Q-jet/Vacuum Advance tuning questions
 

Good evening everyone, don't kill me here, but this is questions about the 383 Small Block Chevy that we just built and installed in our 1983 K10 4x4 Chevrolet, but as many of you know, my family and I own and build a lot of Pontiacs as well.

Everything on this 383 build is brand new, Carb to Pan, as well as the new 700R4 trans and Converter, so it has some bugs to get worked out, like most new builds often do. We just got the engine broken in, and ran great through the whole break-in process without a problem, so now today we have been trying our best to get the timing set and carb adjusted to run and idle it's best, but have ran into an issue I want to see if you Carb and Timing guru's can help us solve. We first set the timing with the Vacuum Advance unplugged, set it to 12° Initial. We then adjusted the Mixture Screws on the S.M.I.(Sean Murphy) 800 Q-Jet(#17084231) until we got the highest Idle Speed we could achieve, which ended up being right at 6 turns out from seated. We then adjusted the Curb Idle Screw to achieve an 800 RPM Idle. Here is where the problem begins .... when you plug the vacuum advance back up the the "ported" vacuum port, to the left(drivers side) of the Fuel Inlet, it throws a bunch of timing back in the engine, because, for some reason, that port seems to have vacuum on it, even at 800 RPM Idle? That of course, causes a bigger problem, because each time you place it into gear, the idle drops low enough that it loses the vacuum off the port, and drops the added timing from the vacuum can, causing it to drop so much RPM it stalls. We bought what was suppose to be a 2300 Stall 12" non-lockup converter for the 700R4 when we built it as well, but it seems to drag the engine down an awful lot when you put it into gear, like it doesn't have much slippage, which causes the problem to be even worse it seems. You can get it to run without stalling when you put it in gear, IF you leave the vacuum advance unhooked and readjust Idle RPM to 900 or so, but the engine still drops down to 600-620 RPM when it goes into gear. Can someone tell me what the cure for this problem is please? Why does that port have enough vacuum on it to pull the advance can, even down at 800 RPM? Other than this, it seems to be doing pretty well, so far atleast. Here is a little more about the combination


383 Cubic Inch

350 block
"Zero" Decked
4.030" Bore
3.750" stroke Eagle cast crank
5.700" Eagle SIR floating rods
Speed Pro Hypereutectic -12cc pistons
Total Seal Plasma Ductile rings
New Promaxx "Project X" 185cc aluminum cylinder heads
2.02/1.60 stainless valves, 115lb seat pressure/315lb Open
Felpro 1010 Head Gasket .039" thick
Quench Distance- .039"
Lunati VooDoo 10120704 Camshaft
281/289 Advertised Duration
233/241 Dur@.050"
.538"/.560" Lift(with 1.6 Rockers)
110° Lobe Separation
Camshaft Installed at 104° Intake Centerline
Compression- 10.16-1
Performer RPM Intake(Non-AirGap)
S.M.I #17084231 800cfm Q-jet calibrated for this engine
Factory GM HEI recurved by Cliff Ruggles
1 5/8" headers, dual 2 1/2" exhaust
700R4 Transmission with Transgo 2&3 kit, converted to non-lockup
Torco 12" 2300 Stall converter, non-lockup
4.10 Gears
1983 Chevrolet K10 4x4 truck

Any help would be greatly appreciated

You may need more bypass air so throttle blades come down and cover the ported port @ idle
Also, I ran that cam also in a build and it may want more idle fuel than what produces highest idle.

What are the details of the engine build, compression ratio, cam specs, heads used, etc?

Did you verify that the distributor port on that carb is actually ported vacuum? Some were not.

What distributor? What has been done to it? What VA is it using?

I would add here that we NEVER try to tune idle in and out of gear issues with a looser torque converter. That would be 100 percent a "crutch" fix for simple carb and distributor issues.....

Cliff, I posted all of the specs in my post above .... 10.16 Compression 383 with Promaxx 185cc alminum heads, Lunati Voodoo Camshaft 10120704, spces are in the above post. The distributor is a factory GM HEI unit that we had you recurve for the engine. Thanks for the info about the converter being a "crutch" fix, I know there is another solution for this.

Thanks Shaker, so basically you are saying that after coming out with the mixture screws to achieve highest idle, they may have to be adjusted out even further to increase the mixture even more, even though it might lose Idle RPM slightly? Sorry, I am not trying to question you at all, just trying to get a good understanding

....and the Idle Bypass Air, I have no idea how much this carb has, as SMI didn't give me any specifics on the carb at all, but I do know you can pull a small manifold vacuum hose off while it is idling, and it does really affect the idle RPM much at all in either direction, but would that be telling me if it needs more Idle Bypass Air or not? Does that test work the same as adding more Idle Bypass Air? Can one simply add Idle Bypass by drilling the primary blades? If so, what size hole would you recommend starting with? Of course, I wouldn't do that, unless it comes down to knowing that is for sure what it needs. Thanks for the help

I assume you are setting idle mixture with the vacuum advance connected and all unused carb ports are plugged?

