Marvel Carburetor...Help!!

[Guest sndtubes]

I have a 1930 Series 40 Buick with the dreaded Marvel Carburetor. A little background first:

The carb was rebuilt completely about 5 years ago by "The Old Carb Doc".

The manifolds have all been refurbed and the heat tube replaced. The heat system is totally disabled. The opening to the exhaust manifold has been completely sealed.

The fuel pump is working fine and was rebuilt when the carb was. I have added a 6 volt "helper" fuel pump for when the car wants to vapor lock.

The engine was rebuilt about 15,000 miles ago.

Ok, here's the problem. I cannot get the car to run right at high speeds without running the choke at least halfway out. When it is really hot (like yesterday), the choke has to be nearly all the way out for the car to run. If I push the choke in, the car starts popping and backfiring through the carb as if it's running too lean. I have tried and tried adjusting the air valve spring and I just can't seem to the the car to run rich enough at higher speeds. If the air temp is reasonable (mid 80s or so) the car runs fine with the choke all the way in, but still runs better on the highway with the choke partially out. It's easy to find the "sweet spot".

My question is this: Is there something wrong, or is this just normal for this kind of carburetor? Maybe it just isn't possible for this Marvel carb to deliver enough fuel for this engine at high speeds? Or could there be something else going on?

Would I be just better off by changing to a downdraft like a Rochester or Carter BB-1? I'd like to keep it as stock as possible, but it's a driver and that's most important to me.

Thanks!!

[tblack]

I have shared a number of Marvel horror stories on different posts before you go crazy with the carb check your points.

[Guest sndtubes]

I have shared a number of Marvel horror stories on different posts before you go crazy with the carb check your points.

I did. None of them matched my problem.

Not a very helpful response.

[Pete Phillips]

Vacuum leak somewhere?

Pete Phillips

[jbbuick22]

Check the heat riser tube in the manifold. They will cause a big vacuum leak.

JB

22-6-55 Sport Touring

[Guest sndtubes]

Check the heat riser tube in the manifold. They will cause a big vacuum leak.

JB

22-6-55 Sport Touring

Well, the heat riser tube was replaced by a machine shop who specializes in working on manifolds. There shouldn't be any vacuum leak there or really anywhere else I can think of......... The windshield wiper motor is diasbled and there is a tight fitting plug on the wiper vacuum tube. No leaks there.

[Bob Engle]

Rig a vacuum gauge into the wiper vacuum line and check what kind of vacuum you have at idle and running speeds. If vacuum is too low, intake manifold gasket leaks, tubes bad in the heat riser, or vacuum wiper line leaks are the most common causes. Burned valves can cause low vacuum also. A compression check will find this problem.

If vacuum is good and ignition timing is correct, then it is most likely a fuel problem. You could have plugged jets in the carb causing the engine to run lean. a set of 60 to 80 drills can be used to open the jets. The opening in the jets can be increased in size with these drill also. Be very carefull with opening the jets. It's a pain to solder the jets shut and redrill them.

Bob

[Mark Shaw]

I agree with Bob.

To check for vacuum leaks around the intake manifold, spray WD-40 around the gaskets while the engine is idling. If the engine speeds up, you found a leak....

[Guest sndtubes]

I'll try looking for a vacuum leak at the intake manifold. It's the only likely place there could be one. I'm thinking that the jets may need to be opened up as Bob suggests. Since the car has ALWAYS run like this, I think its more of a problem like that rather than a transient problem like a leaky manifold gasket. I'll try the WD40 thing tomorrow and report back.

[jscheib]

Michael,

My '32 has a similar problem but not nearly as severe and I think my problem is the vacuum line to the wiper which I need to address. I will also review the other hints, so thanks to all for posting. I know you cover a lot more ground then my car.

John

[Guest sndtubes]

Michael,

My '32 has a similar problem but not nearly as severe and I think my problem is the vacuum line to the wiper which I need to address. I will also review the other hints, so thanks to all for posting. I know you cover a lot more ground then my car.

