Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: dave at hsp on May 10, 2014, 12:46:42 PM
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Howdy! I'm new here so forgive my mistakes.
I talked to Cliff about this a few days ago and he gave me some info of what to do, so this is sort of an update. First some back ground. This is an '80 Camaro, 305 automatic, 350 turbo with a 2500 stall converter, 3.73 gears. The cam is too big for this application, I know. But, it's in there and that's what I'm gonna make work.
I rebuilt the engine. It's .030 over with flat top pistons, so the compression is now 10.2 to 1. Stock heads. New Comp Cams Thumper cam, lifters, springs, retainers, locks, timing set, installed at 102° intake CL. Its .479 and .465 lift and 227 and 241 @ .050. The carb so far is as follows:
idle tube: .042
Idle down channel restriction, brass thread in : .063
Lower idle air bleed, brass thread in: .052
upper idle air bleed: .070
Accelerator pump discharge holes: .028
Main air bleeds ( base plate) .140
Main air bleed (air horn) .120
I have the lightest spring in the power valve that comes in the Edelbrock 4 spring kit.
It will idle warm at about 1000 in park, pull it in gear and it drops to between 500 and 800 with a big chop, which is OK, but the idle is stinky rich. Wing the throttle and it rev's right up. No hesitation or stumble. The vacuum at idle in park bounces a bit between 5 and 8 inches. In gear it's barely 5 inches.
My question is, where do I go next to back the idle screw off about 1 turn to get the throttle plates to where I'm not exposing so much of the transition slot, and still maintain about 800 rpm at idle, not so stinky rich? How big is too big on the main air bleed in the base plate?
Thanks for any comments, ideas, answers.
Dave
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You did not mention if you have idle bypass air, Are you familiar with this? With a cam like that, you will most likely need it.
Are you experiencing nozzle drip at idle?
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Yes, I am using idle bypass air. The main air bleed in the base plate is at .140. That is my main question; how big is too big on the main air bleed in the base plate? (for the idle bypass air)
I also didn't mention the timing. The timing is set at 14° initial, 12° vacuum advance, 16° centrifugal, all in at 3500 rpm. The vacuum advance is full vacuum, with a can that comes in at 3 inches and is all in at 8 inches of vacuum. That is part of the problem. Trying to carry enough vacuum to hold the vacuum advance at idle, so the idle won't drop below about 800 rpm. Once I loose the vacuum, I loose the rpm being held by the vacuum advance. If I turn up the main idle screw, the butterfly goes into the transition slot and pulls fuel from the booster.
I don't see any drip from the booster at the current setting.
I did check the AFR this afternoon with my LM-2 and it was 13.0 in park at 750 rpm and 13.5 to 13.9 in drive at 550 to 600 rpm.
That was fully warmed up, and it did have a slight, very slight stumble on acceleration. No problem with a quick wing of the throttle.
Thanks.
Dave
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Dave, I always like to plumb my vacuum advance to a ported vacuum source, and then set the initial timing to suit. I believe Cliff recommends this in his book. I'm not sure if this is your primary problem, but it is worth trying.
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the idle afr sounds pretty good? like a computercontrolled car almost...
What about afr at cruise and wot?
on a sidenote: I have the same LM-2, great little instrument. How far up the tailpipe do you mount yours ? I homemade a bracket for mine, it goes about 2" up the tailpipe, so I often have trouble getting trustworthy readings in idle. ( 25:1 ). No problems once driving though.
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I'm thinking the idle is too lean, try setting idle to max. vacuum, & see how it is. O2 sensors don't work very well at idle with a big cam every misfire reads excess oxygen & reads it as lean.
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ejowrench, thanks for the reply.
I have not tried ported vacuum. I always set up a big cam engine with full vacuum. Let me tell you why. I try and set the initial at 12 to 16, then run full vacuum and add another 12 to 14, then pull in the centrifugal with another 12 to 14, so I end up with 40 to 42 total at 3500 in the end. I really shoot for the end result as long as I can get my initial good enough to start hot, and the full vacuum at idle so it WILL idle.
I think if I run ported at idle, it will idle even slower because of not enough advance. Make sense?
Dave
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Marx3, I haven't driven it and watched to see where it's at cruising and wot. The roads have been wet and I really haven't had time. My dad was diagnosed with stage 4 cancer 4 weeks ago, so I've been pretty busy with him.
As far as the sensor mount, I tig welded a bung in the collector on the passenger side, just past the end of the header.
Got my 02 sensor screwed in there. This is the first time I have used the LM-2, so I'm still learning.
I ran the car down the road Sunday with the kid that owns it, but like a dummy, I didn't have the LM-2 plugged in. It runs really good though. Good throttle response and smooth at wot. I hope to get some readings this weekend, if it ever stops raining.
Thanks!
Dave
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77cruiser, by idle too lean, do you mean the idle tubes? .042 was the biggest long bit I had, so I had my daughter order me more bits up to .050 today. (she is a sales rep for MSC industrial supply) So, once I get those, hopefully Friday, I was gonna go up .002 at a time to see what happens. How big is too big on the air bleeds in the base plate? I'm at .140 now and was gonna go another .002 at a time.