I would adjust the idle mixture screws for lean-best idle with the engine fully-warm, trans in gear, wheels blocked/park brake engaged.

What did Cliff suggest for initial timing on "his" distributor? Is that why you set it at 12 degrees initial? What is total timing? How much vacuum advance is available?

I know that Cliff is not a fan of excessive initial timing as a crutch for poor fueling; but I might have expected a few more degrees of initial than 12.

Have you tried connecting the vacuum advance to manifold vacuum instead of "ported"?


You should contact SMI about that, not a bunch of internet yahoos, even if we are talented and handsome.

IF (big IF) that truly is "ported" vacuum, it shouldn't have any vacuum at idle. Thus the suggestion for bypass air instead of having the throttle open so far. But SMI should have handled that when they built the carb.

This doesn't make sense. Throttle position affects whether a ported vacuum source provides vacuum, not RPM.

The only way I can figure this is when the RPM drops, your manifold vacuum reduces below what the vacuum canister needs to begin the vacuum advance curve; and that's typically pretty low--on the order of 9-ish inches of vacuum, sometimes less.

So the bigger problem is "why does the engine have so little vacuum?"

Can you say again what happens when you unplug a vacuum source when idling?
Idle picks up a lot or no real change?
Do you have a digital tach on your timing light?
You should pull the carb off and make sure choke is fully open and no fast idle contact, make sure idle speed screw is in contact with throttle shaft and look and post a pic of how far open your blades are @ idle.
Don't drill anything at this point, gather calibration sizes like bypass air hole size in base.

When you set the Idle at 800- 850 RPM, with the vacuum advance connected, and you pull the vacuum advance hose off of the port that is normally a ported source( port that is drivers side of the fuel inlet), the engine will immediatley lose a bunch of RPM and stall, but you can put your finger over that port, and keep it from stalling, the prt definitley has vacuum on it at idle, it will suck your finger to it. If you are fast enough, when it is at low RPM after pulling the vacuum advance hose off, you can slip the vacuum advance hose back on it, and it will stay at a slow idle until the engine is once again rev'd, then it seems to gain enough vacuum back on the port, that it once again pulls the vacuum can, adding timing back, and will regain it's faster 800-850 RPM idle, because it will not drop the vacuum can up at that idle speed, until you pull the hose off again and repeat. Yes, we use a digital Dial back timing light with the tach on it. The Timing light we are using is the Innova Pro 5568 light. We are not running any MSD or CD ignition boxes to cause any interference.

Thanks Schurkey, yes, we set the timing to Cliffs recommendations, which was 10-12° Initial, and while we haven't gotten to the step to check Total Timing as of yet, Cliff said at 10-12° Initial, it should have around 30-32° Total, and he limited the vacuum can to add another 10-12° at cruise. We will have to check manifold vacuum tomorrow to see what it is.

Do two things.

Remove a small manifold vacuum hose that is NOT going to the distributor VA and see if it likes it. If the engine speeds up some, lower the idle speed back to where it was. Re-adjust the idle mixture screws and see if things work better?

I'd also try gently placing your hand over the choke housing or tipping-in the choke flap and see if that helps.

Either of those tests will tell you what it wants in terms of idle fuel, more idle bypass air, or both.

What I do know is that 383CID, 10 to 1 compression and a 233 @ .050" camshaft on a 110LSA is going to REQUIRE significant idle system modifications from the late model truck carb. Your problems are mostly related to that part of the equation and once sorted out it should work fine. Even with that said cams that big on tight LSA's make idle tuning difficult as you are finding out. Not impossible, you just have to give the engine what it wants.

I'd also add that if you set the initial timing at 10-12 degrees, idle speed at 750-800 rpm's it should be making at least 12" vacuum, 13-15" would be a lot better. If not the engine isn't liking that cam much.......

Thanks Cliff, we tried removing a small manifold vacuum line that wasn't the Vacuum Advance, but the Idle speed didn't really change much in either direction doing that test. However, we did try tipping in the choke flap at Idle, and it does pick up Idle Speed about 70-80 RPM, but adjusting the Idle Mixture screws out to provide more fuel mixture doesn't seem to have the same effect. We were out 6 rounds with the mixture screws, and it seems to idle best there(highest RPM), but the mixture screw feel awful loose(no spring tension) if you try to back them out any more than that. We also have your Q-Jet book, and did the "Tip In procedure" that you explain up at 2000-2200 RPM, and tipping the choke flap in raises Idle Speed aroung 70 RPM doing that test as well, so I am assuming the APT is adjusted correctly for a baseline ....

Do we know the sizes of idle tubes, upper idle air bleeds, downchannels, lower idle air bleeds, mixture needle holes and idle bypass holes?