John

Oooo, I'd love to own a 32! My vacuum wiper port is currently disabled with a plug. It doesn't leak at all. But a vacuum leak somewhere could cause problems. Unfortuantely I don't think my problem is a vacuum leak.

[50jetback]

Well, the heat riser tube was replaced by a machine shop who specializes in working on manifolds. There shouldn't be any vacuum leak there or really anywhere else I can think of......... The windshield wiper motor is diasbled and there is a tight fitting plug on the wiper vacuum tube. No leaks there.

Did they replace the heat tube with a N.O.S. item or make up a new tube from stock materials?

The internal diameter of the tube is critical to the carb settings, if the walls of the tube are thicker than the original ( which are quiet thin and why they rust/burn out ) it will make setting the carb difficult.

The air valve spring is also critical and shouldn't be altered - i.e. don't try and stretch it. New ones are available and should always be used when rebuilding a Marvel carb.

The other thing you can check is to make sure the heat riser casting hasn't been cracked when the replacement heat tube was pressed in by the machine shop. I had this occur with my 1928 Roadster and it exhibited the symptons you describe.

[Buick Downunder]

I did. None of them matched my problem.

Not a very helpful response.

Michael - I think the post from above meant 'check your ignition points' - not check your 'posts' which is a good call too. If points are too far open or weak, sometimes you need to 'choke it up' to get it to run OK.

I have had similar problems with the marvel on and off - but it is almost always muck in the carb, points or timing. If you suggest that yours has ALWAYS run this way, and requires excessive choke, than it sounds like a jet or vacuum related problem, as others have said. You reallt can't go to wrong with a marvel if you know the basics (and these too have been well described here by others).

Vacuum leaks must be tested with a guage to be sure. Jets can be tested with a drill. I would suggest that you go to the effort of checking these before running in circles on the twenty or so things that it could be from here.

Best of luck.

[Bob Engle]

If you go into the carburetor, you should look at the tubes that hold the jets. I have seen a number of these with cracks in the tubes. This is caused by embrittlement of the brass over the years. They can be soldered by a patient person. If these are cracked, fuel can leake out below the venturie and you get a poor fuel air misture. Choking the carb will increase the manifold vacuum and improve the condition a little bit.

One other item, If you do have an intake manifold gasket leak, do NOT just tighten the manifold bolts. You will only succeed in cracking your exhaust manifold. Follow the proper procedure for installing a new gasket. If you remove the manifold, check that there is a guide ring in each runner to the cylinder head. If any of these are missing, it will increase the chance of an air leak.

Bob

[Guest sndtubes]

Quick update:

I squirted WD40 all around the intake while the car was running. No change in engine speed, so I dont' think the problem is a vacuum leak at the intake gaskets

I put my hand over the heat tube inlet. I could feel no vacuum there, so I dont' think the heat tube is leaking

I can put my hand on carb air intake while running. The car immediately dies. So, I'm not so sure there is a vacuum leak. I'm going to try to find a vacuum gauge and check the vacuum at the wiper inlet port. At present that port is closed off with a small hose (not leaking)

I'm becoming suspect of the air valve. I'm wondering if it is working properly. When the carb was rebuilt a few years ago, the spring was replaced with the correct one.

More updates soon. Thanks for all of the great suggestions.

[Mark Shaw]

Mike,

You should also check the float level which should be just below the lowest jet.

[nsbrassnut]

Marvels.

I recently had my first experiences with these dreaded beasts. I friend recently picked up a neat 1930 Essex roadster which had been stored un-used for the last 25+ years. When cleaned up and first run it barely run and then only on near full choke. It took three sessions for us to figure out the basics. It has the die cast body and air valve. Problems, the air valve would not move freely on the hinge pin. Careful filing of the seal ridge on the air valve flapper was require to restore free movement. Ran but still required choke.

