OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs 375 popping

Hi Guys,
Got one I am not 100% sure about before I progress.
I acquired Briggs and Stratton 375 Sprint Engine (It�s a slightly older briggs with the starter clutch). A very good engine from my experience.
Model 98902 Type 0363-01 Code 94092003
I�ll do my best to describe the problem.
So far I have:
Cleaned the carb and replaced the diaphragm
Replaced Pull rope
Replaced spark plug
Replaced the oil
Replaced the bottom oil seal
Upon getting the engine back together it started pretty easily and sounded quite good. Once the engine was put under load it started to pop ever so slightly from the exhaust and once the engine was quite warm (10 mins or so of heaving mowing) it lost some power. Not a lot but noticeable.
When restarting the engine is started and chugged and after a few seconds it eventually got back up to the required speed.
As a result of the initial test I proceeded to check the tappet clearances
Exhaust - .20mm
Inlet � couldn�t fit the .04mm gauge.
At this stage I am thinking about laping the exhaust valve only and ever so slightly grinding and lapping the inlet valve.
Wouldn�t mind some pointers. I�ll put some photos up tomorrow!

Specified valve clearance is inlet 0.006" (0.15 mm) and exhaust 0.008" (0.20 mm). The Briggs Easy Spin feature holds the inlet valve open slightly during the compression stroke, so you need to set the engine slightly past top dead center on the compression stroke, so that the piston has moved down the bore about 6 mm. If you still find very little inlet valve clearance, you will have to shorten the stem of the inlet valve. The engine cannot be expected to run with effectively zero inlet valve clearance, and if you try it will pop in its inlet pipe, if it runs at all. You may have burned the valve if you have been running it like that.

Begin by lapping both valves until there is a clean band of metal showing all the way around both valve head and valve seat. The clean band should be about 1 mm wide on both surfaces.

The exhaust valve clearance is already correct. However if you confirm that you have little or no inlet valve clearance, you need to shorten the stem of the inlet valve by about 0.15 mm. If you have a bench grinder, you can do this by clamping a straight-edged piece of metal to the tool rest, using a square to ensure that it is exactly at right angles to the side of the grinding wheel. This piece of metal will then be your guide for grinding the valve stem. Grind a tiny amount off the tip of the stem, using the guide to keep it perfectly square against the side of the grinding wheel, then just drop the valve back loosely into the guide with the engine slightly past top dead center on the compression stroke, so that the piston has moved down the bore about 6 mm from TDC. Use the feeler gauge to measure the tappet clearance, holding the valve firmly against its seat. Keep taking tiny amounts off the tip of the stem until you have the required 0.15 mm of tappet clearance. Do not be tempted to remove more than a tiny amount at a time, or you will overheat the valve stem and burn it, so that it gets a blue colour on the surface where you have been grinding.

It will then be time to reassemble the engine, check the valve clearances again to be completely sure, then start it and check for any irregularities of performance.

Thanks Grumpy. Last night I lapped both valves and re-installed the exhaust valve.

Lapping the inlet valve gave me a .06mm clearance however I dont think this is enough to justify not griding the valve. It got too late to consider grinding the inlet valve last night so I'll finish it up tonight.

Both the valve stem and seat cleaned up nicely after lapping so I hope this engine operate well once back together. Sorry I forgot about the photo's for today.

Nothing else I should check before reassembly?

The important thing is to finish up with 0.15 mm inlet tappet clearance. Normally, lapping the valve will make the clearance less, not more, because it settles further into the valve seat. So, you need to grind the tip of the valve stem. Do not let it get hot when grinding. If it goes blue, it becomes soft - a bad thing for the tip of the stem which is continually being hammered by the tappet. Also ensure that the tip ends up flat and square to the stem, of course.

Thanks Grumpy. The engine is now back together and both valves have the required clearances. I made sure to only grind very small amounts as instructed. I also ensured the valve did not overheat and had a smooth finish on the tip.
I have now invested in a valve spring compressor as getting them out was a little difficult. Installing them using Joe's trick with the cable ties was relatively easy.
One I other thing I wanted to understand was the slight splutter on startup before I did the valve job. I am assuming this was primarily associated with the little to no tappit clearance on the inlet valve?
I will give it a run tonight and let you know how it goes.
I have some before and after photos that might help others:

We won't have proved the diagnosis until we see the mower exhibit the fault, then have it run properly after only making one change, Jaffa. If you change more than one thing at a time, you'll never know what the problem was. (Of course most mechanics doing a job on someone else's mower for money, don't care what the fault was. They just make a broad diagnosis, fix everything it might have been, and as long as the fault has gone, why worry?).

Popping sounds from engines are usually leaky valves, but can be incorrect ignition or camshaft timing. If your description was a bit off, and it was actually a sharp spit rather than a dull pop, the problem would probably be lean mixture, but we've worked together before and I think your description would be more accurate than that. On a Briggs or most other engines with electronic ignition, the only thing that can change the ignition timing is a sheared flywheel key, which only happens when the mower has hit a pretty serious obstruction. Changes in camshaft timing are very common when the engine has been dismantled, but otherwise uncommon on engines with gear-driven camshafts such as your Briggs. If you had an OHC Honda there, with a belt-driven camshaft, incorrect camshaft timing would have been the very next thing to check after the tappet clearances, because it happens to those engines quite a lot.

