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Collection, Preservation and Display of Old Lawn Mowers

Suffolk 75G 14 25A zenith 13 tca2 carb problem revisited yet again!

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Hi

Advise needed!

The motor has been overhauled (BTW I have two of them )

After playing around with different carbs and cleaning up a decent one which is not worn and now appears to have all its channels clear, I have this problem:

I can get the engine to tick over quite slowly and evenly but if I wobbly the the board it's mounted on, it speeds up, idles very erratically, vibrates violently seems to spills petrol from the vent on the the top section of the carb, then eventually calms down to a slow civilized tick over.

I not sure but I think it may be something to do with the either the float and needle seating I think they might need replacing although the needle looks ok, difficult to tell - I don't want to replace anything which is ok.

I have made sure there are no air leaks by replacing gaskets: that was a problem once as it would stop if the strangler was opened.

 

Forums

wristpin Mon, 19/12/2016

As you suggest, it does sound a bit like fluctuating fuel levels . The construction of the carb doesn't lend itself to conventional pressure testing so I suggest  checking the float for leaks - give it a shake and listen for fuel slopping about and then immerse it in recently boiled water and watch for bubbles.

** See below

Then examine the needle tip with a magnifying glass for a perfect taper and undamaged tip. Not a lot that you can do with the seat but you could lap the needle to it using something gentle like Brasso or Solveol  Autosol metal polish . Also make sure that the seat is tight in the carb top.

** Very early carbs have a spherical float bearing on a separate needle and seat but hopefully you have a later float and needle combination either with a brass float or a plastic one . On some the position of the needle was fixed at manufacture and on others the needle is located by a spring latch in the top of the float. If the latter, check that the latch is engaged into the groove.

A somewhat way out possibility is a loose wire in the magneto either breaking or grounding the connections between the coil, points and condenser. Later coils have the condenser embedded within them. 

goodgrass Wed, 21/12/2016

Hi wristpin

I have done all the recommended things.

The float wasn't leaking but the needle taper looked rough so I exchanged it for a different float which seems to have a smooth needle taper.

I tried the lapping technique using fine lacquer polishing compound.

However there was no improvement that was discernable.

I have included this video to demonstrate the problem.

https://www.dropbox.com/s/ppfypte8gu2l96l/engine1.mp4?dl=0

I'm sure it's not supposed to run this rough although it idles ok ?

Maybe it does require a new seating and float kit, however it's difficult to tell if there is a leak from the original needle seating where it meets the base ( the sealing washer looks intact)

Is it best to replace both float and needle seating at the same time as they may bed in over a period of time.

wristpin Wed, 21/12/2016

Not quite sure what to make of your video as it doesn't seem to quite match your previous description . It does sound as though the idle speed is a bit low and that it is struggling to make the transition to higher rpm when it seems to run out of fuel. Possibly a low float level or just needing the main jet to be opened up quarter of a turn? I would remove the neon ignition tester just in case it is causing an issue in the sparks department . I presume that you have either broken into the low tension wire from the the coil to the points for your switch, or arranged it to earth the primary but I would suggest removing it just in case. Suffolks of that era usually just had an springy earth  strip to the plug terminal. 

goodgrass Thu, 22/12/2016

I've disconnected the earth switch and removed the ignition tester - they don't appear to be causing a problem.Today It cuts out if I open the choke flap completely. I have notice the clear fuel pipe does not fill up with fuel but there is just a bit at the bottom of the tube near the carb sometimes bubbles appear there from the carb. I'm wondering if there is an additional problem with the filter on the tap restricting flow.

wristpin Thu, 22/12/2016

Quite often  there is an air bubble, not necessarily a sign of trouble. Needing choke is a sign of either lack of fuel or an ingress of air ,i.e. a weak mixture. What happens if you open up (undo) the main jet a turn? It should make a difference and make the engine run rich. If it doesn't you either have a massive air leak or, more likely a partial blockage in the main jet system.  Bit of an after thought, check that the vent hole in the carburettor lower section is clear.

 

wristpin Fri, 23/12/2016

Why not just richen the main jet on your present setup? If it makes a difference it will save the bother of reassembling the "other" engine. If it doesn't it will possibly save you the bother of fitting a duff carb'. Then have a really good look at the main jet adjusting screw and check that the tip is not broken off and jammed inside the carb'.

 

goodgrass Fri, 06/01/2017

Both engines are running without choke and idle smoothly, however if I run them at a higher speed they vibrate like a hardcore compactor . They might behave in a more civilized manner once properly mounted on a mower chassis though. 

wristpin Fri, 06/01/2017

Being bolted to a chassis will absorb some vibration but my test bench is a bit of board held in the Work Mate  with four spikes to locate into the engine's mounting holes to stop it wandering . I just drop the engine over the spikes and don't bother with nuts and  Suffolk engines run quite happily without jumping about  so it surprises me that your's apparently vibrate badly.  The only thing that occurs to me is that the  flywheels may be out of balance but it would take an awful lot of broken fins to do that!