McCulloch MH452 P petrol hedge trimmer won't start

Hi folks.

I've recently purchased a very cheap second hand McCulloch MH42 P petrol hedge trimmer. The seller couldn't tell me much about it as he said he was clearing out old tools of his father in laws.

A few days before buying it I knew nothing about petrol tools like this so have been madly Googling since.

Generally it looks clean and the fuel tank was left empty with a clean looking fuel filter inside. I've replaced the spark plug, all fuel lines and fuel primer bulb. The air filter is very thin and I've cleaned the worst off of it. I've mixed up fresh petrol with 2 stroke oil following the 40:1 ratio on the fuel cap.

It looks like the fuel primer bulb is doing the job of getting fuel to the carb.

I unfortunately still can't start it. I've taken the spark plug out, sprayed in carb cleaner and refitted the spark plug. This has allowed me to get it to fire up briefly, but it only runs for a couple of seconds afterwards. I've also tried screwing the low carb screw in, winding it back out one and quarter turns, and then tried again.

I've removed the blades and checked them over. They were seized together, but I've now cleaned them and they seem to move very well now, even to the point of being able to turn the small gear with my finger which then moves the blades. I've also tried starting the trimmer with the blades removed in case they were restricting it in some way.

Any suggestions are greatly appreciated. Happy to take any photos which might help. Many thanks.

If it start using carb cleaner then it has to be a fuel problem. Maybe the float needle is stuck and not allowing fuel into the float chamber. That would be my first port of call especially if it has been sitting for any length of time.

Before you dive in too deeply, does the engine have good compression when pulling the recoil starter? This is absolutely vital for a two stroke motor.
A useful test is to remove the exhaust silencer and look at the piston and rings that you can see inside.....there must be minimal scoring and the piston ring needs to be polished bright.
As above, I do suspect a fuel problem and rather than rebuilding the carburettor with a diaphragm / gasket set it could be a good move to find a replacement carb from ebay or whatever......usually around a tenner.
John

Many thanks for your comments and suggestions. It does feel like it has good compression when pulling the recoil starter. I did have the exhaust silencer off yesterday morning and thought at the time that the piston looked nice and shiny. I could probably do with taking it off and taking another look to be sure that it looks good like John suggests.

I did get distracted earlier and removed the carb so I could take a better look at it. Photos of the carb below.


We've taken a good look at it for a part number but can only see C1U stamped on it. I've rummaged around the internet and have found the following which appears to be it:
https://agroparts.gr/p/husqvarna-group-467/
This gives some other part numbers so hopefully will help identify the correct carb to buy if I get a new one.

I guess there's no harm in taking this carb apart and checking the diaphragm and float needle. First time taking apart something like this so I'd better have the time to take some pictures as I go.

The C1Q designation shows that it is a Zama carb, actually owned by Stihl now.....somewhere there will be a few numbers written on too.
When you get it apart, you'll find there are two sets of gaskets and diaphragms......often they split on dismantling so new ones are needed.
The diaphragms are next to the carb body - you'll find one has a round metal disc (that's a metering diaphragm) and one has U shaped flaps - that's a pump diaphragm.
Also you'll find a needle valve, spring loaded but no float chamber as such as these engines can run at any angle. Look for a fine gauze filter on the other side of the needle!
The two screws, L for low speed tuning and H for high speed tuning can't be mixed up.
An excellent firm I use all the time is GHS Ltd, plenty of spares, pictures and speedy delivery.
Good luck with your project!
John

sounds like the fuel/air mixture might need adjusting. there should be a screw for this on the carb.

Hi,

I rebuilt a mculloch hedge trimmer as my first project in small engines. I found that the gaskets were impossible to source for the model I worked on. It was the ones between the carb and barrel\air filter I struggled with. I ended up making my own from gasket paper, light oil and a crafting knife.

Cover the carb face in light oil and stamp it onto the paper then carefully cut it out. Pay attention to all the little holes as some are important in carb function. Hole punches help lots.

Lots of good info on YouTube (channel called therepairspecialist iirc) on exactly how a 2 stroke carb works.

I've now moved on to stihl 4mix engines now which are lovely wee things. I'm fully rebuilding an fs130 engine after someone over torque a screw and put an dent inside the cylinder. No wonder it didn't have compression! Have another smaller one that dropped a valve so lots to keep me busy.

Have fun and good luck

Graeme
ETA carb info link

Want to be kept busy, Graeme?
Become a specialist on Ryobi petrol machines
John

Are you sure it's not a 542p hedgecutter?

From a ref chart it's either a c1u m42(or 32)
https://www.stens.com/quick-reference-guides
https://www.yumpu.com/en/document/view/52490607/zama-carburetor-reference-chart-barrett-small-engine

No other stamps on the carb?