defiant1

Ok have a 4BBL Q-jet installed. When motor is running (timing set at 12* BTDC @ 1100 rpm) it seems to be having a vacuum leak at the carb. When I turn in the idle mixture screws all the way in nothing happens (no change in rpm, etc.)

Someone mentioned that I might have a leak in between the top of the 1" phenolic spacer and the bottom of the carb. I touched that gasket with the end of my finger and it is saturated in gas. Do I need a thicker gasket in this area?

I don't want to spray carb cleaner everywhere to check for leaks because I just painted the intake an valve covers and don't want to ruin my hard work. Is there another way to easily find leaks at the carb?

I am in the process of getting a vacuum gauge to help diagnose problem as well.

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oldcutlass

I don't remember if yours was a pretty stock motor or not. I believe your timing should be set at around 7-800 rpm.

@1100 rpm you may be opening the throttle plates therefore above the idle circuit. Lower your idle down to 7-800 and try again.

The wet gasket has me puzzled a bit? Is there a noticeable fuel leak on the inside or outside of the carb?

defiant1

Ok, I will look at the timing again.

The leak does not exit outside the carb. Just saturates the gasket.
When I put my hand over the choke area to see if the rpm changes I notice the gas kind of splash up and hit my hand. Of course this was right before it was starting to die.

I will keep trying to find the vacuum leak.

defiant1

Got a vacuum gauge hooked up.

Timing: 10* at 1100 rpm (per manual-4BBL AT w/AC)
750 rpm - 10.5" of vacuum (manifold) Idles rough
2000 rpm - 18" of vacuum (manifold)

Reinstalled thicker base gasket on carb.

Idle mixture screws when turned in actually start to affect rpm, whereas before they did not. Adjusting idle screws did not improve rough idle-turned them in until idle was worse and backed the off. Backing off the mixtures screw did not improve idle, it still returned to the original rough idle condition.

Not sure what to do with the above info. I do not know what the Hg measurements correlate to.

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svnt442

Do you still have the points dist? If you have an HEI then you need to bump the base timing to 20°.

TripDeuces

An HEI has 20 degrees built in. If you bump initial to 20 degrees also now you have 40 degrees.

defiant1

I have the Petronix III unit with flamethrower coil. I ran this setup with my former 2BBL/intake (same motor w/o head work) at 10* BTDC at 1100 rpm (dist. vacuum capped off).

I just finished my compression test, ran motor to operating temp and took readings:

#1 - 159
#3 - 155
#5 - 155
#7 - 162
#2 - 157
#4 - 159
#6 - 163
#8 - 160

I also checked the plugs and the #1 plug was the worst looking-had a black coat on the ceramic. The remaining plugs had a light to dark brown ceramic coating.

When motor was running I got brave and squirted starting fluid around base of carb. No noticeable difference in rpm occurred.

The local mechanic thought the vacuum was pulling from w/in the carb itself. If that is the case I have no idea what circuit inside the carb controls that. BTW I had every vacuum port plugged with the exception of the PVC and the power brake hose which I used to hook up my gauge.

I noticed this time around my idle Hg reading at 750 was a steady 11".

I thought I had fixed my idle mixture screw problem, but I screwed them in all the way and the idle did not change (well maybe 50 rpm--but it should have made a noticeable difference).

New items:
4BBL Q-jet
Cam: custom grind 217/224 @.050
Edelbrock Performer intake

I am open to suggestions.

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Warhead

I think you're close, the car is going to have a little bit choppy idle, but not that much. You should have more vacuum, but I think the idle mixture may be lean.
Instead of long distance diagnosis, give a call to Mark Smith.
http://www.mjproformance.com/contact.php

You will be able to do more in 1 phone call than 2 weeks here, and he is a master of Q-jets.

Jim

defiant1

Thanks for the contact info, Jim. This is getting a little frustrating. I just came inside from checking the power piston spring.

I took a skinny screwdriver and placed it in the vent tube to ensure the power piston goes up and down (motor off).

With motor on, I did the same thing and the power piston is fully seated. So the power piston spring I am using is not affecting the idle circuit from what I can tell.

Adjusting the idle mixture screws in or out does little to nothing to the RPM.

I am out of ideas.

- 10.5" Hg at 750 rpm - not sure if that is good or bad
- Idle does not improve when I cup my hand over primaries
- Sprayed starting fluid around carb and rpm did not increase

What else can keep the idle mixture screws from doing their job???

