinder seems to have a lot of fuel going to it.. Wet plug and lots of fuel spraying out when I crank it without the plug in..

Seems to me that you are on the right track. 15 seems a little low. I think you need to follow the manual exactly when setting the pop off pressure. The proper delivery of fuel is critical on these engines. Sabr I think does a lot of work on these engines and Doc is very knowledgeable on the carbs. Keep your eye on the plugs and if they indicate it is running lean take immediate steps to correct the situation

 

[JonRusso89]

Well I've got a theory.. Basically, my pto cylinder seemed like it was getting a ton of fuel.. and the mag cylinder was running very hot/lean.. In addition to having low pop off, when I reassembled the pto carb I pressurized and it was bleeding down, it would hardly build pressure. So the diaphragm was holding the needle open.. Just to confirm, I bent the arm down a hair and it built pressure just fine.. So I'm thinking that this needle has been hanging open all along flooding the pto cylinder. My question is.. since this carb was really dumping fuel, could that cause the mag carb to run lean?? Basically "stealing" the majority of the fuel..?? The mag carb had a higher pop off.. so is the fuel taking the path of least resistance? What do you guys think? I'm gonna pick up some rebuild kits tomorrow... Might throw it together in the mean time just to see what happens, now that the pto carb is dumping fuel anymore..

 

[16vmxjm]

I was hoping you could update this with how you got on with the issues you were having. I had the exact same problem this weekend and ended up killing the mag piston. I am guessing I need to have a good look at the carbs, but I was hoping you may be able to tell me what issues you found?

 

[SabrToothSqrl]

No, one carb running rich would never pull enough fuel to starve the other one.

2 strokes do however use fuel as part of the cooling system, so running lean over heats these things fast and bad.

always error on rich, then cut back if you have to.

Running them lean, won't end well

 

[JonRusso89]

I was hoping you could update this with how you got on with the issues you were having. I had the exact same problem this weekend and ended up killing the mag piston. I am guessing I need to have a good look at the carbs, but I was hoping you may be able to tell me what issues you found?

Carb rebuild and some tuning did the trick. Ran great for the rest of last season, but I sold this ski over winter.