Hi all,

is is there an issue running a 78’ 403 carb on my 70’ 350? Ie too much cfm, jet differences etc?? Both engines are stock build

 

Matt, It will probably be just fine. Considering...I haven't done this....
Is the carb in good shape?
Does everything connect correctly with it? Concerning the throttle bracket, Choke, etc?

GM spent a lot of money and time at engineering to put the right carb on its engines. Lots of hours and research and development. It won't be perfect.

I'm sure some of us have worked with worse.

 

^^x2^^ what don71 stated......"probably". You should expect to have to fine-tune the carburetor & it might take some time to pin-point its optimal tuning.

I'm sure you'd like to expect a 'no problem', just go for it. Yet, Q-Jets were designed w/ very specific metering of both the internal venturi series, the primary & secondary metering rods, base plates, etc. Bear in mind, as you move from a 1970 Q-Jet to a 1978 Q-Jet, emissions controls were ramped up significantly & Q-Jets were specifically designed to optimally perform within the ever-increasing emissions scenarios which changed nearly yearly from 1970-1978.

I'd suggest you research your carburetor(s) by number & gain some familiarity with the Q-Jet in general (during the years between 1970-1978). When I have an issue understanding the deltas between carburetors (Q-Jets in particular, but also Carter, some Holley, etc.) I head over to this URL>>>>https://www.digitalcorvettes.com/for...t=88376&page=2

It may state corvettes in the URL, but don't be misled. The folks who put this information together (including Joe Padavano from this CO site) is, IMO, one of the most informative exposes on Q-Jet carburetors. It isn't for the timid, plan to spend more than a couple minutes digesting the information.

 

Yeah I have done a fair amount of tweaking on it already. Seems to still be a little stumbly when taking off. Checked timing religiously since I know most carb problems are usually timing lol. I just wanted to make sure I wasn’t flodding my 350 out with the 403 carb as I have read the 403’s were ahigher cfm. Also seems like I need to keep the choke set even an operating temp to keep her running. Still working on it, I’m sure it’ll be a bit To get optimum fuel delivery. I completely agree with you guys about GM doing extensive research to get each carb setup for best performance for each engine which is why I wanted to get an opinion. I’ve had my share of Quadrajets , no stranger to them but I am new to olds motors. I love my Quadrajets when they’re dialed in right. This 403 carb is on my newly overhauled 350. Carb worked beautifully on my 403 but it’s not playing as nice with my 350. 350 seems to like a rich mixture which I suppose makes sense since we’re talking 1970 motor vs emissions choked 78

 

Matt- There are very specific deltas between the 70xxxxx series (pre-EGR) and the 170xxxxx series (EGR) Q-Jet carbs (these are pointed out in the URL I sent to you previously).
A 1970 Olds 350 Q-Jet should be 7040250 (70xxxxxx series). A 1978 Olds 403 Q-Jet should be 17058253 (170xxxxx series).
The delta in cfm on a 7040250 vs a 1705853 isn't the issue. The issue is going to be the size of the primary metering rods/jets and the curve of the the Q-Jet's fuel delivery (pre-EGR vs EGR). Some people have performed maneuvering steps to accommodate, but you'll need to do your research to determine if this is the way you want to go. The EGR carbs have larger jets, and even larger diameter rods. This changes the fuel delivery curve.
Again, have a close look at the link I sent to you previously. Search for "170" on that link and note the specific deltas between the 70xxxxx series and the 170xxxxx series. Note the delta in primary metering rods/jets, rich/lean delivery (throughout the curve). HTH Cheers.

 

I believe the 1705853 is a 800 cfm Q-Jet, still should work on your 350, with jet and rod changes, that’s the beauty of Quadrajets.

 

Do you mean partially closed ? Please explain.

 

The 403 carb is tuned very lean, the idle especially. I had a 403 carb on a 9.6 to 1 350, it ran awful. Completely recalibrated by Everyday Performance made a huge difference. It has some advantages over your 70 carb, not just the extra cfm.

 

Figured I’d post video (hopefully you guys can see it. I’ll post pic too in case video doesn’t work. It’s the stream of fuel I’m talking about. This is after running. Note: when engine running there is only one stream if fuel as well. Also posted pic of choke setup when engine hot. Also why am I getting smoke/vapor out top of carb? Just fuel hitting the manifold. Carb rebuilt 2 years back but I can see another I future...

