OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines 4hp Briggs and Stratton Quattro

Briggs flywheels normally have two untapped holes on the top surface. The holes are the correct tapping diameter for 1/4" UNC screws. They recommend that you just screw two thread-cutting 1/4" UNC screws into the holes. Personally I haven't found those screws for sale in Australia, and I tap the holes with a 1/4" UNC plug tap. You then pull the flywheel easily with a simple home-made flat plate puller and a pair of ordinary hardware store screws:





You only have to tap each flywheel once of course - after that it's easy.

Great detail thanks.
I took the flywheel off last night relatively easily. The key was not sheared and the top crankshaft seal looked to be in good condition.
I took some photos to help but I left my camera at home. I try and load them up tonight.

I'm open to suggestions.

OK, if we have compression and we have strong, correctly timed spark, it's likely we have some kind of carburetion problem. You reported that the spark plug was getting wet, but it isn't entirely clear that it was wet with petrol, given the problem you were having with oil at the time.

I've very occasionally had a problem with a line trimmer that wouldn't fire but had nothing wrong with it. Once I'm completely sure the carburetor is OK (and I'm not really convinced yours is OK), my standard solution is to remove the muffler and apply an electric drill to the flywheel nut. Spin the engine up to 700 rpm for half a minute or so. The only times that hasn't cleared the engine's throat and made it run, has been when I've been missing something obvious, like I've been trying to start it with water in the fuel system or there's a problem in the kill switch circuit that didn't show up in the ignition test. In the case of your carburetor I have no confidence in the primer working properly, but by spinning it fast for half a minute it should start without any choke or priming anyway.

This is not a routine way to start a troublesome Briggs. In this case though I think there is something missing from our communication, rather than a mysterious problem. If my wild guess is correct and the problem is in the carburetor, there's a good chance some fuel will find its way through during the long period of cranking. Removing the muffler for this test is just to cope with the possibility that the muffler is blocked, which we haven't disproved so far.

have you tried a new/known good spark plug?

He's tested the spark Joe, but I agree, a known good spark plug should be tried. This looks like a case where something that has been tested or replaced is nevertheless, defective.

Some pictures of the flywheel removed for reference.

Let's check the governor, in case the spring is not working. To do that, first set the speed control on minimum speed (idle). Look at the position of the throttle butterfly lever: it should be closed nearly to the idle speed screw. Then move the speed control to maximum and see if the throttle butterfly lever has moved to maximum, without you touching it or any other part of the mechanism.

1) The current carby and tank came off a working machine. The primer is spraying fuel into the intake manifold as it should. The primer bulb is retracting as it should and is not staying pushed in. It still won�t fire even when I put a little petrol into the plug hole. It will pop and carry on for half a second but that�s about it. The engine should fire for a second or 2 when petrol is put in.
2) Last night I thought the kill switch might have a fault (the deadmans set-up). So I removed the setup and replaced it with a standard kill switch setup via the throttle control and no change. I guess that took away a possibility.
3) I can confirm the oil problem has been eliminated since changing the breather valve.
4) I have another carby which I could try tonight off another machine I know works.
5) I tried 2 brand new plugs in other machines that worked perfectly. It made no change when put onto this machine.
6) I put a brand new muffler on and it didn�t make any difference.
I don�t have a catcher for this mower (GMC catchers) as they are the flimsy and prone to breaking� so yet another bad point of this mower.
I wasn�t sure about your method of using a drill as I don�t have the attachment to put it onto my battery drill. I have the socket but no suitable attachment. I did however give it 20 good cranks and nothing put the odd pop or two occurred.

Now that the oil is gone and you are confident about the carburetor, let's start again.

1. Test the spark using the actual spark plug you intend to use to run it. We need to know that you are getting a steady stream of blue sparks across the spark plug, whenever you pull the starter.

2. Test the compression the Briggs way: disconnect the spark plug, tilt the mower so the oil runs toward the crankshaft, and rotate the blade plate backwards until you find compression. Then just flip the plate backwards, to see if it will bounce off compression and turn back a bit in the forward direction. If it will, it should have enough compression to run. Reconnect the plug lead.

3. Make the governor check I described in my previous post. This is important.

4. Remove the muffler, and remove the element from the air cleaner but put the lid-retaining screw back in.

5. Set the controls for start (speed control on maximum), verify that the throttle butterfly is wide open, prime the carburetor with the bulb as normal, and pull the starter a few times.

6. Report back.

Essentially, the "odd pop or two" indicates that it is trying to start. Its failure indicates that mixture, compression, or spark is quite a long way wrong. We need to find out which.

1) Definitely has a steady stream of blue spark. I used a plug from a known working engine.
2) I did the blade plate test and the engine definitely didn�t bounce off when compression was found. I suspect lower than anticipated compression in this instance. Is 100PSI on the low end of the scale?
3) I installed a brand new spring on the governor from a known working engine. When the speed control was set to minimum, the throttle butterfly lever was just about closed. When I moved the speed control to maximum the throttle butterfly hardly moved (only ever so slightly)
4) Done
5) Done � throttle butterfly was not wide open though.
6) No change

Bad news � I thought the starter was dodgy giving me the odd clunking type sound so initially I didn�t really worry about it until I got the engine running reasonably well.
Lets just say after watching one of Joe Carroll�s video�s on YouTube, I confirmed the engine has quite a bad knock. Funny thing is that its exactly the same mower as Joe blew up in the video.
Why it will not fire still is beyond me although the lack of compression seems likely. I tried another working 4 stroke and the blade plate bounced back as you indicated.
Worst case this turns out to be a parts mower but I have learnt a few good tricks along the way. If you think its worthwhile looking into further for the experience I�ll await your advice.

