The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Engine & Drivetrain

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 04-15-2018, 06:11 PM   #1
T and S
Registered User
 
T and S's Avatar
 
Join Date: Dec 2013
Location: Oak Ridge, Mo.
Posts: 286
Holley carb on 250 cyl. adjustment question

I have a 390 cfm holley on a 250 straight 6 engine, carb is new. Engine is fresh rebuild - 9.1 compression, mild comp cam, offy intake, 292 exhaust manifold(2.5 outlet), HEI distributor.
I have trouble with a stumble - letting the clutch out and taking off, it takes off ok, stumbles, then takes off again. I have checked and set float level, mixture screws are at 1.5 turns out, good idle.
It came with .025 accelerator pump nozzles. Have tried .031's and .035's, and have .037's on the way. Still has the stumble, maybe not quite as bad.
Anybody running the 390, have you had this problem, and what nozzles did you end up with ??
Kind of getting frustrated, even thought of trying an edelbrock carb.
Have heard it mentioned this could also be an distributor advance problem - any comment ?? Any help would be greatly appreciated. Thanks in advance !
__________________
If you don't like this pound sand .....
T and S is offline   Reply With Quote
Old 04-15-2018, 07:14 PM   #2
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 9,853
Re: Holley carb on 250 cyl. adjustment question

What is your timing curve like?
What’s initial set at?
geezer#99 is offline   Reply With Quote
Old 04-15-2018, 08:47 PM   #3
T and S
Registered User
 
T and S's Avatar
 
Join Date: Dec 2013
Location: Oak Ridge, Mo.
Posts: 286
Re: Holley carb on 250 cyl. adjustment question

to be honest, I haven't checked it. I usually borrow a timing light,(haven't needed one till now) but I am buying one tomorrow.........
__________________
If you don't like this pound sand .....
T and S is offline   Reply With Quote
Old 04-15-2018, 08:57 PM   #4
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,885
Re: Holley carb on 250 cyl. adjustment question

A timing light and a vacuum gauge are two tools I've used many times on these old trucks.
__________________
68 GMC 250/3 speed Saginaw p/b p/s
69 Chevy 350/350 currently in pieces still lookin for a cab
06 Trailblazer
I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...
68gmsee is offline   Reply With Quote
Old 04-15-2018, 09:10 PM   #5
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 9,853
Re: Holley carb on 250 cyl. adjustment question

Quote:
Originally Posted by T and S View Post
to be honest, I haven't checked it. I usually borrow a timing light,(haven't needed one till now) but I am buying one tomorrow.........
Put your initial at 12 degrees.
Make sure you plug off the vac advance when you set it.
geezer#99 is offline   Reply With Quote
Old 04-16-2018, 09:17 PM   #6
T and S
Registered User
 
T and S's Avatar
 
Join Date: Dec 2013
Location: Oak Ridge, Mo.
Posts: 286
Re: Holley carb on 250 cyl. adjustment question

Bought a timing light. Also got a hold of my engine builder, he is having me drive it to his shop Friday morning and is going to help me out with it. He may strap it to the roller dyno for test and tune, I hope ....... I will let you know how it goes.
__________________
If you don't like this pound sand .....
T and S is offline   Reply With Quote
Old 04-18-2018, 10:15 AM   #7
T and S
Registered User
 
T and S's Avatar
 
Join Date: Dec 2013
Location: Oak Ridge, Mo.
Posts: 286
Re: Holley carb on 250 cyl. adjustment question

Did some research. Found a really good article from Super Chevy about vacuum advance, how it works, and how to tune. About %99.99 sure this is my problem. The HEI distributor worked fine on a stock engine, but not so much on my modified motor. I for sure am no expert on this sort of thing, and I am sure my engine builder is - he builds race engines for a living. Going to his shop Friday ....... http://www.superchevy.com/how-to/eng...r-drivability/
__________________
If you don't like this pound sand .....
T and S is offline   Reply With Quote
Old 04-20-2018, 12:11 AM   #8
mattfranklin
Registered User
 
mattfranklin's Avatar
 
Join Date: Dec 2011
Location: Moraga, CA
Posts: 480
Re: Holley carb on 250 cyl. adjustment question

For me, as long as I can hear it if it knocks, I advance until I can hear a light knock at WOT and then back it off just a little.
__________________
(Very) Slow-Going Build Thread: Stock 1970 Short Step with Stock 1970 LT-1

http://67-72chevytrucks.com/vboard/s...d.php?t=567340
mattfranklin is offline   Reply With Quote
Old 04-21-2018, 07:44 PM   #9
T and S
Registered User
 
T and S's Avatar
 
Join Date: Dec 2013
Location: Oak Ridge, Mo.
Posts: 286
Re: Holley carb on 250 cyl. adjustment question

Success!!!!!! Took truck to Baldwins (engine builder) this morning. I was only there a hour or so. He checked the timing and advance, was dead on, working fine (so much for my guess). He took it for a drive, right away he said it had a "lean miss". Changed power valve from a 5.5 to a 5.0, changed main jets from .051's to .065's. Wow what a difference, runs really good now. Case closed.
__________________
If you don't like this pound sand .....
T and S is offline   Reply With Quote
Old 04-22-2018, 10:06 AM   #10
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,885
Re: Holley carb on 250 cyl. adjustment question

Quote:
Originally Posted by T and S View Post
Success!!!!!! Took truck to Baldwins (engine builder) this morning. I was only there a hour or so. He checked the timing and advance, was dead on, working fine (so much for my guess). He took it for a drive, right away he said it had a "lean miss". Changed power valve from a 5.5 to a 5.0, changed main jets from .051's to .065's. Wow what a difference, runs really good now. Case closed.
It's good you found someone with knowledge of Holleys. There aren't many carb people left out there.

I for one have never worked on Holleys or owned one so I looked up the power valve and found a good article on how to select the right one for a given engine. Might help someone with similar problem.

https://www.racingjunk.com/news/2015...ey-carburetor/
__________________
68 GMC 250/3 speed Saginaw p/b p/s
69 Chevy 350/350 currently in pieces still lookin for a cab
06 Trailblazer
I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...
68gmsee is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 01:44 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2018, vBulletin Solutions Inc.
Copyright 1997-2013 67-72chevytrucks.com