 

It looks as if you have an electric choke. If the choke air valve is not opening after a period of time, then I suspect you have an issue with voltage to the choke or the electric heater coil is defective.
Verify you have 12 Volts on the wire, then verify there is a good ground (as in no gasket between the black choke coil cover and the carb).

 

The choke will open more if I adjust it to, but it needs to be that choked to keep it running. I’m really concerned about only 1 steam of fuel, is that normal ?

 

Carb could be dirty, causing primary side to only feed fuel to one side. Also, the power piston might be stuck in it’s bore, with engine off, use a small screwdriver to carefully depress the piston, It should pop up if its free.

 

I took the Airhorn/top plate off today abd cleaned all passages and checked the needle and seat, float level and accelerator pump and all looks good. Piston is moving as it should. Am I correct in my assumption there should be 2 streams of fuel coming out?

 

There should be two.

 

OK, so you have to restrict the amount of air to keep the engine running. Sounds as if the mixture is too lean and the air restriction is required to make it richer. When you had the air horn off, did you see what primary jets and rods were in there?

 

Matt, a few mods you'll need to do: increase idle tube restriction, increase idle down channel restriction, increase idle mixture discharge diameter, increase jet, richen secondary metering rod & change hanger from M to something between A-J. All the mods are in Cliff Ruggles book, some on Youtube. Besides being a very lean carb from the factory, today's 10% ethanol added fuel requires an almost 5% fuel increase across the entire fuel curve. Best of luck with your project.

 

Would only having one stream of fuel when pulling back accelerator be causing my lean condition??? The jets were a 73 and rods 55

With the mods that need to be done would I be better picking up a carb from 1970 that is correct for the 350? If the 78’ carb is too lean for the engine

 

No, that is a superior carb. Unless your motor is stock, the 70 carb may need tuning as well but will be closer. I believe my 78 Qjet had 73 jets and 44K rods, yours is very lean. You need to do the idle mods mentioned above to idle the way you want. The one stream could be a plugged passage, it will definitely make things worse. Contact Ken at Everyday Performance. He does complete retune and rebuilds, throttle shaft rebushing, even replating. His prices are fair. Unless you can find a 70 core that is in good condition, get your current carb reworked.

 

My motor is a 70’ 350 with #6 heads, all stock including the larger dish lower comp pistons on the 250 horse 350. My cam is a Melling Mto1 which is an RV cam with a 474 lift vs the 400 lift factory cam. So it’s basically stock. Changing out the jets and rods are a pretty simple swap. Before I go nuts modding this carb I want to make sure a simple rod/jet swap wouldn’t fix it. I will pick up cliffs book, I’ve been hearing about that book for years

 

You should have 2 streams. I am also noticing that you may have one of those later "aneroid type" Quadrajets. According to what I read in Cliff Ruggles book, they can be finicky to tune. Since you already have a problem, and if you do in fact have one of those "Aneroid type" carbs, I would find a good "earlier" Quadrajet and start from there.
I would highly recommend Cliff Ruggles book before you go much farther. You can order it online and have it in just a few days.

 

I’m going to attempt to find out why I’m only getting 1 stream of fuel. If I can get that to shoot 2 it may help out a lot. I’m not expecting it to but I’m going to get that situated before I swap parts when I haven’t fixed an underlying issue first. My first instinct is accelerator pump is bad but I’m still getting 1 stream, I’d think if that was shot I’d get nothing. Just bought cliffs book. My carb is a 17059253 so a 78’ 350/403 carb

 

Blocked primary nozzle, could be trash, also,check the gasket to assure all of the passages line up.

 

Something appears clogged on one side.. Its showing up more visibly by one stream. The primary circuit is probably clogged and that's why you have to run the choke partially closed.

 

^^^ Saved me a lot of typing.

The '78 carb is a better unit than the '70 design. Don't go off looking for a '70 carb, make the one you have work PROPERLY. START by fixing the known issues--accelerator pump circuit has problems, the choke coil isn't set properly, and--probably--the idle mixture screws aren't adjusted properly or there's a plugged idle fuel passage.

In MY shop, that carb would get stripped, soaked, all the passages would be verified with aerosol carb spray or compressed air. Re-assemble and see what problems remain. THEN start modifications/tuning AFTER the known problems are fixed.

