Thread: 1st lumpy idle build, and cards I've been dealt

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1st of all, I've read and searched for a couple of hours here, learned quite a bit, and looks like a guy can get set straight here. So with that, I'll go on.

I've built many kinds of engines, but this is my 1st experience with a rough idle cam.

Neighbor brings me his all stock 76 Nova Concours 350/350, and this pile of parts: comp cams 270h kit, w/ springs, and retainers, Edelbrock performer intake, 1 5/8" primary long tube headers, and roller rockers.

I pull the 882 heads, he takes them to the machine shop,no fancy valve job, had them use the new springs w/o dampers for cam break-in.

I put it back together with regular fel-pro, cam goes in straight up, comes out 1 1/2 degree advanced.

He is advised to start out with the stock q-jet, so ok.

Stock HEI, set 10 degrees initial.

Very cool sounding idle at 700 to 1000 rpm. But need that while in gear, of course. I eventually increase neutral idle to as much as 1600, but when he puts it in gear, it still gives up and dies.

I drop it in gear and get it moving, and it sounds like it gets a dead miss under load. Haven't gotten it on the street yet, as cam break-in is just now completed. Oh, yea, header mufflers and no pipes right now.

Now I've read here about matching cam with SCR, but of course I didn't have that opportunity. I've got the three key crank sprocket, so maybe I could improve things there if I had to. Don't see any vacuum leaks, and of course the stock q-jet is suspect. Could it just be lean? Neightbor has 2200 stall converter to put in, we're thinking that's a have-to.

I'm wondering how else I might tune to help this deal out.

any thoughts?


Thanks,

Rootsgroup

Welcome to here! Lots of sharp guys I've learned a lot from... My first guess would be too lean... That cam should idle fine at 800 in gear... Looser converter will help. If mix is right but still won't run could try advanced key way will give more low end at expense of top end (I think I got that right!). What is the compression?

Others will have better advice...

-Chris

Welcome - this is a great site and I think you'll find us fairly outspoken on everything from cars to "you name it"..

I think you have way too much cam for this application. The 882 head is not going to work. It is a low compression, open chamber head. The 270 degree duration and higher lift is too much for the heads. It will be difficult to drive occasionally, much less a daily driver.

Their web site offers the following description:
“Magnum™ Cam
These cams are very aggressive, designed for performance oriented vehicles where they will see little street use. Stock components are generally incompatible Changes are needed in rear end gear ratios, headers and the torque converter.

270H Magnum™ is a great cam for high performance driving in street machines. The 270H has a rough idle in small blocks and a noticeable idle in big blocks. This is the largest cam you can use with a stock torque converter. The 270H will not affect power accessories, but should be used with headers and a 4 BBL. carburetor. This is the perfect cam for cruising.”

You may be able to use the stock converter - but the 882 heads are your problem.

Just my two cents - others will chime in.

Good Luck,
Glenn

Motor needs minimum 9.5:1 static compression ratio to work with this cam. Agree with Denny, 2000-2500 converter. Ditch the Performer intake. Use RPM or Stealth. Agree with Glenn, there are better heads out there.
http://www.barrygrant.com/demon/default.aspx?page=28

http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html
http://www.4secondsflat.com/Spark_plug_reading.html
http://www.ngksparkplugs.com/tech_su...qs/faqread.asp

Originally Posted by glennsexton

270H Magnum™ . The 270H has a rough idle in small blocks and a noticeable idle in big blocks. This is the largest cam you can use with a stock torque converter. The 270H will not affect power accessories, but should be used with headers and a 4 BBL. carburetor. This is the perfect cam for cruising.[/I]”

If it can be used with a stock converter then this cam should idle in gear fine, and I think they are talking the performance aspect of when power turns on, not if it will idle. I think you have other issues, fuel, spark or vacuum. Check for Vacuum leaks, and ensure your PCV valve is working. Dead miss on one cylinder is mechanical or ignition, find dead cylinder and do compression test to see if cylinder is OK. I wouldn't blame the heads up front until everything else is checked out. Heads may not be optimum for combo, but people have been overcamming smog heads for years and the cars still run, there just dogs.

Wow, thanks for the welcome and the great replies.

Jetstang, yes, you've got the idea. Like skids72 said, if I see the need I could use the 4 degree adv. keyway just to keep from trashing the dcr so bad, have I got that right?

On the dead miss, I don't get the impression it's a particular cyl, but rather a problem of lean/poor velocity/atomization/no tailpipes. As I think about it now,just about everything is killing low end velocity, so that's my challenge.

