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Old 06-13-2019, 01:39 PM   #1
Redtail
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Default Persistent intake manifold leak

Hi Guys,

It seems I have an intake manifold leak that I am having trouble fixing.

The main symptom I am trying to cure is the engine stalling when coming to a stop. I have a brand new Zenith carb and I have tried raising the float level all the way to the point that it starts to leak fuel with no success.

I have replaced the manifold gaskets with the late 31 style, torqued to 35 ft-lbs from the inside out, tried installing with and without exhaust glands all to no effect. I had the manifolds decked as a unit at a cylinder head shop so I know they are flat, but I still think there is a leak at the rear intake manifold port.

With the engine hot and at a high idle, when I spray brake cleaner around the rear intake manifold port the RPMs drop significantly, then come back to normal when the brake cleaner evaporates.

Any suggestions what to try next? I hear some people use manifold sealant but that doesn't really seem like a long term fix. I don't think the carb is at fault so I will set the float level back to where it was when I got it. I don't have a problem spending $200 for a new intake and exhaust manifold set, but I don't want to just keep throwing parts at the issue without really understanding what is going on.

Thanks,
Zack
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Old 06-13-2019, 01:51 PM   #2
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Default Re: Persistent intake manifold leak

You haven't mentioned inspecting for cracks – did you check the manifold and block for cracks that might be admitting air?
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Old 06-13-2019, 02:04 PM   #3
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Default Re: Persistent intake manifold leak

Thanks for pointing that out, I did my best to look for cracks in the manifold and didn't see anything obvious. However, I didn't take it any further than a visual inspection ie: I did not have the manifold magnafluxed. So I suppose it is possible there is a hairline crack somewhere I can't see.
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Old 06-13-2019, 02:17 PM   #4
marc hildebrant
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Default Re: Persistent intake manifold leak

Zack,

I had a similar problem with the Zenith stalling at stops. I changed my carb to a tillotson and the stalling stopped.

I suspect that I have some blockage in the air passages of the Zenith. Maybe you do too.

Marc
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Old 06-13-2019, 02:31 PM   #5
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Default Re: Persistent intake manifold leak

If the report of the idle dropping when the rear intake manifold is sprayed is accurate, that would be a pretty clear indication of a vacuum leak rather than a problem in the carb.

The dropping idle speed indicates that the brake cleaner is temporarily sealing the leak. I mention this because another common tactic is to spray starting fluid or carb cleaner in the area. In that scenario, the flammable vapor is sucked into the leak and burned, causing RPMs to increase. But here since the brake cleaner is non-flammable, it decreases RPM by reducing the supply of oxygen.
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Old 06-13-2019, 02:34 PM   #6
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Default Re: Persistent intake manifold leak

My thoughts are either hairline cracks somewhere that open up or the GAV needs to be opened up when you come to a stop. Ooooh, is the block flat where it bolts up?


Mike
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Old 06-13-2019, 02:40 PM   #7
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Default Re: Persistent intake manifold leak

It's possible you could take the car to a sympathetic auto repair place, since vacuum leaks are still a thing that afflicts modern cars and they should have equipment to diagnose them. Alternatively, applying flammable or non-flammable sprays carefully with a straw may help pinpoint the leak.

I like the idea of a crack that might only be opening up when the manifold is bolted to the block, maybe something very close to the bolt site.
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Old 06-13-2019, 02:55 PM   #8
Kurt in NJ
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Default Re: Persistent intake manifold leak

How was the manifold surfaced---a machine with cutters ---or on the big "belt sander"----the belt sander can round edges, has caused problems for work that I have done, have the surfaces been checked with a straight edge
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Old 06-13-2019, 03:06 PM   #9
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Default Re: Persistent intake manifold leak

Quote:
Originally Posted by alexiskai View Post
If the report of the idle dropping when the rear intake manifold is sprayed is accurate, that would be a pretty clear indication of a vacuum leak rather than a problem in the carb.
.
That's my thought as well. At first I suspected the carb, but after swapping to a new carb and trying different float level adjustments I am 90% sure the carb is OK.

I have not put a straight edge on the block, but I thoroughly cleaned the gasket area and did not see any cracks.

I have one other piece of information I didn't mention earlier because I'm not 100% sure about it, but the engine seems to stall out more easily when it is really good and hot. The first few minutes of driving after a cold start are usually better.

How does this sound for an action plan:
1. Remove manifold and double check block for straightness and cracks
2. Order new intake manifold, gaskets, glands etc (can I install a new intake manifold with an old exhaust manifold or should both be new?)
3. Install new parts and adjust carb float to the book standard
4. If the problem persists bring it to the local Model A mechanic. I'd be out of ideas at this point.

Thanks!
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Old 06-13-2019, 03:12 PM   #10
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Default Re: Persistent intake manifold leak

Quote:
Originally Posted by Kurt in NJ View Post
How was the manifold surfaced---a machine with cutters ---or on the big "belt sander"----the belt sander can round edges, has caused problems for work that I have done, have the surfaces been checked with a straight edge
It was surfaced on the big belt sander. The manifolds are straight across the ports checked with a straight edge but I didn't check for rounded edges.

