Let's build a Stromberg 97 3x2

I'm putting together a 3x2 intake set up for a early sbc and I thought it would be a good idea to make a thread showing the steps start to finish. I haven't put together a 3x2 system yet nor have I used any Strombergs, but I do have an idea of what needs to be done. I would love to be schooled, hear tips, and feedback.

I'm using 3 new "Genuine Stromberg" repops (not speedway's) so there won't be any need to rebuild. Intake is an Edelbrock C357 on top of a 283 with a 327 crank. The center carb will be bolted on "out of the box", but the outer two will need to be made into secondaries. So I figured I would start on the outer two.

 

I didn't want to buy any secondary bases so I ordered 12 degree throttle plates from Dickester27 here on the HAMB. They should be here in a couple days. In the mean time I'm going to take out the idle screws and epoxy in some 1/4" 10 32 screws. (like the standard with metric?).

I plan on leaving in the choke plate but lock it wide open.

 

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Shocked to see how badly those plates fit. I built this and had different "sloppy-shaft" issues. Drilled them out for larger shafts. Looking forward to seeing more.

 

Yeah I was just a little upset over the fitment with the price paid on these new carbs. I'll deal with it though, after going through the paces tracking down enough cores and rebuilding other carbs, I'm more than happy to have new ones. Everything else on them are top notch and the throttle shafts are good-n-tight.

 

Second carb torn down.


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Third carb is for the primary. Need suggestions on jet size....

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Remember the 2 vacuum holes in the top of the bases, fill with epoxy. Bump your primary to 48 and leave the 45's in the secondaries.

 

Ah yes, thanks Dickster27! On my way to grab some screws and epoxy. I am also going to make some block off plates for the secondary spots and get the motor running on the primary...I'll be ordering some jets from you Dickster27.

 

So many schools of thought on this subject. My Fenton intake with rebuilt 97's and Segal progressive linkage. All vintage stuff. Unlike a "rochester style setup, my end carbs have idle circuits,43 jets, 69 power valves. If i was plugging anything I would do it with lead, not epoxy, easier to remove if you have to. Thats what we did at the carb shop I worked at. 1957 283.




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Why not just screw the idle screws all the way in? And why do you need to remove the power valve?

 

Sorry not removing power valve. On the idle screws I've heard it both ways. I don't think one is better than the other. I plan on tracking down some more cores so it would be nice to have some extra needles.

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Grasshopper... You would have been far, far ahead to have just run them with direct linkage.

 

This is very interesting.

 

Made a couple of block of plates. Just thin sheet metal so I'll use some 59 bases to keep even pressure on them.

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Who's linkage are you referring to Max?

 

What are the block plates for? Are you planning on firing up with just the center carb and tuning it?

Edit: missed the part where you said you would be firing up the primary carb.

 

Yup. About to turn over the motor on just center. Tune, then introduce the secondaries.

Running progressive linkage.

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Maybe a mechanical advance dizzy?? I thought Strombergs provided minimal vacuum.

 

Was reading bunch of reports going between manifold or port vacuum for these set ups. The intake has MV provision so I'm going to try that first and see how it goes. If it doesn't work then I will tap a hole on the primary base and go from there. Open to all suggestions and opinions.

Max-I know that going non-progressive would be easier as long as you can tune all three together, but I want to give progressive a shot first. I do have two SUs on a volvo 4 banger that are non-progressive that run great.

 

I got the engine running on the center carb. I can get it idled down to about 700 just fine. It does seem to fluctuate a bit, maybe 10 rpms, but its smooth. Very snappy response so far though I think that I can get it dialed in a little better. I need to get a regulator to bring the fuel down to 3 or 4 pounds.

I was kinda of thinking that one 97 may not have enough cfm and that I would have to switch to non-progressive but it doesn't seem to be the case.

 

Should I strive to get the idle down to say 500?

 

Hope you've budgeted for new pistons!

 

Hey uncle max, others list you in the group that knows these carbs, and i respect that. But how about some input rather than cryptic responses.

Are you saying this because using one carb will run to lean? I'm ears. I haven't changed anything on the outer carbs yet and i can be schooled to run them non-progressive.

I'm a body man by trade, never claimed to be a mechanic. When someone asks about metal fabrication or paint and body work and i decide to chime in on my own, I'll put some effort into my response.

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cool

 

You need 1.5 to 2 pounds max.

 

Thanks alchemy.

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Hi everyone – especially the guy at North Texas Customs. Sorry, but I couldn’t find your name, anywhere. I usually stay away from commenting on here as I don’t believe the HAMB should be the place to promote our businesses. But having read a few things that I (and Max) found surprising if not alarming, I’d like to add a few words that might help you and others set your 97s up on a Chevy motor. Everyone is entitled to their own opinions, of course, and we respect that. These are just ours. I’ll try to work through the subjects covered in turn.

First off, thanks to North Texas guy for buying Genuine Stromberg. We’re a small company and we appreciate every sale. Remember you can always contact us – usually me personally – at tech@stromberg-97.com if you have an issue or a query about setting up new 97s.

