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1926 Buick fuel/air ratio issue with modern 87 octane gas.


AzBob
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Hi all,

 

Recently got the 1926-26 back on the road and am having some fueling issues. The Marvel carb has been rebuilt and the "Gasoline Adjustment" on the bottom of the carb was adjusted to the notch (one turn) as per Shop Manual. The air screw was adjusted next for smooth idling, so far so good. The engine runs good as long as I have my foot in it. When I let off or try to maintain a steady speed, It tends to buck and sometimes pop back through the carb. This goes away if I pull the choke out about halfway. So I am thinking AH HA! lean mixture. So I open the "Gasoline Adjustment" to 2 turns open. This helps, but still runs smoother with the choke out approx. 1/4 of the way. 

 

My question is should I continue to open the "Gasoline Adjustment" past 2 turns open, or open up the "Low Speed Nozzle" (jet) with the next size wire diameter drill. To rule out the ignition system, new spark plugs and points were installed and adjusted to spec. 

 

All suggestions and ideas are welcome from anyone who has been down this road.

Edited by AzBob (see edit history)
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AzBob,

    

It is common to maintain about 1/3 choke until a "cold blooded" Buick engine warms up.  I often run for a mile or more before I can push the choke all the way in.  Generally, all Marvel carbs will run rich when adjusted to run correctly at idle when the engine is still warming up. Therefore, it is very important to get the engine up to temperature before making any adjustments. So, get out and run it around the neighborhood first. Start with the big knob even with the end of the tang that clicks in the grooves of the knob. Start with adjusting the fuel supply needle valve at the bottom of the carb 1 1/2 turns out to start.
With the engine warmed up and the choke fully open, retard the spark and adjust the air screw (the big knob) to the left until the motor hesitates, indicating the mixture is too lean. Then turn it to the right a few notches at a time until the motor runs smoothly.
Then, open and close the throttle to see if it hesitates or stalls on acceleration. If it does, then turn a few more notches to the right until the hesitation is eliminated. With this done, advance the spark and terrorize the neighborhood again, or take it out on the road and run it for at least an hour before checking the plugs to see if it is running leaner. You could also just mark the air screw to see how the new setting differs from where it was. You can try increasing the bottom needle valve to get more power, and repeat the air screw process until you get the most from the engine.
Good luck,

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Mark,

 

Thanks for posting your Marvel carb tuning procedure. I followed it to the letter and engine now runs with less hesitation at partial throttle. Before I can complete the fuel/air tuning, have to replace the seal on the gasoline adjustment packing nut as it has began to leak after my repeated fiddling with it.

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All right, gasoline adjustment gland nut leak repaired using cotton string packed with plain bar soap (Model T fix) it works!***

 

Followed Marks' procedure to the letter and turned the "gasoline adjustment" on the bottom of the carb to 1 and 1/2 turns open. Drove for 2 miles for warm up. Adjusted the air screw per instruction, every thing sounds fine, no hesitation when blipping the throttle. However, out on the road, still likes to have some choke.  So I am thinking if I turn the air screw in a couple of more clicks, this should enrich the mixture to eliminate having the choke out. Here is my dilemma, turn in the air screw or open up the gasoline adjustment on the bottom of the carb more. Will try more experiments tomorrow. I am at 5300 ft. elevation and one would think the mixture should be adjusted towards the lean side.

 

*** Addendum 10-12-15***

Found the cotton string soaped packing to work for a short time, only to fail again as the soap breaks down. My latest fix was to take fuel line hose and separate the inner core and cut to length to fit inside the gland nut just below the threads. So far, this has been a success with absolutely no leakage from the "gasoline adjustment" (low speed jet needle valve packing nut).

Edited by AzBob (see edit history)
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I got an email from Ray Brown a couple of weeks ago regarding Marvel carb testing.  He is doing tests on his buick with an O2 sensor installed to test different air valve springs & jets.  His results will be published in Dean Tryon's 16-18 Buick Newsletter.  I expect we will see that the original springs or  jets may not work well for modern gasoline.

 

Stay tuned...

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If the 26 is like the 29, with a pre-heat pipe from exh. pipe back to heat riser, you should ensure you're not sucking air in through this pipe or have tiny pin-holes in the heat riser tube as this will require you to run with choke pulled part way out.