Oh well, I will give Mark Smith a call tomorrow and see if I cannot make heads or tails of this.

Thanks again

Shaun

Warhead

I'm not over the fender, but here is my line of thinking:
The power piston may not stay fully seated, with primaries cracked.
Primaries cracked because it won't run with plates closed.
You may need a different primary piston spring, or to drill the plates.
Talk it over with Mark.
Jim

defiant1

Jim, I did not get a chance to call Mark today (got busy at work).

Here is what I did when I got home:

I retorqued my intake manifold.

I plugged off both valve cover grommets and left the hose for the PCV attached to carb and left the other end open.

I started motor and it was pulling a steady 11" vacuum with hose open. When I put my finger over the hose and plugged it the vacuum jumped to a steady 15". Not sure what this exactly means but just an observation.

Ok hooked up everything else back up and ran motor. Set the timing at 14* at 850 rpm. The vacuum at this setting is a steady 12.5". I played with the timing up and down for awhile and this seemed to be the happy medium. The motor has an occasional slight even shudder at times but it was not abrupt. Although a shudder to me still screams vacuum leak...so still not sure.

It would run rougher when I turned in the idle mixture screw all the way in, but still would not kill the motor or make a substantial decrease in rpm.

It dieseled a little when I shut it down. But I guess this could be attributed to making multiple adjustments to the timing.

I might be gaining, but still not completely confident I got the idle figured out.

On another note: Does it matter what port on the carb (Chevy Q-jet) I hook the PCV hose to? I am using the one in the front on the base plate, it that right? I plugged the rear port on back of carb and am using the intake manifold for the power booster.

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MDchanic

That PCV hose should NOT be a vacuum leak at idle. You should not notice a difference in vacuum with it connected or plugged. Now, if you've got just the hose, with no PCV valve on it, then, yes, of course it will bleed off vacuum (ie: allow extra air in without any fuel), but if you put the PCV valve in the end of that hose, and you pinch the hose with ViseGrips to close it off, the vacuum should not change noticeably. If it does, you've got a bad PCV valve. Traditionally, the way of telling whether the PCV valve works is to see if it rattles - if it does, then it's good, but if it's acting like a vacuum leak, even if it does rattle, then it's bad.

- Eric

Warhead

X2 what Eric said. Inexpensive part, too.
You should be at 15" with a good one. Your explanation sure makes me think it's bad.
You have it hooked up correctly.
Readjust everything (again) once the new one is in.
Check timing with the vacuum advance disconnected.
Keep us posted.
Jim

defiant1

Ok - did the PCV test. Left PCV valve in cover -- pulled 12". Pinched hose and it dropped 1 full inch to 11".

Did the same thing with PCV valve out of cover and the same results. Is 1" too much, I assume it is based on your explanation above. BTW it does rattle, that is why I did not think it was bad initially.

I will get a new one as they are not expensive and go from there.

Thank you gentlemen.

MDchanic

The vacuum dropped when you pinched the hose?

Your car is possessed. You need an exorcist, not a mechanic.

- Eric

defiant1

Nice, I will make sure I put that in the auction ad when I sell it on eBay. Got to be worth a few thousand more being "possessed".

Should I have capped off the breather grommet on the other valve cover or would that not made any difference at all with the vacuum reading?

MDchanic

No, it really shouldn't matter. The PCV valve should be minimally active at idle, if at all, so whether the hose is pinched (closed off) or not should not make a noticeable difference.

In your case, pinching the hose and closing it off seems to have made the motor behave as though more air was getting through the hose, which is, of course, impossible.

It would appear to me as though there IS a leak of some sort through the PCV valve (there must be if the vacuum changed when the hose was pinched), BUT the additional air, rather than reducing vacuum by leaning out the mixture and by breaking the vacuum that's supposed to exist on the inside of the throttle plates, increased the vacuum.

In this case, you would have to consider the possibility that something is making your idle mixture too rich, and that by allowing in more air, you are getting it back closer to a stoichiometric ratio, which is making the engine run better (faster with the same degree of throttle opening), which is raising the vacuum.
It would be logical to experiment with controlled vacuum leaks to test this hypothesis.
Remove the PCV valve from the end of the hose, and use your fingers, or a pair of ViseGrips (can be adjusted in small increments by turning the screw) to pinch the hose completely closed, then allow it to open more and more, watching the vacuum gauge and the tach at the same time.
If you find that the engine runs better and better as you open the hose, then begins to run worse as you open it even more, you've got a sense of how much it will need to be leaned out when you find the problem, and you know to look for things that might cause a too-rich mixture, such as high float level, broken idle mixture screw tip, hogged-out idle mixture screw hole, clogged idle air bleed jet or passage, etc., rather than looking for things that cause a too-lean mixture.

