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Rebuilding carb, do I use sealant?


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I find that the surfaces on those old carbs are not machined as well as they should be and I also think the carb 'warps' as the engine heat reaches it. I tried double gasketing and sanding the surfaces on a hard surface to flatten them, but nothing seemed to stop the leaking around the main 2 parts of the body.

I dont' remember how I found out about this sealer but I do hang around my local Dodge dealer parts counter. You can buy some very good cleaners and sealers directly from Chrysler that you can't get from the aftermarket parts store. GM also sells some really good stuff at their parts counter behind the sales office. I think a lot of people miss this resource.

Anyways, I use a black RTV silicone sealer that Chrysler mechanics themselves use in the shop to seal trannies and engine oil pans. It is gas/oil proof. I have used it for many years so this is not something I just started using. My carbs and oil pans on my old cars are leak proof now and I could not stop the leaks using any method or product from the aftermarket guys. Only this RTV from Chryco finally stopped the leaks.

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You should not need any sealer if the carb is in good shape.

There is one exception, some old carbs had cork floats that were sealed with shellac. Sometimes it was necessary to dry them out and repaint with fresh shellac.

Shellac will not do anymore. The alcohol they put in the gas will dissolve the shellac.

The solution is to paint the float with gas tank sealer.

One more tip. If you coat the float bowl gasket with silicone sealer on both sides and let it dry it will not stick to the carb. This will allow you to take the carb apart and reuse the gasket if necessary. Otherwise it is apt to stick to the carb and get torn.

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Silicone sealer swells up with gas on it and will dissolve and get into the fuel system. I just read that on a tube yesterday, Brand was "Permatex". I have seen it happen.I would not use it myself. Read the product info.

Bob

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They never used a a sealer when new, if it is leaking it is usualy because the float level is to high/or it is warped ,if you have to sand it , i have found that a piece of flat glass with approx 320 wet and dry sand paper does the job nicely.

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Ok, well here's the deal, I already put the carb back together with a little sealant before I read this. I did use it very sparingly and tried to keep it away from the actual openings so hopefully it will be ok. I've got it all back together and I fired it up. It seemed to run good, but I'm trying to make sure it is adjusted properly. It's not smoking like it did before but I want to make sure I've got it dialed in.

I'm looking in the mechanics manual and it says to put the spark lever half way down and rev the engine quickly. (do I leave it at a fast idle or immediately return the throttle to the closed position? Then it tells me to turn the adjusting screw counterclockwise (up) until I hear a popping sound and then clockwise until that is overcome. What are they talking about when they say adjusting screw? The bell crank stop screw? Because the only other screw I see is the idle screw.

Thanks!

Dave

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By turning the idle mixture screw towards the lesn side it will cause the popping they are refering to. A pop is usually caused by a lean condition. So they want you to set the idle mixture just to the rich side of the pop.

I am only talking from the experience I have with the outboard motors that I worked on in my past life. Theory should be the same. Lean is lean.

Good luck !

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I only see the bell crank stop screw and idle screw (for engine idle speed). So I'm assuming the bell crank screw is the one I need to be turning? That seems to be the most logical since it says to turn the screw "up" until the engine begins to pop. Now what about the revving, do I just hit it and release or do I maintain the engine at wide open? It's backfireing when I release the throttle after a sharp ramp up like that.

Thanks,

Dave

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It is not often that you would be goosing these engines. But I would try both ways and see what happens. Sometimes backfiring can be caused by exhaust leaks. (or intake)

I am probably the wrong guy to be giving advise about this carb, I only have the general knowledge that they drumed into me at service school for marine engines.

Funny what thoughts are back in the old brain when I am trying to figure something out, Dont remember what I had for breakfast or what I did yesterday.......,,,,,,

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I understand! haha I'll just fool around with it, I'm almost positive that it's the bell crank screw the book is talking about since it refers to turning the screw "up". I think the reason I wasn't getting any change in the popping is because I still had the cable hooked up to it so it wasn't allowing it to close any further. The exhaust leak is definately a possibility too, I knew I should have ordered the gaskets with my last Myers order! It just wasn't backfiring before I worked on the carb, but then again I wasn't ramping it like this either.

