First time posting. Can usually search and find answer, but haven't found anything on this. 49 Mercury with recently rebuilt 9CM flathead; bored .125; mild cam; Offy heads; Sharp 3x2 manifold w/94s, progressive linkage, fuel pressure set to 3psi, choke plates held fully open, pv in center carb blocked in ends, end carbs have idle screws seated, throttle shafts tight and plates appear to be sealing good; MSD Unilite dist; 5sp manual trans. Issue is when coming to a stop, clutch fully disengaged, the idle will drop to around 700rpm then within 2-3 seconds ramp up to 1100-1200rpm and stay there. Sitting in garage I can get the idle stable at 700rpm, but as soon as I take off down the road, idle comes down then kicks up at first stop. Engine starts and runs good otherwise. Just can't get this idle issue under control. Ideas or suggestions appreciated.
Throttle linkage may be binding or being put out of alignment by bad / loose motor mounts or something else contacting linkage as vehicle is in motion. ??
I agree with center of the galaxie. If you're able to maintain a rpm idle setting still then its either the movement of the engine or the linkage causing the change. Are you running all three carbs? If so thats alot of carbs to be fooling with that you don't need. Consider shutting down the center carb and running the outer banks. I realize this would effect your powervalve performance.
Pretty sure it's not linkage related as its all free and motor mounts are new so shouldn't be any engine movement. Besides, it will come back to normal idle for 2-3 seconds before kicking up. Hadn't thought about running on outer 2 carbs, blocking the center. When outers were blocked engine was running out of air about 3200rpm given overbore and cam. Progressive linkage bringing 3 into play pulls strong now as far as I'm comfortable pushing it (5500?) Throttle plates are stock. Holding bases up to light I only see a very thin light line around plates - I'd say maybe .002". Should they seal tighter than that?
YES, Start your car with out your air cleaners and put your hands over your shut of carbs while running. You will probably find 2 leaks.
My experience is that the mixture screws need to be removed and passage way blocked. Make block off plates for the 2 outer carb and try it again ,then remove one plate and try it then block off that carb and unblock the other one.
Did the idle perform ok when you had the end carbs blocked so it only ran off the center carb? I'm going to guess that it did. My experience agrees with Roadster1927. When you add the extra carbs, the throttle plates will leak due to imperfect seals and the idle is affected. Apparently, you can buy modified throttle plates which seal better and eliminate or minimize this problem but I have never tried it myself.
Is the center carb right for a '49? The '48 and earlier carbs are wrong for the '49-up distributor, the advance goes all wonky. OK, MSD dist., just noticed that. If it has a vacuum advance, hook the line direct to manifold vacuum if you are using a '49-up carb with ported vacuum.
You might check "Vintage Speed" on google...he sells plates & stuff 4 multi carb set ups. Will talk with you too. Neat web site. mike
Depending on your intake runner design, closing off the two outer carbs may effect a lean condition at the outer cylinders. Am running two 97 carbs on this with a Msd. Does the condition continue with the air cleaners removed?
Roadster nailed it! Couldn't feel any airflow with a hand over the outer carbs. Still thought he made sense so used some duct tape to seal the outer air horns - yeah, I know, probably not the smartest way, but was fast and worked. Idle was amazing, even cold, which its never done. Ordering new secondary throttle plates from Speedway today. Special thanks to Roadster - and everyone else offering suggestions.
I'm not a "3 Deuces" guy, but I'm kind of curious. For those of you that have been through this, is there any special procedures that need to be done to get a better seal with the new throttle plates? Do they need to be trimmed, or just installed loose and the throttle plate screws tightened while looking through the carb at a light (which is what I do when I do any carb).
off the internet, Black arrows on carburetor base show where idle circuit passages on the end carburetors are plugged with Permatex liquid solder.
The new throttle plates didn't do the trick for me. They were better than the old stockers, but still too much leaking. I tried leaving the screws loose then getting them perfectly centered, still couldn't get it. I ended up with new baseplates.
Replacing the throttle plates with new ones probably won't seal any better. People who tried to seal the end throttle plates better by removing the idle stop screws and letting the plates close tighter, resulted in the throttle plates sticking in the bores. I believe the vintage speed guys sell throttle plates that have a bevelled edge on them which allows them to close tighter without sticking. Then your idle and low speed performance will improve.
Aren't the end carbs' special throttle plates thicker? So they don't stick as easy? And maybe even a different angle?
Stock loadamatic distributer,regardless of licktronic update ,wont get correct vacume with all ya got.thats a fact you can add to all the other problems you have.
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