I’m after some tuning advice for a Cox XL Scout that I’ve recently purchased and have done some work on to bring it up to scratch as a second mower.
My issue is with tuning the engine. It’s a Briggs & Stratton 11HP cast iron bore four stroke (Model 253707) and the specific issue is that the engine is breaking down under load when I’m mowing.
Before tuning I replaced the spark plug (recommended gap is correct), fitted a new air cleaner element and dis-assembled the carby and cleaned it (I had to replace the main metering needle in the bowl as it was the wrong one fitted).
I’ve found what I think is the appropriate tuning guide in this forum (I’ve attached the guide to this post) and have tried to follow the tuning procedure (as per page 8 in the guide) and I also checked and adjusted the remote throttle/governor (as per page 7).
It seems to tune up OK as I follow the procedure but when I actually try to mow with it it breaks down under load when I engage the blades.
The only modification I’ve made that I thought may have an effect is that I’ve fitted an inline fuel filter and that it may be starving the engine at higher revs but I’m not sure how likely that is.
I’m in Geelong, Victoria, and I can’t find anyone at a mower shop locally who wants to have a go at tuning it so I’m trying to get some advice as to what either may still be wrong with the mower, or what I may be doing wrong.
Hi Andrew, Welcome here, sorry missed you post earlier. Only couple of things I could think of is, 1 something is siezed (pulley somewhere) or 2 the governor is not working. Are you able to rev the motor up using the governor arm?
I did have a minute to look at the mower this afternoon and checked a couple of things.
I took the top cowl/cover off the engine because I wanted to check the ignition lead but while the cover was off I engaged the cutter blades and turned the engine over by hand with the flywheel/cooling fan on the top of the engine. It seemed to turn over freely so I think the belt pulleys are OK.
I haven't had it running again yet though to check the governor arm as you also suggested. I did remove the spark plug to have a look at the colour of the electrode, it didn't look overly dark and sooty or whitish so I guess the mixture isn't too bad.
While I had the plug lead off I checked the resistance of the lead from the magneto to the plug end of the lead. The resistance of the ignition lead conductor from the plug end to the magneto was only about 1.5 ohms so that was fine but when I tested from the stainless crimp that connects to the plug the resistance for the entire lead was in the meg ohms and the crimp itself looks very corroded and ordinary. I don't know that the ignition lead is my problem but it's certainly something that needs to be rectified.
My plan is to re-terminate the spark plug end of the ignition lead when I get a new crimp, once I've done that I'll run the engine again and then I'll try your suggestion about the governor arm.
Ok, I've had another go this afternoon with not great results.
I got another crimp for the ignition lead and repaired that, it's tested OK.
I then had another go at starting from scratch to tune the engine.
I was able to get it started but whether at high revs or lower revs the engine is now pulsing. As it's doing that if I look into the throat of the carby from above I can see tiny splashes of fuel landing in the bottom of the carby housing.
I'm not able to get it to run at lower revs, it just stalls.
However, as suggested above I tried to rev the engine using the governor arm and it revved to pretty high revs, certainly much higher than I've had it revving using the remote throttle controls.
Reading around here am I right in suspecting there may be tubes blocked in the carby?
On the plus side this afternoon I was able to get an old McCulloch brushcutter running that's been sitting in the shed for about 10 years so I guess you win some and you lose some.
Ok Andrew, get it running, rev it using the governor arm and then wack you hand tightly over the carby throat, wait till it starts to die, release your hand, let it rev again and repeat a couple of times and see how that goes, it often clears any blockages and sucks them through the carby.
I gave it a go but I'm not sure it's helped. I did as you suggested and after doing it three or four times there was a build up of dark oily looking fuel in the carby throat, I thought maybe that was some residue clearing out of the carby but it still seems to be surging at low and higher revs as well as being impossible to tune. I also tried some carby clean into the carby as it was running but that didn't help either.
I'll take the carby off and pull it apart, apart from the fuel bowl there doesn't seem to be much else that dis-assembles, but I'll try and clean out all the jets and orifices I can see and then give it another go.
I had a chance to take the carby off today and took it apart as best I could to clean it. The only snag I struck was the main fuel nozzle, it was stuck in pretty good. I ground down and reshaped a flat blade screwdriver to fit the slot in the nozzle but still couldn't get it out.
