I finally got the car up and running, but I need help tuning this carb, BADLY. I read a bit thru Google, but need some help and advice on where to start. Motor is a fairly stock 401, carb is an 850cfm Holley double pumper w/mechanical secondaries (4781). Jetted to stock (80/78), and the squirters are 37's. The car idles in park just fine. However, it will only idle in gear in high stall. Giving it any gas off idle results in it stumbling and sputtering over itself. Once it finally catches, it runs like a raped ape up to the rev limiter at 4500rpms (was suppose to be set at 5400, Pertronix LED is confusing uzzled: ). But, it runs a little ragged at anything below 2000rpms. I haven't driven it a whole lot, but I plan on pulling a few plugs. Also, at idle, sometimes it'll kinda jump around in rpms a bit. It'll just be idling, then it'll almost hicup, go up in rpms, and then level out. Any suggestions on where to start tuning this? Thanks for any and all help!
Power valves should be 6.5's. So, jet down what, like 2 sizes? 4 sizes? What should I do about the PV? Any other suggestions? Thanks!
Squirters are way to big also. I would start with a 31's and see if that helps. I only ran 35's in my BBB race motor.
Although there are exceptions, a double pumper isn't usually very happy on the street with a mostly stock motor and an automatic. If you have the chance, go with a vacuum secondary carb. As for your troubles, my guess is in the accelerator pump or squirters. Does it have a 50cc pump on it? If not, a 37 squirter is going to be too big. As already suggested, start reducing the squirter size until the stumble goes away. How does it start? Read this:http://www.v8buick.com/showthread.php?t=206224 It's a problem I recently encoutered. Not saying for sure that it applies to you but it might be worth taking a look at.
Yea, I figured the squirters would be to big, thought I'd give it a shot though. It has 30cc pumps, which again, I know is too small for the 37's. It starts like a champ, just has that off idle stumble. I did pull a few of the plugs. They were a little wet with gas, but they looked good. I'll change the squirters to 31's. Any suggestions as to a starting point with jets?
Start with the jets that holley reccomends.... and the power valve that stays closed at the lowest idle vacume.... plus just a little..... How wild is your cam????? if the engine is overcammed it will not idle below a certain rpm... and it is the cams fault not the carbs.... As for the accelerator pump , go with the reo kit and some normal size squirters, and a brown cam,,, that way you get a loooong lasting fuel squirt... that covers the inrush of air when you open the throttle.... remember you are not using vac secondaries and like Bill says they will cover a lot of problems.... but you dont have them so the accelerator pumps will have to cover the gap.... but at the same time , the engine can only handel so much raw fuel sqirted into the intake without flooding,,,, kind of like walking a tight rope.... the idle mixture screws figgure in on this too... warm the engine up before you start tuning... it is important....
I did start with the jets the carb originally came with (80/78), Cason said they were too big. Cam's a stock replacement from when the motor was rebuilt a number of years back. Didn't have enough coin to do a cam swap while the motor was out this last time. Right now it has pink cams, which I believe is one step below the brown. So, I'll change the squirters to 31's, probably leave the cams for now, tune the idle mixture screws, and go from there. How's that sound Doc? I know your the resident Nailhead/Holley guy, so you'll probably be getting many a PM from me. Thanks again everybody!
When you change jets, dont go more than 2 numbers up or down at a time.... and be sure to get you a little black book and write down the sizes and the change it makes in the performance...that way when you think '' I'll do thus and so''' and when you look in the book and discover that you already did that and it didnt work.... or it did work good,,, then you will be making progress....same with the squirters and the power valve....and dont over look the float settings.... i usually take the choke off , all the way plumb, and just start the car on the accellerator pump shot.....like Monk said, make a change and then read the plugs....and sniff the exhaust,,, and drive the car to ck the performance.... jet down untill you get a lean surge and then jet back up untill it runs right again...and you will be right on...... then turn your attn. to the power valve.... with a stock cam the 6.5 should be just right.... now all that said,,, holleys usually idle rich.... and there is 2 ways to go to lean them out,,,, restrict the fuel going into the idle circuit and /or open up the air bleed restriction.... but be very carefull here... go slow because you cant ''undrill'' once its is drilled....
Thanks guys! I'll be ordering a small set of jets tonight, and the squirters. Hopefully I can start playing around with it sometime this weekend!
