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Checked my vacuum at idle today and was just over 20Hg, which I think is good for a 1954/235 with a Roch B. When I pulled the vacuum advance line out, the idle speed dropped, but did not seem to affect the vacuum at the manifold.
When I was reading the thread below about 235 timing and vacuum, it mentioned that any vacuum registering at the advance line while idling is bad, but another thread I was reading said 7 to 9 Hg at the vacuum advance port is normal. Can anyone clarify? I guess if you are getting any advance at idle from the vacuum advance, then something needs to be tweaked. Once I get my hands on a 6v timing light I'll check to see if the timing has advanced at all from the ball. I just static set the timing on the ball today (after setting the dwell to 60 degrees which was .016 with the feeler gauge) and have not added any advance yet. Runs and idles fine.
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/760819/2.html
Thanks in "advance"...
Sulli

If the dwell is 60 degrees the engine won't run- - - - -the points will be closed all the time. On a 6-lobe distributor cam, there is only 60 degrees of separation between the high spots on the cam. Now, if the dwell is 60%, like it's supposed to be on some distributors, it will be close to 40 degrees.

If the idle speed is adjusted correctly, there should be no vacuum on the distributor line. As the throttle is opened, a port is uncovered by the throttle plate that begins to advance the timing just off-idle. On a stovebolt distributor you don't need a timing light- - -you can watch the distributor housing start to move as the engine speed is increased.
Jerry

Maybe I'm not doing something right in terms of process or I'm not sure how to translate what the digital dwell meter was telling me. I set the timing with a static timing light to just light up when the engine set to TDC (ball aligned with needle and points just open - i.e. light comes on) then turned the engine until the lobe had the points at maximum open. I then set the points to be .016 with a feeler gage assuming I should be good for dwell (points gap is the same as dwell, right?).

When it started and was idling, I hooked up the dwell meter to the #1 wire, which read 59.9 for the dwell angle, and not knowing any better, figured that I was spot on 360/6 = 60. There must be something more to it than that because what you said about points being open for all 60 degrees makes sense - that it could not run.

How did you calculate 40 degrees from the 60 degree reading? I honestly want to know and am not questioning how you got there, just want to know how to make sure I get it right.

Digital dwell meters and point-type ignition systems don't play well together. The digital readout reacts too fast to changes in voltage and you get an incorrect reading. The old fashioned sweep-needle (analog) meters work much better.

Here's a couple of definitions for you:

"Dwell"- - - -degrees of distributor rotation the points are closed, or "dwell together" The most efficient dwell setting for most distributors is a "points closed" duration of 66%. On a 60-degree (6 cylinder) cam that would be 40 degrees of dwell. Some early stovebolts actually had a dwell specification of 39 degrees. Later, it was reduced to 32 degrees, I believe. Most V8's have a dwell specification of 30, which is 66% of the 45 degree distributor cam they have.

"Dwell" is actually an average of the voltage seen at the downstream connector of the coil primary terminal when the points are open, then closed. Follow closely here- - - -this gets a bit confusing. With the points closed, the voltage at the distributor side of the coil will be zero, since one side of the points is grounded, and a few inches of wire won't make a difference between ground and the downstream end of the coil primary. You can check this by measuring the voltage with the engine stopped, ignition on, and the points closed.

With the points open, you'll see full battery voltage at the coil (-) terminal, since no current can flow through an open set of points. Once again- - -don't take my word for it- - - -it's easy to confirm.

Now- - - -with the engine running and a good analog voltmeter at the coil (-) terminal, you will see an average between battery voltage and zero, depending on how long the points stay closed. Since the points are supposed to stay closed 66% of the time, with a 14 volt alternator reading you will see about 1/3 of that voltage- - - -about 4.6 volts. If the point gap is widened and the dwell decreases, the voltage will go up. Don't try this with a digital voltmeter- - - -all you'll get is random gibberish on the readout!
Jerry

If I read it correctly you hooked the dwell lead to the #1 wire. I hope you meant you hooked it to the minus (-) side of the coil.

