carb problems

[TexasRed]

Got a qjet issue. I cannot get the thing to idle down/smooth. It's i believe an 80's qjet. Mr Gasket 1" spacer (that looks upside down). Theres enough slop in the throttle so that's not holding it open plus trying to pull the throttle close doesn't slow the motor down any.

No appearance of vacuum leaks. I'm going to try disconnecting everything non essential to motor function off any vacuum circuit. Fast idle seems to have kicked off. Choke is fully open.

If I try to tune the mixture further out (past 4 turns), engine speed just goes way up. Gas pools in the spacer a little because I have to use it upside down or at least it looks that way. Not sure if that contributes. Thoughts?

Oh yeah: engine is a 383, vortec heads, ~ 9.2:1, 264 isky megacam (.450 lift single pattern), dual exhaust, hei

[bd]

Is this a brand new problem that began suddenly, or have you been living with it for awhile?

Your symptoms really sound like either a vacuum leak or uncontrolled airflow skirting the throttle plates or sucking around the throttle shaft, either because of baseplate wear or because the throttle plates aren't properly indexed (are rotated or not centered in the bores - throttle plates aren't circular, they're slightly oblong).  Study the .pdf diagram below then look down the throat of your carb for any similarities.  I've repaired many a Qjet that required reindexing of the throttle plates; I've rebushed many more.  The secondary plates can also be a problem, but with less frequency, because they aren't cycled as often. 

With the carb off of the manifold, backing the base idle screw off should allow the plates to close completely in the bores and seal all the way around without sticking.  If they stick even a little, or don't contact the throttle bores all the way around, they either are not indexed properly and/or the shaft/plates have too much wear. 

Probably the best starting point is to have a fire extinguisher handy and, with the nozzle tube installed so you can better direct the spray, spray around the carb base and intake manifold with some B12 to determine if there are any obvious external leaks.  Be careful - B12 is highly flamable!  Let us know what you find.

[TexasRed]

The carburetor was built by a guy that builds them and he says it runs really well on his test motor. Unfortunately he's in a different city so he's not five feet away. He's got everything too, four-gas and sun distributor machine.

It was running relatively ok before but I don't know if some of the gaskets swelled up to lower some vacuum leak or what? It still had a similar problem, I had to run ported vacuum advance and play with the mixture screws to keep the idle down. I broke the engine in with a summit racing carb and had no issues there, but I didn't use that spacer, used an aluminum one which is now gone because

I did recently have the carb off because it had to started to seep fuel out of the airhorn a little. I was running an airtex electric 110-e84070 with a holley bypass regulator but I think the regulator had gotten stuck and caused the pressure to go through un regulated. I don't really see it seeping now. The spectre gauge says 3 psi after the regulator and something like 11 psi before.

Anyway, I'll drop by advanced auto to see if I can get some B-12. I've already got a fire extinguisher, NIB.

Thanks!

[bake74]

     After checking what BD suggested.  I know you talked about the guy who built the carb, but is he willing to help you diagnose the carb over the phone if he lives too far away ?

[TexasRed]

OK, an update. I had to get a spray bottle from home depot because the b-12 I got was not aerosol. It seemed to make a difference at the front mounting when I sprayed the stuff. I'm running a 3/16ths or gasket before the spacer and then a thin gasket between spacer and carb. I've got another 3/16ths or so gasket so I used a piece of all thread to cut me new studs (the old ones weren't long enough) and remounted the carb. I checked the throttle bores, I didn't see anything wrong off hand, everything looked consistent.

The problem is better. It's pretty much right on the money with the screw not touching the throttle stop unfortunately. If I get a stronger return spring, maybe that would help? I can pull the throttle a little more by hand, but it doesn't drop that much, less than a couple hundred rpm. I'm going to drive it and see as it seems much better now. I've got the advance hooked up to manifold vacuum but the vacuum can is locked out to around 6 degrees, I'd like to add in 4 more, but I need to find where this extra air is coming from? Would my brake booster possibly be a culprit? Maybe the idle air bypass should be plugged?

Well, the guy who built the carb has helped quite a bit over the phone and internet but he built the carb a few years ago now. I know that some of you guys have run into weird problems due to our vehicles being somewhat of advanced age, and this guy builds the carbs mostly for the dirt track so all their stuff is new and not as complicated as ours.

[Fairlane514]

There is a rubber seal inside the brake booster, you could disconnect and plug the line to the booster and see if that helps. You could systematically pull each vacuum operated item one by one and maybe find the source.

When you spray the B-12 be careful not to let it get sucked into the open choke area of the carb this could give a false reading. 

[Grim 82]

Try moving the throttle shaft around, front to back and up and down without turning it, and listen for air whistling. The base housing can get slopped out and it will suck air around the throttle shaft, and can also cause the throttle shaft to bind and not return to idle position. You can drill the housing out and put new bushings in if that's the case.

[bd]

...It seemed to make a difference at the front mounting when I sprayed the stuff. I'm running a 3/16ths gasket before the spacer and then a thin gasket between spacer and carb. I've got another 3/16ths or so gasket so I used a piece of all thread to cut me new studs (the old ones weren't long enough) and remounted the carb... it seems much better now.

So, you're telling us the spacer flange isn't flat and the thicker gasket compensated?

