Rochester B Choke Bowl/Choke Valve Housing Gasket Holes

[thelawdoc]

Guys, If you have a Rochester B, have you noticed whether your gasket, that sits between the carb's upper section and the middle section, blocks a passageway? I am referring to the choke valve housing ("upper section") and the choke bowl ("middle section").

In the attached photos, please look at my choke valve housing and choke bowl, and the gasket that belongs between them that I want to replace. When I was ordering my rebuild kit I noticed that my old gasket seems to be missing a hole opening that would match up to a passageway that goes from my choke valve section to my choke bowl section. Please observe, that at the location on the old gasket where one would expect to have that hole, there is only a dark tan "spot" that reasonably suggests some conflict was occurring there during operation of the carburetor. When I discovered this, I simply assumed that the previous owner had used the wrong gasket. I attributed my poor mpg to this fact.

When I was ordering my rebuild kit from Mike at Mike's Carburetors, I did not see among his available gaskets one that would match up to my carb. Nor have I been able to find this gasket elsewhere online. When I inquired, he seemed surprised to see such a hole layout and said he does not have such a gasket. He said I could just punch this hole in the gasket he would send me. (He wondered whether the passageway was not simply a "casting hole" meant to hold a mold together when producing the carb. However, I pointed out to him that the passageway does have an exit orifice in the middle section-choke bowl.)

So I query anyone here as to whether they have run into this issue? Am I likely correct that blocking this passageway would certainly have a detrimental effect on the operation of the carburetor? I am a novice 'mechanic' but it does not seem reasonable to me that a gasket block a passageway. I have the Manual, but the diagrams in it are not clear enough to ascertain the holes in the air horn gasket. Thank you.

[68gmsee]

Good youtube video on this carb. Look around 32 minutes into it and it shows the two types of gaskets. (The first link shows it at 32 minutes.)

http://www.youtube.com/watch?v=V2AIN...ailpage#t=1922

http://www.youtube.com/watch?v=V2AINKEauVM

[68gmsee]

If you want more info on the carb itself:

Look for the carb model here http://www.carburetor-blog.com/free-...epair-manuals/

[thelawdoc]

68gmsee, Thank you for responding. I have seen the video a few times already. I think that by now I have seen every online video for the Roch B and I have read dozens of online articles and forum threads for this carb. Mike was on the phone with me and had my photos in front of him when we spoke. I did mention to him that I saw what I believe to be the gasket I need in this video from November 2012. Plus, I also mentioned to him that I thought I saw the gasket that I need included with a different rebuild kit on his web site. But he said he looked over all of his gaskets that might be for my Roch B and does not have one with the 2 holes where I need them. Anyway, it is not a big deal since he has one exactly suitable and I just have to punch out the hole in question. (I don't think Mike manufactures the gaskets himself. Rather, he acquires them and then puts the kits together. So, he will have whatever they send him.)

I posted the thread hoping to hear from anyone who has the Roch B and who does their own rebuilding of it and what, if any, issue they had regarding this gasket. I think the Roch B remains in widespread use and my guess is that there is plenty of rebuild experience out there for it. I was surprised to find the gasket in my carb blocking a passageway, the more so because the previous owner of my truck impressed me as a real hands-on, experienced, kind of guy with these trucks. I, myself, am a novice. I found it so hard to believe my own eyes when I saw the gasket that I even began to wonder whether it was intended as some sort of performance improvement. I spent a lot of time researching online because I was looking to see if there was any empirical or anecdotal evidence to be had in support of blocking that passageway. Of course, there is not. Quite the contrary. My research only confirmed that a carburetor must be maintained precisely as called for by its manual. I sent an email to the previous owner of my carburetor, along with the photos that I posted here, but he has yet to respond.

All in all, if my mpg and starting improves after this rebuild, I will be very happy. Plus, after these several days of research, and doing the rebuild, I feel that I have earned my diploma for Roch B rebuild. I guess that is the name of game with these trucks.

[pritch]

I recently rebuilt a few of these. I went and looked and I couldn't find my left over gaskets in all my mess out in the shop. I might have to pull mine back apart to make sure I didn't miss those. I haven't fired up my motor yet, so I'm not sure if they even work, but I'm pretty confident.

