Can carburetion get any simpler than an "NH"? HELP!!!

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Model T Ford Forum: Forum 2010: Can carburetion get any simpler than an "NH"? HELP!!!
Top of pagePrevious messageNext messageBottom of page Link to this message  By Marshall V. Daut on Wednesday, December 22, 2010 - 10:25 pm:

For those of you who have been following the saga of this 1926 Touring restoration (or as I call it, "resurrection"), it's finally almost done! I'm doing detail work and correcting 101 little things that are loose or need tweaking after a full restoration. But I still am experiencing a slight misfire under acceleration and what I feel is a too rich idle from the Holley "NH" carb.
As a backfill to this restoration, the engine was rebuilt by Rich at Antique Engine Rebuilding in Skokie, Illinois. I know that part of the equation is bullet-proof. Ron Patterson (the Coilman) rebuilt the coils. Scratch them from the list of suspects. The coil box has new wood and is tight. An Anderson timer eliminates a cheapo timer as a potential problem. I've tried two sets of spark plug wires, reading here that some of the new ones were not made properly. No wires from the timer are shorting out against bolt heads. A new exhaust manifold that bolted up perfectly with the intake manifold should be sealing well. Champion spark plugs have been gapped from 0.025- 0.035" in various tests. 0.032" seems best all around.
Considering all the above should be above reproach for causing the misfire and stumbling, I now lean (no pun intended) towards casting a suspicious eye at the thoroughness of my Holley "NH" rebuild. It has been cleaned, including drilling out the passageways according to the very helpful photos posted on this site. I was able to pull wire through the passages just like in the photo. I threaded the passage openings and used small Allen head plugs with sealant to plug up any internal leaks. New gaskets were installed, as well as a Viton tip needle and seat. The spray nozzle seems to work properly, to the point that closing it down kills the engine. Opening more than 1/4 off the seat, however, makes the engine run rich and stumble during idle. I have to set the mixture leaner to idle, but enrichen it out to drive. Very irritating and of course, not correct. What have I missed or overlooked when I rebuilt the carb? Do those symptoms sound as though something's still wrong inside this simple little creature? It galls me that something as simple as a Holley "NH" carburetor is thwarting my efforts at making the engine run right. I mean, how could a carburetor be any simpler? And yet, I have obviously messed something up during the rebuild.
Any ideas what I should check from you guys, who pick your collective teeth with carburetor parts?
Marshall


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kerry van Ekeren on Wednesday, December 22, 2010 - 10:39 pm:

Sounds like a wrong angle on the mixture screw? also check to see how much play is in the throttle butterfly shaft, maybe sucking to much air?


Top of pagePrevious messageNext messageBottom of page Link to this message  By tyrone thomas on Wednesday, December 22, 2010 - 10:41 pm:

Has the seat or needle been screwed down to tight and damaged? Are all ports in body open?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Peter Borland on Wednesday, December 22, 2010 - 10:53 pm:

Just a guess, are the new drillings the correct size?


Top of pagePrevious messageNext messageBottom of page Link to this message  By keith g barrier on Thursday, December 23, 2010 - 12:17 am:

Put a kingston on it and go trouble free the next forty years! Have fun , KB


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tony Bowker on Thursday, December 23, 2010 - 12:23 am:

In some obsqure page of the Ford Handbook it gives the corect way to set up the float level on the NH. Most of us just set the float to 1/4" (from failing memory) and go from there. The handbook details the problem that Marshall is experiebing and instructs how to adjust the float level to get the mixture consistent across the power band.
Maybe someone can remember the page or better still, can instruct on how to do it....


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan Treace on Thursday, December 23, 2010 - 01:00 am:

The Service Bulletin Feb 1924 says to set the float for a distance from the top of the metal float to the machined flange on the mixing chamber, that distance should measure 15/64"

The machined flange is where the bowl meets the carb .


