For everyone who has been following this thread, yes I still have the problem. A short recap..

Engine: Freshly rebuilt 1970 402 .030 stock with Comp Cams 224x230, .510x.520, rochester qjet, stock exhaust

Problem: car will not idle unless idle screw is turned in so far as to open throttle blades. This causes fuel to be pulled through booster venturies and renders idle mixture screws useless.

Diagnosis so far: Carb had been restored by the Carb Shop. I sent it back, they ran it on their test engine, it worked fine. I then tried another Rochester Qjet supplied by the engine builder and it showed the same symptoms.

Checked for vac leaks.....none

Degreed camshaft..lift, duration, and centerline checked out ok..TDC on balancer was off 4 degrees, fixed with timing tape

Replaced wires, coil, distributor, timed at 12 degrees BTDC

Checked for something dragging the engine down. Disconnected torque convertor. No change. Disconnected water pump and alternator drive belts and idle jumped 100 RPM, not enough to allow me to back out the idle screw to solve the problem. Disconnected power steering belt, no change.

Next steps:

I noticed that the fuel pressure jumps between 5 1/2 and 6 1/2 psi on my Summit in line guage. Im wondering if the pressure could actually be spiking above that and causing the carb to flood? Ill try installing a regulator in line and dropping the pressure down to 4 psi.

Im still not sure Im using the correct carb base gasket. My assembly manual (and the Carb Shop) tell me it is a 1/4 inch thick gasket with an open center for my stock intake manifold. This is what I am using currently. Friends tell me it is a 4 hole gasket. I will try a 4 hole gasket I just picked up.

I will pull each valve cover and measure the valve lift of every intake and exhaust cylinder. Im wondering if maybe an intake or exhaust lobe(s) may have flattened out on the camshaft.

Check compression of each cylinder.

Believe it or not I have had this problem for in excess of 3 months! Im surprised I have not thrown in the towel yet. Im really getting anxious now that the car show season is fast approaching. Unfortunately Im only able to work on the car on Sundays and maybe a few hours during the week. I try to make a game plan during the week and then attack the problem on Sunday, so if anyone has any more ideas, send them my way.

Thanks, Kevin

You're having a helluva time with this one. Do you have mild cam, so I would expect it to idle no prob.

The two things I always think of when I hear of a crappy idle, I think of two things: vacuum leaks and timing.

It sounds like you've ruled out the vacuum leaks, but they are hard to find. With the trouble you're having, and the time you've spent on this problem, I would consider remounting the intake manifold with new gaskets. (The heads have not been angle milled, have they?)

Check your idle vacuum. Note the RPM. That cam should idle at 800-850 in park with lots of idle vacuum.

This is the most important recommendation I can give you: Get a performance recurve on your distributor. Have them limit your mechanical advance to about 18 deg, so you can run about 20 deg initial timing. (PLUS vacuum advance, adjustable ones are better)

I've done this will all my small blocks, and the idle is MUCH stronger. IE you can back off the idle speed screw a half a turn or more for the same RPM. This should render those idle mixture screws useful again.

Let me know how you make out!

RH

Ryan,

I am using a brand new distributor (points) type from Accel. I can advance the timing to 20 degrees initial and see if this allows me to back out the idle screw enough to cure my problem, but I didnt think that my cam was that wild to require me to have to do this. Could it be?

I did reinstall the intake manifold with new gaskets after I rechecked the degreeing of the cam. The idling problem was present before and after I reinstalled the intake manifold and did not change.

I have used 2 cans of carb cleaner to look for vac leaks and have not found any. I've heard of another method of looking for vacuum leaks that entails flowing some propane through the carb throat while the engine is running and seeing if the idle increases. Is anyone familiar with this method?

My manifold vacuum is lower than where it should be, but I attribute this to the fact that my throttle blades are open while the engine is idling. Maybe I should check the manifold vacuum with the throttle blades closed just by cranking the engine? What should I get for a reading?