 

I rebuilt the carb about 2 years ago abd did just that. Completely disassembled, soaked, blew out every passage and she was running perfect on my 403. So I’m looking at cliffs site and I see he’s got 44 rods. If I go with a 44 rod should I step up to a 75 jet from the 73? Or higher?

 

Excellent! You know exactly what you need to do again.

 

Figured I’d give an update. Haven’t gotten parts yet so did some tweaking tonight to try track down cause of only 1 stream of fuel. Took air horn off and cleaned passages out again. Float level was low so I brought it up to spec, also gave the APT a counter clockwise 1/2 turn. Verified my check ball for accelerator pump was moving and not Stuck. Power piston moving up and down nicely. Verified all my primary passages are clear of any debris. Still only getting 1 stream of fuel, not sure what the issue is. Also car still idles poorly and wants to die when I give it gas. However when I give it more gas it finally revs again and smooths out when I hear secondaries open. So gotta be lean in the primary circuit?? In any event, just figured I’d share my findings

 

Sounds like you are heading in the right direction and have a good handle on things. Maybe poke backwards with a wire ? It may be time for another soak in carb cleaner. Hang in there, I think you are going to get it yet.

 

Did you adjust the idle mixture screws? No mention of that in your post.
No surprise that you have acceleration problems when four cylinders are going dead-lean due to lack of accelerator pump shot.

 

I did adjust but maybe I’m wrong about what they do. do the idle mixture screws control the amount of fuel that gives you the squirt when you pull back throttle without engine running? I thought they just controlled amount if air entering when idling. Please correct me if im wrong that could be my issue.

 

To even get a reasonable idle, I adjusted the screws 5 to 6 turns out. You can enlarge the passages behind the idle screws, it will help some. Cliff Ruggles book shows what needs modified and has recipes for cam duration and hp. Are you running the stock cam? Try blowing compressed air or run a fine wire through that passage.

 

Went out and backed out the idle screw on drivers side about 5 turns and checked to see if change in stream and still only 1 stream and no change. Cleaned the crap out of it last night so I know passages are clean. Seems like that would only leave an accelerator pump as an issue. I have a spare, will pop it in and see if makes a difference in squirting

I ordered Cliffs book yesterday, should be here Wednesday. In mean time in still tracking my single squirt issue. Figured I need to have carb functioning before doing modification.

Cam is very mild. Melling MTo1 RV cam. 450/474 lift I believe

 

Good news! I put in new accelerator pump and blew out all passages again. Now I’m getting my 2 full streams of fuel. Took it out and shes moving really good now with the exception of 1 issue. After a spirited acceleration it stumbles and wants to die. To clarify if I give it about 1/2 pedal and run it hard up to 40 mph, when I slow down to an idle it wants to stall out. Could it still be a lean condition? It pulls great, slight hesitation in a normal take off but as soon as it grabs, it goes.

Just getting opinions on wether it could still be lean. Anybody else has this problem? Thanks

 

Yep. It’s running lean. I sent you a link. You should read it.

 

Move your APT back to where it was, adjust your a/f mixture screws for the highest vacuum or rpm at idle. Make sure your timing is set right. To answer your question up a bit, the a/f screws only adjust in the idle circuit and have absolutely nothing to do the accelerator pump squirt.

 

...and verify your fuel pressure under load.

 

Figured I’d check back in with an update. Spoke to Cliff last week for about 40 minutes, great guy. We got carb setup with a higher flow needle and seat, new jets but stayed with 73, new 44 rods, idle tube set at .038, one of his H.P. accelerator pumps, and opened up idle air restrictions to .055. Choke opened all way up. She is running great now. Much more responsive, not dying when coming to a stop. Still has a slight stumble when sitting at a light but 10x better. Thinking a slight tweak to idle mix screws may take care of it. But overall very happy with it now thanks to Cliff

 

Glad to hear you fixed it yourself. I like Quadrajets the best. Cliff has always given me great advice. There are other very good Quadrajet specialist and parts suppliers. I personally like dealing with Cliff, just my choice. Timing, mixture screw adjustment, even opening up the accelerator discharge restriction may help with the light stumble. When you turn the carb upside down the transfer slot should just barely be exposed.