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Discussion Starter #1
I'm having trouble getting my q-jet to idle properly, and had the same issue with Edelbrock on this motor. It is a GMPP 350/290hp motor, which I understand really has a cam mismatch for such a low CR motor - 8.5-1,

So the lope is awful.... I love the rebuilt q-jet for running down the road, but it still idles awfully (no vacuum leaks, checked that out, timing at 12*)

SO... I'm thinking maybe everyone that says EFI is the way to go are probably correct.

Looking for recommendations on aftermarket EFI systems for my sbc. Anyone have first hand experience?
 

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Is your q-jet a Rochester or Carter?? Carters aren't worth the powder to blow them, as they were designed for Chrysler autos. Rochester is the ONLY way to go. And, for a Holley, about 600cfm would be about right.
 

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efi=$$$

holley has a nice system as do howell efi both are high dollar
go with a sean murphy q jet it will run ya about 300 bucks or so....
i promise you will never run anything else...:beer:
 

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It's a Rochester, but I think it has an internal issue. The idle is so erratic, set it to idle right in gear, then in Park its up to 1200 or even a little more. Seems like impossible to get a consistant 650-750 idle. Bring it down a little and it drops to 500....
I think you would be very happy with a new Holley.

I would never tell you not to go EFI, EFI is awesome, just much more expensive than a carb.
 

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I had one of the first Holley analog 2 barrel projection systems back around 1989', on a lightly modified 305 in a chevy monza. You tuned it with a screwdriver in the cab. Minimal hookup effort, improved entire output on my engine over quad or Holley 4160, improved fuel economy & smoothed out low end driveability.This simple , small (670 cfm) unit would choke out top end & not supply enough fuel on a large /high hp engine over a good carb, but did great in it's intended application, patricularly for improving driveability.
I still do a lil paying attention to aftermarket setups. Never heard much bad about any of Holleys. Would stay away from Edelbrocks, too many always for resale, hmmm? Howell has a good rep specializing in retro application of factory units & could tune computers for needed applications. Many of the new fast learning setups are probably good such as msd or FAST, but have no personal knowledge on them.
For a reasonable budget on your(modest) hp level, a holley 2 barrel would suffice & be the least expensive aftermarket, next up their 4 barrel, the ultimate their multiport setup. I'd also consider howell's stuff. http://howellefi.com/gm-tbi-products/complete-tbi-kits/universal-tbi-kits.html
If you're unsure about power froma 2-barrel system, Painless Tom's former Iroc Camino was 2-barrel efi'950 commander. It did OK.http://www.painlesstom.com/elcamino.html
The absolute ultimate in sbc EFI is a kit using the late model ls series coil per cylinder setup, sequential multi-port computer system & ram intake, but for the money, may as well swap out the engine to an ls.
Did I make your head hurt more than help?:dontknow:
 

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:dontknow: if it does that with 2 different carbs?? sounds like something else is causing it..:texas:
maybe a sticky pcv valve?
sticky mechanical advance?
leaky/sticky vacuum advance? are you running ported or full??
 

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Discussion Starter #12
Running full right now. Also ran no vacuum advance (per the GM instructions came with motor).

New PCV, New GMPP distributor....

There are some articles on a z28 forum that talk about GM using a 60's style profile cam, which works fine in higher comp. ratio engines.... and how they don't work good in 8.5-1 engines causing a very bad lope at idle (500rpm). I think that is the real problem.... but I don't know.

Still checking other items too

There was another member here with same motor that went to the cam from the 330hp motor and that was actually better suited.

BUT - I think I'll try another PCV .... its such a cheap part. And wouldn't that be a kick if PCV was actually the problem -:poke:
 

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Which Holley would you recommend - GMPP350/290hp - engine came from GM with too much cam for 8.5-1 cr.

I guess the final change/mod would be to break into the motor and swap out that overzealous cam???
It sounds like you strongly feel that the cam is the problem. I don't know enough about your engine combo to tell you that it is or is not, but changing the cam is going to be cheaper than a new carb.

If you decide to buy a new carb here are some recommendations on sizes. Unlike EFI, a carb cannot adjust to different driving conditions (Street/Race). For example a 400 cfm carb on your motor is ideal for driving to the grocery store and back, but a 800 cfm carb is ideal for wide open throttle.

5000 rpm: 430 cfm (Street) 557 cfm (Race)
6000 rpm: 516 cfm (Street) 668 cfm (Race)
7000 rpm: 602 cfm (Street) 779 cfm (Race)

I would go with a 700 cfm carb. The bigger the carb, the worse it will drive around town.
 

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I have the same engine in my 78. I'm using an edelbrock 1806 AVS thunder series. This is my second carb. First was the same but wouldn't idle and adjusting the mixture and idle screws had no change. So they exchanged it under warranty. This carb is a bit thirsty at 650 cfm. Had the same lopey idle and sucked gas like it was free. I advanced the timing until just before it pings on regular gas. The lopey idle is gone and I gained about 2 mpg's highway. I'm not recommending an edelbrock carb just passing along my results. Another thing with the edelbrock carbs is the GM factory fuel pump puts out too much pressure. So instead of using a regulator that has been shown to leak by other members, I used the the edelbrock 1721 fuel pump.
 

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Discussion Starter #15
I have the same engine in my 78. I'm using an edelbrock 1806 AVS thunder series. Had the same lopey idle and sucked gas like it was free. I advanced the timing until just before it pings on regular gas. The lopey idle is gone and I gained about 2 mpg's highway. I used the the edelbrock 1721 fuel pump.
Do you recall what you set your base timing at??
 

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All I did was loosen the dizzy and advance it until the idle smoothed out and rpm's increased. Then I just turned down the idle screw so it's not racing.
 

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I know you said the manual said not to hook up vaccum advance, but I would try hooking the dizzy advance up to a MANIFOLD VACUUM port at the base of the carb. I would then time the engine like Bobby78 said. MANIFOLD VACUUM gives your advance a stronger vacuum signal at idle which longer duration cams seem to like. After you set the timing to the smoothest idle you can get, go back and adjust your idle mix screws on the carb. This might help and best of all ITS FREE! Good luck and keep us posted.
 

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My dizzy is off the ported vacuum on the front of the carb. It's a summit HEI, nothing fancy.
 

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Discussion Starter #19
So I gather that we are setting timing by ear and feel of the motor running smoothly. I know the timing light method calls for plugging the vacuum, setting timing and then hooking up vacuum.

I'm gonna try ported vacuum too. Is that located higher on the carb than the full manifold vacuum?

Right now with no vacuum I'm running 12* base timing. I've run with both NO VACUUM and FULL VACUUM which both appear to have the same irratic idle characteristics.

The guy that build my q-jet wants to provide me a 600cfm Holley to try... I may take him up on that. His expertise is really with the Holleys.
 

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John
Ported vacuum is no vacuum at idle.
 
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