" 1/4" either past TDC or 1/4" back before once TDC is reached." ??? Note that when properly installed, ALL B&S Engine use an aluminum push rod on the valve which is utilized for Compression release. In the case of B&S V twin Vanguard engines, the aluminum push rod is on the EXHAUST valve. 18.5 Intek would be on the Intake. I can send you a Service Manual for your engine IF you like, address below, put in propber format and remind me, engine model number and what you want. I am having commuter trouble so don't be surprised at delayed reply. Walt Conner wconner5 at frontier dot com

***"BLACK would indicate oil fouling"*** Not necessarily the exclusive deciding factor. The black color only indicates carbon. The source of the carbon will be revealed by other facets of the carbon deposits. Carbon deposits due to lube oil will have a glossy look and will be quite gummy/sticky when scratched. Also the built up deposits can be quite thick and uniformly cover everything about the spark plug completely. Carbon deposits due to a very rich fuel mixture are sooty and powdery looking, and when scratched, will readily shed dryish flakes or specks. Often, where a rich mix is causing carbon, you might see places on the spark plug where the carbon has already flaked off or areas where the carbon seems not to be as thick. A poor boy's method for testing/guessing at compression issues can be done using compressed air. If using this technique, you will need both valve covers removed. You will need an adapter fitting that threads into the spark plug hole and accepts to being fitted to a shop air hose. The engine must be rolled to the correct position (piston 1/4" down and beyond TDC point of the COMPRESSION stroke) for the cylinder being checked. The engine absolutely MUST BE HELD FROM TURNING by some means to prevent the piston from reacting to the compressed air. You would have to figure out how to hold it with what you have on hand. An alternative method that does not require engine to be "locked" is described at the end of this description, so read carefully not to get the 2 confused. Air pressure needs to be 90 PSI to 120 PSI for best results. The air hose and all fittings need to be free of leaks as you will be "listening for leakage" in the test (and any air leaks in the plumbing can dull your testing accuracy). Each cylinder must be tested individually and the piston for each cylinder must be in the correct point, which means you will have to go through the locking and unlocking of the holding device for each cylinder. Once the engine is rigged against turning, apply compressed air to the cylinder you are testing. Now, you must listen carefully for air leakage. I use a 3 foot length of 3/8" plastic tubing to pin point the leaking spot/area by using it like an impromptu stethoscope. Hold one end of the tubing near to one of your ears (but don't stick it in your ear) and probe the following places on the engine with the other end of the tube: Get inside the valve push rod area of the cylinder head where the head meets the block. If the head gasket is leaking, that is where the leak will be. If you hear air leaking, and it gets louder as you probe nearer to the head/block juncture, and begins to become quieter when the probe goes deeper into the gallery (has moved past the head/block juncture), you have located the leak. But.....if the noise level continues to increase the deeper into the gallery the probe goes.............the leak may be coming from the opposite cylinder head or rings. So you have to test BOTH cylinders the same way, but one at a time. If you find that escaping air noise heard through the plastic tube increases on BOTH cylinders the deeper into the galleries the probe is inserted, and you do not hear an obviously louder sound when the probe tube end is positioned right next to either head/block juncture seam.......... the air is escaping past piston rings. This test method will also identify valve leakage by placing the probe tube into the exhaust/muffler or the intake/carb locations. There is also an alternative to rigging a holding fixture to keep the engine from moving under the influence of the compressed air. The alternative way is to REMOVE the push rods from the rocker arms of the cylinder you are testing and simply place the piston at BOTTOM DEAD CENTER (BDC). With the push rods removed, the terms compression, power, exhaust, and intake stroke become meaningless, as both valves are closed. The push rods must be marked so as to get them back exactly where they came from. It only requires finger tip pressure to push the valve spring down to remove the push rods. My thoughts on the Vanguards having "head gasket issues" are that the "true Vanguards" (and the 303700 is a true Vanguard) do not have any ill repute for head gasket failure. On the other hand, the Briggs single cylinder OHV 280000 and 310000 engines DO HAVE somewhat a reputation for head gasket blow out. Same goes for the Briggs V twin OHV engines that are just based on having one more cylinder of the same design as the 28 and 31 OHV Intek family (405700 and 445700). This post was edited by mownie on Tue, Feb 4, 14 at 12:02

Thanks for your input folks and sorry I haven't replied before now - had ticked the box "send me an email when you get a reply" and hadn't seen any sowas thinking I had no replies. I can't help but think that fuel shut off issue is the culprit as I checked the oil level before and it was fine - after the engine blew it was low - thought it must have flung some out but this makes more sense. What sort of $s are these motors in the US? - I wa\s quoted a "special" price of $1100AUD for a 15.5 Intek from localshop but there is one on ebay locally for $847 - details are 15.5HP OHV Premium Intek series. Model 31E677.0133.B1 with the std. universal 1" shaft. The deck is 42" and while I have 1 1/4 acres there';s lots of gardens so nota big job - only went with 42" as it was a good deal swcond hand - or so it seemed :( Thanks for your help & any more suggestions much appreciated - Graham

Thanks Walt, it's coming back to me now (I hope). I seem to recall something from another thread about a couple years ago to the effect of the two separate coils AND the little "bulge" in the wrapped harness where the diodes are. I hope the OP is keeping up with this so he can check out that possibility. I believe I can safely guess that if ONE of the coils were "going south" when hot, it could cause the symptoms wb3ffv is experiencing ( misfire, backfire, loss of power, seem to lose a cylinder)? But I see from your explanation of the two coils being connected by a pair of diodes, that if a diode were to "short" (continuity both ways), that would result in one coil "being influenced by the other coil". I am not sure what that scenario would create as far as symptoms, but it would seem that it could "kill" or weaken the spark on the "good diode" coil because the spark energy would dissipate into the coil served by the bad diode. The coil served by the bad diode would still throw a good spark because the other "good" diode would still be keeping the energy from the other coil from back tracking into its coil. Now, if the diode failed and became OPEN, the coil served by it would no longer have a "path to ground" (kill circuit conductor) and when the kill switch was closed, that coil only, would not stop sparking until the engine came to a complete stop. Sound about right??? Makes my head hurt some.