Next, the passages looked good, but we found build up in the low speed jet. Carb cleaner and fine wire cleaned out the barely visible varnish. Started up and pushed the choke in and it stay running. We took it around the block and he said this was the first time it made it without stalling and quiting.

Next round is to better tune the mixture etc to reduce some smoking.

It was a great help that he had a spare carburetor that came with the car. We took it apart and put it together twice for practice to see how to work on the parts before attempting to fix the one on the car.

Key learning, the air valve flapper distorts with age and can bind inside the body. It should be check on occasion by removing the carb from the car and checking for free movement of the valve and piston.

Looking forward to learning more as we get his Essex to run properly.

Drive Safe

Jeff

[Guest lonewolf900]

nsbrassnut

Thank You for your clear concise solution to my problem!

lonewolf900

[arcateq]

Since I cannot seem to find a Marvel Carb. does anyone have a recommendation on a replacement Carb for the 1930 Buick Series 40 ?

[tigersdad]

Jeff -

 

Thanks for your findings in regards to the Marvel carburetor.  I think answers like this will help all of us down the road.

 

Jay

1932 8-86 Victoria Travelers Coupe

[Mark Shaw]

Try here:  http://www.zenithfuelsystems.com/prod_carburetor.htm

 

[147 Franklin Airman]

On 7/28/2010 at 6:34 PM, sndtubes said:

Quick update:

I squirted WD40 all around the intake while the car was running. No change in engine speed, so I dont' think the problem is a vacuum leak at the intake gaskets

I put my hand over the heat tube inlet. I could feel no vacuum there, so I dont' think the heat tube is leaking

I can put my hand on carb air intake while running. The car immediately dies. So, I'm not so sure there is a vacuum leak. I'm going to try to find a vacuum gauge and check the vacuum at the wiper inlet port. At present that port is closed off with a small hose (not leaking)

I'm becoming suspect of the air valve. I'm wondering if it is working properly. When the carb was rebuilt a few years ago, the spring was replaced with the correct one.

More updates soon. Thanks for all of the great suggestions.

Have you been able to resolve your issue with the popping thru the carb. I would be very interested to hear.

[Hubert_25-25]

I would like to know your vacuum gauge reading (mentioned earlier but I have not seen a number).  It should be a pretty steady needle between 18 and 19 inches Hg at idle.  On my car, I also had some fine rust particles in the hollow bolt that is the air valve adjuster.  This was preventing the air valve from moving easily. 

Start with diagnostics.  Buy or borrow these tools if you do not have them.

1) vacuum gauge reading

2) Check the point gap with a dwell meter or feeler gauge.  

3) check the timing at idle with a timing light

4) pull all the plugs and do a compression test of each cylinder

Make a test of the carburetor the last item after verification that the first 4 items are known.

 

Edited September 20, 2021 by Hubert_25-25 (see edit history)

[Morgan Wright]

21 hours ago, Hubert_25-25 said:

I would like to know your vacuum gauge reading (mentioned earlier but I have not seen a number).  It should be a pretty steady needle between 18 and 19 mm Hg at idle.  On my car, I also had some fine rust particles in the hollow bolt that is t

 

 

Is that mm Hg or inches Hg? 

 

18 mm Hg sounds like a glaucoma test result

[Hubert_25-25]

Corrected - inches.

[147 Franklin Airman]

On 9/19/2021 at 3:58 PM, Hubert_25-25 said:

I would like to know your vacuum gauge reading (mentioned earlier but I have not seen a number).  It should be a pretty steady needle between 18 and 19 inches Hg at idle.  On my car, I also had some fine rust particles in the hollow bolt that is the air valve adjuster.  This was preventing the air valve from moving easily. 

Start with diagnostics.  Buy or borrow these tools if you do not have them.

1) vacuum gauge reading

2) Check the point gap with a dwell meter or feeler gauge.  

3) check the timing at idle with a timing light

4) pull all the plugs and do a compression test of each cylinder

Make a test of the carburetor the last item after verification that the first 4 items are known.