The vacuum leak seemed to be the same before and after surfacing.
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Old 06-13-2019, 03:36 PM   #11
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Default Re: Persistent intake manifold leak

Did you check for droop on cylinder4 exhaust manifold gland rings nor gsakets help if rear is drooping
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Old 06-13-2019, 03:40 PM   #12
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Default Re: Persistent intake manifold leak

Quote:
Originally Posted by Truckerjim View Post
Did you check for droop on cylinder4 exhaust manifold gland rings nor gsakets help if rear is drooping
I did not. I will research how to check for droop in the exhaust manifold and add that to my first point on the action plan. For whatever it is worth, the exhaust manifold is not leaking at all, just the intake.
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Old 06-13-2019, 03:41 PM   #13
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Default Re: Persistent intake manifold leak

I will say, I'm having second thoughts about the intake issue, for the specific reason that you described the problem as stalling upon a stop, i.e., upon deceleration, rather than stalling upon idle. Is the behavior that the engine stalls as you decelerate, or is that that you come to a stop and then are sitting there and at some point it stalls?

If the behavior is the former, it might still be carb-related, because most people who report that problem find that the solution involves the carb. Zenith owners sometimes describe this behavior something about the fuel sloshing in the bowl as the car decelerates and starving the engine. Here's a quote from someone on this forum a few years ago.

Quote:
Originally Posted by Mark in MT View Post
My experiences with the Zenith are that if you are idling too fast, it is not running on the idle circuit. This can be because the throttle plate is not positioned right in the bore. If the air at idle is not forced right by the discharge port for the idle, you have to increase the idle screw to get it to run. This makes it run on the cap jet circuit which is more sensitive to sudden stops. Adjust the plate in the bore to get it to cover the idle discharge port and you can get it to idle at 400 rpm and do so while standing on the brakes.
So I dunno, it might be worth exploring the carb situation a little more before you buy a brand-new intake. You may have multiple problems a tiny leak in the intake that's making it run a little bit lean, but also a problem in the carb that cuts the fuel flow on deceleration.
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Old 06-13-2019, 03:46 PM   #14
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Default Re: Persistent intake manifold leak

Quote:
Originally Posted by Redtail View Post
I did not. I will research how to check for droop in the exhaust manifold and add that to my first point on the action plan. For whatever it is worth, the exhaust manifold is not leaking at all, just the intake.
If you've been running it with gland rings, it's unlikely port 4 is drooping. When that happens, you can't get the gland ring to fit in the first place because the hole in the manifold is offset from the hole in the block.
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Old 06-13-2019, 05:25 PM   #15
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Default Re: Persistent intake manifold leak

Be sure the gland rings are not too wide and will not allow the exhaust manifold to seat up tight against the block. If the exhaust manifold will not seat up tight, the intake also will not seat up tight.


Often I have to narrow down the gland rings a bit so the exhaust manifold will seat up tight. This is especially a problem with used exhaust manifolds that have been surfaced. Clean the relief in the exhaust manifold to be sure the gland rings will fit all the way in.


As far as a sealer. I use a small bead of high temp. silicone around each port opening on both sides of the gasket. I have been doing this for many years with very good results.


I have built specialty engines where I used no exhaust manifold gaskets at all. The flanges were thick and machined flat. Just a little high temp sealer and bolt it up tight. absolutely no problems.


I also like the '31 style copper gaskets. I think you have a better chance of getting a good seal.


Also, I think the float level in your zenith Carb may be too high. This is a common problem of flooding over when making a stop. I usually will solve this problem by lowering the float level a bit.


And as stated by others, take a good look at the idle circuit passage and the throttle plate. This is critical.


Again, these things I would do but others may have other ideas.


Chris W.
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Old 06-13-2019, 06:01 PM   #16
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Default Re: Persistent intake manifold leak

Great advice from Clark !! My vacuum leak turned out to be a bad wiper line fitting. Idle mixture MUST be adjusted at minimum rpm. Keep in mind that turning the idle mixture in makes the idle mixture richer since the needle controls air and not fuel. Low rpm mixture adjusting forces fuel through the idle passage in the carb throat (if it is not clogged). Good Luck.
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Old 06-13-2019, 06:19 PM   #17
1931 flamingo
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Default Re: Persistent intake manifold leak

BEFORE throwing parts at it (and yes, order both manifolds if you do that), take it to the Model A mechanic FIRST.


JMO

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Old 06-13-2019, 11:27 PM   #18
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Default Re: Persistent intake manifold leak

A few thoughts.


A majority of the time stall at stops are carb issues.


Make sure all passages are thoroughly cleaned, including using a paper clip. Spraying and compressed airr sometimes are not enough.


Have you tried a known good carb? something that runs properly on a different car.


Typically for this issue you lower the float level in stages up to a float level guage reading to as much as 15/16ths.


Here is some additional carb info,
https://www.model-a.org/


If rebuilding your own carb, try flow tested jets from Renners Corner.
http://www.rennerscorner.com/carburetor.html


Also Berts has an excellent reputation for rebuilt carbs, but the wait time can be long.


Are you running wireless distributor plates? They can do some odd things, including your symptoms.
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Old 06-14-2019, 05:37 AM   #19
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Default Re: Persistent intake manifold leak

No one has mentioned the flatness of the flanges that attach the carb. If they are not flat, and most are not, that tiny air leak can cause your problem. Hit them both with a file and clean them up. Shiny everywhere. Very common problem.
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Old 06-17-2019, 10:14 PM   #20
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Default Re: Persistent intake manifold leak

Quote:
Originally Posted by alexiskai View Post
I will say, I'm having second thoughts about the intake issue, for the specific reason that you described the problem as stalling upon a stop, i.e., upon deceleration, rather than stalling upon idle. Is the behavior that the engine stalls as you decelerate, or is that that you come to a stop and then are sitting there and at some point it stalls?
.
It is the former, it stalls as I am coming to a stop and specifically when weight transfers forward (and fuel in the carb float bowl sloshes around). This probably indicates incorrect carburetor float level. I also know for sure there is an intake manifold leak so I am probably looking at a number of issues playing off each other.
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