[thelawdoc]

Pritch, And what really stumps me is how did my Roch B perform at all with that passageway blocked? I mean, the mpg was crappy, but until very recently, my truck always started up like a brand new Toyota Corolla. As if blocking the passageway meant nothing to her. It was only happenstance that I tore down my carb for rebuild and found that incorrect gasket: a) twice I ran out of gas; b) I did not have the sintered bronze filter in place, and c) I would flood her in my attempt to start with the dirty carb, which caused even more issues, e.g., fouled up plugs. All of the foregoing on top of the fact that I let the truck sit for 4 months. The previous owner rebuilt the carb about 18 months ago according to receipts I found. I have the truck 8 months. So he drove like 10 months with that blocked passageway! When I acquired the truck, I drove her home to Brooklyn from Maryland. She ran and sounded sweet as a happy baby. Go figure.

[pritch]

OK. I finally found my spare Model B part stash:


Counting the two that are on the 292 I'm building for my Panel, I have 3 complete and a 4th for parts.


On the one parts carb I have, I have that same gasket, but that opening you are looking at don't go anywhere:


I am going to take another look, though, because I am looking at the bowl and you are talking about the top, the choke housing.

[bloody hammer]

Here's a photo of the bottom side of my air horn from the recent rebuild.

Mine doesn't have the same holes and the gaskets I got from Mike matched up perfectly.

[pritch]

I went back and looked again, and sure enough, there is not supposed to be a hole there. That is the accelerator pump circuit down inside there and this video from Mike's explains it pretty well, even mentioning that it's the lack of a hole in the gasket that helps hold it all together:

https://www.youtube.com/watch?v=bb_m4c3xS7A

It's super easy to do the test that is done on the video and should be done with a rebuild, anyway.

[thelawdoc]

Guys, I think we have slightly different carbs.

Pritch, if you look at my photos, you can clearly see that the previous owner used a gasket between the airhorn and the float bowl that lacked a hole where a hole should be. Observe the brown spot on the gasket in my photo. That brown spot is the exact spot where the hole ought properly have been.

Bloody Hammer, when I looked into the airhorn's passageway I saw that it, indeed, has an exit orifice deep inside. I shot some compressed air into it and I immediately confirmed the fact. The gasket must have two holes in order to be a correct gasket. Mike agreed with me. He simply did not have in stock this particular gasket. I punched out the necessary hole in the gasket he sent me without any problem. -alden

[bloody hammer]

Alden,

We definitely do have different Rochester B carbs. For example... my fuel line enters straight from the front whereas yours is on the side and has that large opening (for a filter, I assume).

I'm not sure what years the Rochester's were made but I'm guessing there were numerous design revisions throughout the years. I haven't seen any info on how to date a Rochester B, have you?


BTW... this is the rebuild I recently got from RockAuto.... you can see the double gasket and also the bushing they added to the throttle shaft.

This one "seeps" just like the one I rebuilt myself so I may be returning this one.

[thelawdoc]

Bloody hammer, yep. Endless variants of the Roch B. You know, between the numerous variants of the Roch B 7026011 put out by Chevy (variations which do not appear in the service manual), and the errors and omissions of previous owners in rebuilding the Roch B, a guy today could go crazy trying to get the carb in order. I have long ago given up trying to get a manufacture date or precise model-variant number for my carb. For one thing, there is no published record with enough detail to enable one to match up their carb to a date. I have spent many hours sleuthing this. You can come to some range. But that is it. The more critical aspect is simply knowing what you have. And then figuring out what the previous owners swapped in and out. Very tedious. It seems to me that Roch B float bowls and air horns have been mixed and matched over the years by owners, likely in an attempt to remedy the common warping issue. But one thing I feel sure about: there should be no leaks or seepage. Aside from the danger of fire in the engine bay, there is no physical reason why seepage cannot be remedied. And it is my firm opinion that such remedy should not entail doubling or thickening of the gasket. If one lacks a mill, then a file should be used. Not a small file. Large file with a machinist's level, on a jig. And there may be other methods. I do not have the nerve or confidence to heat up any parts and press them into perfect mating. I used a file some, though my carb was really not bad at all at that joint.f Frustrating also is the fact that Rochester does not seem to have published a torque value for the screws that fasten the airhorn to the float bowl. I think that is a critical spec.

Meanwhile, I ran into another anomaly with my carb. When I posted photos of my parts, I neglected to include a photo of my float bowl's throttle body end. I only posted a photo of the float bowl top end that mates to the airhorn . As it turns out, my float bowl has slots on the bottom end that mates to the throttle body. The previous owner again failed to use the correct gasket. He used a regular flange gasket. But it needs a gasket that will accommodate said slots. My understanding from research online is that said gasket will have a "groove." The groove does not go clear across nor clear through the gasket -it is not a slot. It is just a shallow groove, a 'depression' in the gasket, along part of the inside edge, that will permit venting of high pressure from the float bowl's slots into the throttle body bore. This high pressure is said to occur during hot weather slow traffic. Do you have such a slotted float bowl?

