Small Block Chevy Question

[rmb_cobra]

I recently had a leaky head gasket that was beyond my skill set to replace. It was running great, with a little coolant leakage on the headers making the motor smoke like a steam cleaner. While the shop had it torn apart, I had them put in a new cam/lifters to give it a little more power. A moderate Lunati cam kit. The motor is a '69 350 SBC, box stock rated at 350HP. The heads have been ported and polished and am running a 650 Holley. The bad news is that it now runs like crap. Idles too fast (around 1500 once warm), the plugs indicate it is running too rich, and when I accelerate hard it pings like crazy and misses a bit, and the power is 1/2 what it used to be. When the mechanic tried to time it, he said it shorted out/nuked two different timing lights so he guessed at the timing which allowed for a good idle, took my check and said "call me if you have problems."
Any suggestions? I need to take the car back this week. Where do I start?

[xlr8or]

I would start with a new mechanic.

Sounds like he didn't get the distributor setup correct. You may also have a vacuum leak causing the high idle but that will usually show a lean condition not rich. There is nothing that could fry a timing light unless he plugged it in wrong.

Also verify that the plug wires are put back on correctly. Drivers side cylinders are front to rear 1,3,5,7 passenger side cylinders are front to rear 2,4,6,8 and the firing order is 18436572 going in a clockwise direction around the distributor cap.

Also verify the vacuum advance (if you have one) is plugged into "Timed vacuum" not manifold vacuum. For the timing on a 350 you should have 8 to 10 degrees BTDC initial timing with a total of 36 that should be all in by 2500 rpm.

Initial timing should be set with the vacuum advance disconnected and plugged with the idle at 1000 or less.

Hope this helps.

[Hotfingrs]

Too many things come to mind, right off the bat....you don't give the came specs, but porting heads will cause you to lose the bottom end if you didn't need the porting. 650 carb way to small...you don't say what jets and power valve are in the carb..Xlr8or pretty much summed it up in one line.....get a new mechanic

[Bruce Edwards]

Sounds like the timing is set WAY to high. Loosen the distributor and turn it so the engine slows down to were you think it should be say 650 to 850 rpm, I figure you do not have your own light, and check the vacuum gauge on the dash, if you have one. At idle unless the cam is really wild you should have around 18 to 16 inches.
Really need to get a LONG way from that wrench turner he is not a mechanic.
The other folks are right the 650 is to small for a 350 you should have at least a 750cfm vacuum secondary on it.

[MidOHasp]

I'm not challenging anyone's remarks on the carb, just asking a question here:

I've seen an equation in numerous locations that is supposed to help you figure a carb size (doesn't take into account all variables) and that equation put me at 750cfm for my 408 running 6000-6500 rpm.

The guy who dyno'd the motor said we needed a dominator 1150 on it, while the equation I ran actually put me at about 710cfm. I bought a Speed Demon 750, upped the front jet 1 size, and am running pretty good.

Is there a different school of thought on carb size?

JP

[Hotfingrs]

I would say 1150 is to much for a 408....I run a 1050 Dominator on a 421 motor. Just changing the carb dropped my 60' time from 1.52 to 1.45 ...Rule of thumb for carb size is....hook up a vacuum gauge and run a quarter mile....see what the vacuum is when you cross the line......if you are pulling over 1" of vacuum the carb is too small, if you are on zero, the carb is too big. You want to be between 0 and 1"

[Chaplin]

MidOHasp-
You are correct in your thinking. 650 is not too small for a 350. I posted this in another thread, but here it is again.


A 350 with max rpm of 6500 and 100% volumetric efficiency (VE) (which you will not achieve with a naturally aspirated motor), you would need a carburetor that flowed 658.3 cfm.

How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency

(Using this example: 350 CID x 6500 RPM = 2,275,000 / 3456 = 658.3)

Below is from Holley:
Most Street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.

Therefore, assuming you achieved the same VE as a fully modified race engine, (95% VE), the max CFM necessary would be about 625 (95% of 658). Therefore, a 650 should be fine for a 350. You certainly could use a 750, but you may suffer some loss of power and responsiveness on the bottom end because of decreased velocity of the air/fuel mixture.

