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Old 07-11-2013, 08:07 AM
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Another Carburetor tuning thread.

Im starting a new thread cause my old one got way off topic and was hard to extract information from.

I need advice on what exactly to look at in getting my carb running correctly.The motor was built as a race motor for road racing, but since I acquired the car Im making it more of a nasty street car.

Here is my set up:
Chevy Small block 383
Pontiac Motorsports aluminum heads
Lunati Cam .627 lift 260*intake 264*exhaust @.50
308 rear gear
67 Camaro weighing about 3000lbs
MSD Race distributor locked at 34* (Idle to redline)


The current carb is a Dillman racing carburator based off a Holley design, probably in the 850cfm range.
70 front jets
74 rear
3.5 powervalves


I have done the following to try to get it to run right
68 front jets
72 rear jets
New 3.5 powervalves
6 psi of fuel pressure
Floats set
New Neddle and seats 110
Timing set to 34*
Tried to open secondaries a little but fuel seems to dribble from the rear

Here are the symptoms:

1) cant get the idle mixture screws to work, I can literally close them almost all the way (the idle pickes up then starves out)

2) Raise the idle curb screw to get it to idle to the point it is probably running off the main metering block and not the idle circut

3) at cruise under 3000 rpms it seems to be breaking up from running rich, my eyes are burning

4) periodically it will load up and stall when coming to a stop, and the raw unburnt fuel burns my eyes..On other occasion at low speeds the engine seems to hang up and keep revving..


What do I start to trouble shoot next? or is the carb and cam combo just too much for the street?

Please help me get this dialed in and where to begin fine tuning this.

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Old 07-11-2013, 08:12 AM
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first , do the primary throttle blades have a small hole drilled in them. Have you completely taken the carb apart at any time recently?Do you have another carb that you can try?Are you 100% sure you have no vacuum leaks(hard to tell with a moderate cam sometimes.
also try PMing painted
jester or fbird88
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Old 07-11-2013, 08:39 AM
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Quote:
Originally Posted by vinniekq2 View Post
first , do the primary throttle blades have a small hole drilled in them.
Yes.


Quote:
Originally Posted by vinniekq2 View Post
Have you completely taken the carb apart at any time recently?
Yes, several times.

Quote:
Originally Posted by vinniekq2 View Post
Do you have another carb that you can try?
No, unfortunately.
Quote:
Originally Posted by vinniekq2 View Post
Are you 100% sure you have no vacuum leaks(hard to tell with a moderate cam sometimes.
The best I can tell, no. There are only 2 vacuum ports on the carb, and the manifold appears to be sealed fairly well, I see no fluid leaks. But I can try spraying carb and choke cleaner around it. Would a vacuum leak be there sometimes and not be there sometimes?
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Old 07-11-2013, 11:56 AM
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Carburetor Tuning the Scientific Way
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Old 07-11-2013, 12:21 PM
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Do a simple search here, you'll find more carb tuning threads and posts than you can shake a stick at.

Also, decide which thread you are going to post in, the other thread wasn't off topic, it offered good advice right up to today's posts. These threads should probably be merged.
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Old 07-11-2013, 06:51 PM
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the blades being drilled is good info to transfer to the thread that fbird is replying.This is a race carb and maybe should be shelved for now and get a 750 minimum or 850 HP.If you are considering a camshaft change then the 750 is very street friendly.
Like I told you before,I use a 950 and its very street friendly,it replaced an 830 demon that I had originally.
Im very glad to see that you are on track now.
re the street 350? you could do that and sell the 383/jerico combo
probably build a very street friendly 550 hp 406 and tremec 5 speed with the sale of your items(probably have to add some)
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Old 07-11-2013, 08:21 PM
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Originally Posted by vinniekq2 View Post
the blades being drilled is good info to transfer to the thread that fbird is replying.This is a race carb and maybe should be shelved for now and get a 750 minimum or 850 HP.If you are considering a camshaft change then the 750 is very street friendly.
Like I told you before,I use a 950 and its very street friendly,it replaced an 830 demon that I had originally.
Im very glad to see that you are on track now.
re the street 350? you could do that and sell the 383/jerico combo
probably build a very street friendly 550 hp 406 and tremec 5 speed with the sale of your items(probably have to add some)
Actually was thinking of an LS swap. On a side note, I dropped the fuel pressure to 5.5 and put a 2.5 powervalve in it. It is a little better, but still not quite right for sure. The hotter the engine gets, the worse it runs... I did notice that..
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Old 07-11-2013, 08:30 PM
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The LS-427 is a great and fun engine,stock.The aluminum block lets you run more CR.,,,,,,570 HP is easy to get,N.A.
and bigger engine,,,lol
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Old 10-11-2013, 10:11 AM
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I have determined that over the winter I am doing a cam, carb and perhaps manifold swap to tame the motor down a bit. It is god aweful fast from 3500 up but the low end and drivability is not to my liking. Its just too much of a hassle to drive on the street at low speeds, but on the highway it sure is fun I guess I will have to sacrifice some top end power to bring the power curve where I would prefer it and improve the idle and drivability.

