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

At 22° advance when static would cause the starter to struggle.

Set your dial-back to 36° and rev it hard [and set the dizzy there]
When it comes back down to idle, dial the light and see what you have as a result.

This method is best with a Dizzy that somebody has already tinkered with [you dont know what weights or advance stops are in it]
36° to 38° total mechanical advance is your practical limit on pump gas , If it ends up being slightly more [or less] than 12° at idle is not that important.

 

Thanks I’ll do this in the morning. I found an extra set of springs and weights in my MSD street fire box that I could tinker with on this HEI distributor but I’ll start with your suggestion.

prior to replumbing the vacuum and the valve work I noticed it took a few tries to get the truck to start when sitting for a while and it needed the pedal feathered. With the vaccum plumbed correctly it fired up first try and didn’t need any pedal feathering. It must have had too much advance like you said.

For the idle stop screw you mentioned in a prior post.. is that the same thing as the “fast idle” screw or is it the screw on the throttle arm to set idle? I was rereading your instructions to tune the carb after I get the timing corrected.

 

This is the idle screw [blue circle]
The Choke has a rod that moves a cam [blue arrow] that is the fast idle cam.

 

Have you verified that the mechanical advance is working properly? I had a Mallory distributor break an advance weight spring, and the broken piece was hanging the weight up, causing a similar pop on acceleration. Also, I read eons ago that those first year for Chevrolet 1975 HEI's had problems with the module, and the fix was to change to the improved '76-79 style. Something to ponder.

 

Now you have a suspect damper. Will it shift again? Get it antiseptically clean and put the tape on. I wouldn't throw big bucks at it until the timing is well-sorted out. A carb rebuild kit would get you by for a long time, like I said, they are easy to go through. Take pictures along the way.

With the stock cars, one class I ran specified a Holley 4412 2-barrel. I have run them on the street too. Those are also easy to work on and adapters used to be in any parts store.

 

Thanks so yeah I had taken out the HEI that was in it.. it has some wires that looked like they would go to a computer or emissions module and they were cut and it had no vaccum advance. I switched to an extra ac delco hei I had from my 53 Chevys 350 but not sure what year it was from however when I revved the motor up I could see the timing increase to the mid 30s with my dial back light albeit I was off by 10 degrees on the mark. I’ll double check again.

 

Yeah I know it might shift again luckily I made a tool that I can reuse to find the correct mark quickly. I’ll get the timing dialed in and then proceed to the carb and probably will buy the rebuild kit so I can learn something even though eventually I’d like to put a 4 barrel intake/carb.

 

Worse case, the damper may fly apart. They are not expensive and I have used used ones before. Try to do things one-at-a-time right now. The carb kit is cheap and easy. I'm dsylexic (even spelled that wrong) so things like linkages flip around in my mind. I used to make sketches, now I take pictures.

 

Still stuck!!! Alright so update. I replaced the transmission vacuum hose on both ends (its steel in the middle), and added 1 gallon of regular gas from Shell. I found the new timing mark with the piston stop both directions and set to the middle. Then I fired it up and set it to 12-13 deg advanced using the new mark and my dial back craftsman light and distributor vac advanced disconnected (and plugged at hose end coming from intake). I was going to set it to 35 degrees revved up well but I don't have timing tape and I'm not sure how trustworthy the dial back would be at that level? When I was spinning the distributor by ear is sounded best right about where I was so that makes me think the timing is set good.

I adjusted the idle mixture on the carb this time using the manifold port. I read 17 in hg at the highest vacuum and the vacuum was nice and steady. I followed instructions from here

https://www.carburetor-blog.com/knowledge-base/rochester-2-jet-idle-compensator/

It idled well and when I gave it some throttle it responded well in park in my driveway giving it some good amount. Then when I took it for a drive as soon as I gave it anything more than a light touch it started backfiring and it isn't a single backfire but like several pops if I keep trying to push through it even lightly... so I backed off the pedal and babied it back home by not touching the pedal too much.

