Make sure the throttle shafts are tight and not pulling air (vacuum leak). Also check all the air/fuel screws and make sure the tapers aren't clobbered. When you think it's all good, I might then try disconnecting the linkage and running each carb individually before syncing them and reinstalling the linkage.
Stromberg update.....Finally got an opportunity to get the carbs back on the engine today, Had to do some final linkage adjustment but once the engine was warmed up my dead throttle from idle pretty much went away. Not sure what the actual fix was but glad it's resolved. I'll have to wait for better weather here to do a road test and finalize jetting under load. I appreciate all the feedback and advise. Thank you
I'm glad you figured it out. Like B&S mentioned, looks like the rebuilder forgot this crucial step in rebuilding. I went back and looked to see if you mentioned the size of the engine and what else was done besides an intake (EX: heads, cam, porting, etc.). All those variables will be important to know before you start changing jets & PV's. Last night, I was speaking to a HAMBer who is also a Bonneville flathead record holder to talk about suggestions and tips for swapping out my heads. I was concerned about the carbon I saw on piston tops and around the valves. My engine is a 292 ci, Potvin cam with Edelbrock Slingshot and two 97's running 45 jets and 67 PV's. Contrary to OHV thinking, he believes that the more you can feed the flathead, the better it will perform. The thinking is it's such a poor breather that more intake charge will help it out.
@oj Can you describe or illustrate the "transfer slot" you're describing in some more detail? I've got a dual carb set up that is suffering the same stumbling symptoms upon acceleration only I've got brand new Stromberg 81's. I've narrowed down all my issues to carburation but have not been able to resolve yet. (Stock '40 ford motor) no performance upgrades other than the dual intake.
Give me a day or two, I believe I have one of those carburetors and will take it apart to show you along with some illustrations on the principles of how the circuit works.
I'm not that computer savvy to properly show this, anyway here is a rendition of the 'transfer slot' It shares the same feed as your idle circuit. This transfer circuit is aligned to the throttle blade, when holding the carburetor upside down the transfer slot should be slightly exposed (they reccommend the hole to appear as 'square'), this circuit sees vacuum with the throttleblade closed and fuel passes thru the exposed hole - if you look at a used carburetor that area will be washed clean. As the throttle is opened the transfer slot has greater exposure to engine vacuum and greater amounts of fuel will flow - all of this happens in microseconds and quantities are very small - when the throttle blade passes the transfer slot the vacuum is not great enough to get the fuel to flow and the main jets have begun to flow. You idle circuit sees the same vacuum and has been supplying fuel metered by the adjuster screw. The purpose of the transfer circuit/slot is to provide fuel until the accelerator pump shot has happened. If this circuit is not working right the motor will have a momentary lean bog until it gets fuel from the accelerator pump. This photograph is your Stromberg looking in from the top to see the size of the transfer slot It differs from the illustration in that the idle circuit and transfer slot are on the same side, the idle circuit is below the transfer slot (this is true with all the carburetors I know of, I think the illustration separated the circuits for clarity) This photograph shows the carb from the bottom as you'd see it before bolting it down. The transfer slot is seen just below the closed throttle blade, the square hole, and below the transfer slot is the idle feed: I hope this has been a little help, there is considerable internet 'information' if you google 'carburetor transfer slot' Goodluck, Oj
No it isn't. I was thinking the OP had Stromberg 81's, my fault, I believe the 97's have a series of individual holes instead of a slot. Good catch.
If I get to guess, my guess is Carter WD-0. The explanation of the function of the idle transfer slot is very good. While the Stromberg EE-7/8 marked 81 or EE-1 marked 97 had a three-bolt mount; MANY Strombergs have a four-bolt mount. Jon
Regardless of the carb type, thanks for the information @oj. Hopefully helpful for someone else trying to diagnose their issues. An update on my problems was finally solved last night after hours of troubleshooting... I finally traced the issues back to my distributor that was due for a tune-up. After noticing a very strong spark from the coil but a terribly weak spark at the plugs I dug further into my distributor (Stromberg e-fire). Sure enough, a rotor that was making poor connection with cap and points in the cap that were oxidized was giving me very poor spark and quickly fouling out plugs. A quick tune up by touching up the points and making sure the rotor was engaged with the cap made a world of a difference and the engine runs excellent now. Point is, what I surely thought was carburation issues really was ignition problems. Just speaks to how sensitive the fuel and ignition systems are needing to work in sync and it's easy to only focus on one system.
Back to the topic, a stumble will occur when the throttleplate is opened to increase the idle speed because the transfer slot is overly exposed, there isn't enough vacuum to get fuel flowing. If the engine idle speed cannot be lowered then a compromise can be made by drilling a small hole in the throttle blade. This hole becomes a vacuum leak in that it allows air into the carburetor, then the throttle blade can be closed down so that the transfer slot is active again. Lots of carburetors have these holes from the factory, holley offers throttle blades with holes from .093 up to .150 that are typically used on dirt track cars to avoid this stumble, Carter has a different way of doing it on some AFB's. What you can do is drill a small hole, I make them adjacent to the transfer slot, your motor is small and won't need much air at idle, so maybe a .063 hole might provide enough, I expect .080 will be too great. The object will be to close the throttle blade fully when the engine is at proper idle speed.
Pinewood, you said your engine was stock except for the carbs. If you had mentioned the E-fire we might have solved this sooner. Just proves the point that 87% of carburetor problems are the ignition.
Right, I had just convinced myself the issues was with carbs instead of the distributor so I didn't even think to mention it. Next step is to get a vacuum port in the intake so that can dial things in even more.
94 year old guy in our Model A club says “most carbuerator problems are distributor in nature” It always gets a laugh. I will be very interested in your road test results, depending on which “94” you have you may not find the 97’s are an improvement. The 2100 (a 94 model) has throttle plates up to 1.25”. If motor is basically stock then you are going to be way, way over carbueretor’d. I think that for that reason you will need to run jets that are on the rich side. The 94 has a vacuum operated power valve which would be significantly affected by the low vacuum generated with the 2 carbs. If you want 2 carbs “The Look” that’s cool but you would probably be better off with 81’s
Generally - "overcarbed" refers to a carburetor with much more air flow than needed by the engine. Since the carburetor is expecting a certain venturii air velocity dependant upon the size of the engine for which the carburetor was designed; the lower engine displacement will provide less venturii air velocity, resulting in less negative pressure at the bleed-over points, and a lean running engine. Fattening the jets can help. However, if the carburetor has a vacuum operated power system, the lower vacuum may cause the power system to be open when it should be closed, resulting in the engine running rich. Testing is the only way to determine which of the above is true. Jon
Interesting and certainly makes sense. I was of the thinking that "over-carbed" was just too big in general, both air and fuel, so at first blush, adding more fuel seemed to be counterintuitive.
I replaced my rebuilt leaking two 94’s. Rebuilt by reputable guy may I add with two Henkyo Chinese 94 carbs and the damn thing starts right up and idles perfectly cold and has better performance with no hesitation and I get better gas mileage. Bought them as a pair on ebay for under $300 bucks. LGKustoms on YouTube replaced his old leaky carb with one and did a review that was all positive
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