I haven't worked with sb Chevys for years, so the cyl head info you guys talk about on here has helped tremendously. Owner tells me the q-jet's a 750. If's that's so, seems too big to me. Short term, the only thing that might happen is a swap to a 600 cfm Edelbrock. I'm thinking either carb will need some jet work for this combination.

This is a pristine Nova, 55K, and will be only used for occasional shows and cruises, so the "wow factor" of the idle is the main thing. The owner thought the 882's were a good casting. Oops. Cyl head swap is an option long term. So kinda planning carb options around that....

Make sense?

Keep the Q-jet. It's the best design ever devised. The tiny little primaries allow excellent throttle response and mileage, while the huge secondaries feed the motor when you step down on the loud pedal. Best of both worlds. Get some books or DVD's and learn how to tune it. You'll end up with the best carburetor there is and add a little to your own expertise.

Cliff Ruggles is one of the Q-jet gurus that I would recommend for information.
http://www.cliffshighperformance.com...history_1.html

If you Google Q-Jet you'll get more people that actually use the carb every day, and will tell you what to fix and to make it a better carb.

I have run many Q-Jets and tuned them. With the cam you have you'll have to drill out the air mixture screw holes to get a better idle, drill them 1 drill bit size at a time, till you get a good idle. Use number drills, not fraction size.
you'll need to epoxy the fuel well plugs under the bowls to stop dripping if they are, they will leak.

When you start to adjust, make sure you set the idle screw with a feelers gage in the front butterflies(and that the rear butterflies are closed), then only use the air mixture screws to get the highest vacuum, and to get the right idle rpm's. I would put your timming at about 16-18 deg's with a stock HEI, and without a 4 deg key(if you don't have one)

Guru's aren't always the best way to go. A lot of them only know what they read, no real hands on!

Pat

Made some progress today, had a fire yesterday. Turns out that sporadic dead miss is ... you guessed it, plug wires roasting on header pipes. New Moroso wires today. Still burnt thru two plug boots, this is like killin' ants. Got more boots and socks coming, so once we get good spark, we'll start in again.

If it was mine, I think I'd stick with the q-jet. It came on the car new. Neighbor (owner) had some good luck with an Edelbrock carb on a Camaro, so we'll see.

Interesting comment about 16-18 degrees advance. We took a ride this afternoon and it just acted like it was cryin' for more ign lead, I thought. With those smog heads, I was thinkin' it might have room for that.

Thanks so much for the q-jet tips, I knew about the wells and such many years ago and simply didn't remember! Had a Firebird 400 in my teens, tried a big Holley on it once, put the q-jet back on two days later.

So the Nova's tucked away 'til next week, neighbor's already asked, can we build a 383?

Sure, I said.

To be continued......

DennyW, no, not yet. But I will. I was already curious about that. but put it off until we get the ign wires sorted out. Man I don't remember header heat being such an issue. Wires 6 & 8 were about 3/8" away from header pipes, and actually burst into flames! (stock wires). Glad I had an extinguisher!

Neighbor is going to invest in some header wrap as well. Is that the norm for keeping underhood heat in control these days?

For things to get that hot, almost got to think it's running very lean, or timing is retarded....?????

A rich condition will cool the motor, lean makes pipes glow cherry red... 3/8' is more than enough room for wires, lean...
Good info on the Q-jets, wish I was around you guys years ago. I have built good and bad Qjets, bad ones are what I remember and I didn't have a forum of people to ask how to fix them, so I swapped them. I've also had warped plate metering block Holleys in the past, and leaky seats. Holleys look the best, especially dual lines, and two is better than one on a big tunnel ram! I'm stuck in the 80's, or late 70's, lol...

Originally Posted by techinspector1

Keep the Q-jet. It's the best design ever devised. The tiny little primaries allow excellent throttle response and mileage, while the huge secondaries feed the motor when you step down on the loud pedal. Best of both worlds. Get some books or DVD's and learn how to tune it. You'll end up with the best carburetor there is and add a little to your own expertise.

Cliff Ruggles is one of the Q-jet gurus that I would recommend for information.
http://www.cliffshighperformance.com...history_1.html

AND ? Ditch the Performer intake. Use RPM or Stealth. )

Hey Tech Q-Jet's dont Bolt Right onto RPM's and Weiand Stealth's . What Book did you take that From ? Don't buy that one .

Agree with Denny, that one will make more hp and torque from idle to 6000 than any other manifold you could bolt on.

That is also a non-Air Gap intake ? More HP and Torque then any other intake Idle to 6000 ? Yes Q-Jets are good . Lets hear the many Down Sides to the Q-Jet and other Spread bore Carbs ?