More Marvel questions

[drovak]

I'm (still) trying to figure out why my specific Marvel carburetor is giving me issues. Car runs alright if the choke is half pulled, but I feel like I'm giving up power doing that, not to mention it's just not right. 

 

Firstly, what are the correct dimensions for the air spring for a 1931 Series 80 carburetor? My carb casting is a 10-796.

 

 

I figure if the spring is tired, that could be part of my issue. 

 

Secondly, someone at Hershey mentioned drilling something out to improve the mixture. Was it the metering pin jet, marked 84-091-C in the booklet?

 

 

It does appear I have the correct hardware in my unit, so that reassuring. 

 

Lastly, the booklet seems to claim this part is 15-106, a 1/4" channel plug screw:

 

 

Is this really what it's supposed to look like? Does it have any impact on helping to restrict the flow into the passage to the intermediate and high speed jets?

 

 

The diameter of the pin portion measures 0.087".

 

I'd be much more willing to perform irreversible modifications on the metering pin jet if I had a spare. Is this something that is available, or do I need to start talking to a machinist?

 

Are there carburetor shops that can actually test these units on a flow bench?

[Gary W]

Being you have all the correct hardware, first check that your jets are not blocked.  You need to know the proper orifice measurement for each jet.  Don't drill anything out until you know what each jet should be opened to.

 

My recent experience is with my Model "A" Zenith carburetor.  I purchased a jet drill kit and a welding tip cleaning kit. (A series of very fine files like piano wire, each specifically sized). 

 

 

I always start undersize to remove any internal crud, varnish... before going to the actual correct size.

You can affect performance if you open the jets too wide.  

If they are too wide, you may have to solder the tip closed and resize them to the correct orifice.

But don't start opening up jets until you know the specs.

 

 

I could not get it running smooth.  I removed the main jet to discover this fine hairline split.

 

 

This was allowing air in when running, and allowing gas to leak out when sitting. 

I swapped it out for another and she's just fine.

 

So, clean all the jets, study them for any fine cracks or leaks and be sure they are sized properly.

 

Then, check for proper length of your jets, and that your fuel float level is proper.

 

If you have to hold the choke to get it to run properly, it seems that you are increasing the vacuum inside the carburetor creating a greater "pull" on the jets to get the proper amount of fuel.  

Hopefully its just a clogged jet or a blocked internal passageway preventing the fuel from reaching the jet.

[drovak]

Thanks, Gary. I should've given a little background to what has already been done. 

 

The carb has been cleaned multiple times, ensuring jets are free and clear. I replaced the float and adjusted the fuel level so that gas is right below the orifice of the idle jets. I resleeved the heat riser and have checked for vacuum leaks by spraying ether around the intake manifold and such. The air valve spring looks correct for this model, but again, I have no idea if it's been compressed over time. The air valves seem to have a fairly tight clearance against the walls of the carburetor. One venturi block has been replaced prior to my ownership, and the other one appears fine. Wish they had replaced both... I recall from a year or more ago that the jets are indeed the correct sizes for this carburetor, and they do not appear to have been modified in any way. 

 

I might be forgetting something, but that should be most of it. 

[Mark Shaw]

Running at half choke is a typical condition until the engine is fully up to temperature.  If you are driving it less than a mile or so, it will not warm up enough to push in the choke.  

[drovak]

The engine was plenty warm in my drive to the show and back, and opening the choke any more than halfway immediately resulted in backfiring. 

[drovak]

I've double-checked the jets:

Idle jet is marked 120A—correct.

Intermediate is marked 120-E-24—correct.

High speed is marked 120-C-26—correct.

 

I've ordered some brass 1/4-28 bolts so that I can work on turning them into new metering pin jets to play with the flow more easily without harming original parts. I figure it's less risky than tweaking the only intermediate and high speed jets I have. 

 

The air spring measures 0.4" in diameter, 1.5" long, with 14 turns. Does this sound correct?

[32buick67]

Drovak,

You seem to know what you are doing, nicely done so far with checking for correct jets, float elevation, etc. and your spring is correct (don't mess with that spring).

What is your engine warm idle?  I seem to recall its about 340rpm per spec, but it might be better to raise it to about 400-500rpm until the carb and timing are sorted.

Be sure the idle adjust screw and lock screw are in place to avoid chasing ghosts.

 

My initial hunch is that your timing is too retarded, and your venturi IDs might be non-concentric (teardrop or egg-shaped).

An incorrect/non-concentric venturi bore ID will make your engine run lean (not sufficient vacuum to lift and vaporize fuel from the low speed nozzle), which requires choke to compensate.

Can you confirm your carb body casting is 10-196 with 111-3 venturi blocks?

 

Warm idle timing - you can quickly prove whether the timing is slightly too retarded by loosening the distributor screw and slowly rotating the distributor a few degrees and listen whether the engine perks up and idles strong and smooth.  If it does, the lock the distributor and push in the choke and see if the engine idles smooth.  Then pull the spark knob to full retard and see if the engine idle drops, and continues to run smooth.

