Adjusting Carb on 38 Terraplane

Unknown

After doing a valve job, we have been trying to adjust the single barrel carb on my 38 Terraplane 6 cyl. coupe using the vacuum method. We are using a K&N air cleaner and the carb was rebuilt about five years ago. There is an electric fuel pump, but I have a regulator on it to limit pressure on the carb float and needle valve. I believe the timing and distributor are set correctly. What we are concerned about is that max vacuum of 18 inches is achieved only after we back the idle mixture screw out approximately two (2) full turns. The owners manual states that best performance is normally achieved between 1/2 and one full turn.



Are we doing something wrong, or is it possible that the two turns out is correct for this motor's setup? Has anyone else encountered a similar situation?



I would appreciate anyone's ideas and comments on this question.

Thanks,

Paul Caldwell

Jon B

I'm not sure I can help, but I did attempt to adjust my '37 (2-barrel) carb with the vacuum method. My purpose was to see if I couldn't advance the timing just a bit, to save some gas.



First, I think you have to get the RPM's to a certain setting (can't recall, possibly 400 rpm?). Then turn distributor to get max. vacuum reading, after which you back it off maybe 1-1/2 inches. Someone told me this vacuum timing is a rather involved process, though. You apparently have to adjust the timing, then the idle screws, then go back and re-adjust the timing, and the idle screws once again until it's perfect.



That done, I got out on the highway and at the higher speeds, heard the engine knock! (Possibly the pre-vacuum advance engines knock at the higher speeds rather than the lower speeds, under stress, as is normal?). Had to retard the spark, it still knocked, retarded again... Bottom line is, I ended up retarding the engine right back to the UDC mark on the flywheel.



I would be curious if you had any better luck than I did, or did you too end up with the pointer right on UDC (which we could both have done to begin with, and saved our time!).

[Deleted User]

Best thing to do is set and test drive if engine runs fine on road you are ok if not adjust and try another road test. The manual is only good as a base guide now as the fuels you get today are very different from the fuels used when writting that manual.

hudsontech

38TplaneCoupe wrote:

After doing a valve job, we have been trying to adjust the single barrel carb on my 38 Terraplane 6 cyl. coupe using the vacuum method. We are using a K&N air cleaner and the carb was rebuilt about five years ago. There is an electric fuel pump, but I have a regulator on it to limit pressure on the carb float and needle valve. I believe the timing and distributor are set correctly. What we are concerned about is that max vacuum of 18 inches is achieved only after we back the idle mixture screw out approximately two (2) full turns. The owners manual states that best performance is normally achieved between 1/2 and one full turn.



Are we doing something wrong, or is it possible that the two turns out is correct for this motor's setup? Has anyone else encountered a similar situation?



I would appreciate anyone's ideas and comments on this question.

Thanks,

Paul Caldwell

Paul,



Drop me an email at HEThester_nec @yahoo.com (Drop the HET) - I have a 1934-1939 Carter carburetor sheet that tells how to adjust your carburetor.



Hudsonly,

Alex B

Uncle Josh

After rebuilding my WA1s on the Hornet, I had to back them out to 3 1/2 turns to get it to idle.

Unknown

A couple of people have asked me via e-mail about that very subject recently and I am going to respond here so that anyone wondering about things might get some benefit (I hope!)



I am not terribly familiar with the 38 Tplane. But I can offer some general pointers that are applicable to just about anything. I've used this procedure on my 29 Hudson and our 1998 Chevy S-10 with a 388 stroker. And everything in between.



