Maybe someone can help me figure this out.

I'm only getting 18mpg on B100. 75% of my trips are 45-50mph with no stop lights. Even taking off 10-20% for bio, I think I should be in the low 20's

Had a bit of smoke on startup, so I ordered a new set of Monark injectors from Mr Watts. The first time I started it after installing them, the engine was purring like a kitten. It was as smooth as our 606 engine.

The next day, start it up, idle is a little rough. Day after that, the idle is as rough as before. I though there was a clog in the system and maybe it's sucking air. Changed the primary fuel filter (it was clogged). Seconday filter was changed a couple months ago. To rule out the tank strainer, i've swapped the return and supply lines. Still has a rough idle.

I'm thinking the poor economy and rough idle may be the same issue. I can't figure out why it ran so smooth the first time I started it.

I'm going to run the tank down and remove the strainer and also change the secondary filter again.

Any ideas?

You did not say if the was home brewed B100 or Commercial B100. If home brewed it may need better filtration before it goes into the tank.
Also I would not rule out the fact that you Secondary (spin on) Fuel Filter is clogged also. I would change the filter to be sure and put the old filter in a Zip Lock bag. If it turns out the Old filter was OK you can use it again for the next filter change.
There was a couple Threads concerning a good IP timing for B100; this may be something that might improve the mileage.
Anything that helps your car run the way it is supposed to like, Valves adjusted properly, proper IP timing, proper tire inflation, not having sticky Brake calipers that cause you Brake Pads to drag on the Rotor and so on; will improve you milage.

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84 300D, 82 Volvo 244Gl Diesel

You could try bottle feeding it with D2 to see if the idle improves. Might save you the hassle of pulling the tank strainer.

1984 300d turbo 147k
check for black fungus in the clear prefilter, it will plug the nozzles, treat the tank with biobor 1 oz per tank may have to remove fuel tank to radiator shop for cleaning

Humm the primary filter was clogged with a black gunk, but I thought it was junk that was dislodged when I started running biodiesel.

Odd thing about the primary filter. It was a solid white plastic not clear like i'm used to seeing. I wonder if it was the original filter?

So it's possible for the fungus to get past both filters and get into the injectors? That would explain why it started so smooth the first time and got worse after that.

This is homebrew B100, but I filter to 1 micron nominal going into the fueling drum, then 2 micron absolute going out of the drum.

I've read lots of people saying that Bio will wash out the gunk in your stock fuel tank. It may be your case. Also, black gunk could be Algae in your tank. It's possible.

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Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.

How well is the water and other potentially dissolved or emulsified crud washed out? My guess is unhappy injectors. Might pull them and check pop & spray. A clogged filter would cause low power, but AFAIK, no low mileage.

Does the tranny shift when it should?
Compression good?
Valve lash good?

If you have Fungas in your tank there is a good chance the Secondary filter is clogged.

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84 300D, 82 Volvo 244Gl Diesel

how long have you been running bio?

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Well I changed out the seconday filter yesterday and cut it open. It had some black crud in it as well. Started the engine and it was purring like a kitten. Now this morning, back to rough again. Primary filter is clean.

I'm pretty sure my bio is fine. I've sent it off for ASTM testing and everything was within spec. I'm also running it in 4 other diesels with no problems

I've bought the car about 9 months ago. 115k miles. It's been running a blend or b100 since I got it. Put around 4000 miles on it. Starts up with no problems in the cold, so i'm guessing the compression is ok. valves adjusted 3k miles ago. It's been a while since i've had a 123, but the transmission seems to shift ok. It shifts a bit hard when it's cold, but smooths out when everything warms up. New motor and tansmission mounts about a month ago. Brand new compete Monark injector are about a week old

I still don't understand why it starts smooth every time I crack open the fuel system, then gets worse.

I have some biobor on order and i'm going to replace the primer unit.

question: does the return line stop at the bottom of the tank or is there a tube inside the tank. I'm trying to run the tank down to remove the screen, but want to make sure i'm not going to run out if the return line extends up into the tank.

Did you have the injectors pop tested? Swap the in and out lines around the prefilter to eliminate the screen. Where are you located?

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OBK #42

It may be that after you open the fuel system pumping the hand primer primes the system well (and all the air is out of the system); engine runs good but the prime is only temporary.
Thinking about this also reminds me that the Hand Primer it self can develop an air leak; more so if you have the older type that you have to unscrew before you pump and screw back in when you are done (to seal it).

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84 300D, 82 Volvo 244Gl Diesel

I just installed a set of Monark nozzles into my '81 300D with 332,000 on the clock. This car would rock the daylights out of you at traffic lights! I changed the filters, cleaned the tank strainer, and adjusted the valves. All of these things improved the performance BUT the Monark nozzles have solved the problem! Smooth as "butta". No I did not pop test or balance them. I will be building a "grease gun" pop tester next week but for now she's running great!

Btw, the car never did smoke. It just ran rough after warm up.

Kevin