The Stovebolt Site Forums The Shop Area Engine Shop 3 - Stromberg 97's, 302GMC - Carb adjusting help?

I recently rebuilt to stock condition a 302 GMC straight 6. I have installed a header and duel exhaust system. I have a McGurk log intake, that is setup for three Stromberg 97 Carbs. all three are rebuilt with the IDOL jets at .044" (I think, need to double check). To start things off, I originally blocked off the two outer most carbs, and just had one in the center when starting the motor. I wanted to make sure the motor was tuned in, and timing set before intruducing all 3 carbs. I got the motor running and idoling good. I added the two outer carbs, with progressive linkage. The linkage is driving the center carb, and at about 1/4 throttle, the outer two carbs come on. (oh ya, I blocked off the idol circuit on both outside carbs, and have removed the choke butterfly plate on these as well.

With the 3 carbs, It starts ok, but idols rough, almost like it's on two cyclinders. When giving throttle, it backfires out of the exhaust, and runs very rough until I can goose the rpm's up, then it revs up pretty good, and strong from there. But again, will stuggle with idol and will sputter. I have not balanced out the airflow yet, but plan on looking at this, this weekend. What steps can I take to begin to dial this beast in!! Any help is appreciated.

That's cool - a 302 with a McGurk log running 97's. My thought on this is that with the progressive linkage working the center carb first, the outer carbs are leaking vacuum into the manifold and upsetting the mixture. I would try a straight-up linkage so that all the carbs are putting fuel into the mix at the same time.

Formula1,

Boy, that's a lot of carburetion. I sure would like to see a picture of your engine and manifold...it sounds impressive.

Stuart

Formula,

I have a 302 with a howard log and 5 of those carbs. I haven't got it runing yet. I am running a progressive 2 primary, with 3 secondary. The guy who rebuilt my carbs told me to try it out like you. I had concearn about the extra carbs contaminating the mixture at idle. He suggested soldering the idle jets closed. I was also concearned that air would slip past the throttle plates. www.Vintagespeed.com sells 97 carb bases that are suppose to address that issue. Good luck. That sounds like an awesome setup.

John

I have some great shots of my motor and carb setup. How do you post pictures for viewing?

Formula - before investing in throttle bodies which you probably don't need:

Remove the linkage from the outer carbs; but leave the carbs in place. Wire the throttles closed. Some of the aftermarket progressive linkage is really good at leaving one or the other outer carb cracked open.

My guess is this will clean up the idle.

If the idle clears up, add linkage to one carburetor at a time, to determine which is the guilty party.

If removing the linkage does not clean up the idle; remove both carbs, and replace with block-off plates. Readjust the center to get a good idle. Now introduce the outers 1 at a time. The idle adjustment on the center carb SHOULD NOT CHANGE!

In the "for what its worth" category, I would highly suggest solid linkage for this set-up NOT progressive. The 97 is too small for the center to use with progressive linkage.

Jon.

Thanks Guys. I'll try CARB kings suggestions this weekend and will post back my findings.I am not sure how to post a pic, but if anyone is interested in taking a look at what I have, let me know your email and I will forward.

You might also find that one or more of the throttle bodies has a leak in the throttle shaft bore.

email me the shots you have and i'll host them and post them for you. plus i'm getting ready to do a offy 3x1 rochesters and am very interested in seeing your setup!

mwleal@fedex.com

I replaced the throttle arms with new, and reamed them out for the proper size. Just curious, how would you verify a potential leak of there is one?

Update, I disconnected the two outer carbs from the linkage, and made sure the throttle adjusting screw was backed off, and tired the throttles closed just to make sure they did not move. The truck ran considerably better, and drove down the street nicely. But, I had to have the choke cracked open about 1/2 way in order for it to idol and run. Any input appreciated. Lastly, here is my webshots link for photo's. I have tons more, with details that I will be uploading soon.

http://community.webshots.com/user/unser102

Formula1 - you have now verified two valuable pieces of data: (1) that the progressive linkage was giving problems, and (2) a single 97 IS TOO SMALL to be the primary carburetor in a 3x2 for a 302 with progressive linkage.

My suggestion would be either: (A) use solid linkage; or (B) use different carburetors. Obviously, solid linkage would be the least expensive.

Jon.

The carb used on most or all stock GMC 302's is a Holley AA1 2 barrel.Very similar to the 94 model used on Ford Flathead V-8's.Does a Stromberg 97 flow less air than the Holley?

The Stromberg 97 is a model of the EE-1 series. It has a 15/16 inch main venturi. Without looking to be absolutely sure, the EE-1 platform would support either 1 1/16 or 1 1/8 venturi. The larger venturi will flow more air.

The Holley AA-1 is similiar. There are a number of different internal venturi sizes. Again, without looking, my guess would be the 302 would use the largest internal venturi available in the AA-1 frame.

Actually, a large Zenith single barrel carb was also used on the 302. The single barrel was a size 3 with a large venturi.

Jon.

Carbking,
Thanks for the continued help. Just so that I understand, what your suggesting is to remove the progressive linkage connecting the center carb to the two outer carbs, and replace it with a solid link, thus allowing all three to open, close identically?

Yes, the carbs will need to be synchronized, but set them to idle on all three, and run on all three. If you wish to discuss, call me during normal business hours, and will be happy to explain the synchronization process.

Jon.

I have a pile of Ford/Holley 94's on my bench that have 15/16" venturi. I bought them with the intention to do a 3x2 setup on the 302.

