Bog/Black Smoke on Olds 350 at 3000 rpm - Hot Rod Forum : Hotrodders Bulletin Board
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Old 12-28-2004, 07:16 PM
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Bog/Black Smoke on Olds 350 at 3000 rpm

First time out in our 1939 Olds street rod with 1971 Olds 350 and hit a wall at 3000 rpm. Here is the setup:

The 350 is stock except for the following changes:
- replaced the 2 bbl intake and carb with a Performer intake
- installed remanufactured 4 bbl QJet bought through eBay store for 350 ci
- installed Performer Plus cam (204/214 @ .050, .448/.472 lift)
- installed 3.42 gears running on 27 " tires
- installed used HEI distributor and 8 mm wires

Compression is supposed to be about 8.2:1; cylinder pressures were around 150 psi with original cam. Haven't checked since putting in new cam. Engine had 106K easy miles on it before I got it.

Here's the problem/question.

We took it on the road for the first time last week (just to the alignment shop), and after warming up got into the gas a little and she started bogging and putting out black smoke at about 3000 rpm. Didn't want to run above that rpm.

Back at the shop also noticed she didn't want to run much over 3000 rpm even under no load. Sounded pretty good with blip of the throttle but not at sustained throttle open.

Air valve seems to open fully when throttle blipped.

Checked secondary metering rods and they are CP (.0567 in. dia at 90 deg air valve opening) and tip length M.

We had some rods out of a 1988 Olds 307 marked DD (.1047 in. dia from 60 to 90 degree) and tip length LL. From a sheet I have on QJet secondary rods, there is a caution not to use LL rods on performance engines with air valve openings greater than 70 degrees. (Why; too lean at open throttle if 90 degrees open?)

Anyway, we swapped the DD rods into the 350 and she seemed to rev much better, to 4500 without bog, setting in the shop. We didn't take it on the road (still no door latches, doors just taped shut).

Vacuum advance seems to be working okay.

Since the leaner rods seemed to help, I am guessing that the CP rods might be too rich for this cam.

Any opinions on what size rods should be used in this very mild 350? And should we get those DD rods back out?

Any other causes we should investigate?

Thanks,

John

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Old 12-28-2004, 07:41 PM
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You could check to see if the distributor is a tooth off. It will rev good but will be a dog.
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Old 03-17-2009, 09:06 AM
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Just found this post, maybe I have some relevant info. Check the spring on the secondary airvalve. If it is to loose or broken, the air valve will flop open and cause a bad bog. on the pass side of the carb you will find a set screw and a allen head setscrew. loosen the allen head set screw while holding the other set screw. tighten the set screw until the airdoor closes, and then tighten 3/4 turn more. lock the adjustment with the allen stescrew. Good luck!
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Old 03-17-2009, 09:17 AM
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Quote:
Originally Posted by JohnTN
Air valve seems to open fully when throttle blipped.
Seems that the issue has been resolved.

Last edited by cobalt327; 03-18-2009 at 01:24 PM.
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Old 03-17-2009, 11:39 AM
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Note that the original post was from 2004.

Despite my belief that the distributor was working okay, we took it out and found it was gummed up and not advancing. After cleaning it up the car would rev without black smoke.

But, the car still had little power. We then discovered the cam had been installed 45 degrees advanced -- and it still ran. We reset the cam and power and gas mileage increased significantly.

Been on the road for 7,000 miles now.

John
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