Which one did I blow up that time? I think there are 3 or 4 there, the old gmc went off with a pretty good bang sayonara Chinese Briggs.....

yep it was the gmc one! mine makes exactly the same sound. To date I have never found one that bad.

Any suggestions on the not starting would be appreciated! I am sensing compresion given grumpy's advise but I'll wait and see what he thinks unless you have a thought?

I'll be interested in Joe's comments too, Michael. It seems though that your engine has little compression, and a major clunk internally. It sounds like a partly broken (one side of the big-end bearing only) connecting rod, which sometimes happens to Briggs engines that are run without oil, though I think it is more common for both sides of the big-end bearing to break. However from previous threads, especially Joe's posts, about early Chinese Briggs engines, I think the combination of this seeming to be an early production Chongqing engine, and it having a major internal clunk, suggests there is a very good chance that at least one of the connecting rod bolts was not tightened in the factory, and what you have now is a badly damaged connecting rod. If the crankpin is OK, it is possible that replacing the rod would get this engine back into operation.

My guess as to why there is very little compression, is that the loose, mangled big-end results in the piston not travelling far down the bore on the intake stroke, so the compression ratio might only be 3 to one, instead of 6 to one. That is not enough compression pressure for the mixture to ignite.

You might find it interesting to open the crankcase and find out what is actually going on, and whether my guess is correct. Joe is the expert on what you might find in there.

The lightweight blade of a gmc mower gives the false impression that they have compression, I concurr wit hthe thoughts of maybe the conrod bolts never being torqued enough at the factory, and the very obvious fact that they are very pourous castings.

I would say the problem with the compression would be a HUGE ring gap, I have pulled some down before with a (no bullcrap) 5-10mm ring gap, they will act like they are trying to fire up but they are very weak. ill see if I can digup a photo of at least one bad ring gap case I have seen...

this was a 3 year old briggs and stratton with poor compression and just a tad smokey, however I have seen worse.

I agree the not starting, and very little compression, could be just ring gaps Joe, but what about the internal clunks? Peter reported that they sound like your bad Briggs with the loose connecting rod.

Personally I like to do the oil-through-the-plughole trick to check for ring leakage before opening a crankcase.

Grumpy and Joe,
Thanks for the comments - It makes for an interesting thread. I havent had a chance to pull the engine down yet as I've been doing a few other jobs.
I am interested in what you mentioned Grumpy about putting a teaspoon on oil down the plug hole to see if the compression increases. If I read that right, id like to try it.

I will pull the engine down when I get some spare time so I can learn from it.

Funny thing is I got given another GMC which has a 375 Briggs that actually has a snapped rod. It must of went off with a loud bang. What I didnt understand though was the engine had plenty of oil (I am thinking however the owner must have added some oil after it went bang) Nevertheless I'll eventually pull it down to ensure I get all the usable parts off them both.

The oil-in-the-plughole trick is a standard old-time mechanic's test, Peter. It works on the principle that engine oil is fairly viscous, especially when you first put it into the engine (before it warms up, if the engine is warm). It therefore takes a while to be forced past the piston rings by compression pressure, when you pull the starter. This means you can do a compression test, then put in the spoonful of oil and immediately to another compression test. If the pressure increases by more than say 10-15 psi with the oil in there, the rings are leaking. If it is more like 40 psi, which it commonly is with worn out rings, you reach for the spanners.

Just remember not do do something I did a couple of years ago. I was doing the oil trick on an OHV Honda. Those engines have 8:1 compression and therefore take a bit of rotating, especially if you are doing the test with the decompressor disabled, as I was. I therefore used a very large two-handed back-geared electric drill to spin the engine by the flywheel nut. After I got the second reading (the one with oil in the cylinder) my mind must have wandered because I removed the compression gauge from the plughole before I let go the trigger of the electric drill. I was wearing a brand new ODK-logo white T shirt at the time ....

Thanks Grumpy I did some checking on the engine and I am now pretty sure the rings are stuffed.
On a separate note since we were talking about loose rod bolts I thought I�d add a couple of pictures. It is very interesting to note the bolt connecting the rob was loose, supports Joe�s comments.
The engine had plenty of oil when it came into my possession. I was pretty sure it was stuffed and the photos supported my findings.
I have completely rebuilt a number of two strokes however I haven�t split a case on the briggs engines before so I�d appreciate some of your comments around each of the parts and key things to consider about repairing the engine. I have watched a few videos and it doesn�t seem that complicated but I have no doubt there are some special things I should consider like usual.
At worst out of this exercise I have a perfectly straight crankshaft that might come in handy for a future project.
Oh and I haven�t got the piston out of the bore yet.

Funny point - I have never snapped a boss - they must be made of rubbish

While it is commonplace for a side valve Briggs to break its connecting rod, it is nearly always for one of three reasons:
1. Engine run without oil.
2. Engine run with governor disabled, resulting in gross overspeed.
3. Engine incorrectly assembled.

The state of the crankpin is the most likely issue as to whether the crankshaft can be saved. So far as the rest of the engine is concerned, my recollection is that it is an early Chongqing Briggs with a broken connecting rod. There seems little point in regarding it as anything but scrap aluminium. And I don't know what the broken piece is, in that sixth picture.