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Old 07-11-2020, 02:00 PM   #1
Praxiss454
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How to assess whether a carb is worth rebuilding?

Hi folks,
Been a while, but after burning up my first camshaft after my rebuild I was frustrated and took several months off. New cam and lifters are in the engine and its running great!

I have a spare Rochester Model B carb, which I was considering ordering a rebuild kit for. But I wonder, how you assess whether its even worth rebuilding a carb. Some of the parts, as I understand, are not serviceable, such as the bushing/pin that holds the throttle mechanism. (i don't know the right words here). Just wondering what you all do when making this decision.

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Old 07-11-2020, 05:42 PM   #2
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Re: How to assess whether a carb is worth rebuilding?

As for the throttle plates, the cast iron base plates wear like...iron. If it is currently running, you can spray that throttle shaft (both sides) and see if the idle speed changes. If it does, you're a candidate for bushings. I don't think I've ever had bushings put in an iron base plate, only the aluminum and pot metal ones like Q-Jets and Edelbrocks. For the rest, if you take it apart and it isn't corroded into next week, it'll probably be OK to rebuild.
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Old 07-11-2020, 10:26 PM   #3
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Re: How to assess whether a carb is worth rebuilding?

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Originally Posted by Steeveedee View Post
As for the throttle plates, the cast iron base plates wear like...iron. If it is currently running, you can spray that throttle shaft (both sides) and see if the idle speed changes. If it does, you're a candidate for bushings. I don't think I've ever had bushings put in an iron base plate, only the aluminum and pot metal ones like Q-Jets and Edelbrocks. For the rest, if you take it apart and it isn't corroded into next week, it'll probably be OK to rebuild.
Here are a bunch of pictures of the old one I took off when I rescued the truck from the field it sat in for years. Not sure even where to start.

If you look at where my finger is in the first picture, the old timer who has occasionally been helping me with advice suggested this part might be shot and can't be serviced. I was leaking gas a little bit, but I honestly think it was coming from higher up, not here. I could be wrong. He's forgotten more than I'll ever know about these old cars.







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Old 07-12-2020, 11:38 AM   #4
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Re: How to assess whether a carb is worth rebuilding?

Gas leaking at the throttle valve shaft comes from a flooding condition. That could have just been a stuck float or needle and seat not closing. The needle is usually made with a resilient tip which can deteriorate and allow flooding to occur. It doesn't necessarily mean that the shaft/bore are all that worn, since there has to be some clearance for the shaft to move, and gas will wick right on thru that small a crack. A kit isn't that expensive, so maybe throwing a kit at it will work just fine. It's already off, so take it apart and see what's inside before you buy a kit.
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Old 07-12-2020, 12:45 PM   #5
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Re: How to assess whether a carb is worth rebuilding?

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A kit isn't that expensive, so maybe throwing a kit at it will work just fine. It's already off, so take it apart and see what's inside before you buy a kit.
Sounds like a plan. On my old motorcycle forum, the guys used to soak the carb in pine sol to clean them. Any reason why that's a bad idea here? Or what would you do?
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Old 07-12-2020, 12:58 PM   #6
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Re: How to assess whether a carb is worth rebuilding?

Ya know ya can’t forget something if you didn’t know it or understand it in the first place. LOL!
I’d soak it in this stuff.

https://www.summitracing.com/parts/umi-cc3k

From your pics, that carb looks pretty good. I’ve started with way worse.
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Old 07-12-2020, 07:32 PM   #7
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Re: How to assess whether a carb is worth rebuilding?

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Ya know ya canít forget something if you didnít know it or understand it in the first place. LOL!
Iíd soak it in this stuff.

https://www.summitracing.com/parts/umi-cc3k

From your pics, that carb looks pretty good. Iíve started with way worse.
Yeah, that's the stuff! Just don't leave it on your skin for long.

Never heard of using Pine-Sol, though it is a pretty strong soap. Then again, I actually know people who have "boiled out" a carburetor in a pot on the stove. Bet they had to buy the wife a new pot, after that.
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Old 07-12-2020, 08:55 PM   #8
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Re: How to assess whether a carb is worth rebuilding?

