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Old 04/18/2007, 08:13 AM   #1
miller511
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Cool Tuning issue on the CS 347

Hi all,

Fired up the new motor on Monday. Initially, I had some trouble getting it started. But it finally roared to life. :-)

Since then it really takes a while to get it to fire up (sometimes it won't). I suspect that it's a timing issue that's causing it to not fire immediately. Maybe I need to upgrade my ignition to get a hotter spark?

Question for you guys with a similar setup -
What should the initial timing be set to? I have it set to 10 degrees before top dead center (BTDC). I've read that it could be anywhere between 6 and 15 degrees BTDC.

FYI- The cam specs are lift .509/532 duration at .05" is 222/232 with 112 degrees separation. I'm using a 600cfm carb.

Thanks, Jeff

Jeff Miller-
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2007 Black GT/CS Coupe
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Old 04/18/2007, 08:28 AM   #2
joedls
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The 347 in my GT/CS has very similar cam specs. I run 10 degrees initial timing. It starts up right away. My guess, from your description, is that it is not the timing. If you had too much timing, it would kick back at you, not necessarily take a long time to start. This sounds to me like it could be a fuel mixture problem. What carb are you running?

Joe

HP numbers are good and all, but they're like asking someone how much they can bench. What difference does it make, if I can still kick your ass.
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Old 04/18/2007, 09:13 AM   #3
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Best e.t. : 11.042, Fastest speed: 124.08, Best 60' : 1.615, 0 to 100 in 6.98

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those cam specs are a little more mild than mine:
.544/.560" with 232/242 duration at .05" lift (peak horsepower 6500rpm)

i dont know what your compression ratio is....but could it be your starter?...i bought a high torque starter to crank over mine because it has a 10:1compression ratio...i believe it takes more torque to turn over the 347 than a 289 or 302 and i know it takes more torque to turn over motors with high compression ratios

as far as the initial timing goes....right now im at 16 degrees...but it might ping if i drive it on a hot day...i've had it set between 10-14 degrees before, so your 10 degrees should be fine.

i have a holley 4779C (750 mech sec) and i pump the gas pedal two full times and i just tap it once more as im cranking it and it starts, then i give it a little gas initially to keep the idle up because i do not use a choke

what spark plugs are you running? what is your spark gap? what is your ignition coil?

im running autolite 3924s with anywhere from .040" to .050" gap with a MSD Blaster Coil
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Old 04/18/2007, 12:01 PM   #4
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Joe/David,

Thanks for your input.

I'm running the stock starter (from the previous 302 motor). It does crank the motor (but maybe not hard enough?)
I DO notice if I crank it too long (like 15 seconds), the starter kicks back and makes a "noise".

spark plugs are Champions (can't recall what number at the moment)

gap is .034 (maybe I should gap them larger?)

ignition coil is stock (and the distributor housing is stock with a wide circle cap added up top) maybe need a hotter spark with the MSD blaster?

carburetor is an Edelbrock 600cfm (brand new)

Let me know what you guys suggest.

Thanks, Jeff

Jeff Miller-
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2007 Black GT/CS Coupe
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Old 04/18/2007, 12:28 PM   #5
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Quote:
Originally Posted by joedls View Post
If you had too much timing, it would kick back at you, not necessarily take a long time to start.
Joe,
I want to get clarification of too much timing. Too much timing meaning it would be further away from TDC when it ignites? Like 15 degrees BTDC is too much compared to 10 degrees BTDC? (I think this is what you are saying).

Thanks, Jeff

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Old 04/18/2007, 01:52 PM   #6
davidathans
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Best e.t. : 11.042, Fastest speed: 124.08, Best 60' : 1.615, 0 to 100 in 6.98

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with a stock coil i dont think you should gap any larger than the .034 where you are at now, because your coil might not increase the voltage high enough to jump the gap

if your starter is acting funny, sounds like it is...it might be your problem

another thing it could be is the carburetor....if you just took it out of the box and sat in on your intake manifold, your tuning could be way off....i have spoken to people that have bought demon carburetors that were rediculously lean for their application right out of the box....it wouldnt be a bad idea to take the car to someone like Bob Jennings (North Hills, CA) he is an expert with distributors and carburetors and i guarantee he could figure out whats up with your car and fix it...his number is 818 894 3811, the man has over 50 years experience
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Old 04/18/2007, 03:03 PM   #7
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Thanks for the info David.