 

That's interesting. I can only get 15 "Hg which indicates late timing on my gauge but unable to get it any higher. Been living with this for a long time now & have to run with the choke out.

[Hubert_25-25]

15 inched Hg is not adequate, so do a few other tests. 

1) Is this reading using a tee to keep the vacuum tank in service, or is the gauge attached directly to the intake manifold and the engine is just running on the fuel in the vacuum tank?  We need to make sure that the vacuum tank is not causing any vacuum leak reading issues.  

2) If you can get the car to idle around 700 rpm or lower, put a timing light on it.  Top dead Center TDC is the 1-6 line on the flywheel.  Retard the spark lever fully.  At this speed, the mark should be at two flywheel teeth from the 1-6 line.  The goal is 8 degrees before TDC. 

       Then 2 more related tests:

a)  move the spark advance lever slowly. the 1-6 line should get more teeth away from the timing mark.   This indicates that your manual advance is working properly.   If during the test it crosses the 1-6 line - then your initial timing is at 8 degrees after TDC, or the spark linkage is installed incorrectly.  

b) Now go back to full retarded spark.  increase the engine speed slowly. the 1-6 line should get more teeth away from the timing mark.  This indicated the centrifugal spark advance is working.  

3) Have you inspected the potmetal venturi in the carburetor for growth?  Will the air valve lay smooth against the inside of the carburetor bore and still have a "tail gap" on the venturi end. 

Hugh

Edited September 21, 2021 by Hubert_25-25 (see edit history)

[147 Franklin Airman]

Hi Hubert

Thanks for your detailed explanation. 

My model 29-25 doesn't have a vacuum fed fuel supply, it has a fuel pump. 

Apart from skimming the block & head, reseating the valves, I have tried using a down draft single barrel Rochester carburetor that had been calibrated with the same result. Done a smoke test looking for inlet leaks. Blocked the inlet for the vacuum wipers. 

Has the fibre cam gear ever been known to slip on the cam shaft? 

My idle jet is a little off centre in the venturi. 

[Hubert_25-25]

If you installed another carburetor, and the vacuum reading was unchanged, then I would rule out the carburetor.  The venturi air block looks like it was replaced though it is oddly copper colored.  The off centered venturi hole is not ideal but I do not know the real effect of that.  Next time the carb is off and the float is removed, put a light behind it.  You should see an even air valve tail gap against the venturi block.  Then no light between the air valve and the body bore (photo below).

 

I do not think the probability of a slipped timing gear is very high.  The original timing gears had steel center hubs with fiber teeth.  The replacements are all fiber.  I have not heard of keyway failures with the newer camshaft gears.  Below is a description on how to check the valve timing.  The inlet valve on 1 or 6 should be just beginning to open before TDC (Top Dead Center)(the 1-6 line). This is about the width of a flywheel tooth or less. 

Remove the valve cover or the side cover.   From the 1925 Buick shop manual (with some paraphrasing).

"The point at which the inlet valve begins to open can be accurately determined by inserting a piece of letter paper between the valve rocker and the valve stem.  Move the paper in the gap. " Roll the engine by hand.  "As soon as the paper can no longer be freely moved, the valve is beginning to open."    

So the inlet valve begins to open just miliseconds (the width of a flywheel tooth) before the engine rotates to TDC.  On the flywheel picture, Below the 1-6 TDC mark in the window is before TDC.  Above the 1-6 line is after TDC.   

Notice in the last engineering drawing, the inlet valve opens when the piston is 2 thousandths of an inch before TDC.

 Hugh

 

    

Edited September 22, 2021 by Hubert_25-25 (see edit history)

[147 Franklin Airman]

6 hours ago, Hubert_25-25 said:

If you installed another carburetor, and the vacuum reading was unchanged, then I would rule out the carburetor.  The venturi air block looks like it was replaced though it is oddly copper colored.  The off centered venturi hole is not ideal but I do not know the real effect of that.  Next time the carb is off and the float is removed, put a light behind it.  You should see an even air valve tail gap against the venturi block.  Then no light between the air valve and the body bore (photo below).

