Silver Century plus not revving up

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Keith.P
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Re: Silver Century plus not revving up

Post by Keith.P » Thu Sep 14, 2017 3:04 pm

I'm not convinced its a carburettor calibration problem, Did I read that the motor ran fine in the test tank, but not on the boat, I can see the picture of motor on the boat, no tiller extension, so you're sitting right at the back, sorry stern of the boat, how deep is the motor sitting in the water, is the thrust block keeping the motor vertical as it looks very far forward.
The only differences I see from tank to water are, how vertical and deep the motor is, also it was running in gear in the test tank?
You can go back to talking about jets now Charles.

rstumpy
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Re: Silver Century plus not revving up

Post by rstumpy » Thu Sep 14, 2017 5:24 pm

Ran the same in tank and on river (ok but not great) without prop engaged...chugged when under a load and only to +/- 1800rpm at full throttle.

I shimmed the float about an eighth inch last night. Started on first pull cold in the tank, ran up ok in neutral. Will test under load fishing today.

Keith-
The transom has a shim block built into it so motor is set to it's angle not the transom's. Made a shaft collar to adjust depth to the proper height with gear and I in the boat. And yes, I have a tiller extension that just slips on.
Last time on the river while fiddling with things, managed to get her smoothed out and running proper by closing the fuel tap and vent nearly all the way for minimum flow...seems that points to a fuel issue, and since there's too much it points to the carb....at least in my wee mind. Am I missing something?...wouldn't be the first time...

Thanks for re-posting the actual jet measurements Charles. I shall file them away for future reference.

Off to test gull and chase salmon now...

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Re: Silver Century plus not revving up

Post by headdownarseup » Fri Sep 15, 2017 7:15 pm

I'm in agreement with Keith for the most part.
I'm not convinced it's entirely down to float height either. If there's fuel in the bowl it should still run no matter how high or low that level is. True, once the fuel level becomes too high it will over fuel and start to bog the engine. 1800 rpms +- is really struggling for one of these century's. That's not that much above idle speed for one of these.

It has to be something a lot more simpler than that. I'd look a bit more carefully at the pointed end of the needle to make sure it's located inside the bowl cover properly, and at the other end (which looks like a trumpet) make sure it is actually stopping the flow of fuel when you gently press upwards. Back to basics with this i feel.

If that doesn't cure things, i'd look at swapping out the jets with another set of 10:1's as a next solution. Not that i know anything about old engines, but we in the trade NEVER place anything inside a brass carb jet, no matter how tempting it might be to get a measure of things not even one of those tiny little files for unblocking gas torches. That's a big NO NO. It's possible that you might have inadvertently scored or scratched the inside of the jet with your indexed bits and upset it's flow rates, and thus be in the situation you have right now.

If by chance you have another carb to hand that you trust is clean and trouble free, fit it and try again on the water. This should at least give you a baseline to work from.

Jon

rstumpy
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Re: Silver Century plus not revving up

Post by rstumpy » Sat Sep 16, 2017 12:41 am

Well...I managed to catch my allotted three salmon yesterday but the seagull behaved as before.

Jon-
We check jets on old aerocraft carbs this way...but carefully and not with the "sharp" end. That being said I only put it in the tall end of the jet, not the inner part...and It was acting this way before I checked the jet and after...zero change...so no apparent effect regardless.
Pressing the tickler results in fuel dribbling out as it should...let it up and fuel flow stops.if I shim the float any more the tickler won't be able to engage (wasn't planning on doing that of course). I will pull it apart again, put the float back into the single notch, and carefully examine the rest once more as you suggested. In operation the float as stated above appears to be shutting of flow as designed.
I've only had one other century with a 46n amal, quite some time ago, so they're not second-nature to me like the villiers on the 40's are. Gulls are rare sights around here, so finding a known good spare not likely. This carb is however in great apparent condition and clean as a whistle.

Yesterday she started first pull, choked for two seconds. As before was four-stroking at all throttle settings and wouldn't run up to higher revs under a load.
Anticipating the possibility of the cure not being found I'd made a thin shim/clip for the fuel tap out of plastic that barely holds it open (so it doesn't vibrate closed). Once again after I nearly closed the tap she smoothed out, tached-up, and I made a two mile run upriver at various throttle settings quite happily. Plenty of water from the pump, and very minimal smoke for a century.

