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31 Nash 880 twin 8 backfire


31nash880
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You probably have ignition and carburetor issues.......and with two problems at the same time, it’s going to be difficult to sort. The entire installation looks how shall I say.....trying to be polite and friendly..........not well done. Replacing any carburetor isn’t an easy task, jamming something on a car without consulting an expert.......and there are very few........is very problematic. Unless you put the car back to 100 percent stock......you will be chasing you tail for a long time to get it right. Backfiring from both ends indicates possible  ignition wires crossed, hung valves, bad valves, lean burn misfire, puddling of fuel in the manifold........it’s just too much going on to be able to offer much constructive help.

 

Distributor caps are available if you look.......I see them fairly regularly offered for sale, but they won’t be cheap. I’m guessing the distributor is dual point........has it been synchronized on a machine? 

Edited by edinmass (see edit history)
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We have generally found that backfiring through the carburetor is caused by faulty ignition, IF the carburetor is stock.

 

Having said that, the 1931 Nash 880 used 2 different downdraft carburetors, a Marvel DO, which was Marvel's first attempt at a downdraft; and a Stromberg DXR-2,probably a better carburetor than the Marvel, and was Stromberg's second attempt at a downdraft.

 

Stromberg freely admitted that the D series carbs (basically updraft technology on top of the engine) were not very good, and issued service replacement ONE-BARREL carburetors for all of the D series Strombergs.

 

But even a poor single-barrel carb is probably much better than running a two-barrel if running through an adapter.

 

Have no idea about an AFB two-barrel. AFB stands for (A)luminum (F)our (B)arrel.

 

Carter did produce an ABD two-barrel, but the carb in your picture is a Rochester 2-G series. If the intake plenum is reconfigured for the 2-barrel, and the 2-barrel is running without an adapter, and the 2-barrel is internally the correct size for the 248 low RPM engine; then it should work well, but lots of "ifs".

 

Running better when cold can mean that the carburetor is richer when cold due to the choke and is running lean when it warms up. It could also mean, a possible vacuum leak that shows up after the engine warms. It could also mean a coil, or condenser breaking down when hot. Or many other possibilities.

 

I would agree with Ed, you probably have both ignition and carburetion issues. And going to be a chore.

 

I would suggest trying to borrow a known working SINGLE BARREL carb from maybe a 240 CID Ford or a 250 CID Chevrolet for testing; and then test the ignition. Not saying it would help, but where I would start.

 

Jon.

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Doesn’t matter. The twin plug was 90 percent sales gimmick. Unless you have access to test equipment, it’s going to be guess and try. I look at all those modifications and cringe.......as a fantastic mechanic would be challenged to get it correct. People without extensive pre war car experience are going to drown trying to repair it. I would attack it by making the fuel system perfect first, ignition seconded, and then carb last.

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It sounds like the car is running lean. Yes you can run the car on either bank of plugs  by isolating the left or right side by unplugging one coil.  Its going to take some  time consuming process of elimination of different  components. 

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The Nash units I am familiar with is the next larger series with the updraft carb. With all the alterations you're swimming up stream. If I was trying to fix it, I would do so on a chassis dyne and a five gas exhaust analyzer. That way you can see everything to make a better diagnosis. Problem is, very few people have access to test equipment. Crazy things like coils wired backwards could cause the running problem at temperature.........along with endless other things........ trying to sort it in the garage or driveway is going to be fifty times more difficult than having all the test equipment accessible.....and even then it won't be easy. Not trying to kick you.....but all the changes make for a problem not too many people today can successfully fix. As we say around our shop.......you need a very talented craftsman to service the car.

Edited by edinmass (see edit history)
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First things first......do a dry & wet compression test........with a fully charged battery with all the plugs out........unless you have good compression across the board, everything else is a waste of time. Start by posting the numbers please........Ed

 

PS - use a good guage, not a Chinese POS. 

