FDC bypass info (yes, again)

Trying to bypass the FDC due to lots of fuel in the oil so hopefully this will fix the problem. Please check my work here, I don't want to tear up anything. I read all the FDC posts and just want to verify that I did it right (or wrong).
1. I installed the fuel line from the final filter directly to the hydraulic head inlet.
2. I plugged the FDC fuel inlet and outlet.
So what do I do with the fuel return line that comes out of the overflow valve? Plug it?
Thanks

Yes, plug it....and make sure to turn down the fuel delivery some...

O.K. from what I understand here I have to turn the fuel adjustment nut counterclockwise to reduce the fuel flow. How much do I turn it?
Thanks again

Hmm...perhaps 2 full turns, the screw should move in about 1/8 of an inch.

I just did mine due to fuel dilution. I removed the T from the fuel return and used a 90 elbow vice plugging. On my FDC I removed it from the pump and turned stop plate adjusting screw in .100" to reduce fuel delivery (using two 1/2" wrenches, one may need grinding to fit, loosen locknut and turn adjustment nut CCW to reduce fuel) . After this I found it was difficult to start (need to hold the accelerator to the floor while cranking), but there is an increase in power during normal operation.

Yes, it does get less fuel at start-up, since the FDC adjusts when it's in the loop. When bypassed you start out with fixed fuel delivery.
I had a potentiometer installed on one FDC and monitored the movement of the piston and at start-up it's "wide open" and gradually adjusts according to the viscosity. When the fuel warms up it adjusts and delivers more fuel, since the viscosity becomes lower. When working (not leaking), the FDC is a good thing IMHO, patented by Continental Aviation in the early 1960's.
Discussed this in the "Biodiesel" article in the MVM 4-5 years ago.

Thanks for your help guys. The good news is I got the FDC bypass and oil change completed and took it for a test drive. I ran the fuel adjustment in about .100 which appears to be about right as I have a little more power but not much. It still starts easily without having to push on the accelerator and the idle speed is the same.
The bad news is I drained out about 7.5 gals. of oil. I think I put about 400 miles on the truck since I first noticed the low oil pressure so I hope I didn't cause too much damage.
Hopefully the problem was with the FDC but I'll definitely be checking the oil level every time I go for a drive.

Wouldn't it also be wise to turn down the droop screw setting? The droop screw controls fuel delivery from 0 to 1,600 RPM and the main fuel delivery controls delivery at 1,600 to red line.

When I started turning down the fuel on my LDS engine I was only turning down the main delivery setting. It wasn't until I started turning down the droop screw that I started noticing reduced smoke.

Glad I read this, now I know why I have to give some throttle to get mine running. It runs great, but just stumbles at around 500 rpm 3 or 4 seconds before coming to life.

My FDC was bypassed by Uncle Sam & they left the plastic line on there & looped it back into the T. I think I'll remove the fittings & install plugs. Need to look at it a little closer & see what they did, seems like a potential failure having that plastic looped so tight.

O.K. where might the droop screw be and how much would I turn that one down? I don't have all the manuals yet.
As far as smoking goes I don't get any at idle, just a little when I step on it.
Believe me, all the help I get here is GREATLY appreciated.

The droop-screw is inside the housing, under the FDC. To adjust it, you need to remove a plug(pipe-plug) in the back of the housing(it faces the firewall). In that hole, at an angle, with a ball-end allen wrench, you adjust the droop-screw. I would suggest starting your adjustments with 1/8 turns.

I just searched "droop screw", lots of results, must be a picture in there somewhere. I am using a borrowed computer, I don't have my pictures or camera available right now.

The TM for the pump is " TM 9-2910-226-34 "
It is here under the resource's tab, all the info you need is in that TM

The recalibration suggested above (by yours truly) simply compensates for the bypassing of the FDC. Correct me if I'm wrong, but the droop screw adjustment should not have anything to do with that and is a separate procedure.

Bypassing the FDC, does not require an adjustment of the Droop-screw.

If you don't adjust the fuel, will the motor be over-fueled to a dangerous level?

Yes, unless you have a pyrometer to monitor EGT....

You guys may very well be right about the droop screw not needing to be adjusted when bypassing the fuel density compensator (FDC).

The reason I brought it up is because when I received my LDT engine the FDC had been bypassed at the depot. Judging from the high exhaust gas temperatures (EGTs) and heavy smoke I was seing I felt the depot had failed to turn down/recalibrate the pump after they bypassed the FDC.

After turning down the main fuel setting twice (one flat each time) and not seing any appreciable difference in reduced power, EGTs or smoke I turned down the droop screw 1/8 of a turn. When I turned down the droop screw I finally saw a difference although I'm still considering turning down the settings ever more.

It is my belief that the droop screw controls fuel delivery from idle to 1,600 RPM and the main fuel setting controls fuel delivery from 1,600 to red-line. I figured the low end (droop screw) delivery was too rich and therefore causing excess smoke and early temperature rises.

I know just enough about this to be dangerous and I welcome other logic. In other words, I reserve the right to to declare that I am wrong.

I should also mention that the manifold pressure rolling load tests that I took (as per TM 9-2910-226-34) indicated a need to turn down the droop setting. Whether this was because of the FDC being bypassed I do not kow.

Where would be all be without the Steel Soldiers website. This is the one venue where we can get together and sort out topics like this. Thanks for your input.

My truck had hard starting problems until I figured out the fuel pump was not pumping. It was running but not pumping. The pump was very loud and was not a normal sound for an in-tank mounted pump.

SoylentGreen said:

Trying to bypass the FDC due to lots of fuel in the oil so hopefully this will fix the problem. Please check my work here, I don't want to tear up anything. I read all the FDC posts and just want to verify that I did it right (or wrong).
1. I installed the fuel line from the final filter directly to the hydraulic head inlet.
2. I plugged the FDC fuel inlet and outlet.
So what do I do with the fuel return line that comes out of the overflow valve? Plug it?
Thanks

Click to expand...

It looks like you are getting a lot of 1st hand info. I have been considering it and here is something I got off of this site some time ago, maybe it will add something:

Resurrecting this old thread for clarification...I see members suggest turning down main fuel adjustment when bypassing the fdc and leaving the droop screw alone. The document posted above suggests turning UP the main fuel and decreasing the fuel via the droop.

So which is it? I have a truck with a bypassed FDC and it smokes big time. I lowered the delivery via the main fuel adjustment at least 2 full turns with no decrease in smoke at idle.

I am in agreement that the droop needs to be adjusted, but what boggles me is how the document says to increase the fuel delivery by the main adjustment first.

At this point I probably need to get a boost gauge and make the adjustments by the book, but I think we should clarify the procedure required. If the document is right, then what most members have suggested is wrong.