Thanks!
Dave
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Dave, so what you are saying is you set the initial timing at 12degrees, the vac advance at idle adds another 12 degrees, so at idle you have a total of 24 degrees?
When you slowly open the throttle with this set-up, the vacuum drops down, retarding your timing when it should be advancing. On ported vacuum, as soon as you start to open the throttle, the vac advance starts to quickly advance the timing, and then the centrif advance comes on, a nice smooth fast advance curve. Make sense?
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Something like that, yes.
The centrifugal advance starts to come in early and is all in at 3500 rpm, so the timing never really backs up much.
What you say does make sense and I'm gonna give it a try this weekend. I'll let you know what happens.
I've been doing it this way so long, maybe it's time for a change.
Thanks!
Dave
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Not necessarily the idle tubes, but could be. How far out did you have the mixture screws? Did you open up the holes for the mixture screws?
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Jim, I had the mixture screws out about 3/4 of a turn.
I opened the screw holes to .106 from .096.
UPS just dropped off a box of long drill bits from MSC.
So I guess I'll be drilling the idle tubes some more this weekend. I'm gonna go a little bigger on the bypass holes in the base plate too. Unless anyone has a better idea.
It's pretty close, but could still be closer.
Thanks!
Dave
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3/4 of a turn was thinking should be out 3 or 4 full turns. Maybe more try that first.
Set idle to best idle or highest vacuum.
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3/4 of a turn WAS the best idle and highest vacuum.
Dave
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Have you tried giving it more timing? Go as far as can while still getting improvement in the idle, you can always limit the mechanical after you've determined where it idles the best. You may not get it to work with manifold vac. as you know the vac. drops off too much when you put it gear.
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I finally got a chance to work on this thing again today.
I went from .140 to .150 on the main air bleeds in the base plate by the butterflies, went from .042 on the idle tubes to .051. The mixture screws are at 1 turn out. That's where it idles the best with the highest vacuum of just above 4". I changed the vacuum advance from full vacuum to ported. Didn't really see much difference except for a more steady idle as opposed to a slight up and down before due to the vacuum at idle being up and down. I'm gonna leave it there for now.
My LM-2 is reading 13.0 at idle in gear at 550 to 600 rpm, and 12.5 in park at 650 to 700 rpm. It sat there and idled in gear for about 5 minutes with a nice chop. Never stalled or even acted like it wanted to stall. It still stinks rich at idle, not real bad, but it seems to run fine and revs right up. I'll go take it for a drive in a while and see how everything else runs. Sitting here in the shop at 3500 rpm the LM-2 reads 12.1 to 12. 5.
I still would like a leaner idle, but right now, I'll take what I can get. Not bad for a way too big cam.
Dave
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Try 2 more things the timing & try adjusting the mixture screws with it in gear to best idle.
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Well, I gave it a little more timing, and since it's a small cube engine, I'm sure it won't mind. I left the vacuum to the distributor on ported, not full, adjusted the mixture screws at idle in gear, (the way I always do) and they are still just a tad over 1 turn out.
I ran it down the road about 5 miles and it runs really nice. The AFR is 14.5 at 3500 rpm cruising. I got back and let it set and idle in park for about 10 minutes. 750 rpm and 13.2 AFR. Dropped it in gear and sat there and idled at 600 rpm with 13.3 to 13.5 AFR for about 5 minutes. Nice chop to the idle. Sounds like a pro stock. Kinda cool and the kid that owns the car really likes it. When I drove it, from a stop, I smashed the pedal to the floor and it just kinda hazed the tires and went. Chirps the tires when it shifts and pulls hard up to about 80 mph when I let off. Pretty strong for a 305.
Anyway, I let it go and the kid has driven it now for a couple days and has a BIG grin on his face. That's what we were after. After getting some miles on it now, the idle doesn't smell so rich anymore and will idle all day in gear, and no hesitation on acceleration.
Thanks for all the help and advice guys!
Cliff now has another recipe for the next book.
Dave
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I went from .140 to .150 on the main air bleeds in the base plate by the butterflies, went from .042 on the idle tubes to .051. The mixture screws are at 1 turn out
There are no "main airbleeds" in the baseplate, did you mean idle bypass air?
Having to go to .051" idle tubes indicate a lot of idle airbleed for the application. In many cases it's easier and more effective to reduce or install smaller idle airbleeds so there isn't so much emulsion air at idle when the vacuum signal is really low. Non-emission and Marine carbs are set-up like this, and put down a TON of idle fuel with pretty small idle tubes and down channel restrictions......Cliff
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Yep, idle bypass air, not main air bleeds, sorry.
I had reduced the idle air bleeds to .052. That's why I asked. I wasn't sure which direction to go, so, I just did some 'spearmentin.
If I get the car back later in the summer, I'll reduce them a bit more and go smaller on the idle tubes.
That's why I ordered a couple extra from you the other day.
Oh well, it runs pretty good now and you can't wipe the smile off the kids face with a crow bar.
I'll let you know when I get the car back.
Thanks!
Dave