I'm not sure what to look for at this point..... I think the timing is set right. Some ideas:

1. Maybe the new mechanical fuel pump from Autozone is putting out too much fuel pressure and causing an issue on load? I had measured 11 psi with the oreill loaner gauge... though who knows how accurate that is.

https://www.autozone.com/external-engine/fuel-pump/p/delphi-fuel-pump-amf0001/340206_0_0

2. I'm confident when I changed the valve seals I put everything together correctly. I used the firing order and teeter/overlap technique adjusting the companion valves... at zero lash (up and down not twirling the rods) I used 3/8 additional turn for preload. Maybe I need to get the valve covers off and loosen until clacking and add 1/2 turn method?

3. I had set the valve lash too tight possibly the first time around after doing the valves or maybe not (I had twirled until can't spin then 1/2 turn) I wonder if I would have wiped out the cam lobes driving 5 miles? I am confident they are set right this time around. I suppose I can take the valve covers off and observe rocker arms and while I'm at it set the lash while running.


Outside of that I'm wondering what else I can check.. is there anything I can look at from the top of the carb? I feel like the backfiring has to be related to something I changed. I'm going to trace the spark plug wires, take the distributor cap off and look at rotor, etc. to make sure I didn't connect anything incorrectly but I highly doubt it based on how I have the looms set up.


I read in another site of someone having a similar issue when replumbing the vacuum lines which I had done:

"I am having the same problem with my car. I was working on mine the other day with some friends and we found that the guy i bought it from had two of the vaccum lines crossed. One of these was goin to the vaccum advance. Now i have a backfire under slight load when the engine is still cold. Started investigating the vacuum and found that there is hardly any vacuum at the small left hand port, plenty at the brake booster port and what seems like plenty at the small right hand port."

I wonder if I have an issue with the vacuum... I don't think I have a leak.. maybe the carb has something dirty in it? Right now I have my brake booster and TH350 modulator lines coming right off the intake itself. On the carb I have the PCV valve coming from the back base. I have the vacuum advance coming off the passenger side ported port (it used to be off manifold).

 

Clean and rebuild the carb! You're still leaving a big potential problem on the table.

BTW, when setting the valves as they are closed, you just lightly spin the pushrod until you feel zero lash and a SLIGHT resistance, then tighten a half turn and you'll be fine.

 

think I found the problem and I’m the biggest idiot ever!

 

It’s with shame and embarrassment I have to write that I think I found the problem. The idiot (me) who installed the HEI distributor (to replace the electronic controlled junk that was in it before) ended up installing it 180 degrees off. As if that’s not bad enough this was installed almost 5 years ago. I’m not sure how it ran but maybe all the other things that were wacky ended up allowing it to run and fixing them is now making my 180 degree offset become an even bigger issue.

I read some other threads where some had a similar issue and said to check the distributor isn’t 180 degrees off. Jogging back in my memory I started thinking very possible I installed it 180 off! I remember what happened.... I was going through a terrible girlfriend break up (I’d say second worse so far) and I wanted to get my mind away from it so I looked for something to do and decided to swap out the distributor since I had an extra one I took out from my sbc 350 I had on my 53 Chevy….. I don’t think I was in a clear minded state for a few months remembering back…. Ironically I decided to take care of the valve seals (which unfolded all the rest of this work) just a couple months ago because I was dealing with a different gf breakup.

Morale of the story I should go surfing more when girlfriends become exes and not touch cars… in theory it seemed it would be a great way to get my mind away and keep busy. At least I learn even if it’s the hard way both with cars and hopefully with past significant others!

I’ll report back once I orient it correctly, retime and adjust the idle mixture. Luckily my timing mark is spot on still, and I rechecked using the piston stop tool I made!

 

if it was 180 off it wouldn't run. clean your carb. accelerator pump is probably shot.

 

I don't know if it's 180 off per se' but it was 180 off from 1. It gets better....... I didn't even have it in the right firing order. With the engine at TDC (I used finger in hole on #1 to verify air came out and on compression stroke) the rotor is pointing at #6 spark plug wire and at 1:30 o'clock . From there I traced each wire and marked them and the order is 6, 5, 7, 4, 1, 8, 2, 3. Looks like #2 and #4 wires were crossed in addition to the 180 deg off. Maybe by sheer luck this thing ran the last 5 years with that order.

I remember a little more now from what happened. With the old distributor I traced and marked all the wires before changing them and did one at a time. I thought the #1 wire should be at the 5:30 position on it but I mirrored what was on there because I thought there must have been a reason the #1 wire was where they put it (I didn't at the time know you can set it anywhere you like as long as the rotor points to it at TDC on compression stroke).

