carb problem finally solved- square top

[Doc Dave]

Several months ago I posted about a vexing problem I was having with my engine not being able to accelerate normally. This all occurred after a carb rebuild, using a kit from VPW.

After a lot of trial and error, research and advice from several folks on the forums, I finally was able to correct the problem.

I have summarized the issue below. I hope this results in someone else not having the same difficulties.

This summarization was taken from advice given me by Bob Jones and Vaughn Payton.



"Square top carburetor secrets"

The square top carburetor has a vacuum operated accelerator pump system. If this does not work properly, the engine will sputter and cough when you try and accelerate it.

The accelerator pump is spring loaded. In other words it is always at the bottom of the fuel bore. When the engine is off the leather piston is at the bottom. When you start the truck, vacuum from the manifold pulls the piston up. This is why you need to choke a cold engine to help it start. The accelerator pump does not work until sufficient vacuum is applied. As you know when you accelerate the vacuum drops way off. If you still have vacuum wipers you know they stop when you hit the gas. So the theory is hit the gas-lose vacuum-spring forces down leather piston-fuel shoots into the engine.

Here's the problem: The rebuild kit that is sold is for a different carb (I don't know which one). They make modifications to the accelerator piston length and add a few new gaskets then sell it to you. Which is OK because you just can't find these kits anymore and all the other parts work fine. The spring in that kit seems too stiff and the kit’s acc pump shaft leather is also too hard and stiff. So two things occur, the vacuum cannot overcome the spring tension and the piston is too tight in the bore. I used an old piston and spring from another carb and the engine came to life. If you have an old carb remove the piston and soak the leather seal in any kind of oil for a day or so. Install it with the used spring and you should be happy. Note: if you need a carb rebuild on your square top, save the old acc pump spring and accelerator pump shaft with its leather seal to use with the new parts.

Also, you need to check the accelerator piston cylinder in the air horn for smooth action of the piston. Sometimes these carbs get oxidation built-up from setting for many years. If the piston does not move up or down without sticking slightly, then you need to take a brake cylinder hone and hone the cylinder until the piston can move freely. If the piston has any oxidation, it to needs to be cleaned with fine sand paper as well. Same for the piston/rod in the carb body, the cylinder has to be smooth and free of any roughness. Two items that are very important; the air/fuel passages must be open (not plugged) and the gaskets must be in good shape

[Gordon Maney]

Good job on that, and thank you for the detailed coverage.

A lifetime of carburetor work [....and I am 57 as I write this] has formed in me the habit of using as many of the old parts as I felt was appropriate.

Probably the biggest benefit of a so-called carburetor rebuild is the cleaning. A general cleaning and a verification that all internal passages are clear and are capable of flowing as defined by their diameters.

If the needle is rubber tipped, then that is a good thing to replace, too. If it is an entirely metal needle, then very often the needle and seat are just fine. Certainly it is possible for a metal needle to wear, to establish a relationship with the seat the effectively changes the functioning float level, but that is not very common.

A lot of people, unfamiliar with float adjustments, make what they consider to be a correct adjustment, causing a carburetor problem due to the float level being wrong. In the vast majority of instances, a carburetor's float level is correct. If a person is measuring float level and is unsure if it is correct, chances are good that it is correct.

The next real benefit of a rebuild is the new gaskets. Having said that, it is good to observe relative gasket thicknesses, as in some carburetors a change in gasket thickness can affect float level. Also, a difference in the thickness of a fiber washer under the needle seat will change float level.

So, although I could not have, did not, foresee your particular spring problem, I think this stands to me as an example of the benefit of using old parts that worked well before.

A vacuum operated accelerator pump has a hard enough time working as it is, without such obstacles as you discovered.

Persistence paid off for you....

[Joe Flo]

I'm glad to hear you got your carb going Doc. That was an excellent summary you wrote of the problem.

Gordon,

What you said about float level adjustments is so true. I have adjusted floats using the gauges in kits only to find it wasn't right or I adjusted it wrong. My motto is "If it ran before with the floats set this way, then it must have been ok."

Sometimes there is a "bath tub ring" in the float bowl or on the sides of the floats of old carbs that will give you a clue as to whether the float level or the needle valve were ok or not.

Joe

[Doc Dave]

Joe,
That is an interesting observation about the "bath tub ring".
Between what you say and Gordon's comments, I appreciate that fact that we made the right decision when my mechanic and I decided to leave the float level alone on my carb after we measured it and it looked "close to specs", so we decided to leave well enough alone.
As they say down south, "if it ain't broke, don't fix it".

[maineSS]

I'm wondering if you can still get the correct accel pump springs for these carbs. Dodge made two different spring tensions, and both were lighter than the later B & B carbs. The previous owner had the carb rebuilt, and the hesitation problems I've had sound stiff-spring related. Unfortunately, he didn't keep the original carb parts.

[Doc Dave]

I don't think you can buy the original "weaker" spring.
I tried, and VPW sent me a new looking spring that was still too stiff.
But, a few of the guys on the forum helped me out and sent me a few used springs from their parts bin.
In retrospect I should have kept all my old parts, and then I would have been O.K.
One person said I could take a stiffer spring and anneal it by heating, and then letting it cool slowly, thus making it softer. At least that is the theory. I did not have to try it though.