Not hardly an expert in this area. But you mentioned a lower than normal vacuum. Had a problem on a 76 where I put on what I thought was the correct carb. Fought hard with it and could not get vaccum up. Plugged off everything at the carb and still had low vacuum. Turned out manifold vacuum was lowered by the PCV system. Friend of mine plugged and re-drilled the carb base and got vacuum up.

Kevin,

Try increasing your initial timing like I mentioned above. Reduce your mechanical timing to about 18 deg (it's easy with the accel dist, right?). I've performed this simple fix for a few friends, and it worked every time. Insufficient timing will cause poor idle vacuum. As you turn your distributor counter-clockwise, listen to your idle speed increase!

It's not that your cam is radical or anything, it's just that most sbc performance books recommend this timing curve for a performance engine.

I had a similar cam (228/235@0.050" hyd) in a 10:1 355 with a rochester, and it idled like a top with about 12 inHg at idle. My 260@0.050 (11.5:1) 355 had about 5.5 inHg at idle.

"TRUST ME." (famous last words)

RH

Kevin,

I just had one more idea...

I remembered the experiences with my Rochester carb. You may be overcoming your float with that much fuel pressure. I'm not sure what they can take, but I had a problem with this when I changed a fuel pump, and to adjust the float level on a rochester, you actually have to remove the top, lift out the float, and BEND the float down. If the timing doesn't work (it will!), you could consider either lower your fuel pressure, or dropping your float level.

RH

I saw the propane test on shadetree this weekend and it worked for them,have'nt tried it myself.

Kevin, give Ryan's fix a try. I think you've managed to eliminate everything else. Maybe your cam profile is dictating that you dial in more initial advance.

Rancon, I used to live near Victoria, just on the other side of Shiner in the big city of Moulton. I worked at Victoria Machine Works for a brief time and then moved on to Leonard Kovar Ford, back when it was Swain-Kovar Ford in the late seventies. Watch the Victoria Advocate for any Chevelles that come up for sale, if you don't mind. Can't access their classifieds online, and I'm always on the lookout for another bargain. Email me if you can. My email address can be found by clicking on the mail icon above any of my posts. Kevin, thanks for letting me piggyback this message on your thread. Rancon didn't leave an email address.

[This message has been edited by Randy Mosier (edited 03-06-2000).]

This weekend I bumped the timing up to 20 degrees initial. Still had fuel dribbling.

Connected vac advance to manifold vacuum. Fuel almost stopped dribbling from 1 side but still dribbled from the other.

Of course I now had 32 degrees BTDC total timing now at idle (850), which allowed me to turn out the idle screw and elimiante some of the problem.

I then checked the rocker arm movement to make sure I didnt have any flattened cam lobes. All looked well.

Tried to check compression. Did cylinder 1, 155 psi. Couldnt get new Craftsman compreesion guage out of spark plug hole! Ended up accidentally pulling hose and plug adapter apart. Will have to return tool and recheck next weekend.

Didnt get around to installing fuel pressure regulator on line and dropping pressure down to around 4 psi. Will get to this during the week.

Since I eliminated some of the fuel dribble by bumping up the timing, I am wondering if the remainder of the fuel dribble could be caused by the erratic fuel pressure, which bounces between 5 1/2 and 6 1/2 (sometimes 7) at idle as shown on the Summit in line gauge. Maybe the fuel dribble is being caused by more than one thing?

Heres a thought...Has anyone tried this cam with stock exhaust manifolds, heads, intake, and a rochester carb? Maybe this cam needs headers, a better intake, and a different carb to perform?

What exactly is your vaccum reading at idle. If it is low then it may not be able to overcome the spring that pushes the primary piston up and opens the jets to fuel enrichment when the car accelerates i.e. vaccum drops. Pull the spring and clip a few coils out of it. Did the carb shop ask what cam you were using?........Ward

Ward, the last vacuum measurement I took was at with timing at 12 BTDC with vac advance plugged. Vacuum was 13" but slowly drifted from 12 to 14 in an irregular pattern as the engine surged and stumbled slightly. I assume the vacuum would be higher if I could close off the throttle blades.

