Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Ozzmann on December 03, 2010, 03:11:21 AM
-
Just finished rebuilding my Carter 4988S, with Cliff's kit, sitting on top of my 406 Pontiac. All I can tell you about the motor for sure is the cam is a Comp 270H Magnum, factory everything else it would appear.
I followed recipe #2
jets------------------073"
prim.rods--------------43
idle channels---------.053"
idle tubes------------0385"
upper idle air bleeds---.051"
lower idle air bleeds---.070"
idle air bypass-------.096
mixture screws-------0.96
sec. rods -----------Cliff's
main body air bleed-------0.70
main air bleed air horn--------0.70
Put carby back on car and it started pretty easy, but as soon as I started to adjust the fast idle cam, it died.
Would only run with the fast idle engaged, till it warmed right up. Didn't do this previously.
Once running I played around with the mixture screws, could wind one right in with no change, but as soon as I did this with the other mixture screw it died. After restart then both would kill the motor when fully seated.
If I wound them out too far the motor would stall??
Seemed happiest around 1 1/2 turns out.
I couldn't get car to idle without the primary throttle flaps slightly opened, as I had full vacuum at the vacuum advance port. This was the whole reason for the rebuild.
Anyway kept playing with the idle screw and mixture screws hoping to close the blades, but only managed to stall motor, then it wouldn't restart....just flooded I think.
Where Have I gone wrong, I know I stuffed up something. ???
Oh yea I also noticed a couple of times when it stalled when cold it shot fuel up out of the air horn about 4-5 inches into the air.
-
Sounds like it is flooding. Probably something between the needle and seat. It's also possible that the incoming fuel was holding the float down.
I see this once in a while testing carbs here on our test engine. I shut off the fuel supply, then pulse the electric pump. This does two things, it flushes the needle/seat, and allows the float to raise up and get control of the fuel level in the bowl......Cliff
-
I'm going to take the top off and just double check the float level and the needle and seat.
I pulled the small wire clip off, as you said in the book it's not needed.
Do you think the idle circuit is enough? I just can't get that ported vacuum happening.
When I pull the plug off the port pipe , the engine increases idle speed a lot.
Do I have a leak some where?
-
You should have enough idle fuel with the specs given above. You may want to go to .055 or even .058" with the idle downchannel restrictions.
The symptoms you describe indicate that you may have a vacuum leak someplace. That carb set up has enough fuel for a pretty hefty camshaft.
What is the vacuum reading at idle speed?
Did you verify that the throttle plates are fully indexed and seated during the rebuild?.....Cliff
-
I haven't got a vac gauge handy at the moment, will have it back on Monday, a mates using it.
I was only getting around 10-11" at idle before hand.
The throttle plates did close and seated nicely during rebuild.
-
Think I found the cause of flooding, had no spark, I must have flooded it while attempting restart.
Damn dissy!
I took the idle down channel out to .058, still cant get the throttle plates to close enough to kill the ported vacuum.
When I was rebuilding the main body, the idle bypass air void holes were plugged, so I drilled them out to .140, do you think I should open them up more? ???
This is really starting to annoy me >:(
Oh and thanks for all the advice Cliff, your a legend.
-
(http://www.fcfsquad.com/pcw/images/gallery/8/Picture%20042a.jpg)
I have another query, Is choke pull off meant to have full vacuum at idle ?
After having a closer look at my carb the ported outlet and the choke pull off both connect to the same void area in the main body.
If this is the case I will never get a ported vacuum, correct or am I just confusing myself.
-
Ok so I'm a little slow, it would seem this whole exercise in trying to get my ported vacuum working correctly was a waste of time, I have no ported outlets.
I should have looked more closely at the carb first time I had it apart.
Still not sure if my throttle plates are closing enough at idle now, how can I check that, with no port vac Cliff?
What can I do with my vac advance, I really wanted to connect to ported.
With full manifold when I shift into Drive the revs drop a good 200rpm and car almost dies, that's why I wanted to get the ported working to try.
Can a ported outlet be put on to this carb?
-
In order to get manifold vacuum to work, you will need a can with a really low tension spring in it.
