OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters McCulloch Trimmers & Brushcutters Adjusting screws brushcutter

The brushcutter starts, but then revs and stops. Sometimes it runs for a short time and if i pull the trigger up it will stop. I tried adjusting the three screws but no change. Could you help please?

First, can you put the two mixture screws back where they were in the first place please? If you don't know where that was, we may have to do a start-from scratch tune when the fault is fixed. That isn't difficult, but if you can put things back as they were it will be a bit quicker.

The two most likely causes of your problem are first, a bad spark plug, or second, the wrong air/fuel ratio. I suggest you try a new spark plug, and if the problem persists, try removing the air cleaner in case it is blocked. If neither of those changes causes an improvement, clean and reinstall the air cleaner (for safety, in case the engine spits) and try loosening or removing the fuel tank cap (in case the cap vent is blocked. Be very careful not to spill fuel, of course - that stuff is dangerous, especially near a running spark-ignition engine.) All three of those possible causes are common every-day faults. In case you are wondering, your symptoms seem to match the third fault best, but I suggest you check these things in an orderly way in case you have more than one fault. (Vented caps on brush cutters have a one-way air valve, and during the off-season the valve often gets stuck shut when oil residue in it dries up or oxidises and becomes sticky.)

Thank You. I`ve been following the instructions in the manual (attached), so i`ve probably got it wrong. (1) I will get a new spark plug. (2)I have cleaned the air cleaner in warm soapy water. (3) I`ll check out your suggestion on the fuel cap vent. So, i need some guidance on how to set the screws first? I should mention that the brushcutter was working ok when i was doing the edges but it suddenly stopped and now this problem.

If you followed those tuning instructions when you adjusted the carburetor you should be in good shape as far as carburetor tuning is concerned. I had suspected that you had tried to make trial and error carburetor adjustments rather than following that procedure.
1. If the spark plug has run more than say 15 hours, it probably needs replacing.
2. If it is a foam air cleaner, these normally have to be dampened with engine oil to be effective. However you should follow the manufacturer's directions if you have them.
3. Since your brush cutter was running properly then stopped suddenly, the fuel cap vent becomes a less-likely suspect, but needs to be checked. You also need to look at the fuel pick-up in the tank in case it has become blocked. If there is any sign of sediment in the tank, it needs to be cleaned out and the fuel pickup replaced if it is the strainer/filter type.

4. In general your symptoms sound more like fuel starvation than anything else. Since you also report that the carburetor tuning adjustments seem to have no effect, this suggests that the problem is a fuel restriction upstream of those adjustments. This could be inside the fuel tank, which means the cap vent, or the fuel pickup filter, or the fuel pickup hose could be split inside the tank, or the fuel hose from the tank to the carburetor could be leaking and therefore sucking air. Alternatively it could be a blockage in the first part of the carburetor, which would most likely be in the fine gauze filter at the entry to the fuel pump cavity. Lastly, it could be that the fuel pump diaphragm has failed, so the pump is supplying less fuel than is required. These are all fairly common faults, and fairly easy to fix.

Thanks again. I`ll do those tuning instructions again and see how it goes." Turn counter clockwise one turn" does the mean a full 360?

Yes, it means 360 degrees. Be gentle when you fully close the needle screw into the seat (i.e. turn fully clockwise), or you could deform the needle, which will make it impossible to adjust sensibly (i.e. becomes too sensitive). Most of those chainsaw-style carburetors have an initial adjustment of either 1.25 or 1.5 turns, so yours is a bit unusual, but I don't know what brand or model it is.

Thanks, i saw the name "Zama" on the carby. Tried resetting it but the same result. I`ll have to go to the mower shop to give it a good overhaul, it`s an older machine.

Hello again. i`m still trying to solve this problem of starting/reving and stopping. It was working ok and then suddenly stopped. I put in a new filter for the petrol tank but no change. I saw a CJ8 spark plug at the shop, is that the same as a DJ8J i`ve always used?

DJ and CJ spark plugs are both Champion types. DJ types are taper seating, and CJ types are washer seating. This means they are very definitely not interchangeable.

As I said 2 posts back under Item 4, your problem seems consistent with a failure to feed fuel to the carburetor jets. I set out the most likely causes in that post.

Thank You

Mine is a MC Culloch proscaper 11-HD, had it for a long time. Diaphragms look ok to me but i`m no expert.

Terance, the diaphram in the carby may look ok but they go hard, which means the fuel will not pump correctly, I would replace the diaphram and the needle and see how it goes.

Thank You.

After a new DJ8J plug and petrol tank filter i cleaned the air filter with petrol and a lot of dirt came out and i can see daylight through it and that made a difference. I was then able to start the brushcutter(it kept going) and adjust the screws and had some semblance of tuning and idling. After a while i`m back to hard to start and stopping. i examined the air filter( a type of felt/flock)see attached, and it is now absolutely dry. Re your no. 2 above>"If it is a foam air cleaner, these normally have to be dampened with engine oil to be effective. However you should follow the manufacturer's directions if you have them." Should i now dampen this filter with oil, any kind of oil?

I don't know the answer, because I can't identify the material. Both wet and dry types exist. The urethane foam type is used dampened with engine oil. The issue is whether putting oil on it would cause the fibers to cling together, thus flattening the filter into an airtight object, the way a wet dog's fur contracts and lays flat. Foam does not contract in this way, and is used slightly dampened with oil.

If your filter is now clean, it will not interfere with you starting and running the engine.

Thank You. It`s not urethane foam. After i cleaned the air filter i was able to start it and it kept running (contrary to previous stopping) so i could do some tuning, and that led me to thinking it must have been the filter was blocked. I`ll give it another go.

If it kept stopping due to air cleaner obstruction, it was running very rich. That means it should have run decently if you had screwed in the mixture adjustments far enough, but power would have been down due to not getting enough intake air. Anyway, with a clean air filter, you should be able to follow the recommended procedure for mixture adjustment and get a feel for how it is going to run.

Thank You. I`ll set the screws back to scratch and then one full turn anti-clockwise and try to set the screws accordingly.

"Two tubes from petrol tank to carby". Could please you tell me, does the tube with the tank pickup filter on it, connect to just under the primer bubble and the other tube down below that or the other way around??

The tube that begins with the tank pickup filter is the one that carries fuel from the tank to the carburetor. That one needs to connect to the carburetor's fuel intake. Try pumping the primer and seeing which carburetor connection sucks when you do it. That should be the one to connect to the pickup filter.