Hi all, I have a Briggs and Stratton 5.5hp Intek OHV Rover as in the photo. The previous owner said it would not prime, so I removed the primer button and holder and thoroughly cleaned out the housing and the mesh filter. The engine now primes correctly and starts first time every time, but it has a misfire, and no matter what I have tried I cannot seem to track down the cause, I will go through what I have done to try to eliminate the problem. Could be a lengthy explanation. I have tried 2 different ignition coils, 2 different brand new spark plugs, another carburettor, replaced all the relevant carburettor gaskets and brand new air box gasket, new air filter cartridge and pre filter foam. I soaked the stripped down carburettor in carby cleaner and let is soak for a couple of days then re-assembled with new gaskets, needle and seat are working perfectly, governor spring and linkages are fine. Grit on flywheel and coil contacts have been thoroughly cleaned down and airgap reset. So basically I have replaced the coil, spark plug, air filter cartridge, carby gaskets and thoroughly soaked and cleaned carby including using fine wire to check all carby holes are clear as well as using compressed air to blow them out. Compression is excellent and it doesn't blow smoke and the valve clearances are ok.
Last edited by MickyD; 09/11/2109:13 AM. Reason: spelling errors
No mate, i've checked the wire all the way from the coil and the kill connector clears the plate easily. As far as the gaskets go I have put one behind the carby mounting plate where it bolts to the engine, I have put one on the front of the mounting plate before I put the large insulator gasket on and i've put one in front of the insulator before the bakelite spacer block goes on and i've put one between the carby and the spacer block, so there shouldn't be any air leaks. I've even checked for a loose/worn flywheel keyway and checked the bottom seal. I suppose as an elimination process I could remove the cut out wire altogether and see what happens, just pull the plug lead off to stop it, that would eliminate any issues with the wire.
Hi wce, just finished doing that, have had flywheel off, key and keyway are fine, wire is fine, insulator seems good. It's a mystery. Could it be a valve seat issue? I've tried everything else. I've regapped the air gap on the flywheel again as I had it off.
I guess its now a case of determining if it is a misfire ( as in a clear plug misfire ) or another issue causing that effect. Does the issue happen through all rev and load ranges ?
If it was me I'd be removing all the fuel from the tank and carby and run the motor on start ya bastard , with the air cleaner off spray the fluid into the carby then when the motor starts keep spraying enough fluid so the motor runs on the starter fluid for 20 seconds or longer if more than 20 seconds was when you previously had the misfire.
Doing this will tell you if it's mechanical or fuel related.
First thing I would do is another plug - I have had a couple recently that wouldn't behave until another plug with a slightly larger plug gap.
Think these take a bkr5e plug?
The other thing I had a bit of luck with was a quantum with similar behaviour was to arc up the RPM to 3400 and spray this down the carby until it nearly stalls but doesn't. Do that about 5 times.
Then put back to idle and spray until it stalls. Leave it half hour and start it back up
Also it would be worth checking that the head gasket isn't leaking slightly and the exhaust port /muffler isn't blocked with carbon ,rust ,etc , you could even run the motor with the muffler removed just to see if the misfire is still there .
Carbon buildup in the combustion chamber and around the valves can cause a misfire.
Used the engine conditioner, still did nothing, I am now going to pull the head off and check to see if the head gasket is blown, I have heard they have a habit of blowing between the cylinder and push rod chamber.
Thanks Max, I saw that on Youtube actually. I've pulled the head off and it was warped slightly, it was higher at the corners where the bolt holes are and dipped in the centres. I put the edge of a steel ruler along the head and it sits flush at the bolt holes but you could see daylight easily under the rest of it, The valves are seating fine and the head and inside motor are really clean, no carbon build up at all, so i've cleaned all that up and have since machined the head so there is no daylight showing and just waiting on a new head gasket and i'll reassemble it and try again.
Last edited by MickyD; 14/11/2104:41 PM. Reason: spelling errors
Head Gasket finally showed and I put in on after machining the head down. Valves sealing ok, re-assembled and re-gapped valves, started it up and same thing, so it's going to the tip.
It makes it a lot easier if you had a spare running engine that you could bolt the carby onto this one to rule out a fuel supply problem and replacing an ignition coil from a good running motor helps as I've swapped coils from old motors and they still didn't work.
I take it when the valve clearance was adjusted the exhaust valve was fully open then the intake clearance was adjusted and when the intake valve was fully open the exhaust clearance was adjusted otherwise the clearances are wrong because of the automatic compression release mechanism.
Some of the carbs have a drilling that is vented to atmospheric pressure ( between the carb and head) and if people use the wrong gasket or install the gasket upside down it will effect how the motor runs.
If the motor is mechanically good (compression) and fuel and spark are good it should run well ,sounds like you have confirmed the mechanical side to be good unless the cam is worn out or a ring is stuck in the piston land or the cam timing is out and I would think you have already checked the mechanical side.
Valve guides can come loose and move and Valve seats can come loose.
Though I did mention try a larger plug gap, one last thing to double check is a pretty small gap. I have heard some of these are pretty particular. This one is 20 thou factory spec and if you run the more common 30 thou it will act odd apparently.
