Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: corellian corvette on February 11, 2013, 11:22:53 PM
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Hey everyone! Hoping the experts here can help me diagnose a problem.
Recently had the motor rebuilt in my '78 Trans Am. It was basically a mild/stock 400 rebuild, with flat-top pistons and a Comp Cams XE262 cam. We'd struggled with some bad cam choices (XE 274) but could not get the car to idle properly and had no low end.
After the installation of the XE262 things are much better, but not perfect. We can now at least get the car to idle, but there is a distinct surge at idle of ~ 100RPM, with a pretty steady and rhythmic drop of 5 - 7 hg of vacuum (hovering around 15, going between 13 - 17). It drops down ~ 100rpm then picks right back up.
Although the car does idle OK, I have to turn the idle screw pretty high until it almost starts to drip through the ventruis. Right now I have it at about 850 RPM in neutral, with the idle-mixture screws about 5 turns out, and the APT about 4 turns up. I can't get the car to idle faster than 850 rpm without starting to open the primaries too much and making them drip. Which of course, causes another type of surging.
Carb is the original 17058278 Pontiac Carb, originally jetted with 74 jets and 45K rods, now jetted with 77 gets 45K rods to increase the idle mixture.
It feels like I've done everything right, but I can't figure how to eliminate this surge. With the 74/45 combination, if I started to cover the choke idle would increase. With the 77/45 combination that's much less, but it still seems like I really have to run out the idle mixture screws and the APT just to keep the car running, and I still have this surge.
I'm running about 19deg initial timing (mechanical) and about 40 with full-manifold vacuum just to smooth things out. If I remove the manifold vacuum (which I am doing for testing purposes) and idle drops to ~ 700 rpm.
I'm really confused how to fix this problem. I feel like I know my way around Q-jets, but I can't figure out this surging. 13-15hg of vacuum should be providing enough signal to the idle circuit - but man I have to crank this carb to the extremes just to get idle but it still surges.
I should point out this carb was running like a CHAMP before the engine rebuild. Of course, I had a worn and rounded cam (hence the rebuild) so I was pulling 20+hg at idle. I had no POWER, but an awesome idle :)
Any thoughts?
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It sounds like you need more idle fuel to me. You could have a plugged passage or the idle circuit is just too small for the new cam.
Changing jetting won't have a huge effect on idle if at all. When I change jets I frequently don't even have to touch the idle screws.
Now, when you did the rebuild did you run a wire through the idle tubes to make sure they weren't plugged?
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How would you suggest fixing that?
Thanks!
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Run some thin wire through the idle tubes to make sure they're clear. If they are clear and you still need to upsize, you'll need to pull the tubes and size them up. I would actually just get a replacement set of tubes from Cliff.
My 78 350 has a similar cam, and I actually sized the idle circuit up quite a lot to get rid of off-idle surging.
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Thanks Shark Racer
I have Cliffs book, too, but a little more specifics would make me feel better about potentially ruining my carb :(
I've done a ton of rebuilds but never messed with the internal passages.
Which tubes are the idle tubes again?
What size are they stock?
How do I think about re-sizing them? What size upgrade should I order?
Thanks!
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Typically from the factory they run .32-.35ish.
Mine were about .34, and I started at .36 and went to .38.
The idle tubes are in the main body, one on each side of the float well. There is a "path" that leads to the down channel restrictions. Basically two brass wells, ~1/8" diameter. The one that is mostly flush with the top of the main body casting is the idle tube, the restriction is about 2" deep and you'll likely have to remove them to change the size.
The down channel is the larger hole, about 1/4" below the top surface of the main casting.
(http://elucidus.smugmug.com/photos/i-NtxgDbj/0/L/i-NtxgDbj-L.jpg)
The tubes are next to the corners of the float, the down channels just slightly forward.
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Thanks again. I'm going to ask a bunch of Q's so I don't bugger this thing up.
I was reading an article on HPP that talked about doing this, but the condition they were dealing with was the car was running too RICH.
In my case, it appears to be running too LEAN. So wouldn't opening the idle air bleeds make the carb leaner?
Again, I *really* appreciate all the help and guidance here. I don't mind tackling this at home, but I don't want to start drilling into things without knowing exactly the outcome.
As it stands now, it appears I need to somehow improve the amount of fuel the engine gets at idle, as it's running lean. How would you order the steps of drilling out the tubes to analyze this problem?
Thanks SO much!
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I should also point out, after reading several other tuning articles, it seems I need to start by upsizing my IFR (Idle Fuel Restrictors). Are those the ones in the body of the carb, or the ones that are pressed into the carb top?
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They are basically a pair.
