Ported vacuum

[47 dodge 1.5 ton]

I am trying to determine if I’m pulling enough vacuum for the timing advance to work properly on a dual carb set-up I have (really just question if it’s fast enough). I am still getting a slight stumble just off slow idle. After alot of tinkering, I believe carbs are where they need to be ( very smooth in all rpm ranges)I’m only using the ported vacuum of (1) carb and don’t believe this is enough when the accelerating from idle. Seems to be good accelerating above idle rpm’s although I still expect a little more improvement.  With idle set faster, it’s useable.

[Sniper]

Since you tell us nothing about the vacuum advance can being used or the amount of vacuum you have I am not sure how we can help.

 

But if you have the FSM it will tell you how much vacuum you need.  For example,

 

 

[Dodgeb4ya]

Your truck of course never had a vacuum advance distributor from the factory...because of heavy truck loads requiring a lot of wide open throttle...and spark knock etc.

So a distributor # ID is necessary for proper advance #'s.

Those being only approxumate because of mod's to your engine and it being in a some what heavy truck...

Maybe the step up pistons are sticky/ or no gasket under them...partial accelerator pump jet blockage....maybe a small jetting issue...plenty to check out.

 

[FarmerJon]

I think the best I can do is give you a list of things to check, and a handful of questions to answer. 

First off:

What engine, and what condition is it in? Stock '47 dodge? Stock compression?

Mechanical fuel pump or electric?

Can you put a T in the line and get a fuel pressure reading, idle, tip in, and revs?

What intake?

What distributor are you running? 

When was it last out and mechanical advance cleaned? 

Does point plate move freely? Many get gummy and slow to move, or seize altogether.

Are points set by gap or dwell?

What is the condition of the vacuum canister? New, rebuilt or old? Many units have a nut that can unscrew from the end where the vacuum comes in. This allows the spring to be swapped and or washers to be adder or removed to tune the amount of vacuum needed to activate. New or rebuilt may no longer match factory specs.

Are you running stock Ball&Ball carbs? Have you tried different settings on the accelerator pump?

What is your base idle set at with the stumble?

What is your vacuum at base idle?

What is your base ignition timing?

 

Next, after checking all of that, especially the distributor advance plate...

 

Rig up a soft line from the distributor to carb with a T to a vacuum gauge in the cab.

Record vacuum at all conditions. 

You could even try the vacuum port on the second carb instead,  perhaps one sees more than the other, or is partially blocked? Seems unlikely,  but easy check.

Use a vacuum pump and timing light to verify the amounts of advance at the various amounts of vac you are seeing on your drive.

You may want to use grid paper and plot it out. If you can, do the same with your mechanical advance....

Remember, half of all carb problems are actually ignition, and half of all ignition problems are actually the carbs

 

Good luck and let us know what you find

 

Edited January 31, 2023 by FarmerJon

[47 dodge 1.5 ton]

Thanks for these responses, going try and fill in some questions;

 engine is a stock 236, compression 90-95 psi all cylinders, factory dual manifold with factory equalizer tube, matching truck Carter b&b, stock idle jets, opened main jets .004”, accelerator shooting full on both through plate, currently middle position accelerator pump setting. Currently 3 1/4 psi fuel pressure but have tried 2 3/4-4 psi, gauge installed in system near carbs after regulator with new electric pump, manual pump removed from system. I don’t see much change in psi free reving engine. New tank and 5/16” lines up to carbs. 
 Exhaust- Langdon cast iron Headers, (2) 2 1/4” x 4 ft pipes w/ glass packs.

 Ignition- 12v alt, new MSD coil, current distributor is from I believe a truck 251 with vacuum advance( I will check number) seems to move free, without much wear. Currently have points (feeler gauge set) may try pertronix or other recommendations for electronic? Holds position with hand vacuum pump. I only have (1) carb hooked to vacuum advance. I am reading 2 degrees advance at low idle.
  Manifold port vacuum measures 17” steady at low idle, carb ported measure 4” steady on both carbs checked individually, not tied together. I have not checked vacuum at set upper rpm’s but runs smooth. 
I do not have a tach hooked up but may add something temporary. I Have (2) factory oil bath air cleaners that suck oil when filled to line. Runs leaner when they are off so settings are with them on.