 

I do not think the probability of a slipped timing gear is very high.  The original timing gears had steel center hubs with fiber teeth.  The replacements are all fiber.  I have not heard of keyway failures with the newer camshaft gears.  Below is a description on how to check the valve timing.  The inlet valve on 1 or 6 should be just beginning to open before TDC (Top Dead Center)(the 1-6 line). This is about the width of a flywheel tooth or less. 

Remove the valve cover or the side cover.   From the 1925 Buick shop manual (with some paraphrasing).

"The point at which the inlet valve begins to open can be accurately determined by inserting a piece of letter paper between the valve rocker and the valve stem.  Move the paper in the gap. " Roll the engine by hand.  "As soon as the paper can no longer be freely moved, the valve is beginning to open."    

So the inlet valve begins to open just miliseconds (the width of a flywheel tooth) before the engine rotates to TDC.  On the flywheel picture, Below the 1-6 TDC mark in the window is before TDC.  Above the 1-6 line is after TDC.   

Notice in the last engineering drawing, the inlet valve opens when the piston is 2 thousandths of an inch before TDC.

 Hugh

 

    

Many thanks for this information Hugh. It gives me something concrete to work on as the problem is driving me made having spent a lot of money trying to resolve it. 

I will have to try a locate a workshop manual somewhere as the one I have is not as detailed as the one you have. 

[147 Franklin Airman]

On 9/22/2021 at 5:34 AM, Hubert_25-25 said:

If you installed another carburetor, and the vacuum reading was unchanged, then I would rule out the carburetor.  The venturi air block looks like it was replaced though it is oddly copper colored.  The off centered venturi hole is not ideal but I do not know the real effect of that.  Next time the carb is off and the float is removed, put a light behind it.  You should see an even air valve tail gap against the venturi block.  Then no light between the air valve and the body bore (photo below).

 

I do not think the probability of a slipped timing gear is very high.  The original timing gears had steel center hubs with fiber teeth.  The replacements are all fiber.  I have not heard of keyway failures with the newer camshaft gears.  Below is a description on how to check the valve timing.  The inlet valve on 1 or 6 should be just beginning to open before TDC (Top Dead Center)(the 1-6 line). This is about the width of a flywheel tooth or less. 

Remove the valve cover or the side cover.   From the 1925 Buick shop manual (with some paraphrasing).

"The point at which the inlet valve begins to open can be accurately determined by inserting a piece of letter paper between the valve rocker and the valve stem.  Move the paper in the gap. " Roll the engine by hand.  "As soon as the paper can no longer be freely moved, the valve is beginning to open."    

So the inlet valve begins to open just miliseconds (the width of a flywheel tooth) before the engine rotates to TDC.  On the flywheel picture, Below the 1-6 TDC mark in the window is before TDC.  Above the 1-6 line is after TDC.   

Notice in the last engineering drawing, the inlet valve opens when the piston is 2 thousandths of an inch before TDC.

 Hugh

 

    

I think I have found my problem. The piece of tape in the photo is the position where No 1 inlet valve opens. 

Why it is so far out is something I need to find out. I also wonder how the engine has ever run like this. 

[Hubert_25-25]

So I see a line just under "ADV-17" .  That is 17 degrees before TDC.  Then it looks like a blob of white at about 30 degrees before TDC.  That says your cam is advanced either 17 degrees or 30 degrees when it should be at about 1/2 degree advanced. 

- Did the timing gear hub move, or the keyway move?

- Did a person not put the gear mesh in the correct spot?  Maybe sold the car when they could not get it to run right? I remember that I spent a lot of time trying to figure out that the gears were meshed in the correct place.  I should have done the inlet valve check as verification that the gears are meshed correctly.  