Edited by edinmass (see edit history)
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For what its worth:

 

Dyke's Automotive:

 

Back-firing through carburetor:

"Generally speaking back-firing is caused by a lean mixture which burns so slowly that the flame continues until the opening of the admission valve again, when it ignites the incoming charge in the intake pipe and shoots back to the carburetor. While an over rich mixture will also burn slowly, it rarely ever will cause back-firing. Another cause of back-firing is, of course, the faulty timing of the valves. Or in fact a leaking valve. As a general rule, back-firing is due to one or more of the following causes: (1) very slow explosion or weak mixture. (2) very late explosion; (3) a spark occurring during the intake stroke; (4) the intake valve partially open during the power stroke; (5) premature ignition.  ...Slow combustion is caused by a lean mixture. A late explosion is caused by a weak or retarded spark. Nos 1 and 2 are the usual causes."

 

Popping back or spitting through the carburetor: "... mixture is too weak and more gasoline is required."

 

As Ed has stated there is a lot going on. The trick is figuring out what isn't the problem. Go through one system at a time - one modification at a time.

 

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First,thanks for all suggestions. All are taken into account. Going to start will removing potential air leaks.(the spider web). Let warm and adjust valves,run one ignition set then other . See what we get. 

Mr. Harper, you mention slow burn of gas. Could this happen with too high octane gas? Running 92 octane non ethanol. 

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Fuel is not your problem........as of right now, go step by step. Do a compression test as stated above........the guys here are happy to help.......but shortcuts are NOT the way to fix a car. We will walk you through it the best we can....but some things just need an experienced set of hands on them to get them right. Best, Ed

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Time for a follow up. Adjusted valves with motor hot. Surprising how far they were off. Helped a great deal. Played with timing. Got to run nice with back fires happening when throttle opened fast. 

Since all sounds good,breaking down and spending on points,condenser and the distributor cap (expensive but new). Also was running incorrect spark plugs.

So wait for parts. Looks like we may save it.

Thanks for all the help.

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  When you do the compression test the numbers will give you an idea of the general condition of the upper end of the engine. If you find  some anomalies the numbers will give you a solid point of reference for making a plan for future work on the car. If you want to pursue matters further a leak-down test may be in order.

  When you have your compression test numbers, it would be a good idea to record them in your maintenance logbook for future reference.

 

  Good luck. 

 

 

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^^^ What Jim said.

 

Leakdown test would be very helpful, but do the compression wet/dry test first.  If you have a leaky or stuck-partially open intake valve, that could explain your backfire.  In leakdown test (google for more complete info), air hissing at carb means bad intake, at tailpipe bad exhaust, at oil filler cap bad rings.

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On 2/13/2022 at 8:06 AM, 31nash880 said:

Time for a follow up. Adjusted valves with motor hot. Surprising how far they were off. Helped a great deal. Played with timing. Got to run nice with back fires happening when throttle opened fast. 

Since all sounds good,breaking down and spending on points,condenser and the distributor cap (expensive but new). Also was running incorrect spark plugs.

So wait for parts. Looks like we may save it.

Thanks for all the help.

Could be as simple as the accelerator pump or its adjustment.

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On 2/14/2022 at 3:41 PM, 31nash880 said:

Just had head done so that should be good. Explain dry/wet test please.

 

Wet/dry is to do a compression test right after taking the sparkplugs out of the engine.  Record the values as this is the dry test.  Then squirt 2 or 3 shots of oil into each cylinder and do a compression test again.  Record the values, this is the wet test.

 

If the values do not change, then there is no basic problem.  If the values go up after putting in the oil, then there is a piston/ring issue.  Either worn or cracked rings.

 

If all of the values are lower than specifications then you have a worn engine that needs to be rebuilt.  If  you have one or more cylinders that is outside of the +- 10% you need to determine the problem with that one cylinder. 

 

If two adjoining cylinders are low, then you probably have a bad head gasket.

 

Hope this helps.

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With your twin ignition, I assume you have just one compression gauge, so only remove one spark plug per cylinder for the compression test and (later) leakdown test.  Be sure to ground both coils if you have to have the ignition switch on to operate the starter.  But if you have a kick starter that operates with the ignition switch off, no need to ground the coils.  I recommend full open throttle while cranking.

