Hey fellas, so i have a quick question. Im running a weiand tunnelram with 2 600 AFBs on top of a very built 327. I just got it drivable last night, and noticed that when i reach about half throttle, before the secondarys, its almost like the carbs run out of fuel. Wicked terrible bog. Any suggestions?
Are you talking when you hit it off of an idle or a gradual acceleration to half thottle and then hitting it ? If its off an idle it because of the weak va***e signal from the tunnel ram .1200 cfm of carbs is a bit much for a street driven 327 cubic inch IMO ...
truth is - switch to holleys - in the end, you'll be better off - try jetting in both directions (rich, lean) in big steps to see what happens
off idle its fine. Is when gradually accelerating, about half to 3/4 throttle right before the secondarys, it acts like its completly out of fuel. get past that and it flys!
Maybe it is .Are the floats set high enough ? How much fuel pressure do you have at the carbs ? How do the plugs look ? Im more into Holleys myself and not so much AFB's ,but i will try to help ...
floats are set at 3/8ths from the top, Fuel pressure should be around 7 psi at the carbs, but not sure. Im going out to look at the plugs now. The distributor im using is a mallory YC 310
What you need is a pair of 450 Holleys. Imagine that I just happen to have a pair and need a pair of 600 AFBs. I'm with George on this one. Try adjusting the accelerator pumps you you get a stronger squirt.
2 600s is not too much if you have a BIG cam. I ran 2 600 DPs on a 289 SBF with big ported heads and a huge cam, it flew. With a small cam, 2 390s were too much and needed heavy secondary springs to be driveable. Sorry I know squat about AFBs, can't help there, except that it sounds like the power circuit might be leaning out at high rpm and small throttle openings. Try a heavier spring to keep it open at higher va*** levels? (is heavy right? Or lighter?)
Click on this.... http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1405_manual.pdf It might take just a sec to load, it is a PDF file. This is the operating manual for a 600 Carter/Edelbrock carb. Pretty much everything you will ever need is in there...Jets, metering rods, transition circut, everything. Once you truely wrap your head around how these are supposed to work, it's all downhill from there. Good luck, -Abone.
I respectfully disagree .There is a formula for calculating optimum CFM based on cubic inches / desired rpm range .Sorry but theres no getting around it .Yes a small motor can work with 1000-1200 cfm ,but not ideal at 5500 rpm or less which is where most street driven cars will operate 99% of the time and is where most big cams will start to come in .Not a good choice for a street car IMO .Just for comparison im running low 11.40's with a single 750 carb and a very mild 408 cubic inch motor ...
AFB/Edelbrock carbs have only one accelerating pump - its done all the squirt its going to do by the time you're at half throttle. Agree with Bigchuck - get the Strip kit that has a variety of step-up rods, jets and springs and then start experimenting. Stock carbs are set up to respond to strong vacuum signals when used in single carb applications, not in the softer signals encountered in multiple carb set-ups.
Are they vacuum or mechanical secondaries ? I agree with some other's on that the 2-600 cfms-is too much fuel for a 327 ! 2- Holley 450s will run much better and they are much easier to tune . Those AFB's aren't the easiest to tune either . Tunnel Rams always look Bad *** as Hell but aren't worth a **** on the street ! Just way too much fuel all at one time ! The tunnel ram is best suited for the drag strip ! A progressive 2 x 4 set up with spacers would work much better on the street ! I had a tunnel ram with 2 Holley 450's and ran great at the track . Wasn't worth a **** for a Saturday night cruise in town ! Just my opinion . Retro Jim
i just had this same trouble witgh my tunnel ram ..i have a vac advance on my distributor try pulling the vac hose off adjust ya timing ,i just did this and it fixed the trouble ...i thought i had the timing set perfect but it wasnt ....nbow the throttle is really snappy ..good luck
There's a formula that say you can't exceed 150 mph in a 1/4 mile from a standing start too, formulas aren't everything, and they all require ***umptions. VE comes into play, a high VE engine of the same size will require much more carburetion than the same CI engine of low VE to operate properly. Cam and heads affect VE dramatically.
I figured his 327 @ 100 % which im sure its not that high if he is driving it on the street and a 600 - 650 would be best in theory. Formulas arent an absolute proof ,but time slips are .I knocked a half a second off my car that the original builder couldnt with the same basic parts ,just by correcting his ***umptions with mathmatical facts ...
I agree 100%. If the 327 is as built as you say, go with the -660 center squirters (part #4224). To me, there is no better tunnel ram carburetor because a tunnel ram application is exactly what they were designed for. I had a pair on a very stout 327 and they kicked ***. That engine would rev like a 2-stroke. A tunnel rammed 327 with an M-22 and 4.56 gears is about as cool as you can get. When you roll into the throttle you had better be ready to grab the next gear because **** is happening at the speed of sound.
