Author Topic: Qjet 17057253 on a 380 Olds stroker  (Read 306 times)

Offline Seff

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Qjet 17057253 on a 380 Olds stroker
« on: November 24, 2018, 02:21:00 PM »
Car: 72 Cutlass
Engine: Olds 380 stroker 9.5:1,  Ebrock performer RPM intake, stock 7a heads with 2" intake valves and a bowl blend port job. HEI connected to manifold vacuum.
Cam: 221/228@.050 on a 112lsa, hydraulic roller cam.
Idles at 13" at 750 RPM.
Carb: 17057253 rebuilt by the third recipe. Soft PP spring.
5 speed manual transmission.


Symptoms: (With APT at 2.5 turns up from bottomed out)
Cruises lean (16-18:1 AFR), both in 4th and 5th gear. More so in 5th, where vacuum is lower due to lower RPM.
Gets rich (12:1 AFR) when engine braking/letting off the pedal.
When going up hill and maintaining speed (as in, a slight increase in throttle opening), the engine leans out.
The opposite, going downhill with less throttle opening, fattens up the mixture.

Symptoms: (With APT at 4 turns up from bottomed out)
Cruises at a perfect 14.6-14.8:1 AFR.
Leans out worse when going up hills
Even fatter when engine braking


My idea is that the jets are too small, forcing the APT to keep the needles too far up to maintain a good cruise AFR - this makes the PP have too little travel to work properly - this means the needles can't bottom out under high vacuum during engine braking, and that they can't rise enough to fatten properly at a moderate throttle increase.

Am I totally off?

Thanks.


Offline Frank400

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #1 on: November 24, 2018, 04:05:44 PM »
Numbers are one thing, but the question is: Does it run good ?

Leaning on slight acceleration (power piston still down) is normal from what I've always seen, at least on all the Qjets I've built, all of them dual main air bleeds like yours.  It's something you never notice until you have an AFR gauge.

  Out of curiousity, what size are your jets and primary rods ?  Have you measured your main air bleeds ?

   What is your "heavy load" AFR ?  while staying on the primaries.

As for the richening while deccelarating, two things that I can think of; throttle opened a bit too far at idle, exposing a little too much of the transfer slot, where it can pull lots of fuel on decceleration.  You can give the motor the air it needs at idle by enlarging the idle air bypass and closing the throttle a bit with the idle speed screw.

   Not pretending to be an expert, but third recipe for such a cam at 112 lsa might be a little on the rich side ?

 
« Last Edit: November 24, 2018, 04:27:12 PM by Frank400 »

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #2 on: November 25, 2018, 01:28:08 AM »
No, I don't feel it runs right.

MABs are 0.070".

I built it to the third recipe based on the vacuum at idle.

Heavy load AFR is up there in the 16.8-18 range.

73 jets and 44 tapered rods.


I have idle bypass air currently, though it's not terribly large.

Offline Cliff Ruggles

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #3 on: November 25, 2018, 05:01:07 AM »
I've built hundreds of 1976-1989 Olds carburetors with that jet/metering rod combo with perfect success, so suspect you may have other issues.

Float level is a big player, as is fuel pressure and size of the fuel inlet seat.

I don't like or use manifold vacuum to the advance for most well thought out engine builds and you need to look at how much timing it's adding and how strong the spring is in the vacuum unit.

Might be dropping out timing as you increase throttle angle and engine load which effects engine performance in the "normal" driving range.

Also look at the mechanical advance curve, should be adding nothing at idle but start right off idle and all in by about 2800-3000rpms or so.

For the later Olds units I seldom tough any of the airbleeds, so if you opened up the lower MAB you may need to add a little more jet to make it happy but it still wouldn't be nearly as far off anyplace as you are seeing with a .070" bleed in that location......Cliff

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #4 on: November 26, 2018, 10:52:18 AM »
Float and seat adjusted. Throttle blades are nigh totally closed at warm idle, do I dobt think the transfer slots are active?

Timing starts at 13 btdc at idle and totals at 36 at 3000 RPM. Vacuum adds 18 degrees.

Will test now.

Unrelated question: I live in a cold climate, and drive year round. Would hot idle compensator help my cold carb drivability?

Offline Cliff Ruggles

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #5 on: November 26, 2018, 11:17:31 AM »
HIC is for really hot climate/conditions where the engine is so heat soaked that it needs some additional air to speed up and help cool things down at idle, like stuck in traffic, for example.

In a cold climate you need a working crossover in the intake manifold, hopefully it's open.

Where is the float set, what size fuel inlet seat and how much fuel pressure?....Cliff

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #6 on: November 26, 2018, 01:05:30 PM »
Crossover is blocked. Aluminum intake ought to have decent heat transfer, once warmed up?