  Drivability is ok when accelerating from anything above idle. Stumbling when moving at idle, then hitting throttle, sometimes popping if I hit it too hard. 

 I know that I have not answered all questions but will shortly. 

 

Edited February 1, 2023 by 47 dodge 1.5 ton

[kencombs]

Maybe up the idle speed a tad, and definitely open the idle mixture a quarter turn each, maybe another 1/4 if that helps but doesn't fix it completely. 

 

I don't think the off-idle stumble is related to vacuum advance timing, but a degree or two initial may help.   The vacuum advance only works after the engine comes up to speed and the vacuum rises.  On initial opening it will drop sharply, then increase.

[FarmerJon]

10 hours ago, 47 dodge 1.5 ton said:

Thanks for these responses, going try and fill in some questions;

 engine is a stock 236,

All of my information is for the 217/230, so adjust as needed

compression 90-95 psi all cylinders, factory dual manifold with factory equalizer tube, matching truck Carter b&b, stock idle jets, opened main jets .004”, accelerator shooting full on both through plate, currently middle position accelerator pump setting.

How does the stumble react when you put them on setting 1 or 3? Better or worse?

Currently 3 1/4 psi fuel pressure but have tried 2 3/4-4 psi, gauge installed in system near carbs after regulator with new electric pump, manual pump removed from system. I don’t see much change in psi free reving engine. New tank and 5/16” lines up to carbs. 
 Exhaust- Langdon cast iron Headers, (2) 2 1/4” x 4 ft pipes w/ glass packs.

 Ignition- 12v alt, new MSD coil, current distributor is from I believe a truck 251 with vacuum advance( I will check number) seems to move free, without much wear.

Currently have points (feeler gauge set)

Check and adjust the points with a dwell meter. Point gap is a great way to set up a new set of points, but dwell is what actually matters. My book shows dwell as 34.5-38. When I set my points by gap, they were not in this range.

may try pertronix or other recommendations for electronic? Holds position with hand vacuum pump.

What vacuum does it start to move at, and what vacuum does it max out at? And how much timing does it add?

I only have (1) carb hooked to vacuum advance. I am reading 2 degrees advance at low idle. 

Factory says initial timing can be increased up to 4*.  I would try that, and would even try 5.5. You may be limited by the weight of your truck, or how you use it. This should bring your manifold vacuum up.
  Manifold port vacuum measures 17” steady at low idle, carb ported measure 4” steady on both carbs checked individually, not tied together.

Are you seeing the 4" with throttle plates closed at idle? Typically ported Vacuum sources see 0 vac at idle, unless throttle plates are open too much. Or are you seeing 4" max with throttle opened?

I have not checked vacuum at set upper rpm’s but runs smooth. 
I do not have a tach hooked up but may add something temporary. I Have (2) factory oil bath air cleaners that suck oil when filled to line. Runs leaner when they are off so settings are with them on.

  Drivability is ok when accelerating from anything above idle. Stumbling when moving at idle, then hitting throttle, sometimes popping if I hit it too hard? 

What is the idle set at when it stumbles? You may just need a slightly higher idle speed than factory. Try upping it 50rpm at a time. For '53 cars, factory calls for "about 450-500"  for a modified engine, 550 doesn't seem unreasonable. 

 I know that I have not answered all questions but will shortly. 

 

Expand quote for more diagnostic questions in Bold

 

Are you running resistor or non resistor spark plugs? What gap are they set at?

 

If it were mine, I would check and set points by dwell, bring initial timing to 4*, up idle to 500, and try the accelerator pumps at #3 positions. 

 

You may need to check that your throttle plates are closed enough by reseting the throttle stop screws and bringing idle up with the mixture screws as much as possible. Looking at a spare carb, it doesn't seem like the vacuum port should be seeing much vacuum at all at idle. I have never put a gauge on mine though,  so I could be wrong.