Properly adjusting that carburetor depends a lot on your timing setting. Ideally, you want to set your timing curve so that you are running at maximum efficiency throughout the powerband. This means having both the proper inital and the proper final, or fully-advanced timing settings. The absolutely correct way to set ignition timing is at full advance, and if you have a vacuum advance unit, with it disconnected and plugged. The problem on a 38 Terraplane is that you probably don't have a specification for such. But if you want to experiment, you're going to need a timing light, and preferably one with adjustable advance. If you don't have one with adjustable advance, you'll need to get some timing tape or something and make some reference marks. If you've got a 6 volt system and no 6 volt light, so what? Sit a 12V battery on the ground beside the car and use it. Hook her up and run the engine up to the point where you see no more advance. At that point the strobe will be steady on a reading no matter if you increase throttle beyond that point. I wouldn't be surprised if it was around, say, 28 degrees, and 2500 RPMs. Note whatever that timing reading is. Obviously you want to get the maximum burn out of that high-test 87 octane fuel you're running. The dirty way to find out how far you can go is go out and lug the thing in high gear at about 10 mph uphill. When it starts to ping, you've got to back it off a little. When you do, throw the timing light back on it and note that final reading. Do this as many times as necessary to find how much advance you can put into her without causing problems. Don't worry, it's okay to ping a little during this process, just don't rattle the thing unnecessarily. That's going to be as far as you can go.



Now, hook up your vacuum guage and with it at its specified idle speed, adjust the mixture screw to achieve the highest vacuum reading. You'll find that as you turn the screw in, the vacuum will go down until the engine dies, and as you turn the screw out, the vacuum will increase to a point and then flatten out, so that more unscrewing doesn't result in more vacuum (hopefully!). You want to set it up right at this point-- where the screw is turned out just enough to achieve maximum vacuum, but not beyond that.



If you want to stop here, that's fine, and you'll have a good running car. But if you want to really make it sing, here goes.



Make sure you've got that total timing number you figured out earlier written down. Let's say for kicks that it's 28 degrees. On vehicles without a vacuum advance, you're going to get to this number by adding your initial timing setting, let's say 4 degrees, to whatever is being put in by your centrifugal advance (in this case, 24 degrees, 24+4=28.). I'm not familiar with your distributor, but I imagine you've got an upper plate, and below that a pair of advance weights and springs. The tension and travel of those little springs is what's going to regulate how much centrifugal advance is going in. Bring back the vacuum guage and at idle, advance the distributor until you achieve the highest vacuum reading, and then retard it about one pound. This is the initial timing setting your engine wants to run at-- and the good news is this will work on any engine, with any camshaft, and any modifications. This setting will provide you with the best economy and off-idle throttle response.

If you've got a low compression engine, such as my 29 Hudson, you're not going to be able to get the thing to ping on 87 octane no matter what you do. In that case, this idle-vacuum method is the only thing you can use; the drive-it-till-it-pings choice is out of the question. (also in this case, I'd also check your timing reading against a known reading-- such as for 29H we know that it should be around 4 flywheel teeth BTDC-- and make sure you're not too far off.) Now using your timing light, run it back up to full advance and compare that number to what you figured out earlier. If it's the same number (or very close, say within a degree or two), great! If it's less, say 24 degrees total, then pull those little springs out and stretch them a little at a time until you get your total reading back. [A sidenote here: If you do this repeatedly and it doesn't seem to make any difference in total timing, then you are limited by the physical travel of those weights. That's okay. Just advance the distributor a little to get your total timing reading back.] If it's more, say 30 or 32, then you either need to shorten the travel of the weights, which can be done by installing a heavier/shorter spring or shortening what you already have (just bear in mind that shortening the spring also decreases tension).



Once you have done this, you are set up for the best inital AND total timing. You've just put a performance curve on your distributor. Now go drive the fool out of the thing.



Best Regards,

James

Geoff

Here is my simplified "Earometer" method of setting, without either a timing light or vacuum gauge. As there is no vacuum advance on a '38 Terraplane, the timing needs to be slightly ahead of t.d.c. To set this, loosen the clamp, and turn the distributor slightly clockwise to retard. The engine will most likely slow down slightly. Note the position, then turn it slowly anti-clockwise to advance, until it the engine speeds up and starts running rough. Back off to about half way, or until the engine runs smoothly. For the idling mixture, it doesn't matter a toss whether it is two or one turn out, just wherever it runs smoothest and fastest. If too far in the engine will buck around and stall, if too far out it will lope, and emit smoke out the back. Of course you will have cleaned and set the points before you do any of this. If you have a timing light, you need to have the firing at 4 teeth b.t.d.c. for best performance. There should be a nice smooth advance curve as the engine speeds up. If you notice "pinking" at any speed, you will have to retard the distributor slightly. Good luck,

Geoff.