Opening up the idle air bleeds will add emulsion air and tip the scales more lean. I can honestly admit that I don't understand why a different sized bleed has different characteristics from another. IE, you can get the same mixture with bleed A and tube A, or bleed B and tube B.
At any rate, the down channel restriction may also be called an idle fuel restrictor. It works together with the idle tubes. I would up them both in pairs. There are some recipes in the book that explain different sizes. I'd start by sizing what you have and then go up a thousandth at a time, maybe more. I ended up having to go up QUITE a lot on mine.
Considering you are over 5 turns out and still seem to be running lean, I would think you need to go up a couple steps. However, it's not easy to "shrink" a hole, so ... :)
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OK using Cliff's book, I went ahead and pulled the measurements for my car. My harbor freight drill set doesn't have some of the smaller size increments, so I'm going to get a better set to get more specific
Idle Tube: .046 fit, but not .052 (my next size up)
Idle Down Channel: .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .055
Idle Bypass Air: None.
The upper air bleed seems really small based on the specs I've seen.
Any thoughts on this?
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The restriction in the idle tube is at the very bottom of the idle tube - I doubt it's .046. That's HUGE. You need to be measuring at the necked down section which is ~2" below the surface.
(http://www.cliffshighperformance.dreamhosters.com/images/10-09%20images-done/Idle-Tubes-09-lg.jpg)
Pic above is from the parts Cliff offers: http://www.cliffshighperformance.dreamhosters.com/parts.html#a
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How long are those tubes? The smallest bit I have is .040, but they are super short so I want to make sure I'm getting them all the way down (note - I have not removed the tubes)
Given that I need to make the idle RICHER - what would be the best bet? Drill out the IFR (Idle down channel)?
What about blocking the Air bleeds? I notice my system has 2 - one at the top (.070), and a smaller one at the ventrui (I believe this is called the main air bleed or lower air bleed) which is very small (.055).
Could it be as simple as just blocking the upper idle air bleed which would enrichen the fuel mixture?
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You really need to size them as close as possible to determine where to go. Like I said in the last post, the tubes are about 2" long.
Personally, I'd open the IFR and tube up as matched pairs.
Cliff sells screw in idle tubes, which would probably help with speeding the process up. Prying the tubes out is kind of a pain.
You'll need a 61-80 drill bit set, and pulling the tubes will require a drywall screw, .090 punch and a pair of pliers. Probably some heat too.
(http://sphotos-a.xx.fbcdn.net/hphotos-ash3/75327_912174133634_1600172536_n.jpg)
I took that photo that shows all the tools I used to enlarge the idle tube restriction... :)
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OK thanks for that note - I measured again and the idle tubes are exactly .042. I have a .043 and it doesn't fit.
I also realized that I was measuring the MAIN idle bleed, not the LOWER idle bleed. Since I don't have the baseplate off, I can't measure it but I'm assuming it's the larger size.
These would be my new measurements:
Idle Tube: .042
Idle Down Channel: .046 fit, but not .052 (my next size up)
Upper Idle Air Bleed: .070
Lower Idle Air Bleed: .070 (assuming)
Idle Bypass Air: None.
Given that I need to make the idle circuit RICHER, it seems like I only have a few options.
1. Trying going up a size or 2 on the Idle Down Channel.
2. Try blocking the upper idle air bleed. Doesn't this mix with the fuel going down the IDC to lean it out?
It already looks like my idle tube is plenty large based on some of the "recipes" in Cliff's book - it appears it's the down-channel that severely undersized.
Make sense?
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I'm still really surprised your idle tubes are that large. I would go up, VERY slowly on the down channel restriction until you solve your problem. I might consider pulling the idle tubes anyways and necking them down to 036.
Do you know the static compression ratio of your engine?
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We used flat-top pistons so I believe I'm in the 8:1 ratio, with the XE262 cam which was recommended after we couldn't get the XE274 to idle AT ALL.
For giggles, I also measured the idle mixture screw hole, which is also VERY small, .055
My guess is to move up .010 on each hole - .065 on the idle mixture screw, and .56 on the idle down channel. Those numbers look more like what I see in some of the upgrades in the book.
I've also heard when drilling the idle mixture screw, you should look at the hole in the baseplate and make sure it's not also restricted.
Does that seem like a logical idea?
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You should open it up a little bit, and you may well try that first; it's the easiest of the mods to do. I'm running mine at .090.
So my recommendation would be to start with the mixture screw holes, try that, then the DCR. I would definitely sneak up on the DCR, and not go straight to the 056, as your idle tubes are quite large.
As for the restriction - once you've drilled the hole out, it's pretty much guaranteed not to be blocked. :) Just clear any chips out of the way...
