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Old 09-14-2017, 01:26 AM   #1
BigRandy
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I could use some Quadrajet Guru help

I searched and found this post on the transition spot and would like to know if this could be my problem. http://67-72chevytrucks.com/vboard/s...ight=quadrajet

I have a 1974 Chevy C10, 103,430 miles and a 454\TH400\3.73 gears that hadn't seen much love in years. It ran like crap and the MPG is 6.9 on the highway at 65MPH. The truck flooded easily and the warm idle would vary from day to day between 800-1100 in park and 400-600 in gear.

Last weekend I rebuilt the Q-Jet but didn't notice or change the jets. After the rebuild the engine really came alive. It has an broken old style choke with the heater on the intake but it doesn't seem to cause any trouble once it's warmed up.

The problem, other than the 6.9MPG is that it runs rich even with the idle screws out at 2.5 turns each. If I turn them in it gets the shakes.

To give you an indication of how rich it's running my eyes water and today while it was idling in the drive way I goosed it a couple of times from 800RPM to maybe 3,000. The dual exhausts are dumped under the bed and point down. After moving the truck I found some very black-sooty spots where it had been parked.

Timing is set to 12BTC and I have a curve kit with medium springs in an HEI. At 2000 RPM the timing is too far off the mark to know exactly where it's at. If I had to guess I'd say 24'ish. I know that's not much help but it's all I have right now.

Does this sound like a timing or carb issue? I need to make a 450 mile trip with it in a couple of weeks and would like to get maybe 10 out of it. I don't know if that's possible but I am certain it's still running rich.

Thanks!
Randy
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Old 09-14-2017, 03:56 AM   #2
WorkinLonghorn
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Re: I could use some Quadrajet Guru help

I'm definitely NOT a guru, but I've been running Q-jets only for the last 27 years and the only times that I've had a rich condition it was one of three things; 1) Choke plate not fully opening. 2) Air filter totally clogged up. 3) Float valve stuck open with some particle of crap. I would smell gas and see it overflowing out the vent. Fixed on side of road a few times.
Maybe there are other causes, but it wouldn't be the idle setting that causes massive soot discharge out the pipes IMO. Timing would not cause soot either. I've heard that too much pressure from the fuel pump might cause the bowl to fill up too high and cause a rich condition but I have no personal experience on this.
Did you set the float level exactly per the instructions? Jets are well seated? I like to fill the bowl while the carb is on the bench and see if there are any leaks but like I said, I am not very knowledgeable, maybe just lucky. -BA
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Old 09-14-2017, 08:46 AM   #3
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Re: I could use some Quadrajet Guru help

Some things to check are fuel pump pressure. Too much above 5psi and it may be pushing past the needle valve.

Also when doing the rebuild did you check the well plugs for leaks? Qjets were known for leaking well plugs causing symptoms like you have. Anytime you rebuild one you should separate the top and bottom and if you suspect they're leaking either re-epoxy or install the aftermarket well plugs.
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Old 09-14-2017, 09:12 AM   #4
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Re: I could use some Quadrajet Guru help

Are you setting your idle mixture with a vacuum gauge? With what you're describing I would expect the gauge to be wandering all over the place in the low range at idle.

Sounds like a carburetor issue. Check fuel well plugs as pointed out, if you pull the carb and it's soaking wet underneath they're leaking. Fuel pressure as well. They can be epoxied. Check the choke mechanism and make sure it's working correctly on the cold-hot transition.

Adjusting the mixture screws before verifiying fuel pressure and leaks will be an excersise in futility. They won't do much if the fuel is being pushed past the needle.

FWIW I love quadrajets on street engines. I will tune them but let Sean Murphy rebuild them for me because he does some additional internal modifications that wake them up.
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Old 09-14-2017, 09:45 AM   #5
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Re: I could use some Quadrajet Guru help

You have a combination of problems.
You need to verify tdc #1 is accurate on your balancer/timing tab.
Then set your initial timing (with the vac advance disconnected).
On your carb running rich, you shouldn't smell it when it's idling. You might have the float too high, choke not open, needle installed on the float wrong or a bad needle/seat.
Quadrajets handle 7 psi pressure so most pumps work. THe wells usually leak after the motor has been heated right up but can leak if they're installed wrong. Epoxy works but the ultimate cure is to drill and install plugs.
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Old 09-14-2017, 10:12 AM   #6
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Re: I could use some Quadrajet Guru help

A very common problem is that unless you have a brass float, they do tend to soak up fuel after awhile. that causes the float to ride lower in the fuel bowl that it should and would cause a rich condition even though you set the float level correctly. If you did not replace it, I would recommend doing so and then reseting the float level. IMO, I don't think your current timing setup is causing the rich condition.
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Old 09-14-2017, 01:00 PM   #7
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Re: I could use some Quadrajet Guru help

You guys are great and I appreciate all the advice.