I'm thinking it could be the carburetor, too. I'll look at that when I get home. The DVD that came with the Edelbrock carb is unbelievably detailed. One of the first things it talks about is that you shouldn't need to do much of anything to it other than hook up vacuum lines and maybe set the idle screws.
I'll need to study it more to see what to tweak (to richen up?) the mix.

-Jeff

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Old 04/18/2007, 03:27 PM   #8
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Quote:
Originally Posted by miller511 View Post
Joe,
I want to get clarification of too much timing. Too much timing meaning it would be further away from TDC when it ignites? Like 15 degrees BTDC is too much compared to 10 degrees BTDC? (I think this is what you are saying).

Thanks, Jeff
Yes, that's what I'm saying.

Joe

HP numbers are good and all, but they're like asking someone how much they can bench. What difference does it make, if I can still kick your ass.
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Old 04/18/2007, 05:34 PM   #9
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I would guess carb also. I know you're supposed to be able to run them right out of the box, but with the huge variety of engines they might be put on, I just don't see how that's possible. If you haven't tuned a carb before, I'd definately take it somewhere where they can adjust it and put an exhaust gas analyzer on it.

Steve

The wannabe formerly known as an owner.
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Old 04/18/2007, 05:50 PM   #10
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Hello,
This is a subject right up my alley. I tune and assist people in starting new engines in the Seattle area.

Jeff,
A couple questions and then some thoughts.

I assume it starts hard when it is cold.
I assume you used the distributor from the 302.
What year was the 302 or better yet what year is the distributor?
I assume you have a vacuum advance on the distributor.

I do not think it is a starter problem, but it does sound like you need a new starter. Using your starter for anything longer than 10 or 15-second bursts is not advised. Engines, as you want this one to do, should light right off.

With the cam you are running, I assume you are looking for horsepower. I would recommend at least a 650 or 750 carb for this combination. A 600 could work on a 347, but I would cam it more mild and would work in a 5,000 RPM peak or less. This engine could make good torque with a 600-CFM and a milder cam.

I did not see a mention of what heads are on it or its estimated compression ratio.

Lets talk timing. A stock distributor works with a stock cam at about 6 degrees before. That is what I call "initial" advance. The centrifugal advance and the vacuum advance add to this number. Regardless of the distributor most 10 to 1 engines work best at 38 degrees to 40 degrees of "total" advance. I would recommend that you put a degree tape on an engine that has this must potential and you are looking for performance. You may have a "fluid" damper or a similar performance damper that already is 360 degreed.

So I want you to get it started and hold the rpm up to about 2,500 to 3,000. This will ensure that you have all of the centrifugal in and with no load the entire vacuum in. Ensure if you have vacuum advance that the hose to the carb is plugged in. You want to see what the "total” advance is. If it is not 38 to 40 degrees before then you need to increase the initial. Hopefully you are not using a smog distributor with 2 vacuum diaphragms. Get rid of that thing. Very common in 1968 or 69.

Now it gets more complex. I would assume that if you have a single diaphragm distributor from the 67 and older or non-California car, you should see 38 degrees of total advance.

You motor will more than likely run much better with more "initial" advance. Somewhere near 12 to 15 degrees before. If you dial that in it will have way too much "total" advance and ping like hell. So for your motor I would recommend either a MSD or Mallory distributor with "adjustable" centrifugal advance. I would shorten up the "centrifugal" amount of advance so you can run more "initial" advance. Whew!!! Following all that. The goal is enough initial advance to make the motor happy at idle with enough total to make HP!!! But we have flat spots and crab adjustments to talk about.