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Today's update,

New plugs, points, condensers and distributor cap. They were fun to find and don't ask the cost.

Had ordered plugs but they were incorrect. Seems motor has a later head installed. Used plugs for a 1934.

Have never checked engine numbers.

Everytime we work on it, it sounds better.

Photos show compression numbers. No change wet/dry. Could not fine sheet when i originally did this but know there was a great increase. Guess rings are freeing up.

Only thing mechanic is worried about is play between upper and lower parts of distributor shaft. 

It is sounding good.

20220220_142356.jpg

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Update: ran it some yesterday.  Still have a backfire issue . Mechanic is going to check lift on cam lobes to see if that is an issue.

Carbking, if you see this, still wondering about a direct bolt on replacement carb. 1931 nash 880 twin ignition . Still using a Carter from 60's with adapter. 

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On 2/14/2022 at 3:41 PM, 31nash880 said:

Just had head done so that should be good. Explain dry/wet test please.


Hate to tell you this.....that means absolutely nothing. More poor work being done today than you can imagine. Figure a 20 percent chance it was done right.

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Numbers posted are ok but not stellar. Stuck rings can but rarely get better. Understanding running problems and diagnosing them are not simple.......too many cooks spoil the soup. Without years of experience playing with pre war cars you fighting a huge up hill battle. With non stock components.........it probably never ran right to begin with. You need a diagnostic procedure to eliminate things one at a time.....guessing and speculation is not only unproductive........it’s making things hopeless. You need professional help......and it’s gonna be expensive. Good enough or “ok” isn’t either.......a fuel system, ignition system, and a valve train/compression system is right........or it’s not. If you brought your car to my shop, I would only take the job if the entire car was put back to stock.......regardless of cost. Very few people are capable of making that car right with a different carb........very few. And most of them that say they can have absolutely no clue. Where are you located?

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Wet / dry can show a valve sealing issue. Wet will increase the reading. Backfiring through the carb is 90 percent incorrect firing order, eight percent bad valves, and two percent stuck valves. Also severe lean misfire is another one percent tossed in there somewhere. 

Edited by edinmass (see edit history)
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On 2/27/2022 at 12:41 PM, 31nash880 said:

Update: ran it some yesterday.  Still have a backfire issue . Mechanic is going to check lift on cam lobes to see if that is an issue.

Carbking, if you see this, still wondering about a direct bolt on replacement carb. 1931 nash 880 twin ignition . Still using a Carter from 60's with adapter. 

I checked Carter, Stromberg, and Zenith; none of which offered a replacement for this engine.

 

The entire run of the Marvel DO (DN) downdraft carburetors was:

 

1930 Oakland V-8

1931 Oakland V-8

1931 Nash 880

1932 Pontiac V-8

 

These were NOT Marvel's finest achievements!

 

Like the Carter U-1 (also used on Nash) and the Stromberg entire D series, these carbs were transitions from updraft to downdraft, and basically used existing updraft technology, and sat it on top of the engine.

 

I have never seen one of these for Nash that I can remember. I did buy one from a salvage yard some 35 years ago marked Nash, but inspection revealed it to be 1932 Pontiac, which, since I was a T/A for POCI at the time, made me happier than had it been for Nash.

 

Just for grins, I checked the specifications on the 4 applications above. The calibrations on the 1931 Oakland and 1932 Pontiac are close; and the calibrations on the 1930 Oakland are even closer. Of course, the linkage arms are different. The 1930 Oakland has a cast iron air intake, while the 1931 Oakland and 1932 Pontiac have zinc alloy air intakes.

 

I have the 3 versions you are not looking for.

 

I really do not believe that installing a Marvel will end your problems (maybe make them worse?). Re-read Ed's comment in the post above about backfiring through the carburetor, and the percentage of probably cause.

 

EDIT: both the Nash and the Pontiac bill-of-material list the bowl cover as part number 56-521. I have the Pontiac version and it clearly shows "Model DO". So the DN which I have seen referred to in some reference material would not be shown on the carburetor. It will show the Model DO on the bowl cover.

 

Jon

Edited by carbking (see edit history)
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