Float set to 1/4", fuel inlet seat is .135, using a factory fuel pump.

The changes to float and inlet made no difference to the issue. Would a vacuum leak cause such problems?

Offline 77cruiser

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #7 on: November 26, 2018, 02:52:51 PM »
How cold of a climate? Should have the cross over open & hot air off of the exhaust going to the air cleaner. If you only drive it down to freezing or so it'll be good but if colder, maybe not.
Jim

Offline Cliff Ruggles

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #8 on: November 27, 2018, 03:43:27 AM »
Unblock the crossover and see if things improve some.  Complete waste of time to run a cold intake on a dedicated street car.  It's a wet-flow system and the heat is your friend with that sort of thing.  Even GM used a heated intake on their TB engines well into the 1990's.

That will help your scenario, but I still think you have other issues.  Right to start with the A/F ratio should go rich with increased throttle angle as you change the pressure differential above and below the throttle plates.  By design the carb will fatten things up and the power piston raises to the highest position.

Make sure you have the correct length primary rods in it even if you got them here.  One of my hired help some time ago tossed an entire order of 44B metering rods into the bin with our 44 late model tapered rods.  A few sets went out before we caught the error.

Also verify metering rod height in the jets and make sure the PP arms are level and exactly even.  Every single time we get a carb in here with similar issues the hanger arms are bent up, down, and never even.......Cliff

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #9 on: December 05, 2018, 01:24:30 PM »
I haven't unblocked the crossover, as the passage in both heads is filled up with poured aluminum. The temperature of the heads after a test drive was 182-185 F, the intake under the carb was 142 F, and the carb body itself was 125 F.

I measured the primary rods to 2.400" precisely, end to end.

The arms of the power piston were the same distance to the bottom of the power piston, so I assume they're straight as well.

Rods sit with the tapered part in the jet.

While looking for vacuum leaks, I noticed that idle picked up when I pulled a small vacuum hose. I got the engine to idle with 16"@850 by leaning out the mixture screws a quarter of a turn, but I couldn't replicate it after having turned the engine off and on again. I once again had 16" after having test driven, just when coming to a stop at a roundabout/traffic circle.


During the test drive, I noticed that the car would run at around 14.6-14.9 when cruising. When climbing a slight incline, or increasing speed, it'd lean out to about 15.5-15.9. This corresponds with a weaker vacuum signal compared to cruise. Adjusting APT moved the cruise AFR, but not the hill climb AFR.

Having found nothing wrong with rods or PP, I swapped the green PP spring for a light blue PP spring, but didn't have time to test drive.


Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #10 on: December 06, 2018, 05:58:44 AM »
Seems like I've been banned in the mean time.

Offline 77cruiser

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #11 on: December 06, 2018, 07:09:25 PM »
Is this a dual main air bleed?
Jim

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #12 on: December 06, 2018, 09:08:36 PM »
Yes, I believe it is.

Offline Cliff Ruggles

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #13 on: December 07, 2018, 03:08:49 AM »
Nothing wrong with seeing A/F ratios in the 14-16 range at light part throttle and even with a light load climbing a grade as long as the engine doesn't go "flat" or loose power. 

At some point engine load becomes great enough and throttle angle increases enough where vacuum falls off to the advance and the power piston raises up to richen things up.

When this happens you should see the A/F ratio quickly drop some and no lag in engine power.

If you have places in the engines power curve where you increase throttle angle and it goes lean, surges, hesitates, etc, then some additional tuning is needed to richen it up in that range or the carb has a fundamental issue someplace.

I've tuned many hundreds of the later Olds Q-jets and never once had any issues in the areas you are seeing problems with.  We usually end up with 72, 73 or 74 main jets in them and use the APT to fine tune part throttle A/F accordingly.

I actually use a 1977 Pontiac Q-jet on my own engine, basically identical to any 1976-1978 Olds Q-jet with a different airhorn and it has been flawless for decades.  It's also been on 4 different engines with many changes to those engines taking the power level from about 350hp to almost 600hp, and still using the exact same jet/rod combination on the primary side.

My engine doesn't have a working crossover in the intake either so in cool/cold weather it takes a few minutes to heat soak before it's completely happy.......Cliff

Offline Seff

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Re: Qjet 17057253 on a 380 Olds stroker
« Reply #14 on: December 07, 2018, 08:19:16 AM »
Cliff, I remember you commenting on your combo before... And make no mistake, I'm grateful for any and all comment!

I think at this point it's prudent to take the carb apart again and measure all parts again, to eliminate the factor of human error when I was building the thing. That's a project for tomorrow.

Would you still recommend third recipe idle system modification? Or take it back to stock?