carburetor help on a fd620

[SixGunNick]

Hey guys I am having trouble getting my carb dialed in it seems to be running rich and it stalls out sometimes when I'm plowing snow or rev the motor up high I just had new seals and float needle put in also had it cleaned very well,I also have fresh gas in and new plugs .When I rev up the motor in nuetral and hold it steady at one spot it sounds like its hitting a rev limiter and chugging does anyone know what adjustments I could start with and is the adjustment that has the black plastic on it the idle adjustment?
Thanks ,Nick

[JohnF]

What is your elevation and did you happen to notice the main jet when you cleaned it?

The plastic capped screw is an air screw to adjust the mixture for the low idle circuit. You adjust your low idle RPM via a screw stop at the throttle linkage.

Oh yeah, did it run fine before the carb work and now it doesn't?

I've seen them surge when lean. For me it means I need the next larger jet (I change elevations a lot) but it can run lean if there is an air leak, like the manifold gasket is broken.

It can also surge when rich. This you could probably smell but it will also smoke a bit and cough soot. This could be fuel leakage in the carb; although in this carb I don't know where, unless the float valve doesn't work. Could also be a stuck/broken choke.

Another edit

It also happened to me when I think my fuel pump was going out. I think it probably surges when the engine fuel consumption exceeds the pump capacity.

[SixGunNick]

Yes the exhaust is coughing soot the choke seems ok cuz when I pull it out it will kill the motor I had the carb rebuilt and cleaned professionally and it did run great before I had the work done but I was getting gas in my fuel before that's y I had it looked at there is really nobody around here that services argo so I'm doin it myself

[SixGunNick]

And I did mess with the linkage screw and the black plastic screw so maybe I got them setup wrong I hope!

[JohnF]

Yes the exhaust is coughing soot

We'll assume rich then.

the choke seems ok cuz when I pull it out it will kill the motor

That tells us it closes, we need to peek in there and make sure it is opening all the way.

... but I was getting gas in my fuel before

gas in the oil, maybe??? There's a thread on here talking about that. That doesn't seem like a carburetor issue though.

so I'm doin it myself

No problem...

And I did mess with the linkage screw and the black plastic screw so maybe I got them setup wrong I hope!

So let's start working through this...

Don't go thinking I'm an expert - I'm just a guy that has his carburetor apart a bunch of time. So, with the caveat of "This is what I would do"...

I don't have a black plastic cap. I think it is to limit the screw travel. If it comes off I would take it off for this. Carefully screw it in until it seats - not tight, you can break things - just until you can tell that it seats. Then back it out 1 3/4 turns. Maybe 2. A little rich here is our goal.

Now, if it already idles high (RPM) I would leave that adjustment alone but I want it to run at idle somewhere around 900-1000 RPM so I may give it a twist to higher RPM.

Start it. Adjust the idle RPM down until you think it is a little higher than where you want it. I dunno, 900 RPM. You're probably just using your ear right now, that's okay and having the idle a little high helps to hear the changes, too high and you aren't in the idle circuit anymore. Let it run so that we do the rest when it is warmed up.

Be aware of hoody string, loose long sleeves, hair, any long stuff that can get caught in moving parts. Don't have them. You are going to futz around the engine while it running. You'll be close to the fan. Had an uncle help me once that forgot that and stuck his thumb into the fan - RRRAAAAAPPPP - scared him but the fan is spinning the direction that just scares ya and doesn't cut off the thumb.

Now very slowly turn the air screw in. We should have had the engine running lean so you should hear the RPM increase and smooth out and then eventually drop. You are searching for the point where the RPM peaks and the engine runs the smoothest so you might have to go in and out a bit. Make small changes (1/8 - 1/4 turn) and wait a bit for it to stabilize. Once you find that point - BAM, you're set. I guess put the cap back on.

Now adjust the RPM down to where you want it. Mine is about 780 (I have a tachometer) but this needs to be low enough that you can shift without too much grinding. Sometimes the lights blink a bit at idle.

That's your idle circuit. There are pilot jets involved in this but as far as I know they aren't ever changed. I suppose you could play around here but I change elevation probably more than most here and, doing the above, it idles just fine.

Let's see what that does for you. But if we need to go further...

The other adjustments you have are float level and main jet. Float level will affect how the engine acts when you go from idle to WOT - it is sort of a part of the accelerator pump function, and the main jet affects mixture of the main fuel delivery circuit.

