B&S Backfire Through Carb And Won't Start Hot

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  #1  
Old 08-18-13, 02:38 PM
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B&S Backfire Through Carb And Won't Start Hot

Engine is a Briggs 12.5hp, Model 28V707, Type 1113-E1, Code 020628ZD in a Murray model 40508x92 riding mower. This is not an OHV engine. Model number is for an aluminum bore engine, but it is a cast iron sleeve engine.

Issue is a lot like this thread below that was never resolved: http://www.doityourself.com/forum/ou...backfires.html

Observation
Mower is hard to start when cold, but runs well once started. Mower will not start once hot.
With air filter removed, flames will backfire through the carb when engine turned over.
Sometimes engine will die unexpectedly shortly after cold start.
Will plow through tall grass when running.

What I have found and fixed
Intake valve had zero clearance. I pulled the head and removed the valve. I had to grind .009 inch to achieve a .005 clearance with piston at .025 after top dead center. I also removed quite a bit of carbon deposit from the intake port and used a wire wheel to remove carbon deposit from the valve stem. Valve seat looked good, so I left it alone.

The exhaust valve is .007 and should be .009 minimum. I don't have a valve spring compressor so I left it alone.

Cylinder had no signs of wear or scaring, so I left it alone.

Valves were functioning freely and opening and closing as should be. I removed all carbon from the head, installed new head gasket and torqued head bolts to 15 ft lb.

Carburetor had rust in the float bowl, but the float valve looked new. I disassembled all parts and cleaned them with carb cleaner. I sprayed carb cleaner through all the ports to make sure they were not plugged and sprayed 80 psi of air through all ports. I found the choke lever was not closing the choke properly, so I adjusted it to function properly. A new foam air filter and paper inline fuel filter was installed. I also replaced a cracked crank vent tube.

New Champion RJ19LM spark plug was gaped and installed. I get a good blue spark with cold or hot engine. After running, plug reads a dark tan color on the insulator and black carbon on the outer surface. Plug does not wet foul after failure to start.

I have pulled the flywheel and inspected the key. There are no signs of damage. I cleaned grass and dirt from around the charger magnets and removed rust from the outer coil surface area. I gaped the coil at .013.

Something that makes me go hmmmmm
As noted above, the specs for this engine model number indicate the series 280000 should be an aluminum bore, but in fact it is a cast iron sleeve. I have seen the sleeve with my own eyes, plus the mower body hood advertises the mower to have a cast iron sleeve (mower body, not engine cover).

The reason this makes me go hmmmm is the B&S specs for the series 280000 L Head aluminum single cylinder valve clearance is: Intake .005-.007, Exhaust .009-.011. However, the specs for a cast iron L Head single cylinder engine is: Intake .007-.009, Exhaust .017-.019

I have researched internet post ad nauseam about this symptom, but nothing is working When I saw flames backfiring through the carb I really really expected to find a bad flywheel key throwing the timing off. Not the case; that key looked perfect :NO NO NO: Any help appreciated.
 
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  #2  
Old 08-18-13, 03:19 PM
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Two things come to mind and it centers around the zero lash you found on the intake valve. One is the valve was either recessed into the seat or the lift on the cam is worn.

You might pull the intake again, put it in a drill and spin it to check for warp age. You can spin it against a fixed pin. It that is good put a little metal grinding paste (or a smear of toothpaste) on the valve, put it in the seat, and turn it. Then look at the seat where the valve made contact to make sure the valve is seating.

I was just rereading this. You had the larger lash spec on the engine you think you may have. It wouldn't be a worn cam, then. It would be more likely the valves or the seats are worn out. Also the seats could be loose in the head.
 
  #3  
Old 08-18-13, 04:02 PM
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Yes, I guess I am going to have to pull the head again and check the seating. I'll set the piston at TDC and see if I can turn the valves. If so, they are not seating well.

After I pick up a Briggs valve spring tool 19063, I'll take those valves out and give them your spin test.