I told the Carb Shop what cam I was using. They said that they test ran my carb on their BB Chevy with a similar duration roller cam and it worked fine. I even shipped the carb back a second time and they test ran it again.

Is there a way I can test to see if the spring that you are talking about is the problem before cutting it? Does someone make replacement springs? Can I remove the spring totally and test it that way at idle?

Kevin there is a way to check and I have it in an e-mail from another list so you or any one alse who wants it can e-mail privately and i will send it..........Ward

Barry I did a copy and paste so here is the directions. Note though my engine is not done yet I am supposed to get 12"-15" of idle vaccum and it was suggested to me to use the 8-4 power piston spring part #7036019 the next lower is the 7-3 part number 7029922. If you can get springs through a GM dealer, you can pick the vacuum they will activate at.
If you can only come by generic springs, try and get the tallest ones you can, so as a minimum, you have plenty of spring to work with. The way I tell if the spring is too tight, is to have the car at idle, in gear, warmed up (someone with foot on the brake is good, too) and insert a small feeler, or something like a model paint brush handel, and insert it down through the vent. you can easily touch the top of the hangar for
the primary metering rods, and feel if the
hangar/piston assembly is lifting as a result of too much spring pressure vs. vacuum holding it down. I normally start out with the stiffest one, then cut off a coil at a time until the assembly is fully seated during the lowest vacuum ill encounter.....
which is low prm/hard braking. Then cut off 1/2 more coil, and it works fine.
yup....it is a pain in the butt, but well worth it with this setup, youll find that for a cam that draws about 16" of vacuum at idle in gear, the power piston will activate at about 12-13 or so. Hope some of this helps
give me a yell if you need any more info...........Ward

Kevin,

I had one of those Summit Fuel Gauges do the same thing to me. Always bounced around & never was accurate. I ended up hooking up another gauge & it solved that problem.

Good luck with the rest of the trouble shooting!

Jake

------------------
Black 66 SS396 www.chevelles.com/feature/september99.html (http://www.chevelles.com/feature/september99.html)
Tubby's Drive In
members.xoom.com/Pgh_Knights/ (http://members.xoom.com/Pgh_Knights/)
Pittsburgh PA

Kevin,I think I kinda had a problem like yours.My boosters were leaking fuel and the screws had no effect and to get it to idle normally i had to lower the float level tremedously til I figured out what was wrong.
I was running a holley on a performer manifold and had a severe vacuum leak.at the base.I put the edelbrock square bore adapter.It's this copper thing.You said you might not be runningthe right base gasket so I'd try that.Just my 2 cents.

Ward, you may be on to something. The symptoms that I have could definitely be related to the power piston not getting enough vacuum to pull it down. I just found a web site for Carbs Unlimited and ordered a 3 spring set.

Why do you check the power piston at idle with your foot on the brake? Is this to give you the lowest possible vacuum that your car will operate with the power system not coming into play?

What do the numbers mean, for example 8-4? Does this mean that the spring begins to extend at 8" and fully extends at 4"?

Barry you are right on both counts. There is a book called Rochester Carbs by Doug Roe that is sold by Amazon.com among others that is really good. Hope the springs help...............Ward

hi, I just went through a similar problem,I have a 402,10.1 comp. cam is 288-296 dur. 510 lift torker intake holley 750 accel dis. msd6 box hooker headers with flowmasters.for the last month it would only idle if adjust to 1500 then put in gear drop to 1000 rpm running auto with 2500 convertor.everyone told me it must be to much cam or not enuff convertor,or vacum leak because idle kept moving around.I checked everything still idle terrible then two days ago pulled distributor took it to shop put on machine to my surprise dist. base plate was moving around,this is a brand new accel out of box, so I put the old chevy single point in and now it idles great at 1000 in pk 900 in gear, so if you have another dist. check it out.I didn't think of dist. because it was new, learned a lesson again.

