OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs Quantum 40

I have a Briggs that was running very well and just stopped. I have fuel, spark and compression, re-sealed the inlet tube, "O" ring replaced, and I can't get even a hint of a kick with starter fluid. Bit stumped, all I can think is timing has moved on the shaft. Any other thoughts before I remove the flywheel?

These do need a little compression at least to start and if you have that at least it must be a timing issue. I can only recall one mower that I've had with similar symtoms and it had a sheared flywheel key.

All I can think of Tiger, but I would be surprised if that is what it is. I'll pull it apart in the morning

NormK,

First question is how much compression? Finger over is plug hole is not a valid test. As with side valves the ACR can become over active at start-up due valve decrease clearance. This is usually a cause of shut while running but sure a give you fits when trying to restart.

Since spark plugs can and do fail under compression load, you have tried a known good plug? They will spark fine out the engine even when bad if they are under compression load. Also if testing without a plug the coil high voltage should be able to jump a .150" spark gap when is turned @ about 350 rpm.

Flywheel don't usually shear if the flywheel is properly torqued; unless, you hit something with the blade. As for if the flywheel key is sheared a quick test that doesn't require a lot disassembly is to remove the engine recoil shroud and spark plug. With these removed insert a wooden dowel through the plug hole and bring the piston to TDC. Now with the piston at TDC look at the flywheel magnets and coil legs alignment. They should be fairly align. There will be a slight mis-alignment but not much. With practice you will get a feel for when the flywheel keys are sheared by just checking this way and save having to remove the flywheel nut or ratchet starter. If you do remove either one do torque them back to spec which should be 55 ft-lbs. Under torquing will lead to key shearing or over-tightening can damage the flywheel.

Hi AVB, if it is the ACR is there an easy fix for that without splitting the cases?

The simple answer is no but don't need to drop crankcase either. Explained below.

First you need check your valve clearances as most these side valve uses a bump or ramp ACR grind on the camshaft; though, a few uses a swing arm version. When swing version fails most times it causes too compression at start-up. If the valve clearance are too tight then you would need to pull the cylinder head. This is because in order to increase the valve clearance you must remove the valve so the stem tip can be squarely filed down a few thousandths of an inch. Be careful when doing this as you can quickly over do it. It is also best to go ahead and do a valve grind job before final adjustment; just makes sense.

BTW if you work many engines that those swing out ACRs be careful as they can really jerk back a recoil rope violently hurting you. I have have learn to first pull slow on any engine that comes in to check excess start-up compression. I spent a month recouping from a badly sprained fingers a few times.

I would suggest to you to obtain a copy of the Briggs single cylinder L-head service manual. The single OHV are in a different manual actually a few different ones and which to use depends on which OHV engine. Drop me a PM with you email ad if you would like me to send you a PDF copy. It is about 11 megs.

Thanks AVB, I know about the swing arm version, I have one of those here and when it decides it wants to pull your fingers off it will. Is there a fix for these, I realize it is a case split job.

The only fix on the swing arm version if the swing arm goes bad is a camshaft replacement. I have been replacing quite a few of the 310000 series cams because of the ACR failure (bad design in my opinion). Usually the ACR completely comes apart and the electric starter can no start the engines even the valves properly adjusted. I have one case where the ACR fragments actually took out the oil slinger/governor gear.

I have seen several of the smaller cams where the hair spring fails again the only option is a camshaft replacement. Probably wondering why I called it a hair spring, it is because looks a strand of hair and easily missed.

The main to make sure on all designs is the valve clearance is first setup correctly as most it is it and not the camshaft ACR. Only after this is done and you still don't get ACR bump then you look at the camshaft. This ACR bump can either on the intake or exhaust just depends the engine design.

BTW I subscribed to the checking of the least invasive things first or what is called the KISS system. KISS = Keep It Simple Stupid. No reflection on the experienced tech but beginners can over complicate things.

AVB I would say you have nailed it, something I just thought about was when I was rotating it by hand yesterday with my finger over the plug hole I noticed a strong suction on the inlet stroke so to me this means the inlet valve is not opening. I should have paid more attention to that at the time

Doesn't look like that was the answer AVB, compression is fine, valves opening and closing as they should. I was expecting something like a broken cam but that all checks out as it should, got me a bit stumped at the moment. I will swap the carby, but I don't believe that could be the prodlem because even with a bit of fuel or strater fluid down the carby I don't get the slightest indication that it might fire

I would come over to help but I don't think either one of my Emus (Walker or Daisy) can swim that far; though, both have been in my pool. Smart birds because they know to use the steps to get out.

Hi Norm,

If it was me I'd be giving up on this one. Usually unless the mower has specific sentimental value to me or I am extremely bored I will strip it down and use it for parts at this point (I get the impression you get driven nuts until you solve the problem though). I'm sure you know this already but over time I have found the plastic air cleaner housing warps and these carbies rely VERY heavily on a good tight seal here. I usually find chucking a 2nd carb gasket on fixes most hard to start issues on Quantums as it builds up the thickness and seals all warped areas.

Just my thought; I really have nothing of value to add past this point.

Pete

Norm swap the ignition coil with another one and it should fire right up. I have found a fair few quantums with dead coils. Bent valve is a long shot but it does happen. Have you checked compression with a good meter?

Hi Jaffa, I have done all the normal things like swapping coils, compression gauge is a good one. Can't believe this thing, I had 2 spark plugs that tested fine, after half stripping this motor I decided (mainly because I had run out of ideas) to put it back together, put in a third plug and it fired straight up. Sometimes the simple things will trip you up. One problem is these plugs are most likely made in China to Chinese standards

This type problem is what I referred to earlier about plug not firing under compression and the use of a known good plug. To me known good plug is one that is working in another engine.

I have seen new out of the box plugs to fail. This is why you need a spark gap type tester to make it not the ignition coil. Those little glow (neon) type are not a good indication of the spark strength, they just a present of a spark.

I had one engine on air compressor that would be fine at first start up with empty air tank, fill the tank, and throttle down valve engages. Then when time came for it to throttle back up die. The plug simply could not fire under the extra compression load.

I also have seen engines be knocking at part or full throttle like a rod was loose or carbon on top of the piston to be only a failing plug. I also replaced quite a few NGK CMR6H and CMR7H plugs had failed this way. Note: these are 10mm plugs.

With experience comes wisdom is old adage but true many times that why many of us old techs are so good at repairs. We build on our previous experiences and like you have probably made the same mistakes, we just learn not to repeat things that don't work over and over.

Personally I have learn to do my non-invasive tests first as they can save a lot time and expense. I am firm believer in the KISS system.

I had a similar experience with a Chinese Honda on a go cart. The engine started and ran well with the wheels off the ground. However, once it was under load it refused to rev as though the air filter was blocked. The problem turned out to be the Chinese spark plug that was failing under load.