BTW, I have seen two different ways to check valve clearance on the internet for the flat head engine. One is to take the clearance check with the piston at TDC and the other is .025 after TDC. Does it matter on this engine?

Also, I just did a cold engine compression check and I get 80 psi. Is that about where it should be?
 
  #4  
Old 08-18-13, 04:24 PM
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.025 is with a compression release. If the exhaust valve is held slightly open at TDC, then snaps shut at .025 = compression release. 1/4" or 1/2" past isn't going to break anything. You just need the comp release out of the game. It has it's own little gizmo (ACR) that does that stuff. It's for easier starting. If it's messed up, it causes its own nest of problems.

It will run fine on 80#. An engine in good shape will have over 100#. A lot of it depends on what is dropping the compression. If everything is in great shape except the intake valve, it won't run as well as if the problem is from ring blowby, i.e.. Two strokes will run a little higher.
 
  #5  
Old 08-18-13, 06:40 PM
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Just came back from the garage. I pulled the carb off. It is disassembled with all rubber and plastic pieces off and will now soak in carb cleaner solution for a day or two.

Valves seem to be seating well. I can turn them with the piston at TDC, but it is hard to do. I also stuck a small flash light in the intake port with the valve seated and I couldn't see light with out raising the valve a bit.

This just doesn't seem to be a valve problem anymore...sure would have made a lot more sense if that fly wheel key would have been sheared off
 
  #6  
Old 08-18-13, 07:02 PM
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If you're about fifteen degrees before TDC on compression (both valves closed) can you turn the intake valve by putting your thumb on it and twisting. Then go to TDC with the valves closed and turn one revolution, going past TDC. Does the exhaust valve open fully?
 
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Old 08-18-13, 07:12 PM
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At 15 BTDC I can not turn the intake with my thumb. I can turn intake by pressing thumb and finger against surface, but it takes some effort. I can turn the exhaust with my thumb only, but that also takes a little effort. The exhaust is a rough surface, so my thumb grabs it a bit easier.

After one revolution, the exhaust valve fully opens.

I have also ran the starter with the head off and observed the pistons and valves. All are functioning as expected.
 
  #8  
Old 08-18-13, 07:33 PM
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It may turn out to be in the carburetor. The thing that stands out is the lash on the intake and how far you had to go to get it. That and the discrepancy on the specs/engine you have.

When the carb gets back on it, maybe the backfiring will be gone. That type of thing can be caused by hot carbon (preigniting), advanced ignition (sheared flywheel key) the intake not sealing fully,the exhaust valve not opening, or the carb fuel air mixture.

Let us know how you make out.
 
  #9  
Old 08-18-13, 07:46 PM
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Yours doesn't have the compression release.

80# is very low... low enough to cause hard starting. That, combined with the popping out the intake indicates a valve problem or ignition timing problem. I don't think your valves are seating properly, either because of a seat/valve face issue, or a clearance issue.

Just to make sure the valve timing isn't off, do the valves "rock" between the exhaust and intake stroke at the exact same time that the piston reaches TDC?
 
  #10  
Old 08-19-13, 04:24 AM
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Cheese, I will check the rock condition this afternoon.

When you say rock, do you mean slight movement?
 
  #11  
Old 08-19-13, 09:55 AM
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One closes and the other opens at exactly TDC between the exhaust and intake stroke. The point where the one closes and the other opens should coincide with the point the piston reaches tdc, and if it happens just a fraction earlier or later, the cam is out of time.
 
  #12  
Old 08-19-13, 03:48 PM
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There does appear to be a valve overlap.

On the exhaust stroke, with the exhaust valve open and the piston on the up stroke, the intake valve begins to open when the piston is about 1/4 inch from TDC. At TDC, the intake valve is about 1/4 way open.

I can put a degree wheel on and give you specifics when the intake valve opens, or I can put a dial on the piston and measure inches before TDC the intake valve opens if desired. Maybe it would be better to just post a picture?

