Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: RMA on July 07, 2023, 01:11:49 PM
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Model: 7040251
1970 Cutlass SX 455 TH400. Rear is probably a 2.56 or 2.78. Complete stock rebuild last summer (200 miles) except 0.03 overbore and comp cam hyd. flat #42-114-3 (so called factory muscle). LSA 113, Intake/exhaust dur. 308/308, Int/Exh Duration at .050”: 233/233, Intake/Exh. Lift: 0.296/0.296.
After rebuild Poor performance, idled in park ok, idle dropped in gear & continued to stall.
My carb guy purchased Cliff’s book and parts.
Changes made - Parts from Ruggles:
Idle Tube .039
Idle Down Channel drill .059
Upper and Lower Air Bleed .070
Idle Bypass Air drilled .110
Mixture screw through hole drilled .090
Main Air bleeds plug & drill .040
Primary jets .070
Primary rods .026, .048, .061 (was .026, .038, .062)
Secondary rods .045, .084, .132 (was .053, .102, .134)
Secondary air valve ¼ turn from zero
Power valve unstuck and new “red” spring
High flow needle & seat, new cam on secondary air valve
After above changes very slight improvement. It will idle in gear if idle set to 850-900 rpm in park. Initial timing = 12 BTDC (tried 8-14, 12 seems most stable). Running rich, black soot in pipes, burns your eyes. Manifold vacuum at idle erratic 10-14 inHg (advance plugged). Checked for vacuum leaks multiple times w/ carb cleaner, none found. A/F mixture screws very little to no effect. Secondaries will not come on (due to lack of vacuum?). Also as rich as I’m convinced it is, it still seems to gain a few rpm if I hold hand over the carb. PS – it ran smooth as silk/great acceleration (broke tires loose 1-2 shift) prior to the engine rebuild/carb work! Any advice is much appreciated. Thank You.
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What is the static compression ratio of the engine?
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Engine builder estimate is 9 to 9.5:1, running 93 octane. Thanks
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Is the cam degreed?
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Yes.
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Just for yucks try about 20 initial.
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I did try up to about 16 degrees, (eyeball it Pointer stops at 14). Idle got worse. I could try again with a dial back light.
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Your main problem is the cam selection. Not enough compression for that much duration even in the big 455 engine.
It's going to want a LOT of idle fuel, bypass air and initial timing to be happy.
The exact same engine build at 10.5 to 1 compression would be making a LOT more vacuum and pretty happy with that cam, but even then it wouldn't be making a lot of vacuum at idle speed and still require some help from the carbs idle system to be ideal......
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Thank you for the feedback. Any suggestions on initial timing? Also, any ideas why the secondaries will not kick in? I have full travel on the throttle linkage. Thanks
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Bumped the initial timing to 20 she seems happier. Mixture screws out about 6 turns in park Idle is 750 and drive around 600 which is an improvement. Check on vacuum tomorrow. Hoping for better weather and take a drive. I appreciate all the feedback! Thank you.
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Do you have a limiter so you don't have too much total?
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No.
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Third post I responded to on here this morning where folks have too much camshaft for the CID and compression ratio of the engine so it's liking, wanting and responding to a BUTTLOAD of timing at idle speed.
Running the initial timing clear off the scale plus a LOT of idle fuel and idle bypass air to make the engine happy tell you the cam is bleeding off too much cylinder pressure at idle and low RPM's.
The fix is more timing and more idle fuel, although more of a "crutch" than a fix, that's what has to happen with these things.
Here is a basic "rule of thumb" to tell you if your cam selection is about right for the engine parameters. Set the initial timing to 10-12 degrees. With the engine fully warmed up lower the idle speed to about 700-750 rpm's and pull a vacuum reading on it. If it isn't makes at least 12-13" vacuum you need less overlap or more compression, or a little of both.
Many engine builds I end up custom tuning here can't even get down much below 900-1000rpm's with that test, so the work begins. I end up going into both the carb and distributor to provide more initial timing, less total timing with a shorter mechanical advance curve, and have to modify the VA as well to get the total timing at light engine load where it needs to be. The carb is going to require more idle fuel and idle bypass air as well.
Although the end results will be 100,000 times better than when the vehicle showed up here it's still more of a "crutch" fix for a fundamental issue of having too much overlap and/or late intake closing for the static compression ratio of the engine.
Not saying it in a bad way, that's just the laws of physics with these things and how we fix them. Besides, who wants their new "high performance" engine build to idle like the 305 engine in a 1987 Chevy pick up truck?........FWIW.....
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Cliff, thank you for the advice. Unfortunately my engine builder seems to lack your level of expertise at least with this engine. My advice for anyone needing engine work, do your research. Of 5 builders I met with this one seemed the most knowledgeable/experienced. Thanks again.
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It's really not their fault. I wasn't hitting "home-runs" with these engines right out of the gate.
However it didn't take me too long to figure out that bigger camshafts REQUIRE more compression or you only degrade idle quality, hurt throttle response, kill off low end power and push power up higher in the RPM range.
To offset the losses one needs to INCREASE the static compression ratio apprx one full point for every 10 degrees added duration. This helps keep things happy at idle and greatly improves street manners, and much easier to tune without having to run the timing clear off the scale at idle speed and add a buttload of idle fuel to the mixture screws.
You also have to keep in mind that there is so much miss-information out there with these things one quickly gets confused trying to build a high performance street or street/strip engine.
The biggest enemy of bigger cams in these engines is the idiot who came up with the 9.5 to 1 threshold for compression ratios for pump gas. That's just STOOPID!
Lowering compression then installing bigger camshafts is going backwards with these things and it NEVER works out well for the end user......FWIW......
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Thanks Cliff I appreciate the advice.
It's getting hard to find people that are knowledgeable and passionate about these cars. thanks
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You are most welcome.
It's better when I get involved with these things BEFORE the engine is placed in service, which seldom happens.
I've lost count of how many ill fated engine "builds" have been brought here over the years for custom tuning, or that I've build carburetors for to get them working well.
You'd think with all this good info out there these machine shops and engine builders would be able to make better decisions with these things, but sadly they do not......
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Quick update. I checked the vacuum in park its 10-12 and in drive drops all the way down to 6-7 in. Confirmed with the timing light the vacuum advance is actually retarding the timing in drive about 6-8 degrees. Seem typical for this combination? Has anyone had any experience with an adjustable vacuum canister? Confirmed with a hand pump the current canister starts to advance at about 7 in. Thanks
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If you are trying to use manifold vacuum to apply the advance it's a MUST to use an "adjustable" unit and you are going to have to set the "all-in" timing it applies BELOW the lowest vacuum reading you're getting at idle or it's going to drop-out some timing when a load is placed on the engine (trans placed in gear/vacuum falls off some).
Despite how popular that method of tuning is, it's more of a "crutch" when you simply don't have enough compression for the cam right to start with. As much as I've tried over the years I've never been overly happy with running the timing clear off the scale at idle speed to make an engine happy. Can it be done, yes, with enough persistence and making more fuel available to the idle mixture screws you can get it to work OK with those methods.
Also keep in mind that the carburetor is equally as important in getting a good end result when things are less than ideal for CID/Cam/Compression. Engines making low vacuum at idle will need more idle bypass air, and more fuel available to the idle mixture screws......
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I'll look into an adjustable canister. I currently have the idle mix screws out 5 to 6 turns. Sorry I'm still learning - what's the best way to increase my idle bypass air...? Thanks
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All of that information is in my book, with pictures....
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Ok. Thank you.