17.5 hp intek lawnmower engine backfire

jimmyw

Hi, could any of you give me some pointers as to the cause of the above problem. The engine did work since it was overhauled earlier this year,but it was laid up for a while,and it won't start at all now,even though it ran for a few minutes initially. It's a very loud bang.

The spark is good and the compression appears to be good while turning the engine by hand with the plug in. The engine does seem to spin over at a fast pace on the starter with the plug in also.

I checked to see if the float and needle were working as it should,and it is.

Any thing else it could be?

fox0512

...given that all the silly stuff is ok it would be timing...worn lobes on a camshaft will give you lots of backfiring at lower to mid way rpm but will be less noticeable at higher rpm.. Briggs 17.5 intek are prone to this issue at 450hrs work on-wards...check valve clearance and no movement of valve guides....a guide that has shifted can cause a bent push rod.....

Regarding the silly stuff I would check the fuel solenoid....when your turn the key you should hear a click from the solenoid as it opens to allow fuel through....if no click sound then check if you have 12.5v to your solenoid... highlighted in yellow box )

jimmyw

Would you believe it,and I had a slight suspicion(easy to say now of course) that it may be the flywheel key?.........................
.well it was and it happened TWICE!

It just proves that the nut on there needs to be VERY tight. I ruled it out in my head because the flywheel still turned the crank,but there must have been movement enough to feck up the timing.

Working on engines is still a learning process even though I thought I knew alot a bout them :-P

Thanks

fox0512

jimmyw wrote: »

Would you believe it,and I had a slight suspicion(easy to say now of course) that it may be the flywheel key?.........................
.well it was and it happened TWICE!

It just proves that the nut on there needs to be VERY tight. I ruled it out in my head because the flywheel still turned the crank,but there must have been movement enough to feck up the timing.

Working on engines is still a learning process even though I thought I knew alot a bout them :-P

Thanks

You can find the torque specs online for the flywheel at briggs home page.... yeah the slightest movement there will throw engine out....we learn as we go....if you dont have a torque wrench you will see the old line on the magnets on the flywheel where it meets the coil if u follow...try get those matching up...

jimmyw

The specs says that it should be 135 nm of torque.I gave it a little more for good measure and so far it's working ok.

fox0512

jimmyw wrote: »

The specs says that it should be 135 nm of torque.I gave it a little more for good measure and so far it's working ok.

Thats the job! always nice when it turns out a simple and inexpensive fix......

jimmyw

My word, just discovered the dam thing is now leaking oil in what appears to be somewhere under the metal shield on the bottom of the engine.It wasnt there while I was putting on the shield a short time ago.Its like its coming through the head, not on the seam like the metal is porous??

I didnt see it myself but it was described to me like that.Its leaking even when the engine is off, just before a chap came to buy it!!!!!!!!!

:mad::mad::mad::(

Dont have the engine now, bugger! She has the last laugh now.

fox0512

Ah noooo!! When you say the bottom of the engine are you talkin bout where the blade it fixed to shaft or at the head at spark plug?

fox0512

Sorry have u now.....when you are looking at the front of engine at the head is the leak on the right hand side?

jimmyw

Sorry I can't really help you as I didn't see it but the brother just happened to see when it was too late.
You know the metal shield that fits on the bottom of the engine,it's leaking somewhere between the head and this shield.

I was working on this engine for a good while now and I didnt see anything. I know it would be hidden under the shield, but there was no sign of any leak.

I really hate this sort of thing as it gives the impression that I was incompetent installing the engine back together.

RIGHT WHEN IT IS BEEN SOLD!

fox0512

Harsh... i know the feeling....if the head or cylinder is not damaged and head is torqued properly it has to be the head gasket, oil will leak from the right hand side from the push rod galley...if gasket is gone between galley and cylinder you will have a slight leak when not running and smoke when u fire it up.... you will have a stronger leak if the OHV cover is lose....it can get messy too sometimes at fuel pump if head gasket is gone and smoke when dipstick is removed.... badly worn or incorrect size piston rings can cause it but you would still have smoke on start up...

you prob know all this and im boring you to tears! lol.....its a strange one!.....maybe theres a crack in head....if you have a compressor you can insert bout 10psi ish at spark plug and at top dead center on compression stroke only(both valves closed)....this will tell you fairly quick where your leak is!!

Im South kk..... your more than welcome to drop over if needs be...

jimmyw

Thanks.,no your not boring me, its the other way around ;-).

I was careful to torque the head both in the correct sequence and to the correct torque figures. There is new piston,rings and head gasket plus sump gasket and ohv cover gasket so you would think that it would pretty much rule out all leaks:-/.

I know the engine was not run much since the rebuild,but you would think that the leak would be apparent at some stage.I don't like selling the machine with this kind of fault, but too late now as the engine is gone.if it is a crack,then I couldn't really have spotted that but
It leaves a bad taste in the mouth for me,and I don't even own the engine

fox0512

yeah thats ****ty....happens tho....

shareboy

Hi. I have a B&S Intek OHV 17.5. Recently serviced it with standard service kit pieces. I accidentally place the fuel filter the wrong way around and she cut out with a loud after fire band. Turned the fuel filter the right way around and she started up but when I turn it off now or when I get off it when it's running the loud after bang is still there. Any help on how to resolve this problem or even what is it?

fox0512

The filter on the wrong way would have no short term effect... The backfire is cause by some fuel entering cylinder/exhaust after you turn off machine.. Your carb should have a fuel solenoid, you will clearly see it at the end of carb bowl with 2 wires leading to it...if solenoid does not close off jet in the carb after turning off mower this will cause your backfire.

If you have a fuel shut off valve then do just that or clamp fuel hose...remove solenoid from carb.... when you turn ignition on the pin on the solenoid should drop and rise when you turn ignition off... when testing a solenoid always ensure you give it 10 min ish to make sure the pin stays up or down... a solenoid losing magnetism can make the pin rise after some minutes use and close the jet.

shareboy

fox0512 wrote: »

The filter on the wrong way would have no short term effect... The backfire is cause by some fuel entering cylinder/exhaust after you turn off machine.. Your carb should have a fuel solenoid, you will clearly see it at the end of carb bowl with 2 wires leading to it...if solenoid does not close off jet in the carb after turning off mower this will cause your backfire.

If you have a fuel shut off valve then do just that or clamp fuel hose...remove solenoid from carb.... when you turn ignition on the pin on the solenoid should drop and rise when you turn ignition off... when testing a solenoid always ensure you give it 10 min ish to make sure the pin stays up or down... a solenoid losing magnetism can make the pin rise after some minutes use and close the jet.

Thanks for that. Also my cutting deck is squealling when starting up and stopping. Would lubing up the belt help?

fox0512

The squealing when blades are engaged is just the belt making traction on the main pulley/clutch... this should only last a sec or two, any longer than that and theres belt issues...the squealing on stopping may just be the brake pad making contact with pulley to slow blades (if its a manual engage system)...if no noise when blades are in operation? I wouldn't worry much....