 

Bill McLaughlin

1929 Buick Roadster

Supporter of All together Parking by Era

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Bill,

 

Thanks for the tip regarding the heat riser tube. I checked it for leaks when I had the manifolds off and the carb out for rebuilding. Couldn't find any leakage.

 

Mark, will be real interesting what Ray Brown comes up with his O2 analyzer. 

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I have to run the air bleed screw in a LONG way to get my '27 to drive well, I believe a stiffer spring is in order to have it operate correctly any where near being flush to the tangs, mine is in that far it is not even on the ratchet, same result with 2 carbs, and 2 heat risers that have both had tubes replaced, with it in that far, car runs very well, idles well, and reasonable fuel milage and no overheating

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The more posts I read on tuning Marvel carbs it seems that every one of the Marvels seems to have its own sweat spot and this spot may change. For example I've just put a new kit in to our carb (many thanks to the Carburetor Shop in MS) and the 26-25 now has a completely different set up to where it use to be. Admittedly the new air spring in the kit looked nothing like the"original" one as the new one was shorter and stiffer than the old one. Also as recommended in other posts the flap controlled by the air spring has now been adjusted to sit against the throat of the carb rather than sitting 2mm proud. But with that said it's now running happily at 1 turn on the low speed needle and the air screw is screwed in to the bottom of the ratchet. Whereas before the kit it was running half a turn on the low speed needle and half way out on the air screw. I'm not sure what this all means and whether there is a rhyme or reason for any of it but I'm sure someone out there will know.

Cheers

Neil

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I thought for sure it was going to be that heat tube rusted out with holes.  Since Mark Shaw changed his '24 out in front of us all at the Rochester National Meet (and being a 1923 guy, I had no idea there was that pipe in there on '24 up, since I don't have one)  I've help two others with the same problem, where both were ready to sell the car since they could not get it to run.  Both plugged it off and ran great.

 

If I had to bet, I'd guess that 95% of all the main air spings in Marvels have been yanked out of shape over the years by someone at some point.

 

My Marvel hates to go for a trailer ride.  The bowl empties and the float bangs around on the trailer ride since there is no fuel to dampen it and finds a way to hang up while dry that never happens when sitting in the garage dry.

 

Knocking on wood here as I have a 600 mile VMCCA Tour in 2 weeks.

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With all this great information, I am going back to the drawing board to check the heat riser tube again as well as the carb internals, perhaps even send it out to a professional rebuilder. In addition, am checking the intake system for vacuum leaks as well.

 

To date, starts great, idles great, blip the throttle, no hesitation, Out on road, only runs good with load on engine. Going downhill, bucks and pops / backfires, same on level roads.  Will keep posted. No adjustment on the carb appears to remedy this problem.

 

Just completed a compression test to rule out any problems with the valve train.

No. 1 = 65 psi.

No. 2 = 60 psi.

No. 3 = 65 psi.

No. 4 = 60 psi.

No. 5 = 61 psi.

No. 6 = 60 psi.

Edited by AzBob (see edit history)
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Bob,

    The current symptoms indicate a fuel delivery issue.  The float bowl level should be about 1/16"  below the level of the low speed jet when the float closes the inlet.  Also, check the fuel supply lines and fuel filter for restrictions.

 

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Problem solved! :)

 

Mark, you were correct, the problem was traced to a "fuel delivery issue". I had replaced the old cork float with a new nitrophyl float. Apparently, the nitrophyl has a higher buoyancy than cork. Anyhow, the float level was way too low. Adjusted float to raise the fuel level to approx. 1/16" below the low speed jet orifice.

 

The other issue was the fuel metering pin linkage from the throttle was too long and holding the pin hard closed on the metering pin jet. I shortened the linkage to just get the metering pin off the seat. This made a huge difference and eliminated the surging and bucking at cruising speed.  :)

 

While I had the carburetor disassembled, checked all of the jets for cracks, none were found. Checked jet sizes and all were correct.

 

I want to thank all who replied and offered suggestions. Certainly helped sort through the troubleshooting process.

 

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Edited by AzBob (see edit history)
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