Now I'm thinking to run the wires correctly... and just debating putting the rotor so its at the 5:30 position at TDC just to keep consistent with how most do it. When I checked my SBC 350 on my 50' shoebox Ford it looks like the #1 is at the 4 oclock position and not the 5:30.. so I guess some set it where they want......

 

it doesnt make any difference where 1 is pointing. swap wires 2 and 4 and run it.
Couldnt remember if it was 5/7 or 2/4 but if you swap them it'll sound fine at idle and light throttle but it'll start backfiring under heavy load.

 

already pulled all wires out and will just get rotor to 5:30 position now that I’m this far and rewire. Everything I read said swapping 5 and 7 would cause backfire under load but maybe 2 and 4 does the same. Also possible I swapped 2 and 4 during valve work recently..

 

Life is good (for now)! Reinstalled distributor and rewired with #1 at 5:30 o clock. It fired right up. Put 13 deg initial timing and adjusted carb and now I’m parked just about ready to enjoy the warm weather and stand up paddleboard in the bay.

Thanks everyone. It ran way better on idle with the vacuum replumbed. No more blowing smoke after changing valve seals and no more stutter when accelerating. No more needing to feather the pedal on startup. The backfiring was likely #2 and #4 backwards after doing valve seals. I don’t think it was actually 180 off since I just realized you will feel air out of the spark plug hole even on exhaust stroke just not as much.

The only thing left that leaves me wondering is if the mechanical fuel pump truly is putting out too much pressure and it’s going to give me issues soon or if the gauge I got from oreilly was off. I ordered timing tape to check/adjust total timing later.

 

Here's an update, somewhere along the way I must have messed something up after it was running great on July 19th and again on August 7th.

On August 7th I added a Holley 12-803 regulator (more like restrictor) which has 4.5-9 psi range and I set it to 4.5. I then re-adjusted the idle mixture and it ran as good as it did on July 19th and I drove it around and it behaved. I wanted to add the regulator for peace of mind.

I've been working on my shoebox since but yesterday I wanted to get back to this truck and check total timing, and even try running the distributor off manifold vacuum instead. Here's what I did:

YESTERDAY:
1. I fired up the truck without making a single change and it idled fine then I shut it off.
2. I realized there is an inline filter in the carb which I took out and it was the bronze stone type. It had a lot of gunk and silicone pieces in there that I cleaned out.
3. I set the distributor vacuum advance on manifold then fired it up and it idled nicely and the exhaust had a little bit of a deeper lower tone!

TODAY:
1. I took the #1 spark plug out and used my piston stop to recheck the marks on the balancer and nothing shifted then put the plug back in. The factory mark is off exactly 10 degrees but I had already discovered that so nothing changed! I added timing tape.
2. I wired up a tachometer temporarily so I could check total timing, and it was working fine after firing it up then shutting off.
3. I fired it up again and it idled nice but when I drove it out of the garage it was stumbling a little. I drove it around the block and it felt awful when giving it gas and I barely made it back to my driveway.
4. I raised the fast cam idle a little to see if it would help anything and it didnt.
5. I switched the vacuum advance to port and it had the same issue where it stumbled although not as bad but still not great at all.
6. I went to NAPA and got a brand new stone filter and installed it and it had the same issue. I drove the truck back in my garage and left it alone.
7. Lastly with the engine off I checked with the air filter off and there are two good streams when I actuate the throttle.



I'm not sure what I messed up but my only thoughts on what could be wrong are:
1. When I cleaned up the gunk out of the stone filter, and then later when I put a new one in, maybe I changed the flow characteristics and need to reset the idle mixture screws? Otherwise maybe the float bowl is getting filled too much with a better flow through the filter especially with 4.5 psi?
2. Maybe I messed up my distributor when I ran the tachometer temporarily?
3. Lastly perhaps bad gas? I've been filling it one gallon at a time usually once every other week or so.

How would I check if the float bowl is getting too full? I know there are vents gas would flow out of but does that require looking in with the air filter off while it is running?

 

S/B: 1-8-4-3-6-5-7-2

 

Totally, it is 100 percent set correctly now. I think my issue must be carb related right now.

 

Yeah, that carb looked knarley... those are very easy to rebuild or replace.

 

Did you rebuild the carb yet?

