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#1
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Can't get any low end power
I have 383 that was rebult last year. It has a 454 lift, 268 duration cam in it and a 670cfm Advenger carb. 3.23 gears.
It runs really good and I'm guessing it could do the 1/4 mile in about 15 seconds. (I figured that because I ran 16 seconds @ 85 mph at the track with my old carb and now it only takes 15 seconds to get to 85 mph) The problem I have is that I can't get any power off the line. I think I should be able burn rubber but it won't. Does anyone know if this is a timming issue or maybe the accelorator pump or the air/fuel screws? HELP. |
#2
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As you had the engine rebuilt, what is the static compression ratio?? Also, that carb is to small for a 383. I've never run a 383 with anything smaller than a 750 vac secondary. You should also look at upping the gears to something around a 3.91, as it will give you alot more low end than a 3.23 will.
My old '68 Charger in factory original condition (orig carb and engine needing rebuilt) can run the ¼ in 15.10 with the 3.23 peg leg. A fresh engine with the right carb should be able to make it into the high 14's (or better, with the right gears) |
#3
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I should still be able to take a good layer of rubber off those tires with my 3.23 gears. The compression is 135.
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#4
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Have you looked at the timing? what dist. are you using? When i first put my 340 in my Dart i used the factory elec. unit with an orange box. That car was a dog and would barely spin off the line. The timing curve was horrible. I switched to a MP distributor and all I can say is WOW. No hope of traction in first now. CHeck it out.
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#5
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Here's some quick tips for you,
1. Pull that stock POS distributor out and have it curved to bring in full advance 36 degrees at about 2250. With a minimum of 32-34 at 1800. 2. Get a fuel pressure gauge on it and get the pressure at the finish line. Must be 5.5-7.0 3. Put a vacumm gauge on it and see if you have any vacumm at the stripe, should be ZERO If you have vacumm you need more air (Bigger carb) 4. Do a plug check take a couple of good close up pics of the porcelin from the side and top for me. ...I laid my hands on an AMX 2 weeks ago and moved him from a best of 12.71 to 12.57......a 440 Cuda and he went from 10.05 to 9.95, a month ago I reworked a Firechicken and it went 12.70 from a best of 12.98......touched another 440 and took him from a best of 11.68 to 11.22....it's magic I just can't explain it. I leaned on a Buddy's car on Saturday and he ran a 10.68 on a 10.70 dial...ooops sorry Kyle.... You can email direct at cuda66273@hotmail.com |
#6
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Hopefully the cam is indexed correctly. If so then the ignition timing is definitely the first thing to look at. "Smaller" carbs generally give real good low-end. That 670 may be a little small for all out high RPM racing but it should give you great low end as it is. Concentrate on the timing, that will get you the most gains.
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#7
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I have a stock distributor. Does anyone know what the total timing is at stock? And what should initial timming be?
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#8
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Quote:
Also, what intake are you running? What is the stall speed of your torque converter? |
#9
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i've the same question as ehostler... are you using the stock torque converter? if so, that could possibly be killing your performance.
you also said you rebuilt your engine... did you change cranks?? (like from a cast to steel) if the weights are wrong in the converter, one thing that could happen (this happened on my 340) is the converter, if weighted wrong, could cause pre-ignition, and very bad/slow launches. just some things to chew on... good luck! BTW: i also agree, you should change your rear ratio to at LEAST 3.56's...thats what my brothers 440/727 combo has... it'll keep up with my 340/727/4.10's in the 1/4 mile, but it's got better top end speed... bad traction on launch too... |
#10
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Won't turn them over?
My first thought is the torque converter. I've seen some stock converters stall at about 1200 and that isn't good for any performance engine. The short stroke of the 383 isn't good for strong torque so it needs to rev. You need at least the Road Runner converter or better. I like the 340 converter for street, it's suppose to be the same as the Road Runner. Mine stalls at 2300 and works with street cams well.
Just thinking out loud. BTW: Mom had a 68 Plymouth Wagon, 440, auto and posi. It wouldn't spin the tires either. But it would leave 2 black marks for over 50' and fly for a heavy car. I use to love to pick on Camaros with a guy and his girl. It's great to see the look on their faces to lose to a wagon in front of their girls. |
#11
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I like the carb - should be great for the street. On the street, I wouldn't go shorter than 3.56s. I'd be looking at the ignition related things.
You should still be able to burn some rubber from a standing start by punching it. You should be able to tune it to that point. |
#12
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Re: Won't turn them over?