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Yeah, that's the stuff! Just don't leave it on your skin for long.

Never heard of using Pine-Sol, though it is a pretty strong soap. Then again, I actually know people who have "boiled out" a carburetor in a pot on the stove. Bet they had to buy the wife a new pot, after that.
I had some pine sol on hand so giving it a bath now, will see how it works and report back. I disassembled the carb and submerged everything but the floats (they float, who knew?). Gonna get a rebuild kit from Summit I think.

Anyone know if this is the appropriate rebuild kit? I assume so. https://www.summitracing.com/parts/s...-size/3-8l-230
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Old 07-13-2020, 09:48 AM   #9
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Re: How to assess whether a carb is worth rebuilding?

Did the carb have a tag to help select the rebuild kit? If not, Rochester B carbs had 2 different style accelerator pump plungers, round and flat. Yours looks like it should have the round one based on the fuel filter location, but checking doesn't hurt ... better safe than sorry. If you have the round plunger, the kit you selected should work, but with one word of caution. The round style plunger came in 2 lengths (2-13/16" vs. 2-3/4"), if the kit comes with a replacement plunger, confirm the length is an exact match or there's going to be issues. What condition is the float? Is it in one piece and with no holes? Fun fact, 90% of all Rochester Bs are warped at the horn, so don't be surprised if it leaks a tad at the front. Have fun with the rebuild.
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Old 07-13-2020, 10:43 AM   #10
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Re: How to assess whether a carb is worth rebuilding?

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Did the carb have a tag to help select the rebuild kit? If not, Rochester B carbs had 2 different style accelerator pump plungers, round and flat. Yours looks like it should have the round one based on the fuel filter location, but checking doesn't hurt ... better safe than sorry. If you have the round plunger, the kit you selected should work, but with one word of caution. The round style plunger came in 2 lengths (2-13/16" vs. 2-3/4"), if the kit comes with a replacement plunger, confirm the length is an exact match or there's going to be issues. What condition is the float? Is it in one piece and with no holes? Fun fact, 90% of all Rochester Bs are warped at the horn, so don't be surprised if it leaks a tad at the front. Have fun with the rebuild.
Ok so I'm not crazy. This carb has a flat post on the accellerator pump plunger. How do I find an appropriate rebuild kit? Everything is still soaking but perhaps later today I'll put things out and take some more pictures.
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Old 07-13-2020, 10:59 AM   #11
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Re: How to assess whether a carb is worth rebuilding?

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How do I find an appropriate rebuild kit?
Looks like this one from Summit is right for flat shaft: https://www.summitracing.com/parts/smp-123a
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Old 07-13-2020, 11:13 AM   #12
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Re: How to assess whether a carb is worth rebuilding?

I have two of these rochester model b carbs. The one on the truck has a round shaft accellerator pump. The spare one has a flat shaft.

Is there any reason why one is preferred to the other? I could rebuild either one. I tore the gaskets on the flat shaft carb while disassembling so that one will require a rebuild kit in any case to be useful again.
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Old 07-13-2020, 11:46 AM   #13
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Re: How to assess whether a carb is worth rebuilding?

I've cleaned carbs in 50/50 PineSol and Water. If you leave it in too long, it will remove the protective coating on the carb (if it has one).

Worn throttle shaft bores can be re-bushed, I've done a few. Your local parts store probably has bronze bushings in a size you can use. If you don't have the tools, and machine shop can probably bore the holes out for you for cheap.

In my opinion, if the throttle always closes to the same place (you don't have a different idle speed every time you close the throttle), I wouldn't worry about the shaft bushings.

Anything is rebuildable.
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Old 07-13-2020, 01:52 PM   #14
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Re: How to assess whether a carb is worth rebuilding?

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Originally Posted by Praxiss454 View Post
I have two of these rochester model b carbs. The one on the truck has a round shaft accellerator pump. The spare one has a flat shaft.

