Infernal combustion engine needs exorcism

Having tried every trick in my home mechanic’s skillset, I’ve decided the BCS 715 rotovator is actually an infernal combustion engine and needs to be exorcised.

I’ve spent three days repeatedly stripping down its fuel system, checking it methodically, testing it, rebuilding it and successfully running it in the workshop only to find it throws a hissy fit when wheeled out to tackle the potato field.

I’ve run it for 10 minutes on idle and 10 minutes on three-quarter throttle in the workshop, without a blip. But if I run it for more than 60 seconds in the field, it coughs, splutters, pops and dies.

I’ve checked and cleaned the petrol filter.

I’ve checked and rodded all the fuel lines.

I’ve dismantled, checked and rebuilt the fuel cock.

I’ve removed the carburettor bowl and watched petrol pour through the needle valve.

I’ve partially connected the bowl, seen it fill to the manufacturer’s specified level and seen the float cut the flow.

I’ve successfully blown air through all the carburettor’s ports and jets.

I’ve checked the tolerances of all valves, including the throttle and choke. I’ve checked they move smoothly.

I’ve carefully reassembled the fuel system, ensured all the seals and gaskets are in place, and torqued all the fasteners to the manufacturer’s specifications.

I’ve started the engine on the second or third pull of the starter cord. (Two or three pulls are needed to pull fuel through the system as it doesn’t have a primer.)

I’ve had the engine running sweetly in the workshop, from cold all the way up to operating temperature.

I’ve had the infernal machine purring away as I walked it out to the field.

And then, just as I contemplate putting it to work…

Vrrrooom, putt putt putt, vroooom, putt, putt, vroo-ooo-oom, puttitty putt putt, putt, PUTT!



Back to the workshop. Check, double check and triple check the workshop manual.

Work through the fuel system again.

Get the engine running again.

Take it out to the field and…

Vrrrooom, putt putt putt, vroooom, putt, putt, vroo-ooo-oom, puttitty putt putt, putt, PUTT!

Several hundred imprecations and mutterings later, I decided it was time to reach for bell, book and candle…

20 Responses to “Infernal combustion engine needs exorcism”

  1. My husband, The Car Guy, wonders if it might be the governor?

    • The governor is working correctly. When the engine is under load, I can see the governor spring pushing the drive rod inwards as it attempts to increase fuel feeding. Sometimes the fuel feeds and the spring pulls the drive rod back until the the engine speed stabilises but at other times the engine dies through lack of fuel. To me, that indicates a fuel feed problem. It may be a partial blockage of the main jet or a small air leak, with little effect when the engine is not under load but causing fuel feeding to be insufficient under load.

      Demonic possession sounds better, though.

  2. Is the fuel fresh as petrol goes off?

    Is the spark plug fowling?

    Is the spark healthy as beleve it or not as the cylinder pressures goes up as the load goes up it gets harder for the spark to jump the gap. Which will lead to it running fine on no load but once the load goes on it breaks down.

    Have you tried running it in the pitch black. To see if you can see if the HT lead is tracing to earth.

    • Yes, the fuel is fresh. Yes, the spark plug is sparking. It’s dry, grey and the gap is correctly set. And the HT lead isn’t leaking to earth.

      I’ve checked much more widely than the fuel system, but the indications are that it is a fuel feed problem. If I remove the air filter assembly and I’m quick enough to spray a fuel-air mix directly into the air intake as the engine falters it immediately picks up again for handful of revs. If it was the spark plug, that wouldn’t work.

      I did an RAN marine engineering course many years ago, so I know something about internal combustion engines, plus gas and steam turbines. A steam turbine powered rotovator would be interesting… :D

  3. Is there a flake of rust swilling about in the bottom of the fuel tank, that finds its way over the out let of the fuel tank ?.I get that in and old fordson tractor every now and then.

    • There were a few small rust flakes in the fuel filter after I first filled the tank for the season’s work. I cleaned the filter and did all the rest of my start-up maintenance.

      When the problem subsequently manifested itself, I drained the tank, checked there were no obstructions in the outlet, rodded all the fuel pipes and dismantled the fuel cock. All were clear—that’s why I said there was fuel coming through the needle valve and filling the bowl.

      At idle, the choke valve is open and the throttle valve is closed, with the piston draught creating a vacuum next to the slow running port. Fuel is clearly coming through the slow running port as the engine idles well.

      Under acceleration and without load, the increasing throttle opening partially destroys the vacuum, fuel stops coming through the idle port and instead comes through the progression hole. Fuel is clearly coming through the progression hole as the engine responds immediately to throttle pressure.