Quote:
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#13
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"No power off the line?"
I worked on another members Cuda last night...what a beautiful car!!!!!!but similar problems. When he backed it in I had to have him shut it off...even I was choking on the fumes....and I can breath raw Nitro! I set up the carb with my vacumm gauge and used my Snap-on dial timing light, heres what I found... Initial timing was about 20 and max was up to 44...yikes! After we got it set to a max of 35-36 the initial was at 9, this distributor has vacumm advance problems and irratic curve issues. We ended up plugging off the vacumm advance and running it straight mechanical this took away the stumble vastly improved the throttle response, took away all the raw fuel dripping from the tail pipes, eliminated engine run on and let it start with a quick bump on the starter. Although it's still not right it's ton's better, the next plan of attack will be to have the distributor curved, a Blaster coil, MSD box, remove the two? ballast resistors. Did I mention how beautiful this gun barrel grey immaculate 70 Cuda is?.......made an Old Man drool.... |
#14
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Thanks for all the replies. Does anyone think it could be the accelorator pump? Maybe to much gas off the line?
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#15
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That could hurt as well, however answers to the other questions would help.
What is the actuall measured compression ratio (9:1, 10:1, etc), not the result of a compression test. If the compression ratio is too low for the cam, you will kill the low end. Also, what intake are you running? A mis-matched cam/intake will kill performance. What is the stall speed of your torque converter? Too low of a stall speed will kill the low end. |
#16
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I don't have a clue what the compression ratio is. My guess would be 9.1 or 9.2 to 1. I'm using a mopar low rise dual plain intake manifold. The cam is close to a stock hi performance cam for a 383. 454 lift with 268 duration. The converter is stock.
I think the problem is with the Carb. I did have it set properly so that I could burn out but this wasn't consistant. One day I would burn rubber then the next day I couldn't. And I didn't change anything. I think air temp changes makes a big difference in the way the car runs. When I floor it when standing or driving at low rpms it kind of coughs or makes a loud puff noise and then the engine wants to die or does die. That is why I'm thinking it might have to do with the accelorator pump. I had no low end power with my old 600 cgm edelbrock carb and then disconnected the accelorator pump and it ran better off the line but not great. This new Holley carb does the same thing but it has the puffing/coughing problem also. |
#17
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that sounds like timing to me... the puff is more than likely a mis-fire through the carb (intake valve open when the cylinder fires). this is caused by bad timing, or so i have experienced....
just thinkin out loud... |
#18
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Bad timing will cause it to back fire!! Oh No there go your power valve, unless it has the power blow proof built in protection. You may want to go with a heavy vacum spring for the secondaries bbrl's! If the spring is to light when you hit the throttle it will stumble beacause of too weak spring! But spend the mney and get a new MP distributer kit! Well worth the money! (I think alot of it is your timing)
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#19
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The noise I'm hearing is not a back fire. As I said earlier, this problem didn't occur with my old carb. It only started to happen when I switched to my Holley carb. That is why I think its a carb problem.
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#20
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have you checked for vacuum leaks?? i agree with 6packin, play around with the vacuum springs, ya never know what could happen. it may get worse, and it may run better...who knows till you try...
good luck! |
#21
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Sounds like the power valve is blown. That happens, when the engine coughs through the carb. It could also be related to the torque converter being too low of a stall speed.
Like others have said, upgrading the ignition couldn't hurt. |
#22
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What's the power valve?
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#23
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OK here's my 2 cents worth....
1. I think your running lean when you stomp on it, check to see if your accelerator pump is working. 2. What is your initial and full advance and what RPM is the max coming in at? 3. Get a Holley book and look at the schematic, you'll see the power valve in the primary metering block, it's easy to change...about 20 minutes and a 3 for degree of difficulty. Might as well pick up a float bowl and metering block gasket....you'll need them. 4. Power valve selection...from what I've read here I would go with about a 6.5 valve, this valve will dump fuel as soon as the engine vacumm drops to 6.5 or below. If the power valve is too low like say a 4.0 the engine vacumm may not get to that point and not allow the valve to dump the fuel your engine needs which will give it a lean condition and give that smokers cough. If it's too big like say a 8.0 it may be dumping fuel too early and cause a bad stumble on the launch. Does this make sence? 5. Get a vacumm gauge and set the idle mixture screws to get the max vacumm at idle. You'll have to go over them a couple of times to get it bang on. Better yet....bring it over tonight and I'll fix it for you...I have everything in stock and I only charge 1 bottle of ET (Early Times) per tune up |
#24
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For those of you with Edelbrock or metering rod type carbs the little springs under the metering rods are like your powervalve. The lighter the spring the quicker and more the rods are lifted giving you the fuel you need when you stomp it to WOT.