The service manual indicates the 250 engine to take the Rochester B 7026011; metering jet size .058"; Idle vent open to .055". The manual does not indicate the float bowl as slotted on its bottom for venting.

My carb's parts exhibit the following:
-No idle vent adjustment screw at top of airhorn. Instead, a removable plug is in the hole.
-Pressure venting slots on bottom end of float bowl.

My carb has no model tag. But the parts have the following numbers on them:
ROCHESTER BC {But I do not have an electric choke nor an automatic choke}
7025382 C-3 {This number is stamped on the fuel bowl.}
22712 {This number is stamped on the throttle control bracket.}
3 57 {This number is stamped on the edge of the flange of the throttle body. The '3' is stamped backwards.}
7004522 {This number is stamped on the edge of the flange of the throttle body}
BC 22952 {This number is stamped on the bracket for the choke control}

So, all in all, this simple carb can keep a guy busy.
Alden

[67 Burb]

My 51 Pontiac uses a Rochester BC carburetor - with the automatic choke. I put a kit in it a few years back, and one of the gaskets left over has the hole in it like you need. It is NAPA / Echlin kit #2-5158. The other pic is the original gasket from my carb.

[thelawdoc]

67 Burb, I managed the gasket for the airhorn-fuel bowl by simply punching out the hole that I needed. Now I have to modify the gasket the kit provided me for the fuel bowl-throttle body connection. An online photo seems to show that it should have a groove that goes about one quarter of the way around the inside edge of the gasket, and about one third down into the thickness of the gasket. But the photo is not perfectly printed and I am still in the process of gathering this spec. Wish me luck with the scalpel. -Alden

[JJH Jimmy]

I recently had the same issue from a Rochester B that is on my 51. I had already discarded the original gasket so i could not tell if it covered the hole or not. I just put the new gasket on covering the hole and it runs perfect. If a remember correctly either the top portion or the lower portion does not have a passage way through it so i figured it would not make any difference.

[thelawdoc]

JJH Jimmy, you recollect correctly that the passageway lacks an exit. Only the airhorn has an exit at the end of its passage. As far as it making a difference, well I know too little about carb engineering to venture into that. But I did see a brown spot at the site where that hole should be. And I did have terrible mpg. So, now that I have the correct gasketing, I will soon find out.... Alden

[bloody hammer]

Alden,

I ordered several different gasket from Mike when I was first getting started on the rebuild. This one here seems to have the additional hole you mentioned. Check it out...

[thelawdoc]

Bloody hammer, yep. That's the one. Mike was out of those on the day that I placed my order, but he sent me one that was identical but for that hole. I easily punched out the necessary hole.

Bloody hammer, my float bowl has slots on the bottom end that mates to the throttle body. (The previous owner again failed to use the correct gasket here.) This gasket will have a "groove." The groove does not go clear across nor clear through the gasket -it is not a slot. It is just a shallow groove, a 'depression' in the gasket, along part of the inside edge, that will permit venting of high pressure from the float bowl's slots into the throttle body bore. Do you have such a slotted float bowl? And if you do, what gasket did you put on?

[bloody hammer]

Originally Posted by thelawdoc

Bloody hammer, my float bowl has slots on the bottom end that mates to the throttle body. (The previous owner again failed to use the correct gasket here.) This gasket will have a "groove." The groove does not go clear across nor clear through the gasket -it is not a slot. It is just a shallow groove, a 'depression' in the gasket, along part of the inside edge, that will permit venting of high pressure from the float bowl's slots into the throttle body bore. Do you have such a slotted float bowl? And if you do, what gasket did you put on?

I think the bottom of the float bowl is the one thing I didn't take a photo of when I rebuilt it. Here are the three gaskets I removed though...

[thelawdoc]

