PeteZ28

Hey I was wondering it anyone has tried this. Using an aftermarket carb with the original computer controlled distributor. I have an 84Z28 with 350, M5, ZZ4 roller cam (1.6 rockers on intakes) with ZZ4 intake, Edelbrock heads, headers, etc. It's currently running with the orig cc qjet but not as smooth or strong as I believe it ought. I have worked on it quite a bit and seems like its lean. I've gotten it to idle OK but it could run better.

I've been wondering if an aftermarket holley or Edelbrock carb would work while leaving in the orig distributor. I like the idea of a having spark knock sensor (and spending less money). I'm thinking a resistor can be put in the circuit to the solenoid and the TPS can be mounted to be actuated by the accelerator pump. I was curious if anyone has tried this.

thanks,
Pete

naf

iTrader: (7)

https://www.thirdgen.org/forums/carb...cant-done.html

five7kid

iTrader: (14)

That was a pretty crude effort, but it worked. I would get lean SES lights at 40 MPH cruise, which I never got to go away. Interesting the q-jet run richer under those conditions (as evidenced by no lean trouble code), but still got better MPG.

The rest of the story is I put the q-jet back on, tweaked the secondary tuning (choke pull-off had a small leak), and now the q-jet is just as consistent.

If you do change the carb, don't consider anything but a Holley double pumper. Anything else would be a downgrade from the q-jet.

You didn't say if your lean condition was from the primaries or secondaries. If primaries, it should be setting a trouble code. If secondaries, change the rods & hanger.

PeteZ28

Thanks for the help guys. It feels sluggish on the primaries. I'm still adjusting the AV on the secondaries (I used the B hanger and DR rods). Anyway, I figured someone else had tried that. I'm not in a big hurry. As you mentioned holley carbs are calibrated to run well on a mild perf engine without major tuning. I'm afraid with the cc qjet that opening up idle bleed and restriction circuits may be necessary to avoid that lean feeling low speed sluggishness when opening the primaries. I'm not getting any codes. SES light stays off. I figured I could eventually get the secondaries right. Not sure about he primaries.

Some other question came to mind. This is an original L69 car. Would the knock sensor be prematurely retarding timing with the change from 305 to 350? I think I used a new sensor for the L69.

I was wondering it the TPS could be adjusted on the qjet to bias the primaries richer, or would the O2 sensor compensate?

Five7kid I noticed your using the original dual snorkel air cleaner. Do you think the somewhat smallish air filter in there is a restriction?

Also, I'm using the stock fuel pumps, electric in back, mechanical upfront. Do you think they are adequate at WOT?

Thanks again for any input.

Pete

naf

iTrader: (7)

The KS could be pulling timing. The KS and ESC module are different for the 350 vs 305. Try disconnecting it. The L69 ECM shouldn't run the self-test for the KS that the later ECM's do.

Adjustments to the TPS, in my experience, only bring in timing earlier/later. The ECM sets primary mixture based on TPS and Vac sensor readings, then adjusts them based on O2, except, of course, at WOT.

The stock, single snorkel air cleaner is definitely a restriction for even the LG4.

Your stock pumps should be more than adequate, providing everything is operating correctly and there are no kinks/restrictions.

Have you adjusted for proper dwell on your carb with the change? Do you know it's going into closed loop? Timing set correctly/TPS voltage at idle near 0.40?

five7kid

iTrader: (14)

The air filter isn't smallish. The distance between the air cleaner top and carb air horn is the bigger issue. I found a top that sits up a little higher but still allows the hood to close.

PeteZ28

Thanks guys for the input. I recently did check the mcs dwell. At idle it was reasonably close to 30, although it would vary occasionally and come back. I'm thinking if it stayed in close loop dwell would not vary.

I haven't checked the timing or TPS recently, but I haven't driven the car much since I set it several years ago. I was going to check the timing and maybe advance it 2 deg. I set it at 6 btdc last time.

Yeah I kind of figured the O2 sensor would do the final adjust. on the fuel.

Thanks again for the info. Gives me a few more things to think about.

naf

iTrader: (7)

The varying dwell tells us it IS in closed loop. The dwell will start varying slightly as the ECM re-writes the blocks based on O2 readings. The ECM commands dwell based on TPS and Vac sensor readings then adjusts dwell for those settings based on O2. A simple 'learn'.

The biggest problem I have with the ccc-qjet is it's sensitivity at idle to a little bit of cam, especially tighter lobe separation. A mechanical carb could be tuned to run slightly richer at idle under the cam but the ccc-qjet is still trying to trim the mixture. Pull your TPS plug while it's idling and I bet it smooths out.

I'm running an XE262 in my '87 and find that I get my smoothest idle with less timing (0-2). It will idle in gear at 650-700 but still 'hunts'. I recently finished a build with a 350 with LO5 (truck TBI) heads and cam with '87 ccc-qjet and it is rock steady at idle. Really impressive daily driver, 'course it runs out of steam around 4000 rpm, but I've averaged better than 24 on two tanks so far.

Try adjusting your timing a little each way. You've got an '84 LG4? It shouldn't have come with a knock sensor and will have a less aggressive timing curve than the later models, hence base setting for yours will be higher than for my '87.

five7kid

iTrader: (14)

I'm running an '86 LG4 ECM w/knock sensor and an aftermarket chip (forget whose chip it is). Not a significant difference between the stock and aftermarket chip, but with the ZZ4 cam, 9.7:1 CR, 2055HKR headers, TPS & IAB set to factory, base timing at 10 degrees, it idles quite nicely at 650 RPM in drive.

naf

iTrader: (7)

10 degrees? Was base on the '86 (I'm missing that year) zero? I'm afraid to run that much with my vortec heads, in fact I experience some deceleration stumbling when I run it near 6 that I attribute to my lack of EGR. Base on my '87 LG4 is zero.

Pete's vehicle is most similar to yours and originally an L69 (not an LG4). More aggressive timing than the LG4, WITH KS. Not sure what the base was on the L69?

five7kid

iTrader: (14)

Base for the LG4 was 0 degrees, 6 for the L69.

I would get pinging with the LG4 with the timing advanced to 6 when the EGR ports were plugged up. Cleaning up the EGR passages (actually, installing the ZZ intake manifold) solved that problem and it ran fine with 6 degrees base until the 305 was souped up.

But, with the more aggressive ZZ4 cam and mile high altitude, the 350 has been fine with 10 degrees. I started with 6, stopped at 10, never tried 12. 85 octane pump gas to boot.

PeteZ28

Correct, the varying dwell indicates its in closed loop. Sometimes its too late at night to be on here. Yeah, I remember 6 deg advance base on the L69. Anyway, I tried disconnecting the KS without much improvement. It appears the choke is not opening all the way lately; fast idle would not dis-engage. That may be the reason for poor power the last couple of times I drove it. I think the air-valve is probably opening too early also. I'll look at these things before I move the timing around.

thanks guys for your input.