Technical SBC 350 with 2 barrel rochester rough on acceleration/idle

Hi All, I have another open thread related to my 2 barrel rochester on my SBC 350 regarding choke but thought I better start a new thread.

I have what appears to be a 1975 sbc 350 with 2 barrel in my c10. The last time I drove the truck in March/April it was stuttering a bit when I pushed the gas pedal a bit though nothing extremely bad but I could tell little by little it was getting worse. Here's the work I did to the truck since then:

1. Changed the spark plugs and saw #3 and #7 were extremely filthy with oil/carbon gunk, to the point I can't imagine there being any spark. I changed all plugs. The last time I changed them was in november 2018 and I probably put less than 500 miles on the vehicle since then... maybe not even 300 miles.

2. I decided to do the stem seals (o-rings) added umbrella and positive seals as well. (What a pain for the last passenger side where I barely had any room due to the block being far back in the truck). I set the valve lash following some guide online following the firing order, etc. I'm pretty sure I set it correctly at the time.

3. I changed the breathers, new valve cover seals and new PCV valve.

4. Checked the fuel filter and it was just fine and checked fuel pressure and it was somewhere around 5 psi. Also made sure the fuel filter was in the right direction when putting it back.

5. Next I noticed the pedal linkage on the throttle only opened it 3/4 of the way so I added an extension with a piece of steel and two bolts to basically make the lever longer so there is more throw when pressing the pedal.. with the pedal floored it is maybe 90 percent or so all the way open on the throttle at the carb... don't know if this was a good idea but I noticed going up hill sometimes I was pressing the pedal as far down as it would go and wondered if the engine was pulling all it could with just a 165 hp rating for that year or pedal not correctly configured.

6. I adjusted the fast idle screw a bit to get it to not want to shut off right away after changing the pedal linkage. When I fired up the truck at this point it felt different in sound, would pop/backfire and when I rev'd it in park it was a little rougher.. later I found the mechanical fuel pump was failing but not sure if that was the cause.

7. Changed the overflow coolant tank (broke the plastic one reaching over to do the #8 valve seals).

8. I noticed the truck would pop/backfire when starting up after all that work (before I changed the fuel pump) and checked the timing and I have it set at 12 degrees initial (with line to distributor plugged) and it didn't change from the last time I had set it but thought maybe I shifted the distributor when I was cranking like crazy on #8 spring to change the valve seal. I have an HEI distributor and am not sure what total timing is but I measured around 35 degrees with the vacuum line connected back to distributor and reving it a little bit (though not sure what rpm without tachometer) using a Sears dial back light. Eventually it stopped popping/backfiring when starting it up but not sure why.

9. After a couple weeks later (this was last weekend) when I had time and stopped working on the 50 Shoebox I fired it up and was ready to drive it and noticed fuel leaking everywhere coming from the mechanical fuel pump (seems like at the casing where the top/bottom go together). I changed it with the Delphi AMF0001 (from autozone) and checked fuel pressure and it read 11 psi. Note I had to take the fuel filter out to put the fuel pressure gauge tee in. When I blow through the fuel filter it feels totally free flowing.

10. I changed the oil and filter. I noticed no more leaking from the front area after this (I think harmonic balancer) so maybe there was fuel in the oil and it thinned it out.

11. I pulled the speedo bullet out to look at the gears and confirm the number of teeth (to check a different issue with speedo off a little bit) and put it back in and put the fluid that came out right back in the trans.

12. Today, with the truck warmed up and where I thought was a good fast idle setting, I set the idle mixture screws on the rochester carb by turning them in all the way then 1 1/2 turns out then using a vacuum gauge teed into the line going to the distributor advance. I watched the vacuum go up (didn't move a whole lot) until it was max for both screws going out then I screwed both in 1/4 at a time until the vacuum started to drop then went 1/4 out on both. At this point I noticed the vacuum was roughly at 15 in Hg and on the vacuum gauge it says "late ignition timing" in that region... not sure if that applies is something worth noting.

13. I checked and the choke butterfly is stuck in the open position... I feather the pedal when warming up. The divorced choke component is missing...I'll deal with that later.