I should also ask if your distributor advance weights and springs are moving freely?  They will start to move around 400-500rpm if I recall correctly from memory.

If the engine idle does not perk up, then the timing was likely pretty close.

 

Additional carb setup factors for your engine which can influence airflow and vaporization - 

• Each air valve tail clearance to venturi spacer block face = 0.018-0.022"
• Each air valve seating to carb body ID bore wall should be tight and allow little to no light around the valve
• HS jet clearance from carb body ID bore wall should be 0.017"
• Correct venturi spacer block #111-3 (applies at idle and other speeds)
• Correct metering pin jet 84-091C (only applies above idling speed)
• Metering pin diameter 0.075"
• Venturi bore ID 0.0343" - use a caliper for this, its surprising how much this bore shape and size matters
• Follow the Marvel manual and set the air valve screw to be flush with the metal retainer tab to start your tuning
• Also follow the Marvel manual with the carb in hand to ensure the proper amount of air valve free play (1/16"-1/8")

The brass etc. doesn't change over time - I highly recommend avoiding mods to brass and other parts which are nearly original factory condition.

Focus on what changed in the last 91 years...

Remember, these cars originally ran well to sell them - only three dependent variables changed over time - venturi blocks swell and change geometric shape, fuel is different today, timing is always changing.

 

Here is some info about the Marvel setup which is interesting and might be helpful in the future.

I think you are not far from a well-running engine, good luck!

[drovak]

27 minutes ago, 32buick67 said:

My initial hunch is that your timing is too retarded

I've set the timing a couple of times statically, as per the book. I haven't tried using a timing gun, nor have I assessed if the distributor advance is working correctly, but I should probably look at that. That said, I've never complained about idle performance, except when I richen the mixture with the air screw to the point where it doesn't backfire, the idle mixture is way too rich. 

 

29 minutes ago, 32buick67 said:

venturi IDs might be non-concentric

One venturi block has been replaced. It is concentric. The other one may show a slight eccentricity, but it's very slight. The bore is 0.345" or so at the tightest, best I can measure. Maybe it'll make all the difference in replacing the remaining original, and I'm certainly not opposed to getting a new one. Who has fabricated these for the '31s? I met someone at Hershey who seemed to specialize in the earlier models. 

 

 

36 minutes ago, 32buick67 said:

Can you confirm your carb body casting is 10-196 with 111-3 venturi blocks?

Confirmed. 

 

 

 

But, despite the singularly replaced venturi block, it looks like the idle jet sits off center.

 

 

The high speed jets seem to sit further away from the walls of the bore than 0.017"—seems it's double that, perhaps. 

[32buick67]

The venturi block for cyls 3,4,5,6 looks like the replacement- can you confirm the ID is hourglass shaped and the narrowest point is the 0.343" dim, and what is the widest dim at the top and bottom of the hourglass?

Also, the narrowest neck point of the venturi bore ID must be approx 0.172" up from the bottom of the block, otherwise the LS jet elwvation wrt the fuel level in the bowl will cause compression and not vacuum on the fuel sitting on the top of the LS nozzle well.

 

The nozzle needs to be centered in the ID, otherwise wicking and condensation buildup can occur, which block vaporization.  Either the block wasn't made properly and/or the LS nozzle needs to be bent slightly (not difficult, just patiently slow).

 

I spent a lot time researching venturi blocks for our cars, and I have new replacement 111-3 venturi blocks available if you think yours might need to be replaced.  There is a bit of info on the forum about this topic, search for Marvel venturi.

[drovak]

I would love to go ahead and replace both venturi blocks with new ones. While the replacement one I have is hourglass shaped and seems to roughly conform to your dimensions, I still think it wasn't quite right given how the idle jet sits off center. I'd rather give new blocks a try so they both match before I attempt bending an idle jet to be centered. Feel free to PM me. Thank you very much for your help!

[Mark Shaw]

23 hours ago, drovak said:

opening the choke any more than halfway immediately resulted in backfiring. 

This is a sign of weak fuel supply to the carb.  I suggest you remove the bowl cover and watch the fuel level as you increase throttle to see if the fuel level is reduced while accelerating or increasing choke.

[drovak]

12 minutes ago, Mark Shaw said:

watch the fuel level as you increase throttle

I was doing this a lot when adjusting the float level. I took out the electric fuel pump, which had too much pressure, and found an original fuel pump to rebuild at last year's Hershey. I discovered a pinhole leak near the top of the fuel pickup tube (the pickup is nowhere remotely close to original, as I've mentioned in another post) and repaired that for the time being. I haven't measured the pressure, but I feel like the mechanical pump is having no trouble keeping up with demand. However, next time this is all assembled, I will pay close attention to that. 

[Larry Schramm]

Drill kit seller.

 

https://www.snydersantiqueauto.com/product.htm?pid=986732&cat=41942

[Grant L. Meredith]

On my electric pump I added a Holley regulator with a T brass fitting to add a gauge. I run about 1-1.5 psi.