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Collection, Preservation and Display of Old Lawn Mowers

Replacement Engine for Atco Royale

I own a reconditioned Atco B24E.  Sadly the Briggs and Stratton Engine is producing quantities of black smoke and having reground the valves, with no noticeable improvement I am faced with the prospect of either reconditioning or replacing the engine.  Advice from BriggsBits is as follows:  

The nearest current replacements would be;
Electric & Pull start - 6.5HP Vanguard E/S @ £514.80
Pull start only - 800 OHV 3/4" crankshaft @ £264.00
UK Shipping on either £17.72.
They can be found on the page linked below
The crankshaft position and footprint is the same as your original sidevalve engine however, please carefully check the dimensional information given with the listing in order to be sure that its differences would not interfere with any chassis structure.
The electric start seems a bit too expensive, but I would be interested in any advice on whether I would be likely to run into  difficulties if I were to purchase either of these engines as a replacement.  Removing the old engine is straightforward, but I don't want to run into any signnificant reconfiguration of the mower chassis.
Has anyone had experience of a similar project?  Are there other replacement options? All advice gratefully received.



Dengiebob Wed, 10/04/2013

by the sounds of it your bore is a little worn or it could be the piston rings or both lol. it is a easy fix but it would need a engine strip down and a trip to the local machinest and a new piston and rings to but it right i have a hayter rough cut mower with a briggs 7 horse engine that we have used for twenty years and it was a good ten years old when we got it and thats on its original engine it smokes a little but i think they all do a bit!! they go forever if you keep the oil changed. the briggs side valve is one of the most basic engines to strip and rebuild. while you was there you could grind the valves in and you would have a new recon engine i bet there are strip down guides on you tube to help

wristpin Wed, 10/04/2013

Black smoke would suggest an over rich mixture rather than oil burning but if you are intent on a new lump look at Loncin engines. Loncin have made engines for Honda and BMW  - good quality and keenly priced.

hillsider Thu, 11/04/2013

I am inclined to agree with the suggestion by wristpin that the black smoke likely to be caused by the fuel mixture or a carb problem. It would be better to investigate this before comitting to a replacement engine. Do you have an engine manual for this engine if not they are a good investment and from memory are not too expensive? Failing that if you post the Model, Type and Code numbers for your engine we may be able to point you in the direction of some setting advice for the carb fitted. Ray

Carruchan Fri, 12/04/2013

I did actually order up some a kit for the caruburretor, take it apart and reassemble with new gaskets etc.  It didn't seem to have much effect and as I recall, when I tried to make adjustments, it did not seem to respond according to the text book descriptions -  and it was very difficult for me to find an optimum adjustment. 

I would have to confess however that when I last left the mower in for a service I was firmly told that - to my shame - I had let the oil run very low.  I have become used to engines which  leak or burn very little oil and so had not taken the trouble to check.   Prior to service I had thought it was running not too badly, but it would seem the fact that there wasn't clouds of black smoke was because there was no oil to burn and not because the engine was running smoothly. 

wristpin Fri, 12/04/2013

Obviously you are the man on the spot but oil burning is usually white or grey smoke; black is usually a sign of over fuelling. The fact that your engine is not responding to carburettor adjustments would tend to support this. Are you sure that the carburettor is not flooding?

Carruchan Sat, 13/04/2013

Thank you for your insistence on paying attention to the colour of the exhaust.  Having not started the mower for about 10 months, I can now see that the information I was providing was inaccurate, so this morning I went out and started it up to remind myself exactly what was happening.  

First of all, it started on about the 3rd or 4th pull and I would say initially ran without much obvious exhaust, but once the engine had warmed and the choke had been taken off settled into a pulsing [would this be "hunting"?]  with white smoke being expelled with each pulse and the governor linkage pulsing with the same rhythm. 

More concerning to me however was that once the engine had warmed a little it also started a backfiring, with the frequency of backfires pretty much following the pulses of white exhaust smoke.  

I do by the way have a "Single Cylinder L Head Repair Manual"  but this covers a range of engines and carburettors and I haven't found it that easy to follow. 

I am quite interested in the idea of doing a complete strip down of the engine but not having much spare time at the moment I am a little daunted by this - would I need special tools?  Would it be sufficient to rebore the cylinder and fit outsize piston rings?  Or could there be wear in the crankshaft requiring more radical attention?  

Thanks to all who have taken an interest in this.  Further comments and suggestions welcome.

wristpin Sat, 13/04/2013

Everything is possible with unlimited time and cash! However if you are talking reboreing (BS actually recommend honing) and possibly crank regrinding I think you will find that the economics just don't stack up - especially if it's your first foray into serious mechanicing!

I'm still not convinced that you have a serious problem but remote diagnosis is not easy  and you might be wise to try and find another amateur with more mechanical knowledge. A dealer or professional repairer always has to play safe and do the job that won't return to haunt him or be unsatisfactory for you. I spent over thirty years in the trade and it was always a balancing act between what was possible and what made economic sense for both parties. As a post retirement "Fred in a shed" I am now doing jobs that I would never have done while in business because time is "limitless" and I'm working for beer money!