So it goes. Everything is clean. Everything is set up to proper spec. Starts great and doesn't die. AND runs great IF I restrict flow at the tap to almost nil. All consistently. This is beginning to become an irritant. The fact that it runs great by restricting the flow seems (to me) to eliminate electrical and carb jet issues, but ????. That's why I decided to come here to the Cathedral of BS and ask for guidance from the greatest seagull minds on the planet....

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Charles uk
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Re: Silver Century plus not revving up

Post by Charles uk » Sat Sep 16, 2017 9:54 am

The tall end of the Amal 46N carb power jet forms their equivalent of an emulsion tube & I believe is the same ID in all 3 sizes of these jets.

I've run out of sensible suggestions as to a cure for your over fuelling, you might have to try a chapel visit.
Make it idiot proof and someone will make a better idiot.

Keith.P
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Re: Silver Century plus not revving up

Post by Keith.P » Sat Sep 16, 2017 9:59 am

It seems to point to the float valve not shutting as you are controlling the fuel by the tap.
Reseat the needle, like you would a car valve maybe, or replace the float chamber completely.

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Charles uk
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Re: Silver Century plus not revving up

Post by Charles uk » Sat Sep 16, 2017 10:40 am

I wonder if someone has cleaned up/shortened the bottom of the throttle slide body (damaged threads) so the banjo bolt will catch the threads again, raising the height of the float chamber.

Remove the power jet again & measure the distance between the jet seat & the bottom face of the body.

0.860" on a new throttle slide body.
0.062" thick new fibre washer.

Clutching at straws now!
Make it idiot proof and someone will make a better idiot.

headdownarseup
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Re: Silver Century plus not revving up

Post by headdownarseup » Sat Sep 16, 2017 3:18 pm

If i may, as Charles has said (quite rightly) if for any reason the banjo bolt threads on your Amal 46n 2 jet carb are in any way dodgy (quite a lot of them are now) it's possible there might be some interference with the some of the components inside the carb body, but i've just checked on the 5 i have as spares and i come up with the same answers. NO CHANCE of any interference in this area at all as the banjo bolt itself is hollow and more than capable of coping with some degree of "fudging" in here. Generally it will be the female threads inside the carb BODY itself that are suspect.There's nearly 1/2" gap before there is any interference with anything, (only the bottom of the 2 jets) AND (more importantly) the float needle isn't anywhere near the banjo bolt in the first place. Not on any of the 46n carbs that i've got anyway. (don't know what Charles is on about!!! i think he's getting confused with villiers carbs somehow in all this bless him)
Gasket thickness isn't really going to make a lot of difference either at both sides of the banjo bolt, but no leaks in this area is definitely GOOD however which way you can keep it leak free, all depending on how good the threads are by now.
Amal 400 series carbs are completely different again,(possibly where Chas. is getting confused) but as we've now seen some pics of your motor we can confirm it has a 46n 2 jet carb. FYI the float needle is located inline with the fuel line itself and nowhere near the banjo bolt on any 46n 2 jet carb.

Back to basics then.
Your carb is over fueling no matter what you seem to try, although with some improvement and fiddling with the fuel tap things seem to get happier. So, back to the float needle again and remove it from the bowl and look inside the body of the bowl itself at its seat (the trumpet end of the needle) Both parts (needle and seat) are usually made from brass and if there's any corrosion or gloop in there that's at least 1 answer to the overfueling. It's not seating properly and fuel is flowing past its tapered seat inside the bottom of the bowl to the point its almost flooding the engine. Nothing to do with the primer on the top of the bowl cover. All this does is push the float downwards (off the needle seat at the base of the float bowl) and the fuel continues to flow until it shuts off when the float reaches it's pre- set height, a bit like the cistern in your toilet. The second reason is the jets themselves for whichever reason, something is not right with them. Someone might have been inside this carb before you as previously mentioned. This is half the fun with trouble shooting. Not knowing what's been done beforehand!

While you're in there check to make sure the banjo bolt has its little filter (small round one with a reversed domed top) as without the filter in place it can (sometimes) upset the fuel flow with too much or too little fuel, whichever the case may be. These carbs are finicky at the best of times, but still fairly straightforward to get to grips with. Plenty of spares out there if you get stuck, alternatively a replacement carb might be a better way to solve all the problems.