Bloody hammer, thanks. Actually, we "shouldn't" have slotted float bowls. If I understood and recollect your truck, we both have 250's and Roch B's running pretty much stock. The published material is pretty clear that our trucks never got slotted bowls -at least not from Chevy. Previous owners swapped it in either because they were having issues in hot weather when sitting in traffic, or had a warping issue. Anyway, it is what I have so I have to deal with it. The gasket you show with the notches is, to the best of my knowledge from tedious research, indeed intended for the slotted bowls. It seems to me that, because it is not like the OEM gasket that had the groove (described by me previously), it is obviously a modern, cheaper-to-produce replacement. What has me stumped right now is that the gasket you show just prior to the notched one is also for slotted bowls. There seems to be absolutely nothing from a Google search in the way of specs for these gaskets. I have run across a guy who I believe to be perhaps one of the last living experts for the Rochester B. His name is Jon and he writes under "Carbking" on another forum. His advice is to use the grooved gasket as called for by Rochester. His company does make the gasket but one can get it only when giving his company the carb for rebuild. Regrettably for me, I was nearly done with my rebuild when I came across his extensive expertise. However, I feel sure that if I can come across a sample of this gasket, I can spec it and duplicate it by hand. Anyway, I cannot meditate over my carb anymore. It is time to get it back in the truck. I put on the gasket that you show just above the notched one because I do not feel confident about the notched one for the slotted bowl as compared to the one with the holes. I can always swap it out. The slots are really just for venting high pressure that seems to cause issues for the Roch B when idling in hot congested traffic. I made one very minor mod when rebuilding my Roch B. The rebuild kits give what appear to be very cheap clips for retaining the choke rod. The kit also includes small 'horse shoe' clips for anyone who does not like the tiny clips. I do not like either of them. When I used the clips, one of them slipped off when I was working the choke rod after oiling. When I swapped on the flat horse shoe retainers, one simply fell off. I noticed that they lack an ability to sit flush and lock around the choke rod's detent. They are a joke. In the end, I used both clips together (flat horse shoe under the clip) and I was satisfied that, for the mean time, it will be good. The old clips seem much stronger and the only reason I did not use them is because they are pretty rusted and I fear the rust will soon go through. But I will keep this issue in mind and eventually figure out a way to retain the choke rod in a more secure fashion. The second thing I intend on once I see that my carb is good is to use a thread locker (red) on the bolts that fasten the airhorn to the float bowl. (I know it will take heat to free them for removal.) I could not find any torque specs for these bolts and I believe that when using a gasket on a Zamak casting, the designers ought properly have published torquing specs and grade of bolts. For now, I tightened them 'good' even though I do not like doing things that way. Also, for the moment I am not putting in the sintered bronze filter. I do not believe that one should use this exceedingly fine filter without having a fuel pressure gauge between it and the carb. These filters are so fine that they can clog up really fast and shut you down with no notice. This is a filter for which you really want to have a spare in the truck. At the same time, it also seems to me that, without this filter, the carb will be under higher than spec fuel pressure, which might be a flooding factor. Until I do up a new housing that will entail a fuel pressure gauge fitting at the carb, I will add an inline filter between the pump and the carb to reduce pressure somewhat. Ok, so I want to tell you -and the other guys here, thank you for your time and attention. You guys gave me knowledge -which I consider to be a valuable thing, and you guys gave me confidence -which is just as valuable. -Alden

[bloody hammer]

Alden - Mine is 62 Step with a 235. I posted the gaskets in order from top to bottom...

airhorn to float bowl

float bowl to throttle

throttle to insulator/spacer*

* I've seen it said that a gasket is not needed here but I put one on anyway since there was one there already.


I agree about the clips that came in the rebuild kit... pretty flimsy. Luckily, the ones I already had seem in good enough shape to reuse. Are these the same kind you had on yours before you rebuilt?

[thelawdoc]

Bloody hammer, so I did mine as you did yours. But, for perfection, the bowl-to-throttle gasket, if you have a slotted bowl, should be one with the groove. I cannot tell from the photo whether your gasket has that groove and thus whether your bowl is slotted. I am not going to worry right now about the slotting issue. It is really just for venting high pressure when idling in hot weather for a long time, like when stuck in traffic in a heatwave. Your clips differ from mine somewhat. I told Mike that he is giving us junk with his clips. I was able to use one of my old ones. The one in the photo immediately below is the one that I did not use because it seemed to have been played with too much and lost its form. But it is clearly a better quality clip than the one in the kit which has insufficient 'spring' in it. No big deal. It would be easy to fab up some clips. In fact, it should be pretty easy to even fab up a complete choke rod linkage. I just wanted to finish my carb already. The second photo below is of my completed carb. You can only see the lower clip of the choke rod; the top one is hidden with this camera angle. My first real automotive mechanical job in my life. Actually, the carb itself was no story. It was the research that took up most of the time. Plus, as a novice with this stuff, perhaps I was overthinking things a bit. (I am an aspiring amateur machinist.) If the carb performs as I hope it will, I will give it a nice paint job and keep it. And in the future I will fab up my own rebuild parts and gaskets. And I want to fab up a leather plunger. First I want to confirm that the Roch B can be made perfectly leakproof. I am paranoid about gas dripping onto my exhaust manifold. -Alden