  If you really have a knackered engine check if there is a short block available - that will give you all the critical moving / wearing parts onto which you bolt your flywheel, ignition system, carburettor (if ok?) starter and cowlings. However a lot of manufacturers have now lowered the price of complete engines and done away with short blocks. If it gets to that stage I would have a serious look at Loncin engines, I know of several machines that have been re-engined with them with no problems whatsoever.

I may have missed it in your posts but the starting point for finding out availability/cost of any BS parts are the Model, Type and Code numbers carried by every BS engine . These identify your engine right down to the day of manufacture and even the shift/production line it was made on. Armed with those numbers you can access the exact parts list for your engine and do your sums.

hortimech Sat, 13/04/2013

Yes that would be hunting and there could several things wrong with your engine but most likely it is a blocked carb, fuel nowadays goes off very quickly and leaves deposits behind which have probably blocked the carb. You will need to remove the carb, strip and clean it, you may have to find somebody who has an ultrasonic bath to clean it properly, also if it is the carb I think it is, it does not have an adjustable main jet, it is fixed.

Please post the model, type and code numbers from the engine, these should be stamped into the airshroud.


Carruchan Sat, 13/04/2013








3048 01


I think you are correct in saying that the main jet is not adjustable.


Here's a video of it running which may help to diagnose what's wrong:

In the final sequence I add a little choke to demonstrate that this enables it to run more smoothly which seems odd given that I'm told "hunting" is symptomatic of a mixture that is too rich - and I thought that choking the carburettor would further enrich the mixture.

The backfiring doesn't seem quite as obvious on this video as in real life but is a definite feature of current running.

wristpin Sat, 13/04/2013

Hunting is much more likely to be as a result of a weak mixture and the fact that you can smooth the engine out with a little choke confirms this and  backs up Hortimech's suggestion of a gummed up carb. As he says, you have a fixed main jet but if it's the original carb there should be an adjustable idle/slow running jet (118 on the illustrated parts list) and it is sometimes possible to use this to compensate for not enough going through the main jet.

Considering the age of your engine  I guess that it is entitled to be fairly worn, particularly if it has not had regular oil and air filter services!

Decision time I think!

PS, Tried to run your vid but it says PRIVATE!

wristpin Sun, 14/04/2013

It gets odder and odder but here's what I think!

The state of the exhaust and your report of black smoke (although I couldn't see any in the vid) definitely suggest a rich mixture but the hunting and being able to smooth it with choke say weak mixture!

You do have an adjustable slow running jet, it can be seen in the vid just behind the plate carrying the throttle and choke controls  - in fact there is a little semi circular cut out in the lh edge of the plate to facilitate access with a screwdriver.

Could it be that your work on the carb has cured the over fuelling and left you with the slightly weak mix and hunting I wonder? Try smoothing it with that adjusting screw. If it cures it and the machine performs ok  i would say job done and go mowing.

If it doesn't help I would give it a  top end overhaul/ valve job ie head off, fuel tank and carb off, remove valves, reface if necessary/ equipment is available, lap in, set gaps and reassemble. It's a fact that no amount of fiddling with the carb or ignition will get you anywhere it you have poorly seating incorrectly gapped valves.

I will just throw one other thing into the equation because the old brain has just clicked round and remembered it but as your machine has apparently worked ok for over 20 years it probably doesn't apply. When Atco switched from the old Tecumseh H50/60 engine to the BS which must have been around the time yours was made they did have a hunting problem - in fact there were service bulletins from both BS and Atco detailing the suggested cure which involved drilling an additional hole in the plate on the throttle shaft and fitting an additional spring to damp out over sensitive throttle reaction at idle.  I can possibly dig out the bulletin but hopefully it it will not be necessary.

Carruchan Sun, 14/04/2013

The black smoke, if you remember, was an error on my part:  when I checked, the exhaust is    white which I suppose is consistent with some wear in the engine.  

I have already - last year - done a top end overhaul, set valve gaps and reassembled and it was the failure of this to improve things which led me to think I should replace the engine.

I have made fairly exhaustive attempts to adjust the slow running jet but couldn't discern any notable variation. 

However!  The theory that the mixture is lean and the carburretor needs to be overhauled seems a good one to me.  I did have a go at cleaning the carburettor last year  but to be honest, it was of a type I had not encountered previously and had features that I felt unable to take apart.  Here is a picture which will help to explain where I got stuck: 

Note the two tubes, one short and one long, which drop down into the fuel tank.  The carburettor is mounted on the fuel tank and the long tube   I assume draws fuel.  I'm not sure what the purpose of the short one is.  The top part of the long  tube is metal - brass I think.  The bottom part is plastic and has a fine mesh filter housed in a hexagon.  [I am guessing this plastic tube might be part 611]  I would guess this could very quickly become clogged with old fuel.  The plastic section is held on the  brass tube by a little metal clip.  I couldn't see a way of removing this which would not have caused damage to the assembley and so I left it alone.  I would be very tempted to have a go at removing it however.  The hexagon makes no sense as it fairly clearly does not screw into place.