I would like to know which jet you have and the elevation where you are running. The jet has a number stamped on it. It is behind the anti-afterfire solenoid - the only thing with an electrical wire. You should be able to see this without taking the carb apart, well, except for taking the solenoid off.

To remove the carb - unplug the fuel pump - start the engine - and let it run until it starves. (just to empty as much of the fuel as we can)
Two wing screws takes the cleaner and filter apart. 7 screws removes the top plate - there is also a breather tube at the back that pushes into a tube underneath, remember that when putting it back together. 2 12mm nuts hold the carb on. The anti-afterfire solenoid has a connector at the far end if it's lead (I hate that!) - it just presses in and pulls out. The throttle and choke linkage are wires (one will probably have a spring over it) on the left side. With you left middle and index finger hold the linkage up into the holes on the control arms - you want the linkage to go up and come out of these holes when you lift the carb out and not fall into and go further onto the control arms. Lift the carb out - jiggle and wiggle so that solenoid clears stuff. Go straight up so that you don't spill what little fuel is still in there on everything. If there was a spring on the throttle linkage then you'll stop at some point after the linkage wire frees to get the spring out with needle node pliers. After I get the carb out I usually go into the street (I have one) and turn the carb upside down to dump out what little fuel is left inside.

Now take the solenoid off (don't loose the copper gasket), grab a flashlight (and for me something to magnify ) and see if you can read the number on the jet.

Assembly will be the opposite - when we get there.

[SixGunNick]

Hey john thanks for all your help I think my carb is somewhat dialed in it does not seem to be running rich now starts up fine,idles great but when I throttle up it still has rev limiter effect happening as I can see it revs up then a shot a fuel shoot in from the top of the carb and the linkage starts bouncing,and then does it over and over again.I had seen on another post where it was said that maybe it could be the linkage springs are worn out and its letting it bounce like that,my fuel pump seems good and fuel filter is working properly

[Bazooo guy]

Buying a universal fuel pump from an automotive store???
Cars with fuel pumps are usually fuel injected and the pump puts out high pressure.
That's why they use a fuel regulator and return line to the tank. Without a regulator the fuel pressure will push the inlet needle open on any carb and flood the engine. Have I been misled all these years or do they have a pump that doesn't require a fuel regulator now?
Now I probably shouldn't be commenting on this thread but I can't believe what I'm reading.
It's very hard to diagnose a fuel problem from miles away and I'm sure everyone wants to help but this is getting ridiculous.
I don't want to offend anyone and by no means am I an expert at anything related to this post so maybe I will just shut up.
If you think the pump is bad get a guage and the specs on the pump and check the pressure before you buy a new one.
And if you had the carb off are you sure the springs and rods are in the right place?

[JohnF]

Buying a universal fuel pump from an automotive store??? ... Have I been misled all these years or do they have a pump that doesn't require a fuel regulator now? ...

Don't know if you've been misled but you can get a low pressure fuel pump from an auto parts store.

Here's an example.

[Bazooo guy]

I don't think the jet size will cause the problem you are having. i can tell you I have no experience with elevation tuning but the difference in jet size shouldn't cause it to surge as you describe. Adjusting the float level is very important IMO.
It's easy if you follow the manual and makes a big difference if not set properly. The thing that worries me is fuel in the oil.
My neighbors lawn tractor filled the crankcase with fuel and it wouldn't even turn over with the starter.because the inlet needle was stuck open. Prior to that I saw Him cutting his lawn with black smoke coming out the exhaust like a diesel engine. Have you looked at your plugs to see if both cylinders are rich? If only 1 plug is fouled you could have issues with valves or rings on the fouled cylinder. Fuel delivery is important but the engine needs to breath and have good compression. If both plugs are soaked with fuel it's probably the carb.

[Bazooo guy]

Yes the exhaust is coughing soot the choke seems ok

doesnt sound like its running lean.

It can also surge when rich. This you could probably smell but it will also smoke a bit and cough soot. This could be fuel leakage in the carb; although in this carb I don't know where, unless the float valve doesn't work. Could also be a stuck/broken choke.

Totally agree with that.
And now

The surging is a lean condition in the main circuit which is controlled by the main jet. If you are sure that jet was clean then order the 112. There is no other adjustment.

Yes install the 112 it's the standard jet for that engine Someone has changed it.
If your engine is fd620D the valve cover gaskets are here:
http://tewarehouse.com/7-02498