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Author Topic:   Lean condition
dode
Member
posted May 20, 2002 08:35 AM
Ok...first of all...this is the engine we are running:

355
9:1 comp with 993 heads
cast iron intake
quadrajet

Our problem that we have yet to figure out is that we are getting a backfire (lean backfire I think) and a serious lean condition in upper rpms (above 4000) under load, but only on the passenger bank. I can't get it to do it under no load, so it is very hard to diagnose. My question is what would cause this? I am running out of ideas of things that would cause this. The only thing I can come up with is that on the new heads we bought, they didn't put the upgraded valve springs in and we are getting some valve float on that side. Could that be it? We are running a pretty mild .484 lift cam, and the springs are supposed to be good to .600" of lift. Anyway, I would appreciate any help I could get. Oh, and the timing is set at a base of 12 degrees and everything in the advance system seems to be working just fine. After checking the plugs, the ones on the drivers side look great. I am pretty much baffled. Anyone have any ideas?

John

smayo964
Member
posted May 20, 2002 12:26 PM
Hey Dode. Yeah, sounds like you might have a bad spring, or lifter in there. Have you checked or replaced your plug wires on that side? For about $50, you can get those Z28 springs, which are rated for .551 lift, or in that area. For the lift you have, they're perfect for the buck. Plus, they'll handle RPMs up towards 7K. Had something typical with my engine early last year. Changed wires, plugs, pick up unit, coil, etc., and still the same. Finally changed springs and behold, instant performance and no breaking up under load. But, just food for thought, I'd look at the wires also. Just my couple pennies.


dode
Member
posted May 20, 2002 12:45 PM
Well, I don't think it is just one spring...I think it could be all of them on that head. We bought the heads from Midwest Motorsports, and they are supposed to have the Z-28 springs on them. I wonder if somehow one head didn't get them. We have put new plug wires on. I haven't tried a different cap or coil yet, but could that cause a problem on only one bank? I pulled all the plugs after the last race, and all of them on that bank showed very lean, while the other bank was almost perfect. If it were the springs, how would you go about proving that? Thanks.

John

KPLugnut
Member
posted May 20, 2002 12:51 PM
Have you checked that there isn't a problem on one side of the carb, possibly? It's a thought anyway...

and it's easy to check valve spring pressures without disassembling the head, too. Wouldn't hurt to check that first before
yanking heads off the motor or anything.

I'd make sure there isn't a vacuum leak, internal carb problem or something like that first, but valve springs could cause your symptoms as well.

Hope ya find it.

KPLugnut

dode
Member
posted May 20, 2002 01:02 PM
How do you check valve spring pressures?

I can't guarantee that it isn't the carburator, but if it was, how would I go about diagnosing that? I am a fuel injected ford guy, so I am trying to learn here. It is a brand new rebuilt quadrajet that has 4 races on it, no known problems up until last week.

KPLugnut
Member
posted May 20, 2002 02:12 PM
Valve spring seat pressures can be checked on the head with a spring tester that is designed to mount to the rocker stud and is somewhat similar to a torque wrench. It's not the most accurate method, but it works for what you need to find out.
I didn't mean to say it had to be a carb problem, but it's something you may want to look into. One thing I found on a carb one time was that when you remove the top of a quadrajet and flip it over, if any of the 4
brass tubes that stick out of the underside of the carb top has fallen out, particularly the ones that are air fuel feeder tubes for the secondaries, it will lean out one side or the other of the carb.

Just something to check...

KPLugnut.

dode
Member
posted May 20, 2002 02:18 PM
hmmm...ok...where would I find one of these spring testers? Also, as far as the brass tubes go, anyway I could see a pic of this? I think I know what you are talking about, but like I said, I know very little about carburators.

John

KPLugnut
Member
posted May 20, 2002 02:38 PM
Valve spring testers are available all over. Summit, Speedway, Midwest, almost anywhere speed parts and tools are sold.
As for pictures of the brass tubes in the quadrajet top, probably your best bet is to get your hands on the Rochester carburetors book written by Doug Roe a few years ago. It's a good starter reference and will help you out a lot. I believe Amazon.com still has them.

Hope that helps,
KPLugnut


dode
Member
posted May 20, 2002 02:47 PM
Actually, I already have that book thanks . I will check out midwest. I have their catalog right here. Yes, they do have them. Thanks. I will look into it. More tools...what's a guy to do?

I should be able to turn the carb upside down and see these 4 tubes right? What if one is missing? Is it hard to replace?

John

smayo964
Member
posted May 20, 2002 04:03 PM
Hey Dode. Don't know if your last question was about taking the whole carb off and turning it over. I do believe KPlugnut was refering to taking just the top half off and looking for the tubes. It's been a long time since I've used the rochester, but I do remember what he is refering to. If a tube has fallen out, it should be laying in your carb for your retrieval. Also, a man can never have enough tools. hahahahaha Good luck.


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