 

8 hours ago, kencombs said:

 

I don't think the off-idle stumble is related to vacuum advance timing, but a degree or two initial may help.   The vacuum advance only works after the engine comes up to speed and the vacuum rises.  On initial opening it will drop sharply, then increase.

I agree,  but with ported vacuum timing does not ( should not) drop on initial opening, because it is seeing near 0 at idle, and when throttle starts to open, it gets 10" or so of vacuum, applies advance, then as throttle opens more, and vacuum drops, it backs off again. Obviously if throttle is ripped open, this brief spike in timing happens very quickly, or not at all, if the advance cannot respond in time.

This is why some people (and companies) used manifold vacuum for the advance. It acts as you described,  which imho makes more sense, and typically better fuel economy, if properly tuned.

Edited February 1, 2023 by FarmerJon

[47 dodge 1.5 ton]

Plugs are set at .028” with feeler gauge but have tried more open and closed. 
 The vacuum advance has a approx 1 1/2” length hex that the line is connected to. You mentioned this is adjustable? I’ll have to look at the numbers later tonight but I wasn’t aware of that.

 The manifold port is centered on the crossover tube, I would guess the reading would be higher if inline to each carb. I have that hooked to the brake booster but maybe can tee off? I would think that would cause a tremendous advance at low rpm though? How do they plumb/control that amount?

[FarmerJon]

I have to pretend to work for a little while, here is how it adjusts- more shims= more vac needed to reach max. Less=less

Edited February 1, 2023 by FarmerJon
Adding stuff

[47 dodge 1.5 ton]

Yes— perfect! Mine has a longer hex, that one shows to be turned but guessing the same. Thank you. 
  Back to the 4” carb setting. I found the closer to 2”, the worse the stumbling. I started with the plates closed, guessing they are open a hair while tuning though. I would be interested it what others have the idle vacuum reading set at with duals.

Edited February 1, 2023 by 47 dodge 1.5 ton

[vintage6t]

Thinking out of the box.  I wonder with 2 carbs = 2 accelerator pumps if you might be temporarily flooding the engine off idle. Do the plugs look like it might be running rich?

[FarmerJon]

2 hours ago, 47 dodge 1.5 ton said:

 The manifold port is centered on the crossover tube, I would guess the reading would be higher if inline to each carb. Shouldn't be much difference.

I have that hooked to the brake booster but maybe can tee off? I would think that would cause a tremendous advance at low rpm though? How do they plumb/control that amount?

To be clear: I have not tried tuning my Mopar 6 with manifold vacuum yet, nor have I gotten my dual cabs set up yet.

But I have had great luck on other vehicles.

Also: I suspect your stumble is 75% likely to be a accelerator pump tuning issue, hard to say if it is going rich or lean from info given.

If you run e10 gas plugs will be difficult to read. Best bet would be to try the 1&3 positions on the pump arm and note if it gets better or worse.

 

So I think playing with advance vacuum source tuning may be getting you off in the weeds.

 

But you are correct,  using manifold vacuum for the advance adds full vac advance to idle. Depending your distributor, 14-19* + initial 2-4*, you could see as high as 23*

It is crazy to wrap your head around at first, but it is actually what it needs. At idle, volumetric efficiency is very low. So more spark lead can provide a more complete burn. There is no load, so it isn't likely to ping (although if your initial is too high, it is certainly possible).

Then as the throttle opens, vacuum falls, say to 5-8" and the advance backs off to initial 4*+ maybe 5-8* vacuum advance. Load is pretty high here, so 9-12* is hopefully not pinging. From here forward,  it works the same as ported Vacuum advance. 

[47 dodge 1.5 ton]

Ok— found a dwell meter at the farm that looks like it hasn’t been used since the 70’s. Next is to figure out how to use it and measure some base line values. Hoping I can get some free time to work on it over the weekend.

  My gut feeling is it’s not getting enough fuel at that moment because adding fuel and having the idle rich definitely is better. 
 Still scratching my head on setting up a timing light meter in the cab, vacuum meter is no problem.

 From Dodgeb4ya’s comment, I may look at the distributor I removed when I first got the truck running without the advance(thought I was upgrading) and I don’t really know for certain mine is a truck model(I need to get the number and do some searching) I do now have an assortment to choose from.

