# Merry Tiller problem



## Lincsbodger

Hi

Ok, i got a problem, and whereas i can take a Cortina 2 litre Pinto engine to bits in the dark, im somewhat less au-fai with small petrol engines.

This is a 5 HP Briggs engine I want to put on a Merry Tilly frame to replace the poor little 2.5 HP engine that tries its best to turn the soil but tends to overheat after 20 minutes.

This 5 HP engine has no indication of model. It will only start if you pour petrol into the carb throat, and keep doing that, until its warmed up. Keeping it running is a very exact setting on the choke. The engine runs pretty slowly, about 150 rpm, but will run faster if you manually open the throttle. I suspect the carb is blocked, because its easy to get a flame and backfiring from the exhaust (indicting weak mixture), and then stalling.

Ive had the top off the diaphragm chamber. there are no holes in the diaphragm, but its clearly aged and stretched. Ive blown through the visible holes, all seems clear.

Does anyone have any pointers on what elase i can do, or what to look for? Is it worth finding a new valve diaphragm ?


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## bser

From your photos ,look to the left of the instruction sticker on the left hand side of flywheel cover(shroud)you'll see some numbers.It will say Model-Type-Code. Being it's a 5hp it will probably be something like 120000 or 130000.When lokking up parts you will need all of these no. The code is the yr/mon/day engine was made,ex. 090214.To solve your problem you may have to remove carb from tank and clean screens at the bottom of pickup tubes, also replace fuel pump diaphram on the left hand side of carb.There is a plate with 3 or 4 screws holding it on. With your engine no. go to www.partstree.com and you can look up your parts needed.HTH


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## 30yearTech

The diaphragm on this carburetor is strictly a fuel pump, and it draws fuel from the tank up into a little cup that has a dip tube where the fuel is drawn out of and metered to the engine. It's likely that there may be some restriction in this pickup tube or metering circuit. This carburetor is adjustable with a small adjustment screw on the side of the carburetor opposite the fuel pump diaphragm. Turning the screw counterclockwise will increase the fuel mixture and turning clockwise will decrease the fuel. 

I would venture to say if your engine is running at all, it's running faster then 150 rpm, it usually needs about 600 rpm just to make spark. It could be very weak on compression or a problem with the valves. Looking at the pictures you posted, this engine looks very old and most likely has points and condenser, which may need maintenance. The model, type and code numbers are on the blower housing just in front of the fuel tank and are almost visible in your pictures.

Best of Luck... :thumbsup:


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## Lincsbodger

hmm ok thanks for that so far.

the label actually says:










and its doesnt seem to have any number anywhere else.

Presuming that in fact it WILL run, do you think the valve diaphragm is ok? Is it a case of they work or they dont work and no halfway point?

Ive blown up the dipper tubes in the tank and they blow freely. Where else can it be blocked?

also is part 205 a jet ?

Also the setting of the mixture screw and the choke is pretty critical.

ok, on the theory most of these engines ar ethe same, browsing through a few of the early model numbers at random, the carb is pretty much the same, ie like this:










the diaphram is part 394.

Are they all as similar as they look in the diagrams? Comparing this to mine it pretty much looks like it.

Also, is the screw in the upper left, part 205, a jet ?


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## 30yearTech

The model type and code numbers are stamped into the steel housing right next to the sticker, I can just barely see them in the picture. 

#205 is not a jet, just a pivot screw for the throttle linkage, the actual adjustment screw (jet) is slightly to the rear of the pivot screw and is referenced by # 681 in the parts breakdown.

A loss of compression, or an air leak between the manifold and cylinder could cause the type of problem you are experiencing, so it would be a good idea to check these out.


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## Lincsbodger

30yearTech said:


> The model type and code numbers are stamped into the steel housing right next to the sticker, I can just barely see them in the picture.
> 
> #205 is not a jet, just a pivot screw for the throttle linkage, the actual adjustment screw (jet) is slightly to the rear of the pivot screw and is referenced by # 681 in the parts breakdown.
> 
> A loss of compression, or an air leak between the manifold and cylinder could cause the type of problem you are experiencing, so it would be a good idea to check these out.



Ah, i didnt look close enough!!! Thanks!!

Yes, your right, an air leak like that would create a weak mixture, good thinking


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## Lincsbodger

Ok, ive had the carb to bits, I understand how it works now, correct me if im wrong.....

The key to it is the diaphragm. It functions as the only fuel flow control device apart from the metering jet. One side of the diaphragm is exposed to the carb vacuum, and moves in and out with the venturi pressure variants. The other side of the diaphragm effectively forms the fuel chamber, and theres two flaps that form valves. As the diaphragm is sucked in by the venturi vacuum, it effectively increases by a small amount the volume of the fuel chamber on the other side of the diphragm, and draws fuel out the fuel tank in the same way the squeezy primer bulbs work on chainsaws. When the revs start to drop, the diaphragm relaxes, forcing drawn in fuel out the other hole and into the venturi, via the metering needle. Thus wobbles in the engine speed translate into wobbles in the diaphragm, which translate into anti-wobbles in the fuel flow, thus maintaining the engine speed.

There is effectively no other moving parts (apart from the throttle flap and linkages). If you examine the diaphragm, its stretched around where the spring flexes against it, and so is being pushed up against the cover of the diaphragm chamber and thus failing to pump fuel effectively. The engine runs on a weak mix because the fuel still flows round the fuel chamber by vacuum suction from the venturi, and thus introducing a weak residual flow, hence causing a weak residual mix.

The choke works by limiting the air flow, causing a greater vacuum and drawing more fuel though the fuel chamber, exactly as in a larger carb.

I therefore confidently predict that replacing the diaphragm will fix the problem.

The engine number is 130202, which is in the same block number as the exploded schematic, so that was a good guess 

Ill keep you all posted


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## 30yearTech

Yep, that's pretty much the way it works, except that the vacuum does not draw any fuel up into the area where fuel is metered from. Fuel can be pumped there via the diaphragm, or if the tank is full or near full it can just flow or be splashed in from engine vibrations. It may be that your fuel pump is working but not at a great enough capacity to keep up with demand from the engine. A new diaphragm may indeed take care of your issue.

Best of Luck.... :thumbsup:


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## Lincsbodger

Seems to work ok now, diaphragm was £1.20 and an 16 mile round trip.

Thanks for advice.


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