I could be totally off base with this, so anyone else out there - check me on this and confirm that it makes sense, but it's the only explanation I can come up with for vacuum decreasing when the PCV hose is blocked.

- Eric

defiant1

Just got off the phone with Mark Smith from M&J Performance. Very nice guy that is very helpful and knowlegeable about Q-jets.

Again, just like in the forum it is hard to diagnose over the phone but he had the following suggestions:

He feels the throttle plates are open too far at idle, interupting the idle circuit (What Jim already suggested above)
He said this would contribute to the following issues:
Low vacuum
Idle mixture screws having no affect

He suggested I:
Replace the power piston spring with the lightest spring to help keep to ensure the needle stays seated
Ensure that the throttle plates are covering from the middle to lower third of the transition slots (vertical slots in bore).
Idle Bypass Air hole should be drilled a little larger. Mine is .081" He recommended .093" I will drill the hole incrementally until it runs the best.
And verify that fuel is not dripping from nozzle

Thanks Jim for pointing me Mark's way, hopefully I can figure this out soon.

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defiant1

Made some adjustments:

Drilled out the idle air bypass to .086 from .081.

Installed lighter power piston spring.

Adjusted throttle plates so no more than .040 of the transition slots was showing.

Hooked everything back up. Ran motor to operating temp and kicked it off of the choke. It died. So I attempted to hold the throttle at a steady rpm and adjust the timing at the same time. Could not get it to idle. Wound up turning the throttle screw in 1 1/2 turns.

Idle mixture screws were set 5 turns out from seated.

It ran ok (still shuddered) at 850 rpm/14* BTDC and held 12" Hg for vacuum. I pinched the PCV hose and the vacuum did not change (Eric should regain his sanity at this point ).

However, do to the fact I adjusted the throttle plates 1 1/2 turns I am sure I am no where near the .040" I originally set it at.

Oh then I leaned over the fender and looked down the carb. It looks like I have nozzle drip as well at idle.

Soooo, do I increase the Idle Air Bypass hole diameter even more? Do I back off the idle mixture screws even more? I am gaining. Vacuum is a little better, motor runs a little more consistent...rough idle but consistent.

How do I rectify the nozzle drip? I am assuming this is happening because the throttle plates are open too far at idle.

If that is the case, how can I get the motor running effectively with throttle plates nearly closed???

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dingusboy

Are you sure the leak isn't coming from somewhere else. I had a leak I couldn't find. I replaced the carb gasket twice. Over torqued it once. Still leaked. A friend who is a mechanic looked at it and found that the choke housing was leaking from a plug that blew out behind the hot air intake.

defiant1

I used the thicker gaskets that came with the kit to help overcome any "level" issues with the carb and intake mounting surfaces. I sprayed around the carb and intake and no increase in RPM was noticed.

All my plugs have either been plugged off or tapped and set screws installed.

defiant1

Drill out the IAB to .096. Idle mixture screw 9 1/2 turns out--I started out at 5 and the more I turned it out the less nozzle drip I had. Decreased my nozzle trip by 2/3. Before it was gushing out, now it is a drip. Throttle screw still turned over 1 full rotation to get to run at idle. I am still assuming the throttle blades are open too far leading to the nozzle drip.

Any other suggestions. Vacuum is around 12.5" I am slowly improving this reading with every adjustment I make.

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oldcutlass

Man your carb is messed up. Your idle mixture screws should be no more than 2 1/2 - 3 turns. There should be no drip. And yes I believe your off the idle circuit.

Warhead

Shaun,
What is your float level at?
What does Rochester want it at?
That's the first thing I look at if I have fuel dripping from the boosters like that.

Hang in there, you're getting there.

Jim

overthehill

Reading down through this posting brought back flashbacks of my vaccum nightmare on my 66 starfire 425. turned out to be the valve guides, replaced them and was able to set/adjust back to normal and it runs great.

defiant1

Got metric thread on the screws, I read where the amount of rotations differ from standard threads.


Jim- Float is currently set at 3/8". I had it at 1/4" based on Cliff Ruggles book. I have not been able to find any Rochester specs for my carb (1980 Q-jet).

I had the heads rebuilt, so I hope it is not a valve guide issue.

