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Collection, Preservation and Display of Old Lawn Mowers

Yet more suffolk issues

Hello all,

Suffolk is still playing up, it is putting up a good fight that engine, I can get it to idle on choke but when I rev it dies, Off choke it is hunting and revs are all over the place. Is it just a matter of adjusting, I dont really know what to adjust it to, leaner or richer and which direction is which on the adjustment screw?




redfernmowers Wed, 12/09/2012

OK let's give this old girl a jump start.

it's funny going  back on one of these carbs after dealing with a refurb on a client's restoration job a few weeks back, to which I'm extremely happy with both the results of the work and my client's joy and pleasure.

the zenith carb can sometimes be a fiddly little bliter to deal with. when setting up a carb like this, the first screw where your float chamber is (facing 9 o clock)  is nearly closed. so to do this, firstly screw in until it stops but don't put pressure on it. as soon as it feels it's touched, that's it. now unscrew 1/8 turn initially. the top screw adjuster past the float chamber is 1-1/4 turns so that you should get a decent mix.

now start your engine in idle. if it stalls, adjust the screw to the float chamber anti clockwise (slacken) 1/8 turn so then giving it a total of 1/4 turn. I find that sometimes this helps a little bit depending on the carb's condition. 

when in choke and off choke, the engine should run stable. then it's a fine adjustment stage with the engine running full. you can listen to tone changes in the engine when adjusting the fuel mix screw at the top. the key thing to remember here is dialing the carb to be as efficient as possible, also ensuring that the engine is fitted to the machine in question so that if you're using clutch shoes like you'd have on a qualcast / suffolk colt / punch, the idle rate does not accelerate causing the clutch shoes to open out and turn the drive head. when in idle, tick over should not engage any drive parts otherwise it can put pressure to the engine and stall it. this is done with the fuel mix screw as discussed.

I wish I was in your direction to come and help you out but wales is one of those places... too many bits of wool, not enough people lol.


HenryB Wed, 12/09/2012

are there any other possible causes to the revving up and down, not idling. which are not carb related?

oldiron Wed, 12/09/2012

Just to weigh in on this a little if i may, The above knowledge of the carb on that engine is far superior to mine so is probably worth following. Start with the basics, it does have all the hall marks of an air leak on or around the carb,check flanges or couplings to see if the seal is good. Check it out,Run it, if no good or different check back in and we could have another wild stab ion the dark lol.


redfernmowers Wed, 12/09/2012

good thing I popped in to update some details.

I always assess a series of aspects to an engine's performance:

1: fuel intake and efficiency. your fuel tap has a filter which could be shot. replace the fuel tap with it's filter go from there. I would replace the fuel hose from tank to engine as fuel can solidify causing poor fuel consumption and contamination.

2: Spark Plug. If you're using the original spark plug, replace with a new one. this will help with ignition control.

3: carburettor seal checks. check the seating of the carb to the engine to see if there's damage. this is done by unscrewing the two set screws and carefully removing the carb. check the gasket for wear or damage. if damaged, replace. reinstall the carburetor and test. assess the gasket to the float chamber for damage. ensure the gasket is a snug fit and the two screws are set in and reasonably tight. DON'T OVER TIGHTEN.

4: Oil level and quality. once an engine's stripped, serviced, etc, new oil should be loaded and to the correct capacity. over filling will affect performance and cause leaks now and again from the compression plate.

compression is always worth checking out and in this instance is very important. assuming that you've done a general clean of the piston and exterior of the valves including decoking, valve grinding is a necessary part of service. Valve grinding helps both the valve and it's mating surface to create a seal, also a good, clean point of flow. the average clearance gap between push rod and valve is 6 thou.  you need a valve grinding stick, coarse and fine grinding paste, a spring compressor, long nose pliers and a fine pry tool to help remove the spring with the compressor. this wouldn't cost any more than about £50 from 

in grinding valves, you remove the valves one at a time. remove the valve, clean it and the valve hole. now apply a finger dab of coarse valve paste to the underside of the valve and NOWHERE ELSE. ensure the shaft of the valve is clean. push the valve back in and with the valve grinding stick apply pressure so it is sucked into place, turn for a number of minutes and withdraw. Clean both areas and then apply fine paste and repeat the process. After cleaning the valves and re-installing the springs, re-install the valves and find the hole for the pin to locate in and insert the pin, back out the spring compressor carefully so the spring plate drops to the pin.

something worth doing here is to replace the gasket for the compression cover. this is an area where trouble can really  happen. 

with some patience, this engine would thrive again.


hillsider Thu, 13/09/2012

Hmm, are you absolutely sure that the contact breakers points are both clean and set to the correct gap of 18 thou?

I once had trouble setting up one of these engines that gave all of the indications of a carb problem but after fitting a replacement carb to no avail I decided to remove the flywheel to have a good look at the points. Cleaning and re gapping fixed the problem instantly ( there was a good spark but the points gap had closed up) this allowed the mixture screws to be set up as described in previous postings in the technical part of this forum. 


redfernmowers Thu, 13/09/2012

I completely agreee ray. have come across this issue before and yes, the ignition points do need a service. it's important to ensure they are a polished finish.  I use one of two methods to keep these contacts polished. the first is 320g wet and dry paper, the other is emery cloth fine abrasive. . the trick I use is to use  the abrasive on both sides to even polish the contacts, then (if using emery cloth) reverse so the emery cloth itself is exposed, this will polish further. then a clean sheet of paper passed through a couple of times does the last cleaning phase. this will ensure your breaker contacts are in perfect condition. the clearances can be 18 thou or 20 thou and you should have a strong spark that should be continuous.

it's very important not to get oil or debris on the contacts, so after you've removed the flywheel and have direct access to the contacts, I advise the use of disposable latex gloves.


HenryB Thu, 13/09/2012

I will give the points a good clean, When I first got the engine it had no spark so It was re gapped then, not much cleaning, could the fact that I have over filled the engine's oil so what remains of the dipper catches the oil surface?

I tried to remove it so I could replace it but no luck.