buZy Posted October 27, 2004 Share Posted October 27, 2004 My sbc 327 cuts out under loaded power. When I go into the secondaries the engine starts to stubble around 4000 rpms plus. Other than that starts and runs reliable. So far I checked the compression and float bowl levels changed the spark plugs cap and rotor but that didnt seem to help too much. Any thoughts on what I should check next would be a great help. Thanks! Quote Link to comment Share on other sites More sharing options...
Guest 305240 Posted October 27, 2004 Share Posted October 27, 2004 I assume you are running a carb. If so, what size? What are the jet #'s? What year car? What size is the fuel line? Sounds like it could be starving for gas, or it may have a coil breakdown. Point system or HEI? We need some more info to help us get pointed in the right direction. Quote Link to comment Share on other sites More sharing options...
strotter Posted October 27, 2004 Share Posted October 27, 2004 Also, is this a new motor? Or is the problem something recent? Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted October 27, 2004 Share Posted October 27, 2004 At first glance it sounds like your carb is oversized. Get a vacuum guage on it and then if the vacuum approaches 0 and the gas pedal is not at the floor, you're oversized. IF you have 3+psi of vacuum and you're floored, this is not the issue. Even if you aren't sized correctly, it will still be fine, just don't press the pedal further than your vacuum can allow. A vacuum guage could help you to deturmine this. Otherwise it could be insufficient pump flow/pressure, clogged fuel filter, clogged air filter. Points could also cause this at high RPM. Go electronic for the ignition. It could also be a valvetrain issue. Quote Link to comment Share on other sites More sharing options...
Pyro Posted October 27, 2004 Share Posted October 27, 2004 Is the vacuum advance on the right port? It makes a big difference! I have found a lot of bad running chevys with the dist. vac advance on the wrong port. Use the port on the carb that DOES NOT have vacuum at idle. Quote Link to comment Share on other sites More sharing options...
buZy Posted October 28, 2004 Author Share Posted October 28, 2004 This is not a new motor. The carb I am running is a Holly 4150 650cfm. I dont know the jet size yet. The guy I bought the car from a guy who didnt know much and just installed it new. The car is a 71 240z. All New Fuel lines size is 3/8 with AN #8 fittings typical. The pump new . Malloy 110 comp, New filter high flow filter which exceeds the pump flow. Mallory unilite dist with vaccum advance. MSD 6 series ignition. The vacuum tap is a drilled fitting in the intake below the carb. Performer rpm intake and maybe cam. Engine runs out good to 6500 max. Do I try the vacuum reading while driving or in static neutral? I hope not it's valves or a broken spring, engine seems quite basically, No real loud noises. Im trying the easy stuff first before pulling valve covers and or wasting money etc. I'm hoping its carb or dist. I know I can take the dist to a speed shop and have it tested but if thats fine how do I ballpark my jetting. Does anyone know the basic jet size for that carb and engine? Quote Link to comment Share on other sites More sharing options...
Guest 305240 Posted October 28, 2004 Share Posted October 28, 2004 Sounds like you might be a tad over carberated to me. A 600 would be fine. I'm running a 302 and the 600 I have is actually a little too big. Put a timing light on it and check it with the vacume plugged. Check your timing at idle. then, put the vacume line back on the dizzy. see how much the timing will advance. As said above, you don't want any vacume pull at idle. Check the port you have the line attached to and see how much you are pulling. Hopefully it isn't pulling anything, but I think it is. The port you need is on the carb it's self. Quote Link to comment Share on other sites More sharing options...
Pyro Posted October 28, 2004 Share Posted October 28, 2004 It sounds like you have a pretty nice set up. I think the 650 carb is a good size for a 327. Chevy put 750cfm Q-jets on them from the factory. However, are you sure it is a 650? I don't recall a 4150 offered in a 650. I think it need to be a 600 or 750. It could also be carb power valve issue. What manifold vacuum are you running at idle? Have you checked float levels?? I ending up jetting my 4150 750 cfm carb on a 350 with a rpm intake to 69's up front and 76's in the back. And I used a 6.5 power valve because my engine only make 12 inches of vacuum at idle. FYI, the power valve number used should be 1/2 the idle vacuum. It sounds like your dist. vacuum advance is on the wrong vacuum source. The manifold is not the place to get that. It would be better to not even use the vacuum advance then to use it wrong. Try setting your initial timing up to 20 degrees and disconnecting the vacuum advance to see how it runs. Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted October 28, 2004 Share Posted October 28, 2004 To see if your just overcarbed, put that vac gage on the tapped fitting on the intake manifold and watch it when you have the problem at full load not in neutral, on the road. IF it goes below 2psi or so and you're still not even close to pedle down, then you just need to watch your pedal. Carbs are pretty good at regulating themselves unless it's way overdone. You can get a formula for how much CFM you need for a given cu.in. Check the web. Everything else sounds good, if it's not fuel starvation, check your entire ingition system from coil to plug before you rip the engine apart. Under load only above 4000? My suburban did that and I assumed it was an oversized aftermarket 4 bbl, but it did it at any speed when I opened it up too far. Will it do this at lower RPMs? If not then it's probably not carb size, it should be worse at lower RPMs. If it happens every time you open secondaries no matter what, check the setup of your barb, get a book etc. Quote Link to comment Share on other sites More sharing options...
