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Old 08-14-2020, 02:52 PM
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Default Carb or vacuum issue???

A little back story

I installed a brand new 468 in my 70 Judge this past spring. Stroked crank, HYD roller( old faithful), Iron heads(ported 13's) stock intake, Rochester Quadra jet (750cfm) ...Build is 9.5 CI and I run 91 to 93 octane gas. Timing is at 13 initial with 34 all in. I am running a ready to run MSD dizzy and I have about 15 inches of vacuum at idle (750 rpm) It has run flawlessly for 3 months until Tuesday when I decided to go for a ride after supper. BTW it is a 4 speed.

Started it up, (no issue as the car had run on Sunday) backed it out of the garage and headed out. Tried to kick off the choke (about 1000 rpm ) and no go. Took it for a jaunt on the highway up to 4000 rpm and came back after 10 minutes. Oil pressure and temps were normal. High idle remained.

Let the car cool down, pulled the ram air pan and air filter off. check all vacuum connections , all ok ....moved to tighten all four carb bolts to manifold. I cant discern any manifold leaks, no fuel weeping or leaking.

Then it got funny.....I then took a screw driver and tried to lower the idle on the carb.....Did Not Work. Set screw can go all the way in and out and the idle remains high. Unhooked the electrical line to the choke....Nothing. Car will start easily, runs fine and shuts off with no back fire. The only thing I have noticed is there is a stumble if you try to stab the accelerator whether on the peddle or from under the hood.

This carb is 3 years old and has never been an issue.....

Is it dirt or something else ? I am thinking I will take it off and blow it out with air and check the jets and rods....

Kinda weird

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Old 08-14-2020, 03:08 PM
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Curb idle screw wont slow it down if the throttle shaft is on the fast idle cam.

Choke un-plugged or not openning all the way would have the front of the fast idle cam raised up.

It's right there behind the pull-off, in between the choke stat housing and carb body. Try pushing it down with your finger.

Clay

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Old 08-14-2020, 09:52 PM
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thanks....I will give it a try

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Old 08-15-2020, 03:27 PM
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My brand new QuickFuel Slayer does the same thing.

I can let it completely warm up and with the idle speed screw set to around 500 rpm it idles in park all day and seems normal when you whack the throttle. When you drop it in gear, it is a touch too slow add wants to stall in gear. If I do an 1/8th turn on the curb idle screw, it jumps straight to around 1500 RPM. Turn the idle screw back an 1/8th turn and it drops back down right to 500 RPM with nothing in between. It's like an on-off switch. When it went to the fast idle, I tried pressing the fast idle cam down thinking it was just catching and sticking on fast idle and it doesn't drop down. This has me stumped also as it is doing this fully warmed up and off the fast idle cam.

I think I've got to increase the idle air via the secondaries and see if that will solve it as something weird is going on with the throttle tip in off idle also. I have to feather it to keep from stalling and then I get an abrupt "wake up" as it gets past the stumble.

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Old 08-15-2020, 05:55 PM
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Quote:
My brand new QuickFuel Slayer does the same thing.
Is that a Qjet? Holley? Other?

How many turns out is the curb idle screw?
Sounds like it's out of its range?

02Hawk, electric choke?

Is the choke blade actually off? (and the lever controlling the fast idle cam working correctly?)

Sounds like the throttle is on the choke cam. The hesitation can be a few things, accelerator pump, secondary air valve spring setting, etc.

Could even be the throttle blades are hanging on the carb base gasket?


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Old 08-16-2020, 01:31 AM
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Hi John: You need to make sure you have 12 volts on the wire for the electric choke. As was mentioned, it sounds to me like the choke is stuck on. You may also need to set the tension on the choke. Come by tomorrow and I'll set it for you.

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Old 08-16-2020, 02:19 AM
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Quote:
Originally Posted by johnta1 View Post
Is that a Qjet? Holley? Other?

How many turns out is the curb idle screw?
Sounds like it's out of its range?
It's a QuickFuel Holley style w. vac secondaries. I'm going to bump the curb idle screw again so it jumps up to the 1500 rpm idle and take the carb off the see where the throttle are plates are in reference to the transfer slots. The fact that it it goes straight from 500 rpm idle to 1500 rpm with just a slight turn of the screw puzzles me as the carb responds normally to mixture screw adjustment and is set at around 1-1/2 turns out (max vacuum w. gauge).

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Old 08-18-2020, 06:59 AM
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Any time you experience high idle and lack of control of the idle speed and have ruled out any vacuum leaks, or sticking/stuck throttle plates check the timing.

