Bogging need help! - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 10 (permalink) Old May 27th, 02, 04:24 AM Thread Starter
 
Join Date: May 2002
Posts: 140
Hi there, I've got a 78 Z28 with a 350 and a 4 speed tranny. Stock Q-jet on an Edlebrock Performer RPM intake with headers, flowmasters with no tailpipes.
I get a strange hesitation when at around 1100 to 1300 RPM when accelerating a quarter throttle in nuetral or in gear, if I rev it up it will go up the RPM's with out a problem but if I start the "bogging" at quarter throttle and continue the car will back fire and start idling really badly.
I've tuned the carb almost perfectly, choke works great, I get a nice smooth strong idle. The problem occurs when the engine is at normal operating temp with the choke off. I've also changed plugs, wires, and had the ignition timing set to spec. Also I do not have any vaccuum leaks. I can't figure it out.

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1978 Z28 350
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post #2 of 10 (permalink) Old May 27th, 02, 08:59 AM
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Join Date: Mar 2001
Location: Tucson, AZ, USA
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Personally...I would use an older Q-Jet..
But... You need to check your power piston..since this takes care of part throttle transistion...
Check the power valve...it may need to come out and be cleaned up VERY lightly with 600 emery with WD-40....very lightly clean in the vertical direction....
You want to make sure the power valve does not bind and check that it moves freely..
Also, make sure you are using the correct spring under the power valve...
Edelbrock sells a small set of springs at different Hg" .....if your vacuum is strong you would want the strong spring..
Make sure accelerator pump level is on the inside hole for the rod....
Also make sure your transmission kick-down cable is set so to come in properly...
The Q-Jet secondaries have an adjustable spring that need to be set as well...make sure to have allen-key in the spring lock and tighten spring 1 turn from 0 tension..this needs some small experimentation...
Regards
Chris

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post #3 of 10 (permalink) Old May 27th, 02, 05:45 PM
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Location: Ashby,MN, USA
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Q-jets have a power valve? i thought that was a Holley only thing.

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1971 Nova(looks like 69 camaro from underneath!)
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post #4 of 10 (permalink) Old May 28th, 02, 06:39 AM
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Location: Westlake, OH 44145
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When did it start doing this? Sounds Ignition related to me. Check the wires goiing to the pickup coil( light tug) they tend to break inside the insulation and then sometimes have contact as the enging load changes and the vac adv moves cause problems

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67 prostreet BB Camaro
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.
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post #5 of 10 (permalink) Old May 28th, 02, 07:17 AM Thread Starter
 
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Thanks for your help guys,

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1978 Z28 350
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post #6 of 10 (permalink) Old May 28th, 02, 04:12 PM
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Location: Seattle, WA
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Green-

Ive got a similar problem in one of my cars. My problem is with the distributor. I'm not getting the appropriate spark advance, thus causing a backfire and a horrendous bog at hte lower rpms. This all started when I replaced my distributor with a used HEI. I am thinking that it is worn advance weights or springs on the distributor. It could also be a worn distributor shaft (too much "play" side to side). I havent pinpointed it yet....but I would do some testing of your distributor.

Get your timing lite out, and check your timing at different rpm points. Have a friend rev and hold the engine at a constant rpm (like idle, 1000, 1500, 2000 etc) while yuou check the timing. this will see how your spark adavance mechanism is working in the distributor. While your checking, look for spark scatter, or check to see that the distributor can hold the spark steady at each rpm test point. If it cant, and rpm is being held constant, you could have a worn out distributor shaft or another problem with the distributor.

Can u give us some details on your distributor?

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post #7 of 10 (permalink) Old May 28th, 02, 04:14 PM
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Green-

When you do the spark timing checks at the various rpms, unhook the vacuum advance and plug the vacuum hose. Then do the same test with the vacuum advance active.
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post #8 of 10 (permalink) Old May 29th, 02, 02:53 AM Thread Starter
 
Join Date: May 2002
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Thanks for the info esahlin,
The distibuter is the one that came with the car. It's an HE.


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1978 Z28 350
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post #9 of 10 (permalink) Old May 29th, 02, 05:32 AM
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Green Hornet-

If you are able to check the timing at various RPMs, check to see that your getting about 33-36 degrees of advance (with the vacuum advance disabled) at 3000 rpm or so. THis amount is usually the best setting for max power, of course it depends on your engine combo etc. But this range is usually close for most engines.

When you go to WOT (wide open throttle), vacuum is zero so the vaccum advance is not active at WOT...hence the reason for checking advance without the vacuum hooked up.

If you find that it doesn't look like your spark advance is acting normally, you can purchase calibration kits for the HEI. These kits contain different weights and springs to modify how much spark advance comes in at different rpms.

I'm not an HEI expert, but I guess you could possibly have an HEI ignition module problem.

Let us know what you find after checking the distributor.
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post #10 of 10 (permalink) Old May 30th, 02, 08:48 PM
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Here is some info on "tuning" the aprk advance curve on an HEI.
http://www.boyleworks.com/ta400/psp/distcurve.html
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