Kevin,

Different power valve springs will not stop your carb from dripping fuel at the boosters. This problem is caused by too much air going through the venturiis. You could leave the jets and rods out of the carb and it will idle the same. Won't drive down the road but the idle won't change at all. What intake do you have? The stock cast iron or an aftermarket?

Tom

Tom, I have the original cast iron intake with the original qjet carb.

Kevin, Does your intake have an exhaust heat passage that runs across the front of the carb mounting pad? Looks like a groove 3/8" or 7/16" wide, at the ends are holes that go down into the exhaust heat crossover passage that goes between the heads.

Also, look at the bottom of your carb near the rear bolt holes. There are several chambers there, sometimes these cavities are ported into manifold vacuum. If they are, the correct gasket will be crucial to seal the possible vacuum leaks.

You may want to inspect the manifold floor area under the carb, this is what you see if you look straight down into the manifold with the carb removed. I have seen several that had cracks in the floor allowing a vacuum leak from the exhaust heat crossover passage into the manifold. Really kills the idle.

Tom

I tried to find an old q-jet to tear down and see where the power piston gets its vacuum from. No luck, so I'll have to guess. Ward may be on to something. You've been concerned about the base gasket all along. I remember you saying something about a groove in the bottom of the carb being closed off by the gasket. Maybe this is where the piston gets its vacuum? If nothing else, try taking a small rat tail file and cutting an opening in the base gasket so that the groove is exposed to the intake plenum.
Tom may be on to something also. I've seen holes in the bottom of intake manifolds that were created by the exhaust gases in the crossover passage. If the gasket mod doesn't work, you might want to pull the intake and give it a thorough inspection, or even try a different one

[This message has been edited by Randy Mosier (edited 03-24-2000).]

I know what groove you are talking about in the intake, mine doesnt have it. I did pick up some different carb base gaskets to try this weekend and also installed a fuel pressure regulator so I can crank the pressure down to 4 psi or so.

I also picked up some power piston springs and was going to fool around with those but Im getting different opinions on whether or not this will make a difference at idle.

I have tried another QJet on the car and it did the same thing. This QJet was a race ready carb belonging to my engine builder and was known to be working properly.

Id like to hear more about this power piston spring and if it could possibly cause my fuel dribble problem at idle before I start replacing the spring.

Im also considering that maybe this cam doesnt work well with the combo I have: stock original cast intake, exhaust manifolds, and qjet. Im tossing around the idea of a cam swap to a milder grind, but I thought this was a somewhat mild grind already.

Worked on it this weekend:

With timing at 18-20 BTDC, fuel pressure at 4 psi, and a new style (4 hole) base gasket the nozzle drip stops completely from the passengers side and barely dribbles from the drivers side. The idle is fairly smooth at 800 RPM, drifting 25 RPM in each direction.

Once I increase the fuel pressure past 5 lbs the needle on the fuel pressure gauge begins to bounce erratically between 5 and 7 psi and the drip increases from the drivers side and just starts to spit fromt the passengers side nozzle.

It seems like the problem may be related to 2 different factors; a combination of fuel pressure and timing.

I am wondering why the fuel drips more from the drivers side than from the passengers side?

Im thinking that the float setting I am using, which is the stock setting, may be a little too high. Can anyone recommend a proper setting?

I dont want to have to run a fuel pressure regulator. I have a stock replacement pump from Year One (Carter I think) and a stock fuel line, Id like to keep the stock look. Shouldnt my needle and seat be able to withstand more than 5 psi of fuel pressure?

By the way, I stuck a small ruler down the front vent and rested it on the metering rod hangers to check the power piston spring. At idle there is enough vacuum to bottom it out.