I don't like doing this, ported vacuum works fine for what you are trying to do.
The port is easily added to the main casting in the factory location. However, it is very difficult to add the source hole in the throttle body, as it MUST be precisely located just above the primary throttle plates at idle. The source location is critical, as the port must be uncovered with the slightest movement of the throttle off idle.
I use a really sharp punch to locate the hole, then drill it with a very tiny drill bit, about .040". I've done a lot of them here, but it's an easy thing to mess up unless you have some practice.....Cliff
-
In order to get manifold vacuum to work, you will need a can with a really low tension spring in it.
I have a Accel breakerless dissy with a adjustable vacuum advance.
It's just the spring tension you can adjust, changing how much vacuum is required to enable the advance, would I be correct in thinking that. I don't see how putting a allen key in the pipe is going to change how much advance I get.
So if I have to keep manifold vacuum, I want to limited how much, correct?
I believe it's adding to much at the moment, which is the reason it dies in rpm so much when put into drive.
Please correct me if I 'm wrong here Cliff, I am doing a lot of assuming.
-
Cliff as well as answering my above questions, could you please advise on these 2 as well.
My carb looks exactly the same as the one on page 23 of your book. The opening on top at the rear of the air horn, should there be vacuum there?
I only just noticed today that I'm getting about 3" there.
To answer your earlier query I am getting a full vacuum reading of 11" at the front port on carb.
My next query, when I removed the bung from the front port the rpm increase a lot, , and I was able to turn mixture screws out another turn and back off the idle screw to the point that it didn't contact the throttle linkage at all.
Does this mean I could use more idle air, if so what do you recommend?
-
Very few q-jets will have vacuum at any rear ports in the airhorn. Most of those ports were to take in fresh/filtered air for the hot air choke. I've worked on several early 70's Old's carbs that had vacuum in the airhorn at one of the rear fittings, so it was used on some models.
In any case, make sure that you plug all vacuum fittings and correct any vacuum leaks.
Take a look in the carb at idle speed and see if any fuel is coming off the boosters (nozzle drip). If you don't have nozzle drip, and have full control of the idle fuel via the mixture screws, you do not need any addtional bypass air (in most cases). Some engines still like a lot of bypass air, for one reason or another, so adding it can help clean up the idle and further reduce how much transfer slot is exposed to gain more control of the idle fuel via the mixture screws.
Experience has taught me when to do this, over leaving it alone, so it's pretty much a case by case basis.
In most cases I do NOT use manifold vacuum to the advance at idle speed. A well chosen ported source is used instead. Quite a bit of information about this on the NET, and some folks will say that you MUST use MVA or you just don't know how to tune or what you are doing.
From my experience, having to run the initial timing clear off the scale to get the engine to want to idle well, tells me that the basic components (compression/cid/camshaft) were poorly chosen, and even more likely the carburetor does NOT have enough idle fuel capabilities at idle speed.
What I recomend to do first, is to set the carb up for the application, then do some tuning to see if the engine likes/wants MVA, or is fine with a lower base timing setting.
The real trump card in attempting to use MVA with heavily cammed engines, is that the timing falls out easily at low vacuum readings, requiring an adjustable advance or one with a really low spring tension.
Most folks, even some "experts" who debate this topic on the NET, so not even fully understand how the vacuum advance works. The ONLY difference between ported and manifold vacuum as far as the vacuum advance is concerned, is that timing is applied at idle and coasting with MVA. A well chosen ported source does EXACTLY the same thing everywhere else.
I chuckle when I read threads where folks try to indicate that a ported source continues to add timing beyond where a manifold source would fall off. Common sense would tell anyone looking at this topic, that ALL sources under the throttle plates at any give throttle angle, engine speed/load, would show the same reading if a gauge were placed on them.
An even bigger laugh comes when folks try to indicate that the advance could be applied at heavy/full throttle. When the throttle plates are on end, the reading(s) are near or at zero, or at least well below the spring tension found in any vacuum advance every produced.
The biggest laugh of all comes when we read a thread where the owner of a car switched from ported to MVA, or visa versa, and now his engine makes a TON more power at full throttle.
