Running Lean Still......

[ChevyIIMan]

Ok refrencing back to this thread https://www.pro-touring.com/forum/showthread.php?t=9299

The same condition exist. Aside from that thread I have put on new instake gaskets, verified the strightness of the heads and intake. Pressure tested the intake for leaks, swapped carbs, and so on. The car got put on a back burner for a while but im ready to tackel it again.

My next steps are to swap the distributor out and bypass my MSD box. Any more ideas?

[paul67]

When you changed the exhaust to the larger bore did you increase the jet sizes.
paul67

[paul67]

In the old days we used to increase the jet size up 1 for after market air filter and up 1 size for hedders as the car is now breathing easier .
paul67

[ChevyIIMan]

No I did not up the jets but I put a few hundered miles on the car before my trip to Texas and then made the 900 mile trip there with out a problem. Also it runs lean at idle, jet sizes shouldent affect that should they?

Also I went from Dual 2.5in exhaust to a single 3in, so in effect I decreased the exhuats flow by a bit if anything.

[cad]

Your plugs look to be on the rich side.

White porcelain is not the only indicator of running lean.
Just for a little back up, here are a few sources for reading spark plugs:
http://members.uia.net/pkelley2/sparkplugreading.html
http://www.dragstuff.com/techarticle...-pictures.html
http://www.dragstuff.com/techarticle...ead-plugs.html

To me, based on the strap, I would say that your spark plugs are on the "too hot" side based on the bluing of the strap, and the jetting is rich based on the base ring, but the pics are hard to tell.
A 750dp is BIG on a 355 with less than 10:1 compression.
If it is dieseling, I would not advance the timing. I would lower it.

[cad]

If it is lean at idle, the engine is making more than 5-6 lbs of vacuum at idle. If it were making less, the power valve would be open and allow a rich condition. What are you basing "lean at idle" on?

[ChevyIIMan]

IT makes 7 at idle with a working guage oin last check and I base running hot at idle on the fact that fireing it up and letting it idle it will deisel plus 2 O2 sensors read lean.

As for the plugs being too hot before I left for my trip the same plugs read fine, perfectly tan with no discoloration they have for 10,000 miles without issues. Same with the jettings. Nothing has changed. And for lowering the timing its alredy lower then I have ever run it. For the life of this motor it ran @ 16 initial and ran quick at that. Now I have it set at 10 initial.

The 750 may be a bit big but I experience the same results with a 650. I see no reason why I would need to change heat ranges or jettings when nothing else on the motor has changed.

[cad]

I asked about the vacuum because your previous posting string stated 5-6, then 13 in another post.
If you are lean at idle, you should turn the idle mix screws out an equal amount, maybe 1/2 turn each.
http://www.holley.com/TechService/Library.asp
As far as tan coloring, you should NOT base jetting on that. The base ring is the jetting indicator. If you've driven on the street and idled the car, the plugs will be tan or black.
The strap on the plugs indicates the proper heat range. The strap on the plug you displayed was blued like it had been under a torch. The heat stripe was too low on the strap (near the base ring). It should be closer to the middle. Too hot of a plug can contribute to detonation/pre-ignition/dieseling, whatever you want to call it. It is the cheapest swap to try at this point.
My 2 cents.

[ChevyIIMan]

The car always ran a tad on the rich side but it never deisled and the 02 sensors never read lean. I NEVER changed the jetting. Oh and the vac thing turns out was a bad guage. With a new verified working one I get 7 and when you turn the idle screws that wont change, same with both carbs ive tried. One a 650 jetted smaller. Doesnt change the fact that if I start the car let it idle for a bit the car will deisle when I shut it off. Thats not a jetting problem correct?

Why would all of the sudden my jetting be 3 or 4 off or my plugs be 1 or 2 heat ranges to hot? Durring an 1800 mile trip I NEVER touched the carb and it ran PERFECT half the weekend.

[cad]

So, it seems to run lean, but the plugs say not. The 650 and 750 acted the same, so its prob not the carb.
A F P....air fuel spark.
Lean does not make a car diesel.
Fuel is on the rich side based on your past experience...
You have a MSD ignition, but you have not said if you bypassed it, yet.
MSD has a t/shooting article referring to what Andrew mentioned in the previous string:
http://www.msdignition.com/1troub1.htm#runon
Does the article apply? Have you done that?

[ChevyIIMan]

I have not been able to bypass the MSD yet I will this weekend when I get a chance to work on it. As for the MSD run-on troubleshooting i have checked that. Made no diffrence. Also remember 2 seperate O2 sensors are still showing lean.

[cad]

What mfr/brand of O2 setup is used?
Where are the sensors placed?

[ChevyIIMan]

One is a Bosch 02 Sensor located in the collector of the drivers side header. The other was a wideband (not sure of brand) sniffer type in the tailpipe.

I understand why suggestiong of plugs are jetting are being made. But it just doesnt add up that it would work right for SO LONG and then al the sudden be WAY off

[Damn True]

Sounds to me like you might have a vacuum leak or something clogged in the carb.

[cad]

I am at a loss...
For what it's worth, I don't think your plugs represent a lean condition.
Dieseling is generally low octane or stretched timing chain.
If you haven't tested your O2 sensors, I would take it to a smog station for a quick test (if possible and cheap).
Bosch's webpage states the sensors can go bad without warning and cannot guarantee testing methods:
http://www.boschusa.com/AutoParts/FAQs/OxygenSensors/


Also, for what it's worth, here's a pretty good webpage on carb basics, which I am not assuming you dont know, but others following the thread may benefit:
http://www.bob2000.com/carb.htm

[ChevyIIMan]

Well im back at it I pulled the head inspected the guides and the vlaves and seats and they all seem ok. So its back on the car with a new head gasket, maybe that was the issue all along. Anyone else have any ideas we rules out the carb, all gaskets but the head gasket, distributor, msd box, vac leaks.