Hello, I'm new to this forum and look forward to contributing what I can. I have a question about my car, a '79 244 with the B21F (K-Jet). It's got about 260,XXX miles on it.

Since I got the car a couple years ago, it's always seemed to prefer 93 octane gas. With 87 octane, it is a bit more sluggish, and, when under a load, makes a sound that can only be described as glass marbles rubbing against each other. I suspect there's something off with the timing. Could it be related to the distributor? Or maybe the camshaft is worn? Everyone else I know with a 240 runs regular 87 octane, as I think the normally aspirated engines were designed to run on it. Any thoughts?

I've read all the posts and here's what I what I think after reading your last post in this thread--the one regarding knowing the gas tank has rust (an important factor not mentioned in the opening post)---by all means you need to check the timing as the noise you describe is exactly what severe ping sounds like. Either borrow a timing light or bring the car to a quality shop and ask for that one task to be performed. If it's within spec--fine (except now you know you must look elsewhere). If not, it must be adjusted. You'll need a 13mm socket, 12" extension and ratchet to loosen the locking bolt for the distributor. With 260k don't be surprised if the distributor doesn't want to turn--they do tend to seize up in the block. Freeing it up can be a project in itself. It wouldn't surprise me if someone cranked up (advanced) the timing in the past to to skirt or mask an underlying problem - rust in the tank leading to clogged filter and injectors. A lean mixture could be contributing to the sluggishness and ping. I'd remove the filter and pour the contents into a clear container. If there's any sign of particles in the gas the next thing I'd do is remove the injectors and test them--they may have a terrible and reduced spray pattern. Professional cleaning/rebuilding or replacement would be called for. If the tank rust is causing or contributing to your problem that aspect has to be addressed as well (either treat the tank with POR15 or replace it. My main daily driver is a 1980 245 w/M46 bought at 123k--now at 258K and have always used regular (87) except for when towing with the car. In most areas in the U.S.A. the gas now has alcohol added which has good anti-knock characteristics. -- Dave

Also, I don't know if this is important or not, but there is no back pressure in the exhaust; it's free-flowing. Both mufflers and the cat had rusted out, so we did it cheap and ran straight pipe. I don't know how lack of back pressure would cause knocking, though.

I'm still a bit confused as to how clogged injectors or a clogged fuel filter would cause detonation and produce the knocking sound. Can detonation be caused by an overly lean mixture? I experienced a lean mixture before when my tank pump bit the dust; the main pump was buzzing and the car would stammer and buck under a load, but no knocking that I was aware of. What are other common causes of detonation with K-Jet or other CIS systems? Also, the knocking sound seems to be most prominent in higher gears, i.e. when it's in 4th and I increase throttle. Thanks again for the help.

-Aaron

Thanks for the detailed response. Yes, I noticed the tank rust when I replaced my in-tank fuel pump a couple years ago. I also replaced a faulty injector around the same time (it was causing a misfire). I didn't do anything to the other 3 injectors. I don't own a timing light so I'll have to borrow one or take it to a mechanic. But given the rust in the tank, and the age of the car, I have a feeling it may be the injectors or the filter. Can you provide a little more detail about POR15? I'll probably have the car off the road for a while (I'm trying to buy an automatic 740 wagon) so I can address the fuel tank. Would it cost less to have it professionally cleaned than to replace it outright? Thanks again- Aaron

I've always run 89 octane in my 1980 245DL B21F...I can remember riding in it as a child when it was fairly new and hearing the engine pinging going up hills before my parents started putting 89 octane in it. I think while newer 240s do fine with 87 octane, even a well-tuned B21F from late 70s/early 80s will often do better with 89.

Zack
1980 245DL M46 296k
1988 745T+ M46 242k

I had a 79 245A. Even with everything to spec it seemed to have a little extra pep with mid grade 89. 91 didn't make that much difference but with 87 it was lacking that little bit of responsiveness. So I guess once you have verified that everything is up to spec, do a seat of the pants test and decide which you prefer. I don't think premium 91 is necessary to do any "cleaning out". Buy a good grade of top tier fuel and add some additional tectron. If you want to buy the no-name fuel, throw a little techtron into each tankful, you'll end up paying about the same as you would for top tier fuel in the first place. Even better if you can find an excuse, go for a sustained long distance run with a tank of good fuel with techtron added. No, I don't own Standard Oil stock, I have no financial interest. The product seems to have smoothed out injector and related problems for me in the past which is why I continue to use it.
--
1988 245A. Past: 1979 245A; 1987 244 M47; 1971 164E

welcome to the site.

You wrote your question well. The other post previous to this covers pretty much the answer.

I would like to add that you may be able to move your timing back a couple degrees. You are allowed 2 degrees plus or minus of the spec of 12 degrees btc.

As far as pep it is hard do judge if the engine could be a little tired or not. Do a compression check to see where you stand over all.

I have a '78 with over 290k and it still does fairly well but it's not a 2.3 but it getas good mileage as my '86 or my '91. The 2.1 helps.

If you have a o-2 sensor you may want to check the output voltage after its warmed up. It could be running the engine leaner than needed.

How are the plugs burning? Gray, sooty black or oily carbon like?

Again welcome aboard. Your on the best site for ease of use.

Get back to us. We all like to know if we help or hurt. :>)

Phil

Thanks for the response. Plugs seem fine. How is ignition timing adjusted? I know it involves the distributor, but I'm not sure how it gets adjusted or how much movement would be required.

If necessary, it's adjusted by rotating the distributor one way or the other.
The "if necessary" part requires a "Timing Light".

I'd recommend you do some browsing at www.k-jet.org and/or googling some of these generic topics.

Here's a start for you from that excellent site.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

I can only guess that the sound of 'glass marbles' is knock/pinging with the low octane gas.
The '79 K-Jet should run fine on regular, but when the car was made, it was better grades of regular available, like 89 octane, so use the grade that eliminates the knock. The '79 and later B21F has higher compression than the earlier models, so it is a bit more sensitive to the grades of gas.

With all other things in top order, one thing that could contribute to knock is carbon build-up in the combustion chamber and/or water in the gas tank from simply being old. Run premium gas with a pint of STP or Techron and take for a romp on the highway and burn the carbon off.
K-Jet is sensitive to the quality of gas and aged gas will do all sorts of bad things to the way the car runs. Run the Techron at least 4 times a year.

Thanks for the reply. It could be related to the gas tank; I know it's got rust on the inside. Perhaps water as well? I suppose it wouldn't hurt to run some high octane fuel with a cleaner.