 

I have not, I've been trying to give my chopped 50 shoebox a lot of love so I can get it to some shows!

I went through and checked everything I could from what I changed last time and the ignition seems all fine, timing is fine, I checked wires again, swapped cap, rotor, ignition coil, etc.

I think I found the problem! Here's what I think happened:

I pulled out the in-line carb stone fuel filter and there was a ton of junk big pieces of silicone/rubber or something else in there that seems like someone would have put in there since larger than the fuel line diameter. Once I cleaned it out I would get the stumble on acceleration. I put a new filter in and same issue.

I measured fuel pressure in July after the Holley 12-803 regulator and I was getting around 3 psi just before the carb (so just before the in-line stone filter). I measured again this morning and I'm seeing 6 psi at the lowest setting I can get with the regulator. Just what I suspected earlier it seems with clearer flow the pressures are different and maybe the Rochester is seeing too much. All because of some stupid junk mechanical fuel pump I replaced that doesn't put out the right pressure since it came from overseas with no Q/A! I had measured 3 psi with the old mechanical fuel pump before it went bad but that was with junk in the filter! I wonder if someone purposely put some pieces of silicone/rubber to reduce the pressure? Seems crazy but you should see everything else I've found wrong with this truck when I got it! This truck had been south of the border from the previous owner for repairs!

I assume this is the cause of the stumble I'm getting when I lightly press the pedal in drive.

Wonder if I should purposely put junk back in the filter and see how it runs

 

Maybe some junk got through and is sticking the float/needle/seat?

 

Well I carefully took out the filter (it is the bronze stone type) and put it back in so I don't think anything would have gotten past the filter into the carb if it wasn't already in there past the filter. I can see that the flow characteristics would have changed with a cleaner filter.

Here is a link to what the filter looks like by the way:
https://www.carburetor-parts.com/filter-30-53

I also made sure it's installed in the right direction.

 

You know, lots of guys here spend a bunch of time trying to help the shall we say, less experienced?
If they don't listen..it's all on them.
Good luck.... I'm outta here.

 

Well thanks for the previous and very useful advice. Rebuilding the carb is on the list I just am really gasping for air right now from being so busy with work and responsibilities. I'm hoping to rebuild the carb around the holidays when I should have some breathing time or at least find someone to send it for rebuilding, or even just go to a Holley or Edelbrock with different intake.

I believe the increase in fuel pressure after cleaning the filter and then it not running right is an outcome of the fuel pump and regulator combination I’m running. I bought another fuel pump from Napa this afternoon and will give that a try in hopes it’s not another piece of junk 10+psi outputting part.

I’ll also inspect the needle and bowl.

 

Be extra careful with Chevy fuel pumps , cause most ain’t E10 proof ,if they are old !!!

 

Yea I’m realizing now thats likely why I had to change it out earlier this year when it was leaking fuel everywhere!

 

Problem solved!!! My intuition was correct. This wasn't a case of a lack of experienced person not knowing what they were doing (wasn't sure if Mr. Mark Yak was trying to say that about me in earlier post #116 although I appreciate his help and suggestion to rebuild the carb but I knew that wasn't the crux of the issue).

As I suspected when I cleaned the fuel filter the better flow changed something and the Rochester 2G carb wasn't happy. I adjusted the Holley 12-803 regulator as low as it would go and had 6 psi. I drove the truck around the block and it was alright, no stuttering, however that's not good enough for me!!!!!

I went to Napa and got the FUP B0186P fuel pump for a 1975 C20 350 engine (that's what the stamp code on the block checked out as) and installed it and without touching the regulator was reading 2.5 psi. I adjusted it up to 3 psi and it idles nice, revs nice, and put it in gear and no stumble! I'm running off manifold for the distributor vac advance. I'll fine tune timing tomorrow and call this chapter good and get back on my chopped 50 Shoebox. Perhaps over the holidays I'll rebuild the carb or ditch it and get an edelbrock or holley and appropriate intake manifold.

As I suspected and complained about earlier, the Delphi mechanical fuel pump I got from Autozone for the 1975 Chevy C20 350 engine is a POS made in china part.. it was putting out north of 11 psi before I added the regulator... my sanity for the past couple of weeks was a casualty of crap quality overseas manufactured parts not a lack of experience. FTW!