Erratic RPM Forty Featherweight.

[cassini]

Hello to the Forum.

I haven't used my engine in quite a while, but put a new throttle cable onto it last year when the old one snapped. I seem to get full carb barrel(?) movement via it.
Anyway, on Monday, motoring across Plymouth Sound with my 1976 Bing Carburettor Forty Featherweight on the transom of my 1965 GP14 dinghy,
I noticed the engine note would change randomly. The most obvious symptom would be that it would be 'on song', then, with no change of throttle,
the RPM would change down a little. Twisting the throttle control to full didn't help as it hadn't moved from full in the first place!

Later on it might recover to a higher RPM. Not a huge difference you understand, but quite noticeable and I need everything I can get out of this motor into tide and wind.

Also, coming back up the Plym estuary later, random surges in RPM beyond beyong normal limits, which clear quickly (second or two). Now this was not the same as surges in RPM caused by a following sea which I also experienced earlier, when the prop managed to come out of the water on a wave, as the estuary was nearly flat calm.

The fuel wasn't 100% fresh, it had been stored for some months in a plastic can, but fumed profusely like fresh petrol does, as it was decanted into the tank. Anyway, it was well mixed.

I did read a comment on the web that old Seagulls can have problems with the fuel float in the carb and run very rich in consequence, causing erratic behaviour. I can't see why that would cause a problem as I thought the fuel flow is determined by the needle position and load. I am not sure how to test this though. I have had the float chamber off, it seems fairly clean and the float itself seems fine, plus it moves on its pivot without noticeable friction.

Any ideas? I was going to fuel the tank and then manually move the float up and down to see if it really does cut off fuel from the float chamber when in the full position, but was wondering if people think this might be a plausible cause and if not, what might be?

By the way, I got rid of maybe 4 litres of 25:1 on a trip of approximately 9 miles? That's about 10miles/gallon! The fuel consumption on the way out was about twice that on the way back as due to heading into an 8-10kt wind and a spring tide at mid-flow, but even correcting for that, I make that very roughly 13-14 mpg in neutral conditions. Is that about right for a lightly-loaded 14 foot dinghy???

[1charan]

Hi Cassini,

Reading through your post, my first suggestion was going to be to clean the carburettor completely.
The second suggestion being fresh fuel.
It seems your fuel is not that old, and your carb is clean, but still, it sounds like a fuel problem. Take the carburettor apart completely. I usually put everything in a sonic cleaner with water and some dishwash soap. And while you are at it, take the gas tap too.

Fuel consumption; I have a featherweight too. I use less than one liter per hour, maybe even per two hours. That is at nearly full throttle, going about 5 km/h as measured with a gps. So your fuel consumption sounds excessive.

If it is not something with the fuel going in, than maybe it has to do with a power loss?
Do you have a compression meter? Or maybe you can feel the compression when turning the flywheel by hand?
Was your speed OK?

Good luck,

Charan

[cassini]

Hello Charan,

Thanks for the reply. I shall use fresh fuel next time but I don't feel that's the problem - it was fuming and quite volatile. I will take the carb apart and clean it but it doesn't appear to be too bad - I've had the barrel out already and in fact had it out and cleaned last year when changing the throttle cable.

I was making slow progress for the outbound part of the journey, walking pace at most. However, on the way back the speed seemed faster 5-6 knots? The engine didn't behave any differently, it was just the wind and tide being with me (although high water was only an hour away when I started on my return journey so it was getting quite slack). I certainly felt the speed on the way back was very acceptable, considering the size of engine.

My fuel consumption is a concern - I was annoyed to find myself having to refuel mid-journey. I need a better method of refuelling than a can with a spout - perhaps there are flexible fuel hoses with a hand-pump/bulb I can use so I don't have to stop the motor to refuel from a can?

I do have a compression tester but hadn't thought of that. I'll give it a go. Teaspoon of oil in the spark plug hole, hold tester to hole and then pull the cord, right?

[Stelios_Rjk]

Maybe the fuel tap is leaking?

[headdownarseup]

1976 40 featherweight

has it got the gold top wipac ignition? (check/clean and regap your points thoroughly)
spark plug ( i prefer the D16) again correctly gapped.
is the carb tightened correctly on the stub of the cylinder. this can cause the odd air leak which leads to erratic running

give the motor another run on the boat, if there's not much change to performance/economy, then look at the compression with a view to rebuilding the powerhead.

firstly check the basics before you end up spending your hard earnt!

jon

[cassini]

There's no leak from the tap.

I don't know what the ignition system is and can't look until the weekend. However, I had the recoil starter housing off last year and looked at the points and gaps and they appeared OK. I'll look again though.

The plug is gapped to 0.5mm, though it usually looks a bit blackened - is that normal? I'm used to sand-colour looking plugs in four stroke engines. It is in a helicoil repaires thread, but it seems sound and there is a soft metal sealing washer for the plug seat. I'll replace it anyway.