The truck seemed ok in my driveway at idle and I could step on the pedal a bit and I didn't feel anything bad but a little vibration. When I took it for a drive and pressed the pedal I could feel way more jerky/vibration behavior as it was accelerating even when I would press it very light. It did seem to help a little with a light touch vs pressing the pedal hard. At a stop light it seemed to idle a little rough and I had to feather the pedal a little bit and when I accelerate from a stop it was quite jerky/rough. A couple of times I felt a funny very brief stall maybe when the transmission was shifting gears or maybe something with the pedal linkage, I could feel a little pulse on the pedal during that very brief stall. I definitely didn't have that issue before. Going home uphill it didn't see to have quite the same power before I did all the work/fiddling.. I feel like I barely made it home overall. When I made it back to my driveway it seemed ok in park.. when I rev it I only felt a little bit of the vibration but not that much like in drive.

Does anyone have any recommendations on where to start looking for possible problems? Maybe I should put my pedal linkage how I had it? Is the 11psi with the new fuel pump possibly causing some problems? Could I have messed something up with the pedal linkage mod or its doing something funny with the kickdown cable on the trans, or maybe not set the valve lash correctly after the valve seal work? Perhaps I didn't set the mixture screws correctly and miscounted or didn't have the gauge connected properly?

 

What does the hose on the front of the carb go to

 

The lower one right? Vacuum advance but in this case, it’s most likely going to the thermostat housing were there is smog related thermal deal that controls spark advance vacuum signal.

 

Shouldn't be vacuum on the line going to the distributor at idle. If there is, it's advancing the timing

 

I think PCV valve for that lower one. I'm posting more photos now.

 

Here are some more photos

 

Oh hmm. I thought you're supposed to plug the line going to the distributor when setting initial timing at idle. Isn't that to keep the vacuum going to the distributor?

 

Front of carb was in question, not the rear.

 

There's manifold vacuum, and venturi vacuum. Manifold is constant, venturi is part throttle. If you're setting initial timing, there shouldn't be any vacuum to the advance. But if you're setting total timing and your vacuum is hooked up to your distributor, it's gonna reflect that additional 8⁰-10⁰ from the advance. You can pull the hose off the advance and stick a screw in it, and see what your initial timing is, compared to with it hooked up. Also test drive it with the hose plugged.

 

Thanks here are some photos.

 

Thanks, so when I checked the initial timing, I plugged the line going to the distributor and with a dial back sears craftsman light I saw the 12 degrees I had set it to a while ago. With the line connected back to the distributor and revving it constant a little bit (not sure where 2500 rpm or so would be) I saw it go to 34-35 degrees total.

 

Anyone think I should buy a fuel pressure regulator and in-line gauge while I'm at it given I had a reading of 11 psi with the Delphi? I'm not surprised with a made in china part... it was cheap... I could try another one and maybe my luck will be better but experience tells me better to have more control with a regulator.... I wonder if that's causing the issues at all that I'm seeing.... I'm nervous it may cause other issues based on other threads I've read about the rochester wanting 5 psi

 

That's about right. But if you do have vacuum going to the advance canister, and it's not working, the diaphragm is probably shot in the canister. Mine is unhooked. They say it helps fuel mileage, but I don't drive a hot rod and worry about mileage lol

 

11psi sounds freaking crazy to me! Should be flooding like crazy.

 

Ok I see it now. Your PVC hose should be off the back of the carb at the base/throttle plate.

 

So I borrowed a fuel pressure gauge from Oreilly and checked with the old mechanical fuel pump and read about 3 psi or so... maybe it wasn't 5. I borrowed the same one again after I installed the Delphi pump and it read 11. I just recognized the vacuum gauge also does fuel pressure so I may check again with mine to see what it reads. I read on some review from Autozone that someone read 10 psi with the same pump and blamed it on being a china part with no quality control... wonder if mine is really that high or a faulty gauge I borrowed.

 

Check it with that other gauge, could actually be that far off, I've read it here before. Seems to me that 3psi would be wonderful. 11 should blow the needle and seat through the vent tube lol

 

Thanks just checked and the brake booster line is plugged in there. Do you think it will make a big difference moving the PCV to the back? I guess I could buy a tee fitting of some type..

 

So in other words if I had 11 psi I probably wouldn't even have been able to drive it like I was able to albeit all the really rough acceleration/vibration?

