OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Intek 5.5HP backfires on (not) starting

Hi, A chap bought back a Rover "Professional 50" I'd sold him some time ago. It has a 5.5HP intek engine with OHV's. He couldn't get it to start and neither could I. However, it would start and run happily if I squirted some petrol into the carburettor or if I blocked the carburettor air intake with a rag but no luck otherwise. I've cleaned the carburettor twice and now it will kick and backfire back out through the carburettor. Pull start action is smooth so I don't think there is any timing problem there and it runs well if you start it with a prime. I was thinking it might be a valve but I still have doubts about the carburettor. The engine markings are 124602-0111-01-050823-FA. Is the consensus valves or carburettor fault.

send us a pic mate

Maybe check carby gaskets? sounds like it might be sucking air from some where in the carby.
Just a thought..

I'd say carby related issue going by your diagnosis but check the valve clearances anyway.

Also check for a partially sheared flywheel key.

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flmtxLX-nfBhU7y.pdf

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/BInnCFXBnfBhU7y.pdf

Best fit for the symptoms is very lean mixture. As I read what you've said, you have trouble starting it unless you squirt fuel in, but once started it runs perfectly. In other words, you have a problem with the cold enrichment system.

At this point I don't know whether you have a primer, a manual choke, or an automatic choke. As I read the manual, they seem to have made versions of the engine with each of those three. Whichever you have, it appears not to be working as you are calling on it to work. So, next step is we need you to tell us just what you have there. I already have a strange sinking feeling that you may have the ReadyStart automatic choke - I think that is the most common cold enrichment system for the 120000 series vertical crankshaft engines. That system is capable of working acceptably, but is often misunderstood and mishandled by tenants. In order to comply with emissions requirements, it only chokes the engine when it is completely cold - after being out of use overnight for example. Once the engine has run for even a very short time - perhaps a few seconds - you do not get any choke application at all on restarts. Normally a mower is run until you need to empty the catcher, and by that time the engine is properly warm and does not require choke. However experimentally-minded tenants may stop it before it is fully warmed up, and then they are just out of luck as far as being able to restart it is concernced.

If you have a ReadyStart system, and you confirm that your problem is that the tenant can't restart it when it is half-warm, you seem to have a choice between retraining the tenant, or fitting a similar carburetor with a manual choke or primer bulb.

Grumpy, If I reading the IPL correctly it does not have the ready start choke but could the manual choke or primer system. As you probably already aware of the primer system a bad habit of becoming warped or deformed causing the primer not to work. Sometimes doubling up on its sealing gasket can work.

The reason I suggested checking the flywheel key is that sometimes at low start rpms a partially sheared key can cause backfiring as the spark may advanced a little too much.

AVB

Thanks AVB, I think you've moved us further forward. I too only saw the manual choke and the primer system in the IPL, but I saw this in the service manual:


I agree of course that the manual primer system is somewhat prone to not working and needing an overhaul. I also agree that with enough ignition retard, the mixture can still be burning when the exhaust valve opens, which is a form of backfire. I've also found though, that many members say "backfire" when they mean "spit", which is usually lean mixture, though it could be incorrect valve timing or severely advanced ignition timing. To advance the ignition that far, I think the key would have to be completely sheared and have caught on some part of the flywheel so the flywheel was still being driven. Unless you know the person or can put your hands on the machine and look at it, the quickest route to the solution is probably to list the possible causes rather than pick a favourite guess at the beginning.

That why you got ask if it thru the carburetor or the exhaust when simply say "its backfiring".

Talking about after fire as I call it I had Kawasaki engine nearly set my pants on fire when it shot about foot long flame out the exhaust due to failing ignitor. With that I going back to lurking.

It seems the only thing retarded is me. I should have checked the simple things first. It might be fuel. The photo's below show new fuel and old fuel samples (I don't think the pictures highlight the difference as well as true vision). The old fuel was very cloudy even and though it has a fuel filter in-line I don't think a filter would stop contaminated fuel but is there for debris only. I have drained the tank and carburettor and it now starts. I'll try it again in a few hours just in case it is a fluke. I know bad fuel can cause starting problems but could bad fuel cause backfire through carburettor?

The bad fuel may have contained ethanol, and there may have been water in the tank, so now the fuel contains hydrated ethanol. Certainly you can't expect the engine to start well in that situation, since it changes the required air/fuel ratio and also will not flow through the jet properly.

Australia does not use the various ethers that are added to a lot of American fuel. They cause a need to add stabiliser to fuel that will be kept for even a few months. However some of our fuel does contain ethanol, which is hygroscopic and can cause trouble. Typically when it becomes hydrated it changes appearance, and to some extent smell, so if you are alert you can probably guard against what happened. However my preferred solution, for several reasons, is to avoid fuel containing ethanol. Water does not mix with petrol, it lays on the bottom of the tank, and in the bottom of the carburetor float bowl, typically stopping the engine from firing or running at all.

Hi Grumpy, that little jet on the left you can see in the picture (is it an air bleed?) was blocked when I first cleaned the carburettor but it still wouldn't start after cleaning it. Once started, with a good fuel prime or choked with a rag, it would start and run on that bad fuel without any problem. I think this fuel is at least part of the problem but I may not be out of the woods yet.

The restricted hole on the left provides air both for emulsifying the idle mixture, and for the emulsifier above the main jet. Let's wait for more data to try to figure out whether the bad fuel is all that was wrong, but it may have been.

It started on the 3rd pull just now. That is an improvement but I'm still not happy. I would like it to start on first pull. More tinkering needed.

Are you sure the gaskets between carburetor and cylinder head are in perfect condition?

Yes, It is quite a thick and sturdy gasket and it looked to be in good condition.

You may already be aware but with the OHV Briggs engines they use a slightly different design primer gasket to the standard quantum engines. I have done a couple of these intek�s now and I couldn�t get one pull starts without the proper Briggs OHV primer gasket. With the proper gasket I was getting 1 pull starts on every occasion even when unused for the few weeks it took to sell.

The difference is only slight in design but you will see the standard primer gaskets don�t fit 100% correctly when you install them onto the air filter casing.

I put a photo below
Just a suggestion!

That gasket is not the one Grumpy was referring to. He was referring to the other side of the carburettor. The air filter gasket that I have is in poor condition so you might have something there.

I think Jaffa knew that sparker, but AVB has suggested that the primer bulb system is a likely source of the problem. It seems to be a prime candidate for overhaul. (Yuk, a pun, now people are going to mistake me for an INTP.)

Lol. Anyway, I started it this morning and it chugged into life on the second pull. I've been starting it with the air filter housing removed and therefore the primer bulb also off, just for testing. I don't understand what that primer bulb does exactly. It doesn't pump fuel because it is not connected to any part of the fuel line but just sits on the outside of the carburettor. Does it just allow more fuel into the carburettor by temporarily lowering air pressure? I can't see how that "primes" anything. Can someone please explain to me how it works?