Jerry, thanks for the detailed explanation. Your last point about connecting to the (-) terminal followed by Coilover's comment about clipping to the #1 wire is probably where I went wrong. The digital mmeter came with several different leads/probes, one of which clips around the plug wires like for a timing light and that's the one I used.... Sounds like that should be used for checking rpm, and I should have been using the battery type clips with one on the (-) coil lead and the other to a ground?

For my first question about vacuum - the rpm does drop when I pull the vac advance line from the carb, but the manifold vac looked to be unchanged at 21 Hg. Does the rpm drop mean that there was enough vacuum to pull the advance at idle and therefore raise the rpm? If I set timing to have 0 advance at TDC, and assuming all else is good, at idle the timing should still read zero with the vac advance connected and the engine idling, correct?

Have you tried connecting the vacuum gauge to the line you disconnected from the distributor? That's the simplest way to see if there's any vacuum on the distributor at idle. Ideally, there should be virtually nothing there until the speed is raised a little, then it should increase gradually as the RPM goes up. Just as a precaution, even on engines that supposedly idle with no vac advance, I disconnect and plug the distributor line BEFORE setting the timing. Then I recheck the timing marks with a light after locking everything down and reconnecting the vacuum.

On a long hill or during passing, etc. the vacuum advance goes away and the engine only has whatever timing advance the centrifugal weights are providing. The only reason to have vacuum advance at all is for light throttle cruise, to improve fuel economy. Anytime the engine is under any load at all, vacuum advance will be nonexistent.
Jerry

Try setting the initial timing a few degrees advanced. Then you can slow down the idle speed with the carb idle speed screw. That will probably close the throttle enough to close the port to the vacuum advance. Make sure the idle mixture screw is adjusted correctly.
George

If you have a timing light for 12 volts, just use you modern car battery and clip the timing light probe to your truck. The light doesn't care where it gets it voltage from.

The main thing with setting timing and carburetors is each engine is different. Don't be afraid to tinker with the settings and don't get hung up on numbers. George has a very good suggestion, advance the timing a little till the engine speeds up, then adjust the carburetor idle speed back down. Next, adjust the mixture screw to smooth out the idle. You may have to go around couple of times till it feels right. Once set and running, verify by driving, then check with a timing light so you know how to reset it in the future.

Modern gas is much better then when the truck was new, so the settings are different, even your location can make a difference due to fuel formulas.

Just so you know, there are two types of vacuum ports on the carburetor.

1) Full time vacuum, this is direct manifold vacuum under the throttle blade. It's always there as long as the throttle is partially close. Even at full throttle you will have some vacuum ( very little, but some ).

2) Ported vacuum, this above the throttle blade. It will only be present when the throttle is open enough to expose the port to the air rushing through the venturi of the carburetor.

If you have vacuum at a ported vacuum line at idle, then you either have bad gaskets in the carburetor, mismatched carburetor parts, or the throttle blades are open to far.

Jerry - I'll check the idle with the advance port plugged (I just left it open when I pulled the line) then I'll check the vacuum at the advance port and see if it is pulling any vacuum at idle. Then I'll check with the timing light after connecting the advance.

George and Joe - thanks for the tuning tips. I'll bump it 5 degrees once I've confirmed I don't have vacuum at the advance port, then plug it and see what happens. Have not messed with the mixture or throttle lever screws yet, and will fiddle around this weekend.

Thanks!
Mark

So this weekend, I re-checked the dwell, and got some more reasonable numbers when using the negative side of the coil and a ground. I guess there is an art to setting the points gap because I was able to hit 37.4 and 42.3, but never seemed to hit 40 degrees. I'll keep working on that this weekend.

Checked the vacuum at the advance port and it barely read 1hg. Is that low enough to be considered little to no vacuum? I have not started to mess with any of the adjustment screws on the roach-B yet, and plan to get my dwell set, then adjust the timing and see what happens before I mess with the carb.

Now about messing with the carb...
I opened it up because I wanted to make sure I had the floats set correctly since it leaks (probably a warped air horn), and was curious about the jet size while I was in there. Found that is a #52, and my engine is a 235 from 1954. Did some poking around the internet and found that it is supposed to be a #58... Can anyone confirm a #58 correct for a roach-B on a 235? I have a #56 and was thinking about trying it out this weekend. Seems to run and idle ok with the 52, but it does run out of steam pretty quick when I try to run up the rpm's. My biggest concern is that if I make that big of a jump in the jet size that it may create a whole mess of other issues.