[Junket]

I am with xlL8or change the mechanic if the thing he sugested don't work might look at the timming gear on the crank it has 3 positions might be on the wrong one for your applaction also if your cam is not over 240*@ .050 the 650 carb should be good.
Ken

[RallySnake]

RMB,

You have had some EXCELLENT advice here. I like the 650 Holley and use that for autocross and a 670 Avenger for the street. If you are drag racing ONLY, then a larger carb may help. I had a very similar problem last year. My vacuum advance on the distributor had ruptured, allowing a vacuum leak.

The way to test the vacuum advance (assuming that you have one): Remove the distributor cap and vacuum hose from the distributor. Twist the advance plate against the spring with one hand. Put a finger over the vacuum advance hose fitting tube. Release the plate, allowing it to return to the normal position. After a few seconds, pull your finger off the tube. You should hear a "pssst" of air suck in. If you do not hear it, try a few more times. If it is working correctly the vacuum should be maintained for a whole minute. If there is no pressure generated by this procedure, your diaphram is ruptured and you will need a new vacuum advance.

Paul

[rmb_cobra]

I appreciate all the great advice, and timely as well. If I can tear myself away from work/family for a few hours, I will head to the garage and get started, at least with the obvious stuff. It seems to idle fairly well, albeit fast. It is just when I get moving it craps out. Timing off, combined with a possible vacuum advance leak, maybe.
I spent some time into it yesterday, and the gorillas had no sense of what they were working on. Numerous wires were routed wrong and cooked on the headers, various things were loose. Scary.
Thanks again for the help. I will update the crowd once it runs strong.
RMB

[Bruce Edwards]

Ok I have to put up my 2 cents on this again. I feel that the 650 is a bit small for a built up 350 Chevy or a built 351. We have used 750 vacuum secondary carbs on our engines for years and never had a problem. The advantage of the vacuum secondary is you can tune it to the engines needs unlike a double pump carb. I understand Holley's table and why they share the information. The number of return carbs because of over carburetion was very high a number of years ago. I was also in the parts business for a few years in the early 80's and one of the biggest problems we had was folks buying 750 double pumpers for their street engines. Like duh they run like crap when you floor it and all that fuel shoots down the intake and floods the engine not to even talk about the over size jets they came with.
As to over carburetion fellas I run a 6 pack on a 340 in my 73 Charger. The factory set up for the 340 was the same as the 440 six pack. Center carb is 350cfm and both out boards are 500cfm each. You do the math that works out to 1350 cfm on a small block. HOLY CRAP BATMAN that baby must be way over carbureted by Holley standards!!!!!! Ok so the car weighs two tons and runs 12's in the quarter and till I parked it I drove it every day to work and back. Point is with a vacuum set up you can get the advantage of a larger carb for spin up and still have it tuned to your engine size. Can not do that with a mechanical secondary.
That does not mean I would put a Domonator on a street car. My only point is that a 750 will work great on a 350 engine with out being to large.

[trularin]

The size of the carb is:

(Max RPMs to be turned X the CID)/3456

He has a 350 and lets say he wants to turn 7000 RPM.

( 7000 x 350 )/3456 = 709

A 750 CFM carb should be fine.

I think the engine needs to be looked at by someone who knows what they are doing.

Just my $0.02

[Bruce Edwards]

Trularin

THANK YOU!!!

[Chaplin]

A 750 vac secondary will work on a 350, and you could also use a 900, but to what benefit? As noted, a vacuum secondary will only take as much fuel as the vacuum draws- and even assuming 100% volumetric efficiency, you can't use all 750 cfm. And unless you're drag racing, I'm guessing most people aren't turning their motors to 7k on the street, so a 650 is more than enough- as explained in my previous post. Plus, with a 750 you will have a drop off on the bottom end- but, yes you can use a 750 or even a 900 with vacuum secondaries if you want.