Anyway, I cleaned it all out, poked some fine wire into the nozzles and gave it all a good spray with carby cleaner. The float wasn't quite set parallel so I reset that and then put it all back to together and re-installed it onto the mower.
Still no luck. The main issue seems to be that no matter how I adjust both the main metering needle or the idle mixture screw the engine runs way too rich. Today, after starting the engine, I wound the main needle all the way in and the idle needle into about 1/2 a turn out from fully seated and while looking into the throat of the carby I can still see tiny splashes of fuel landing on the sides. It's also obvious from the fumes and smell that it's running very rich.
The thought has occurred to me that maybe the float needle is not shutting properly and constantly allowing fuel into the bowl but I've replaced the needle and checked that the float doesn't have any fluid inside it due to a possible hole in the float itself. As I said above I've also set the float level as per a tutorial I've found on an American site for the carby.
I'm not sure what else to do. The only component I wasn't able to remove and clean was the main nozzle but I don't know that it could be causing all this anyway.
I've spotted replacement carbs on Ebay for $60 delivered. They're Chinese knock offs but I'm considering getting one and trying it. They're certainly a lot cheaper than the $250 or so I saw for a genuine Briggs & Stratton item.
Does anyone have an opinion on whether I've exhausted the possibilities with my existing carby and should be looking to replace it?
If you wind the high speed needle in all the way and it doesn't lean out/start to die, you have an over fueling issue. Being that you cannot get the main nozzle out is an alarm bell for me, it has little holes in the sides to emulsify the fuel that goes through, if they are blocked it won't run right. in saying that it should still lean out when you wind the needle in. Check that you have the right needle in it. There are two types, one is a solid needle with a viton rubber seat (green) and there is a neoprene needle with a solid metal seat. they won't work the wrong way around.
Reading you're comments to me it sounds like you have only picked up the flooding issue since changing the needle, and I personally would say you're original problem is likely to be a blocked up carburetor. Getting the stuck tube out is not impossible, I usually put the whole thing in a vise upside down, gently heat the alloy around the tube with a small blow torch and soak the inside with wd-40 or lanolin spray. I find the best then is with a really good fitting screwdriver gently work it undone, with heat it will come. just don't strip it! there is a tool to remove stripped ones. I have done it a couple of times, finding someone in a mower shop these days will be pretty difficult.
I am yet to use a china carb, have one here for an engine but ran into other problems. If you have play in the throttle shaft it is definitely the way to go, otherwise I don't think they are ever quite as good as genuine. I have heard of a fair bit of success with them though.
The main nozzle is the only bit I haven't been able to remove, it looks like someone before me has had a go and even though I modified a screwdriver tip to fit nicely in the nozzle slot I haven't done it any good either.
I've double checked the main needle spec, the Briggs & Stratton document that has exploded views of all the parts says the main needle is Part No. 396568. When I first got the mower it had the needle with the green seal so I replaced it with the correct one.
I've ordered a new carby off Ebay, aside from the nozzle issue the existing carby has quite a bit of surface rust inside the fuel bowl and on the choke baffle plate. I think given the damage to the main nozzle my most likely option will be to use a screw extractor to remove it so once I factor in the cost of a new nozzle I might as well spend the $60 and hopefully have a replacement that's good enough to be able to tune properly.
I'll remove the old carby when I have a minute because as well as trying to get the main nozzle out I also want to check for play in the linkages as per your suggestion above.
I'll post back on my progress in case anyone is interested in reading about me muddle my way through this.
Received my new carby and fitted it to my mower. Had a bit of a hiccup in that the OEM fuel tap didn't want to screw into the fuel inlet port on the new carby. It was a coarser thread than the original and after visiting a couple of places to try and find a fitting a bloke at Enzed found a 1/4" BSP fitting that while not the same thread exactly still wound far enough in to make a seal.
After fitting the carby while I was messing around the throttle cable decided to snap on me. Got a new one from the mower shop and paid more than I should but didn't care, I just wanted to get it done.
Set the high and low metering screws one and a half turns out and the mower fired up straight away. Tuned both screws until it was running pretty well and then went out for a test run on the front lawn.
It's running really well now, no breaking down at high revs under load and I'm really happy with the way it cuts.
