EightballZ

hey guys,

i need your opinion on this...

after the performance rebuild on the 455 (now 461) you can guess that the idle circuit of my qjet won't allow enough air to flow for a decent idle w/o exposing the transfer slots etc.

to get around 800 RPM i need to open up the primaries wide enough to get nozzle drip.

i already ordered a set of drills etc. to open up idle air bypass and all the other stuff (i.e. idle tubes, bleeds etc.)

my plan is to close the primaries enough to stop nozzle drip and start from there...like not touching the slow idle screw during the process of modifying the idle circuit.

good idea?

float is set to 1/4" ...no flooding or any wet spots around the carb. you can tell that it needs more air by pulling a vac line on the intake..idle gains around 100 RPM and is smoothing out

ELY442

You should contact www.cliffshighperformance.com. He knows his Q-Jet. Good luck.

EightballZ

thank you...i already have his book and literally read through the entire forums on his website. unfortunately he's kinda busy and it takes time to get a reply so i was hoping someone on here can help me out in the meantime

ELY442

I was on the phone with him last week about rebuilding my Q-Jet Edelbrock Performer RPM 1910. You have to call him early like the first thing he's open and he's really helpful.

Marx3

Cliff is the man!
I never understood what your initial question was though...
As far as I remember, the Edelbrock Qjets were set up pretty generously with large idle tubes and DCR. They also had pretty large idle bypass. So what turned out to be the problem?

EightballZ

its a stock qjet and the engine demands more fuel/air @ idle after the performance rebuild. i need to open the throttle plates too much to get a decent idle..but then its dripping from the primaries, cause it starts to use the primary circuit (due to throttle plate angle).

that is the problem...solution is to open up certain vents, tubes and so on to get more fuel and air into the engine while the throttle plate angle is low enough to not activate the primary circuit.

and the question was...is it ok to close the throttle plates to the point the where dripping stops and start modifying the idle circuit till you reach the desired RPM and vac? or in other words...for now i closed the throttle plates enough to stop nozzle drip with the stock idle circuit configuration (but only get around 500 RPM). is that the maximum throttle plate angle i can use for setting the idle even after modifying the idle circuit to increase fuel/air delivery...w/o running into nozzle drip again?

kinda tricky cause with the correct idle adjustments (not touching the throttle plate angle) you may gain RPM and vacuum. this could lead to nozzle drip again since you don't want the throttle plates completely closed.

dc2x4drvr

I've enlarged Q-Jet idle tubes on several carbs, start small and go from there, I've used bits as large as .033, but started smaller. Be sure to use grease on the bit, and blow out the passages after drilling.

83hurstguy

I've also seen each primary throttle blade drilled with a single hole for idle air by a very reputable q-jet builder. This is typically discussed as a mod for holley carbs, but it must apply for the q-jets as well. I don't really know anything more about it than that...

Marx3

Ohh I see. I dont know why, but I thought it was an Edelbrock 1910 Qjet. If so, it should not be giving you idle problems, as their stock calibration could feed a 455 with a pretty big cam.
btw drilling holes in the throttle plates is not the best way to modify a Qjet's idle system. SImply adding idle by-pass to the baseplate ( very easy ) is a much better way to go at it.


.033 idle tubes would be like the stock size for a Chevy 305 :-) A big displacement engine with a big cam wil propably need something like .038-.040 tubes, mid .050's DRC, .070 bleeds and about .110 by pass in the base plate.

droldsmorland

Before you start drilling check and adjust the float level and drop. With today's fuel having a lower boiling point and less heat tolerance compared to 1970 gas, you need to drop the level a tad below spec. I like my float drop a little lower as well to compensate on the other end. "Pull over" from the idle circuit is common with old level spec on today's gas. Tel-tale signs are, it runs rich and rough at idle. Try it before you go to full on modification. Dont cost nuthin.

Octania

TSB [Technical Service Bulletin] from the factory recommends 1/8" holes in 1968 W30 carb primary plates as part of a process to make it behave better. I have seen this on other QJet carbs which appeared to be factory issued.

On my 403, I followed the book and drilled the undrilled Idle Air Bypass channels to around 0.130" but that still did not seem to do the trick. It was not until I ran manifold vacuum to the dist'r that I was able to close the throttle down and still maintain idle and vacuum. Before that, the vacuum was like 5-7" at idle, which presumably popped up the power piston which made it richer which further killed the vacuum and then it would stumble and die. Open the throttle via the idle screw and it would eventually idle ok... at 1200... but not below. And by then it is taking fuel off the primary nozzles.

Proper timing setup made all the difference. It's a 78-ish Toro 403 with a 258? Comp cams stick, ~9.7 compression with #6 heads, and pre-computer HEI. Stock 78 Toro carb gone thru by me with both books handy. PS there was no gunk under the idle fuel transfer tubes and pulling them out was painful and gave no benefit. And, this carb was untouched for years outdoors [underhood] to the point where I had to extract the accel. pump with beasty pliers.

