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Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 3:35 pm
by justin m
Well my neighbors son bought a dual mag 220 GPU not too long ago. He ran it quite a few times but he just was'nt happy with the performance. It ran rough sometimes and it seemed all over the place. He took it to a well known airboat builder to have the mags looked at. They were in time and would lay you in the dirt if you got bit by it. He noticed that the misfire did'nt stay in the same place. He was confused and was'nt quite sure what to do. He called around and got different opinons from people. Well they ended up calling Sensenich. (This is where it gets a little confusing) Sensenich told them the prop he had on his engine was'nt the right prop and something to do with the balance or harmonics in the engine and the prop, was causing the engine to miss all over the place. :scratch: Well he ordered a new prop from Sensenich and put it on the boat. The boat cleared up for a few minutes then started missing again. They took the prop back off and rotated it counterclockwise to the next set of bolts. Started it up, it ran fine for a few minutes then started missing. Removed the prop again and rotated to the next set of bolts. Started it up and has not had any problems out of it since then. It's been a couple weeks now and it's still running fine. I'm having a hard time understanding how this is possible but from what my neighbor told me, it did happen. Has anyone ever heard such a thing? If so, feel free to share your experience!

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 6:54 pm
by Comanche-pup
Sounds like a dirty carb


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Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 7:55 pm
by Birddog247
Sounds like a good story.

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 8:06 pm
by Scarecrow
Birddog247 wrote:Sounds like a good story.
That's Loxahatchee for-----Yeah right---wink wink. :lol: :stirpot: :rebel:

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 9:13 pm
by midnight rider
Had it happen more than once. Prop hub is prob out of tolerance and causing bad harmonics. I would send the hub back and blades with it to be checked. Shouldn't have to clock a hub for it to run right

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 9:34 pm
by Coverall
I have a left handed hammer and a board stretcher in the trailer if it will help. :)

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 10:02 pm
by Ol Yeller
It's spring time so it could be mud chiggers. :roll:

Just kidding, hope it continues to work good.

Re: Prop causing GPU to misfire????

Posted: Wed Apr 29, 2015 10:02 pm
by Ol Yeller
It's spring time so it could be mud chiggers. :roll:

Just kidding, hope it continues to work good.

Re: Prop causing GPU to misfire????

Posted: Thu Apr 30, 2015 12:30 pm
by twister69z
And then big foot drove the boat away! Seriously it is a intermittent problem in the magnetos , carburetor, or a cylinder loosing compression.
Twister69z
Troubleshoot it on the trailer find the problem.

Re: Prop causing GPU to misfire????

Posted: Thu Apr 30, 2015 1:08 pm
by terrible ted
Well on air craft it usually for hand propping the engine.

Re: Prop causing GPU to misfire????

Posted: Thu Apr 30, 2015 4:41 pm
by FLA BOY
It could also be an intake gasket. It may be running better now because of the warmer weather causing it to swell.

Re: Prop causing GPU to misfire????

Posted: Thu Apr 30, 2015 5:33 pm
by John Fenner
There could be an exhaust seat bouncing out and in, have a leakdown done.

Just so you know, harmonics can play dangerous games in reciprocating engines, seen a couple cracked cranks due to 3blades on Lycoming 540s the cracks run lengthwise through the flange and toward the slinger, main seal was original leak diagnosis but leaked with new seal. Four cylinder Lycomings get similar effect with 4blade props.

That said the harmonics can in fact have an affect on engine performance, chatter in the timing gear train causing lifters to bounce and pump up, yes if all 12 dry valve lash are set perfectly then I would say impossible but the intake seat(s) could be hammered into the head causing a theoretical longer valve causing the issue you experienced. Clocking a prop should have no affect as balance is just that, balance.

Re: Prop causing GPU to misfire????

Posted: Fri May 01, 2015 5:11 pm
by justin m
This is just what my neighbor told me so I figured I'd ask to see what everyone else's opinion was. Supposedly the guy checked it out for him as any other airboat mechanic would. Leak down, carb clean, mags gone through, plug wires, new plugs, checked for intake leaks, ect. I don't want to put the name out there of the builder because he does jam up work and I don't know how accurate the story is lol I was just having a hard time understanding how rotating the prop to different bolts caused it to work fine. Could be true, could be a jacked up story. All I know is I've seen some weird stuff happen to engine in my line of work. I'm not an A/C mechanic but all engines are engines.

Re: Prop causing GPU to misfire????

Posted: Sat May 02, 2015 10:00 am
by midnight rider
Just FYI. I had a two blade prop from a reputable prop company that was causing bad harmonics and vibrations thruout the engine. After time and time again of checking rechecking , clocking and reclocking i sent it back and the hub was so far out of tolerance that they sent me a new one and redid my blades. No issues after that. I've seen it many other times too especially with stainless rigs where they crack a lot. It doesn't really cost anything to send them in and have them checked.