Very strange problem, maybe cam timing?

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  #1  
Old 08-14-03, 01:36 PM
matm347
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Very strange problem, maybe cam timing?

Hello all, new the forum, but not to engines. However, this is somewhat baffling me at the moment.

I am working on a co-workers lawnmower with a 4.5Hp B&S "easy start" engine.

The story behind it is that it was given to her because the owner hit some sort of pipe and has not ran since.

When I got it and tried pulling, it felt like it had WAY too much compression, she suspected a bent main shaft but it's not.

I pulled the plug to check for spark, it seemed weak but enough to get at least a sputter. But I noticed that the engine pulled smoothly, normal for one with no plug, no dragging at all.

Put the plug back in and I get the same feeling of super high compression, it's enough to almost make me loose grip on the pull string.

I pulled the head and the valves seemed to move like they should, but I dont know what else could be causing it.

It doesn't back fire, and it is getting plenty of fuel.

I have never been completely inside a small engine, so my question is how possible it would be for the cam to jump timing? Is it gear, chain or belt driven?

Or should I tell her its a gonner?

Thank you for any info!
 
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  #2  
Old 08-14-03, 04:55 PM
Terminator20's Avatar
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Hi there welcome to the forum!

I am no expert on small engines yet. But it sounds like to me that your piston is either bent and or scraping against the sylinder wall, or if that is not it than you might have a major clogging somewhere not allowing enough oxygen to enter the silinder thus possible causing the super compression. First thing I would do would be to check the engine for anything clogging air intake, if all is well and there are nothing clogging the engine, then I would procede to the next step and taking the whole engine apart. Don't be afraid to do that. You will have no problem putting it back together if you put the all the screws and bolts in there own seperate containters with a label to tell you where they go.

PS: Before taking engine apart, look at the blade and shaft below. Make sure that the blade is not scraping it self against the deck and make sure there are no objects including grass obstructing the blade or shaft below the mower. I am sure you will figure this out soon.

Well thats my opinion and I'am sticking to it!
 
  #3  
Old 08-14-03, 07:46 PM
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You might try looking at you flywheel key. Most likely it's either sheared or partialy sheared.
 
  #4  
Old 08-14-03, 08:49 PM
keiser
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I second that!

Originally posted by Jack B.
You might try looking at you flywheel key. Most likely it's either sheared or partialy sheared.
This is my understanding of timing with the small engine.

The key sets the timing accordingly by lining up the flywheel on the shaft so the magnets on the side of the flywheel pass by the magneto at the proper moment. If the key is sheared even the slightest it would effect the performance. It was created in case of just such an incident, to cut down on costly repairs.

Thank goodness for the flywheel key as I think we have all hit something at one time or another...

This is what good flywheel keys look like,

Flywheel keys

Btw, doityourself.com sells them if this does turn out to be your problem or you could most likely get one from your local mower retailer.
Click Here
 

Last edited by keiser; 08-14-03 at 09:28 PM.
  #5  
Old 08-14-03, 11:56 PM
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Hello matm347!

Welcome to the small engines forum!

You have a sheared flywheel key, as suggested. Remove the top engine shroud and the flywheel to replace the key. This will solve the problem. Check first to be sure the crankshaft isn't bent. if it is, the engine may not be worth trying to repair.

Let us know how it goes!
 
  #6  
Old 08-15-03, 12:27 PM
matm347
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Ah yes, this makes complete since. The "MAJOR COMPRESSION" I feel is simply pre ignition due to the rotated flywheel.
I will tear into this simple fix and save the mower.

Thank you for the info!!! I will post results.
 
  #7  
Old 08-18-03, 11:20 AM
matm347
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I pulled the flywheel off this past weekend and saw that the pin looked a little out of shape, but still OK. I hammered out the slight distortion and have the same results as before.

It does not seem to be getting any combustion at all. I pulled the muffler to see if the cycles seemed to be working OK, I did notice the exhaust sucking right before a release, I assume this normal to help pulse the exhaust and the emissions as well???

My next step is to check into making the spark a bit stronger, maybe a condenser?

Any other suggestions as to what this might be?
 
  #8  
Old 08-18-03, 09:17 PM
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Matm,

if the key is even faintly distorted, it is no good. It will throw the timing off and cause the problems you have. A new key is obviously needed here. As to wether or not that is the entire problem, I'm not sure, but you won't be able to further diagnose until it is replaced. They're only 99 cents.
 
  #9  
Old 08-19-03, 08:11 PM
chevman
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DOES THE ENGINE HAVE OHV
 
  #10  
Old 08-20-03, 11:39 AM
matm347
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No, it is the regular B&S Flathead.
 
  #11  
Old 08-22-03, 12:11 AM
HumanFireHazard
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try doing the elimination thing...
but well if you say it fells like there is huge compression and its not igniting... HEY!! wait... check of the coil is not screping the fly wheel... or... maybe there is a lot of build up in the head actulay causing high compression...

but my guess would be... the part that the magnet goes by the coil... it scrapes... take the head of the engine off and give it a check...
 
  #12  
Old 08-22-03, 04:50 AM
mikejmerritt
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Two more possible reasons that woud make an engine hard to roll through. Loose blade (acts as second flywheel) or to much clearance on the exhaust valve which should be .011. To much exhaust valve clearance will cause the "easy spin" to turn into nearly impossible to spin.....Mike
 
  #13  
Old 08-22-03, 08:01 AM
HumanFireHazard
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how would too much clearence be bad?
 
  #14  
Old 08-22-03, 04:26 PM
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Mike makes a couple of good points. A loose blade will cause this problem. The "too much clearance" thing works like this: If the valve has too much clearance, it won't open at the correct time, and will allow too much compression to develop before it opens, causing hard starts. This is something that probably wouldn't change just from hitting a stump, but after checking everything else, it's something to look at.
 
  #15  
Old 08-23-03, 05:22 AM
mikejmerritt
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If you were to roll an engine through and watch the exhaust valve as the piston comes up on the compression stroke you will notice that it jumps up for a second about 3/4 through the compression stroke. This is the famous B&S easy spin function and is caused by a small high spot on the back side of the exhaust cam lobe. If there is to much clearance on the exhaust valve the high spot doesn't push the lifter all the way to the valve giving the compression release......Mike
 
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