Needle seat and carb jets is where i'd be concentrating to begin with. The rest i think by the sounds of it are ok.


Charles, i'm sorry old son but all jet sizes are not the same ID. Seagull or otherwise i'm afraid. Bigger hole means MORE fuel flows through that little hole. Smaller hole means LESS fuel flows through that little hole. Temperature and atmospheric pressures will dictate how the carb should be tuned.

Another car related video for you to look at that might help with the basics. Seagull carbs work in much the same way as any other carb out there, except they're far simpler without all the fancy bits that other carbs on bigger engines sometimes have.
https://www.youtube.com/watch?v=QNNwxrBGzYA

I hope this helps
Jon

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Charles uk
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Re: Silver Century plus not revving up

Post by Charles uk » Sat Sep 16, 2017 4:52 pm

I was talking emulsion tube diameter & correct me if I'm wrong aren't they all the same ID? Jon & yes your post does help confirm how much you know about Seagull carbs.

I think I'm only the bloke who repairs deceased 46N carbs (as throttle slide bodies (S46/081) are no longer available) so I obviously know nothing about them.
Make it idiot proof and someone will make a better idiot.

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fleetingcontact
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Re: Silver Century plus not revving up

Post by fleetingcontact » Sat Sep 16, 2017 9:03 pm

Hey Stumpy

I've got a 46N carb that is not needed, but also untested...if you want it you can have it for the postage. Send a PM. 8)

rstumpy
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Re: Silver Century plus not revving up

Post by rstumpy » Sun Sep 17, 2017 2:20 am

Thanks for the input folks...just got in and checked messages. I'm reasonably proficient on this stuff but this one has had me buggered.

Charles, a visit to the chapel couldn't hurt at this point. I'll yank it and check those measurements...that being said the threads, etc on this Amal are in excellent condition, so I'm being careful with it. No appearance of cobbling any of the threads, etc.

Jon & Keith, I'll pull the float chamber assembly and go through it again, maybe gently lap the needle and seat. The screen in the carb is in great shape and clean.
With the tap on the carb doesn't dribble fuel unless tickled...if it did seems to me the issue would be obvious. It must be the problem, but wouldn't the carb be dribbling fuel while not running with the tap on if the needle/float weren't doing their job? Doesn't dribble a bit. Grrrrrr. Off it comes again.

Fleet, wow!...what a generous offer...thanks so much! That would at least give me another baseline to work from. Will PM you this evening...thanks again.

Gull juju has struck me again...after being away from them for a few years I have another as of today. A '67 FPL that has seen minimal use...chrome on the tubes is near spotless. Was on display in a maritime museum for several years, then acquired by the guy I bought it from who stored it for a decade. Couldn't say no to $30 US.
Unfortunately the gearbox and bracket were removed to facilitate shipping and somebody misplaced the driveshaft/impeller. Sooo...if anyone knows of a clapped-out FPL with a serviceable shaft/impeller...this will be perfect for my boy scout's sloop. Will post in proper section.
Off to wrench on the SC...again :-).

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fleetingcontact
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Re: Silver Century plus not revving up

Post by fleetingcontact » Sun Sep 17, 2017 4:47 am

I may also have a driveshaft - no promises - does anyone know what length it should be please?

rstumpy
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Re: Silver Century plus not revving up

Post by rstumpy » Sun Sep 17, 2017 6:22 am

Martin,
That would be brilliant!
All I know is it's looong. Would assume it's six inches longer than a standard but my assumptions regularly get me in trouble.
Bob

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Nudge
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Re: Silver Century plus not revving up

Post by Nudge » Sun Sep 17, 2017 9:14 am

perhaps you better explain the purpose of a float chamber Charles.
Some of the beginers may not understand
"THE KING OF BLING"!
Is it better to over think, than not think at all?

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Oyster 49
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Re: Silver Century plus not revving up

Post by Oyster 49 » Sun Sep 17, 2017 1:52 pm

I had a similar problem with my SD when I first ran it up after rebuild. The needle was not cutting off the fuel and the float bowl ended up overflowing. I changed both needle and float bowl top, and all was then ok.

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