Piston rings need help

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  #1  
Old 06-13-07, 01:35 PM
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Piston rings need help

I have a 12 years old craftsman lawn tractor with a Briggs Stratton engine model 28n707 type 0171 01. New air filter, new fuel filter, new breather gasket, new spark plug, cleaned carb., changed the oil, crankcase breather still look good... I have trouble with a lot of engine smoke. The smoke is white and black and smell like gas, It getting worse when i engage the cutting blade. I let the engine run without the air filter and smoke came out of the breather hose to the air filter chambers. I talked to the tech at a local small engine repair shop and they suggested that i have a bad piston ring and the smoke came back to the crankcase and back to the breather hose. Is it difficult to change the piston rings. any suggestion?????
 
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  #2  
Old 06-13-07, 08:10 PM
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I'm not familiar with the B&S engine, but I'd like to point out the word "difficult" is a relative term. It depends on how experienced and adventurous you are with mechanical stuff. Rebuilding a small engine, or even a car engine, is fairly straight forward. It doesn't take many special tools... other than a valve compressor and piston ring expander. Both are only a few bucks. The rest is just ratchets, wrenches and busted knuckles.

If you've never rebuilt an engine before, or even if you have, a service manual can be a big help. If your engine is smoking, I'd say worn rings are a good bet. You've probably lost a lot of compression and power as well. Once disassembled, you will need to take the crankcase, cylinder head and crankshaft to a machine shop.

It's a good bet the cylinder is scored and will need bored or honed. Honing will take out small imperfections. Boring will take out large ones. The machine shop will measure the cylinder and tell you what it needs. If it needs bored, you'll have to buy oversized pistons and rings to match. They are typically 0.010", 0.020" or 0.030" oversized.

Have the machine shop check the crankshaft too. If it's rough, it will need polished. Polishing doesn't change the diameter. However, if it's "real" rough it will need turned. Turning typically removes 0.010" from the diameter. In that case, you would need to buy a matching 0.010" undersized piston connecting rod.

You should also have the machine shop check the cylinder head for flatness. Aluminum heads sometimes warp due to heat. They can also get warped if you tighten the head bolts in the wrong sequence. The machine shop can mill the head flat again if necessary.

Your valve guides are probably OK, but you might as well have the shop check those as well. It only takes a minute to measure stuff and make sure it's within spec. Now is the time to replace it, if it's not.

You can buy all the parts in a master rebuild kit (gaskets, piston, valves, etc.). Connecting rods are often part of the rebuild kit, but sometimes you have to buy them separate. In some cases you don't have to replace the connecting rod, but I always do. They aren't that expensive. For reference, the rebuild kit for my Kohler engine contained everything needed and cost $135 (eBay). Our local machine shop charges $35 to bore a cylinder, $20 to mill a head, $20 to turn a crank and $24 for a complete valve job.

Rebuilding an engine can be scary the first time, but it's not really that difficult. Just follow the service manual and go one step at a time. You'll do fine. Spending a day or two tinkering and a couple hundred dollars is a lot cheaper than buying a new mower.

Oh, the machine shop guy tells me that dust getting past the air filter is a common cause of worn piston rings. He's seen engines that had to be rebuilt in less than a month because the air filter was left off and the rings wore out from abrasive dust.

Good luck,
Barry
 
  #3  
Old 06-14-07, 02:17 AM
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If you are sure you have the breather assembly installed correctly (not upside down) for proper oil drainage purposes then you will have a carburetor in need of a soaking and reconditioning. The white smoke and fuel smell is a clear indicator. Forget the complete engine rebuild, I doubt you need this based on what you have described. To rebuild the carburetor you will need a good bath type cleaner (I like Napa's # 6402), a good spray carb/choke cleaner (I like Briggs # 100042) and compressed air (not absolutely necessary but nice to have). You'll then need to determine if the carburetor on the engine is a Walbro or a Niki and purchase the correct rebuild kit and also the correct intake gaskets and possibly a new float for the carburetor. Do you have a local small engine shop you can turn to for parts and advice?
 