OK, progress.

Tipping in the choke flap raised RPM so we know it wants and needs more idle fuel.

Not sure if you are willing to go into a "custom" built carb? I'll be kind here and not roll anyone under the bus but have seen enough carbs "set-up" for engines like you have that aren't making the grade to know what has to happen next.

There is still more to do here before all that anyhow.

We haven't taken the additional timing away the VA is adding. My guess is that your engine combo is going to be one of those that requires a LOT of initial timing AND a lot of idle fuel to be happy.

The tip-in at idle test already tells and no response from the mixture screws backing them WAY out tells you it needs more fuel at idle. This is a common problem using relatively "big" cams with tight LSA on of these engines. Might want to discuss the issue with the carb "builder" before modifying it. That's the best way I know of to void any warranties and to have them evade any responsibility for coming up short on getting it set-up right.

It's no secret that I am NOT a big advocate of running the initial timing clear off the scale to "crutch" a carb that isn't making the grade in the idle fuel delivery department for what it's being used on. You'll read all about buying a "special" vacuum can to put the VA timing ALL in well below your engines vacuum value at idle speed. This basically makes up for not having the carb right in the idle fuel delivery department. Here I go after the carb first, then timing, but in most cases I don't have to run butt-loads of timing to make an engine happy even when it's got a pretty "hefty" cam in it.

Next time it's up and running do a vacuum test at idle speed and during the tip-in at idle test and report back. That usually tells me how successful idle fuel system mods are going to be and if I'm going to have to add more initial timing as well........

Too bad you aren't closer I could fix all this in less time than it took me to type my responses to your issues.......

PS: I need to apologize here for missing all the engine specs before my first response. I had just read Dragncar's response on the WAY TOO LONG RUNNING thread on the RAIV cam in the 428 engine with a FUBAR'd shortblock that nobody is doing anything about and isn't about to get repaired correctly in the next couple hundred years or so.

I was literally LMAO and spewing my Bourban and coke all over my lap seconds before I saw your thread and slipped right past the engine spec stuff!....

I don't know the specifics on those specs, but I just sent Sean at SMI an email asking for them. Hopefully he will respond with the details, and I will post them here.

Thanks for all of the good info Cliff, we will do some more testing soon. I also forgot to mention, that the Performer RPM intake we are using is the Edelbrock 7101, which is a squarebore style, so we are using the Mr Gasket #1932 squarebore to spreadbore spacer to mount the Q-Jet on it. We did this because we already had the intake on hand, but other people I have read using them says they can cause issues some times, not sealing the passages of the baseplate of the carburetor correctly, depending on the year of Q-Jet being used. Others say they are extremely restrictive, and you don't really notice how restrictive the spacer is, until you look at it from the bottom side, with the Q-Jet set on it. I have seen others say it can cause issue, because it is an open spacer, with no divider, like a normal Q-Jet intake has, so you lose the advantage of the dual plane intake. I am sure that it probably isn't all of our problem, but may contributes... maybe we should start looking for an Edelbrock 7104, which is the same intake manifold, but the Spreadbore version. What are your thoughts?

We went ahead and grabbed a new Edelbrock 7104 Performer RPM intake to put on it, that is made for the spreadbore Q-Jet. We found a good deal on a new one, so we went ahead grabbed it. I would still like to hear your thoughts on the carb adapter

Yes sir! 🙂

No one is concerned that idle mixture screws are a full six turns out and the engine still runs better with the choke flap closed some?

The later carbs have a very fine metric thread pitch so it's apprx 2 to 1 compared to 10-32 threads. The holes under the idle mixture screw are also tiny, and unless his builder opened them up some six turns out isn't alarming.

I've done a LOT of testing with intakes, spacers, etc.

I would NOT use any type of "open" spacer or adapter on a dual plane intake. If you do take the time to "slot" the spacer or adapter and install a full divider. I make them here from a piece of 1/8" thick aluminum flat bar.

It's OK to allow one side to see the other slightly, and a "notch" between the secondaries is beneficial, but if you allow very much it can and will create all sorts of running issues including "goofy" idle quality. Even worse we've ran into secondary transition and tuning issues as well.

In contrast a single plane intake pretty much requires at least a fully open 1" spacer especially with a spread bore carb on it. Having the throttle plates too deep or uneven depth in the plenum areas can cost a LOT of power and vehicle performance. In ALL cases on engine of any power level we gained considerable power on the dyno adding a 1" open spacer nicely blended into the plenum area.

Spacers on dual plane intakes not so much and I don't consider them worth the time and/funds unless you have adepuate hood clearance.

I'll also add here that when you move the carb UP, and have to move the air cleaner lid down you MUST have a minimum of 3" clearance to the top of the carb where the air cleaner gasket sit. Any less has always lost power, and even cause transition issues going quckly to full throttle that would not tune out........