Primary/Secondary
Now, onto the primary/secondary issue. It is my belief that you cannot regard three 97s in the same way as a factory Rochester tri-power. They are completely different carburetors. It is far wiser to ask how the 97 can work best for your particular application. Max is right. Back in the day, people mostly used a direct linkage with all three carbs the same, and yes, you can set the carbs up to work like that with great results. What we tend to do now is use a progressive linkage with all three carbs pretty much the same, rather than the traditional idea of one good primary and two 'dumb' secondary carbs with giant main jets, but no power valves or idle circuits. I guess it’s a sort of half-way house solution. We still call them primaries and secondaries, but only in terms of where they sit and which open first.

Looking at your 3x2 progressive then… We recommend you leave all three carbs working, with idle circuits in all three WORKING so all of the cylinders get even fuel distribution at idle and even further up the rev range (97 idle circuits don’t really ever turn off). On those dual-plane Chevy 3x2 intakes, the center carb can supply all eight cylinders, but they’re pretty old designs, unchanged since the 1950’s, so the better access you can give all cylinders to even fuel/air distribution the better. You don’t need lean ends – even flathead 3x2s can see that if you’re not careful.

Using this set-up, we tested a hot 355inch Chevy motor back in late 2010 in Arizona (around 2000ft elevation) on a new Edelbrock intake and it made 345hp at around 5000rpm with better fuel efficiency than a Holley 600 4bbl. The intakes max out at 5000rpm. They rev to about 5700 but the power and torque is flat after 5K. Torque was about 380ft-lbs. We were testing something else at the time so at the end of the session we literally used the 97 carbs out of the box with stock 0.045 jetting and number 65 power valves, and the air/fuel ratio was pretty good from idle to 5K. We did find that you could disconnect the outer carb accelerator pumps and it made no difference. You didn't need the outer pumps (squirters) or the extra top end enrichment from the outer carb power valves either (they come in when the accel pumps hits the pin on the top of the power valve at about half throttle). If you epoxy the PV holes up, and leave the accelerator pumps linked, you will get some hydraulic pressure in the pump well, even with the little relief valve in the bottom of the pump. Plus you pretty much wreck the carb for alternative use. We have developed a ‘dummy’ pump for use in a future project so you can keep the look of your pump and pump link. We did not spend a lot of time on this test and it could be that we could have adjusted the tune to keep all three PVs and pumps working. I expect we’ll test this set again soon, so I’ll report what happened on a different 330hp GM Performance crate motor.

I might add that we have many customers running this same kinda system including one buddy here in the UK who leaves black tyre lines at every local stop light. 345hp in a lightweight '32 Ford is pretty scary! And they really do rev up quick with the progressive linkages.

Non-fitting throttle plates.
Every new 97 is checked at the factory for many things, including throttle plate fit. If your picture was with the throttle stop screw backed off to allow the plates to shut, we’ll gladly look at it again for you, though remember, in our recommended 3x2 set-up, you will have all three carbs slightly open when you set the idle speed. We don’t see a need for 12 degree throttle plates.

Other stuff.
I’m working my way through the comments here….Leave the chokes in. 97s need them to ensure reliable vacuum signal at the emulsion tube tips. You can lock them open with our 9537K-L kit…..Thanks for your kind words : “Everything else on them are top notch and the throttle shafts are good-n-tight.”…… The holes in the top of the base that run diagonally to the idle ports are for fuel and air mixture from the idle jets and idle air bleeds above in the top of the bowl…..Listen to F-6Garagerat. He speaks wise words! ….I don’t see a problem with firing the car up on a center carb only, but you won’t have much cfm (162) so don’t rev the damn thing high or you will hole your pistons. More on cfm later….Fuel pressure. Aim for 2.5psi. You can go lower but not much higher.

Distributor vacuum
The 97 provides no ported vacuum for a distributor advance. I recommend an all-mechanical distributor because we have had customers connect the distributor to the manifold and it runs like sh*t, won't idle and throws out black smoke because the distributor is fully advanced at idle. We have seen vacuum taken from the rear plug on the 97 base, but our consultant at Competition Fuel Systems in AZ says, “we need the vacuum advance only at light load/part throttle conditions (cruising), and none at idle or full throttle. Only a port on the front of the throttle bore at the same height as the transition fuel port accomplishes this”. We are working on a new base to achieve this, right now.

CFM Requirement
People get far too hung up on cfm. The important thing to remember is that maximum cfm is only important at max revs. If you do the traditional math for cfm requirement, you might decide that three 97s at 162cfm each is not enough air, though evidence on the dyno and road disagrees. And I suspect that’s because we all overestimate the volumetric efficiency of our engines. Remember I said that the Edelbrock intake test stopped at 5000rpm. That wasn’t the carbs. It was the manifold. And as we said earlier, how fast do you want to go? At idle your Chev could run on a lawnmower carb because the requirement is cylinder volume x revs. Low revs = low cfm requirement.

Having said all that, we are in final testing of a new big 97 tripower that flows over 750cfm from three carbs. And these are more of a traditional primary/secondary setup, because that’s how they were designed. They are the same as 97s on the outside but totally different inside. And best of all, they haul ass….But more of that another day…

 

Thanks for this - it means a lot coming from "the horse's mouth".

 

WOW, 750 CFM! I need to start saving for some for the TWIN. Thanks Clive, Rocky