Problem: Buick 231 V6 w/390 cfm Holley - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-28-2008, 01:50 PM
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Problem: Buick 231 V6 w/390 cfm Holley

Backwoods Arizona Chevy guy here, still haven't learned to speak Buick very well. Help me out, guys!

A Buick 231 came stock in my 84 El Camino (California version).

Did the following off-road-only (we sure wouldn't want to do anything illegal) conversion:

Edelbrock performer 4 barrel manifold
new Holley 390 cfm Carb, vac secondary spring not changed
stock camshaft
new standard (not CA) even fire HEI distributor
timing at 15 deg BTDC
AC R45TSX set at @ .60
cat converter has been gutted

Couple of "carburetor aces" told me the 390 Holley should work fine on this motor, even with the stock cam, and I understand that this is a popular set up for Jeep engine conversions.

Engine idles and runs smooth, but has weak acceleration & bottom end, bogs with full open throttle and only gets 17mpg. I was hoping to get at least the same 25mpg that the original CA set up got.

Although out-of-the-box Holley aftermarket carbs usually come with rich jetting, the spark plugs are showing almost pure white insulators----trying to jet leaner seems like a bad idea. Still mystified that inside of tail pipe is sooty. Have checked and rechecked and found no vacuum leaks.

Could it simply be over-carburetion? If so, what is the ultimate combo of manifold/carb for best gas milage? When this project began I was thinking maybe a hotter cam, but with the high gas prices now, I'm more focused on economy

Could the stock California cam be the problem? I did a similar conversion on a California Chevy 305 with excellent performance/great bottom end/gas milage results (of course that was 4bbl to 4bbl), but have been real disapointed with the results on this 231.

Don't know where to take it from here....hope you guys can help me out.

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Old 08-28-2008, 02:30 PM
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Check accelerator pump function and adjustment.double check floats.Being to lean as indicated on the plugs could cause all these problems as will a low float level(which will in itself lean out the carb).And contrary to popular belief to lean WILL cause poor mileage just as to rich will.Also check your vacuum and compare to the rating of your power valve Holley,s website has good info for this.Myself I just take the idle reading subtract a couple inches and go.If the power valve has a low rating such as 6.5 it will not open till way late on a stock cammed engine and this will also cause your concerns.Then after all this is set if needed go to a stiffer secondary spring.The carb is not to large as vac secondaries basically self size based on load , extremely difficult to overcarb with one.
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Old 08-28-2008, 03:17 PM
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Thanks barnym,

I put a new kit (Holley brand) in the carb to replace the blown 6.5 power valve. I did think it was really strange for the power valve to go out on a new carb. At idle the vacuum is showing 15 inches which sure seems on the low side. Float levels and accelerator pump are adjusted correctly. Your description of vac secondary Holleys being "self size based on load" sounds right on in my experience with small & big block Chevys where I got excellent milage & performance with larger carbs than the "sizing formula" called for.

The poor gas milage when running lean is a new one to me, but actually the plugs are reading leaner than I think they should. Too lean would explain the way the car is performing too; doggy acceleration and bogging on WOT. My past experience in putting new 600cfm Holleys on small blocks was that they always needed to be jetted down on the new out-of-the-box carb, so I expected this 390cfm would be the same. Maybe not.

From everything you say, going with richer jets may cure the problem. Thanks a lot.
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Old 08-29-2008, 09:57 AM
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The reason lean mixtures past a certain point will kill mileage is ypu have to use to much throttle opening to get enough power to cruise reducing vacuum ,opening power valves when they shouldn't be etc.That being said 15 inches on a stock cammed engine does not sound right.Possible retarded ignition timing or timing chain stretch, restrictive exhaust?also try adjusting the idle circuits using the lean drop method:
1.Set your base idle speed
2.hook up a good test tach capable of detecting small changes in rpm(old dwell tach works well).
3 adjust each mixture screw for the beat idle speed then lean it out by turning the mixture screw in till it drops 50 rpm.
4.Repeat the procedure this ensures correct idle mixture.Reset idle speed.
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Old 08-29-2008, 09:58 AM
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Also try hooking your vacuum advance up directly to intake vacuum this should help a good bit.
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Old 08-29-2008, 12:00 PM
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I had similar problem with my chevelle. I'm running a pair of 390 holley on a tunnel Ram. I had a little bog when I took off. I bought the plastic pump cam kit and tried different cam on the accelerator pump on the Holley 390. It turn out the blue cam and setting from #1 to #2 worked perfect. The bog was gone. The 390 do run a little lean because my tail pipe was very light gray. It was not pulling through out the midrange. I pulled the heads off a found some of my valves were white.
The jets were replaced from #51 to #57. In your case, I would change the spark plug to a colder one, advance the timing a little. I hope this helps.
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Old 08-29-2008, 07:51 PM
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barnym & lg1969: Thanks for your valuable help, bro's. I've got an Air/Fuel ratio guage and some jets on the way. Won't get to it for at least 2 weeks but I'll be back to let you know how it worked out.

Thanks again!
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Old 08-30-2008, 10:02 AM
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buick v6

I have a buick v6 in one of my T buckets. mine is an odd fire. I got it out of a crashed vega . the previous owner did a / 30 over pistons ,,,performance cam and new manifold ...4 barrel holly . He had problems because they didn't do the spark plug wires in the positions shown in the shop manual. when he got it straightened out the vega was super fast. he ran into stuff 2 times before he towed it to my shop. and left it . he owed me $ 300 for welding work I had done...It's nice in the T bucket. 5 speed and 3.70 gear 9 inch
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