This may be a dumb question, but could too small of a jet (overly lean condition) contribute to hard starting? Every now and then it will start on it's own, but generally I need to squirt a little gas into the carb first. Pulls just over 20hg at the manifold when idling.

Are you using the choke for starting? Particularly on a cold start, that's what the choke is for- - - -to make it rich enough to start. Pull the choke out all the way- - - -DO NOT do a flamenco dance on the gas pedal- - - - -and crank until the engine fires. That should happen in 2-3 seconds, max. (Please remember to turn the ignition on- - - -lots of flooded engines are caused by choking with no spark) Once the engine fires, push the choke knob in halfway and let the engine warm up at fast idle. That's built into the choke linkage. Choking pulls gas through ALL the passages- - - -idle and high speed.

Hot starts should happen without choking, unless the carb has been percolating raw gas into the intake. In that case, hold the throttle wide open and crank until it starts.
Jerry

Yep, I pull the choke, turn the ignition on, (never have stepped on the gas), then step on the starter. I found out by accident that the starter is always hot (ignition on or not) a few months ago, and started a thread because I thought something was wired wrong.

I will make sure that I pull the choke out fully from now on, because I don't think I'm pulling it out all the way after reading that it should be closed up as soon as the engine is able to idle on its own. I don't let it turn over much longer than 3 seconds at a time (if that long) because I'm worried about engine wear and usually only try 2x's before I put gas down the carb. I only add a small amount and it fires right up.

Thoughts on the 52 jet size?

I think that is the size for a 216, and I guess someone before me could have drilled out the 52, but I'd have no way of knowing. I may see if I can visually notice a difference between the 52 and 56 that I have now.

Until you put some mileage on the vehicle and you can get a look at the condition and color of the spark plug insulators, tinkering with jet sizes is premature. Running a 216 carb on a 235 will make the mixture run rich, even with a normal-sized main jet. Plugs running the right mixture will be somewhere between light tan and chocolate brown, Bright white is too lean- - - -sooty black is too rich. The main jet doesn't begin to affect the fuel mixture until the engine is WAY above idle speed. Idle mixture is controlled by the adjusting screw near the manifold flange. I like to adjust the idle mixture for the best vacuum reading at the main vacuum port (wiper hose) for any particular idle speed, and then go approximately 1/8 turn rich (CCW) from there. Richening the mixture slightly after finding the best vacuum reading helps smooth the transition from idle to part-throttle operation.
Jerry

I agree. .052 is too small for a 235. They generally run .055 to .058. I had to drill one out too .062 one time to get it to run right. If your throat is warped you better make sure you get vacuum to the power piston. I would try the .056 jet. I bet it will run much better.

If the distributor is Delco 1112388, that is the distributor for 54 with manual transmission. Uses a steel gear. 1112396 is for 54 automatic. Uses a cast iron gear. Both use what Delco Remy specified as a “I” cam angle range which was 38-45 degrees of dwell. You were pretty close at 37.4. The 1112403 was original equipment for 55 and up, but the service replacement unit for the earlier models. Those model distributors use a dwell of 28-35 degrees. The 55 and up cam for the points is different from the cam used in the 54 and earlier distributors.

My reference to a 66% dwell angle is simply a rule of thumb to get an engine running close to right if a definite specification isn't readily available. Whenever using an exact spec. is possible, by all means do so. Since most 216 and 235 distributors will interchange (even if the gear material isn't right) it's hard to know which spec. to use unless you research the part number of whatever distributor you have.
Jerry

1946-1964 Chevrolet Engine Tune-Up Specifications

Dwell is shown

The dreaded Model B warp issue will not cause an external fuel leak. I will cause an internal leak which will cause the engine to run rough.
An externally leaky Model B is caused by the float set too high or trash preventing the float needle from seating.