EightballZ

it already has idle bypass air. i can take the measurements later this week. i'll let you know.

i also heard that drilling the throttle plates is something that you do after modifying all the other stuff and still dont have enough air at idle....so one by one. right now its pulling around 15" @ idle but far from steady at 500 RPM. power piston is seating well.

heres the combo btw:

- 455cui .030 over
- Stock Qjet 7040252: .071" jets and .043" tapered metering rods, "DA" secondary metering rods, .135" needle/seat (cliff sent me that configuration)
- CR 10,25 : 1
- Stock points ignition - recurved: 14° initial
- Edelbrock alu heads - intake 2.125 exhaust 1.625
- Eagle H-Beam Connecting Rods 6735O3D
- Autolite Racing AR 3911 spark plugs
- ICON Premium Forged Pistons IC886-030 + Rings
- Comp Cams Custom Ground hyraulic roller cam 232/236@.050 on a 114 - advanced 8°

Marx3

I recently rebuild an early 70's Buick 800 cfm unit for a guy that har a 1970 Buick 455 ( cr around 9.5:1 ). It had a 235/241@.050 on 110 LSA cam in it.
Specs for the carb were: .040 tubes, .055 dcr, .110 bypass, and the air bleeds were stock for the application. Can't remember their size, can check it in a couple of days, but they were propably around .060-.070 both of them.
The engine idled great, pulled about 10" hg and the engine ran flawless. Your roller on 114 lsa should help lift the vacum level, I guess. Btw 15" at 500 is great for that big a cam, I would say, though I am not exactly a cam expert at all.

oddball

It's idling at 500? Holy smokes!
I drilled and tapped the idle bypass holes in the throttle body to accept 1/4" set screws. Then I can drill the set screws and just swap them in and out to try different sizes. Way easier than trying to put material back if you go too large.
I've always had to make a change and see where the idle settings lie. It's a time consuming process.

EightballZ

i would not call it an idle with 500 RPM ...that's what i get when i close the throttle plates to the point it stops dripping fuel off the mains...rather 550 than 500..but you get the point.

cool idea to play with set screws..i might give it a try

Marx3

Haven't your measured any holes in the carb yet?
I can't believe the carb can't be set up to make your engine idle perfectly.

EightballZ

Still waiting for the drills to take the measurements. Gonna pull the idle tubes on sunday maybe.

IIRC those qjets got a pretty lean idle setting when it comes to all the vents, tubes and holes used as the idle circuit. You can play a bit with the idle mixture screws to increase fuel delivery but to get more air you are kinda limited..

Marx3

What is the carb number? Edit: oh never mind, I re-read the threads:/) I don't know how generous Olds calibrated their carbs I 1970, but my guess is it could be okay as it is, just needing minor adjustments.

Yes you can turn the mixture screws out, but the available fuel will still be limited by the size of the idle tubes. The APT setting will also effect the idle quality some.

Gary M

On mine the idle fluctuated up and down (455 w ported and blended heads, mild cam, under 10 compression pistons. The mechanic drilled out my idle circuit a little at a time till he got it to an acceptable idle. He said you can go too far so he drilled and ran until he got to the sweet spot. On mine it was 40 thousandths that worked but it varies per your setup.

EightballZ

so its a 7041250 qjet. from what i've learned from cliffs forums is that they came pretty lean on the idle circuit to meet emission standards

reply from Cliff:

"It will need larger idle tubes, DCR's, and some additional idle bypass air for that cam. Leave all the idle and main airbleeds stock.....Cliff"

EightballZ

all right...today i opened up the DCR's and idle tubes just a bit...and gone bigger with the idle bypass air. it really had an impact on the idle. now i have a steady 800 RPM idle w/o nozzle drip.

Marx3

Great! So what sizes did you go to?

EightballZ

idle tubes went from .035 to .039 and DCR's from .047 to .059.

idle bypass air should be around .073 for now

idling around 16" vac

however ..if i pull a vac hose RPM its still increasing by 100 RPM in idle...but starts shaking. maybe i should open up bypass air and DCR a bit.

but the effects so far are great..no more nozzle drip, placing the transmission in gear won't effect idle quality dramatically and throttle response improved.

Marx3

.073 bypass aint much at all. I Think about .100 would be even better. I would up it some more a bit at a time.
I don't think opening up the dcr would have a positive effect. .059 is pretty huge.

EightballZ

Sorry i mixed that up ..idle bypass air is around .095 now

Marx3

Oh okay:-) i don't think you would benefit much from going larger on the bypass. 15" hg is pretty good:-) bless the roller and the 114 lsa

Rocketbrian

I've been following this thread with interest, however, I'm unfamiliar with the letters DCR. What do those letters stand for and where are DCRs located and their purpose? Thanks.

EightballZ

DCR is down channel restriction:

http://image.highperformancepontiac....buretor_tuning

Marx3

Afaik the purpose of the dcr is to control the vacum signal the idle tunes sees.

Octania

I had the same problem following this discussion, the TLA's were not explained. You should always define your TLA's when first using them. What with cut and paste available nowadays, you could even easily replace all the "DCR" instances with "Down Channel Restriction." Or at least one. The link to the photo helped greatly.

I have read the books and all that but my memory could not recall what the heck a DCR is.

TLA or course is a Three [or two] Letter Acronym. Very popular:
FBI
CIA
AFL/CIO
PDQ
WTH
LOL
CBS
etc.

Rocketbrian

Thanks for the explanation and the photo, helps a lot. I may have further questions as I get into this more. Nice to know I wasn't the only one struggling with the acronyms.

bw1339

Thanks to this thread I finally decided to modify the idle circuit of a QJ that I have on an AMC 360 that would just not idle without a lot of throttle.

The primary drip is about 90% gone and and it idles very nicely at low RPMs

Thank you!

EightballZ

nice to hear it worked out for you..

gonna post my recipe maybe next week..i employed the APT (adjustable part throttle) and need to play around with it.