Hey guys, I've got some tuning issues/questions/frustrations for ya..... I have an OT SBC in my 62 Biscayne that I'm trying to get dialed in. Im having some issues with idle quality and power, and after doing some routine tests I have some questions. Here are the quick facts: Engine is a 79 350 out of a truck. 4 bolt main. Date stamp says 1980 but it has a drivers side dipstick I didn't build the engine, so I am not positive of cam specs. I have a edelbrock rpm air gap on it, and just swapped a 600cfm edelbrock carb (1406) for a 750cfm quick fuel slayer. It's got HEI. Fuel pressure is just under 6.5psi. Float levels are right in the middle of the sight glass at idle. In the process of putting on the new carb, the engine ran a bit rough and not all that well. I did a vacuum test and it came out really low. 8 Hg at 900 rpm. I did a compression test today and it's good. Max 175psi and min 168 psi. I noticed that when I removed the vacuum gauge the idle speed increased, the dropped down when I put the vacuum cap back on the carb port. I loosened and re-tightened the carb in a cross cross pattern. I have not yet touched the air/fuel mixture, but understand I need to use the vacuum gauge to do this. I want to redo the timing as well and make sure it's good. My question is this: what do I do first? I have had trouble in the past with the engine running really sluggish at 12 degrees and 800 rpm at idle. I had to up the idle screw to 1000 to get it to smooth out. Do I set the timing and idle speed first and then adjust air/fuel? Is 1000 at idle too much anyway? Will backing out the AF screws increase the vacuum THAT much? The engine never really ran that well to begin with, so I don't have good data to go off of. It's had an issue with gas in the oil since its been running, and I thought this was do due the old carb leaking raw gas into the cylinders from an overly rich condition. Sorry for the rambling. Anyone have any suggestions on where to start?! Thanks, Mark
I would lower the float level to just seeing gas in the sight glass and fuel pressure down to 6. What was the fuel pressure with the old carb ? Can you tell how much advance is built into the distributor ?
Sorry, my post was confusing. I replaced the edelbrock with the quick fuel. I ran 6.5 with the edelbrock too. I'm hoping that the gas in the oil was the result of too much fuel pressure. I need to get it running right and I'll do an oil change and monitor. Based on quick fuel's literature, the 6.5 is what they recommend. Edit: I ran the old carb without a fuel pressure regulator at first, based on the manufacturer saying the pump would provide 6psi from the factory without regulation. I then read a bunch of stuff saying that they actually produce more, so I put a regulator on.
The distributor is an MSD street fire. The mechanical advance is 22 degrees at 4000 rpm. I have the vacuum advance plugged in but can't remember what I set it to. The stop plate is not installed in it though.
Thanks! It's a mechanical pump 110 gph. I have it feeding a basic fuel pressure regulator. (Not bypass)
If the mechanical advance is only 22 degrees with no vacuum advance hooked up, you are 12-14 degrees retarded. 34-36 degrees mechanical is a typical setting for a small block chev. At 22 degrees a small block will usually be slow and run poorly and tend to run hot. Hopefully a simple adjustment on this will perk it up big time.
I do have the vacuum advance plugged into the timed port on the carb. When I set the timing before I believe I set it to 36 degrees once it stopped advancing (not sure of the rpm). It was hard because I did it solo, so I had to adjust the distributor then work the throttle to see where it ended up. I don't even remember if I plugged the vacuum hose back in before doing the timing advance setting. It sounds stupid just typing these words. I need to re-do it in the morning.
too bad that you put on a new, bigger carb before you figured out what the problem was. you do not need a 750. do not need a high performance fuel pump either. you can speed up carb to adjust timing by using the idle speed screw. do a leak test around intake manifold to try to find out if there is a vacuum leak. how do spark plugs look? check timing to set to factory specs now ( online books say 8 degrees before top dead center using factory motor timing marks as a starting point.). if you do not have one, get a "bent" box end/socket wrench distributor tool so you can tighten down distrib while are looking at timing marks with light. you can get pretty close on timing, then play with fuel mixture, according to instructions that came with carb.
Jalopy Joker is right on. 750 cfm is a ton of carburetor. I also believe you may have a vacuum leak. Air cleaner off, car at idle, any open vacuum line plugged... place a hand over the air horn, like you're trying to snuff the motor out. If the idle picks up and the engine smooths out, you have a vacuum leak.
What would a more reasonable CFM be for thus engine? I heard somewhere that a good rule of thumb is to double the displacement (I realize that would be 700 CFM in my case).
