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Collection, Preservation and Display of Old Lawn Mowers

Suffolk Punch 30s 1991 Dellorto Carb - Rough Idle

Hi,

Recently aquired a 30s from around 1991 with a Dellorto carb that was non working til i renewed the spark plug and now it starts every time but runs very lumpy/rough to the point it's almost like its banging.  I have temporarily cut off the fuel filter in the tank as it was blocked, decoked head, changed the spark plug and cap with NGK, air filter fine, only two adjustable screws on carb so gently wound in then out 2 turns for both.  I read somewhere that the governor if the wire connecting it to the throttle is in the wrong hole that can cause problems but it only seems to go in the hole that doesnt have the return spring in due to the wire hitting the electrical connector on the inlet manifold.

I can take some pics later and will upload a youtube of how rough it runs.  Just to add I am able to rev the engine ok. 

Did a video if it tonight https://www.youtube.com/watch?v=qmB0bty7UH0&feature=youtu.be

Forums

wristpin Fri, 13/03/2015

Don't want to sound too discouraging or negative but those Tecumseh derived Suffolk / Qualcast engines never really ran sweetly even when fresh out of the box, let alone some years down the line with a bit of unknown service history thrown in.

To stand the best chance of sorting it I would start by reseating the inlet and exhaust valves and setting the valve clearances. Follow this with a thorough clean of the carburettor and then see how it runs. 

Even though the governor may have been fiddled with it is unlikely to cause a misfire. You can perform a quick visual check by using the handlebar control to increase the revs fully and watching to see if the governor arm makes any attempt to close the throttle on the carburettor.

 

wristpin Fri, 13/03/2015

Don't want to sound too discouraging or negative but those Tecumseh derived Suffolk / Qualcast engines never really ran sweetly even when fresh out of the box, let alone some years down the line with a bit of unknown service history thrown in.

To stand the best chance of sorting it I would start by reseating the inlet and exhaust valves and setting the valve clearances. Follow this with a thorough clean of the carburettor and then see how it runs. 

Even though the governor may have been fiddled with it is unlikely to cause a misfire. You can perform a quick visual check by using the handlebar control to increase the revs fully and watching to see if the governor arm makes any attempt to close the throttle on the carburettor.

 

suffolkpunch30s1991 Fri, 13/03/2015

Hi,

Thanks for that and I have kind of bench marked it by looking and listening to another 30s on youtube. The carb has been done so may have to look at valves etc.

Im just wondering if the magneto is faulty or the points need cleaning as some say they have points and others not.

greensleaves Fri, 13/03/2015

Hi There i have purchased  for tecuseh  H25 H30 H35 H40 Relacement carb for Dellorto  £10 including postage

For  A brand new carb ON EBAY This should cure your problem as it has a main mixture screw which is

adjustable beneath carb bowl . Will let you know the TEST results  IN A FEW DAYS .  

suffolkpunch30s1991 Fri, 13/03/2015

Hi,

Yer that will be interesting as i was even thinking of putting a minimoto carb on but didnt really want to go down that route.

One other thing that's come to mind is the spark plug i bought was a NGK BR4LM and looks like it should be a B6HS so just wondering if it's the wrong spec plug causing the problem?  The old plug that didnt work was a ALM or something like that and it was dead. 

wristpin Sat, 14/03/2015

Fairly certain that engine will have electronic ignition, no points and condenser. A faulty ignition unit is more likely to cause a misfire at high rpm and as I read your post it revs up ok. 

I like the suggestion of fitting a carburettor with an adjustable main jet as it allows for a bit of compensation for engine wear and tear or  a bit of fuel contamination. The downside of all the emission compliant non adjustable carbs is that they are fine when the engine is in good health but can't cope  when things are less than perfect.

suffolkpunch30s1991 Tue, 17/03/2015

Correct spark plug fitted and although the mower sounded better still a misfire which isnt great.

Took the flywheel off and inside there it was a bit oily, greasy with dirt so gave it a good cleanup.  Inside the flywheel was written in marker pen "new type" so must have not wanted to get flywheels mixed up with older type!  Gave magnets and pickups a slight sand as well as the clean.

Giving it a go tomorrow as its a bit late to start a mower up at 10:30pm

Will update tomorrow.....

wristpin Tue, 17/03/2015

Ref your comment regarding  the "new type" flywheel. This possibly  refers to the flywheel for the electronic ignition engines being different from that fitted to the older points ignition ones OR to the fact that Suffolk/Qualcast used two different electronic set-ups ( inductive or capacitive) requiring different flywheels. The inductive coil/armatures had 3 "legs" and the capacitive 2 but I'm not sure how to id the respective flywheels - possibly had a part number cast into them. Also, not sure whether a mismatch between coils and flywheels would result in no spark at all or just a rough running situation.

suffolkpunch30s1991 Wed, 18/03/2015

Hi,

it's the one with 3 legs and on testing tonight it's still misfiring a little and sounding rough.  I did notice in the dark that the thin wire that comes from the magneto that plugs in to the top of the carb/intake manifold sparked a few times with the intake manifold body.  I did try unplugging this and it made no difference.

I think the problem is the magneto and wondering if it's worth replacing the thin wire and also replacing the HT lead at the same time.