Thanks for all the advice!

Dave

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Profyrfyter</div><div class="ubbcode-body">Ok, well here's the deal, I already put the carb back together with a little sealant before I read this. I did use it very sparingly and tried to keep it away from the actual openings so hopefully it will be ok. I've got it all back together and I fired it up. It seemed to run good, but I'm trying to make sure it is adjusted properly. It's not smoking like it did before but I want to make sure I've got it dialed in.

I'm looking in the mechanics manual and it says to put the spark lever half way down and rev the engine quickly. (do I leave it at a fast idle or immediately return the throttle to the closed position? Then it tells me to turn the adjusting screw counterclockwise (up) until I hear a popping sound and then clockwise until that is overcome. What are they talking about when they say adjusting screw? The bell crank stop screw? Because the only other screw I see is the idle screw.

Thanks!

Dave </div></div>

An old time mechanic taught me the same tuning technique. He would slam the throttle open quickly and let the engine rev up for a second or 2, then close the throttle. This was with the car in neutral.

He told me when the carb was adjusted right the engine should "bark" right out. If it stalled, stuttered or hesitated, it was too lean. If it "blubbered" and revved up slow, it was too rich.

Normally you would start on a rebuilt carb, with the fuel needle out 1 1/2 turns. Screw the needle in gently until it hits bottom then back it out 1 1/2 turns.

This should allow the engine to start and run. Then, turn the needle in (lean) until the engine hesitates or slows down, then back it out 1/2 turn.

Now do the "bark" test. If there is any hesitation back the needle out another 1/4 turn and try again until the engine "barks".

Of course the engine needs to be warmed up and the choke off before you adjust the carb.

After you adjust the mixture, use the other adjustment to set the idle.

You will probably need to touch up the adjustment once you get the car on the road, take a drive of 5 or 10 miles and get it thoroughly warmed up. But after that it should hold its adjustment until the next tune up rolls around.

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First of all thank you Chocolate for taking the time out to answer my questions. I have been a bad DB owner and I'm not a member of DBC club yet. I know I know, I really need to.

I was messing with the car this afternoon and I couldn't figure out why the car would fire then die after a second or two. No matter how much throttle I gave it, it would die. I pulled the float chamber top off and found that the chamber was dry! so I filled the chamber with gas and then fired it up. Then it died again. Oops, I left the petcock off at the bottom of the vacuum tank. I opened the petcock up and then started it up again and ran just fine until it the float chamber ran dry again.

I don't think the fuel is getting transferred from the vacuum tank to the float chamber. I just replaced the vacuum tank springs and cleaned the inside of it out also. Any ideas?

Thanks!

Dave

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I am also a newbee, but I will take a stab as I am going through my fuel system to figure out why it is 'starving' the engine when getting up to speed. Anyway, back to your issue.

My guess is #4. The tank venting is not closing; which means no gas being pulled in

1. Check the line from the petcock to the float chamber to make sure it is not obstructed in any way.

2. Then I would pull the plug off the top of the vacuum tank, fill with gas, leave the plug out and see if it will run longer (until the tank empties).

3. I would check for good vacuum coming off of the engine; without vacuum the gas can't be pulled into the tank.

4. Then I would make sure the vent tube and the vent hole on the vacuum tank are lined-up and the passage is clear (mine was partially blocked when I got my car; so be sure to test it with some air pressure. Also make sure it closes well (because if it is open then the vacuum from the motor will just pull air from the vent hole and no gas will be brought into the tank. If the rod is not seating well you can use some toothpaste (not the gel type) to fine 'grind' the build-up off.

That's my two cents but you already have the heavy hitters on the board helping you and I completely defer to their experience. Hope this helps.

Richard

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  • 1 year later...

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