seuadr 488 #1 Posted July 18, 2014 Hi all, I apologize if this has been covered - i searched for "onan power loss" "520 power loss" etc and didn't find anything that didn't talk about losing the valve seat on the rear cylinder. symptoms: loss of power with 48" deck engaged, either when i hit a little thicker grass ( about ankle deep, not wet ) and also when rolling up a hill while the pto is engaged. on flat ground, i drop from 3k~ish rpm to 2k~is rpm. with the PTO engaged and going up hill it drops to 1.5k~ish rpm and acts like it is going to stall. Observations/Conditions: new fuel, with stabil in it from a 5 gallon jug that i also used a couple weeks ago with no issues in the same tractor, and in other equipment (generator) plug wire (at least for the front cylinder) may have some cracks, when i grabbed it to troubleshoot if the power loss was related to the rear cylinder it gave temporary Michael Jackson syndrome ( I yelled OW! and danced around a little bit :D ) oil seems fine, changed about 5 hours ago with a 10-30W high detergent oil (temporary for a couple changes to help "flush" the motor, since it is new to me) troubleshooting steps i've taken: pulled the front cylinder plug - stumbles a second but keeps running. pulled the fuel line on the engine side of the filter, fuel flows freely out of the filter, i don't think it is restricting it too much (but it is cheap and i am open to replacing it just because) i'm not sure if this is a single problem or a couple problems causing a single observable issue. i'm gonna replace the plug wires (they obviously need it) either way - but i'm not sure where to go with the rest of this - good carb clean? a different direction? i'd like to keep this motor going - i've got a motor to replace it with, but that is going to take some work to "do right" and i'd like to have the time to do it correctly instead of having to shoe horn it in there in the middle of mowing season or snow blowing season. Thanks, Jared Share this post Link to post Share on other sites
P3x 55 #2 Posted July 18, 2014 Do you have a Tach installed? If so, does it develop 3600 RPM at full throttle? What does the tach read when you feel the power loss? If the engine is in good shape and RPMs are dropping the governor would be 1st on my list to check out. Share this post Link to post Share on other sites
SousaKerry 500 #3 Posted July 18, 2014 It may be a fuel supply problem. I always start with a good carb clean and a rebuild kit. Even if it is not the problem it sure helps. Next check the the fuel cap to ensure that the vent hole is not plugged by running the tractor with the cap loose or off. Also check flow through the fuel pump by disconnecting the fuel line at the carb and cranking the engine and running the fuel into a clear pop bottle or something. Share this post Link to post Share on other sites
Martin 2,130 #4 Posted July 18, 2014 Check the governor sensitivity. Share this post Link to post Share on other sites
seuadr 488 #5 Posted July 22, 2014 (edited) ok, thanks for the replies, guys. i've got a carb kit on the way, probably should be done anyhow. Do you have a Tach installed? If so, does it develop 3600 RPM at full throttle? What does the tach read when you feel the power loss? If the engine is in good shape and RPMs are dropping the governor would be 1st on my list to check out. does not go to 3600 rpm, but that is my fault. the throttle cable was slipping when i got it, and i was not sure what to adjust it to, so i ended up making "full throttle" 3200. i'll try adjusting it up to 3600 and see what happens with the engaged RPM at that point. she is due for an oil change and i shouldve done a major cleaning of the carb and replaced the plug wires when i bought it so i figure if that clears it up, great, if not, well it needs to happen anyhow. Edited July 22, 2014 by seuadr Share this post Link to post Share on other sites
546cowboy 301 #6 Posted July 24, 2014 I would do a compression check on both cylinders. An Onan will run pretty good on one cylinder. Does it spit fuel out of the carb? Share this post Link to post Share on other sites
seuadr 488 #7 Posted July 24, 2014 no fuel out of the carb, and runs with either cylinder unplugged (guess how i found out it needs new wires ) Share this post Link to post Share on other sites
Cable 179 #8 Posted July 25, 2014 Compression check is the logical next step. Share this post Link to post Share on other sites
seuadr 488 #9 Posted August 13, 2014 update on this - i decided to replace the wires and rebuild the carb. also replaced the bigger of the governor springs with a new one from onanparts. parts came in just before i left for vacation, so i was not able to get to it until today. after replacing and rebuilding, i noticed that the tractor wouldn't start. checked for spark, yep. airbox was off, so air should be acceptable, looked back at the carb fuel bowl - empty. i thought perhaps i'd re-assembled the float wrong, but it looked correct and only appears to fit in one way, so i checked for fuel flow at the carb - nope. no fuel flow back to the other side of the fuel pump, so i decided to replace the fuel pump with an electric one when i got a new inline filter. i like gadgets anyhow, so i figured it couldn't hurt. now i have flow all the way to the carb, and the bowl in the carb has fuel in it. now it starts, but then IMMEDIATELY dies. it does this regardless of choke position, throttle position or apparent interaction from the governor ( i pulled the spring back off and replaced it with the old one, also just left the spring off all together, same symptoms.) not really sure where to go with this one, oil pressure switch possibly? I have the oil right at the full line, and changed the oil less than 10 hours ago. i still have not tested compression, i do not have a compression tester and haven't gotten around to borrowing one yet, but would low compression cause the motor to start and die as soon as it starts? thoughts? Thanks in advance for your time. Share this post Link to post Share on other sites
seuadr 488 #10 Posted August 13, 2014 i am a dirty, dirty liar. the carb bowl was NOT filling - well it kind of was. it filled fine with the carb top off, but when i put the needle back in, i didn't notice the slot in the float, and instead installed the needle by itself THEN the float - so no gas when it was all assembled. seems to run fine now, in fact, really nice (but i haven't had a chance to mow, i need to get a switch for the fuel pump and button it all up) i'll keep you updated on problem! Share this post Link to post Share on other sites
seuadr 488 #11 Posted August 15, 2014 seems to have solved that problem - ran up and down hills mowing deep grass no problem today. only issue i had was not enough gas to finish the job! next i will be doing a dry/wet compression test - now that it runs at higher RPMs I've noticed it smokes a lot more.. i'm not sure if i have a valve leaking by, rings, general slop in the motor or something else. i would assume that abnormal compression numbers and/or compression that improves with a wet test will tell. Probably will do a full valve adjustment and re-ring/hone it at the same time, unless the compression test seems normal, in which case i guess i'll be asking you guys what else causes a motor to smoke? Share this post Link to post Share on other sites
km3h 543 #12 Posted August 15, 2014 It is always good to hear that a problem is solved. Glad you were able to work through it. Share this post Link to post Share on other sites