I agree with David, you may want to seek his professional tuner. The advice on spark plug gap with a stock coil is spot on. Do not open the gap with a stock coil.

You did not say what tranny you have. Auto get vacuum secondary. Manual you can use mechanical. The 4779 holley would be perfect for this cam and engine if you have performance heads, 10 to 1 or more and short gears (3:50 or lower) and are looking to go fast. I am unsure what you what you want to do. Go fast or mild motor. With a 347, I think you want to go fast.

Remember... " There is more to life than gas mileage"

Give me all the info on you cars components and whether burying pukes driving Chevy's is your goal!!!

Rob
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Old 04/18/2007, 08:46 PM   #11
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Quote:
Originally Posted by robert campbell View Post
Hello,
This is a subject right up my alley. I tune and assist people in starting new engines in the Seattle area.

Jeff,
A couple questions and then some thoughts.

I assume it starts hard when it is cold.
I assume you used the distributor from the 302.
What year was the 302 or better yet what year is the distributor?
I assume you have a vacuum advance on the distributor.

I do not think it is a starter problem, but it does sound like you need a new starter. Using your starter for anything longer than 10 or 15-second bursts is not advised. Engines, as you want this one to do, should light right off.

With the cam you are running, I assume you are looking for horsepower. I would recommend at least a 650 or 750 carb for this combination. A 600 could work on a 347, but I would cam it more mild and would work in a 5,000 RPM peak or less. This engine could make good torque with a 600-CFM and a milder cam.

I did not see a mention of what heads are on it or its estimated compression ratio.

Lets talk timing. A stock distributor works with a stock cam at about 6 degrees before. That is what I call "initial" advance. The centrifugal advance and the vacuum advance add to this number. Regardless of the distributor most 10 to 1 engines work best at 38 degrees to 40 degrees of "total" advance. I would recommend that you put a degree tape on an engine that has this must potential and you are looking for performance. You may have a "fluid" damper or a similar performance damper that already is 360 degreed.

So I want you to get it started and hold the rpm up to about 2,500 to 3,000. This will ensure that you have all of the centrifugal in and with no load the entire vacuum in. Ensure if you have vacuum advance that the hose to the carb is plugged in. You want to see what the "total” advance is. If it is not 38 to 40 degrees before then you need to increase the initial. Hopefully you are not using a smog distributor with 2 vacuum diaphragms. Get rid of that thing. Very common in 1968 or 69.

Now it gets more complex. I would assume that if you have a single diaphragm distributor from the 67 and older or non-California car, you should see 38 degrees of total advance.

You motor will more than likely run much better with more "initial" advance. Somewhere near 12 to 15 degrees before. If you dial that in it will have way too much "total" advance and ping like hell. So for your motor I would recommend either a MSD or Mallory distributor with "adjustable" centrifugal advance. I would shorten up the "centrifugal" amount of advance so you can run more "initial" advance. Whew!!! Following all that. The goal is enough initial advance to make the motor happy at idle with enough total to make HP!!! But we have flat spots and crab adjustments to talk about.

I agree with David, you may want to seek his professional tuner. The advice on spark plug gap with a stock coil is spot on. Do not open the gap with a stock coil.

You did not say what tranny you have. Auto get vacuum secondary. Manual you can use mechanical. The 4779 holley would be perfect for this cam and engine if you have performance heads, 10 to 1 or more and short gears (3:50 or lower) and are looking to go fast. I am unsure what you what you want to do. Go fast or mild motor. With a 347, I think you want to go fast.

Remember... " There is more to life than gas mileage"

Give me all the info on you cars components and whether burying pukes driving Chevy's is your goal!!!

Rob

Holy cow Rob!!!

OK. Here goes-

Heads are Edelbrock Performer RPM w/60cc chambers
Compression ratio is 10.47:1
Distributor is vacuum advance and I believe is original 1968 vintage, but not completely sure. It does have two vacuum ports, although the one closest to the distributor body has always been capped off.
Damper is not 360 degree marked or fluid. it is marked zero to 40 degrees.
Rear end gears are 2.79
Tranny is auto C-4

So, it's sounding like the 600cfm carb simply isn't getting enough fuel to the engine?
What about my newer stock mechanical fuel pump? Could it be not pumping enough fuel? It worked fine on the 302.