I get a LOT of vehicles brought here with similar issues and often there is a little "creep" in the timing curve allowing some timing that doesn't fall out when the engine comes back or tries to come back to idle speed. This makes idle tuning and engine speed control difficult if not near impossible.......Cliff

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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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Old 08-18-2020, 10:02 AM
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Quote:
Originally Posted by Cliff R View Post
Any time you experience high idle and lack of control of the idle speed and have ruled out any vacuum leaks, or sticking/stuck throttle plates check the timing.

I get a LOT of vehicles brought here with similar issues and often there is a little "creep" in the timing curve allowing some timing that doesn't fall out when the engine comes back or tries to come back to idle speed. This makes idle tuning and engine speed control difficult if not near impossible.......Cliff
Agree, Timing and how the advance curve is set-up can cause lots of "ASSUMED CARB ISSUES". The use of very soft Mr Gasket "HOT ROD" springs being one of them. Touch the throttle and you get a big leap in timing. (engine rpm)

Tom V.

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Old 08-18-2020, 07:56 PM
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In some cases maybe, but I would have to say not likely in mine. Car ran without issue with the previous carb right up until the swap. My problem started the moment the new carb went on.

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Old 08-19-2020, 05:59 AM
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I absolutely LOVE those cheap weight/spring kits folks have been installing for decades because I get paid to remove them and put them in the round file (trash can). I haven't seen a single one to date that works as advertised. For some reason that I will never understand folks are trying to get ALL the timing to these engines very early in the RPM range. Actually the better job you do at choosing engine components/parameters, the LESS timing and fuel the engine will need to be happy.

It's one of those things where old habits are hard to break. Despite the year being 2020 and lots of good info out there on timing requirements for these engines there are still folks "stuck in the 60's" that think these engines need the vacuum advance removed and super-light springs used in the distributor to get the timing "all-in" by 1500-2000rpm's. Even some very knowledgeable engine builders still do this, then wonder why they have to tune the carburetors to bring in a LOT more fuel in the "normal" driving range.......

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Old 08-19-2020, 09:37 AM
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Really would lean toward the choke mechanism, or secondary linkage causing the problem. Also, take a good look at the new base plate gasket to throttle blade interference for possible hanging up and holding the secondaries slightly open

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Old 08-19-2020, 11:51 PM
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thanks guys........at the end of the day, the choke was set a little rich, backed it off a bit and it is running perfectly again

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Old 08-20-2020, 12:53 AM
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I wish mine was that easy but it's not. Had a lean backfire pulling it back out of the garage yesterday. Never done that before. Throttle plates exposing the transfer slots so will have to add idle air at the secondaries. Will also change to 5.5 power valve as it is super fat at idle.

I haven't had a chance to do anything but inspect at this point. Trimming out a buddy's condo he's selling and he's being completely anal expecting fine casework quality on baseboard and shoe that's going to be caulked and painted anyway. Aye cabron ... never volunteer.

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Old 08-30-2020, 03:17 PM
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Well, I found my issue ... operator failure. I was thinking " QuickFuel bulds these carburetors for the sole purpose of installing on modified engines with headers, intake and performance camshafts. I have a mild cam (234/242@.050/108 HFT, why should I have an issue?" I thought, what have I done that is out of the norm?" Then the light went on.

I had coated my polished carb with SharkHide aluminum protectant and did not fully mask off the air bleeds. I left the plastic hat on which loosely covered the main body but apparently nto tightly enough to prevent overspray from getting into the air bleeds. I put a couple drops of lacquer thinner on them and blew them out with compressed air.

Runs like a clock again. Cooler heads prevailed. It runs much stronger than it did with the Holley Street Dominator, idle to part throttle transition is smooth as glass.

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Old 08-30-2020, 05:07 PM
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I have fixed a few other Holley carbs that had some form of spray coating added to the carb external surfaces. They also had blocked air bleeds.

In those cases I ran the proper drill bits thru the coating and then cleaned the carbs properly. They also ran great afterward. Some of the Corvette Carbs and Mopar Carbs
for the 6-pack installations seem to get that "spraying" by builders (who will not do the proper cleaning and re-dichromate coating" process.

Tom V.

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Old 09-02-2020, 11:23 AM
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Took my '71 out for a drive yesterday. I can't believe how much stronger it runs with this Quick Fuel Slayer on it compared to the Holley Street Avenger it replaced. Where it used to light the tires up gradually (linear to throttle input) while rolling into the throttle in low, it is almost futile to stand on it hard in low now due to wheel-spin. There is absolutely no hesitation on secondary opening, just a huge power build like you flicked on a nitrous bottle. Kickdown to low at 30mph is a violent sideways adventure and slowly rolling into second around 30 mph without the kickdown still spins the tires at will.

I'm really liking this carburetor.

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