McCulloch 435 only runs when primed

[BeefStew1332]

I have a McCulloch MacCat435 saw that I have been trying to sort out for a while now, but no matter what I try, the saw will only start if it is primed with the bulb, or if fuel is dropped straight into the carb. It will then run for a few seconds then die again. I have cleaned and rebuilt the carb. replaced all the fuel line, made sure the fuel filter is not bunged up and even replaced the carb with no avail. I’m no genius in this field, but it’s something I’m trying to get better at, so any help whatsoever is greatly appreciated.
-Henry

 

[Bob Hedgecutter]

Not real familiar with the model- but......
Was it starting before you removed the carb and did a carb clean/kit?
Are you 100% sure the lines going to the purge valve (primer bulb) are hooked up correctly? Could be you are pushing air into the carb- not drawing fuel up into it.

Not sure if these have a separate impulse line or impulse through the intake block- but no impulse from cylinder and the carb cannot/will not pump fuel.

 

[BeefStew1332]

Not real familiar with the model- but......
Was it starting before you removed the carb and did a cark clean/kit?
Are you 100% sure the lines going to the purge valve (primer bulb) are hooked up correctly? Could be you are pushing air into the carb- not drawing fuel up into it.

Not sure if these have a separate impulse line or impulse through the intake block- but no impulse from cylinder and the carb cannot/will not pump fuel.

It had the same issue before I touched the thing, it came from my grandad who definitely did not see maintenance as a first priority… everything I’ve done has been in an effort to sort the issue.

I’m fairly sure all the lines are hooked up correctly, unless I’m being dim (very likely) im assuming it must be good as when I pump the primer bulb the fuel has no trouble coming up from the tank, into the carb.

The impulse comes straight from the block, I knew it worked but I double checked and it definitely has impulse from the cylinder.

Thanks
-Henry

 

[Bob Hedgecutter]

Righto- sounds like you have covered the fuel delivery issues- except maybe a new tank filter.
Next place Id be looking is air leaks- by pressure and vacuum testing the saw.
Its a plastic clamshell and they are known for the cylinder coming loose from the engine pan and sucking large volumes of extra air into the system to mix with the atomised fuel- effectively drowning the fuel in air and air does not explode good with watered down volatiles.
If you do not have the means to test- look under the saw- some have the 4 main bolts visible that will hold the cylinder to the plastic pan- others have an internal alloy pan, so need to be fully removed from the plastic case to find the bolts that hold the pan to the cylinder.
Tighten those bolts and see if it helps- may not cure as the liquid gasket might have failed and needs replaced- but it might improve things enough to help it run.

I have had a Husqvarna 345 arrive here with running issues and one of the 4 bolts had come loose, rattled out and had been lost!

 

[Bob Hedgecutter]

I have a McCulloch MacCat435 saw that I have been trying to sort out for a while now, but no matter what I try, the saw will only start if it is primed with the bulb, or if fuel is dropped straight into the carb. It will then run for a few seconds then die again. I have cleaned and rebuilt the carb. replaced all the fuel line, made sure the fuel filter is not bunged up and even replaced the carb with no avail. I’m no genius in this field, but it’s something I’m trying to get better at, so any help whatsoever is greatly appreciated.
-Henry

This cleaned and rebuilt the carb bit- are you sure the components all went back in the same position, same orientation.
Is the pump side of the carb back together correctly, carb body, gasket, diaphragm and then top cover?
Are the screens on the metering side sitting correctly in relation to the carb body?
If there was a small mesh screen in the hole located under the needle valve- was that screen clear and not blocked/varnished?
Did you clean and reassemble using old bits- call it good, or did you fit a new carb kit?

 

[BeefStew1332]

This cleaned and rebuilt the carb bit- are you sure the components all went back in the same position, same orientation.
Is the pump side of the carb back together correctly, carb body, gasket, diaphragm and then top cover?
Are the screens on the metering side sitting correctly in relation to the carb body?
If there was a small mesh screen in the hole located under the needle valve- was that screen clear and not blocked/varnished?
Did you clean and reassemble using old bits- call it good, or did you fit a new carb kit?

Thanks for the suggestions.
I’m definitely sure all the carb bits are back together correctly, I used all brand new pieces when rebuilding it. The carb I replaced it with was not brand new, but was in much better general condition, I also put new pieces on that one to be sure it would be alright.

I’ve tried everything you’ve said so far, and replaced the instant gasket on the bottom of the cylinder (can provide more details), but still I find it has the same problem. Because of this I went back to the carb, replaced it and fiddled with it just in case the 2 that I had used had a common blockage or something like that, but still I’m stumped.

For a very short period of time I removed the fuel filter (made sure the line was still in the fuel and not just sucking air) and tried running it to see if the filter was restricting the flow, but no luck.

I’m considering buying a brand new carb, but I can’t quite see why that would fix it, as I’ve now tried 3 or 4 different carbs, all in good nick, but the saw still has its troubles. I wouldn’t be surprised if after all this time it is something tiny and incredibly simple that I’ve missed, but I just can’t work it out.

Thanks,
-Henry

 

[Bob Hedgecutter]

The spark arrestor screen in the muffler is not totally plugged with carbon?

 

[BeefStew1332]

The spark arrestor screen in the muffler is not totally plugged with carbon?

Nope, I’ve cleaned that out as good as I can.

 

[scoredwall]

I have a McCulloch MacCat435 saw that I have been trying to sort out for a while now, but no matter what I try, the saw will only start if it is primed with the bulb, or if fuel is dropped straight into the carb. It will then run for a few seconds then die again. I have cleaned and rebuilt the carb. replaced all the fuel line, made sure the fuel filter is not bunged up and even replaced the carb with no avail. I’m no genius in this field, but it’s something I’m trying to get better at, so any help whatsoever is greatly appreciated.
-Henry

Fuel tank vent breather valve thing? Does it have one? Check that.