Hot Rods A few Questions for Modern Carb Experts

If this doesn't belong here, I understand. But I trust the info I get from this site more than any place else.

Given the below details, would a DP be a better choice than a VS?
The engine is a 350, 10.3:1, Comp XR294HR cam, with Edelbrock Performer RPM heads and air gap intake. Timing is 37* at 3000RPM, with 10* vacuum added from a manifold source. I'm PRETTY sure I've got the timing dialed. Idles like a champ, buit only makes about 11" of vacuum due to the cam. Converter is a 2400-2600 B&M unit, but It will only flash to about 2000rpm without moving the car. This is on a TH350 trans. Rear gear is 3.70. Never weighed the car, but I reckon it's around 3400-3500 pounds with me in it. I built the car for street/strip.
I don't want to say too much about how it performs now, but if I'm looking for added performance in the wrong place here, feel free to say it.

Thanks in advance!!

 

The answer might depend upon the size of either carb....which you didn't tell us....

 

How much initial ?

 

I was expecting that. It's got a 750 Holley Street HP on it now. I'd likely go with a 650 double pumper, and likely another Street HP. I like these carbs. I'm right on the edge of cubic inches and useable RPM to be between the two. It will pull to 6500rpm, but realistically I'm shifting between 6000 and 6250.
I've tuned the 750 fairly well, but I'm sure a pro would change some things. I'bve got a fairly light spring in the secondary, that's where it seems to be the happiest. I've messed with pump cams to no end and haven't really noticed an improvement over the stock orange cam on hole #2. I'm looking for more of a kick in the ass down low, but it may not be the carb's fault.

 

I’d think that cam needs more timing Initially…

 

16* initial, so 21 mechanical. Any less and it labors a bit in gear. It idles like a champ and is nice and snappy free revving.

 

It likes all the initial I can give it, but even with 10* vacuum (mechanically limited), it will ping in third gear at lower RPM, so this is the compromise. With no vacuum advance, it's a real pig at anything less than WOT. I've never been able to get it to idle clean and cruise without that third gear ping at the same time. So I'm just careful about putting my boot into it at lower rpm in third.

 

Have you tried the vacuum advance on a timed source for comparison? If so, what were the result's?

 

I ran it that way initially. Idle quality suffers without the additional vacuum, and it was lazier to rev.

 

The way I read your post, you have the timing retarded. You shouldn't be setting your timing with the vacuum advance connected. You probably need about 35 degrees at 3000 rpm with the vacuum advance disconnected. You can and probably should run a vacuum advance, just don't use it to set the timing.

 

Did you try bumping the initial up a few more degrees while keeping timed vacuum?

 

Your engine specs. are close to what I'd be running a Double Pumper. It's actually kind of on the edge.
But then I like a challenge in tuning.

Hell, I ran a D.P. on my little 262 Chevy V-6 (in my 67 Chevy II wagon) years back.
Actually, to pass the CA smog tests, I used a stock, Edelbrock 500cfm carburetor.
BUT, to drive, I used a modified (added annular boosters) Holley 600 D.P.
The Holley was MUCH more fun to drive. While the mileage was down from the Edelbrock, the
drivability, (read that fun)...was UP with the Holley.

Mike

 

LOL my brain is in the gutter, I was thinking about a different DP.

First determine why to use a double pumper. Most of the engines that we run are just not extreme enough to need a double pumper. I have normally used them on a manual tranny car with steep gears and a ton of cam. I am probably not thinking the same as everyone else on this and may even be wrong in my usage.

A vac secondary carb is usually harder to dial in, you play with springs and not with cams for example. But once dialed in that are tits in most applications.

That said, if I had one and it was the correct CFM value I would use it.

 

It’s 37* without vacuum, so 47* total max.

 

I have tried that, it will diesel after I shut it down. Even now it will do it on the rare occasion. Doesn’t seem to labor the starter though.

 

That's a ton of lead. But if that is what you need that is what you need.

Do you happen to have a timing tape on it? (graduated balancer ) If you do you can accurately check your full advance with a simple timing light. (sorry I do not like the high tech lights I am a backward bastard).

That said I'll take your word for it and that is a ton of lead. I am not sure still that you need a double pumper but I would not be afraid to use one, like I said setup is easier.

@Mr. Sinister I just caught your last post. Run On can also be caused by idle speed. If you are idling a bit fast (like up around 1000 RPM) that could also cause your run on.