Top of pagePrevious messageNext messageBottom of page Link to this message  By jack daron-Indy. on Thursday, December 23, 2010 - 01:09 am:

Marshall,it is easy to check the throttle flapper (idle adjustment) and be sure one of the vent holes is open toward the engine. If both are covered,it will affect carb performance.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Mullins on Thursday, December 23, 2010 - 01:54 am:

Intake manifold leak


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce Peterson on Thursday, December 23, 2010 - 08:25 am:

Agree with Bill. You likely have a gland / ring that is not sealing well. Another possibility is a pinhole in the manifold.

Finally, I don't see any mention of spark plug type or gap. If you have an excessively wide gap the plugs can foul quickly.

Also, the little modern plugs with thread adapters can be troublesome, I've seen several folks go through 10 spark plugs in a day before throwing the spark plug adapters in the trash.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Anthonie Boer on Thursday, December 23, 2010 - 08:47 am:

Marchall : Did you try another Timer [Tiger timer]
Toon


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenny Edmondson, Indy on Thursday, December 23, 2010 - 10:02 am:

Marshal, "Opening more than 1/4 off the seat, however, makes the engine run rich and stumble during idle." I don't think that sounds like a manifold leak. Sounds like there's an issue in the spray nozzel area or...

Normally the adjustment for the needle valve is 1 1/8 to 1 1/4 turns. If your only turning it a 1/4 turn and getting too rich then something appears to be wrong in the carb. If you had a vacuum leak, the adjustment would probably need to be greater than the adjustments above. If your float level is too high or the needle vavle and seat don't shut the fuel off properly, the fuel will puddle in the bottom of an NH swayback bore. With that said, a lean mixture will make an engine backfire through the carb.

So what I think is happening is at an idle the mixture is rich from the pooling fuel in the bottom of the bore and your controlling the fuel delivery with the small opening of the spray nozzel and needle vavle. Once you accellerate above idle, the puddled fuel in the bore is used up and then the carb starts running lean. The Replacement needle valve seats do not seal in the carb housings well so that's probably your issue.

Contact me off-line if you want me to send you a carb to try or want me to take a look at yours. It will only cost you for shipping.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Marshall V. Daut on Thursday, December 23, 2010 - 11:10 am:

Yes, Kenny, that sounds like the problem's explanation. In response to other suggestions first, I'm sure the intake is sealing and I've tried a wide range of spark plug gaps, settling on about 0.032" as the best for idle and accleration under the present circumstances. The float is 1/4" from the top casting's surface.
I would like to pursue your suggestion, Kenny.
Assuming this is what you describe is happening inside my carb, what is causing this? I intend to "lap" the needle tip today with valve compound, although the tip is smooth and without burrs or ridges. I wonder if the jet orifice may be oversized? It doesn't take much of a spray nozzle movement to affect the engine. When I read that 1 1/4 to 1 1/2 turns off seated is normal, my 1/4 turn seems way off somehow. I tried to swap needles from another carb, but it is too long. The spray needle in this NH is short, much like the Wizard carb pictured in the MTFCA caburetor book. I wonder if there has been a mixting of parts between a true NH and a Wizard? Since closing the spray nozzle down kills the engine and opening it 1/4 turn allows the engine to idle and accelerate (until further enriching is necessary), I assume these parts are still doing their job, whether they came in this carb or not.
So, what do you suggest to restore proper operation and achieve a normal adjustment range? Is the problem most likely with the spray nozzle and seat That may be the key to this lean-rich alternating problem.
Marshall


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenny Edmondson, Indy on Thursday, December 23, 2010 - 02:50 pm:

Marshall, I air test all the needles and seats I install. Turn the carb body upside down in a vice, with the needle and seat installed I put a light oil between the seat and carb body and down in the needle area. I hold a light pressure on the needle with my finger and then apply air pressure to the fuel inlet with a blow gun. Look for air bubbles.

If your mixture needle is short then you have a Simmons or a damaged needle that someone has cut down. The tip should be an angle like in the drawing above. If the orifice is larger than normal, that will have an effect on performance.

Check these items and let me know what you find.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce Peterson on Thursday, December 23, 2010 - 03:31 pm:

Marshall,

Not sure where you live but certainly there must be another T owner in your area who would let you borrow a known good carb. It would take you ten minutes to see if that is really the problem or not. If you are anywhere near me I have one you can try.


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