The problem is better. It's pretty much right on the money with the screw not touching the throttle stop unfortunately. If I get a stronger return spring, maybe that would help? I can pull the throttle a little more by hand, but it doesn't drop that much, less than a couple hundred rpm.

A couple hundred RPM drop is significant.  You should reindex the throttle plates, AND while you have the carb off check that the secondary kick link that ties the primary throttle shaft to the secondary shaft (the link both opens the secondaries and ensures that the secondaries close completely - it's adjusted by bending and a PITA) isn't keeping the primary plates from closing all the way.

...I need to find where this extra air is coming from?

I agree!  Did you remove the vacuum hoses one-at-a-time like you said you would?

Would my brake booster possibly be a culprit? Maybe the idle air bypass should be plugged?

It's possible - but not likely (when boosters fail they usually cause dramatic symptoms).  You can check it, though, by removing the vacuum hose and pugging the vacuum source.

[Irish_Alley]

if it was the booster you would hear it sucking near the master cylinder and a bad booster will cause the truck to stall. i had mine fail while trying to stop for a red light. killed everything lucky i didnt have anything in front of me when it happened

[TexasRed]

The carb flange may not be perfectly flat anymore either. .032 isn't a very thick gasket. I just didn't want to have to cut new studs.

Truly pulling the throttle closed by hand probably didn't do 100 rpm worth of a difference, and it was quite a bit of force. I didn't see the plates outta alignment, but maybe. . .

Well, I didn't remove every vacuum line one by one. But I did use the ol' needle nose pair of pliers on some of them. The brake booster line had no effect. The PCV line did, oddly enough. It didn't stall out, but it seemed to have a little effect. I had already removed the line going to the HVAC black ball of doom and plugged it. I put my hand over the airhorn, and the engine sped up, so's I guess that definitely means vacuum leak. I removed one vacuum line, it was a weird one. Almost like a vacuum line/electric wire. Had stripped colours. It looked like it went to my A/C stuff or something. The engine seemed to slow down some when I pulled and plugged that one. Put my hand over the airhorn, but it still ran, almost no change. Maybe the secondaries are opened a crack? Anyway, when I get some time, I'll check a few more things.

[bd]

At the onset I overlooked the most obvious questions:  1) What is your idle RPM?  2) Do you have an automatic or manual trans?

In the meantime...

Truly pulling the throttle closed by hand probably didn't do 100 rpm worth of a difference, and it was quite a bit of force. . . .

^^^ Based on this, throttle plate registration, in itsself, probably isn't the issue.

...It didn't stall out, but [the PCV] seemed to have a little effect.

PCV is a metered air flow, so blocking it should cause an RPM drop.

I removed one vacuum line, it was a weird one. Almost like a vacuum line/electric wire. Had stripped colours. It looked like it went to my A/C stuff or something. The engine seemed to slow down some when I pulled and plugged that one.

^^^This is curious.  Can you post a pic?

I've got the advance hooked up to manifold vacuum but the vacuum can is locked out to around 6 degrees, I'd like to add in 4 more....

^^^This is also curious.  Why not use ported vacuum?  What happens to your idle RPM if you disconnect and plug the vacuum supply to your distributor advance?  And, where do you have the base timing set?  As a conservative rule you should aim for roughly 32* - 36* total advance (base + centrifugal + vacuum), depending on your build and who you talk to.

It's always possible that your idle issue is cumulative and contributed to by several minor issues, any one of which, in itsself, wouldn't be a problem.  Hopefully, we've provided some food for thought and further investigation.

The main thing in finding a solution is to not do too many things at once.  It's easy to get dispersed and start second guessing yourself.  Approach one thing at a time.  Stick with methodically checking for vacuum leaks, then timing issues, then carb problems, and so on.

I have one final question for the night.  Are you using any oil, or getting an ashy deposit buildup on any of the spark plugs?

[Irish_Alley]

your rpms should change if you unhook the brake booster

[jaredts]

Gas pools in the spacer because its upside down?  Do you have a part number for the spacer you are using?  Are you saying you can look down the throttle bores and see the edge of the spacer and gas is accummulating there?  Are you sure its a spacer and not a carb. adapter?

[454Man]

Maybe the well plugs are leaking or you blew a gasket wit the fuel pressure being high. Just tryin to throw out ideas:)

[TexasRed]

I'm trying to idle at 750 so it drops to 600 rpm in gear.

I don't think the PCV should be active at idle. Should I try one from a bigblock corvette?

Pictures of the wire:

http://i57.photobucket.com/albums/g207/hippiegohome/P4170096.jpg

I think maybe it's some kind of ESC thing??

My idle was around 750 with the idle screw backed out all the way and no vacuum advance. Ported is okay, manifold should give me better idle smoothness. My total advance (mech+ initial) is around 32 degrees. I run 12 initial. Vacuum advance is different. I'd like to get that to 10 degrees, so I'll adjust the limiter plate. If you search ignitionman on google or ixquick or something, you can find where he talks at length about ignition advances.

No real oil usage, it's still only a couple thousand miles old.

Other comments:

Gas pools in the spacer because the gasket would wick it some on the inside. part number: mr gasket 3406. Ain't no adapter. I only saw the gas when I pulled the carb. I hear that's what others' experiences are too.

IrishAlley: even if it's plugged? Part of the vacuum line is rubber and I just used a pair of pliers to close it off.