Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

  A newly acquired 5hp 70’s Ariens snowblower started fairly well over several days with test starts and running the engine for a minute.  Then it started a bit harder and just barely putted along having very week combustion after 10-to 16 cycles and died.  It’s been about the same every since then.  It won’t start easily now with gas through the carb.  I can get it going with gas through the plug but it only sputs along for about 10 cycles, very faintly and dies.

   It has 60psi compression after 5 pulls cold.  There was no to  very faint spark so I replaced the points which were pitted.  The spark is good but the same thing of running faintly for a short time and dies.

   My test of the coil was a little fuzzy so I replace the coil but the same thing happens.  It only starts by dumping gas through the plug and it does so in an unusual but consistent way.  I have to pull about 10 times and I hear a faint put or two with a slight breath of smoke.  The next pull or two a few more puts, the next pull about 6 to 10 puts, the next a couple then nothing.  I can do that over and over.  What’s unusual is that it never fires quickly but takes many pulls. 

   If I change the plug often it does not change things.  I check the spark and it seems very fine. I have a plug with the arm cutoff so the spark jumps to the rim.  I test the tester on other machines and their spark is about the same as on the 5hp. 

   Gas is not an issue, spark is not an issue, air is there.  I pulled the head and the gasket is fine.  The valves look ok.  The intake lightly covered with black carbon, the output a light brown ash.

   The timing is set as best I can set it.  The points gap is correct.  I don’t have a dial indicator so guessing BTDC, take off the point wire and use an ohmmeter so just get the points to open at BTDC.  I’m sure I’m at the right cycle as I’ve checked it over and over with a flashlight through the plug watching the valves and watching the key turn on the crank. I’m probably not on the money for BTDC but think I’m close enough.

   The only thing I can think of is if there was a compression leak from a wall crack or rings.  With 60 psi I think I should be running with no load fine.  It should at least run and sound like a good motor even it did not have any power. 

    I’ve obviously missed something so the reason I’m posting.  This business of running fine then within a few days very hard to start or no starting with running only for a short period and with very week combustions has happened before with a lawnmower and a chainsaw.  All very quickly deteriorated to non-starters unless gas was dumped into the plug.  When they did catch and only after many pulls they ran only for a short time and very weakly.  Since it’s happened three times it must have happened to someone else and be a fairly common thing but I’ve never seen a write-up on the web similar.

trouts

   Since I'm confused about what's going on with the weak compressions and my thinking it's due to a compression problem I came up with a test which I think is valid.  I hooked up an air compressor through an adaptor to the spark plug hole.  I got the engine at TDC on the start of the power stroke.  Both valves should be fully closed.   40-50 psi of air was pumped in and there was leaking from the carb muffler area.  After taking off the carb and muffler, air was coming from the exhaust port.  On trying to block it off with my thumb there was significant pressure against my thumb. 

   After thinking about how that's possible I thought possibly the tappet might be lifting the valve.  There was 8 thousands clearance which is the max so ok.  There must be a poor seating between the valve face and seat.  I lapped the valves but did not pre-grind them.  I don't have the tools for that.  I figured a good lapping would be enough.  On inspecting the face and seat they looked ok with a magnifying glass.  ?? I guess not.  I think I've found the problem but if anyone has any comments about the test please let me know. 

  Will re-lapping be enough?

trouts

trouts2 wrote:

trouts

  Will re-lapping be enough? no, if there a that much of a leak u got a serouis problem, is this the same engine that would'nt run ?. check and make sure the seat isn't loose in the block, and when your have compressed air in the cylinder the engine sometimes wants to turn a bit make sure its staying on TDC u may half to hold.

Niper99

   If you were reviewing the job done by someone in your shop then you’d probably be right.  You could count on the guy that did the lapping to have done the job right.  I think I did not do the lapping right.

    The valve face was grayish.  It seemed smooth and even and the seats similar.  I figured since I still had a leak I might not have done the lapping well enough.  I tested them with water in the ports and there was water at the valves.   So poor lapping job.

    There was enough margin and lash to do them again so I did and decided to stay at it for a while and do a better finish also.  I’m using Permatex which is a course and fine together mix.  When you grind the compound breaks down to a fine grit and you finish with the finer broken down particles. 

 

   I ground quite a bit and got the same gray even looking finish.  I then did not add more compound but let it break down and ground with that for a long time. After a while the face top and bottom part started to get polished and reflective.  The middle of the face was gray still.  That told me the face was somewhat concave and the seat somewhat convex.  I ground more but slowly and evenly and eventually the whole face got ground and I think flatter and smoother than previously.  The full face ended up shiny and smooth. 

 

   I checked the job with water in the ports and no water leaked out of the valves.  So I think the lapping was ok this time.   I put the thing back together and it would not start.  Gas and spark were fine.  I took off the carb and muffler and put air in through the plug again with the cylinder at TDC (and a board to hold it there).  There was a lot of air coming out of the breather port.  I drained the oil and there was a lot of air coming from the oil check hole.  There was no air felt on my thumbs at the intake and exhaust ports now.  I think the valves are ok but I have a ring problem.  It was there all the time.  I think the motor had both a ring and valve problem. 

 

    I’m seat of the pants guessing at the air flow through past the rings because I don’t have a proper leak down tester.  At 50psi in it seems like a bunch leaking through the rings.  When I block the oil input there’s a lot of air blowing out the breather. 

  

   I ordered rings this morning, they’ll be sent out Monday and few days later installed and I’ll see what happens.  I’m still fairly new at engine internals so I’m pretty sure got the diagnosis wrong and the lapping wrong.  Hopefully I’m no the right track.

trouts 

Crew,

   There were a lot of problems with this engine and fairly difficult to deal with.  The carb was messed up, valve problems, and intermittent spark causing some confusing problems.  When this was posted I was still ramping up on rebuilding so this engine got put in the background several times.  Over time I’ve rebuilt 5 engines and finally got back to working on this engine again. 

    I relapped the valves, water tested them and put in rings.  It sill sucked, barely started and ran weak.  Since starting this thing I’ve picked up a leakdown tester and after testing this one last week found a big ring leak into the crankcase.  Evidently besides it’s other problems which were worked off it also has a worn bore.  You can’t win them all. 

    The good part was I’ve learned quite a bit and rebuilding and getting strong engines – aside from this one.  I’ve now ordered a decent bore gauge which should be here in a few days and prevent any false rebuilds in the future. 

     Thanks for all the help it’s very much appreciated.

Trouts2

I assume the bore was oval shaped.  This is a fairly common occurrence caused by piston wearing  as a result of poor lubrication.  When it wears, it begins to rock  in the bore usually wearing front to rear.  It's not a readily observable condition but, as you've learned, can seriously effect engine performance.  Was the cylinder beyond a re-bore or just not worth the expense.  I work on old 1970's two stroke motorcycle engines.  When originally produced, they were designed to be over-bored several times.  Max number of over-bores on the Yamaha RDs is eight (2mm).  Which is incredibly robust considering that you can get 10000 to 20000 miles per over-bore providing the bike is ridden sensibly and good quality oil used.      

What does you local machine shop charge to oversize the cylinder?   I have not done it in quite some time..   but if you fine a hungry machinist it can be done cheap..

Depends on you location and their workload, you may want to give them a call.

Friiy