Thank you all for the additional info in the last few posts. I think I may have finally beat this hard-start issue! (at least for today). I found that I have an AC Delco car distributor 1112396, so with a 38 to 45 degree dwell range, I'm sticking with the 42 dwell that it is set at currently so I have a little hotter spark. Since I wasn't having luck starting before, I went ahead and reset the timing to zero using my static timing light by adjusting the distributor to make it light up as the ball aligned with the pointer, then readjusted the points to .017. I used the "octane selector" to dial in a few degrees of advance, screwed the mixture screw all the way in (gently) then backed it out 1.5 turns, and was ready to see if it would start.

This time I pulled the choke ALL the way out, and boom, she fired up! I nursed the choke closed until it could idle on it's own and started checking things. Dwell ended up at 42.2, so I called that good and I hooked up the vacuum gage. It was reading just over 20hg so I adjusted the RPM's from 660 down to 510, then started to fiddle with the idle mixture screw. It got just over 21hg so I kept going to see if I could get more vacuum. Once I was sure it was maxed out, I backed it down till it started to run rough, then I brought it back up to maximum 21hg plus 1/8 turn more (per HRL) and checked the revs again to make sure it was at 500.

By now, it was warmed up, so I turned it off, spent a few minutes cleaning up a bit, and tried to start it again. This time I didn't use the choke and it immediately came to life and settled in to a nice idle. I almost took it out for a spin, but my sag wagon (wife and kids) are out, and I want them to follow me because I am still not 100% sure I won't get stranded again.

If this turns out to be a reliable set up, all I need to do next is figure out the carb leak. The floats are adjusted per the adjustment chart that came with the rebuild kit (1 9/32"), plus another 1/32" to compensate for the 10% ethanol (read that in a post from carbking). I haven't figured out how to check fuel pressure given that I have a hard steel line going to the carb, so I haven't ruled out too much pressure even though it is the old glass top pump. The float needle is clean and the needle's red o-ring gasket is seated properly. I blew into the fuel inlet and held the needle seated (gently) and it seemed to hold when I blew into it. When I looked at the exploded view of the roch-B, and noticed that there may be about 5 parts that I am missing between the air horn and the fuel inlet brass fitting (can't tell if that is B specific or if it is BC or BV specific). Right now, my fuel line goes into the brass inlet and that is screwed directly into the carb with no thread tape. See attached image for the parts that I definitely don't have on my carb (#'s 45 through 49). Anyone know if I need these or at least part #46?

Thanks!!
mark

Took it for a spin, and it runs way better than it has ever run. No stumbling, and it was ok with a bunch of throttle and some higher revs. I was able to go 50mph and it felt like it could go faster, but I didn't want to push it. It feels pretty respectable now.

Thanks again,
Mark

One suggestion- - - -set the timing last- - - -after doing all the point adjusting you intend to do. Every one degree of dwell change (which changes with point gap) causes a 2 degree shift in timing at the flywheel. Always adjust dwell or point gap first, then set the timing.
Jerry

Finally figured out the fuel leak. The seat for the fuel line flare on the brass inlet fitting (see image) has a pretty good groove on it so the fuel line flare could not seat properly. Bought a new inlet and it was such a tight fit that when I was threading the fuel line into it, the darn thing squeaked... Believe me, I partially threaded it several times to make sure that it was not cross threaded or stripping the new part. It was just super snug. Pulled the choke, fired it up, and no leak at the inlet or at the air horn.

Went out for a spin and made the first trip to the gas station for a fill-up. Thanks for everyone's help on getting this thing rolling.
mark

Congrats on your hard work! Well done!

I owe any success to the help from this forum!

So, about the #52 jet... I have driven it around for a couple of 20 minute runs with sustained speeds of 30 to 45mph to see what the plugs would do with the suspected small jet and I think I can say it is running lean for a few reasons.:

1. It got pretty hot and was tough to start (carb, air filter and fuel line were hot to the touch)
2. I saw fuel percolating in the glass bowl pump and the level was lower than usual
3. Pulled a plug and it is pretty white (see attached)

My question is - is the new #56 jet I have too big of a jump for a plug that looks like the one attached? From what I have read, the 235 with a Roch B is supposed to have a #58.
Thanks!
Mark