GAZRNR Posted October 28, 2004 Share Posted October 28, 2004 The 650 should be fine on a 327...maybe just a tad over sized but not to much. If you are hitting a stumble when opening the secondaries and then it cleans up it could be a lean condition. I have not used a holly for a few years but there a couple things you can try to fix this: 1) adjust the acceleator pump to give a bigger squirt of gas and or jet up 2 number sizes on the secondaries. this hsould fix it. Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted October 29, 2004 Share Posted October 29, 2004 I hate to blame the carb, but if it's just added and a holley, then check it out. Holley's are hard to set up out of the box. Quote Link to comment Share on other sites More sharing options...
A. G. Olphart Posted October 30, 2004 Share Posted October 30, 2004 Like Pyro and GAZRNR say, carb size isn't the problem. So. is this an old 4777 double pumper? The early ones ran 71 primaries, 76 secondaries, later ones went to 67 and 73. Holley has a nice free booklet to download here: http://www.holley.com/tMap.html#TechServ . (You want the carburetor numerical listing). The 'list' number should be on the choke housing of your carb. If you aren't sure of your carb, clean it up and feed it a genuine Holley kit. Make certain that the jets are not partially closed down by deposits while you have it apart, and blow out all the passages. If you don't want to soak it in carb cleaner, Berryman's works well, but it contains some pretty nasty chemicals. How do you know the engine runs happily to 6500? Personal experience, or the guy who sold it to you? If not the carb or ignition, that pretty much leaves tired valve springs. Good luck... Quote Link to comment Share on other sites More sharing options...
buZy Posted November 2, 2004 Author Share Posted November 2, 2004 Thanks for all the information everyone! Well after pulling out my hair for days trying to find the problem I took the car to a friends shop. Put the engine on the ingition scope and found the ignition was all over the place. Wasn't even close. Problem found. I know now its either the dist, the msd6AL or the wiring configuration. So I download the ignition instructions from msd and going through that now. Then it goes back on the ignition scope for a final check. He said my motor is good and strong. The bore scope showed I do have domed pistons and should be running race gas. He also said due to the ignition errors it's down about 100HP! Oh my god! I thought this car was fast before! Cant wait to wake up! I'll keep you all posted! Brian Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted November 2, 2004 Share Posted November 2, 2004 It's always electrical! I should have stuck to my guns. Quote Link to comment Share on other sites More sharing options...
buZy Posted November 4, 2004 Author Share Posted November 4, 2004 Yup electrical! So I ordered up a new msd mech adv distributor #85551 blaster coil and new 8mm plug wires. Next rewire the msd 6AL igintion. With my luck that will need replacing too. My mechanic friend said the mallory unilite distributor I had doesn't work well with my msd adj spark timing control #8680. I found that kinda hard to belive, but took his word for it. The old unit was worthy of wear it appered and IF it needed repair might as well replace it with matching components. A drop in Z bucket at this point. By this time next week i will have that stuff installed and will let you all know what happed. Quote Link to comment Share on other sites More sharing options...
buZy Posted November 12, 2004 Author Share Posted November 12, 2004 Well everyone I got the motor run properly. All said and done it was my msd adjustable timing box that was the problem. After installing the new dist, coil, wires , plugs and rewiring/checking the ingition circit I had no spark. So I rewired again this time without timing box in the loop and bingo, it ran great on just the msd 6AL. It's almost too fast now!....well almost. Once again, Thanks for all your input and help. scumdog of the universe. Quote Link to comment Share on other sites More sharing options...
ezS10V8 Posted July 14, 2014 Share Posted July 14, 2014 I had almost the same problem. Engine ran fine but at around 4k it would cut out, like I was hitting a rev limiter. Mild Build 350 with Accell HEI in cap super coil. I replaced the carb replaced the fuel filter full tune up replaced the alternator replaced the battery replaced the whole fuel tank Rewired the entire truck redid the entire fuel line layout ( nice electric pump, return line, and fuel pressure regulator ) after following tons and tons of advice threads. Was getting ready to give up entirely on the motor as the internet had convinced me it was a problem in the valve train and I had another 350 that was almost completely stock in the garage. I took the tach out of it to give to my stepfather because it wouldnt rev so I didnt see a point in having one installed. Soon as I unplugged the tach the motor wouldnt even idle, had to totally re-adjust the carb, after doing so, every time the engine fired the entire truck would reverb, she has no problems hitting redline instantly now while driving down the road. just like I wanted when I did the build. It's almost impossible to keep the tires from squealing if I have the wheel turned at all now and I can use the full tach range... all because I didnt think that a silly tach wire could cause all of this headache. My symptom was that really hard acceleration would make the engine act like it was hitting the rev limiter, and if you walked the accelerator it would cut out @ about 4000rpm. I was convinced forever it was fuel delivery because the RPM cutout wasnt always exact and it was a little eratic. A few days the engine ran fine, then back to nothing. Sitting in the driveway I could rap it out with no problem. Quote Link to comment Share on other sites More sharing options...
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