I would expect a little valve overlap, but it does look excessive
 
  #13  
Old 08-19-13, 07:25 PM
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Cheese,

The more I check the valve timing the more it looks like you have identified the problem.



In this picture, the piston is 3/8th inch before TDC on the exhaust stroke. The exhaust valve is open and the intake valve is at the point where it first begins to open.



In this picture the piston is at TDC on the exhaust stroke. As you can see the exhaust valve is still slightly open and the intake valve is way far open This explains flames backfiring in the carb

As I rotate the crank through all the strokes, the valve timing is out of sync at each one.

So, I guess future posts will include pictures of the engine tear down Hopefully there will be a picture of misaligned timing marks on the cam gear

If not
 
  #14  
Old 08-19-13, 07:46 PM
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What is the history of that engine? Are you working on it for someone?
 
  #15  
Old 08-19-13, 08:17 PM
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Yes, my mother and father-in-law found this rider mower for sale. It's probably around 10 years old, but the owner was only asking 50 bucks. They have about an acre vacant lot they were mowing with a push mower, and were interested in something to ease the job.

When I went to check the mower out for them it started right up and the engine sounded good. So I recommended they buy it and I would fix it up for them.

So I performed the usual like power wash the mower, battery replacement, new air filter, new plug, changed motor oil, sharpened the blades, replaced the mower belts, beat on the mower deck with a large hammer until it was straight again, aired up tires and took it for a test run. Everything worked well even though it took a few tries to get the motor running.

My in-laws mentioned the mower would not start after it was hot without letting it set for a long period. A few weeks later, I helped out with the mowing and had the same experience...started cold and ran great until I finished mowing. Then it would not start even if I beat it like a rented mule

This weekend I was going to help out with the mowing again, but first I wanted to adjust the valves to fix the no hot start issue. Research on the internet lead me to the valve issue conclusion. Sure seems to be a LOT of people complaining about mowers that won't start when hot without any resolve...I wonder how many might be due to engines shipped from the factory with misaligned cams

Anyway, after nothing suggested on the internet would fix the problem, I posted the question here.

Kudos to you guys...I think we are the right path
 
  #16  
Old 08-19-13, 08:33 PM
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I'm thinking someone has been into this and has the cam off a notch or two. The valves should both be shut when the piston is at TDC on either stroke. If one is not, the valve timing is off. On engines with timing belts, even car engines and other engines, I sometimes use this method to time the valves with the engine rather than worrying about looking for timing marks and lining them up. Works every time.
 
  #17  
Old 08-21-13, 08:08 PM
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And the answer is...cam gear misaligned one tooth



This is the alignment I found after splitting the cases. Not good :NO NO NO:



This is the correct alignment

Now for more fun findings...



In my earlier thread I posted a picture with the cam misaligned and the piston on the exhaust stroke at the point were the exhaust valve first started to open. I stated it was 3/8" before TDC, but it is actually 7/16". In this picture the cam is aligned, which moved the piston within 3/16" before TDC.


Next photo please



In this picture the cam is aligned and the piston is at TDC on the exhaust stroke. The intake valve is open, but not as much as the misaligned cam photo. The big difference is on the misaligned photo the exhaust valve was slightly hanging open. But, with the cam aligned the exhaust valve is closed

So the conclusion is there is a valve overlap even when the cam is aligned properly

So why the intake valve overlap My guess is the cam profile and cam timing are designed for low end torque



You can see the intake cam from this picture. There doesn't appear to be any duration built in the ramps on either side of the cam profile It is basically snapping the valves closed very quickly once off the lobe. This plus the intake overlap probably help lawnmowers plow though grass.

Anyway, I should have the engine bolted back by this weekend and I will let you know if life is good or life is banging my head on brick walls
 
  #18  
Old 08-21-13, 08:53 PM
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Valve overlap has nothing to do with timing of the cam but their relationship is significant in the application of the engine.. The latter is indexed on the crank while overlap is the relative position of the intake and exhaust lobes on the cam.