I got a donor 30S arriving Sunday so will probably swap the carb first to rule that out and if still rough swap the magneto with associated cables as long as its the one with 3 legs!

 

 

wristpin Wed, 18/03/2015

Your "thin wire" is the kill wire from the coil that is grounded to stop the engine when the throttle lever is moved to the stop position . In the run position that wire should be insulated from ground and should not be sparking at all. If that's what it was doing, disconnecting it should make a difference! - odd.

Have a look at the back of the ignition coil . if you can see that the hard outer casing is starting to part from the "potting" material in which the windings and electronic components are embedded it is usually a sign of intermittent or imminent failure. I have found that a period in the airing cupboard or on top of a radiator dries the coil out enough to effect a temporary cure but as soon as damp re-penetrated the coil it is back to square one. Even filling the gap with sealant or adhesive fails to effect a permanent cure.

As far as your donor machine goes, just check that it runs ok before making the swap or you may just be importing another set of troubles!!

suffolkpunch30s1991 Wed, 18/03/2015

Hi,

Thanks for the pointers as it's helping me out quite a bit and you may be on to something regarding the back of the ignition coil as where the HT lead goes in the material does look to be coming apart so this could be where damp is entering.

I'll check the donor over before the transplant as your right that I could end up with more troubles!

cheers

wristpin Thu, 19/03/2015

The HT lead screws onto a threaded spike and is not a very close fit. so a small gap around it is quite usual . It is a gap opening up around the perimeter of the back of the coil that I have found to be indicative of a failure.

I've just dug out some old Atco/Suffolk/Qualcast service bulletins which make quite interesting reading particularly the suggestion of opening up the plug gap to 40thou in certain applications and the check list for Wipac electronic ignition issues .

https://www.dropbox.com/s/ajrvuul2oiq66qu/Suffolk%20and%20Wipac%20servi…

greensleaves Fri, 20/03/2015

Hi There  after  digging out old ali engine from back of the shed  decarb plus lapped in valves. Fitted new carb

from ebay  total cost £10  WITH ADJUSTABLE MAIN JET . NOW THIS ENGINE WAS AN  UNKNOWN CONDITION  TO MYSELF  .IT MADE A VAST DIFFERENCE TO THE RUNNING QUITE GOOD SEEING HOW OLD THE ENGINE WAS RUN FOR 2 HOURS UNDER LOAD & NO LOAD .MUST SEE HOW IT SETTLES DOWN MORE TESTING QUALCAST ISSUED A SERVICE BULLETIN DATED  B/Q 73/25 DATED 1985 WHERE THIS PROBLEM WAS WELL KNOWN   A VARIABLE JET KIT Q00959. WAS ON OFFER THEN BUT IS NOT NOW AVAILABLE TO MY KNOWLEDGE .AS  OTHERS  HAVE STATED QUITE CORRECTLY QUALCAST DID TELL SERVICE ENGINEERS TO SET PLUG GAP TO 0.40"  BUT REVERTED  BACK  TO NORMAL SETTING ,THERE IS NO REASON NOT TO TRY THIS DEPENDING ON THE COIL STRENGTH . THIS WAS THE ISSUE WITH SUFFOLK OR QUALCAST FITTED WITH DELLORTO CARBS P/S I AM GOING TO FIT THE OLD CLEANED OUT CARB BACK ON TO NOTE THE DIFFERENCE BETWEEN THE TWO.

 

suffolkpunch30s1991 Sat, 21/03/2015

The process was:-

1.  Collected donor mower and started it although it was in a sorry state of affairs.

2. Tried using magento and flywheel from it in mine bad same bad result.

3. Tried also using carb and same result,

4. Transfered engine (very easy) and while at it replaced clutch brass bush as worn out.

5. Runs better but not convinced so will try and put the carb that came with the engine back on and run that.  Think i'll start to look at a mini moto cross carb for a tenner on the bay as well.

greensleaves Sun, 22/03/2015

Hi There  just a thought  is your ali engine mechanical gov or air vane type  then you if it was air vane type replace it with A  Venith  13. TCA  2  FITTED TO LATER  SUFFOLK  MODELS   ALTHOUGH THIS SEEMS A STEP BACKWARDS  ITS WORTH A TRY. THE ALI ENGINE WAS REALLY A PIECE OF  S*** .P/S IF YOUR ENGINE IS OF MECHANICAL GOV BUILD  THIS IS NO GOOD FOR YOU.

suffolkpunch30s1991 Mon, 06/04/2015

After fitting the correct size mini moto carb all is well and also no stinking smell!  Awaiting an air filter then that's it so will keep this one as now have some nice spares.

just bought an Atco 17s Balmoral so new problems and loads less info it seems on the net.....

gtc Wed, 08/04/2015

Good to hear that you've had success.

Re: "just bought an Atco 17s Balmoral so new problems and loads less info it seems on the net", it will be interesting to hear how you go (in a new thread) with that comparatively recent model.

alistairstubbs Mon, 01/06/2015

What carb did you get off ebay and was it a straight forward swap.  My 43s will only run on full choke or ticover on no choke.  Is there a way of adjusting the jet on none variable jet carbs?