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

Any chance you either have the wrong timing tab for that balancer or the balancer outer ring has spun enough to throw off the timing?

Personally, I'd set the timing light aside and play around with the distributor position, letting the engine tell you what it likes. If it doesn't change anything for the better, reset with the timing light. Your timing light might be working perfectly but if the balancer or the timing tab is off ...

 

Thanks, I'll look into that. I noticed the truck had way more stumble on acceleration after doing valve seals and the other work I mentioned and I had the timing in the same place as it was before I noticed the stumbling getting worse. I had checked with the timing light to make sure I didn't move the timing when I was using the valve spring compressor tool which rubbed against the distributor. I feel I must have done something to make the stumble worse... it was there before but slight.

 

Forgot to also mention at idle in park with the hood open inside the engine bay everything sounds pretty much fine... I guess I need to hear some youtube video clips for examples of good/bad sounding engine.

 

EOIC, exhaust opening, intake closing. 18436572. Start with number 1, when you see the #1 exhaust pushrod start to come up, adjust the #1 intake to zero lash, then 1/2 turn preload. Move to #8, EOIC, follow all the way through the firing order. Then you'll be back to #1 to adjust the exhaust valves. When the intake starts closing, adjust the exhaust valve. Etc. Best method ever. If they're too tight it won't run worth a shit.

 

thanks i pretty much did exactly that but maybe I tightened them too much. I tightened until i felt the up and down play of the push rod go away and then tightened and twirled until they stopped spinning. Then I tightened an additional 1/4 to 1/2 turn.

 

Pull the plugs and give it a compression test! It the valves are too tight it will be down in compression.
If one [or more] cylinder is down, squirt some oil down the plug hole.
If the compression comes up.........it is ring seal! If the compression stays the same it is leaking past the valves.

Once you've identified the cylinder pull the rocker cover and check for loose rockers ,it could also be a valve stuck open.[which will rattle]
And with the valve closed ,run a straight edge across the valve springs to see if one is held open.[lower]

 

Thanks, that's a good idea. I'll borrow a compression tool from oreillys. I'm guessing most of them allow you to crank from inside and check the gauge after done cranking and you let air out of the gauge to reset it to zero before going to the next one?

 

I made all the changes to the vacuum routing. Reset the valve last using 1/4 turn preload after zero lash. Fired up and it’s a night and day difference first crank and it idled way smoother not even touching the pedal to feather it like before. I listened and don’t hear any valve clatter but shut the motor off so as to not make a big mess with oil. Anyone think 1/4 preload is fine? Or should I maybe go in and add another 1/8th or even 1/4 turn to get to 3/8 or 1/2 total? I read some prefer 1/2 turn and some for a street driven motor will go 1/4.

I used the firing order fraction method watching the valves to teeter and set companion cylinder valves.

I guess I could set valve lash while running but want to avoid the mess.

I’m tempted to throw the valve covers back on and test drive and go back to add more preload later if necessary but I didn’t hear any clatter.

 

dang that's a lot of effort....I just adjust both of them at TDC, turn the crank 1/4 turn, do the next pair, etc, then go back and give them all another half turn.

You should be ok with 1/4 turn. Maybe one will get loose after a while, maybe not. Also sometimes worn out old rocker nuts will back off a little, then it's time for a new set.

I'd just put the covers on and call it done, until you hear noises.

 

Thanks I just went with another 1/8 turn to get them to 3/8 total… it took me a little bit of time but less loss of sleep! I read somewhere I may get a slight bit more power with 1/4 turn vs 1/2 turn but I’m guessing that applies more to a race engine?

 

Update so I took this thing for a drive... before that I put the valve covers back on after resetting the lash and going with 3/8 total turn of preload after zero lash.. I also removed the extension I put on the pedal linkage going to the carb so it was set how I had it before. I pulled it out of the garage and let it warm up and then used a vacuum gauge to set the idle mixture screws again on the carb. All the way in then 1 1/2 turns out and then 1/4 turn in on each until I saw the vacuum reading change. This time I was seeing 10 in hg of vacuum vs the 15 in hg of vacuum but this was also after applying the corrections suggested below and re-routing the vacuum lines to the correct locations on the carb. To adjust the carb idle mixture screws I put a tee in to the line going from the carb to the distributor vacuum advance and put a gauge there. Prior to this it was going to manifold so I can see why I am not getting 15 in hg this time. I assume when setting the carb I'm supposed to monitor the vacuum between the distributor and carb?

