[red97corvette]

I have a 4518 that has been in the family since new and always well maintained. The last few years however the mower has become increasingly difficult to crank and prone to stalling and generally seems to be down on power. The engine will crank and idle (somewhat) but any attempt to throttle up and the motor just dies. I can hear some backfire through the air cleaner. This mower has never really been in storage, it has been in use since new.

Compression checked out fine, 155 PSI on both cylinders. Spark plugs are new, correct NGKs. Battery is new. Fuel pump clicks as it should, flow rate of fuel at the carburetor looks good but fuel filter has not been checked or replaced. Research lead me to become suspect of the CDI module and the cold solder joints so I bought a cheap aftermarket CDI for testing. I can tell no difference between the factory CDI and the aftermarket one. The carburetor has been completely disassembled and cleaned, although there were no signs of varnish or buildup found. I did notice the linkage between the throttle and choke baffle in the carburetor was missing so a replacement was fashioned. The choke baffle return spring was also weak so an external spring was added. The choke appears to be working normally now.

Should I go ahead and open the factory CDI and inspect for weak solder joints? Finding no difference between the factory and cheap aftermarket unit leads me to believe they are either both bad or not the source of trouble. I've read of timing belt slippage, could this be part of the issue, especially with that backfire coming through the air cleaner? It is just odd because there are moments sometimes lasting multiple minutes where the engine sounds pretty good at idle, nice and smooth and no backfire (still can't throttle up though). These are rare these days, but it happens. If the timing belt was slipped I'd think it would be all or nothing. Just today it was running ok at idle and I managed to drive it around the yard at idle speed for probably 2 minutes before returning to the shop just as it was starting to bog down and sputter and then died, thankfully right back in it's parking space.

I posted a few YouTube recordings of the mower running if that is helpful for diagnosing what might be going on. Thanks everyone in advance!

May 10, 2022
May 10, 2022

 

[ST1100A]

Check your valve clearance first and adjust if necessary. After running a few minutes, they will heat up enough to expand enough if the clearance it tight and they won't seal properly, that can cause rough running, rough idle, 'spit back' through the carburetor if there is a tight intake valve. They are adjustable with little adjusters on the cam followers.
If they were never adjusted by now, good chances are they are out of adjustment.
Some of those engines were known for the timing belt tensioner/water pump pulley-Aluminum to wear and jump a tooth on the timing belt, that could cause a problem with the cam timing out a little bit.
Honda came out with a new water pump assembly with a steel pulley that is replaceable on it. The original pulley was Aluminum non replaceable, so if it wore, the water pump assembly had to be replaced. The newer pumps used a steel pulley that was replaceable, so you want to check your cam timing by lining up the crank timing index mark, then go up and check if the cam timing mark on its pulley lines up with its index mark.
If it jumped a tooth or more, you want to reset it, or replace the water pump in case the water pump aluminum pulley is worn, check that pulley when you are checking the cam timing.

 

[lmichael]

Sounds like a timing issue. I'd be pulling it (the engine) and getting after the t/belt and water pump and so on

 

[red97corvette]

Thanks guys, looks like I'll be pulling the motor this weekend.

 

[red97corvette]

OK, so I pulled the motor, not too bad, maybe an hour. Took the timing belt cover off and if I understand the timing marks correctly my timing has not jumped. I've attached two pictures.

Any other thoughts on what I can inspect while I have the motor out?

Thanks!

 

[red97corvette]

After a careful cleaning I dropped the engine back in the frame. Before I attempted to crank it and while I knew everything was at TDC I went ahead and opened the valve cover and inspected the valve adjusters. I didn't have the specs with me at the moment but it was clear with a feeler gauge that both intake valves and the exhaust valve at the front of the mower were so tight I couldn't get even the thinnest shim into. The rear exhaust valve had a gap of approx .3mm. OK, so ST1100A was definitely on the right track with the valves needing adjusting.

I backed off the retaining nuts and using a pair of pliers backed out the adjusters one-by-one and reset them as best I could (never done this before). Placing a feeler shim between the set screw and top of the valve stem I turned until snug but not binding and carefully tightened the retaining nut. Intake set to .12mm, exhaust to .2mm as per the manual.

After two starts with ether the motor picked up and was idling. Valves are definitely not 100%, but a huge difference in idle. I shut the mower down to top off the radiator and it fired back up almost instantly, a huge difference from the normal very hard start. There is a lot of valve chatter but no spit back through the intake. I was able to bring the RPMs up fairly easily although I must need to adjust the throttle linkage because at full throttle the engine is at about half speed. I can fiddle with the lever under the hood and bring the engine up to full speed by hand, something I couldn't come close to doing before.

Any better process for adjusting the valve clearance? I searched the forum here but the search results are all for other make/model tractors. Is there an obvious way to get the engine to TDC without yanking it out and pulling the timing cover off?

Thanks guys! I'm optimistic!

 

[dave_r]

You can also tell using a stick or screwdriver through the spark plug hole to feel when the piston gets to the top. Note, some engines may want you to put it slightly before or after TDC to set the valves, as their compression release system will slightly open a valve right at TDC (IDK if this applies to your specific engine).

And you can watch the valves, when rotating the engine in the right direction, to determine if it's on the compression stroke (which is what you want) or betweeen the the exhaust and intake strokes.

 

[ST1100A]

After a careful cleaning I dropped the engine back in the frame. Before I attempted to crank it and while I knew everything was at TDC I went ahead and opened the valve cover and inspected the valve adjusters. I didn't have the specs with me at the moment but it was clear with a feeler gauge that both intake valves and the exhaust valve at the front of the mower were so tight I couldn't get even the thinnest shim into. The rear exhaust valve had a gap of approx .3mm. OK, so ST1100A was definitely on the right track with the valves needing adjusting.

I backed off the retaining nuts and using a pair of pliers backed out the adjusters one-by-one and reset them as best I could (never done this before). Placing a feeler shim between the set screw and top of the valve stem I turned until snug but not binding and carefully tightened the retaining nut. Intake set to .12mm, exhaust to .2mm as per the manual.

After two starts with ether the motor picked up and was idling. Valves are definitely not 100%, but a huge difference in idle. I shut the mower down to top off the radiator and it fired back up almost instantly, a huge difference from the normal very hard start. There is a lot of valve chatter but no spit back through the intake. I was able to bring the RPMs up fairly easily although I must need to adjust the throttle linkage because at full throttle the engine is at about half speed. I can fiddle with the lever under the hood and bring the engine up to full speed by hand, something I couldn't come close to doing before.

Any better process for adjusting the valve clearance? I searched the forum here but the search results are all for other make/model tractors. Is there an obvious way to get the engine to TDC without yanking it out and pulling the timing cover off?

Thanks guys! I'm optimistic!

You can put a pipe wrench on the driveshaft to turn the engine over. Honda suggests to line up the timing marks on the crank or camshafts to do it, but you can do it like Dave_r suggested with a screwdriver in the cylinder to see where top dead center is. That would be a bit easier unless you have a shop manual for the engine to show you how to do it.
You want to set the clearance according to Honda's specs. That engine is pretty easy to get to the adjusters and set them.
Sometimes you can just use the ignition key to 'Blip' the engine over to turn the crank but you'll probably pass the timing marks and don't do that with a screwdriver in the cylinder.
I used to just turn the engine over with a pipe wrench on the driveshaft and watch the position of the camshaft to see when I was on the 'Heel' of the cam lobe, the part opposite of the cam lobes highest point, then I would chack and adjust the clearance.
Once you have them set properly, it should make a good difference on your running problem. Periodically you have to re-adjust the governor, so it is a good Idea to get the Honda service manual for that engine if you can find one at the Honda dealer, they can be expensive, but well worth the money. Get the tractor and engine service manual, they do come in handy.
Also, you want to be able to hear a little bit of valve clatter than no noise at all. If they are silent, chances are they are too tight. If they are a little bit noisy, then you have some clearance in them and that's what you want.
I've also used a vise grip on the driveshaft to turn the motor over, that works. Take the spark plugs out so you're not fighting against the compression.

 

[red97corvette]

ST100A, thank you so much for your advice. I have the shop manual for the GX640 engine and took a look at it tonight, looking for valve adjustment procedure. Advised procedure is find TDC on compression stroke for one cylinder and set clearance for that cylinder, then rotate 360 degrees and do the same for the other cylinder. I can look at the lifters and figure all that out, no problem. I'd been removing the spark plugs and was able to rotate the engine pretty easily by hand just by grabbing the fan blade.

My current clearances aren't correct but on a lark today as I was walking by I did nothing more than turn the key and it fired right up... maybe 3 revolutions before it caught. No choke, cold engine, throttle at full, parking brake set, no butt in the seat, just a passing turn of the key. **** thing fired right up. Not right, but it ran for multiple minutes until I shut it down, no spitting back through the carb.

So super encouraging on progress. I will continue to tune the valve clearances. I will report back, thanks all!

 

[ST1100A]

That's good to know it is working for you again. Starting like a Honda engine should start. They usually fired right up when they were working properly.
Good luck with it and everything and keep us informed on how it works for you.

 

[red97corvette]

Just got back to working on the mower this afternoon. It took me a couple more tries adjusting the valve clearance but when I got it right the engine went back to purring like a kitten! It's all put back together now and just finished cutting my yard with it!

ST1100A, thank you!

 

[ST1100A]

Just got back to working on the mower this afternoon. It took me a couple more tries adjusting the valve clearance but when I got it right the engine went back to purring like a kitten! It's all put back together now and just finished cutting my yard with it!

ST1100A, thank you!

You are welcome Red97. Glad to be of assistance, good luck with it in your future.