lean surge
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
lean surge
i just wanted to make sure i am diagnosing part throttle lean surging on my holley 750 equipped 454 correctly.
cruising speed say 45-50 mph on flat ground just trying to maintain speed. pedal is held steady at just enough of an angle to maintain speed. it feels like my foot is just resting on it.
the engine feels like just for a split second that it loses power then picks back up.
if i press the pedal a hair more it speeds right up. if i stomp it, without hesitation the passing gear kicks in and it takes off.
from all the tuning advice and books etc etc to me it feels like lean surge and it needs a larger jet size. currently it has 72 main jets. does it sound like i diagnosed this correctly.
at one point i read that it could be too much timing but i set it with a dial back light and a recurve kit so that i am getting the 36 degrees right about 2500ish with the hose unhooked.
what do you guys think. reading about something and correctly diagnosing it can be tricky it seems.
cruising speed say 45-50 mph on flat ground just trying to maintain speed. pedal is held steady at just enough of an angle to maintain speed. it feels like my foot is just resting on it.
the engine feels like just for a split second that it loses power then picks back up.
if i press the pedal a hair more it speeds right up. if i stomp it, without hesitation the passing gear kicks in and it takes off.
from all the tuning advice and books etc etc to me it feels like lean surge and it needs a larger jet size. currently it has 72 main jets. does it sound like i diagnosed this correctly.
at one point i read that it could be too much timing but i set it with a dial back light and a recurve kit so that i am getting the 36 degrees right about 2500ish with the hose unhooked.
what do you guys think. reading about something and correctly diagnosing it can be tricky it seems.
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
That will take away all of your vacuum advance.In the vacuum advance is a rod that pulls back and forth in a "slot".(The rod goes in the bottom of the plate the points sit on.)If you restrict how far the rod can slide in the slot you will restrict the amount the can is able to advance.Cutting the vac adv from say 18 to maybe 14.I have 2 little different size tools that I made that I just drop in place but I'm sure you can find a way to restrict it.
Whats the number on your can maybe just changing the can to a more suitable number would be easier for you.Do you know from setting your timing with the rollback timing light how much vac advance you have in the can?
Whats the number on your can maybe just changing the can to a more suitable number would be easier for you.Do you know from setting your timing with the rollback timing light how much vac advance you have in the can?
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From: Florida
It takes a little bit of practice to test for lean without meters, but if you drive a steady speed on a flat surface, very, very slightly let off a very little on the gas pedal and if the carb is lean, you will feel a very slight bit of acceleration. It is just a simple, quick, crude test, but a lot of times it works.
Also the Holleys are very sensitive to accelerator pump settings, and there should not be a gap in the linkage and sometimes even some preload is necessary.
Also the Holleys are very sensitive to accelerator pump settings, and there should not be a gap in the linkage and sometimes even some preload is necessary.
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You may have some crap caught up in the idle circuit/metering block. I was getting a bit of hesitation at cruise in my 68 big block. After eliminating all the usual suspects and throwing tune-up parts at it, I found some small metal shavings (from carb machining) caught up in the metering block. Cleaned it out and now it runs great. Just a thought.
Pro
Joined: Jun 2004
Posts: 700
Likes: 0
From: Lantana TX
It takes a little bit of practice to test for lean without meters, but if you drive a steady speed on a flat surface, very, very slightly let off a very little on the gas pedal and if the carb is lean, you will feel a very slight bit of acceleration. It is just a simple, quick, crude test, but a lot of times it works.
Also the Holleys are very sensitive to accelerator pump settings, and there should not be a gap in the linkage and sometimes even some preload is necessary.
Also the Holleys are very sensitive to accelerator pump settings, and there should not be a gap in the linkage and sometimes even some preload is necessary.
Don't change the jets, your speaking about a transition not WOT. I would start off with larger squirters first a 10 second change before you go and start removing components. What squirter size do you have in there now...front and rear.
I had the same problem and and changed the front to a 37 and the rear to a 35 and problem is gone.
Make a significant change to see if it helps and if it does keep tweaking it.
Let us know how you make out.
Actually before you even start that have you pulled a vac number in gear at idle? Another words we need to check the PV before you do anything.