  I am far from an expert but do feel I’m making improvement, took me a little bit to be able to pull off the spot a few years ago. 
 I do appreciate all thoughts. 

[Sniper]

One thing to remember, regular gas today has a much higher octane level than what your 47 was designed to run so detonation may not even be an issue.

[47 dodge 1.5 ton]

You mentioned something I’d never really give consideration that may/not have effect. I have only been using 89 octane fuel through it with I believe is 5% ethanol. Mainly because that’s what I use in my old tractors for consistency purposes (in my mind only). I know something that only has ~6.1 compression shouldn’t require that.

 What are others using?

[LeRoy]

11 minutes ago, 47 dodge 1.5 ton said:

You mentioned something I’d never really give consideration that may/not have effect. I have only been using 89 octane fuel through it with I believe is 5% ethanol. Mainly because that’s what I use in my old tractors for consistency purposes (in my mind only). I know something that only has ~6.1 compression shouldn’t require that.

 What are others using?

I run 87 w/ethanol if I'm going to burn it up in the next month. If I think it will sit for some time I put in non alcohol gas, I think the pump says 89 octane. My carb rebuild parts say they are ethanol safe but I think straight gas ages better, I could be completely wrong here but just my gut. Certainly is the case with small engines that sit w/gas over the winter.

[Sniper]

I don;t let my small engines sit with gas in them, ever.  I've learned about how much gas I need to put in the tank to do the mowing and I run it dry when I am done.  No more pulling carbs to clean them or having to replace one I couldn't clean out anymore.  My 51 doesn't sit long enough to worry about it.  Now having said that I feel the jinx coming on, lol.  I did fire her up yesterday and let it run till it was warmed up it was 25F out.  I didn't go anywhere, ice on the roads, but I let her warm up.

[Young Ed]

That sounds like way too much work. I just buy no ethanol gas for the equipment and add stabil for the off season. No problems with either mowers or snow blower.

Funny story though this past fall I got delayed and couldn't get the mower to start. So I put it in the warm shop and still couldn't get it to start the next day. After pulling on it probably 50 times and checking the plug I finally gave up and was just going to add stabil to the tank. Well when I opened it it was empty! 

[kencombs]

1 hour ago, Sniper said:

I don;t let my small engines sit with gas in them, ever.  I've learned about how much gas I need to put in the tank to do the mowing and I run it dry when I am done.  No more pulling carbs to clean them or having to replace one I couldn't clean out anymore.  My 51 doesn't sit long enough to worry about it.  Now having said that I feel the jinx coming on, lol.  I did fire her up yesterday and let it run till it was warmed up it was 25F out.  I didn't go anywhere, ice on the roads, but I let her warm up.

I should do that, but don't always.   Way back in the late 50s my uncle was a rep for a distributor that provided parts to a lot of accounts in and near KS.  One of the lines they carried was Lawnboy repair stuff.  He could also get deals on mowers from one of his wholesale accounts so Dad bought a new one.   I wore out a couple of engines on that thing!  Dad had a 1/2 city block and I mowed a number of lawns besides that.  So it ran several day weekly. 

 

One of the things he recommended when he brought us the mower new, was to turn off the gas and let the carb empty, every time it was used.  Remember that iteration of Lawnboys were all two strokes with diaphragm carbs.  Over several years we never had carb problems.  The thinking was that gas in the tank was ok, but not in the carb.  Probably true back BA, (before alcohol).

 

And when I wore the engine down to really low compression, he got a brand new short block from Evinrude/Johnson for under 25 bucks!

 

If we had had today's ashless two stroke oils they would have lasted longer.

[Sniper]

One of the reason's those small engines are more affected than my flathead is that the small ones use a gravity fed fuel bowl.  As the gas evaporates out, and leaves behind it's gum, more fuel feeds in and evaporates off leaving more gum.  Eventually the carb gets all gummed up and doesn't work when you need it to.  With the flathead, once the fuel in the bowl evaporates out that's it, no self refilling there.

[Sam Buchanan]

Good reason to have a fuel shut-off valve on our small engines.......