McCulloch 380 flatback carb start problems

[ATM]

Bought long time rest saw, clean it, get a good spark but then hit to fuel problems.
It fired when spray fuel externally to carburetor intake hole but stop naturally when feed fuel is burned out.
During run it react to throttle. Carb have injector pump but seems that it doesn't have any help for firing either staying run.
I open the carb and found fuel in upper chamber where main needle locate. I been check fuel line and filter + hose and when vacuum the hose the fuel comes from the tank easily. Diaphragms seems OK, no holes or cracks, maybe little "dry".
H and L needles adjusting won't give solution for the problem.
Seems that fuel can't rise up to needle from upper chamber. I haven't opened welch plugs, do not have any spare, so could the fault be behind welch plugs?

 

[Marine-piper]

If you spray carb cleaner into the H&L openings make sure you can see strong spray coming from the jet and secondary. Unless the carb was really nasty you shouldn't have to remove the welch plug.

https://www.sugarcreeksupply.net/co...ulloch-flatback-carburetor-kit-1-40-1-50-1-60

https://www.sugarcreeksupply.net/co...ducts/mcculloch-mac-9-flatback-carburetor-kit
Homelite collector in training [emoji879]

 

[ATM]

I opened the carburator again and noticed few things I do not have enough information.
L-needle line. I assume shape of the needle it's for fuel, not for air. Try to figure how the fuel travels from that to the engine intake.
In fuel chamber there is tiny hole beside the welch plugs. If I put some compressed air to the needle screw hole the air comes out from that tiny hole in fuel chamber.
There is also 3 tiny holes between throttle valve and intake manifold side of the carb. one behind throttle valve which all I think is the fuel line for idle speed. I can't get air through from L needle screw hole there even I close the hole from fuel chamber.
Seems that under smaller welch plug is something that stuck this operation. Does anybody have a picture what is under those plugs?
Haven't looked the H-line yet. Seems I need to clear out the L-line first.
Carb manifold flange formed CV 588661

Some updates. Put the carb in ultrasonic cleaner for 15 min and then put all back with home made upper diaphragm. After several start with external fuel inject I get it run and take revs with feeding all the time primer pump. H and L needles open appx. 3-4 turn both. 10 minutes later found balance where it runs in quite high ilde revs without primer.L open 2,5 turn and H open 3 turn.
Seems that now it get fuel maybe too much cause plug is wet - not even try to fire. After dry the spark is there.

Is there any basic settings for H and L needle where to start and is there any matter if the air filter and cover is removed during test.
This takes effort from man

 

[LiteItUp]

ATM, I also have a 380. My Hi and Lo settings are both one and a half turns out. Saw idles and runs good with these settings once started. It starts hard, I believe due to primer problems.

 

[ATM]

Hi,
Since my last update a lot has happened. I found leaking fuel hose and after replacing it things been gone much better.
Noticed that user's manual says fuel /oil mix 6,25% !! and I been test run it with 3,5% with synthetic 2T oil.
What mix you been used, 6,25 % sound quite thick 2T mix today?
Anyway I have now appx. 6% mix and feels that engine run smoother, sounds softer and it's easier to pull start. No hard back kicks.
L needle is 2 and H 1,25 and air cleaner installed.
Still needs straight extra fuel for first over night start but after that will start without straights, only primer a several pumps.
Idle speed is adjusted so that chain won't run but staying alive it needs still primer regularly period. Full throttle it runs without primer, maybe too rich but it's OK for me.
I am not sure will it die on ilde for fuel or lack of the fuel? Primer helps so I think it's lack of fuel but I need to adjust L needle 0,5-1 turn close and looks what it will effect for idle.

I think it needs a new pump diaphragm. existing can't give enough fuel pressure in low rpms.

Another question from chain. Does anyone have Oregon numbers for 20" saw blade chain?

 

[LiteItUp]

ATM, I normally run 2.5% mixture with a high quality 2 cycle oil. This is a 40 to 1 gas to oil ratio. When these saws were new the oil lubricating quality was not as good as now.