That cam lobe is pretty typical, outside performance products.

You've done a nice job documenting this. Everyone will appreciate it.
 
  #19  
Old 08-21-13, 09:52 PM
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There will be a tiny bit of "overlap" on this engine. Don't be concerned about it, it's normal. The main thing is that the point at which the two valves rock, or reverse movement, coincides with the piston reversing movement as well.
 
  #20  
Old 08-24-13, 09:04 PM
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Well, it looks like we have multiple winners to the What In The World Is Going On With This Engine contest.

We know the timing was misaligned, but was that the end of the story...NO! As it turns out that coil that was producing nice blue flames was just messing with me. More on that later.

Keep reading and I will tell you the full fifty cent tour of failure before success.

After I bolted the cases together I installed the engine on the mower. Then I started work on the valves.



I lapped the valves and valve seats until they had a nice even surface.



It took a little while, but they cleaned up nicely.



Then I carefully ground the ends of each valve stem to the specified clearance.

I installed a new head gasket and torqued down the head bolts...Well, this part of the story really didn't go as easy as I'm letting on, but this thread is long enough already. Let's move on.

Remember that carburetor I started soaking about a week ago?



Ahhh...nothing like the smell of Berryman Chem-Dip in the morning.



Here we are all cleaned up an ready to go.

So, at this point, the cam is aligned properly, valves are lapped and adjusted, and the carburetor is spotless clean. This engine should fire right up, right...NO...fail again!

Being a bit miffed and dejected, I set back for the longest time pondering what the what could be going wrong. After much effort, it actually did start once and ran great through all the RPM ranges. I shut it off and it started right back up. But it wouldn't start again.

I thought it must be a fuel problem...possibly not getting enough fuel to the carburetor. Being the inline filter was the only thing that could block the flow, I decided the new paper filter was too restrictive.



I replaced the paper filter with the standard B&S filter, which is basically nothing more than a screen inside the plastic cover. Weeds can make there way through those filters, so surely gasoline can.

Well it worked...once. Right after I installed the new filter the engine busted right off! But following attempts were failures again.

Searching for ideas, I ran across this article from B&S FAQs: Briggs & Stratton Ignition System Testing | Small Engine Repair FAQ. The coil information intrigued me. I knew coils could be too weak to produce a spark under compression, but I never ran across one.

So the next morning I ran through the checks. First check was to get a resistance reading on the secondary coil. My reading of 4.5K ohms was on the high end of tolerance, but still within tolerance. Next test was to do a spark gap test. I don't have a spark gap tester, but I ran across a YouTube where a guy made his own from a spark plug.



He made one by cutting off the stem of a spark plug. I did kinda the same thing, except I just opened the gap to 0.166 inches like the B&S article claimed. The coil would produce a spark, but it was weak and inconsistent. In other words, it didn't always spark when it should have. At this point I didn't know if the result was because of my home made spark tester or other reasons, but I went on a leap of faith and bought a new coil.



Installed the new coil and hooked up the home made spark tester. The new coil had no problem throwing an arc 0.166 inches. Turned the ignition key and BAM it started first try. It ran up and down all RPM ranges no sweat. Also was able to shut it off and restart repeatedly. But as a true test, I took it out and mowed about an acre of tall grass with no problems and it ran like a champ all day.



So the coil was the last part giving me grief. By the way, the new coil also measured 4.5K ohms on the secondary coil. So a reading within tolerance doesn't necessarily mean you have a good coil. Follow up with a spark test.



And in case you are wondering, here is a live action photo.

Thanks to Cheese and Marbobj for hanging with me. Hope all this helps other out with such problems.
 
  #21  
Old 08-24-13, 10:13 PM
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Great! Thanks for the update, and a good thread. Glad we could help.
 
  #22  
Old 08-25-13, 06:11 AM
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Yup. Congratulations on your success. A great contribution to the DIY forum.
 
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