[Bruce Edwards]

Chaplin,

Drop off on the bottom end? How? At most speeds only the front half of the carb is working and even if it is half the size of the carb that is only a flow rate of 375cfm. So what drop off? Chevy even put the biggest piece of crap ever made on the 350 called the Quadrajet. It ran very small primaries with a manual secondary that was controlled by a air valve plate like the Carter AFB. Granted the nick name they picked up is Quadrabog but that is a case of poor adjustment to the air valve spring rate. Most of them over time got loose so the vale flop open causing the bog problem.
I see by your side bar you have a 427sc ERA. The original 427 came with two Holley 4's that flowed more than enough for the 427 but I do not remember any over carb problems on that engine or the 426 Hemi with two Carters AFB's or the small block Ford with three 2's or the FE with three 2's. All of these were factory Hot Rod engines that had the potential to flow way over there engine size yet all of them were very fast and strong runners for their time.
Sorry my dander gets up on these kind of things and I tend to run on at the keyboard.

[Hotfingrs]

All these formulas are only guidelines....a starting point to picking a carb. Sure the engine will run with a 650, will it see it's best performance...no way.

[MidOHasp]

Bruce,

I think in the past 3 days I've come across half your posts. You seem to know what the heck you're talking about.

Are mechanical secondaries an inherently bad thing? When I put together the spec sheet for my motor, some of the logic I used was more based on what I called 'common sense' at the time but was really not based on a lot of good info.

I chose the Speed Demon 750 double pumper on a 408 turning 6500rpm. The "Holley" number would be 767. We're really turning 6K rpm now, though, and that "number" is 708.

My logic was - it's a stroker so it is going to be a torque monster. It's a light car. Should be able to use everything the carb can supply. It seems to have worked.

I basically had a boner for a double pumper because when I think of vacuum secondaries, I think of stock 200hp V8s from the early 70's. But I've read some info in your most recent post that indicates to me that the vacuum secondary makes sense because you will be dumping fuel/air mixture at the rate you can use it as opposed to however fast the carb can dump it.

In what situations do you say, "double pumper makes sense, no need for vacuum secondaries." ??

I wish I could hang out in the all shop talk forum all day, but I'd get fired. What an education you all provide.

JP

[Chaplin]

Bruce-
I agree that if you use vacuum secondaries you can use a bigger carb or multiple carbs with way more cfm than the engine can ever use, but again the question is what is the point? If an engine can only use 700 cfm why stick 1150 of cfm on it? The extra cfm is wasted and you may see a drop if in power/torque and throttle response on the bottom end. (Explained below.)

As I am sure you are aware, when you use a bigger carb, the velocity of the air/fuel mixutre coming into the carb is slower than it would be in a smaller carb, which may lead to decreased throttle response or a slight drop off power/torque on the bottom end. (To your question, even if you're only running on the primaries, the size of the venturis are smaller in a smaller carb and larger in a larger carb, so regardless of whether you are running on only the primaries, the velocity of the air/fuel mixture will be slower in the bigger carb). As I mentioned previously, if you are drag racing and running to 7k all day long, then you really shouldn't care about a slight drop off in the bottom end, but if you are driving on the street and keeping the revs under 6k most of the time (where most cobra replicas are) you will likely get better perfomance with the slightly smaller carb.

That's all for now. And no problems with dander, just a freindly discussion.

[Bruce Edwards]

JP,

It is my opinion that doubles are best on the track and not the street. They are over kill for most engine set ups. Think about it most folks never drive more than 75 or 80 and most of the time is spent driving around town at slow speed and low rpm. For this type of set up a vacum second is ideal. When you do want it is ready to go but only opens as fast as the engine needs it. Better fuel use and smoother change from the front to all fours.
If you are running at 120+ all the time and the carb never goes back to the front barrells use the double other wise save your money.
To me it is a no brainer. 20+ years of building cars I had to learn something!
By the way thanks for the nice words.

[Bruce Edwards]

Chaplin,

Only a few things. I am glad I have not upset anyone. To your point. First thing a 351W 2v has a carb that flows in the area of 400cfm that means you would need to use a 800cfm 4 barrel to get the same front primary size. I do agree that a large primary is not good for daily driving. I never said I would put a 1150 on a small block as ti is way to large. The point I was trying to make is that "I feel" the 650 is a bit small for the engine he is talking about. The polish job is going to add air flow and a larger cam is going to add to that. The 650 is more than likely a double pump and is going to flood out the engine ever so slightly when it goes from running the fronts to opening the rears. Also the carb would seem a bit small to me in total cfm. Sorry that is just what I have found after years of building engines.
The 650 may be just fine for a bone stock engine but it was my understanding that he had more in it than that.