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JoeyCJ55 06-28-2002 08:16 PM

Adjusting carb (continued)
I decided to make another post, because I went off the original topic and it went two pages.

I've had bad vibrations at idle, and have found it to lie in the adjustment of the carb.

A little info
It's a 1986 4.3L from a k10 blazer
It has a new qaudrajet carb from a late 70's model chevy pickup.(w/a 350)
I've also just set the timing to four degrees(I don't know if this is correct, someone on board said it was 4 degrees a few months back)
It has an auto tranny

LEVE gave me a method of turning in the screw until it drops 100 rpm, then readjusting the idle.
Repeating this until I can no longer drop the 100 rpm.
I have done this, and it will drop that 100 rpm after a turn or two until I have the idle mixture screws ALL the way in, and my idle speed very high(screw is very hard to turn)
I then adjusted out one turn, and it diesels like crazy, and the idle is way too high.

I've also been told to use a vacuum gauge, which I have purchased.
I've been told to adjust it until I can get the highest reading.
If I richen it, it will continue to go up(to and past 26hg).It doesn't stop increasing, and my idle mixture screws would almost come all the way out.I don't want another fire, and I'm thinking fuel will leak out, so I didn't fully unscrew them.

My main problems seems to be that I have to variables, and I don't know how to eliminate one.
My vacuum increases as my rpm increases, either I can richen the mixture, and lower the ilde speed, or lean the mixture and increase the idle speed.I get the same results, and they are two different settings.
I'm just totally baffled..I hate to make another post but I need to go to work tomorrow, and I'd like to be able to drive on the highway, and start it up without any problems(its very hard to start right now)

I don't know if it matters, but I am currently 2 1/2 screws out on my idle mixture and when I put on my air filter, RPM increase about 150.

I've borrowed a timing light, and an inductive engine analyzer(has a tach among other things).
If there are any test I need to do, tell me.

Thank you

GeeAea 06-28-2002 08:32 PM

Re: Adjusting carb (continued)
Joey ...something is getting missed in the translation. How can you have a very high idle when you readjusted it to 800 rpm (which I think is what LEVE told you to) every time you get the 100 rpm drop??

I didn't recheck LEVE's post ...but I know that he gave you an idle speed spec to start off with and to readjust to. If you have a high idle've forgotten one of the steps...

.......or did I read your post wrong???

JoeyCJ55 06-28-2002 08:39 PM

Re: Adjusting carb (continued)
he said increase it to 1000 rpm when resetting
in gear 750 rpm.
When i'm in park, its about 1400 rpm
Maybe my idle speed really isn't that high, but I'm not use to it taking off when I put it in gear.The other reason I said I had a high idle is because my idle speed screw is screwed in about as far as it can go

JoeyCJ55 06-28-2002 10:27 PM

Re: Adjusting carb (continued)
Ok, I've been reading and rereading things on the internet.
I just found a pretty good article.

The only problem I have is how to find out what the idle speed is.
everywhere I read it says "check specs" or "check under hood"
Unfortunatly, I don't exactly have the vehicle it came out of.
Any ideas on what it is, or a guess?

WILL 06-28-2002 11:14 PM

Re: Adjusting carb (continued)
I set my idle based on what feels good. Low enough that I'm not standing on the brake in drive and high enough that it idles smooth.

ozarkjeep 06-28-2002 11:17 PM

Re: Adjusting carb (continued)
do you have a timing light or a tachometer you can hook up and trust?

if not, then your stuck with tuning by ear, which is rarley accurate or the best way to do it.

borrow, rent, the tools, or befreiund soeone who has them ( knowing this board you can find someone local).

idle speed for a V8 automatic is 850 in park, 650 in drive dead stop, that should give you a GO point.

JoeyCJ55 06-28-2002 11:57 PM

Re: Adjusting carb (continued)
yep i retimed it a few hours ago, and I have a tach

Those numbers sound good.I'll set it at those.
The only thing is, where do I start?
When first adjusting the idle, should I have my mixture in all the way, and should I have my idle screw all the way unseated?
any ideas?
i'll be doing it tomorrow, and i'm going to rig it up so it runs tonite.
Thanks for the info

GeeAea 06-29-2002 01:22 AM

Re: Adjusting carb (continued)
Joey ..just do what LEVE said in the other post...but adjust the numbers to suit the situation. The concept is to have a smooth idle that isn't too rich ....yet idles with enough power in gear to drive your available accessories (power steering - maybe you don't have it). Expand the envelope of what you read here to suit the suttle differences of your particular situation.

Oh yeah....did you unhook (and plug) your vacuum advance and THEN recheck the timing?

1. Set the idle screw to idle at 1000 rpm

OK ...set the engine to 1000 rpm

2. I assume you've got two mixture screws.

OK ..put them in sinc with each other (same # of turns out - DONT SMASH THEM TIGHT WHEN YOU HIT BOTTOM - use a light touch -try 2 -2.5 turns out from a "gentle bottom")

3. Screw the mixture screw CW until you see a drop of 100 rpm on the TAC.

OK ..screw one screw in .....half turn ....quarter turn until you see the 100 rpm drop.

4. Reset ilde to 1000rpm

Ok....reset the idle

5. Screw the OTHER mixture screw CW until you see a drop of 100 rpm on the TAC.

OK ..screw the other screw in .....half turn ....quarter turn until the idle drops 100 rpm

6. Reset the idle screw to idle at 1000 rpm.

Ok ..reset the idle to 1000 rpm

7. Repeat Steps 3 - 6 until you can't drop the idle that 100 rpm!

This part is easy enough .....

8. When that's finished screw each mixture screw CCW 1/2 turn.

Still looks straight foward.....screw both mixture screws out 1/2 turns

9. Turn the ildle screw down to 850 rpm for a AT (in gear) or 750 for a Manual transmission.

Now here you might want to modify the # a little if they cause you trouble (launching yourself when you put it in gear and such) ...but I can't see that happening @750-850 rpm. If it does REINDEX "your" numbers so that they work.

Your idle speed screw should be barely opening the butterfly of the carb ....not almost all the way screwed in.

Like Ozark said're going to have to tune it by ear've got a bastard carb on an engine of questionable potency and you may have to stray from the "norms" here.

JoeyCJ55 06-29-2002 09:03 AM

Re: Adjusting carb (continued)
ok i'll do that one more time.
I've done it twice with similar results, and I guess I'll just have to deal with dieseling.
The problem I have with thatmethod is that I can always drop it that extra 100 rpm, so my setting seems questionable.
I think I'll just do that, and get a smooth idle..then adjust everything out another turn or so to kill the dieseling.

You are supposed to recheck the timing after you hook the vac advance back up??

GeeAea 06-29-2002 10:12 AM

Re: Adjusting carb (continued)
You set your initial timing with the vacuum advance plugged. It may or may not increase when you hook it back up depending on its vacuum source, emission related controls, and other variables. If you had a manual for that engine it would probably give you a spec @ an rpm to check the advance of the distributor w/o the vac adv.

If it diesels keep turning down the idle a little at a time. If you can't get it to stop and still idle may have secondary thottle bore/ butterfly alignment troubles. Usually the dieselling, or afterrunning, can be remedied by just turning down the idle a little.

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