The shorter metal tube in the picture, running parallel to the tube just described [the screw shouldn't be in the picture by the way] - is actually two tubes in a single moulding and terminates with what looks like a brass jet, but the location of this makes no sense to me as it would be above the level of the fuel I think.   [The second tube in this moulding terminates with  what looks like a tiny beed - about 1mm diameter  - even more puzzling to me.]

The rest of the carburettor I have had apart and cleaned as best I  could. 

Any suggestions as to how I should proceed?  

I'm back to work tomorrow, so there may be some delay before I make progress with this project, but I'll keep you informed of what happens. 




wristpin Sun, 14/04/2013

The long tube sucks up the fuel under the action of the diaphragm pump on the side of the carburettor and deposits it in the well in the top of the tank so that regardless of the level of fuel in the tank the short tube feeding the jets etc is always drawing from a constant level of fuel.

Both tubes can be removed - they are a stiff push fit- the procedure is covered in your single cylinder L head manual. However I would leave them alone, remove the diaphragm cover , diaphragm (have you renewed this - they go hard and the little flap valves leak and don't pump properly) don't loose the spring and cap, remove the adjustable jet then take the carb body to someone with an ultrasonic cleaning tank and get them to give it a 20 min hot clean with alloy cleaner , turn it over and do the same again.  Then flush it out with carb cleaner and  reassemble (make sure that you get the spring correctly located) - refer to the manual again and then see what has been achieved.     



Carruchan Mon, 15/04/2013

Thanks for the explanation of how it works, which all makes good sense to me.

I took the carburettor into the College where I work today and showed it to the motor vehicle team.  No ultrasonic bath I am afraid [that costs money - we don't have one]  so it was just  a spray with carb cleaner and a blow through with a high pressure hose, which I was assured would do the necessary. [A long soak in a parrafin bath was another suggestion made.]

Having refitted tonight, there is no discernable improvement to the running of the engine.  Here are another couple of queries and suggestions however.

  1. The diagphram seemed quite pliable - definitly not hard.  It is slightly distended where it passes over the little cap that sits on top of the spring and maybe the flaps are leaky, but I couldn't really assess their state. Perhaps it should be replaced?
  2. A couple of people were suggesting to me that the backfiring might be a timing related issue. [In the fading light tonight the flames coming from the exhaust with each report were very visible.] Any thoughts on this?   

Incidentally, the short tube [see my previous post on the configuration of the carburettor] is part of the carburettor moulding and so cannot be removed.  The plastic tube could be removed, though I think there is agreement that there is probably no point in taking this off. 

Thanks for continued interest!

hortimech Mon, 15/04/2013

Typical motor vehicle team, carb cleaner and an air line are no replacement for an ultrasonic cleaner. The fuel and air lines in your carb are very small and are easily blocked and will only be really cleaned now with an ultrasonic cleaner.

As for the possible timing problem, have you or someone else had the flywheel off? if so the flywheel key may have slightly sheared due to the flywheel not being fully tightened.

Another possible problem could be that the valve tappet settings could be too small, only way to fix this would be to remove the valves, but as you have the carb/tank off it will be easy to check, just remove the breather assy that will be visible now.

wristpin Mon, 15/04/2013

If the short tube is a casting integral with the carb body you have a later type carb, possibly not the original for that engine.

I beg to disagree with carb cleaner and compressed air being OK. If the carb is contaminated with stale gummy fuel nothing other than a hot ultrasonic session will get where it's needed and sometimes not even that will do it  - have given up and replaced carbs on some occasions but at £60 + vat ....!!!  You only have to look at the minute size of the progression jets/holes in the carb venturi and the hidden gallery behind them so appreciate the effect of just a little reduction in "flow".

Back firing and flame - weak mixture.

We would always change the diaphragm at service - cheap enough and part of what is known as the CYA philosophy!

Any possibility you can borrow a carb from a known good running engine?

Carruchan Mon, 22/04/2013

Well hortimech and wristpin...I am not greatly surprised that your reaction to the motor vehicle team is   less than impressed, but they were very helpful, so what could I do....added to which they really seemed like my best bet for access to an ultrasonic cleaner....I'll keep looking though.

In the meantime, if I run it with the choke partly  on, will that be problematic?  Running with a weak mixture can I think lead to overheating - what about a rich mixture? Which would I suppose be one risk of keeping it partly choked?  

I don't believe the fly wheel has been off, but I did have the magnet off which generates the spark and regapped it on assembly.  [I think that's the armature - part 333]

I did the tappet settings when I did some work on the valves.  

The reason I don't think timing is a likely cause is that the running issues identified all predate any of my own attempts to correct matters, and I assume with an engine like this that timing is fairly stable.  [In fact my perception is that the problems date from a service carried out by a local firm of horticultural mechanics, who advised me that I had allowed the oil to run very low. Previous to this, my recollection is that the engine was running not too badly, considering its age and considering the fact that it allegedly had very little oil in it.  I won't pretend however that I had taken great care over checking the oil level.]

If you haven't completly lost interest in my case, your further speculations will be poured over with interest.