I've got a DR Trimmer Mower with a B&S 625 190cc engine.
The engine was running perfectly for about 8 hours of use over 3 days last week. I shut it down when I went to eat lunch on the third day and now it will not start, and pops or backfires when I pull the starter cord.
I've gone through the carb a few times, it's got a new air filter and spark plug, and new gasoline. I pulled the plug and it's getting a spark. I can spray carb cleaner through the carb throat and get the backfire.
I removed the cover on the top of the engine and the flywheel key is not sheared.
Does a backfire necessarily indicate an electrical or timing problem?
Did you change the spark plug before or after the problem started. Some of the newer briggs engines take a special gap plug that will foul out in a moments notice, and will appear to spark but won't under compression.
My trimmer is 2 years old. Before I used it this season I changed the filter and plug. DR sent a package with the machine that included a new air filter and a new spark plug.
I set the gap to match the old plug. I wasn't able to find the recommended gap specs.
Is it possible that something has happened to the valves?
I'm no mechanic - obviously, but today I pulled the plug one last time and poured a small amount of gas into the cylinder to try and get it to fire.
Looking into the spark plug hole, I can see one of the valves - possibly the exhaust valve(?). The valve appears open. When I pull the starter cord, the valve doesn't move. Shouldn't it be moving up and down when I pull the cord? There also seems to be less resistance at the starter cord - indicating reduced compression.
Would stuck valves cause the engine to backfire and not start?
Normal hand tools are fine. May need to get a hold of a torque wrench for torquing the head bolts afterwards, If you remove the valve springs that can be a bear to get back on without a compressor, but it can be done.
If less than 2 years old then take the unit to your local Briggs authorized lawnmower dealer and let him figure it out. Because you could void the warranty if it is that new.
I took the machine to a B&S authorized service place today and the mechanic asked if I changed the oil after the initial 5 hours of use. I told him I did not and he then told me that B&S would not honor the warranty since I failed to follow written maintenance guidelines. He also told me that the work that he would have to do would just about equal the cost of a new engine. Anyway......
So I came home and took the head off myself. Sure enough, the intake valve was not moving. I might have done something stupid, but I took a block of scrap pine and a light hammer and gave the wood and thus the valve a light tap just to see what would happen and the valve dropped back into it's seat. And now when I rotate the engine with the starter rope both valves move like they should.
But the damn thing still will not start.
Could I be missing something less obvious....like a vacuum leak....a faulty carb intake gasket......or maybe an electrical issue? When I remove the spark plug and ground it and turn the engine there is a spark. Maybe the gap is too small? Using carb cleaner as starting fluid does nothing.
There could be a chance that the valve that was sticking is not sealing against the seat and you still have a compression problem. Oh and that dealer that said they wouldn't honor the warranty because you didn't change the oil after the initial 5 hours is lying through his teeth. Unless they can prove that the oil went bad after 5 hours and caused your problems they can't deny it. He just didn't want to work on it. Low oil level they can cause warranty problems with. But you have a warranty problem, and a dealer needs to take care of it.
Unfortunately I live in very rural Texas and the next closest Briggs and Stratton is farther than I want to go. At least for now.
Is this a legitimate test for compression?
If possible, perform this simple compression test: Spin the crankshaft is the opposite direction from normal. A sharp rebound on the compression stroke indicates decent and probably acceptable compression. Little or no rebound means that the compression is probably low. (This is actually the only compression test Briggs & Stratton recommends.)
From: http://www.repairfaq.org/samnew/lmfaq/lmtulmbyb.htm
I'm also going to go over the electronic system again......check the plug gap and armature air gap. I did not check the valve clearance, but I'll do that also.
Everything checks out.....that I have the ability to check.
*The plug and armature gaps are spot on, and there's good spark at the plug.
*The air filter is clean.
*The carb is clean.
*I took the head off again and with the piston at TDC both valves are seated tight. I also checked the valve clearance at the pushrod and that checked out too.
It still refused to start.
It seems to me that there is reduced compression, but the piston top visually looked good, so maybe it's a ring issue. When I pull the cord with the plug installed, there seems to be the same resistance at the cord as when I pull with no plug in the head. I don't own a compression tester, or have the ability to do a leakdown test.