Do one more verification that the 1-6 line is TDC.  If so,

Pull the radiator, support and lift the front of the engine.  Remove the timing cover.  Now is a good time to replace the timing cover felt seal with a lip seal.  Inspect that gear thoroughly.  The drawing that I posted earlier shows the keyway slot on the camshaft gear relative to the camshaft.    

Photo below shows the correct puller that Larry DiBarry used.  This is for pulling a gear that has a metal hub.  Not sure how you pull the gear with no metal hub.  You should be able to leave the camshaft in place if you just have to replace the gear.

Hugh

[147 Franklin Airman]

Thanks Hugh. I should have stated that the blob of tape below the 17 deg mark was the point that the valve started to open. I moved the fly wheel just to show its position in relation to the 17 deg mark. 

I will be stripping the car down shortly & will get report my findings. (after I have confirmed that the piston is at the top in relation to the TDC mark on the fly wheel) 

[Hubert_25-25]

More thoughts on cam gear pulling.     Hugh

 

[147 Franklin Airman]

UPDATE. 

I have a dilemma, with the flywheel at TDC, the marks on the crank gear & timing gear are not lining up. They do when turning the crank. Now I'm thinking that the flywheel has been mounted in the wrong position but this does not explain my problem. I don't  have much hair left to pull out. 

Just checked again & it can't be the flywheel off position as piston 1 was at TDC. 

 

LATEST

 

I realigned the crank gear so that cyl 1 inlet valve started to open with cyl 1 piston at TDC. The engine idled badly. Vacuum was 5"hg.

Edited October 11, 2021 by 147 Franklin Airman
Added photos (see edit history)

[147 Franklin Airman]

I have since found that on my 116" the inlet opens at 10 deg & not 2 deg as Hugh mentioned 

[Larry Schramm]

Here is a chart of what vacuum readings can mean on the engine condition and what to look for.  Maybe it will help.

[Hubert_25-25]

1) When the #1 piston is at TDC, the 1-6 line should be on the mark in the window.   
Is this correct?  
 

2). Glad that you found the correct drawing indicating valve opening.  
 

3) the timing marks are not supposed to line up when you are on TDC.  I do not know when Buick started doing that, but it is not that way in 1925.  
 

My computer has crashed and I hope to help you more when I get it back so sorry for the delayed response.  
 

Study the diagram for your year and verify your cam is matching up - not just the gears.  
 

Hugh

[147 Franklin Airman]

10 hours ago, Hubert_25-25 said:

1) When the #1 piston is at TDC, the 1-6 line should be on the mark in the window.   
Is this correct?  
 

2). Glad that you found the correct drawing indicating valve opening.  
 

3) the timing marks are not supposed to line up when you are on TDC.  I do not know when Buick started doing that, but it is not that way in 1925.  
 

My computer has crashed and I hope to help you more when I get it back so sorry for the delayed response.  
 

Study the diagram for your year and verify your cam is matching up - not just the gears.  
 

Hugh

Thanks Hugh 

Hope you get your pc fixed. 

 

So after a lot of moving gears around I have established that the piston is at TDC in relation to the mark on the flywheel and that the camshaft is correct. When I used the paper test it did show as 30 deg out but when I used a dual gauge on top of the tappet spring retainer then the valve started to open at the correct time I. E just before TDC so I'm happy with that. I also removed the cam followers & camshaft and all looks good. This still leaves me with the same issue so I have gone back to the carburetor (photo attached shows the gap I have which is down to wear) I know the issue remained after switching to a downdraft carburetor so I am now picking at straws as there isn't much left for me to check. 

[michealbernal]

The problem with these Marvel carbs is that some of the parts are made of pot metal,,Pot metal adsorbs oxygen from the air and "grows" over time .  It does not take very long either.  From your photos that appears to be what has occurred in this case.  Consider looking back in this forum at some of the prior postings on Marvel carbs.  You might find them helpful.    

You might also try to obtain a copy of the booklet "Carburetion in relation to service, how to overhaul a Marvel Carburetor" by the Marvel Co.  It contains a wealth of information .  I think I got mine from Bobs Automobelia.