Guess again. 2650 lbs empty and I was over 200 even then, so it's closer to 3000. I drove it way more on the street than I ever ran on the track, and plenty of those miles were with a pair of 600s. I had questions no one could answer back then, had to learn it all the hard way thank you very much. Light weight, low gears and a 4 speed will help cover up improper tuning to a large degree, but it all has to be RIGHT if you want the car to perform, not just cruise around. I managed to get 18 mpg with a 328° .552 lift cam that pulled to 8500 and those 2 600s, and it drove in traffic just fine thank you except during torrential downpours when the water tried to drown the engine and I had to keep it at 2500rpm to keep it running. No Sunoco 260 for me, I ran anything on the street, race gas at the track. I had 2 390s on it with a milder 289, replacing the 2 medium riser 427 715s that came on the manifold. No one to ask, just tried it, hoping they weren't too large because conventional wisdom was that 780 was too much for a 289. Lots of difficulty. I started experimenting and got results when I started to change secondary diaphragm springs. I fastened an indicator to the secondary throttle shaft that I could see throught the windshield and went out on the local 2 lane dyno. The secondaries opened all the way, and pretty early. When it finally ran well the secondaries only opened about 1/3 of the way and that opening was maxed at about 4500 rpm, the approximate torque peak of that engine which laid over at 6500. Next engine, more power, and the same carbs. Now the vacuum secondaries opened 100% and that was at only 3500 rpm, way before the engine was "in range" of it's valve timing. That's a very graphic illustration that the same CI engine can have very different CFM demands. Pretty much a slug at the top end too. Long story short, a pair of 600s woke it up big time. Nothing wrong with a tunnel ram on the street but the combination has to be right, and the driver's expectations need to be in line. Put a tunnel ram and large carbs on a motor with too much cam in a heavy car with an automatic with a low stall convertor and it will never run right. Apply it properly, and it's just fine, weight be dammed. Ask the guy who made it work, not the guy who couldn't make it work, eh? The OP stated 'very built' which I take to mean an engine with significant RPM capability, and he's describing a problem at 3/4 throttle. I'm basing my answers on that, which indicates a desire to make it run properly at those parameters, not on making it start in sub zero weather.
Simple fix, adjust the secondaries where they come in a little sooner. The fuel to air in the tunnel ram is starving at mid range. Or you can install a return line to the tank with a manual ball valve / psi gauge to maintain fuel presure thru all ranges of throttle.
sound like to me you have a lot of fuel comming in ,,,,,, and its loading up ,,,, i'd try smaller carbs,,,
Yes, More likely to be the problem than my suggestion. I tend to have a Holley mindset, and it shows here.
Hey guys, got it sorted out. Pulled the plugs and they where white - Lean. I jetted the metering jets down when i broke in the cam thinkin i would be overfueling it. Swapped em back to stock and it cleared right up. Got too much pump shot now, and different metering rods are certainly in order because its a little rich at about 1/2 throttle, but it pulls strong to about 7200 rpm. I got a huge cam. .560 exhaust, .533 intake, 110 lda, and 260 duration at .50. I know its a bit too much carb, but i keep my foot in it quite a bit. My motor sees 5000+ quite a bit, and will pull to 7300. Kinda holding off on it to set things in for a week or 2 before i romp on it TOO hard
Glad you got it sorted out .I always let my plugs tell me what it needs also .Ignition timing, mixture ,heat range ,its all right there on the end of the plug if you know how to read them. Man ,that is a big nasty cam for a 327.Lots of duration .What kinda car is this in ?
m,ee too and I can never remember what those metering rods are called when I need to. Someone said that bog on a Tunnel ram with AFBs is common, but I may take that a step further, bog on a tunnel ram is common period. Once you get 'em sorted out they are sweet as sugar pie but sometimes it takes a lot of patience to get them sorted out. There are some things that should be mentioned, the formula for predicting CFM requirements is a starting place. If everyone is using the formula that I am thinking about it ***umes 100% volumetric efficiency which is pretty basically unachievable. Next thing to keep in mind is that it doesn't take into account port velocity or at least difference in port velocity which is altered by plenum size as well as bore and stroke stroke to rod length ratio port taper or the lack there of and manifold type. As well as many other variables that I'm sure I either haven't mentioned as well as those that I do not know of. We also have CFM ratings put out by the manufacturer as opposed to an independent lab. Tgis becomes important as there are different ways to flow a carb. For instance Holley carbs were traditionally flowed wet as Carter carbs were normally flowed dry. When a carb id flowed wet it normally shows a lower CFM numbers to that of a carb that is flowed dry. The flow numbers for the same carb can be way different to just a little different depending on the venturi design etc. Ok I've muddied up the waters enough for one day I suppose.
YOYODYNE, you nailed it. It's the right combination all the time. And weight is the key also. My truck pulls like an animal and I can still cruise the strip or down near the boardwalk with no h***les. I had one four barrel on it originally, and it ran great. I put the T/R on it and everyone who's rode in it agrees, it's just nasty! Everyone can hear that wicked cam a mile away. That's Hot Rodding!
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