The last time the machine ran it ran fine and I shut it down intentionally. There was no indication that the engine had a major malfunction that caused it to shut down.
After the July 4th holiday, I'll be taking it to a different authorized repair center for DR equipment. This is beyond me.
Did you ever figure it out? I have a 12000 series B&S that lost compression. Turns out there is a little plastic piece (maybe a baffle?) inside the intake manifold which wore down and got sucked into the engine block, jamming the intake valve. I'm still in the process of reassembly but I'm about 99% sure that was my problem. It's worth a look in your case, it could still be hiding behind the valve and not letting it seat properly.
An unscientific way to check for compression is to remove the spark plug and pull the starter while holding your thumb over the hole. The compression should blow past your thumb.
This has gotten to be a bigger Pain in the ***** than it should have been.
Since DR doesn't warranty the engine, I've had to find a Briggs and Stratton authorized repair shop. I even took it to one repair shop who told me initially they were a B&S shop, only to find after 3 weeks of it sitting in their shop, that they actually were not.
So last week I finally found a shop I trust. They called me late yesterday with a voice mail that the oil was really thick in the engine and that the engine had no compression, but did not indicate why. I knew the compression was low when I dropped it off, so that's nothing new to me.
So, as soon as they open this morning (Wednesday 2 August), I'm calling to find out what's up.
The engine has less than 50 hours on it, so I had not changed the oil. A closer reading of the owners manual states that the oil should be changed after 50 hours or annually. So I might get called out for not having changed the oil in time.
I've heard that warranty repair is tough with B&S engines.
Not any tougher than other engine manufacturers, It just has to be an actually failure caused by something that wasn't right from the factory. Most of the time the problem is due to carb issues caused by junk fuel, and nobody pays claims for junk fuel.
the thick oil could be a concern, but that could be an effect not the cause. Blowby debris causes the oil to get contaminated leading to burning and thickening.
So the shop is telling me that there is a "ring issue" causing a lack of compression and apparently evidence of gasoline in the crankcase oil and they point to the use of ethanol laced gasoline as the ultimate culprit.
They are also telling me that B&S will not warranty this engine.
I've priced replacement engines and installation labor charges, and the cost seems prohibitive when new machines cost the same or sometimes less.
So I guess this one is headed for the recycle plant or the dumpster.
I think also that I need an education regarding the use of ethanol gasoline and small engine use and maintenance. Obviously I can't treat modern machines the same way I treated them when I was a kid in the '70's. (I've spent the last 20 years in the US Army and lived mostly in on-post housing with yard maintenance provided by base personnel.)
DR will sell me a replacement engine - a reportedly new EXI (no oil changes needed - ever) engine for $125 + shipping ($39.99) and installation ($80).
The shop tells me installation should take about an hour and cost $80. So given the price of a new machine at $350, replacing the engine makes sense to me.
I would really like to see the inside of this engine. What the rings/piston and cylinder looks like. I could see a leaking carb causing washing of the oil from the cylinder walls causing ring cylinder issues, but don't blame that on ethanol. Straight gas will cause that just the same as ethanol blended fuel. Unless there is excessive fuel in the oil causing thinning I still don't buy the explanation. Blowby past the rings can and will cause gas contamination of the oil.
The gas thinned oil will also cause damage to other bearing surfaces, so crankshaft and camshaft bearings would also be effected.
Unfortunately I'm not the expert you are, so talking to the shop mechanic I failed to ask about the points you bring up. He did mention, and I noticed when I removed the cylinder head, that there was excessive carbon build up on the top of the piston. I attributed this to running the engine rich when someone else was using the machine with a dirty air filter.
But he did mention that the oil in the engine was THICK and NOT thin. Seems to me that gas blowing past rings would do the reverse to the oil and cause it to thin.
Maybe the fuel in the gas was old and had turned into a varnish, thus causing the oil to thicken?
Running with a dirty air cleaner that is clogged can cause dirt to be drawn around the filter, which can and will cause ring and cylinder wear leading to the other problems. that would not be warranty.