I also determined I will bring the motor back to the original engine builder, James Antionete from JA performance Race Engines and follow his recommendations for a cam. I realized driving this car it is a SERIOUS motor and deserves some serious respect, but it is just too much for me in this form. I will update you guys in the spring.
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Old 10-11-2013, 10:21 AM
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The aluminum block lets you run more CR.
really? how?
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Old 10-11-2013, 10:28 AM
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Actually was thinking of an LS swap. On a side note, I dropped the fuel pressure to 5.5 and put a 2.5 powervalve in it. It is a little better, but still not quite right for sure. The hotter the engine gets, the worse it runs... I did notice that..
A lot smarter people then me replying here, but I have chased this same issue on way way less motor mind you. Try lowering your floats more, every time I had the same issue and it got hot it would have the same symptoms, I eventually set the floats lower then conventional thinking, to the point where no fuel would come out, even with a good hard nudge to the car, and this actually solved my issue. For the 3 minutes it takes to try it why not right.
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Old 10-11-2013, 10:32 AM
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Did I miss the reason for the 308 gear with a huge by large cam? I get it when it stays wound up on the road course, but just about everywhere else this must suck? Sounds like lots of low speed/rpm driving which isn't going to help the motor from loading up even with locked timing....I assume that was addressed elsewhere?
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Old 10-11-2013, 11:50 AM
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I dropped the fuel pressure to 5.5
Some of you fellows still don't get it.
MORE FUEL PRESSURE WILL NOT MAKE MORE HORSEPOWER ON A CARB SYSTEM.
Try the pressure at 4 lbs and see what happens. As long as you have enough pressure to fill the bowl, that's all you need. The key is using lines of sufficient size to convey the fuel to the bowl on a continuous basis.
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Old 10-11-2013, 11:56 AM
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Originally Posted by Dajerseyrat View Post
Im starting a new thread cause my old one got way off topic and was hard to extract information from.

I need advice on what exactly to look at in getting my carb running correctly.The motor was built as a race motor for road racing, but since I acquired the car Im making it more of a nasty street car.

Here is my set up:
Chevy Small block 383
Pontiac Motorsports aluminum heads
Lunati Cam .627 lift 260*intake 264*exhaust @.50
308 rear gear
67 Camaro weighing about 3000lbs
MSD Race distributor locked at 34* (Idle to redline)


The current carb is a Dillman racing carburator based off a Holley design, probably in the 850cfm range.
70 front jets
74 rear
3.5 powervalves


I have done the following to try to get it to run right
68 front jets
72 rear jets
New 3.5 powervalves
6 psi of fuel pressure
Floats set
New Neddle and seats 110
Timing set to 34*
Tried to open secondaries a little but fuel seems to dribble from the rear

Here are the symptoms:

1) cant get the idle mixture screws to work, I can literally close them almost all the way (the idle pickes up then starves out)

2) Raise the idle curb screw to get it to idle to the point it is probably running off the main metering block and not the idle circut

3) at cruise under 3000 rpms it seems to be breaking up from running rich, my eyes are burning

4) periodically it will load up and stall when coming to a stop, and the raw unburnt fuel burns my eyes..On other occasion at low speeds the engine seems to hang up and keep revving..


