carburetor help on a fd620

[Bazooo guy]

I thought we were at replacing the missing spring and link rod that looked like a coat hanger.
And what happened to the fuel pump test?

Can we hold the governor shaft or disconnect it and see if it keeps surging?

[SixGunNick]

Hey guys believe it not I think it is fixed , I finally got to mess with it last night and it was the link next to the carb I had to bend it a couple different times and readjust the idle and air and there is no surging it must of had way too much play and I didn't know enough to to that had to be perfect.I am takin it out for a ride this weekend so hopefully it holds up.Thankyou everyone for helping me figure this out
Nick

[JohnF]

I thought we were at replacing the missing spring and link rod that looked like a coat hanger.
And what happened to the fuel pump test?

Can we hold the governor shaft or disconnect it and see if it keeps surging?

Yeah, you can do that but it is harder than you might think. That governor is very persistent. It would be easier if you disconnected the throttle link and manipulated that.

And it is extremely probable that the surge would stop because conditions to cause a surge sans governor are pretty rare involving coupling of the air/fuel inertia and combustion (rpm). (assuming everything else is working, like the ignition system - which, of course, may be where you were headed.)

... and it was the link next to the carb I had to bend it a couple different times and readjust the idle and air and there is no surging ...

Cool, but you don't get off that easy Now we have to figure out why.
You didn't happen to take a picture of the rod before and after?

Knowing the geometry of the control tabs I can't imagine rod shape causing anything except perhaps hanging up on something and I'll group that with "slop" which is coming up.

Slop would act like that old worn out steering on Grandpa's pickup truck. When you drive you feel like you are herding the truck down the road. In this case the governor would be hunting the RPM.

If we assume this and also assume that the slop isn't too much (because too much would be obviously annoying I think) then we ought to see if we can distinguish this kind of surge.

Assume for all cases that the twist grip is held at full throttle.

Fuel delivery surge would be caused by fuel supply not meeting demand. I would expect this surge to happen at lower RPM - but the closer the fuel delivery gets to being able to meet the demand the longer the surge duration and higher the RPM. Also sporadic as this would likely not be dynamically stable. (Actually, it would probably sound like a surge in a surge as there are two functions competing here.)

Mixture surge due lean condition would be dynamically stable - almost a perfectly rhythmic smooth sound that crosses the RPM half way mark. Can't exactly predict the pulse width - probably like 2 to 4 seconds.

Slop - I would expect this one to be fairly high frequency and definitely in the high RPM range. Probably a stable surge if the slop has no stickiness to it.

Remember, we are helping everyone who reads this thread with a surge problem for as long as this thread lives.

So Nick, did any of that sound like your surge?

Did you replace the spring too? That could help in this situation - it would act to bias the throttle decision (always pull the truck left...) which doesn't remove the slop but does remove the hunting. , maybe that's the real reason for that spring. It's still annoying when removing the carburetor.

And I'll be honest here, You said it didn't have power. I have a hard time imagining a condition where slop would have a dramatic effect on power unless the slop was so much that you could actually see the governor arm move while the throttle plate did not.

Was it <warmer/lower barometric pressure/higher humidity/combination> yesterday?

When we know that works we can talk about the next two adjustments. (You'll need a tachometer for setting the high idle.)

[Bazooo guy]

I would take it for a drive and see how it runs.
When you get back check the plugs and see if they look fouled.

If the rod was bent and the wrong length the revs would be limited to a lower rpm and the governor would fight with any throttle advance.

[JohnF]

:
If the rod was bent and the wrong length the revs would be limited to a lower rpm and the governor would fight with any throttle advance.

I'll have to think about this. I might even have to make me a short rod to see it. (down side to being a scientist )

The governors job is to pull against the throttle cable spring to limit RPM. It doesn't advance RPM.
I think a too short link rod would show as a reduced high idle RPM. Basically, RPM wouldn't get high enough for the governor to kick in.

[SixGunNick]

Not sure but I took it down the driveway a few times and back and it seems ok the surging came when I would throttle up at all so bad that I would have no power to turn at all because it would cut out so much I could barely turn (loss of power) and I couldn't really enjoy driving it at all now it seems to throttle up fine and I can also hold it at high rpm and it runs steady I can also turn with no hesitation
I will also replace the spring as soon as I can find one

[Bazooo guy]

In my eyes the governor link pushes back against the throttle when the revs go up and the spring pulls the throttle back to idle but I'm not looking at the carb and don't have one.
Remember the two stroke lawnboy mowers that would surge all the time with no throttle control.
Most of the problems were with the governor adjustment,link,or spring

[JohnF]

In my eyes the governor link pushes back against the throttle when the revs go up and the spring pulls the throttle back to idle but I'm not looking at the carb and don't have one.

This engine has a centrifugal governor that operates a shaft coming up out of the engine. Attached to this shaft is an arm - kinda long, maybe 8 inches - that has, on the other end, a direct link to the throttle valve in the carburetor. The throttle position is set at the point where the (governor spring pressure + governor shaft pressure) = (throttle spring pressure). In the low RPM range governor shaft pressure doesn't play. Pretty much all engine RPM governors work in this sort of proportional control configuration. Wonky springs that don't have a constant rate can wreak havoc with this balance. Engines with a really wide RPM range can be super sensitive to throttle position - enough to exacerbate every condition.

So, throttle spring pulls to open the throttle and the governor acts to close it. For an engine with a set RPM the throttle spring pressure is set and for these machines the throttle spring pressure is controlled by the twist grip.

[Bazooo guy]

So, throttle spring pulls to open the throttle

Did they set it up that way in case the spring fails? And if it gets weak or stretches the throttle would be sluggish or not open.
My throttle return spring pulls the throttle closed and the cable connects to a brake type lever on the right stick.

[JohnF]

Did they set it up that way in case the spring fails?

It's a spring between you and the throttle valve so that the governor has precedence over you. I have noticed that the governor spring is a stouter wire than the throttle spring. This could be for the purpose you mention. In fact it probably biases most every spring failure towards lesser RPM. Hey, neat! I purchased a tachometer and found that at WOT it ran at 3400RPM and the means to adjust this was already at the stop. So I had to embark on tweeking the governor arm on its shaft - an insanely sensitive adjustment - and it took me a couple hours to get it done. I probably should have just ordered new springs. It would have been, to my best estimate, a zillion times easier if it worked.

And if it gets weak or stretches the throttle would be sluggish or not open.

Yep. But let's say both the throttle and the governor spring are weak. Very weak, like short bits of rubber band. In this case I would expect surging, probably wild and dramatic, because the balance of spring pressure acts to attenuate (like a shock absorber) the governor arm. This would be a sort of extreme in the wonky spring scenario and the surging shape would be anyone's guess.

My throttle return spring pulls the throttle closed and the cable connects to a brake type lever on the right stick.

I forgot about the throttle return spring. On the conquest it has no part in throttle valve positioning. It's really just a part of the twist grip and cable - the driver input piece.
If the throttle return spring is a part of the valve positioning, then it adds to, and maybe even is, the governor spring in the function I gave above; assuming there is a governor.