[Gordon Maney]

Always keep old carburetor parts.

Always.

[OLD DODGE]

A spring shop could fabricate new springs from a sample or an accurate drawing. I have springs fabbed all the time. One spring might cost you $75.00. If you have 500 springs made the cost might be $2.00 each.

[maineSS]

I found out that the M37 Carter carburetor also has a vacuum accel pump- wonder if the spring tension is correct for the squaretop? 40 oz is the tension for the squaretop that Dodge finally arrived at.

[maineSS]

M37 carb accel pump spring vs FFPW

I got some used M37 accel springs and compared them with the PW squaretop spring and a roundtop carb spring. The M37 springs were different tensions- one was considerably greater than the other. They were also longer than the PW spring, as was the piston and leather shank. I compressed each spring to coil bind on a scale, and found that the spring in my carb bottomed out at ~6 lbs, the stiff M37 at 10 lbs, and the roundtop carb at 13 lbs. The light M37 spring was corroded at one point, and broke, so I couldn't get a coil bind figure. I'm thinking it might be the one I need, because it looked silver in one spot- like the 40-oz PW spring required for the squaretop. I think the Oz rating must be figured at the max compression stroke of the spring rather than the coil bind point- 1/2 way or so, cause scale readings at that point looked more reasonable. Pic #1 shows the springs and their piston/leathers- M37 at the left, Roundtop in the middle, and Squaretop at the right. I was going to take more pics, but it'll have to wait till the camera battery recharges.

[maineSS]

More Accel Spring pics...

Here's some more comparison shots of the M37, Squaretop, and Roundtop accel pump/springs. Pic #2 is a lineup of the M37 (left), Roundtop (center), and Squaretop (right) accel pump/springs. Notice that both the M37 and Roundtop pump/spring is taller than the Squaretop. Pic #3 is a spring lineup- same order- the squaretop spring is slightly shorter than the M37 and Roundtop springs. I think the M37 spring could still be used, though it would be under some preload. Pic #4 compares the M37 piston (left), with the Squaretop piston (right). Both are the same diameter. If I can get a 40 oz M37 spring, we may have a way out of the Squaretop spring problem.

[maineSS]

The Rest of the Story....

While I had the Squaretop partially disassembled, I decided to check on the effort required to cause an accel pump shot. To my surprise, I found that the spring was too weak- not too stiff! About 1/2 the pump stroke was used up just contacting the spring, and the other half was too feeble to accomplish anything!
I had previously discussed this problem with Sid Beck of Ace Surplus, who sent me 2 M37 springs, and suggested I do some basic Auto Math to figure out how much vacuum pull I had to work with. Excellent idea! The piston is 3/4" dia, which gives an area of .441 sq in, the idle depression is 17-20 in Hg from the Shop Manual, 1 psi = 2.02 in Hg- so 17 in= 8.4 lbs pull, 20 in= 9.9 lbs pull over an area of .441 sq in is 3.71 lbs for 17 in Hg, and 4.37 lbs for 20 in Hg on the piston. This pull has to compress the spring AND overcome the leather friction, while the spring tension has to overcome leather friction AND unseat the check ball to cause an accel shot. I used a mini combination square set at 1/2" (piston stroke) less than the free length of the spring to check the M37 spring- it was 32 oz. The replacement kit spring in my carb was~ 10 oz- pitifully inadequate. I replaced it with the heaviest spring from the Roundtop carb- haven't measured it yet. I'll be back with more info and pics, AND hopefully a working accel pump....

[Steve Pellock]

thanks for the update. please keep us posted

[Gordon Maney]

Really great thread....

[maineSS]

This post will be in several parts, with pics. Hopefully it will help others diagnose accel pump problems. Pic #1 shows what we want- a good accel pump shot. It needs to get out into the carb throat to be picked up and broken up by the incoming airstream- dribbling down the carb wall just won't cut it. Four things have to work together in the Square Top accel pump to ensure a sufficent shot:

First, the pump inlet & outlet valve have to work correctly.

Second, the vacuum piston has to be able to pull the pump plunger up against spring tension, hold it, and quickly release it.

Third, the spring has to be strong enough to unseat the outlet valve forcefully to cause a shot into the carb throat.

Fourth, the plunger has to seal well enough to avoid backflow, without excessive friction.

Carter probably developed the vacuum system for military vehicles that needed to be sealed for deep fording- the usual rod actuated accel pump won't allow that. For civilian use, it was likely a maintainance headache as the pump wore, which likely was the reason the roundtop carb replaced the square top in later PW's.
The first thing you want to check is the pump inlet/outlet valve function. In pic #1, you'll see my fingers pushing the plunger down cause the squirt. Simply push the plunger down til it bottoms, pull up, and push down again. It shouldn't take a lot of effort. If you're not getting fuel in, check the plunger for splits. If it's not the plunger, there may be problems with the inlet valve. Same goes for the outlet valve.
The second item to check is the vacuum plunger and passage. Pic #2 shows the piston in the "down" position before the engine starts. Pic #3 shows it at the top of it's stroke (about 1/2")- engine manifold vacuum holds it up until the throttle valve opens. (to be continued..)