What do I start to trouble shoot next? or is the carb and cam combo just too much for the street?

Please help me get this dialed in and where to begin fine tuning this.

God where to start?

Rear axle is all wrong for the cam; this has a serious race cam which pushes the rev range way up and brings a lot of tuning sensitivity with it. You need to install a ratio that will let the engine wind up on the road to something like 3000 to 4000 RPM at 60-70 mph. You’d probably see a higher top speed and certainly more acceleration off corners with stiffer gears. For long banked ovals, you can run gears like 3.08s and 3.20s with an engine that will turn 9000 RPM. But shorter tracks or road tracks need to be geared to grunt out of the corners. They can be a bigger problem especially if the road track includes a long straight as now you need higher gearing to get the speed; this introduces the transmission as a tuning tool.

Again I see this as more a cam problem not so much a carb problem also to some extent an intake manifold problem. These items are just not working together to where this isn't a good street combination or a lower RPM combination at the race track. A large carb with probably what is a large intake just doesn't have much mixture velocity till you get it up around 3000 RPM maybe more. The signal on the carb is in distress from idle to somewhere close to the torque peak with all the reverse pumping coming from the late closing intake, combined with the high lift which gives more exposure in terms of valve curtain area late into the intake closing cycle. At the other end of the cycle is overlap; with this cam there is a lot of exposure of the intake to the exhaust tract which may well be pulling too much mixture out of the cylinder before the exhaust closes, but that's different problem than the one I'm concentrating on which is reversion of mixture into the intake from the cylinder as the piston is rising on compression. This blows mixture back out the carb which is sucked back into the carb where the carb adds even more fuel. This results in an over rich condition that no amount of fiddling with jet sizes will overcome. The action of reversion can also upset the carb to where the mains dribble fuel all the time even at idle a symptom that you just can't shut off would indicate that. The problem I see is getting control of the reversion.

This is a place where you might see some improvement by adding spacers under the carb to increase plenum volume which will soften the reversion signal. Sometimes going the other way by putting a stuffing block into the plenum to reduce its volume will accelerate the mixture which will push the reversion back toward lower RPM’s. You can also play with rocker arm ratio, lower ratios like 1.5 will reduce lift, effectively reduce overlap and reversion exposure into the intake. Advancing the cam will help reduce some of the effects you're seeing as doing that will close the intake a bit earlier reducing the low RPM reversion a bit, but that will also reduce some the top end power. More advance if this isn't already locked in, max advance from the get-go may help. More compression, you've left off the details needed to compute that except for the head, to know compression the piston type, rod length, deck clearance, and head gasket would need to be known. Big cams with very late intake closing need a lot of compression to get the RPM range under the torque peak to function properly. You can play with the exhaust to reduce its tendency to put wave signals into the intake by way of the overlapped valves. H pipes and termination boxes as well as changes in collector and/or overall exhaust system pipe length and pipe diameters can affect the wave tuning to get it out of the RPM band where the engine is rich. All of this is a compromise; an engine doesn't develop best power by maximizing everything, so you have to play with compromises to get the optimum solution. This can be pretty time consuming. Just changing out parts randomly might offer a solution by accidently getting a combination that works without further effort but often this route just spends money without improving on the problems. But without standing next to this thing, reading your issues I do think this is a parts relationship and dynamics of tuning those parts issue not a jetting problem per-se.

Bogie
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Old 10-11-2013, 09:03 PM
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This was a race car and it has a race engine.It had a race clutch and has a race transmission.
By the time you change everything to street items you will be so far behind dollar wise,you could have had a faster street can with very good manners.your money?
remember the first thread,,,,
If you had sold the race transmission and race engine and dry sump....That would have paid for a brand new 5 speed and a mellow 600 plus horse power street able stroker small block or 700 plus horse power BB.

Its not too late to save some money
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