In the driveway if I revved it up it felt pretty good and smooth and no stutter.. then I left my driveway and when I would give step on the throttle a little I would hear popping noise and it startled me enough to just baby the pedal and barely touch it and drive a few streets over to get out and see if I saw anything obviously wrong. In park I could rev it and no popping noise... then I put it back in drive and headed back home and popping noise if I gave it too much throttle more than just a little touch on the pedal. Could this be caused by the new mechanical fuel pump that I suspect may have too much pressure? I was going to connect my vacuum/fuel gauge but I don't have the right fittings. When I borrowed the oreilly fuel gauge I measured 3 psi on my old/failing mechanical fuel pump and 11 psi on this new one... I wonder if the gauge is inaccurate or if the pump is indeed putting out too much (which I've read is common for any of the parts we get from china nowadays). I was going to buy a fuel pressure regulator (basically the mr. gasket type that are basically restrictors) and see if that helps but I also have little to no room for one.

I also wonder if maybe the fast idle isn't set correctly and causing this issue.. hmmm

Darn I thought I was close this time! It definitely wasn't doing the popping (if its backfiring) before..

 

Idle mixture screws are adjusted off the manifold vacuum port [not the "ported" vacuum port] otherwise you will get a low reading
[This is the blue circle on post # 29]
I always disconnect the vacuum off the dizzy end and jam a golf tee in the hose when I adjust idle screws.

You adjust the screws in / out one at a time to get maximum vacuum [go back and forth a few times]
It should idle slightly higher.
Then you turn the idle a bit [and this should shut off the ported vacuum]

The timing should be checked and corrected Before you tinker with the mixture [I hope you did this after re-routing the plumbing]

So to summarize
Ported Vacuum for vacuum advance
Manifold Vacuum for idle mixtures

 

Thanks! Ah .. so I didn't recheck the timing. First thing I will do in the morning is take out #1 spark plug and get it to TDC (using finger in hole to feel air coming out) and then check to make sure my timing mark on the harmonic balancer is where it should be (based on having it set 12 deg advance in the past). Then I'll start it up and let it warm up and remove the line off the distributor and plug it there to set my timing with a dialback light.

I'll get the timing set then I'll proceed to adjusting the idle mixture screws. Just to make sure I understand right I should remove the hose at the distributor and plug it there. The gauge I would need to tee in to something coming off the manifold? Right now I have the TH350 trans modulator and the brake booster. I'm pretty sure I can make something tee in there.

For the turning of the idle screws I followed the instructions here:
https://www.carburetor-blog.com/knowledge-base/rochester-2-jet-idle-compensator/

It seems to suggest I'm supposed to do both screws at the same time taking turns on each one in 1/4 increments.

 

Do it the easy way....
Disconnect the vacuum advance [an plug the hose]
And use a timing light with the timing tape.

You want about 36° to 38° total advance [at approx 3000 rpm +] Just rev the crap out of it until it doesn't advance any more [you don't need the dial back feature with timing tape]
The idle advance is at the mercy to whatever the advance curve is set inside the distributor [this can be anywhere between 8° to 14° at idle]

Then with the dizzy vacuum hose still plugged , connect up the vacuum gauge to the other port on the carb [below the butterflies circled in blue ] this port has constant vacuum when idling.



The maximum manifold vacuum at idle is what you are seeking [this port ^^^ is manifold vacuum]
After you have the idle mixtures set to satisfaction ,reconnect the hose to the dizzy [other port]

If the idle goes up with the dizzy vacuum re-connected [it shouldn't] just re-adjust the idle screw

It is easy to try... and if you F*** it up simply do it again. You wont hurt the engine .
You are now entering into the "dark realm" of the old timer "black-art" engine tuning

 

Probably missed it, but what jets are in that thing? If it's a 75 carb on a high mileage engine, it's probably too lean.
Big part of your choke problem too, no doubt.

 

thanks will do this in the morning and report back

 

thanks I assume it’s a high mileage engine based on what I saw under the valve covers and how dirty it was. It’s a 75 sbc 350 based on numbers I found on the block but not sure how to determine what jets are in it. The choke issue seems gone now that I replumbed the vacuum lines correctly.

 

Forgot to ask, it's easier to hook up a tee for my vacuum gauge at the intake manifold ports going to the TH350 modulator and brake booster. The hose size going to the PCV at the base on the rear of the carb is a larger size that my gauge hose. Any problem if I go right at the intake manifold itself?