In any case, I ALWAYS recomend for the tuner to work with each individule set-up, to see what settings they like the best?
For most N/A engines with a decent static compression ratio and well chosen camshaft, about 8 to 14 degrees initial (base timing) is sufficient. That setting must be tested with a well heat soaked engine to make sure it doesn't "buck" the starter on hot restarts.
We then set up the mechanical curve to add about 18-22 degrees, all in by apprx 2800-3000rpm's. The curve must NOT start till just past idle speed. This is extremely important. IF any of the timing from the mechanical advance is coming in at idle speed, it typically falls out when the trans is placed in gear. This can cause dramatic drops in engine rpm's at idle speed, and is almost ALWAYS blamed on the carburetor not working correctly.
We set up the vacuum advance to add about 10-15 degrees of timing, then choose what source to apply the advance by testing to see what the engine likes/wants.
Some engines will buck and kick profusely with a LOT of timing at idle speed. Some respond well to it. It's boils down to a case by case basis on what the engine wants. The tuner should keep in mind at this point, that the ONLY difference is that he has the choice to add the timing at idle speed, and when coasting via MVA. A well located ported source adds in the same amount of timing at every other point.
One must make absolutely sure when choosing a ported source, that it is ALL IN right off idle. Many carburetors have ported sources that were designed to run EGR valves, or other emission devices. They have a much higher source location in the baseplate, and do not mimic a manifold source well enough to be used to apply the vacuum advance. Also be aware, that many q-jets have a well located ported source, but it has a bleed-off hole that drops out the vacuum as the throttle angle continues to increase.
I highly recomend to use a vacuum gauge when choosing the source for your vacuum advance, to make sure it is applying the vacuum to the can correctly under all driving conditions......Cliff
-
Very few q-jets will have vacuum at any rear ports in the airhorn. Most of those ports were to take in fresh/filtered air for the hot air choke. I've worked on several early 70's Old's carbs that had vacuum in the airhorn at one of the rear fittings, so it was used on some models.
In any case, make sure that you plug all vacuum fittings and correct any vacuum leaks.
So I should just plug that top port with something? I only noticed it because I could hear it sucking in the air, definately couldn't hear it before I made the mods to carb, maybe enabling the bypass air has caused this leak?
Take a look in the carb at idle speed and see if any fuel is coming off the boosters (nozzle drip). If you don't have nozzle drip, and have full control of the idle fuel via the mixture screws, you do not need any addtional bypass air (in most cases). Some engines still like a lot of bypass air, for one reason or another, so adding it can help clean up the idle and further reduce how much transfer slot is exposed to gain more control of the idle fuel via the mixture screws.
I have no nozzle drip, and and kill the engine by seating the mixture screws. I thought that if you removed a hose and idle speed increased you needed more bypass air, but your saying I would have enough, (in most cases) What should I do, leave the bypass air as is, or add more, I'm confused.
Can I end up with too much bypass air, what is the risk?
Once again I thank you for sharing your time and experience Cliff.
-
I would set the idle with the dist adv. disco'd. You are gonna have to use both the throttle plate adjustment and the idle screws. Probabky wont need too much idle's turned out with the .096 holes. But don't be afraid to adjust the throttle opening. You can always look down in there to see if it pulling fuel from the nozzles. You are not pulling fuel now so open the plates a bit. Even my bypass only carb on my old VW had an adjustment for the air (there was no throttle adj., just air "screw" and the needles). you had to tune both of them. By the way, pulling a vacuum line is exactly the same as adding more bypass air (except for the filtering of the air) think about it. just depends on how big the vac. line is as to how much more air your adding. I have even thought of using some kind of ball valve mounted to a drilled hole in the intake plenum (for air adjustment) and then connect it with a hose to the underside of the aircleaner (for air filtering) it would work like an adjustable bypass air wouldn't it. ( i am nuts i guess}
Pulling a vac. line is the same as adding bypass air just without the filtering. Cliff is 100% correct about having a "ported" source for the dist. advance. Especially for street driving. Race cars dont need vacuum advance but ALL street cars should use vac. advance WITH a "ported" vacuum source. Even if you send the baseplate to cliff to drill it correctly, it would be worth it to have him do that. Always use the ported source. I never seen a Q-jet without a ported source but I guess they exsist. Oh well this is just all my 2 cents worth. Cliff really is the man to inform you. I just write opinions. (but based on some knowledge from my street racing days with a 67 goat and tuning others cars/4x4 trucks round here). Good luck to ya!