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Just a note, are u sure your measuring all the way to the idle tube restriction. unless you have drill bits 2+ inches long u may not be getting accurate measurements like shark mentioned. U can get a set of small wire drill gauges from a decent tool store that are long enough to reach the restrictions that are like 1 21/32 down the idle tube. Taper length drill bits are longer but pricey, precision makes them.
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WE HAVE SOME PROGRESS!
There has been some dialog on this on the PY forums, and I should first correct that I went back and re-took all my measurements by doing a more thorough tear-down. I'm still not 100% sure about the idle tube, but I practiced removing one from a junk q-jet, and used that to gage whether my bit was long enough. The idle tube in the carb was .032, and obviously I could not get my .040 bit down that tube. When I put the same bit in the tube in the carb, it went down MUCH further - almost flush enough I had to pick it out. So I'm still feeling like that tube is ~ .040.
Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: None.
Mixture Screw Hole: .065"
I took a small step tonight and used a pin vice to open the idle mixture screws up to .082, and I used the same drill to put .082 bypass air holes. This felt like a good mid-step. I put the 74 jets back in, lowered the APT, and BOY did it make a difference. Timing is back down to 15 deg, idle speed screw has more adjustment, and the rhythmic vacuum and rpm drop is much reduced. I was able to get it to idle in drive, no MVA, pretty steady at 750 rpm. I still have to have the mixture screws about 5.5 turns out, and I still get a VERY mild RPM increase when I close the choke horn, but nothing like it was before. Lean idle burn smell is much reduced. Vacuum in drive is about 13hg.
New settings are
Idle Tube: .040
Idle Down Channel: .046
Upper Idle Air Bleed: .067
Lower Idle Air Bleed: .070
Idle Bypass Air: .082
Mixture Screw Hole: .082
If a little is good, more must be better, right?
I think I'm going to take ONE more stab at increasing the idle speed and bypass air holes, to bring them closer to the spec others have listed; maybe use the .89 bit on both idle mixture and bypass air to see if that gives me a little bit more mixture screw adjustment and a tad more enrichment.
Frankly I could live with it right now. Would be totally fine, but I just made a HUGE improvement so it seems like one more small step and it might get me there. I'll be very happy if I don't need to mess with the idle tubes and fuel restrictors.
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I'd up the idle mixture screw size to the .089 bit and if you have a very slight up for the DCR I'd open it up just a bit.
I'd leave bypass air alone for now. How far are you opening the throttle blades to maintain the idle you want?
You probably could have gotten that XE274 cam to run at this point. :)
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Thanks Shark Racer!
I'll do that then, first. It's hard because on the PY string people think opening up the idle bypass is also a good idea. So hard to parse the advice.
I'm getting more freedom with the idle screw then I had before. I can get an easy 1000 + RPM on the idle screw without venturi drip (before Max I could get was ~ 850).
But idle screws are still out quite a bit, with a very light lean condition as I stated.
So you think going to .089 on mixture screw, and a very small opening of the down-channel would be better?
The PY forum recommended going to .055 on the Down-Channel, but I'm going to look at my drill-bit options and was thinking of shooting for something more in the .049/.050 range...
RE: XE274. I probably could have made it run, but it was MUCH worse than what I was dealing with on the XE262. I was also told by Lars Grimsrud and other Pontiac guys that the 274 was not a good match for my engine - sacrificing low-end torque for high-end HP, but my stock iron heads and low compression would die out before I was in the sweet spot of the cam. I already can tell this cam is better matched to my engine as stoplight performance is much better. I really had to get into it before to feel any power...
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It's much easier to go bigger than smaller, so that's why I'm recommending stepping it up a little at a time.
I went from 55 to 59 on my DCR and that took me from a little too lean to a little too rich. If I did it over again, I'd go with a 1.45mm bit which is about 57. 2 years away until my next smog test, I'll leave it alone for a bit. :)
As your idle tubes are already quite large, I'd be very cautious going up on the DCR.
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I think we're in good shape.
The next size drill bit I had was .052, so I opened up the DCR to .052.
To compensate, I opened the Bypass Air from .082 to .086.
Finally, I went to .089 on the mixture screws.
I think we've nailed it. Idle mixture screws are perfect at 4 turns out. I did need to crank the idle screw in just a bit to bring it up to ~ 900 rpm in Neutral, but I'm still below transfer slot so no venturi drip.
Idle holds steady in drive at 700 RPM, Vaccum is pretty steady at 14hg.
So I think I'm done at this point. I'm going to drive it around and let the engine break in, and I have a rebuild kit coming from Cliff that I'll put on when it arrives.
Thanks for your help!
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No problem at all... glad to be of help. Now come take care of mine. :)