  #4  
Old 06-14-07, 05:53 AM
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Thanks for the input

Thank you for your input CowDoc, i will purchase a service manual and look into rebuilding the engine. And Puey, I clean out my carburetor good, i spray all the valves with autozone choke and carb cleaner and the engine start with the first turn. What is the carburetor have to do with white and black smoke came out or the engine and the breather hose? Why do i have to rebuild the carb.? Anyway, I have the Walbro float type carburetor and the local small engine shop is only 2 miles from where i lived and they don't give out details advice because they want people to bring their engine to there shop.....Puey,, what type of service manual you recommended for the engine that i have (28N707) 15.5HP OHV and part # for the carb. rebuilt kits. Thanks for all inputs.
 
  #5  
Old 06-14-07, 10:45 PM
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Does the oil smell like gas? That's my bet, and also what Puey61 is getting at. The carb leaks fuel into the cylinder, which seeps past the rings and into the crankcase. There it thins the oil and blows past the rings and most other seals, causing the engine to smoke. I doubt your rings are bad.
 
  #6  
Old 06-15-07, 02:02 AM
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The shop you have in your area may not dole out advise but they should sell you a Briggs service manual for single cylinder overhead valve engines. This is what I recommend you purchase as it is relative to your particular engine and will be of great help and is well detailed.
 
  #7  
Old 06-16-07, 12:12 AM
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Wouldn't a simple compression test tell the story? Low compression=bad rings. Good compression= carburetor.
 
  #8  
Old 06-16-07, 02:23 AM
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Low compression indicates trouble in the combustion chamber, not necessarily the rings. Lets say you have a valve seized in the open position or just a burnt/pitted valve face or a blown head gasket, you'd have no compression.
 
  #9  
Old 06-16-07, 04:31 AM
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How to do a compression test?

How I would do that simple compression test? Do i need any special tools to do it. Anyway, i'm going to pickup a service manual at my local briggs and stratton shop $17 and a carb. rebuilt kit $35
 
  #10  
Old 06-16-07, 05:55 AM
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A compression tester can be purchased at most any automotive parts store, you will need one that has 14mm threads - most do anyhow but be sure. Remove the spark plug, thread in the tester, crank the engine over for 4-5 seconds and read the tester. 60psi or below is low, 60-90 is borderline and above 90 is quite good.
 
  #11  
Old 06-16-07, 03:25 PM
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Puey61, clearly valves could cause low compression and the test is not specific to rings. But if you need to do a valve job on a 12 year old single cylinder engine, you might as well rebuild the engine at the same time. It's already pretty much disassembled. So, if there is low compression, whatever the cause, wouldn't you rebuild the engine... if it were yours?

In truth, I guess that doesn't eliminate the possibility of an additional carb problem though.

Barry
 
  #12  
Old 06-16-07, 05:56 PM
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I would not automatically rebuild the engine if I needed a valve job. A valve job, on your overhead valve engine, requires only the cylinder head to be removed. In order to rebuild the engine, you must remove the engine from the chassis and then completely tear down the engine...Much more work involved! If you can get your hands on a leak-down tester, on the other hand, you could then pinpoint the exact trouble, if any, in the combustion chamber. A leak-down tester pressurizes the combustion chamber and you then listen for leakage past the possible points of origin...muffler, carburetor throat, head gasket and oil fill tube. Air from muffler - exhaust valve problem, air from carb - intake valve problem, and, air from the oil fill tube - ring(s) problem. Now lets say you test such and the air is from the carb indicating an intake valve problem, this may eliminate the carburetor as a possible problem since you would have poor vacuum in order to draw in the fuel charge to the combustion chamber. This does not necessarily eliminate any potential carburetor issues but certainly could.
 
  #13  
Old 06-17-07, 10:52 AM
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Puey, of course you're right. I wasn't thinking it was overhead valves and a leak tester would be ideal. A bit more expensive and harder to locate though.

Even at that, personally, if I owned a 12 year old mower and had to take the head off, I'd finish tearing it down and install new rings. Mind you I wouldn't do it if I were in the business of repairing other peoples engines. As Puey noted, there's more time and cost involved, and you don't fix what ain't broke... yet.

I've rebuilt my Kohler three times over the years for various reasons. Everytime, the ring gap was excessive and I had to bore it twice due to scoring. Hopefully, the new carb will solve Dave's problem. In any case, it's fairly cheap to try. Let us know how it turns out.