-
Now I realize Cliff has much more experience than me at setting up distributors but I prefer full manifold vacuum for vacuum advance. The only time I prefer ported vacuum is with an automatic trans that drags the idle speed down too much when put in gear (usually with perf cam and stock converter). I set them up on a dist machine and try to match the vacuum can to the engine, plus fab a limiter to limit vac advance to 10-12 (crankshaft) degrees. This is after setting mech advance to start above idle of course and be all in at the 2000-3000 range depending on combo. IMO full manifold vacuum advance will give better fuel economy and the engine will run cooler. BTW some early 70's Chevy Q jets have no ported vacuum source. I had a '70 BBC 7040204 carb that way. I suspect it was that way because '70's used TCS, or Transmission Controlled Spark Advance. That was a trans controlled switch that shut off vac advance to the dist unless trans was in high gear.
-
The ported vs manifold vacuum to the distributor deal has graced countless pages on many other websites. The arguements continue to rage back and forth as to which one to use and which one works better?
The improved fuel economy and reduce running temp thing is regurgitated on nearly every thread I've ever read on that subject.
Interesting that after tuning many thousands of different engine combinations over the years, I've never found this to be true in one single instance, and I've seen some pretty hot running, oveheating and detonating engines over the years, and have been asked to help them.
I've done some pretty extensive fuel economy testing over the years as well, clear back into early 1980's. Back in the early 1980's, I was displaced from my family, and had to make a 600 mile run couple of times a month, and I was trying to stretch every penny as far as possible, it made for some very interesting testing.
I had several vehicles over the time that I made that run, and tuned them to DEATH by leaning/richening up the carburetors, and modifying the vacuum units and the mechanical spark advance as well. I'd also swap from a ported to manifold source to the advance.
That's not the only testing I've done, but the results of that early testing taught me two things. Lean mixtures are difficult to burn, and there is no fuel economy advantage whatsoever switching from a well located ported source to a manifold vacuum source running down the highway at 65-70 mph. About all I noticed running MVA, is that the engine rpm's dropped a tad more when the trans was place in gear that if a ported source were used and the throttle plates tipped in a little further.
I've done more closely controlled testing in recent years, and have found that once we increase engine efficiency (optimum compression ratio, optimum quench distance, well chose camshaft, efficient intake/exhaust systems, etc), the engine will not want, like, require, or respond well to a lot of ignition timing anyplace.
Of course we get all sorts of stuff in here to custom tune, and by far and above the very worst are when someone takes a relatively "low" compression engine, tops it with a single plane intake, free flowing headers, and some "whiz-bang" aftermarket camshaft on a tight lobe separation angle. I can't say TURD enough times.
It's just kills me that with all the good information out there these days at your fingertips, that folks still continue to choose piss-poor combinations of parts for their new engine, or upgrading their old engine.
Anyhow, we have, and always recomend to others, when tuning carburetors and distributors, to go after the carburetor first, to make sure it has sufficient fuel across the load speed range for what you are trying to do. Then tune the distributor. 99 times out of a 100, the tuner will find that a lot LESS timing is needed at idle speed once the carburetor is up to par for their particular application.......Cliff
-
Cliff can you put up a couple of pics on exactly where the ported line would need to be drilled in main body and throttle body. I want to have a look, and then decide weather it's worth me having a go at it.
I would send it to you, but as your aware the freight from OZ and back makes it too expensive, not to mention the time frame.
I have no preference to manifold or ported vac, but I believe in my particular case, with the idle dropping so much when put into drive, ported will help me.
-
Don't have any pics. The main body is easy, as there is a pad in the front lower portion (drivers side) of the casting. Drill straight back to intersect the 90 degree passage that comes up from the baseplate.
