Technical Vacuum ports

Hi, I’ve got a couple of questions I’d like to ask you guys to see if you sort out the problem I have with my engine, I’ll try to make it as short as possible.
I put this sbc in my ‘50 styleline after doing a minimal strip down and rebuild a couple of years ago after buying it with the vacuum lines missing so I put them on where I thought they should go, I’ve no real experience with cars,just learning as I go along.
The thing starts on the button and idles fine but I get hesitation when I pull away gently and then it’s ok so I thought I had a vacuum leak but couldn’t trace one so I decided to buy a reconditioned Rochester 2 barrel from Rockauto and replace the intake manifold gaskets whilst I was at it and it did make an improvement for a while but now it’s back where it was.
If you look at my pictures have I got the vacuum ports wrong, I’ve got the distributor to full vacuum and the egr valve on a timed port, would these cause the problem or am I looking at the wrong thing?, I know people will say get a 4 barrel and a new intake but I’d like to keep the engine looking ‘old’ if it’s possible and the other thing is I’m in the UK so I can’t just nip out and get parts quickly, so if anyone has any suggestions I’d be grateful.
Thanks in advance, Steve




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I would try disconnecting the vacuum to the EGR and see if that helps. If so I would just leave it disconnected if that is legal in your state.

 

Check your timing...

 

Many older carbs suffer from worn accelerator pump bores plus the replacement accelerator pump rubbers leave something to be desired. When you first replaced the carb , the pump was able to mate to the old bore , after a while it wears and function decreases . Someone may have a long term fix , but its not me ..LOL simply a thought ....?

 

I second trying it with the egr valve disconnected and the carb port plugged. There is normally some type of delay valve in the egr vacuum line. The way you have it, you get a lot of egr as soon as the port opens.

 

if not required for registration I suggest to remove all smog control related parts, at least disconnect them - with all ports blocked check the vacuum, what reading do you get? check it with motor running around 200 RPM above standard required idle speed - with gauge hooked up try adjusting mixture screws, very slowly, and see if reading of increases RPM - search and find information with pictures/drawings on that year motor that gives specific information on where all hoses run

 

If you are not required to keep the EGR in your area I'd loose that ASAP. And try the vacuum advance on the timed port...

 

His "state" is Great Britain.

Yes, the problem does sound like an accelerator pump malfunction.

 

I think the vacuum advance is connected to metered vacuum, not straight manifold vacuum.

 

The EGR, at the very least should have a temperature switch that wont allow it to get vacuum until the engine is warmed up. A delay valve may also be required. You can play with initial timing setting and try both ported or manifold vacuum to the distributor to see what works the best. I think it probably was manifold vacuum from the factory, but I'm not 100 % sure.

 

See the hose that comes out of the right front of the carburetor? (Toward the camera) Run it to the vacuum advance on the distributor. Plug the hose that is on the vacuum advance now. (Or the tube on the carb where it is attached.) Then start it and watch the rod that connects the vacuum advance to the distributor plate. It should move when you rev the engine. I also see that the right rear carb bolt is missing.

 

Thanks for all the suggestions, today I plugged the egr and checked and set the timing at 8 btdc at 900rpm with the distributor vacuum disconnected, then I ran it with the distributor hooked up to the full vacuum port first then on the timed port second and in both cases the hesitation is a lot less but is still there although when it’s on the timed port there doesn’t seem to be as much popping on the over run?, so I’m going to take out the accelerator pump to measure the cup as apparently there’s different sizes and try a new one.
Also on my inlet manifold the carb is only bolted on with 2 bolts on the diagonal, even the base gaskets only have 2 holes so that’s why in the picture it looks like a bolt is missing.
Thanks again, Steve


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Does the intake manifold have provision for four carb studs? If so, you should be using four to limit the chance of a vacuum leak.

Phil

 

I’m pretty sure I haven’t got a leak around the base, I’ve tried carb cleaner many times and never found one.

After looking at the above picture and especially the barbed fitting , I’ve blocked it off, is it another sort of vacuum port or should I have it hooked up to something like the pcv valve?



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Had never seen a 2 stud mount manifold but there's a twin to not on eBay listed as ' 2 barrel racing manifold non canada 340266 who knew ??
Im guessing the port cast into the manifold was intended for EGR....

 

How did you set the idle mixture screws? The idle mixture can have an effect on the idle to partial throttle transition. By "popping", did you mean backfiring through the carb or through the exhaust?

 

When you get it running again: warm it up , shut it off, get a flashlight, with out motor running, cycle the carb linkage while looking down the throat. You should have a smooth squirt of gas while moving the linkage slow and steady. Check that the linkage engages all the way thru the rotation, float level can effect this too.