Barry
 
  #14  
Old 06-17-07, 05:32 PM
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For what it's worth, I've got a early seventies John Deere 212 that I use quite frequently around my property. While not for mowing, I do use it for snowthrowing in the winter (which is demanding here in upstate NY), towing around a loaded 15 cu. ft. wagon for yard chores and towing around a loaded boat trailer and loaded two place snowmobile trailer and not once have I had to rebuild the engine. Four things that I can't stress enough of great importance to maintaining a long and healty engine life are: 1- Synthetic oil; 2- Cleaning the cooling fins once yearly in the spring - requiring the blower (flywheel) shroud be removed to access the fins; 3- Regular/frequent cleaning of or replacing the air filter (operating conditions dictate how often); And, 4- Keeping the engine compartment clear of debris - I use my handheld leaf blower after each and every use to blow out the compartment. These steps are not time consuming and you just need to get yourself in a routine to do them religiously.
 
  #15  
Old 06-17-07, 09:47 PM
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Engine still smoking

Dear Puey and CowDoc, Thanks for your guys inputs but the smoke now turn to black. Everytime i engage the blade and put some loads to it, the smoke seem to be worse, black and smell like gas. After i mowed my lawn, i remove the air filter and check the breather hose and saw some smoke came out off it and some smoke came out of the carburetor thru the choke valve. I have new air filter, new fuel filter, new breather gasket, cleaned the carb, changed the oil 3 times since the smoke happened, the oil level ok. I never fix any internal engine so i'm affraid that i will mess things up. Do you guys think that it be worth it to bring the tractor to a small engine repair shop and let them do the job or follow the repair manual and try fixing it. Someone suggested that i have a blown head gasket or bad piston rings.
PS i also has new spark plug
 
  #16  
Old 06-18-07, 12:16 AM
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Whether you should work on the engine is a personality and financial question. Do you like working on mechanical stuff? Do you like doing new things? Do you have the time? Can you afford to pay a pro to do it?

Most people could rebuild an engine if they really wanted. I guarantee you're smarter and braver than it is. If you get stuck, do an internet search or ask here. Someone is always willing to offer advice. So yes, I think you "can" do it with a little time and effort, thousands of other people already have. However, only you can decide if you "want" or "need" to do it (i.e to save money). Just remember, life is to short to stress out over a mower engine. If you don't enjoy this stuff, and can afford it, pay a pro.

The first question should be, "Do you even need to rebuild the engine or is it something else?" Maybe you should consider paying a shop just to check the engine out. Once they determine what's wrong, you can decide from there. What happened with the carb rebuild?

Puey, it sounds like we use our mowers quite differently. I have a 1980's JD 316 that I use on the farm. It sees heavy use and about 8 hours of mowing per week. Some of that is more like brush hogging. Sometimes it's so dusty I wear a mask. The oil is changed regularly and I use a compressor to blow off the engine and tranny each time I use it. It gets a full wash down about once a month. My main problem is dust. I go through air filters like crazy. I've even considered rigging up an oil bath air filter. I figured the engine was due for it's latest rebuild when it was burning a quart of oil to 5 gallons of gas. Now tell me those rings weren't worn!

Barry
 
  #17  
Old 06-18-07, 08:05 PM
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Dave,

Have you checked the oil? Again, Does it smell like gas???

The black smoke tells me you have a craburetion problem...NOT an internal engine problem or blown head gasket.
 
  #18  
Old 06-18-07, 09:30 PM
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At this point, I have to agree with Cheese and Puey. Black smoke is almost always related to a carb problem, and what they said about gas getting in the oil makes perfect sense. Try the carb rebuild and let us know.
 
  #19  
Old 06-19-07, 09:07 AM
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The Smoke Is Black And Smell Like Gas

Yes, i checked the oil level and it's right on the mark. The smoke is black and smell like gas. As i put some load to it, the lawn tractor put out more black smoke and smell like gas. I removed the fuel bowl 3 times and clean the floater, the needle and all the valve. I remembered, The first time i remove the fuel bowl to clean. When i put them back, the float needle doesn't shut so i have alot of fuel that came up from the choke valve and enter the crank case thru the breather hose. the I have not rebuild the carb yet because when i remove the fuel bowl to check on the float and the needle, it still look very clean. I bought some carb and choke cleaner and spray them good. If your advise is to rebuild the carb due to the smoke is black and smell like gas, i'll try to rebuilt it this weekend. Thanks for all of your advise.
PS: I already changed the oil, breather gasket and spark plug.
 
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