The rods themselves and the the jets down in the bottom of the bowl control your A/Fuel mix. The two Idle screws in front are for idle mix only and have nothing to do with the performance after you come off the idle curcuit, which comes in at about 1200 RPM on average. If you look in the back venturis you'll see a bar with wieghts mounted on both ends, this controls the opening of your vacumm secondary's. To make them open quicker you can lighten the weights by drilling holes in them. So there's Carb 101, Carb 102 is a graduate course offered in my new book..."Mopar Performance for the Complete Dummy" available nowhere at any price. |
#25
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<<scratch this post, I just read about the backfire, and the day to day difference. I'll leave it whole for giggles >>
My old 340 has a 284/484 purpleshaft that was installed either straight up, or a small 2 degree offset cam key (setup according to an installation guide I guess). Setup this way the car was really doggy off the line, but would really come in about 5 grand. Thinking it wasn't working as well as it could, me and a couple of much more knowledgible buddies (one had an 8.90 daytona, the other a 10.30 Super Street terror) decided to degree the cam. We actually used a compression tester and a handful of cam keys to find what key created the most compression. I think we finally ended up with a 4 degree offset key in CRANK (I believe equates to a 8 degree offset cam key). This caused my 3.55 geared car to go from a granny launch to a tire burning street killer (also dropped about 6 tenths off my ET). Obviously, the best way to degree your cam is with a degree wheel. Besides the compression method above, you can also use the two craftsman socket method (search the posts here - big topic about two years ago). Since your cam is around the same size as mine, I thought maybe someone installed the cam without degreeing it, or degreed it incorrectly. Good luck! Dartman |
#26
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All good sugestions.
This question is for Dartman. If the cam isn't degreed properly why would I be able to cook my tires one day but not the next? |
#27
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First thing I would do is check for a bad sparkplug wire. If you have 135 PSI in all the cylinders I think you should be ok, unless you built a high compression engine and the cam is installed way retarded?
From my low compression 383, it liked alot of initial ignition advance (around 20 degrees), but only about 36 total. I re-curved the distribitor to limit the total advance in the plate so I could run more initial advance without having excessive total advance. The carb may also need to be tuned. If you have a local chassis dyno with a wide-band Air/Fuel meter that would be easiest to figure out if the carburator is running correctly. Last week I dynoed my car (455 HP at the rear wheels with the 451 stroker engine), and the A/F mixture was darn good, but there was a 454 Chevy and a 455 Olds that had really screwed up carbs. The 454 had a problem with the rear fuel level (either a sinking float or bad needle/seat) when he reved the engine (mechanical fuel pump) fuel started comming out the rear fuel bowl vent. The Olds has Edelbrock heads, manifole, and I think carb? The engine ran good to about 3,500 RPM the the A/F mixture was so rich the engine power just went flat. The engine will probbably make 50+ More HP when he leans out the carb. Anyhow, I digress. If the engine is tuned correctly, it just may be the engine was built with low compression pistons and the performance cam are hurting low RPM torque. The best thing to do then is get a high stall torque converter to let the engine get into it's power band. |
#28
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Hi
Power valve breakdown when missfiring! Ive read your opinions and heard it before,,,how truly is it on a scale of 10 you spoil your valve at missfiring? Could it be more or less hurt? Can you by looking at it tell if its ok or not? Could be something to look at after weeks of backfire problem initial when starting up this season,,,,,,,,, Everything seems to work well now ,,engine runs great but who knows what potential you are hiding?? Best regards Olle B |
#29
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I scratched that post about cam degreeing after I read the second page of the post (I posted it after only reading the first) where the day to day difference and backfire comments appeared.
I edited the post with << >> around my comments, and left it whole cause some would find the degree methods interesting. Good luck with your problem - It does sound like carberation or fuel delivery. |
#30
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If you have a Mity-Vac tool you may be able to test the power valve. I just got done checking mine. My Mity-Vac came with a large rubber disk will seal to the front side of the power valve. I was able to verify the diaphram was not leaking, and by slowely reducing the vacuum I could get a good indication of the vacuum when the power valve opens.
I think PowerHouse tools makes a trick adaptor for testing the power valves, but I din't want to spend the extra money on it just to check if the power valve was good. |
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