Marvel carburetor

[Rich Dayton]

Marvel carb problem: 1928 Buick master on road under load ( uphill) will pop back thru dust collector ( air cleaner ) carb rebuilt . New plugs ,points,condenser, set valves, check timing . Cleaned screen in gas bowl and screen on inlet line to vacuum tank . Ran ok before changed carb .( carb float seat not working .)   Help.  Thanks.  Rich 

[raydurr]

I have a 1929 Master so I am familiar with your issue. Even though this condition could be many things , there are a few things one can do to try to isolate the cause.

The wonderful Marvel carburetors can be a pain. I usually do not choose the easy route in life. I do prefer the challenge of originality in this area.

My Marvel sometimes tends to run lean and pop back through the carburetor under acceleration or heavy load . When warmed up and under load , I can correct this by slightly and progressively closing the choke until full power can be made.  If this corrects the problem it proves one thing. It proves that the ignition is working well enough to make power. It also proves that fuel delivery is adequate to make power. The issues that remain causing this lean condition are probably blocked jets , carb venture sticking / being limited in travel or the venture spring not being correct or incorrectly adjusted. I still sometimes have issues getting my carburetor to work correctly. Sometimes the enrichment circuit that the plunger is lifted out of can be a problem too. If choking of the carburetor under load has no affect on performance then ignition and fuel delivery can be an issue. One last thing is to always do your tuning with fuel that is fresh as possible.

[Mark Shaw]

It is most likely starving for gas, but you also did not mention if you checked or replaced the heat riser tube.

[raydurr]

Yes, as Mark stated the heat riser tube can cause total chaos in the tuning and performance department. The heat riser repair and elimination of exhaust gas to it , is a little story of its own.

[Hubert_25-25]

Rich, 

     Since you are new, I am reposting my marvel carb and heat riser information.  I hope it will help you with understanding that there is actually a fair amount of work required to correct the concerns of heat riser pinholes, exhaust valve positioning, and venturi swelling.   The engine should also read 13-14 inches hg vacuum and steady if that helps you with trouble shooting.     Hugh

Edited May 24, 2019 by Hubert_25-25 (see edit history)

[147 Franklin Airman]

Thanks for the info on the carb Hugh although that model is slightly different to mine being on a 29-25 where mine has 3 jets (idle, intermediate & high speed)

I have similar issues and its driving me crazy. My problem is that I can get the engine to idle nicely but when trying to pull off or drive up a hill she will either backfire through the carburetor or the exhaust.

I have done almost everything possible except what is causing this and now I'm thinking that carb has run its life and need changing. I will list what I have done in the hope that someone has an answer.

What I have noticed recently is that to pull off the choke is needed but to accelerate hard then the choke needs pushing back in, so clearly she is running lean but only when at cruising speed.

 

ENGINE WORK DONE

Valve seats ground

Valves cut & lapped in

Head skimmed

New head gasket

Timing adjusted both by timing light & trial & error

 

CARBURETOR WORK DONE

Carburetor stripped & cleaned

Idle needle replaced

Float level raised

Metering Pin seat replaced (worn after years of use) 

All work on carb as listed on Hughs post done.

 

HEAT RISER

New stainless steel tube fitted (now 34mm ID diameter)

Both exhaust ends sealed

Heat riser tube pressure tested

 

Electric fuel pump fitted near the tank in line with the mechanical one.

 

Now considering fitting a Rochester single barrel carburetor.

 

Any suggestions will be very welcome 🙂

Edited June 19, 2019 by 147 Franklin Airman (see edit history)

[Mark Shaw]

1 hour ago, 147 Franklin Airman said:

Electric fuel pump fitted near the tank in line with the mechanical one.

Are you using a pressure regulator?  They are often the cause of fuel delivery issues by not accurately providing 2-3 psi to updraft carbs.  I suggest you try installing a temporary pony tank high on the firewall with gravity feed to the carb.  This will reveal weather fuel pressure to the carb is the problem. 

[147 Franklin Airman]

1 minute ago, Mark Shaw said:

Are you using a pressure regulator?  They are often the cause of fuel delivery issues by not accurately providing 2-3 psi to updraft carbs.  I suggest you try installing a temporary pony tank high on the firewall with gravity feed to the carb.  This will reveal weather fuel pressure to the carb is the problem. 

Thanks for your reply Mark. I only use the electric pump when priming the system prior to starting but its is a low pressure Solex pump.

[Mark Shaw]

Just now, 147 Franklin Airman said:

its is a low pressure Solex pump

I would still run a test with a pony tank to verify fuel delivery is correct.  Otherwise, you are trusting Solex to know what low pressure is.

[147 Franklin Airman]

1 hour ago, Mark Shaw said:

I would still run a test with a pony tank to verify fuel delivery is correct.  Otherwise, you are trusting Solex to know what low pressure is.

I will give that a try. Running out of ideas now, although I have just noticed that the thickness of the gasket between the carb & the heat riser would effect the timing of the opening of the metering pin in relation to the main butterfly on the heat riser. Going to make a new thicker gasket to see if it improves.