As far as I know the motor is all original and the timing appeared correct at TDC when I installed the GM HEI. Piston was TDC and the harmonic balancer was at zero.

The carb has the plastic float and it had some sort of white stain on it but otherwise looked okay. I set the height to the recommended spec that came with the kit but I don't remember what it was but I should because I checked it 4 times before attaching the air horn. I want to say it was 3/8" or 9.5mm.

I saw the well plugs but couldn't tell if they were leaking. Nothing was wet but it had been sitting for a day or so while when I pulled it off so if fuel had leaked by it would have probably evaporated.

No vacuum or fuel gauge available. This was my first shot at a carb'd motor in 30 years so my tool inventory is lacking but I think I'll buy a gauge set and see what else I can find.

I have no complaints with Q-Jets. I have heard many times that they are very good but often misunderstood carbs. I've had a few back in the day and never had any issues but my inexperience with them is likely the main problem.

Is there any trick to sealing up or replacing the well plugs? Are the factory plugs metal, screwed in, just putty or something else? There are currently covered with a brown glob of some kind but I didn't see any signs of leakage. This is an old carb, probably the original 74 that came on the motor, so if they are prone to leaking I would bet this 43 year old toilet has that problem.

Can I clean out the wells and run a tap and seal them with a pipe plug and some locktite? I found some hammer-in plugs https://quadrajetparts.com/secondary...lug-p-121.html but would think threads and locktite might be a better fix.
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Old 09-14-2017, 02:17 PM   #8
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Re: I could use some Quadrajet Guru help

Cliff has what you need.
https://cliffshighperformance.com/Qu...ottom-plug-kit
A test to see if they are indeed leaking is get it nice and hot and then park it for one hour and then start it. If it fires off right away then they aren't leaking. THe carb body has to get real hot to make them leak.

THere's only one accurate way to confirm your timing marks are correct. With a piston stop.
Like in here.
http://www.crankshaftcoalition.com/w...op_dead_center
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Old 09-14-2017, 03:42 PM   #9
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Re: I could use some Quadrajet Guru help

I am no quadrajet guru but here is my take. All your timing and ignition has to be gone through before you work on the carb. If I am correct most others here will confirm.

You have a high mileage motor, that being said a vacuum leek will cause some issues. Check your Vac Booster for brakes, these will be bad affecting idle, then check you vac advance. Either or can be bad with little noticeable or visual confirmation unless tested properly. These are diaphram operated and sensitive to ethanol. They can develop very small pinholes and again until tested properly for function can hold a mass of symptomatic problems.
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Old 09-14-2017, 03:52 PM   #10
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Re: I could use some Quadrajet Guru help

Quote:
Originally Posted by geezer#99 View Post
Cliff has what you need.
https://cliffshighperformance.com/Qu...ottom-plug-kit
A test to see if they are indeed leaking is get it nice and hot and then park it for one hour and then start it. If it fires off right away then they aren't leaking. THe carb body has to get real hot to make them leak.

THere's only one accurate way to confirm your timing marks are correct. With a piston stop.
Like in here.
http://www.crankshaftcoalition.com/w...op_dead_center
It doesn't do that. After the first start it will easily fire right up the rest of the day. I could start it with a hand crank if I had arms like Popeye. However, it will flood easy if I touch the gas first.

I haven't confirmed TDC with a wheel though.

Thanks!

Last edited by BigRandy; 09-14-2017 at 03:57 PM.
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Old 09-14-2017, 03:56 PM   #11
BigRandy
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Re: I could use some Quadrajet Guru help

Quote:
Originally Posted by toolboxchev View Post
I am no quadrajet guru but here is my take. All your timing and ignition has to be gone through before you work on the carb. If I am correct most others here will confirm.

You have a high mileage motor, that being said a vacuum leek will cause some issues. Check your Vac Booster for brakes, these will be bad affecting idle, then check you vac advance. Either or can be bad with little noticeable or visual confirmation unless tested properly. These are diaphram operated and sensitive to ethanol. They can develop very small pinholes and again until tested properly for function can hold a mass of symptomatic problems.
I checked the vac advance when I replaced the distributor and it held but I haven't checked the booster hose. I'll do that next - or just be safe and replace it. I replaced the vacuum modulator on the TH400 and replaced the short rubber hose so I know that's good. All other vacuum ports are either used or plugged off.