The baseplate hole must be right above the leading edge of the throttle plates at idle speed, and uncovered with the slightest movement of the throttle. This provides FULL MANIFOLD VACUUM to the distributor everyplace but at idle speed, or when the throttle is closed.
Folks miss out on this fact, and seem to think you get some sort of different results when using ported vacuum to the distributor when you drive the vehicle.
As simple as this topic is, most "experts" don't even really know how it works.....FWIW....Cliff
-
Cliff, not sure weather you have seen my post on the PY Forums concerning my major off idle breakdown.
I'm pretty sure it's spark related, but a lot of people keep saying it's the carb... what do you think?
I was in the process of tuning the carb when my old dizzy failed, I had a few things to iron out, but it was running, just had to get the secondaries to come on sweeter.
Even now it starts better than ever, idles nicely but give it just a little gas and it starts to misfire badly.
Surely the specs I rebuilt to, give me more than enough fuel, assuming my fuel pressure is good of course.
-
A "miss" under load is almost always going to be a distributor related issue, assuming the compression on all cylinders is up to par?
Another thing that will cause a miss under load is an intake cam lobe going bad.
I would do a compression test, and visual inspection of the rocker arms, and their movements when the engine is cranked over.
If it has good compression, and the valves are all moving fine, start looking at plug wires, spark plugs, and other ignition components......Cliff
-
Cliff, my air horn gasket seems to get soaked with fuel around the accelerator pump, what should I be checking? I've checked float level and needle and seat.
-
hey ozzman, how's it goin "down there" (I noticed you guys got a bit of rain in certain areas- sorry to here about the flooding). As far as the gasket getting wet, it seems like this is THE place that Q-jets like to do this. I do not know for sure how to go about fixing this as I have run into this even after unwarping the body and flatening the top casting. Obviously a thicker gasket would help but I wonder if some fuel is bypassing the accel pump cup. Maybe Cliff will chime in I hope. Seems like this is a common problem.
Hey, goodluck to ya!!
-
If the fuel level is getting high enough that fuel is coming out around the pump, it's a flooding issue.
If the fuel is just seeping out do to being splashed up around the pump shaft, tighten up the clearance between the parts some.
This is very easy to do.
With the airhorn well supported by a wooden block, very gently "peen" the area around the pump shaft until the pump will no longer fit thru the hole.
Then gently drive an old pump thru the hole to size it.
This will minimize any leakage in that area.
If the carb is flooding, pressure and vaccum test the needle seat assembly. I neat was to test them is to put some soap/water mix around the needle and seat assembly. Gently hold down the needle with your finger, and shoot some compressed air into the fuel inlet. If the seat is leaking at the gasket, tiny bubbles will appear.
We get a few in here that are leaking, usually due to damage to the sealing surface, or not getting the old gasket cleaned off. Got one in here yesterday with two gaskets doubled up. The owner of that carb bought all the parts and attempted to do it himself. Did a great job everyplace, except that area.....Cliff
-
Hey Cliff, good info there. I did not know how to do that and would never have thought of it.
-
Should the compressed air be sealed up to the inlet? If so what PSI should be used?
-
No need to seal it up, I use about 100psi air here, thru the inlet with plenty of soap/water mix around the needle/seat assembly.....Cliff
-
I'm finally back to getting the carb tuned correctly, after 6 months on ignition problems.
Anyway the Qjet is running well now except for a slight Light throttle or off idle hesitation.
It dips for a second upon light throttle application then picks up and runs fine, where should I look to fix this? ???
-
Is there a tiny screw in the top of the airhorn under a triangular metal cover?.....Cliff
-
Hello Cliff, no mate I don't have that on my qjet, remember I don't even have a ported outlet!
My carb must have been the poverty pack version...lol. ;D
-
Cliff, what order are the power piston springs you supplied to me in from light to heavy? ???
I have a short orange, long green, medium black and a medium silver/natural color.
Maybe my hesitation is too soft a spring?? I'm using the black one at the moment, it's got more coils than the other medium sized one.
-
From light to heavy, orange, light blue, green, dark blue. The opening points are apprx 5-6, 6-7, 8-9, and 9-10" vacuum.
For most builds I use the light blue spring.....Cliff