By the time you started the thread, we knew that you have a side valve Briggs with a gear-driven camshaft and electronic ignition, and you didn't mention having recently mowed a water pipe, so it wasn't going to be ignition timing or valve timing. We also knew that you had very little inlet valve tappet clearance - and an inlet valve that doesn't close properly was likely to cause popping in the inlet pipe. Right away, we had a single most-likely cause.

It wasn't possible to say for sure that the problem was lack of tappet clearance, but it was highly likely. Furthermore the inlet tappet clearance had to be corrected anyway, or you'd end up with a burned inlet valve, if you didn't already have one. That means if you are playing the diagnostic game that I always play, so that you end up knowing what was wrong rather than minimising the repair time, the first thing to do was lap the valves and increase the inlet tappet clearance to the correct amount. That will probably fix the problem. However if it doesn't, you will still have fixed a serious fault, and eliminated the highest-probability cause of the problem - so you would just be left with the same other checks to make that you faced in the first place.

I'd say there is at least an 80% chance that your problem is fixed, but if it's not, please tell us, and we'll move on to some other checks and tests.

It's important to understand the difference between fixing an engine on the first try so you can return it to the customer immediately, versus finding out what was wrong so that the owner will recognise the symptoms next time. Most of us are here to learn, rather than just blindly fix mowers by giving them all a standard tidy-up. Both repair techniques are valid, but they serve slightly different purposes. Lots of Outdoorking members fix most of their mowers by just tidying them up, but there are still times when they want to know what the actual fault was, so they can recognise it next time.

That is a fair call regarding the process behind proving the diagnosis. I was just trying to understand the possibility of the work done to date (that being the valve job) being a primarily reason behind the engine�s somewhat erratic performance. Whilst some may not agree with my approach that being giving the mower a general tidy up and then looking for further faults, it has worked for me to date. I do it more for the challenge and not for the money. I have a day job which drives me up the wall so fixing small engines is my time.

The sounds made by the engine before the valve repair were most definitely a dull pop and not what I would call a sharp spit. I haven�t been lucky enough to find a Honda at the scrap yard or on gumtree cheap but I should find one eventually. I�d love to find something like Mark �Electric� worked on with you some time ago.

When I removed the inlet valve it was a little burnt but not too damaged in my opinion. It cleaned up quite well before being lapped to the seat.

I didn�t get a change to take the mower for a run last night however I did start it up and it sounded a lot healthier i.e. no splutter on start-up. I won�t be able to confirm for sure the fault is gone until I get a chance to give it a run in the long grass. I should be able to do this over the next day or so.

My mowers are not from customers. I tend to pick them up from tip shops and return the back to serviceable condition. Those which are too far gone I use for parts to repair others. I like the process of addressing one item at a time so I can recognise faults moving forward in a timelier manner.

I agree both repair techniques are valid. This is not a job for me, it is something I enjoy. The fact that I have a hobby that covers its own costs is just a bonus plus buying the odd new toy for the shed helps my ongoing motivation.

The next unfamiliar project I have is a Sanli Lazercut which is just about brand new. Some people knock the Chonda�s but I want to learn more about them before passing judgement. Over the past couple of years no reasonable mower has been unable to be fixed, so this one should be no different.

Once I give it a good test I�ll let you know how it goes.

Thanks Jaffa, I'll look forward to the outcome of your full-scale test of the engine.

My view of chondas has been becoming more positive recently. Provided you are prepared to replace a very few parts with genuine Honda ones if necessary, I am starting to think chondas are interesting and worthwhile. If you have not see this thread of Robs (roebuck), you might care to have a look at it:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=52741#Post52741

I managed to get 20 mins to take this mower for a run last night down the back of my suburb in the long grass. The result was much better than previously encountered.
The mower started easily. I let it idle for a few minutes to listen for any odd sounds and inspection for leaks and other faults. Nothing abnormal found.
I then proceeded to give the mower a good test for 20 minutes or so and it didn�t miss a beat. The engine was stopped and started on several occasions and functioned as a well service engine should.
The list of repairs for this mower was pretty long but I think all worthwhile.
Replaced & Serviced
- Bottom crank seal
- Starter Clutch clean and service
- New Starter Rope
- Valve Lapping and Grinding
- New valve chest gasket (well a second hand one as the existing one was leaking)
- New Blades and Blade bolts
- New boss and boss bolts as the last one was damaged. (got it for $15 bucks so I was happy with that)
- Carby and Diaphragm
- New exhaust � well a second hand one as the existing one was rusted out.
Grumpy thanks for your help.

Thank you for giving us a full final report, Jaffa, and well done on getting that mower back into good shape so it can go back to work.

I'll close this thread.