Tightening up screws on the carb in case of air leaks sounds a good idea. The bottom of the circular air intake flange has been bent backwards sightly i noticed, most likely in a fall from a standing position. It's hard to notice except from the front or rear, with the intake cover off. However, the throttle barrel slides freely.

The compression test seems a must-do as well. Things to do..!

[Hugz]

Spray some WD40 around the crankcase seal/join whilst idling. If there are any crankcase leaks the motor will speed up as it sucks in the WD40. Also could be moisture in the tank that has condensed into water. To check, undo fuel line and let a bit run into a glass jar. Water sinks so there may be the odd clear drop. Good luck

[Lahtinen]

I need a better method of refuelling than a can with a spout - perhaps there are flexible fuel hoses with a hand-pump/bulb I can use so I don't have to stop the motor to refuel from a can?

Look from ebay or amazon for "siphon pump", there's plenty of items to choose from.

[headdownarseup]

when you replaced the throttle cable last year, is there any chance you might have adjusted the throttle slide at all?
i can only imagine your featherweight is running a villiers carb. yes? 25:1 or 10:1? (yours should probably be at 25:1 if it isn't already)
sooty or black/brown plugs may be an indication you're running a bit rich on the mixture.
tweek the needle jet down a little and see how it goes. (no more than 2 turns down from flush)

jon

[cassini]

Hello Jon,

I'm not quite sure what the throttle slide is ;0) If it is the cylindrical item that moves up and down with the throttle position that one can see looking into the air intake, it's position is determined entirely by the thottle cable, isn't it? It seems to go up and down freely.

My Seagull has a Bing carb, according to the SOS serial number identifier info. It's 25:1.

A diagram of the Bing carb appears on this site:

viewtopic.php?f=2&t=1553

That's what I have.

I take it tweaking the needle jet down leans the mixture? (sorry, I know very little about carburettors). I'll see if I can figure out how to adjust it,

regards,

Andy

[cassini]

The good news is my compression tests at 8.4 bar, 125 lb/in2, which sounds fine to me!

However, the engine got so hot during my trip when I turned it to idle to take a look see at landing options at Cawsand, (as it stopped pumping cooling water of course, duh!) that the spark plug slightly melted the rubber spark lead boot. It's still usable (the spark lead) but it'll never pass for A1 ever again ;0)

Onto the carburettor and float chamber next...

[Mike Killay]

Did you look at the plug?
I would not be surprised if it was white.
I had a similar situation with a Mariner 2hp. Turned out that the air screw on top of the tank cap was getting obstructed.

[Stelios_Rjk]

The water circulation is also erratic. Goo in the waterways 90%.

[headdownarseup]

hi cassini.
compression sounds spot on to me.
however, when these motors are left unused for any length of time (6 months to a year or more) unless you religously flush the motor out in fresh water, then the corrosion will start to take a hold in the water jacket.(and fast)
you really do need to be sure you have a clean waterway, free of any obstruction (rust or otherwise) in order to maintain an even temperature.
as you are now aware, when you turn the throttle down to a moderate tick over, waterflow will stop thus the motor gets hot (and quite quickly too if your not carefull!)
what i suggest is this, look through previous posts on how to remove the cylinder head (quite simple and straight forward,just take your time and use a blowtorch to help release the bolts on the head, heat the block NOT the head)
clean and reassemble with a new head gasket.
secondly strip the carb down a give everything a good clean. while your about it have a look at the colour of the spark plug. brown/black and semi wet with fuel is ok, white no good! (shows the engine was running hot/too lean.)
as your motor has a BING carb fitted, the only adjustment you can make to the mixture is by setting the height on the needle jet. once you've removed the 2 screws that hold the top cover on (where the throttle cable enters the carb) extract everything in one go. the needle will be obvious when its out.
try not to alter the setting on the needle jet unless you're absolutely sure this is the problem. its a tricky and awkward thing to mess with. clean first and retest the engine in the bin. clean the fuel tap as well and make sure the filter is not blocked as this will also restrict fuel flow into the carb and result in a lean mixture/hot engine and possibly erratic running! there's also a fuel filter on the fuel line connection to the carb. make sure this one is clean also.
EVERYTHING and i mean everything on the fuel side must be clean. if your motor has a steel fuel tank, rust could be partially blocking the fuel jets.
if it has a brass tank, use caustic soda dip and soak the tank for a couple of days.flush with plenty of fresh water and leave to dry before refitting and filling with fuel.
cleanliness is everything with a seagull. cover the basics first before you start pulling the motor apart.
most seagulls respond very well to a good clean especially in the fuel and ignition department. its when they're left sat in the garage/shed for months/years on end that the deterioration will occur.

any problems just ask

regards jon