Jets and squirters should be here Saturday, although the carb tuning may not be the source of my problems. First thing I realized, dumb mistake, was that the distributor vac advanced wasn't hooked up to the timed spark port on the carb - it was hooked up to a full manifold vac source. Oops :grin: ! After switching that around I fired her up. Let it warm up to about 150* and reved it up on just the primary's, instant vroom. Stomped it to the floor, tiny little studder, then VROOM! I thought I might have fixed the problem so I took it for a little spin. Peeled out of the driveway (hadn't been able to do that thus far, used to sputter and almost die). So I'm crusing along, and the instant the thing gets hot (roughly 200*) it starts to run like poop. I pulled over, opened the hood, and noticed the fuel pressure gauge at the carb was reading 0psi. It was reading 4.5psi in the driveway. Managed to get to the gas station (thought maybe the gauge was inaccurate), the whole ride it was running like crap, surging and just running rough. Filled it with gas and limped it home, still running like poop. So to sum it up, I think it's vapor locking. I've got braided fuel line and AN fittings from the pump to the carb. Any suggestions as to how to solve the vapor locking issue? It's not really routed around any heat sources, just close to the block.
So switching to an electric fuel pump? What should a factory style mechanical pump produce fuel pressure wise? The pumps about 7 or 8 years old. It was reading between 4 and 5psi at cold idle.
You need about 7 lb to avoid vapor lock.... the old trick was to get 7 to 9 lb. and then put a pressure regulator at the carb to cut it back to 4lb. just as it goes into the carb.....
The carb is jetted to the factory specs (80/78). And your correct on the factory PV. I just got my parts from Jegs, so I'll probably start toying with that tomorrow. Going to change the squirters to 31's, and will jet down a few sizes. I thinking it must be vapor lock. Ran like a champ when it was cold/semi warm, the second it got hot it ran like crap. Either that or I might have a possible vacuum leak? I'm going to try and re-route the fuel line from the pump to the carb, and maybe try to fab some sort or heat shield. It's NEVER vapor locked before, so it's got to be because of the new parts. The floats are set properly.
Andy,,,, also ck the coil spark after the car is warmed up.... sometimes when a coil gets hot it will fade away,,,, and the spark gets weak.....
These are the specs for my carb: 4781-5, 850, (F)80,(R)78, 6.5,6.5, 4150. The ignition is a Pertronix Ignitor III setup along with their Ignitor III coil. Both items are brand new. The shooters I ordered did have the little extensions, or tubes as I believe Holley refers to them. Thanks again for all the help, hopefully I can make some progress today.
Well, there's good news and bad news. The good news is this thing runs like a raped ape! Part of my hesistation/low RPM stuttering was due to the Pertronix setup not seeing a full 12 volts. I had screwed up when I initally installed it - idle's and run's much better now. In terms of the carb, I set the floats again (just to be sure, they were dead on) and changed the squirters to 31's. This thing is incredible! Coming from a factory intake, carb, and Dynaflow, the ported dual plane intake, 850 DP, and ST400 make this thing RUN! I don't have a posi, but it sure does like to lay one LOOONG strip of rubber. However, I've got, hopefully, the final real hurdle to get over. It likes to run hot, like 220*. Got up to 230* today at which point I pulled over. Not sure exactly what's up. It's always ran a little warm (200-210*) but never like this. And to top it all off, the second I pulled into the garage tonight, the top radiator hose exploded, getting EVERYTHING under the hood covered in coolant, not to mention my garage floor :af: . So I've got to clean up that mess (speaking of which, anybody got any tips? Coolants so damn oily and sticky, cleaning underhood going to be a nightmare!) and then deal with the overheating issue. After that the mechanical aspects should be finished for now. Just need to get some cosmetic stuff done. Thanks again for all the help guys!
Well there are 2or 3 reasons for running hot,,,, and getting hot enough to blow a hose off is not good... 1.ST,,, plugged up rad.... 2. not enough circulation of coolant thru the engine....3. loss of pressure thru leaks in the system....and not using a fan shroud or having the fan within 3/4in. of the rad in the absence of a shroud....in the days before factory fan shrouds the fan blade had to be within 1/2 to 3/4 in. of the rad... to cool ..... my 64 nailer ran at 185 yesterday ideling in traffic.... You need a 4 core/fan shroud/7 blade fan and pressure in the system to survive....
