Carb Problem

[Woodplayer]

Hi Guys

I've been lurking on this "great" forum for a few months and hadn't posted yet since I was able to find the answers to all my questions. (except one).
(BTW I purchased my latest saw Dolmar 5100s due to the feedback on this forum. Man, after 8 tanks of gas that thing rocks!!):rockn:


On to my problem. I have a 15 yr old McCulloch PM325 (small 35 cc saw) which has had intermittent problems for the past few years.
Seems to have trouble idling, and after shutting off it usually won't start for 1/2hr.(flooded) Back in the summer, I assumed it was
the fuel tank pressure problem, since the fuel tank is mounted overtop of the engine and I did notice pressure when removing the fuel cap.
However I used it again last week (below freezing) and even before it was warmed up much the idling problem was there and it wouldn't start,
immediately after shut down. (no pressure in the tank) I have even disconnected and plugged the fuel lines to/from the primer bulb.
The saw starts and runs great in partial and WOT (high speed screw about 1 1/4, low speed 1 1/8)

It appears that the carb runs very rich on idle. I've noticed fuel coming out in the venturi from what appears to be the main nozzle during idle
as well when the saw is shut off. On one occasion by continually blurping the throttle slightly at a fast idle, I adjusted the low speed
screw all the way in (lightly seated) and it still ran rich. Only after turning the high speed screw in about 3/4 turn, did the fuel appear to stop
flowing from the main nozzle (fast idle chain slightly moving)

I took the zama carb apart, blew out all the passages, nozzles, inlets etc. (which looked very clean) and even tried to very
slighty decrease the metering lever height from it's recommended height of 0 to -0.012" from the carb body. This didn't make any
difference.

I figured perhaps the metering diaphragm or pump diaphragm were too stiff (Seemed alright to me), so I picked up a carb kit.
Both these diaphragms as well as gaskets, metering lever, valve inlet needle, pin, metering lever spring, strainer were carefully replaced
but the saw still has the exact same problem (flooding, excessive fuel at idle)

The only thing that I didn't remove and inspect was under the welch plug. My understanding is that all that could really go wrong in there
is the check valve for the idle chamber or main nozzle could be malfunctioning, but those check valves allow fuel to flow out into
the venturi but don't allow air back into the carb. My problem appears to be related to the main nozzle during idle allowing fuel to flow
into the venturi. Is not the fuel flow to from the main nozzle only during low pressure (vacuum), not during idle or when the engine is off? :bang:

Hopefully what I have posted is understandable, if anyone has any thoughts or suggestions I would REALLY appreciate them!!
I may try removing the welch plug and hope I don't ruin anything underneath.

Thanks Guys!!

 

[Four Paws]

You said you blew out your carb...hope you used low pressure air, or better yet, carb cleaner to clean the passages, etc.

It sounds to me like your impulse line is cracked, split, or unhooked. I have picked up several saws that 'needed crank seals' and siply replaced, or reattached the impulse hose and bingo - idled, cut and started like a dream!

If not the impulse hose, You could have crank seals that have started leaking. You could also spend $5 and rebuild your carburetor - when was the last time you put a kit in it?

Josh

 

[PES+]

The problem is where you didn't go

I may be mistaken but I am almost positive that there is a check valve under that plug the purpose of which is to keep the fuel from dumping from the metering chamber into the throat. It works on vacuum and if it sticks open it dies exactly as you describe.

 

[Woodplayer]

Hi Four Paws


I used a piece of flexible tubing (fuel line) and sealed it against the various passages etc. and blew by mouth to clean.
I may try a carb cleaner but when looking at things with a magnifying glass everything appeared to be OK

I just put the carb rebuild kit in a week ago which made no difference.

The impulse from the crank case doesn't use an impulse line, it seals directly with a gasket on the carb flange to the crankcase.
I would think that if the impulse was weak, it would run lean, but in my case it is rich at idle but I will double check the gasket seal.

Thanks for the suggestions!



Hi Pest

My carb kit did come with a welsh plug, I just have to figure out how to remove the old one without damaging anything else.
I thought perhaps the check valve kept air from entering the metering chamber, but perhaps it also works on vacuum while delivering fuel
from the metering chamber to the throat.
Any suggestions on how to properly remove the welsh plug?

Thanks for your help!

 

[PES+]

I like to thin the plug with a drill bit first but you have to be careful not to drill all the way through the plug. Use a very sharp hard awl or prick punch and carefully tap the point through the plug, then use the point to pry out the old plug. Do not push down through the plug and use the inside of the chamber as a fulcrum. Go slow and easy and it is not that difficult. To install the new plug set the plug in the groove for it and expand the plug by "setting" the plug with a hammer and pin punch in the center of the plug

 

[AZLOGGER]

Pest you are very correct on removal of the welch plug, only thing I do different is to put a little Red Loctite around the step groove the welch plug fits into before I seat it in place, I feel this gives it a better seal. This problem woodplayer is having could also be a leaking welch plug, I have had a couple of saws in the last couple of months that would not adjust out correctly even with new carb kits installed and I found the welch plugs were leaking.
Woodplayer I hope this info helps you out!

 

[Woodplayer]

Well I was able to remove the welch plug using a finishing nail I sharpened to a nice point.
The idle speed check valve underneath looked good, everything was very clean.

Perhaps it was the welch plug itself that was leaking, but that wouldn't explain why it appeared that the fuel was coming out the main nozzle, not the idle circuit holes.
I reinstalled the welch plug sealing it with some locktite.

I guess I will find out tomorrow if it's cured, if not it must be the main nozzle and check valve assembly which appears to be pressed in.

I may take a photo tomorrow if it seems to be a problem with the main nozzle and check valve assembly, since I'm not sure how to press it out?

Thanks again for your help guys!

 

[PES+]

http://www.zamacarb.com/pdfs/TechGuide_2007.pdf

This site will explain how to check and replace your main nozzle check valve

 

[Woodplayer]

Well I gave her a try today after replacing the welch plug, same problem, looks like fuel is coming out the main nozzle during idle.

Here are a few photos from inside the metering chamber as well as the venturi throat that show the main nozzle. (red circle)

Looks like it is pressed in, I assume it would have to be pressed through so it drops out inside the throat, since there doesn't appear to be any way to push from inside the throat.

Thanks PEST for the link to the zama documents, lots of useful information!

I'll see if I can rig something up to use in the vise or a clamp to press out, other wise I may have to take it in to a small engine place and see if they can remove it.

Any idea how tight the friction fit is on these main nozles or any other tricks to pressing it out?

 

[PES+]

Do you have the replacement check valve?

 

[Woodplayer]

No not yet, but I'm thinking of waiting until I get a new nozzle and check valve before attempting to remove the old one....just in case I screw up.

 

[PES+]

Now that I see the nozzle type, once that type clogs they are history. There is no way to clean them reliably. When you get the new nozzle you can do it like I do that type I drill a hole in the nozzle and use a mini dent puller to pull it out then just press in the new one