Marvel dashpot lubrication?

[drovak]

My '31 8-86 seemed like it would idle quite acceptably, but if you give it any accelerator, it immediately backfires through the carburetor. I feel like I've got the float level adjusted adequately, right near the top of the idle jets.

 

Could this be indicative of a loose dashpot? Does it require any sort of lubrication? The spring looks good to me.

 

I am aware of two small pinholes in my heat exchanger tubes, but I suspect those should mainly affect idle performance, considering how small they are. However, I'll be addressing that soon. 

 

I'm pretty sure my venturis look good, as well. Does it look like one was replaced prior to my ownership? One doesn't look like the other.

 

 

 

Another thing I wondered: how soon should the mixture valve open in the accelerator travel? I figure that might also cause similar symptoms if it's not opening soon enough...

 

Also, what are the appropriate jets for this carburetor? When I had it apart before, I seem to recall reading "120" on one of the HS jets. I'll have it apart again to double-check the others. 

 

 

Thanks!

[drovak]

Just checking again if anyone has any thoughts on the Marvel dashpot. How can one determine if it's functioning correctly? 

 

I suspect if the dashpot is slow to move, the mixture will remain richer for longer during acceleration, but will settle out under constant load. 

[Robert Engle]

The more common term for these parts is "air Valves".  You do not want to try any lube on these parts.  With the spring removed, the parts should be floppy loose as you turn the carb 90 degrees.  The most common problem with these die cast parts is that they swell with age and can bind.  there are some other threads on the Marvel carbs that show pictures of this problem.  In essence, if you shine light up from the bottom, you should see a very fine even light gap between the moving piece and the partial round mounting block.  If not, some careful metal removal can easily correct the problem.  Also, the moving parts should lie against the cylindrical wall with no air gap except at the jets.  Careful dressing of these moving parts can correct this problem.  

 

Bob Engle

[drovak]

The air valve assembly is loose, yes. I was specifically referring to the metal cylinder with the spring inside that is adjusted with the air screw, which the manual refers to as a "dash-pot."

 

 

The same thing in similar form exists in the SU carburetors in my MGB (which is lubricated with oil), and most commercial doors that prevent them from slamming shut. 

 

Do you think this is too much gap between the walls and the air valves?

 

[Robert Engle]

There should be no light coming through out toward the jet opening.  The right looks like it might be in contact with the fixed block and could cause it to hang up.  I would carefully dress these areas.  At idle, you want most of the air flowing over the idle jet.  on acceleration,  you want air flowing over the intermediate and high speed jets.

 

Bob Engle

[DGTryon]

Looks to me like one of the venturi's is installed upside down.  I believe the smooth side should be on top.   Minor issue though overall.

 

The dashpot only affects the A/F mixture for about 0.5 sec on an acceleration, similar to the acceleration pump on modern carburetors.  The backfire problem is more likely an air valve spring adjusted too low (lean or too far out).  This is not a sensitive adjustment, move it 1/2 turn increments and see what happens.  I had a 32-57 that ran great with the original Marvel.

 

[Mark Shaw]

3 hours ago, DGTryon said:

I had a 32-57 that ran great with the original Marvel.

I had a 31-57 & had the same experience.

[drovak]

4 hours ago, DGTryon said:

Looks to me like one of the venturi's is installed upside down.  I believe the smooth side should be on top.   Minor issue though overall.

One of the venturis is flat on both sides; the other (I assume original) appears to be cast. That said, checking the manual indicates the venturi is asymmetric, and the newer looking one that is flat on both sides does appear to be upside-down.

 

Note the narrow throat of the venturi lines up with the top of the idle jet:

 

 

In the cast unit, that seems to be the case:

 

 

In the machined unit, it appears upside-down:

 

 

Okay, the pictures make it hard to tell, I'll admit; it's easier when you have stereo vision with it in your hands!

 

 

 

4 hours ago, DGTryon said:

The backfire problem is more likely an air valve spring adjusted too low (lean or too far out). 

That's what I thought, and even with it screwed all of the way in, it backfires through the carb. Giving it half choke fixes it right up.

 

Anyways, at this point, I may be fighting an issue that may soon be solved—the rusty heat riser tubes. I hope to have those fixed soon...then it will be time for reassembly and testing. The question that keeps me up at night is, why would pinole leaks give me so many issues upon acceleration and not at idle, like I'd expect? My plugs were quite black, which to me, indicate a pretty rich mixture. And when it's run recently, it's been 98% at idle. 

[Robert Engle]

When you get it assembled and running, you may want to experiment with adding more advance than the book calls for.  You should never have to make much of an adjustment with the air valve.  1/4 turn from the nominal position is the most you should need.  

 

I plan to experiment with smaller jets this spring.  I know of several other people that say their cars runs rich.

 

Bob Engle