Is there any reason why one is preferred to the other? I could rebuild either one. I tore the gaskets on the flat shaft carb while disassembling so that one will require a rebuild kit in any case to be useful again.
This is one persons opinion ... Rochester Bs were tinkered with throughout their existence, 2 changes were pretty major. First, going from the flat to round accelerator pump plunger, which mostly affects which rebuild kit one needs. The second is the position of the vacuum port. In the early '70s, the port was moved from below to above the carb's throttle plate for "better" emissions. With the vacuum port above the plate, vacuum advance to the distributor is cut off at idle because there's no vacuum with the plate closed, making timing at idle a big mess. This can be easily remedied by routing the advance vacuum line into the manifold vacuum and capping the carb's port. Also can't hurt to run a straight edge along where the top and bottom half meet to see if there's major warp at the horn.
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Old 07-13-2020, 02:04 PM   #15
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Re: How to assess whether a carb is worth rebuilding?

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Also can't hurt to run a straight edge along where the top and bottom half meet to see if there's major warp at the horn.
Humor me... what part is "the horn"? I've heard the phrase but am not familiar.


Quote:
What condition is the float? Is it in one piece and with no holes?
Float appears to be air tight with no holes. There is a small metallic rattle when I shake it around, don't know if that's normal.
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Old 07-13-2020, 05:49 PM   #16
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Re: How to assess whether a carb is worth rebuilding?

The air horn is the top of the carb that bolts to the float bowl. The rattle is probably a small gob of solder from when the float was assembled.
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Old 07-13-2020, 08:20 PM   #17
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Re: How to assess whether a carb is worth rebuilding?

Just hit me, if you haven't checked the jet sizes, take a look. The B was on a number of vehicles, so having the right size installed is important. The jets typically have the size stamped on them, here's a chart to match up to. If you're off by a size or two, you should be okay or you can swap between the 2 carbs if ya' have to.
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Old 07-13-2020, 10:20 PM   #18
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Re: How to assess whether a carb is worth rebuilding?

Well, I'm glad I checked because I have a round shaft accelerator pump after all. My jet is size .057 so that should be fine. I think with my cylinder bore of .040 over I'm more like 235 ci, probably wouldn't notice much difference going to .058.

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Old 07-17-2020, 12:08 AM   #19
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Re: How to assess whether a carb is worth rebuilding?

I was also chasing a leak for a long time on my B. Turned out it was from the fuel inlet fitting (where the filter is) Threads in the carb body were worn, so could not tighten it up enough. Thought carb was junk, but my local parts store had a brass fitting that had longer threads that went further into the carb to unworn threads. No more leak! Thank God there are still some independent parts houses with 'real' parts guys, can't keep our old junk running without them.
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Old 07-17-2020, 11:09 AM   #20
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Re: How to assess whether a carb is worth rebuilding?

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I was also chasing a leak for a long time on my B. Turned out it was from the fuel inlet fitting (where the filter is)
I think this was my source as well. A poorly flared tube, perhaps. Thinking about going to flexible black fuel hose and a good barb or something.
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Old 07-17-2020, 08:00 PM   #21
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Re: How to assess whether a carb is worth rebuilding?

It's as common for guys to ham-hand the fuel filter fitting as it is for them to ham-hand the oil drain plug, with the same result- stripped threads. They take nowhere near the tightening some guys feel is necessary to avoid leaks. Take a careful look at the thread in the casting to see if there are enough there for a good grip. There's a gasket that keeps it from leaking; the threads don't do the sealing...unless someone ham-handed it. Best solution I've found for that (instead of buying another carburetor, since I was po) was to J-B Weld the fitting into the carb body and put on an inline filter (leaving out the internal filter, of course, but I've seen them left in when trouble shooting). If you end up doing that, only buy a metal filter. The glass and plastic ones are famous in a way that you won't enjoy, if they leak.
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Old 07-20-2020, 03:20 AM   #22
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Re: How to assess whether a carb is worth rebuilding?

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I think this was my source as well. A poorly flared tube, perhaps. Thinking about going to flexible black fuel hose and a good barb or something.
If it was me I would avoid adding any unneeded flex hose. A hard line is always a more reliable way to go and is probably not where your problem is. As stated in the last post the seal is a O ring gasket type thing to the carb body. The fitting I was trying to describe has longer threads on it so that it can keep the proper pressure on that gasket. Must be a common problem as the parts store had that part on the shelf. I'm thinking its the same part for lots of other carbs as well.
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