Regarding your idle problem; if the carbs low speed passages are clean and enough fuel is pumped into the carbs metering chamber I would check for a vacuum leak.

 

[FlyingDutchman]

I know the primers on these flatbacks are not serviceable. I cleaned an absolute mountain of debris from mine, it seems to pump fine on the bench when I put it back on the carb, it still does not pump to prime and I cannot start the saw. Does the primer shoot gas into the carb or just prime the bowl? Is it best to delete them? The o-rings are pretty flat, but it does not leak gasoline. The check ball is clean, the rubber seat in the check in the carb seems undamaged.

Sorry for the thread jack.

 

[cbfarmall]

I recall some sort of check ball under that large welch plug. Make sure that isn't stuck.

 

[Marine-piper]

Did you check the 2 links I sent you about flat back carb kits?

Homelite collector in training [emoji879]

 

[heimannm]

The primer should pull fuel from the tank and inject it directly into the bore of the carburetor, keep in mind that the primers are notorious for failing and are non-serviceable. Not a big problem as you can always prime it manually as you have discovered.

I find the L normally needs to be 1 to 1-1/2 turns open and the H are frequently around 1. Next time you are in the carburetor check the height of the metering lever. Follow the link in my signature to a thread devoted to the McCulloch/Walbor "Flat back" carburetor.

If you need welch plugs and are willing to be patient, I can send some to you. Start a conversation with me by clicking on the "Inbox" and "Start a conversation".

Mark

 

[ATM]

Thanks from replies. I need to check metering chamber needle adjust regarding Mark's instructions. When last time carb was out I adjust it to keep more fuel. Another check point is that should metering diaphragm have also 2nd seal above or below the metering diaphragm? At the moment I do not have any extra seal only the self made metering diaphragm from nitrile sheet rubber. Lack of 2nd seal can cause also some unwanted actions.
Needle valve o-rings I need to check and replace new those I haven't pay any attention.

If I understood right the rebuild carb kits are designed for kart use and they are not optimal for saw purpose.

Need to lean my fuel/oil mix from 6% back to 4% .

Primer o-rings are checked and replaced and it works maybe 50% efficiency so it's fine.

Just think that could I made new pump diaphragm, long winter time to try manufacturing it and test it's effect for run.

I am going to look one 1-53. Do those have same flatback or is it loaded with Tillotson.

 

[heimannm]

The metering diaphragm normally has a gasket as well, the gasket actually spaces the diaphragm a bit further from the metering lever and is an integral part of the metering diaphragm. If you don't have a gasket you should try making one from some material around 0.060" or 1.5 mm thickness and placing it against the carburetor body before installing the metering diaphragm and diaphragm plate (cover).

The aftermarket carburetor gaskets and diaphragms are in fact designed for the saw carburetors. Bob Johnson (sells a lot of New Old Stock or NOS McCulloch parts) had some kart diaphragms mixed with his saw parts and that is where I got the one shown in the McCulloch Carburetor thread.

The 1-53 most likely was equipped with a Walbro flat back carburetor from the factory but may have been converted by someone along the way.

Mark

 

[ATM]

Ok. I need to made and add the gasket above the metering diaphragm and made the tuning again, hopefully with better results.

I found interesting problem in chain. It doesn't fit well, some spot it's loosen and when rotating it comes rapidly over tight. I measured the pitch and get 22,5 mm ( sorry European metric units ). I don't know where it's from either the value in US standard. .404 chain pitch is 20,5 mm so I have wrong type chain on it!
Now comes the question: I have McCulloch's original 20" long guide bar ( no tip wheel ) how many drive link should I have in my chain? Now in wrong chain there is 59 links but It should be some more with correct .404 chain. For 24" guide bar links need to be 76 so could somebody know exact number, that helps a lot to order right long chain.

 

[heimannm]

I believe 24" was normally 77 drive links, I don't remember the count for the odd 19/20" bars they often supplied. Problem is there were a number of different size bars, all between 18 and 20" with different drive link counts. You will probably have to wrap a chain around the bar leaving 7 drive links open at the tail for the sprocket.

I suspect the 22.5mm chain is 7/16" pitch.

Mark