 

Dirty old man!!
That’s backwards. VS carbs are easier to tune and are more forgiving when it comes to cfm. I went a little higher on the cfm with my 750 since it will only add the secondary fuel as the engine demands it. All you have to do really is worry about jetting and where the secondaries comes in via the diaphragm spring. DPs are more work to get dialed in nice, but will pretty much always outperform a VS. Traditional logic says a VS for a street car, DP for a strip car. I don’t feel like I’m getting the throttle response I want when I mash the gas, and my thinking is the slower opening of the vacuum secondaries is limiting midrange power. What I’ve noticed is the earlier I bring the secondaries on, the more it likes it. But you can only do so much with a VS and no secondary accelerator pump.

Whether the expense of changing carbs and the time put into tuning is worth it or not is what I’m considering, especially when it runs pretty good now. But I’m never satisfied and feel like it could be better.

 

Balancer has marks engraved in the body. I like these because there’s no room for error. If the marks are off, you’ve got bigger problems.

Idle speed is necessitated by the cam and I set it where it’s happy in gear at about 750-800rpm. 1000 is just where it lands in park. Anything less and it labors to idle in gear. Were I to build this engine again, I’d go smaller on the cam to get better low and mid range power, but man I love that pull up top.

 

Mr. S -

The engine is a 350, so that round disc on the front of the crank, is a "damper", not a balancer.

Mike

 

Are you talking more initial spark timing, or did you mean advancing the cam timing a few degrees?

 

I think a 650 DP could work. Little smaller to help the low to mid rpm when you go WOT from lower engine speed. I think that is what you're looking for on the street.
The slight loss at max rpm is small. As alternative go with annular discharge 750 DP. Annular are better for low vacuum signal. Then you can run a 750 with less chance of bogging. But the 650 will run very well even on strong 350.

 

This statement says a lot. I think you know what needs to happen next, even if it is just to satisfy your curiousity. IMO, better performance is worth the extra effort, no matter where the added performance may be found. The pinging in high gear is what concerns me the most. Have you tried a colder heat range sparkplug to get it to stop, or use a better grade of gas, maybe some octane booster? A looser converter may help some too. OK... I'm done spending you money now...

 

You're right, I have a habit of interchanging the two words.

 

Keep in mind it's only a lower rpm under a load, cruising and stepping into it above 3000rpm is fine. Still, I'd rather not have it.
I run a heat range colder than Edelbrock recommends for these heads due to the compression. I tried 2 ranges colder and it didn't seem to like that. I always feed it 93 octane. I've used Torco booster before, didn't seem to help. 100 octane is available at the pump locally, but it's $$$$.
I've been considering a looser converter, it is a bit too tight for my liking, but it's nice for street manners. For how much I drive the car, I can sacrifice a little of that for better performance.

The ping I think is a product of a lean mixture when the scedondaries aren't open at lower rpm when i step into it. DP might help with that? If I remove my PCV valve, the ping goes away. If I try to tune the ping out, it runs worse. I've had to run one of those high dollar ME Wagner pcv valve due to vacuum signal and not being able to find a parts store unit that worked correctly. It's pricey, but it works great. I've basically accepted it as it is. It's been doing it for years with no apparent ill affects.

 

I pretty much did the same thing on a Barry Grant "gold claw" carb... I switched over to the annular boosters from the down leg ones and it definitely woke the motor up....

 

I think I'm going to try it. I think the 650 is the way to go, as any sacrifice would likely be above 6000rpm, and really the heads and intake are already a limiting factor above that.
This is going to snowball, I can already tell. I'll be into the heads, intake, and converter next. Then might as well throw a strroker crank in it......

Thanks for the input guys, I do appreciate it.

 

I do like those annular boosters, I've got an Edelbrock AVS 650 on the shelf that I played with on this engine. Idle and low end were great with very minimal tuning. but it was lacking up top.

I swear I've seen a Holley style carb on Summit with Annular boosters, but I can't find it now.

 

If it were mine I would limit the mechanical advance and bring the initial timing up 20 to 22 and 36 to 38 total, and most importantly get a looser converter. Contrary to popular belief drive ability wont suffer with a proper converter. then I would go with a h.p. 750 dp. Also to check your stall drive slowly in 3rd gear and mash it while watching the tach.

 

Great screen name!!
On the timing, I've got an MSD distributor with the interchangable mechanical advance bushings. I haven't seen a bushing kit that will let me limit mechanical advance below 19*. What would I do here?

I'm of the same mind on the converter, should have done it years ago. I'm thinking 3000rpm. With the 29" cheater slicks and the 3.70 gear, I'm at just about 3000rpm at 60. Where I get confused is the diameter of the converter.

I can also see justification for going with the 750 after making these changes. Thanks for the input!!

 

The Summit carb, all 3 sizes, have the annular boosters plus a lot of other drivable features. I believe every other manufacturer carb offers them or you can buy them separately and install them.