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Re: Diagnosis: confusion

To: mmcbeth@compusmart.ab.ca
Subject: Re: Diagnosis: confusion
From: Bob Palmer <rpalmer@ames.ucsd.edu>
Date: Thu, 21 Oct 1999 18:01:52 -0700
Mike,

Gee Mike, you call that a long post? Doesn't even compete with the Laifman 
or Cobbmeister. Seems like you have proven it isn't the carburetor. I think 
the MSD is probably a red herring too. You can pull a plug wire and see how 
much spark you're getting. Be CAREFUL though; It'll knock you on your ass 
if it gets a chance.

The problem might be related to ignition timing. Is this MSD distributor 
all mechanical? If you had it dyno'd, then is it correct to assume they 
dialed in the advance curve for you too? Do you have a print-out of the 
curve they set it to, and have you checked it lately with a timing light? I 
would have expected something starting around 12-16 degrees initial and 
twenty more degrees mechanical full in at about 2500 rpm. Have you changed 
the spark advance to see how it affects your problem?

Here's another off-the-wall idea. An intake manifold leak is pretty common 
on a new motor. I've never had this problem myself, but I believe it would 
affect only low throttle condition. I think maybe the way to check for a 
leak is to squirt WD-40,  or something like it around the suspected areas 
while the engine is running. Since I didn't get all my mail this morning, I 
don't know what other ideas have been suggested. Of the two ideas I have, I 
think the intake manifold leak is the most likely. Could be as easy to fix 
as tightening the manifold bolts. BTW, make sure they aren't bottoming out 
- I did have that problem once.

Brgds,

Bob

At 05:14 PM 10/21/99 -0600, Michael McBeth wrote:
>Hi Bob, here was my original post:
>
>I have a riddle for all of the armchair mechanics out there, but it is a
>longish saga so anyone not interested hit 'delete' now.
>
>I installed a very racey 302 into my Mk I last winter and have spent all
>summer trying to sort it out. I am on my third mechanic (four counting me, and
>I did used to be one) with no answer in sight yet. The symptom is an annoying
>'surge' or 'buck' at lower RPM in steady state cruise, most noticeable at
>about 3000 RPM but present at 2000 RPM. This is distinct from the lumpy idle
>that the cam produces, it just seems to erratically fluctuate making it hard
>to hold a steady speed. Full acceleration is crisp, and in fact startling at
>3000+ RPM.
>
>The engine is a 69 302 with a .512 lift cam, ported Dart heads (2.02 intake),
>balanced blueprinted and Dyno'd at 345 Hp by a reputable engine builder. The
>Carb is an Edelbrock 600 cfm, but I have tried two different Holley carbs to
>try to isolate the problem: same symptoms existed. Ignition is an MSD 6AL,
>including the distributor. Ignition is set at 7 degrees initial and 34 degrees
>total advance. We have also tried three different coils, mostly due to the
>mechanics' unfamiliarity with CD ignition patterns, in my somewhat skeptical
>opinion. I have checked the fuel pressure and it seems fine, I put a pressure
>regulator on it to ensure I wasn't over pressuring the float bowl.
>
>Can anyone think of something that will cause my engine to accelerate well yet
>not hold a steady RPM well. I would expect a over rich or lean carb to be
>somewhat consistant, and a faulty ignition to get worse at high RPM or WOT.
>Once off of idle the vacuum is quite steady and it is not too bad even at
>idle, so I doubt a vacuum leak is the answer either.
>
>Help! I am tired of paying for mechanics to make the same guesses that I made
>six months ago, but I still think that even a high performance engine should
>be able to hold a steady RPM once it is in its powerband.
>
>Thank you in advance for your input
>
>Michael McBeth


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