Thanks!
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Old 09-15-2017, 04:36 PM   #12
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Re: I could use some Quadrajet Guru help

No body has mentioned the metering rods. The pri. metering rods must move down with slight pressure & pop right up with out the pressure. You need to check the float. Set it down in some gas or solvent. It should bobble like a cork. If there is any dought replace it. They are not expensive. Make sure to match the old one as there are several different ones. The brown stuff you mentioned on the bottom of the wells are e-poxy that someone has all ready sealed them with. If it restarts as easily as you say you have no problem in that regards. Make sure you use the float level speck to match the carb nimber.
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Old 09-15-2017, 08:45 PM   #13
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Re: I could use some Quadrajet Guru help

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It doesn't do that. After the first start it will easily fire right up the rest of the day. I could start it with a hand crank if I had arms like Popeye. However, it will flood easy if I touch the gas first....
A slow drip into the intake will cause hard starting after it sits for an hour or so but once the fuel is burned off, it will start okay.

Next time it sits for an hour or so, remove the breather and manually open the butterflies. Look down the carb throat with a flashlight to see if there's any fuel puddling in the intake. If there is or if there's dark gray smoke in the exhaust, it's dripping gas somehow.
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Old 09-15-2017, 08:47 PM   #14
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Re: I could use some Quadrajet Guru help

There's a possibility I may have found a problem - maybe THE problem.

When I reinstalled the carb I connected all lines exactly the way they were when I got the truck 3 months ago. Looking at it yesterday I noticed the line to the vac advance didn't look right. When looking at the carb from the front of the truck it was connected to a port on the left which has vacuum anytime the engine is running. I moved it to the port on the right and it idles well and the exhaust doesn't stink as bad. Since I have a mild curve in the distributor my guess is it went way advanced when I reconnected the vacuum line. I haven't put any miles on it to see if the MPG improved but it runs fine and just might be running a little leaner. I also no longer have the idle problem where one time it's fine one day and the next it's 400 RPM too high.

Is my hunch in the ballpark?
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Old 09-15-2017, 10:06 PM   #15
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Re: I could use some Quadrajet Guru help

Sounds like you had the vacuum advance on the manifold vacuum and moved it to ported vacuum. A lot of people like the manifold vacuum over the ported vacuum but it depends on the carb adjustments and engine settings which is better for a particular motor. Personally, I've used both on mine and really never noticed a difference once the idle rpm was adjusted. right now I'm using manifold on my GMC and it likes it.

In your case, I don't suspect it will make much difference in your gas consumption but if it seems better than I'd leave it as it is. Once the you're on the road and off idle, they operate the same.
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Old 09-15-2017, 11:23 PM   #16
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Re: I could use some Quadrajet Guru help

Quote:
Originally Posted by BigRandy View Post
There's a possibility I may have found a problem - maybe THE problem.

When I reinstalled the carb I connected all lines exactly the way they were when I got the truck 3 months ago. Looking at it yesterday I noticed the line to the vac advance didn't look right. When looking at the carb from the front of the truck it was connected to a port on the left which has vacuum anytime the engine is running. I moved it to the port on the right and it idles well and the exhaust doesn't stink as bad. Since I have a mild curve in the distributor my guess is it went way advanced when I reconnected the vacuum line. I haven't put any miles on it to see if the MPG improved but it runs fine and just might be running a little leaner. I also no longer have the idle problem where one time it's fine one day and the next it's 400 RPM too high.

Is my hunch in the ballpark?
So, how much rpm did your idle drop when you switched over?
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Old 09-16-2017, 10:41 AM   #17
68gmsee
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Re: I could use some Quadrajet Guru help

Re. lower rpm's: When plugged into manifold vacuum where it was before, the vacuum canister advances the timing therefore increasing rpms at idle. In ported position vacuum is basically non existent until the throttle is opened to increase rpm's.

Varying rpm's when hooked to manifold vacuum means there's something in the carb still not working properly or you have a distributor problem with sticking advance mechanism or other worn internal parts in the distributor.

If you want to learn more here's a good explanation on ported vs manifold vacuum:
http://www.superchevy.com/how-to/add...nition-timing/

Click on this link and open the "pdf" file to read online or download.
https://www.google.com/url?sa=t&rct=...2iIOJgzWgjuo3Q
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Old 09-16-2017, 10:07 PM   #18
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Re: I could use some Quadrajet Guru help

No expert here either but as rich as it sounds I doubt it could be well plugs or float height. Usually float height or sticking open float will cause fuel to run out the vent and you would notice that. Is the choke flap open all the way when you are experiencing this? Make sure the secondary butterflies are all the way closed when not accelerating. You may also have metering rods not seating properly. Make sure the secondary metering rods are sitting correctly. Once the engine is tuned properly you will need to throw some new plugs in it. Once they are saturated you wont be able to clean them up or get a good burn out of them again and it makes it very hard to tune right. good luck.
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