And the second issue is a distributor that I should upgrade to one with adjustable cetrifugal springs?

Thanks for your help, Jeff

Jeff Miller-
1968 Tahoe Turquoise GT/CS
2007 Black GT/CS Coupe
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Old 04/19/2007, 06:38 AM   #12
robert campbell
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Jeff,
More tonight!! What are your plans? Just a quiet cruiser, or all it can be??? The 600 CFM can work just fine. I have seen bigger motors than yours that work fine with a 600 carb. You cam, heads, and compression are better suited to a bigger carb.

Also, are you running long tubes, shorty's, or cast original exhaust manifolds?

Rob
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Old 04/19/2007, 06:56 AM   #13
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Rob,

I'm leaning towards the middle of the road/cruising end of the scale.

Exhaust manifolds are stock. Intake manifold is Edelbrock Performer.

Thanks,
Jeff

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Old 04/19/2007, 12:39 PM   #14
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Found an interesting carb calculator...

http://www.csgnetwork.com/cfmcalc.html

Jeff Miller-
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Old 04/19/2007, 04:17 PM   #15
robert campbell
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Jeff
I think I have the complete picture now. First lets address the starting. Regardless of total timing that engine should lite right off. Dial in about 10 to 12 degrees of initial timing. You have not indicated whether the carb has an electric or manual choke. You have run it enough that the carb should be full of gas. A normal stock 289 with an auto choke wants 2 full pumps on the gas and then keep you foot off the gas and it should start and kick up on a fast idle. Your 347 more that likely can stand about 4 complete pumps on the gas pedal. It should lite right off. If not then a couple more pumps. Certainly you have tried this. The other guys in the thread have bigger carbs that are more performance based and thus a bit “fatter” (richer) than your carb. They start well cold due to being a bit richer and their accelerator pumps deliver a lot more per pump. They be into going real fast. 12.19 in the quarter is way fast!!!! It is important that your carb have a good choke to help it start. It is lean for you application, but that carb works great on a stock 350 Chevy (puke). They don’t need much gas cause they don’t go fast and are broke most of the time…..

Give me some info on you choke and we can go into way more details. By the way, David suggested a professional. He can really help!! Not sure if he is close to you.

Once the start problem is taken care of you may need to re-jet it. It can work on your engine, really.

Now, I will take the liberty to tell you what I would do if this were my car. Man you got all the pieces to incrementally build a Chevy stomper. At this point I would spring for minimally a set of JBA shorty headers. No clearance problems and way better flow. Those stock manifolds are going to hold you way back. If you are hard set on them, sell the motor and put in a bone stock 302. Long tubes would be better yet, but they can create some ground clearance problems. I have hooker super comps on my 67 mustang and they are very low. I have JBA shorty’s on my GT/CS (under scrutiny on my other thread for authenticity). My GT/CS has the same intake and heads you have with a crane retro hydraulic roller and a Holley street avenger 670-CFM carb. Not as fast as the other guys in the thread, but it gets it done.

Next I would get a Mallory or MSD distributor with adjustable advance curves. A compatible coil or even one of those DUI distributors with the coil built in. Now you can widen the spark plug gap, but don’t go crazy, 040 to .042. Even though you have the coil power most of the guys I work with don’t go anywhere near .050 or above. That 68 distributor is the smog one. Centrifugal advance would need “recurving” to support what you are doing.

I would buy the vacuum secondary 670 Street Avenger for it with an electric choke. Mine is a bit fat on my 302, but it would be perfect on yours.

You have spent a big chunk on this 347 and you need to fit it with the final pieces. Or change the cam and put on stock heads and intakes and put the money elsewhere.

If you make the above changes and later add an AOD and some 3:50 gears, look out. Better have a posi and ask David or Joe about drag radials!!! You will need them!!

Rob
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