 

Describe your “pop” better and where you hear it coming from.

 

Thanks, it was hard to tell, I couldn't get it to pop when revving it in park, I opened the hood and pulled on the throttle even a little faster and I couldn't hear any pop. From inside the cab it almost stutters/jerks when it pops and the pop noise sometimes seemed to be two in a row. I could feel the throttle pedal jerk/move a little during the pop.. I also thought if this is happening in drive maybe its something with the TH350 kickdown cable.. but it could also just be the engine is doing something different under an actual load.

 

If you feel it on your foot via the pedal I’d say it’s a lean back fire. Maybe your weak fuel pump was actually a bad accelerator pump instead?

 

Oh interesting. So the weak fuel pump... I think 3 psi was the correct pressure but it was leaking out fuel from where the top and bottom portion meets. I replaced it because it was spitting out fuel badly.. I did the oil change and filter after that as well.

It could be the accelerator pump is bad as well.. I'm not sure how to tell.

 

Ok, did not know pump was bad due to leaking. I have not been up to date.

 

Surprised nobody suggested a carb rebuild. They are super easy to do, I used to do them on my kitchen table. New ones used to be very reasonable and you get a choke out of it. I'd swap out the fuel pump as well.
https://www.ebay.com/itm/1661527747...Xr/ywGFPBU3Stk9fLDy81PxSWg==|tkp:BFBMuuvulqti

 

Thanks I also wonder if I didn’t set the valve lash right again for the second time but I was pretty careful.

As for the carb rebuild I guess the $90 for the kit isn’t super bad but that’s $90 I could use towards a different intake and maybe an edelbrock carb. Is the 2 barrel Rochester I have that great of a carb I should just stick with it and rebuild? Granted I think the motor needs to come out for a rebuild so a different 4 barrel carb may not be worth it but I don’t want to invest the funds since I drive this truck just 10 miles or so a month mainly with my rowboat to the mission bay part of San Diego. It would be nice to have a little more power even for that short of a journey with the steep hills here.

Wonder what I should do for a fuel pump… get another replacement of the same one from autozone and try my luck again? I wonder if the oreilly gauge I borrowed was bad.

For at least this morning I’m going to check my timing and reset idle mixture screws properly , double check anything else i did and see if the backfire goes away.

 

I wonder if the power valve plunger is stuck. 10 inches of manifold is way to low for a mild engine makes me wonder about the timing tab try advancing the timing and see what happens to the vacuum.

 

thanks I read 15 in hg before replumbing and off manifold. The 10 in hg may be incorrect because I read that off ported not knowing for idle mixture screws I’m supposed to read manifold vacuum

 

I think your carb is likely the culprit for a lot of issues. But I wouldn't put a kit in it. I would find a used 4 barrel intake for about the same amount of money, and then get a decent carb to put on the intake. Something small like a 600cfm with vacuum secondaries.
I also think adjusting your idle settings for a lower vacuum simply made things worse. I've always adjusted idle air screws for the highest vacuum and had my engines run great at that. Your 10" of vacuum is what I'd expect from a engine with a big cam, not a stock 350, so it needs to be adjusted back to the 15" you had before. But forget about that until you swap out the junk carb.

 

Yea that's what I'm thinking.. maybe with $500 I can get a used 4 barrel intake and Edelbrock carb.... then again I don't know if that's a reasonable budget and if that's a waste of money for a high mileage engine I won't drive far or too often.

I think I didn't adjust the idle mixture screws correctly. I do think I adjusted the valve lash correctly though. I'm going to check timing, and reset the idle mixture a little later this morning. The 10" of vacuum was at the ported port from the carb. I bet I'll se 15 at the manifold, unless I'm not understanding something correctly and the ported port of the carb should see 15 at idle.

Thanks!

 

Rochester jets are marked with the actual size i..e. an .065 jet is marked as 65. All you have to do is look at them.
Speaking of.., as mentioned, it's probably about time you went through the carb. They are as simple as you can get.
Not sure where you're looking but that kit is about 20 bucks on Rock Auto.
Pick up a gallon of Simple Green or W/M generic , ..soak the pieces for about a day and a half, and blast 'em off with water and air. This is probably the first thing you should have done when you got the truck, by the looks of it.
You don't need a 4 bbl for this application. If you're lucky, you can find a 69-70 350/ 250 hp , 1. 375 primary 2bbl. which is the one they use for the circle burner restricted classes. Other than that Rochester jets are easy to find. Qjet jets will work too.