I actually kind of figured that out when you said site glass for float level, edbroke carbs don't have a sight glass. You should handle 6.5 psi fine on a Holley type of a carb. You could lower it a bit since you are running a holley Wannabe but you should handle that just fine. Your '79 350 is not necessarily an OT motor, it is just a stroked and detuned 327. You can certainly consider it OT if you like, I am running an '80 block and unless I tell these experts they have not idea what it is. To the masses an SBC is an SBC. Now onto your problem. I had a girl friend once that was a real piece of work, I thought that she would settle out of I gave her one of everything that a "woman really wanted." Turned out all she really wanted was a little attention. To start with stop throwing money at it until you figure out what is wrong with it. Probably all it really wants is a little attention and even if I am mistaken you are going to have to give the bitch a little attention to find out what is wrong. Get your ignition right before you even begin looking at the carb, lets consider that you are running a close to stock motor because we do not know that it is not. Set your initial timing to about 12 and move on with it. make sure that you got good plug wires and plugs. Set your plug gap correctly and make sure that they are clean, not gas or oil fouled. What about the distributer, have you looked inside is anything loose? no worn bushing no sticking advance weights? On most engines you will have to run a lot of lead when the lower bushing is gone or well on its way out, that is until it finally trashes the distributer gear. That is up to par? Now run your valves, hydraulics want to be no lash and 1/4 turn, up to 1/2 turn preload. if you are running solids you are pretty much screwed until you find out what you got for a cam so if that is the case you are going to have to pull the timing cover and if it is not named on the snout you are going to have to pull the cam to find out what you got. But lets make the assumption that it is hydraulic until we know otherwise. Ok you are getting close, light it off now that everything else is proper. Make sure that your float levels are good and then start fooling with your idle speed. if the engine is anywhere close to stock it will want to run about 600-700 RPM with the automatic in gear or 600-700 rpm at idle in neutral if you are running a standard. Lots of guys like to run them real slow, those are the guys that like to replace inserts. You want to get to a point where your butterflies are pretty near closed, otherwise you don't have an idle circuit. Work back and forth on your trim screws, a little here and a little there, your idle should come up as you do this and you may have to keep closing your throttle blades a little at a time. Keep an eye on your vac gauge, you are done when it is pulling the max vacuum at idle as it can. 14-18" is going to be good on a stock 350. After you have done all that come back and let us know where you are at. Then we can tell you new things to look for if necessary. By the way nice looking old Chebby.
Awesome, thank you for the guidance. As luck would have it, on day one on vacation it's sleeting here in Chicago. When I get a break in the weather I'll push it out of the car hole and get to work.
Good advice from Beaner, like always. If I could offer an alternative method of adjusting the idle mixture screws. On adjusting the air/fuel mixture I prefer to use a tach. Start by setting your idle speed to the correct speed, then adjust one side at a time, screw it in/out until you reach the highest rpm reading, then readjust the idle screw to bring it back down to the proper idle speed; then adjust the other side the same way, and then readjust the idle speed. Then do it again, both sides. And take your time, make small adjustments and allow the engine to respond and stabilize. You might even do it a third time, just to make sure. Then after you get the highest idle speed you can achieve with the air screws, and have the idle speed set properly, now go back to the first side and slowly turn the screw clockwise until the idle speed drops 20 rpms. Then adjust the idle speed back up to the proper speed, and then do the same thing on the other side. That's it, you're done. Just remember, you start with the idle speed set correctly, you return to that step in between air screw adjustments, and you finish off with it. But I think the guys that said you probably have a vacuum leak are on the right track. All of Beaner's suggestions are good, but until you find, correct that vacuum leak you're just spinning your tires, cause you're gonna have to go through it all again once you finally do get there. Whether 750cfm is too much for your engine or not is a separate issue, that's not what's causing the low vacuum reading. Your engine may or may not run better/faster with a smaller carb, but at this point that seems to be a different issue. First you need to get it running right with what you've got. Just my opinion, hope it helps.
The Beaner is right on, All I would add is to pull Number one plug and check if the balancer is at the correct place for top dead center . You may have a balancer that is + or - several degrees. Good luck.
Does the car sound like it has a big cam, a lopey idle ? It could be a big cam that is causing the low vacuum and rough idle under a thousand rpm. I also believe your gas in the oil was from to high of a fuel pressure with the Edelbrock carb. Anything much more than 5 lbs tends to let them leak fuel and also creates a hard starting issue. You said that the rpm's increased when removing the vacuum guage, that is because you created a vacuum leak by uncapping the port.
In 1973 the chevy 3/4 ton trucks with the 350 engine came from the factory with a 850 CFM Q jet carb. the 1956 265 engines came from the factory with two 380 CFM carter carbs that's 760 CFM on a 265. A 750 on a healthy 350 isn't too big. To do a proper tune up. You first check compression. You check every spark plug wire with a ohm meter. you grab the front pulley and wiggle it back and forth to determine how much slack the timing chain has. If you have low or erratic compression or the timing chain is loose you cannott tune the engine. Those problems have to be corrected first. Ok if you converted to electronic ignition you need to get rid of the ballast resistor or the resistance wire. Most carb problems are ignition related .
If you have a flat exhaust cam lobe you cant tune the engine and you will have gas in the oil. No matter how big the CFM the carb is. If the needle valve is closing and seating it will not put gas in the oil. to big of primary jets will cause black exhaust smoke on acceleration. pull the valve covers and make sure all the pushrods are in place and all the valves open and close properly.
Use your vacuum gauge for everything here, setting ignition timing and idle mixture settings. 8" is way too low for a stock cam. Everything sort of effects the next adjustment, but get it warmed up and lean out the carb as described previously. Make sure pump pressure and volume is correct, float height is correct before proceeding to carb adjustments or it will never run right. Adjusting the engine timing changes the "draw" so always finish with a touch up of idle mixture if timing is adjusted much. Advance distributor to highest steady vacuum indication. If you increase too far, the motor RPM will start to "wander". Back off about 1" from the highest vacuum indication and tighten down the distributor hold down. Typically around 18" for us flatlanders and a stock cam, Then go back with the carb idle mixture adjustment, tuning for max vacuum achieved. A healthy, stock V8 at sea level will pull a rock steady 19" or 20" of vacuum and should purr - one potato two potato, go for a beer.
X2. I would verify what what the actual TDC is vs. what the balancer indicates. The balancers tend to slip on the band and not be accurate
I'm building a 355 Chevy right now and I had the builder degree the cam. We brought up #1 and verified TDC with a dial indicator. The damper is new. The indicator was installed and we lined up the marks. So confidence is good that we'll get accurate timing out of it. I like to use a vacuum gauge when setting the idle screws, but with as big of a cam I'm running, it may come down to listening to the exhaust and going with that.
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