Hi guys n gals i have a little carb problem with my flatty. i already read and tried a lot but don´t know what to do anymore. Maybe someone can give me i hint what i did wrong My motor is a french flathead with 4" crank. Cooling system is installed. new bubba diz set to 5° before tdc, pneumatic advance disengaged Navarro Heads and 2x2 racing with 2x97 (old big logos) mechanical fuel pump fuel running from pump by rear to front carb with banjo fittings fuel pressure gauge on front carb fuel system is tight, no external leaks both carbs where treated the same way: completely dissassembled cleaned and a premium rebuild kit incl accelerator pump installed new jets and valve (Main 45, Power 63) disconnected the carbs from pedal and each other both carbs have individual return springs throttle plates close well shaft play is ok for me but i´m new to 97s fuel pressure is 3 to 3.5 psi carb fuel level was checked and ok carb inlet valves close successfully against the pump pressure disengaged the accelerator pump for basic carb check for starting i prime carbs with the accelerator pump what helps it adjusted idle on both equally at about 2 turns and idles good
so far so good - here´s the issue: i can only do idle tests right now, no driving. 45 main jets: when i turn the shaft on the rear carb, i have to be slow on opening it from idle as motor seems to die when i immediately fully open the throttle plates. maybe due to disconnected acc.pump? With a little rev´s i can do what ever i want, it runs up great no prob. sounding great. On the front carb, when i open the throttle plates even i little motor dies immediately. in vern´s "how to build a trad. hot rod" the 45 mains where mentioned. but in vern´s "how to rebuild a flathead" 45 for single and 41 where stated for 2x2. thought it could be too rich and bought new 41 jets. i would say idle exhaust smells less fuely so better my guess. But problem stays the same rear carb: getting off idle a bit tricky but ok if away from idle front carb: kills it even when reved up with rear carb i actually realized that when i close the choke plates on either carb engine runs up a little. couldn´t perform anymore tests after i realized that. so the big question is: is it too rich or too lean or is it another problem? what remains to do in my opinion is: do tests with closed choke plates and crosschange carbs to see if issue goes with it. maybe the front carb is crappy? any other ideas? thanks Jan
Remove your linkage from each carb and synchronize them. Unisync. https://www.amazon.com/Uni-Syn-Carburetor-Synchronizer-Synchronizing-Balancing/dp/B073RPZKBX. Adjust the carbs at idle until both read equally. Adjusting the carbs does not mean adjusting the idle screws. We are talking about moving the throttle until both carbs have the same reading. You will find an adjustable air inlet on the sync tool to prevent your engine from stalling. After they are adjusted. Change the lengths of each carb linkage to meet the carb setting. Also I am not familiar with Bubbas distributor although I’ve heard great reviews but.... Exactly how does your distributor advance?? It must advance for proper timing during acceleration. Jets size and power valve size. .043 jet size is normally a good start for dual 97’s. They also have adjustable jets that work well. Power valve size (69). Disconnected accelerator pumps??? Hook em back up. One more thing. DO NOT use those stupid small air cleaners with its element. They starve your engine. If you insist, install a piece of mommas Nylons in there or one of these. https://www.ebay.com/i/163505128824...MI-pfj2-zc5AIVrR6tBh2l8Q_YEAQYAyABEgLDhfD_BwE
https://www.jalopyjournal.com/forum/threads/very-basic-multiple-carb-sync-tech.124624/ Great right up from Tommy(RIP) Pete
"Bubba" supplies both converted GM distributors and rebuilt stock "Load-a-matic" units for 8BA flatheads. Specifically, what distributor do you have?
Try lowering fuel pressure a little (maybe 2.5psi) Other than that, My grand dad always used to say, 90% of carb issues are ignition... Chappy
I keep the pressure under 2 lbs for a Stromberg 97. You say you are throttling the carbs separately? With no linkage, so when you open one the other stays at idle? Isn't the 2x2 racing intake a non-plenum type? So basically you are feeding four ports but the other four are getting idle only? Sounds like that might be a problem. When you rebuilt your carbs did you remove the emulsion tubes to clean them and around them in the housing? There is a surprising amount of crap usually packed around the tubes after decades of use (and dirty storage).
Hey guys thanks for your great replies. - they´re helping a lot to answer the questions: @tubman @Petejoe the dizzy (new) is a GM, turned down and recurved for flathead use, with centrifugal- and intake-plenum-pressure advance. it´s a great diz and it worked great with the original single french carb. Using modified stuff is a part of hot rodding for me. not everything from the shelf @alchemy no i didn´t remove the emulsion tubes as i heard there is a big chance of destroying them. don´t know how to do this and i think you need a special tool for that. But will look it up.. yes no plenum and i ran/run the carbs throttles seperately just for the testing. this was how i found out that the front carb was the issue. but will link´em up for sure. @Petejoe i used no air filters for this tests. but thanks for the hint! ok i will reconnect the acc.pumps. just wanted to reduce influencing parameters. i think, as you say, synchronizing is the missing thing. totally forgot about that and will do it! @prpmmp great i will read that - thanks!! @Chappy444 @alchemy ok i will check my options for lowering fuel pressure Jan
The emulsion tubes are easy to remove. There have been tutorials done here on the HAMB. Basically tap the inside of the tube, screw a small bolt into it, and use washers to screw down and apply pressure against the housing. They pull right out. Press them in by hand with a blunt punch after cleaning the tubes and housing cavity. The curved tubes are difficult to remove and should be just blasted through with high compressed air. There is no outer cavity around those curved tubes to collect gunk.
great that sounds quite easy. Will do so! thanks for your help alchemy curved tubes have been cleaned with carb cleaner and fuel passes good Jan
What you are describing is perfectly normal for an engine without the accelerator pumps hooked up. As soon as you open the throttles, you get a big dump of air with no fuel because under those circumstances there is no appreciable vaccum to cause the fuel to flow. That is what the accelerator pump does...gives it a short dump of fuel to mix with that oncoming rush of air, enough fuel to last until vaccum is reestablished and the venturis can operate properly. Hook them back up and see how she goes.
It sounds like it is running lean as soon as you crack the throttle to me, probably has enough fuel for idle with plates closed but no fuel pulling over in the mains when you open the throttle . Are the float levels correct? i.e. both the same? Is the power valve clean free and closed when it is not needed, if the pump is disconnected and the spring is incorrectly assembled the pump could be resting on one of them that will make that carb run rich. try changing the carbs around front to back if problem moves with the carb , carb is the issue, if it stays in the same place, air leak is a likely problem.
Ive got a flattie with navarro heads and intake and IIRC when i ran jets which were 41s the engine wouldn't run correctly. I think i went to 43s in the end and it was fine, pretty sure i didn't go to 45s.
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