I also have a AA-1 carb off a GMC that has much larger venturi- at least 1-1/8, maybe 1-1/4". There is a very noticable difference. A single 94 is going to really choke a 302.

Also, if you are trying to run vacuum advance to the distributor, you have to modify the strombergs- if they are setup for a flattie Ford the vacuum advance port works backwards. You will have to modify one of the carburator bases to provide ported vacuum for the distributor.

UPdate:
Someone a while ago told me to plug two of the three idel circuits as only one will be needed. That was false. The truck could not run on just one idel circuit, and backfired to beat the band. I unplugged them both, and wal-la, it's running much better, good idel, and no backfiring. I took Carbkings advice and removed the linkage, balanced out the airflow, and got the carbs working together nicely. Now that my linkage, idel circuit are working properly, I have another problem. The truck has a hesitation particularly on hills, or lower in the RPM's. A friend mentioned that it sounded like it was running lean. Can anyone provide guidance on how to determine this, and what steps to correct? FYI, my setup is a GMC 302 straight 6, with three stromberg 97's on a McGurk Log manifold. If anyone is interested in my project, check out my webshots at
http://community.webshots.com/user/unser102. I am thinking of going larger on the main jets, but before I do, would like opinions. Oh ya, I have hot water circulating throught the manifold for a pre-heat condition.

It is possible you have a lean condition, it is also possible you have a ignition timing issue. It is also possible that you have a RICH condition

Maybe an ignition guru would jump in here, and talk about distributor advance on this particular engine.

If it turns out to be lean or rich, give me a call, and I will explain some testing.

Jon.

You might have to reset your float levels on your carbs. I was setting up two 97's on my wife's Model A Ford (I know iat is only a 4 banger)which has a two port Riley and when I first started it up it was running in a lean condition. The carbs were set up with a float level for a Ford V8. With two carbs the engine vacuum was lower making the carbs run lean. I raised the float level on both carbs about a 1/10th of and inch at a time till the car was running a lot better and the plugs looked OK. The more carbs you run the lower the intake manifold vacuum will be and you will have to raise the float level to compensate for it.

John

Cessna195,
I am no expert, that's one of the reason's why I use this forum is to get insite, but I usually can figure things out. By reading your reply regarding setting the float level, I am struggling to understand how this would make a difference?? I rebuilt all 3 97's and used the supplied gage to set the bowl heights. It's pretty straight forward, but possibly I was off a bit. Help me better understand how by raising or lowering a float level would impact this?

Carbking, thanks as usual for your offer to help. Nobody chimed in on the timing, but I am fairly confident my timing is correct. Could you offer some tips on checking the too lean, or too rich statement so that I can begin to tinker a bit to fine tune!

Thanks

Are all 3 carbs accelerator pumps working good? That can cause a backfire when the throtle is opened. A good squirt of raw gas makes for a momentary rich condition that works until the manifold vacuum picks up additional fuel as it flows throught the venturi.

The backfiring is gone, but only a stumbling under lower RPM, and load. I believe the accelerator pumps are working good, but how best to test??

CARBKING, please respond.
Update. The backfiring is fixed. The linkage is now straight, and the progressive is removed. I still get a hesitation when low in the RPM's. When going up a hill, the power is not there. So, I pulled all three pairs of main jets to measure. They are all reading between .046 - .047" diameter. I was told that since there is so much carb, the velocity is not there, and need to open up the jet size. Do you agree, or should I look at something else at this point??

Give me a call during normal business telephone hours, to discuss.

Jon.

You can determine if you're running rich or lean by reading the plugs. Drive on a straight level road at about 35 mph for 1/4 mile, shut off the engine and pull off the road (parking lot?) and read your plugs. Adjust jets accordingly.

As per my last post, Steve called today, and we had a good conversation. The question was at the point where a dialogue was needed instead of a "war of posts" in order to discuss the issue.

To update anyone following this thread, this is a summary of our conversation.

The engine seems to be fine except on low vacuum and low RPM extra load (i.e. hills). Steve has only a few miles on the engine. As fuel requirements are greater during breakin, Steve is going to look forward to decent weather, and put a few hundred miles on the engine on level interstate, of course watching for excessive water temperature. Hopefully, as the engine is broken in, fuel requirements will ebb to the point of the hesitation going away. Probably not going to happen, but engine does need more breakin for final tuning, and we can always hope.

Steve is aware he has way too much carburetor for the street, resulting in low air velocity in the carbs which is the problem here, but is willing to do some fabrication if necessary.

Since it appears that high vacuum metering is about right, changing the main metering jets is probably the wrong approach; therefore, if the hesitation persists after breakin, Steve will augment the main metering circuits by changing the timing of the power valve in the center carburetor. On the Stromberg 97, the power valve is actuated by the bottom of the accelerator pump depressing the power valve plunger, when the accelerator pump is "bottomed". To change the timing, a longer accelerator pump link will be fabricated to lower the pump of the center carburetor in the pump well, thus allowing the power valve of the center carb to be engaged before it normally would be engaged. This will richen the low vacuum mixture without richening the high vacuum mixture.

In the event that there is insufficent articulation in the standard accelerator pump to accomplish this, a new accelerator pump will be fabricated with a longer articulation.

And while I forgot to mention this to Steve, if this still turns out to be insufficient fuel, then the power valve of the center carburetor can be enlarged for a greater volume of fuel flow.

Steve, if I missed anything, please chime in.

Jon.