[Guest]

"Back fire" through the carb is caused by a lean mixture.  "After fire" in the muffler and some times in the exhaust pipe is caused by a rich mixture.  I have no experience with a three jet Marvel,  just 60 years and 500,000 miles with a two jet Marvel so can not help with adjustments.  It does seem logical that the question is why/when/how does your carb go from rich to lean when the engine is running.

Good luck with solving your problem.

[147 Franklin Airman]

Thank you everyone for your suggestions. I thought I would update on what I have found/done.

Since I couldn't solve my issues with my Marvel carb I purchased a Rochester B series carburetor (picture attached) Marvel bottle is the pony tank.

Results:

Using both the main fuel tank and a pony tank the runs but is lumpy at idle and low speeds, she climbs hills no problem but I still have to have the choke out a little. In doing so she is smoking indicating that she is running rich. When cruising at 30mph or slower she runs lumpy but will smooth out when pushing the choke in but only for a few seconds then I have to pull the choke out again as she spits through the carb.

 

Has anyone else experienced this? I have done so much work on this engine but my original problem is still there. I don't have any hair left to pull out.

[Hubert_25-25]

If you have swapped carburetors and the issue still persists, then it is likely an ignition problem.   I am also assuming that you have good compression.  

The firing order is backwards of convention.  1-4-2-6-3-5.  Check that this matches while cranking the engine.  

Since it runs, you should be able to put a timing light on the engine.  Any 12 volt battery will power the light.  Just clip it on #1 wire.  At idle, The timing should be around the TDC mark when you are on full retard on the steering wheel lever.   For 1928, basic timing is at the 17 degree mark before TDC with the lever on full advance at idle.    

Is the distributor rotating the full amount when the spark advance lever is turned the full amount.

As Larry DiBarry also found recently, his plug wires were bad and firing into each other.  They were not old wires either.  Look for sparks jumping in the dark.  If all is good, and you run your hand near the plug wires, they should not spark you.

Hugh

 

[147 Franklin Airman]

3 minutes ago, Hubert_25-25 said:

If you have swapped carburetors and the issue still persists, then it is likely an ignition problem.   I am also assuming that you have good compression.  

The firing order is backwards of convention.  1-4-2-6-3-5.  Check that this matches while cranking the engine.  

Since it runs, you should be able to put a timing light on the engine.  Any 12 volt battery will power the light.  Just clip it on #1 wire.  At idle, The timing should be around the TDC mark when you are on full retard on the steering wheel lever.   For 1928, basic timing is at the 17 degree mark before TDC with the lever on full advance at idle.    

Is the distributor rotating the full amount when the spark advance lever is turned the full amount.

As Larry DiBarry also found recently, his plug wires were bad and firing into each other.  They were not old wires either.  Look for sparks jumping in the dark.  If all is good, and you run your hand near the plug wires, they should not spark you.

Hugh

 

Thanks Hugh

That is useful information regarding the setting of the timing with the steering wheel lever, thank you. I have had to do the timing by feel as could never see the 17 deg mark with my timing light. 

All the plugs were new as are the leads but I have just ordered new ones, I also removed the spark lead cover and separated all the leads in case I had cross firing.

Compression since servicing the head is now 79-82 psi across all cylinders.

Martin

[Hubert_25-25]

Martin, 

     I always paint my timing marks.  A white dot with model paint and a model brush.  Find TDC #1 by the hand crank.  Use scotch brite in the timing hole on the flywheel and you should be able to clean up one of the timing marks, either the TDC or the 17 BTDC.  It is about 2 degrees per tooth.     Hugh

[147 Franklin Airman]

On 7/21/2019 at 6:22 PM, Hubert_25-25 said:

Martin, 

     I always paint my timing marks.  A white dot with model paint and a model brush.  Find TDC #1 by the hand crank.  Use scotch brite in the timing hole on the flywheel and you should be able to clean up one of the timing marks, either the TDC or the 17 BTDC.  It is about 2 degrees per tooth.     Hugh

Hugh

I set the timing at 17 BTDC but she doesn't like it at all, I had to retard it a little to get her to run smoother although she is still lumpy. I assume this is due to the higher octane fuel.

[Hubert_25-25]

Martin, 

     It is not the high octane fuel causing the lumpiness.  There is something else.  

    On my car, the previous owner installed the spark advance to work in reverse  - Like a Model A Ford.  This is easy to do.  On your car, the timing is set using 17 degrees Before TDC, when the lever is in the fully advanced position.   Do this with a timing light at idle.   I know this is a weird way of doing it, but that was how it would have been done , only using a modern timing light.  Now if you put the advance lever in the full retard position - again at idle, you should be around the 1-6 mark or at TDC.  Possibly even in the after TDC position by a few degrees - but no more than 3 teeth.   

One other thing.  Set your timing at idle with the light.  Rev the engine to around 1500 rpm.  You should see the mark on the flywheel move further advanced or even further away from the TDC mark.  That would be how you check the mechanical advance.  

Any idea what a vacuum gauge reading on your engine looks like.

Hugh

[147 Franklin Airman]

Hugh, I meant that the higher octane fuel could be the reason that the timing needs to be less than 17 deg.

I have used a vacuum gauge and she reads a reasonably steady 18 inches of mercury. I believe that a reading between 15 -20 indicates all is good.

I'm going to change the ignition coil next.

Martin