Engine racing

[The Alchemist]

Okay. So I set off in my narrow boat this afternoon, everything seemed fine when I started it up (BMC 1.5) , anyway I just passed a few moored boats and increased the revs a little as I pulled away when the revs increased further without me pushing up on the throttle.

I tried bringing the throttle back to neutral but the engine continued to race at maximum revolutions.

I managed to pull into the side and cut the engine.

Now when I start the engine in neutral after perhaps 30 seconds of normal engine the engine races up to full revs again.

I've checked the throttle cable and everything appears as it should be and nothing is moving where the cable is attached so I'm at a loss as to what is occurring.

Anyone able to offer any advice as to what is occurring or how to address it?

Thanks in advance 😊

[Tony Brooks]

When the injector pumps with hydraulic governors, like the vast majority of BMC 1.5s, start to suffer fuel starvation the governor has insufficient pressure against it to push it down, so it rises towards the maximum fuel position and the engine revs up.  They often then stall, all on their own. 

 

I don't know the history, but if this were mine I would be changing/cleaning the fuel filter(s) and water trap, bleeding it, and seeing if that helped. Do not forget the strainer under the domed top of many 1.5 fuel lift pumps. 

 

Anything that causes fuel starvation can cause it, but the filters and lift pump strainer are the easiest to address. Do not take the rubber O ring out of the lift pump cap and ensure you take careful note of where each of the three or four O rings on the fuel filter goes.

 

The fact it starts normally for 30 seconds and then revs up suggests the governor valve is not sticking open.

 

If that does nt work comeback and give us some history of you and the boat

[The Alchemist]

Thanks Tony. Appreciate your prompt response. I'll get some fuel filters ordered and work through your suggestions.

I'll get back and update you when I've done so.

Thanks again 👍

[Tony Brooks]

FWIW, one stroke on the priming lever on the lift pump should deliver at least half an egg cup's worth of fuel. Testing that might give a clue to further areas to investigate, as may the amount of fuel expelled per stroke while bleeding the filter and injection pump.

 

Also feel and sell the sump oil in case fuel is diluting it. If the level is very high, it eel thin, and smells of diesel, that could cause it to run on crankcase fumes. If you think this might be the case, change the oil ASAP and report back for more advice about where the lack might be.

[The Alchemist]

Very thorough and insightful as usual Tony. Appreciate all your help 😊

[Kingdom Isambard Brunel]

Check that the air filter is not pulling oil from the side breather on the port side of the engine. If it is the engine could run away to destruction. If the oil is diluted with fuel this is more likely to happen. Oil dilution is due to either a perforated diaphragm in the lift pump or, more likely, failed seals in the drive on the injection pump. This is becoming more frequent as the new low sulphur fuel is removing the vulcanising sulphur from the rubber seals making them hard and fail. 

[b00ke23]

27 minutes ago, Kingdom Isambard Brunel said:

Check that the air filter is not pulling oil from the side breather on the port side of the engine. If it is the engine could run away to destruction. If the oil is diluted with fuel this is more likely to happen. Oil dilution is due to either a perforated diaphragm in the lift pump or, more likely, failed seals in the drive on the injection pump. This is becoming more frequent as the new low sulphur fuel is removing the vulcanising sulphur from the rubber seals making them hard and fail. 

 

Good point, but probably not the case here as the OP was able to stop the engine conventionally.  

[David Mack]

12 hours ago, Tony Brooks said:

When the injector pumps with hydraulic governors, like the vast majority of BMC 1.5s, start to suffer fuel starvation the governor has insufficient pressure against it to push it down, so it rises towards the maximum fuel position and the engine revs up.  They often then stall, all on their own. 

Exactly this. I had the same problem as the OP about 40+ years ago. Initially a few random blips of racing, with the engine speed then returning to normal, but after an hour or two the engine speed was rising and falling at random, regardless of anything I did with the speed control lever. The boat had just been craned over Blisworth tunnel, which was closed indefinitely at the time, and in the process the fuel in the tank had been shaken up. The rudimentary fuel filter, fitted as part of a very poor standard DIY engine installation, was completely blocked. It took a few days to sort the filter, and in that time the gruck in the tank had time to settle out again, and after that normal operation was resumed.

[The Alchemist]

So. Here's the update on my engine racing issue. It took me a while to discover but the diesel lift pump was at fault. I've replaced it along with the fuel filter and bleed the system several times! before it finally started for me this morning.

While the engine was down I also undertook an oil and filter replacement and replaced the air filter which I've been meaning to do for sometime.

The engine seems to be running much sweeter for it too. 😊

Thanks for all your help and advice.

My only issue now is with the exhaust back box which needs to be replaced as it's peppered with small holes.

There's a slight problem there though as the existing box is 15" long and has a threaded inlet and outlet pipe which measures 2". I've visited Calcutt boats and they only have a 15" box which has either a 1" inlet and outlet for around £23 or a 1.5" inlet and outlet for around £125, but they tell me there isn't a 2" 15" box .

Anyone able to offer advice as to how I can find one or direct me to somewhere I could get adapters to enable me to install either of what's available!

 

[Tony Brooks]

I think that you might be confusing pipe outside diameter with the BSP size such things are measured in.

 

for 1.5" BSP Google gives this 1-1/2 Inch BSP. 47.80mm, which pretty much matches up with your photo. However, I would have expected Calcutt to realise that, it is something we run across all the time on here. 

 

LGQ who own Aquafax, Midland Chandlers and I think ASAP list one, 1.5" BSP and 15" long, but no price, one from Parts 4 Sale is a bit cheaper than Calcutt, including delivery.

Edited September 22, 2025 by Tony Brooks

[Kingdom Isambard Brunel]

Its definitely 1.5"BSP threads. There is a table on the web with the outside sizes and the BSP size corresponding.

You will have great fun undoing the rusty union and the threads.  The best way is to get them very hot, like red hot, and then fast cool the pipe not the threaded pipe unions with a wet cloth. Then big Stillson wrenches and lots of muscle power.