      Under acceleration and with a load, ie maximum output with the throttle valve wide open, the rush of air should create a partial vacuum over the venturi in the main jet and fuel should be dragged out of the main jet into the air flow while the slow running and progression ports stop delivering fuel. Compressed air blows through the main jet’s ports but it’s possible there’s a small obstruction, enough to hinder fuel flow but not enough to hinder compressed air. Alternatively, there could be a small air leak below the height of the throttle valve: when the valve is closed the vacuum is correctly formed but with it open, there’s too much air above the venturi.

  4. Sounds like there may be a problem with the float or needle valve, something less obvious that you couldn’t see the first time. Floats stick sometimes, but they can also get punctured and fill up with fuel. Needle valves may appear to be free but actually are sticking in one or the other part of their range of operation.

    Best bet, strip and clean the carb again and then test the float for ease of movement by holding the upper carb assembly up to eye level and watching very closely as you move the float up and down with your finger. Watch the needle valve too. Most of the small engine carbs have a tiny retainer that holds the needle valve to the float arm. This little wire should drag the needle valve more or less smoothly out of its seat as you release the float each time.

    If all else fails, order a replacement carb. That will most likely fix it. Certain American small engines of my acquaintance require a new carb every other year or so because of these problems after winter disuse.

    • The float is floating correctly, it doesn’t touch the sides of the float chamber, and I have tested it for ease of movement. I placed the carburettor cover and gasket on the bowl using a single screw to hold it all in place, connected the fuel line to the the union and let the bowl fill. I disconnected the fuel line and removed the cover. Then, with the bowl held horizontally and the float in place, I checked the distance between the fuel surface and the top of the float chamber inside the bowl. The distance was 10.9mm: the manual specifices 10-12mm.

      The needle valve isn’t directly connected to the float. Instead, the float has a small lip that pushes against the bottom of the needle valve to close it. (Effectively, the needle valve’s default position is open. To adjust the fuel level in the chamber, the lip is bent a little up or down.) The float does drop and release the needle valve, the needle valve does drop free, and fuel does flow easily through the needle valve.

  5. I agree, the thing’s possessed! Okay, you’ll need some eye of newt, a few drops of dragon’s blood, a dollop of fesh garlic and some dancing around ‘skyclad’ at the dark of the moon. It (probably) won’t cure the wonky rotovator, and you could end up with frostbite in some odd places (naked dancing is almost never a good idea in Scotland). But it can’t hurt, right? It may, however, cause mass hysteria among the family and any neighbours close enough to witness the spectacle.

  6. I agree – possession is a possibility. Or its just old and lazy and would prefer its warm shed to having to exert any energy.

  7. Do you think a prayer or lighting a candle to Saint Eligius, Patron Saint of Mechanics might help ?

  8. Unfortunately, our 715 has an Intermotor engine with an early carburettor. Some parts are still available for it, but not all. The only solution is to replace the entire carburettor with a later one, at a cost of £147.97. Ouch. That’s a big chunk of money to spend on a machine that’s worth £350-400, especially as it will need a new set of blades soon.

  9. Hi Stonehead

    I had a v expensive printing press once that I came to believe had the ghost /earthly signature of a long gone printer in it! Never did figure out what made it (randomly) run out of register. (Talking precision German engineering here to the thousandth micron.) And we spent a lot of money on top class printers’ engineers trying to find out!
    I’ll put out an invocation to Granny Weatherwax for you – she’s definitely the old crone you need for this task.

  10. Have you tried entering the carb’s part number in Google? Sometimes a unit like that is generic, not proprietary, and once you get away from the manufacturer’s resale front, the price drops.

    Put the whole number in, dashes etc, if there are any, and see what you get.

  11. I’m sure your not anxious to hear another recommendation but I’m going back up to Mick. Clean the carb again with Carb cleaner to get the gunk out – they get sticky – we had the same problem with a generator motor this winter. It’s the ethenol gas that messes ours up – don’t know if you have that in your gasoline.

    • I did clean the carb with cleaner. In fact, I left it to soak in cleaner for some time, cleaned it with an old toothbrush and a thin piece of plastic rod, soaked it again for a shorter time, and then blew air through all the ports before leaving it to dry. As for leaving fuel in the tank over winter, I don’t. All fuel tanks are drained—even though that increases the risk of rust in metal tanks.

  12. It certainly is a confounding problem. My dad was a collector of old cars and I can remember his spending weeks searching out such ghosts, and the considerable increase in my naughty word vocabulary. Have you tried playing with the tolerances? Sometimes machines develop a preference for this to be set wide and that to be set narrow even though they’re supposed to work regardless.

  13. Buy a similar carb of a similar sized engine and I can make an adaptor plate up

  14. It could be an electrical fault, If the engine is fitted with a coil and condenser setup either one of these could be working fine when static running but fail under load, Check the electrical system with a multimeter.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 754 other followers

%d bloggers like this: