Timing frustrations

[Big Red Mach 1]

I don't think I have a choice but turn my attention back at the carb.  This Street Avenger 770 that so many seem to loathe. Stanglover, I definitely need more info on those 3/32 holes your speed shop guy drilled in your butterflies that cured you of the same issue. : ) I'm also definitely taking the power valve back down From the 8.5 at this point. I also need to confirm the original jet sizes and squinter that came with this carb. I think those were swapped also.

In the meantime, I'm also going carb shopping on the web. ;-)

 Hi Big Red, Not that it matters, earlier I thought you said you had a S/A 670, now you mention S/A 770. 

Geoff,

I very well may have slipped and said S/A 670 early on. I apologize for that. Either way, still acting very much like your S/A 670. I'll see what the lower PV does, also see if maybe the Vacuum does come up a hair, and also see what the adjustment to my secondaries does. Unless all goes very well, a new carb is still probably in my future.

 

[Big Red Mach 1]

I wouldn't be too surprised if your vacuum improves after this. Any internal leakage is going to effect how much fuel and air is getting into the intake, which in turn could affect how the carb was tuned and how the engine behaves.

That would be great. That thought did cross my mind if nothing else being just hopefully optimistic. I'll find out today. 

I'll say this, my vacuum would hardly move one way or the other when adjusting the mixture screws. This had to be because so much was coming out from below the plates. What I was doing wasn't making that much of a difference.

Today I either get her dialed in, or I order a new carb. I'm sure I can sell this Holley for decent money and recoup some of the cost. I'm sure there racer that is a Holley lover out there that make this one work better that I have the ability to.

 

[Stanglover]

I don't think I have a choice but turn my attention back at the carb.  This Street Avenger 770 that so many seem to loathe. Stanglover, I definitely need more info on those 3/32 holes your speed shop guy drilled in your butterflies that cured you of the same issue. : ) I'm also definitely taking the power valve back down From the 8.5 at this point. I also need to confirm the original jet sizes and squinter that came with this carb. I think those were swapped also.

In the meantime, I'm also going carb shopping on the web. ;-)

 Hi Big Red, Not that it matters, earlier I thought you said you had a S/A 670, now you mention S/A 770. 

Geoff,

I very well may have slipped and said S/A 670 early on. I apologize for that. Either way, still acting very much like your S/A 670. I'll see what the lower PV does, also see if maybe the Vacuum does come up a hair, and also see what the adjustment to my secondaries does. Unless all goes very well, a new carb is still probably in my future.

 Again I'm probably displaying my lack of knowledge, but I don't think the power valve will have anything to do with the over rich situation other than the afore mentioned leak past the gasket. But, you may have a blown one and needs to be replaced regardless. It only take one backfire and it could be blown. I would go with a 6.5 in the event your vacuum does come up, other than that, I can't offer any more.

To refresh us, what was your cam specs? We know you've pretty much fixed everything else that could cause low vacuum. Cam could be the only other reason I know of that lowers vacuum at idle.

 

[Kilgon]

 Hi Big Red, Not that it matters, earlier I thought you said you had a S/A 670, now you mention S/A 770. 

Geoff,

I very well may have slipped and said S/A 670 early on. I apologize for that. Either way, still acting very much like your S/A 670. I'll see what the lower PV does, also see if maybe the Vacuum does come up a hair, and also see what the adjustment to my secondaries does. Unless all goes very well, a new carb is still probably in my future.

 Again I'm probably displaying my lack of knowledge, but I don't think the power valve will have anything to do with the over rich situation other than the afore mentioned leak past the gasket. But, you may have a blown one and needs to be replaced regardless. It only take one backfire and it could be blown. I would go with a 6.5 in the event your vacuum does come up, other than that, I can't offer any more.

To refresh us, what was your cam specs? We know you've pretty much fixed everything else that could cause low vacuum. Cam could be the only other reason I know of that lowers vacuum at idle.

In a nut shell a power valve is held closed by vacuum.  If you have an 8.5 pv and the vacuum drops to 8.5 or below it will open.  So - if you have low vacuum at idel and a higher number pv you will be dumping extra fuel into the carb causing a rich mixture.  The way to size is take the idel vacuum (if auto tranny have it in drive) and divide in half.  So a 12 inches at idel it would call for a #6.  They don't make even numbers pv so go to the next half number below - #5.5.

 

[Big Red Mach 1]

 

[Don C]

If your carburetor doesn't have an anti-backfire valve in the vacuum side of the power valve circuit any backfire or hard cough can damage the power valve diaphragm, which will then allow full fuel flow through the power valve at all times.

 

[TommyK]

I have heard many times in many places that a good place to have your plates set to is where you "Box" your transfer slots on your primaries and secondaries, particularly if you have mechanical secondaries. My secondaries were not like this, in fact they seemed very closed off. I went ahead and "boxed them out" and you can see from the old lines around the ports how far I had to go to do that. I had to run the screw in a decent amount before they ever became visible. Hopefully this change in setting may help as well.

On most vacuum secondary Holley carbs, the transfer slots are higher in the bore on the secondary side than they are on the primary side. That is why you had to open the secondary butterflies so far in order to get the slots to show. The advice regarding transfer slot exposure only applies to the primary side. You want the secondary butterflies just barely cracked open from the fully closed position which is probably where they were before. If you are trying to get more bypass air at idle you can open them slightly more but that is kind of a last resort as it can make for other issues.

 

[Big Red Mach 1]

I have heard many times in many places that a good place to have your plates set to is where you "Box" your transfer slots on your primaries and secondaries, particularly if you have mechanical secondaries. My secondaries were not like this, in fact they seemed very closed off. I went ahead and "boxed them out" and you can see from the old lines around the ports how far I had to go to do that. I had to run the screw in a decent amount before they ever became visible. Hopefully this change in setting may help as well.

On most vacuum secondary Holley carbs, the transfer slots are higher in the bore on the secondary side than they are on the primary side. That is why you had to open the secondary butterflies so far in order to get the slots to show. The advice regarding transfer slot exposure only applies to the primary side. You want the secondary butterflies just barely cracked open from the fully closed position which is probably where they were before. If you are trying to get more bypass air at idle you can open them slightly more but that is kind of a last resort as it can make for other issues.

Good advice, and I'm seeing that now. Put it back on that way and immediately RPM immediately climbed to about 1,300 through warm-up. (Vac was still at 11) I was able to use the idle adjustment screw to get it back down to 1,000, but it would not go lower. It just wanted to run at 1,000 regardless. In playing with my mixture screws, I was able to get the vacuum up to 14. But the RPM's climbed to 1,200 with no way to lower it at the idle adjustment screw. I know I could have adjusted timing, but I felt like this due more to the secondary plate adjustment than it was timing (which is set at 16). I'll let it cool back off, pull the carb again, readjust the secondaries, and probably bump back up to the 6.5 PV as Geoff had suggested prior now that I have seen that 14Hg is achievable.

 

[boilermaster]

Big red mach1 ,

I believe we have touched on this before, but what pcv valve do you have in Big Red ?

just for the hell of it why not try adjusting the idle mixture with the pcv valve blocked off ?

Would like to see what it does to vacuum reading and idle speed.

After all the pcv IS a metered vacuum leak and you no longer have stock,, well anything.

perhaps you have too high a pcv flow.

Boilermaster

 

[Big Red Mach 1]

Yeah, I've had it installed previously. It's been blocked off all day today. It really doesn't change anything either way.

Latest update is this. Put the secondary plates back were they were. Put the 6.5PV back in. Pretty much right back where I was. 12Hg at high idle now, but can't get any more out of it by turning the mixture screws. I believe this is because the PV is back open now and overwhelming anything I do with the mixture screws. Whereas with the 5.5 it wasn't open and I had quite a bit of adjust-ability with the mixture screws. Just couldn't get the RPMs down because of how far I had those secondaries open. I wish I had the allen head screw for the secondaries. I would have played with them a bit while the car was warmed up at idle. I could probably crack them open a hair more and get better results. Just nowhere near as open as I had them last time.

I'll probably wait until tomorrow to try again. It's 102* out right now, and with the car running in the garage it's a ton hotter than that.

 

[Don C]

The initial idle adjustment for the secondary throttles for 4150 and 4160 is to close the throttle plate all of the way and then opening it 1/4 turn on the idle screw.

 

[boilermaster]

Big Red Mach 1,

look at it this way, with that temp. and idle of yours, you are doing your part killing the virus.

Your pcv valve WILL make a difference when you get the idle mix a little closer.

I could send you a power valve blank (plug) for testing purpose, that way you could be sure what the pv is NOT doing.

You could NOT drive it that way, as it would be way lean when a pv should open.

Boilermaster

 

[Big Red Mach 1]

Big Red Mach 1,

look at it this way, with that temp. and idle of yours, you are doing your part killing the virus.

Your pcv valve WILL make a difference when you get the idle mix a little closer.

I could send you a power valve blank (plug) for testing purpose, that way you could be sure what the pv is NOT doing.

You could NOT drive it that way, as it would be way lean when a pv should open.

Boilermaster

I greatly appreciate that. Before we go there though, I'll put the 5.5 back in and crack open the secondaries a tad more. I probably closed them further than I should have. Hopefully I can get to that tonight or tomorrow.

 

[Don C]

Having the secondary idle set correctly is really critical to the proper operation of the primary idle and transition circuits. The secondary throttle plates are supposed to be nearly closed, open only enough to prevent them from binding/sticking. Open them only 1/4 turn from completely closed. Having them open more allows more air in, requiring the primary idle mixture screws to be adjusted to compensate. More air in equals lower vacuum, not higher vacuum.

 

[boilermaster]

Don C,

I am going to both agree and disagree with you on the idle mixture.

If Big Red is at a point where his idle mixture screws have a zero response, then all bets are off because he can possibly be rich to a point where that mixture no longer supports combustion and that in turn would lower his vacuum reading.

When I adjust mixture screws, I always find max vacuum and then start leaning from there, while watching the vacuum needle drop.

So while your post is 100% true, it may not be in Big Reds case.

if he has too much transfer slot exposed and a ruptured power valve and no response from the mix screws he could

be to the point where  is no longer supporting combustion.

Boilermaster

 

[Don C]

I agree completely about the problems associated with a bad, or bad choice of, the power valve. The tuning problems caused by one is not possible to overcome unless replaced by a correct one.

 

[Stanglover]

On the subject of power valves, all I can say is from what I found on my S/A 670. My engine pulls 17-18" Hg. The carb came with a 6.5 PV. Using the established formula, it should need an 8.5 PV. With the 6.5, it ran well apart from very rich at idle, so much that the wife complained about the gas stink every time I put the car in the garage. So after many attempts at getting it idle less rich, I took the drastic and much maligned decision to drill the primary throttle plates. Next I installed an 8.5 PV and it ran worse on hard acceleration, bogged like crazy. So I reinstalled a new 6.5 PV (the old one had been subjected to a backfire) Since then it has ran very well, My video attached earlier, is with this carb set-up. So the point is, not every time theory is correct. It still comes down to what actually runs better, trial and error and a lot of pissing around.

I am NOT suggesting Big Red go get the throttle plates drilled (yet), but he may need to go buy a load of gaskets and a couple of power valves if that is where his issue lies.

 

[Big Red Mach 1]

So.......I went ahead and purchased a new Summit Racing 750CFM carb. https://www.summitracing.com/parts/SUM-M08750VS

The reviews are pretty darn stellar, and the annular boosters in a carb for that price were hard to pass up. And Summit has an outstanding return policy. I really like Summit. So I'm going to give it a try. I really also wanted to get a carb that has a choke on it. All in all this S/A carb is really all that stands in the way of me enjoying my car that I've barely been able to drive since bringing it home, and I'm tired of fooling with it, especially in the hot garage now that it's 100 degrees out.

I'll of course let you know how it runs, and what readings I get once I get it hooked up.

 

[Stanglover]

So.......I went ahead and purchased a new Summit Racing 750CFM carb. https://www.summitracing.com/parts/SUM-M08750VS

The reviews are pretty darn stellar, and the annular boosters in a carb for that price were hard to pass up. And Summit has an outstanding return policy. I really like Summit. So I'm going to give it a try. I really also wanted to get a carb that has a choke on it. All in all this S/A carb is really all that stands in the way of me enjoying my car that I've barely been able to drive since bringing it home, and I'm tired of fooling with it, especially in the hot garage now that it's 100 degrees out.

I'll of course let you know how it runs, and what readings I get once I get it hooked up.

 That looks like a nice carb, one piece bottom casting by the look of it, so no bottom end leaks like Holley's. 

As I'm not too happy with my carb either, I can't wait to learn the results after you get it set up of course.

On a side note, I just finally got my car out for a decent drive today. It was supposed to rain, but the forecaster was wrong again, turned out a beautiful day. Because I have that new rebuilt rear end in it, I didn't do anything too wild, but I can say all the issues I had last year with that 670, are gone. The thing pulled like a train. No bog, no hesitations and no gas smell when I put it back in the garage. I'm happy for now until the piggy bank gets fatter and the exchange rate improves in my favor.

 

[Big Red Mach 1]

So.......I went ahead and purchased a new Summit Racing 750CFM carb. https://www.summitracing.com/parts/SUM-M08750VS

The reviews are pretty darn stellar, and the annular boosters in a carb for that price were hard to pass up. And Summit has an outstanding return policy. I really like Summit. So I'm going to give it a try. I really also wanted to get a carb that has a choke on it. All in all this S/A carb is really all that stands in the way of me enjoying my car that I've barely been able to drive since bringing it home, and I'm tired of fooling with it, especially in the hot garage now that it's 100 degrees out.

I'll of course let you know how it runs, and what readings I get once I get it hooked up.

 That looks like a nice carb, one piece bottom casting by the look of it, so no bottom end leaks like Holley's. 

As I'm not too happy with my carb either, I can't wait to learn the results after you get it set up of course.

On a side note, I just finally got my car out for a decent drive today. It was supposed to rain, but the forecaster was wrong again, turned out a beautiful day. Because I have that new rebuilt rear end in it, I didn't do anything too wild, but I can say all the issues I had last year with that 670, are gone. The thing pulled like a train. No bog, no hesitations and no gas smell when I put it back in the garage. I'm happy for now until the piggy bank gets fatter and the exchange rate improves in my favor.

That was the other thing that I really liked and put it over the top for me. Gave it the edge over a Quick Fuel which is really just a Holley with a different sticker on it.

Glad you got yours out for a good run. I'll report back once I get the new carb hooked up. : )