 

It would be a testament to the durability of the inlet needle and seat in that carburetor! But stranger things have happened

 

With the advance unhooked, you're showing 35 degrees of timing around 2500?

 

That's with the advance hooked back up. sorry if my wording was causing confusion.

 

Brake booster would go where I circled.

 

I just read that. But if you're at 12⁰ initial, that means you're getting around 22⁰ mechanical. So it's probably right. But I'd check it all in with the vacuum unhooked and plugged just to be sure. And you'd know if it was pushing gas past the needle and seat, it'd be blowing black smoke and falling on its face I'd say. Did you spray around the carb with it idling to see if you've got a vacuum leak? Spray some brake kleen or carb cleaner around, see if the idle changes

 

Appears there's one of the trees in that port and the vacuum advance might be hooked to it. Does that 2bbl have a venturi vacuum port? I'm second guessing myself here

 

One last thing and I'll shut the hell up! I don't own a vacuum gauge, I do own multiple timing lights, but I get made fun of and told "a real man sets his timing by ear"... but I set my idle mixture screws by turning it in until it starts to stumble, then start opening it and counting my turns until it starts to stumble again, Then go to the middle of that. Plus I've always been told there should be a balance there, as in both should be set the same. That's the extent of my mechanical knowledge, now I'll shut up oh, make sure you didn't wipe out a lifter and cam lobe

 

FWIW, I owned one of those from new and it never ran worth a crap. GM was trying to cheap out meeting the tighter emissions requirements by playing with compromise settings in the carb and distributor. As we all know, it couldn't be done.

My truck would start fine, pull away the first time and then stall just as I was trying to merge into the arterial. The dealer just said, "They all do that."

The only way I kept it from killing me was to replace the carburetor and distributor with pre-1970 units.

jack vines

 

Some have two ports (1 full time 1 above idle). There is so much going on I can’t tell. If it’s only got the one, it works above idle (cruz).

 

And @SDrocker Most Rochester 2G carbs should have the vacuum "Ported Advance" [regardless of what the internet and Youtube mechanics claim]
The port is here [below blue Arrow] and not below the butterflies [blue circle]capped off


A lot of carburetor "off the line stumble" is caused by manifold vacuum advance! [and ported will remedy it]
If you set the timing at 12° advance and then connect up manifold vacuum [maximum vacuum is at idle] the timing could advance up to +/- another 8° [approx 20° total at idle]
As soon as you touch the gas pedal and manifold vacuum drops,and the dizzy instantly retards to 12° advance [causing a stumble], then the timing has to climb back up to full centrifugal advance at higher RPM
By having ported vacuum you only have vacuum advance when you back out of the gas while cruising or decelerating.
If your engine runs smoother/better with "manifold" vacuum, simply add a few more degrees into the timing and re-connect it to "ported" vacuum

Also listen to @Johnny Gee about the PCV valve ....It must go to a hosetail at the base of the carburetor [ black arrow on photo ^^^] mine is an early PCV but the port is the same position on the Carb



If the PVC is connected to the port on the rear of the manifold [Johnny Gee posted #23] there will be a constant flow via the PCV and the rear cylinders will lean-out .
So leave that port on the manifold for the Brake booster where it needs vacuum not flow. [Johnny is correct]
The PCV port at the Carb divides into both barrels on a 2G

Here is another view of where the PCV port is on the Carb , on a 350 the PCV is on the rocker cover and the Carb has a hosetail


Also do yourself a favor and buy some "timing tape" for the balancer. And set the total advance at
"36° All-in" at approx 3500 rpm with the vacuum disconnected [The idle advance is simply what you get after setting the total]
Then set the mixture with a vacuum gauge

 

Thanks I’m going to make the changes in how everything is connected to the carb. I have timing tape and a cheap tach I can hook up to double check how I set timing.

I noticed after I did the valve seals and adjusted the pedal linkage the truck sounded different and vibrated more when pressing the pedal.. this was just in park in my garage and never drove it and this was before some of the other work i did including changing the fuel pump. I wonder if maybe the valve lash isn’t set correctly. Although I may need to change how everything is routed on the carb it never vibrated as bad as now when accelerating out on the road…. The fuel pressure being so high with the new pump is the only other thing I can think of.