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Through the exhaust, I’ve got a dual system with no balance pipe


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Those two stud carb mounts were pretty common (in Canada) on mid 70's cars. I worked on lots of them back then.

 

Once I had a problem with banging out the exhaust, when letting off the throttle to change gears. But it was more like a shotgun than a pop. I couldn't hear an exhaust manifold leak, but that was the problem. Yours may be more of a timing issue.
Again (By the way, I don't want to sound all "I know, you don't". I don't know what you do/don't know. Sometimes I just write for any future readers.)
Did you set the idle mixture? By the book or by ear?

The only reason I jumped in here was because I've had that carburetor for over thirty years and I saw that the vacuum advance hose wasn't hooked up like mine.
And the bolt hole. I thought for sure that would fix it. (stumbling going from idle to throttle) The vacuum advance adjusts the timing according to engine demand.
I don't know the chances that the distributor has been recurved. (advance weight springs changed) I put lighter springs in once and it would stumble off the line. I found that the cure was to have one light spring and one heavier. (not as light)
Ahah. (Maybe) Still thinking.
"... timing..."
Check the point gap.

 

I can’t really remember what I did regarding the mixture, I probably just left them as it was when the carb turned up as when I put it on it ran ok so perhaps that’s something else I should be looking at, I haven’t changed the springs in the distributor but I did put on a new vacuum canister and a Pertronix module with one of their flamethrower coils



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What a terrible way to save a few cents:



Especially with that non-traditional, burning hot EGR passage needing to seal by a missing stud.

I must say that although this reportedly was common on Canadian intakes, I was not consulted.

 

Haha. You weren't consulted because they knew you wouldn't approve it.

 

This is correct. You have the vacuum advance connected to manifold vacuum, it should be connected to a ported connection. The difference is the ported vacuum is always located above the throttle plates. I believe the port where the EGR is plugged in is for the vacuum advance.

 

And if you switch the vacuum advance over to ported vacuum you may want to consider adding a bit more initial timing to keep a smoother idle and better off-idle acceleration. Just keep in mind that by doing so you will also be increasing your total advance by a like amount and you may pick up a bit of spark knock under heavy load conditions.

 

Lose the EGR altogether if you can. It's literally recirculating exhaust gases into your engine and only hurts performance. If you have to retain it for emissions where you are, so be it. Apart from that, there is entirely no reason to have EGR on a classic.

There are two schools of thought on the vacuum advance. OEMs in the 70s started connecting it to ported vacuum (any port above the throttle blades in the carb) for emissions purposes. This will only provide vacuum advance off-idle and is "good enough" in most stock engines.

Some tell you it needs to be on full manifold vacuum. This will provide vacuum advance during idle and in most cases you will really see an improvement in idle quality, exhaust fumes, and maybe even fuel mileage, even on stock engines. But to do this properly you need to limit the amount of vacuum advance to 10 degrees max. The adjustable vacuum cans DO NOT limit the amount of total vacuum advance, just when it comes in. Depending on the distributor, some companies make this piece. I had to fab one for my distributor. Without limiting the total vacuum advance, you may get pinging while on the throttle.

Most small block Chevys like 34-36 degrees of TOTAL timing at 3000rpm. So once you have limited your vacuum advance to 10 degrees, you want to set your initial mechanical advance at idle in park to 14-16 degrees with the vacuum advance unplugged and the vacuum port on the carb capped. Mechanical advance will provide the additional 10 degrees. Essentially you want as much advance as you can get without pinging or laboring against the starter.

Now if you're still following me, I will tell you that in some situations, you cannot run full manifold vacuum. l have not been able to figure out why and none of the experts can seem to figure it out, but on full manifold vacuum just letting my car idle, my idle gets weaker and weaker until the engine actually stalls. I'm guessing it's an issue with my particular combo (big cam, low vacuum), but it does this every time I try to swap vacuum sources.

 

Sinister, I respectfully disagree a little. Yes 34-36 degrees of total advance is a good range, mechanical advance and initial advance combined. With vacuum advance disconnected and plugged run idle up to 3000 and rotate distributor to 36 degrees advanced.
Vacuum advance is beyond the 34-36, so when at highway speed high vacuum cruising you may have 46 or so degrees, depending on engine rpm.

Phil

 

You're absolutely right, I had a bit of a brain fart there as all that spewed out. Initial and mechanical advance should be 34-36 at 3k. Vacuum should still be limited to 10*.

 

Just a thought ,some early EGR equipped engines would run lean enough to audibly rattle if the EGR was not working , carbs were jetted lean as I understood it ...