Warhead

Depending on the model number, a 1980 Chevy Quadrajet should be either 3/8ths, or 15/32nds".
What is your model number, I can check it off of the list that I have.

Jim

defiant1

Number is: 17080204

Warhead

Shaun,
Although yours does not show up on my list for that year, the 78 204 model, and the 200, 201, 202, 205, 206 show 15/32nds.
I'm confident that is where it needs to be.
Jim

defiant1

Thanks Jim, I will make the adjustment right now and see what happens.

Thanks again.

Shaun

defiant1

Adjusted float to 15/32"

Fired it up. Kicked it off of fast idle and got it running okay at 700 rpm if I opened up the throttle plates any more it would shake.

I started the idle mixture screws at 5 turns out again. Leaned over and looked down the carb and noticed the same nozzle drip. I backed off the idle mixture screw god knows how many turns until the drip stopped.

So to recap:

12.5" vacuum, 17.5* BTDC at 700 rpm, Idle mixtures screws probably 9-10 turns out, float at 15/32", idle air bypass .096"

If I open the throttle plates any farther it starts to run rough. I played with the timing back and forth and could not find a sweet spot between the timing and throttle.

I could live with the 700 rpm, the only problem is as soon as I put it in drive or reverse the rpm drops to 525 and the motor wants to die.
The current settings are not ideal regardless. Not sure what else to try...

Nothing should be this hard.

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Warhead

And they usually are not.
But they are complicated.

I had to remember that this is a 1980 unit. Even MORE complex.

Did you adjust the Aneroid assembly? Please say no.

There is also another "idle piston (I don't remember it's exact term)" in front of the power piston. **It's the one that you had to install with needle nose pliers.**
How many turns from fully bottomed out is this one??? This can also have a dramatic effect on how it will feed the engine at idle.
If it is out to far, it may have this issue, too far in, it won't run right. I take out the aluminum access plug out of the top plate (it is located where the air cleaner gasket is, front side), while the top plate is off, grind the head of this piston so one of my deep socket nut drivers fit it, and adjust it while running, replace plug once it is done. It needs to be 2.5 turns-4 turns out.

Last resort, call Mark back, get some more feedback from him.
Hope this helps.
Jim

Warhead

Technical term is APT- or adjusting part throttle. It is factory preset, they recommend not to change it. With the cam, and mods, you will have to.

I am assuming you have no leaks in the choke pull offs either.
Jim

defiant1

No aneroid assembly on this carb. I have an non CCC. Just the APT (adjustable part throttle) screw that sits next to the power piston. The APT can limit the power piston seat depth, but from what I understand does not apply to the idle circuit. I readjusted mine from 4 turns from seated to 3.5 turns.

I did call Mark again, he said open the idle air bypass to .096 and ensure I had the lightest power piston spring installed. The only thing he wanted to know that I could not answer was how much fuel pressure (I have no gauge) I had going to the carb. He really did not have a whole lot to add from the other day.

Shaun

cutlassefi

Hey Shaun,

You have the right guys helping you out, I've stayed quiet cuz I'm not a Qjet expert by any means.
But do you have access the another carb? Friend, another car, something?
Just an fyi that cam should idle very well, it's not near big enough to cause a problem.

defiant1

No problem Mark, I am confident the camshaft is not the issue. Carb performs great everywhere but at idle. I will be heading out of town for a few days, maybe I will have a fresh perspective when I return.

Shaun

Warhead

If this does not work out for Shaun, I have a 1970 Olds unit that he can try out, if he so desires.
Jim

oldsonharmont

When you initially described a black varnish on the spark plugs, I thought your mixture could be too rich. Then you mentioned the soaked base gasket. Some Q-Jets leak at the casting plugs underneath the carb, which dumps extra gas into the intake. Have you checked for this yet?

defiant1

Yes sir. I actually epoxied these as a preventive measure. I believe the soaked gaskets was from the nozzle drip that I did not know about until recently.

defiant1

Contacted Cliff Ruggles and he had the following to relate:

"108LSA is not helping the situation. I'd go bigger on the idle tubes, .038 or even .039", .058" down channel restrictions, and .110" idle bypass air. It needs more idle fuel and some help getting back on the idle curcuit.

Don't rule out timing either, it may need more initial timing to be happy. I just cured idle troubles with a Buick 350 yesterday, by adding idle fuel, and recurving the distrbutor for more timing at idle speed.....Cliff"

So I will need to get new idle tubes and open up the down channel restrictors and idle bypass air some more. I will post an update after I make changes.

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