Holly Lean Surge
Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
Holly Lean Surge
I just replaced the distributor on my '74 454, MT, with an HEI unit, and put in new wires and plugs (gapped to .035). From information on other posts, the timing is now set at 36 degrees at 3000rpm, 56 degrees total advance. Idles nice at about 800rpm (about 20 degrees advance), and with the vacuum gauge on the carb base, it draws 6 inches with the carb plates closed (at idle), and a very steady 20 inches with the plates open.
My problem is when cruising along in any gear between 2000-2500rpm I get little surge feelings, like the car wants to accelerate then not. I guess it could be the suspension, but it behaves the same regardless of road condition and mph.
I set the idle mixture screws to max vacuum, which is about three full turns out, and that fixed it some but not completely.
The carb is a replacement Holley.
I was tracking this on another thread, but it was suggested that I repost it specifically for Holley experts.
Any ideas?
My problem is when cruising along in any gear between 2000-2500rpm I get little surge feelings, like the car wants to accelerate then not. I guess it could be the suspension, but it behaves the same regardless of road condition and mph.
I set the idle mixture screws to max vacuum, which is about three full turns out, and that fixed it some but not completely.
The carb is a replacement Holley.
I was tracking this on another thread, but it was suggested that I repost it specifically for Holley experts.
Any ideas?
Racer
Joined: Dec 2003
Posts: 399
Likes: 0
From: Haslet,Texas
If the only thing you changed was the distributor, then my guess would be too much timing. Unplug your vacuum advance (don't forget to plug the port) or back down your timing and see if the issue still exists. Is 6 inches of vacuum normal for your engine? That sounds considerably low.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Timing is too far advanced. The 36-degrees @ 3k rpm would be FULL centrifugal advance with no more to come as rpm increased beyond 3k. If you just cranked the engine speed up to 3k rpm and adjusted the timing to 36-degrees without changing the advance springs, then you are seeing more advance at rpm above 3k. This also gives you an initial advance that is excessive. I also suspect the 6" of vacuum you are seeing at idle is coming from the EGR port on the carb, rather than full manifold vacuum. Or you are tapping the ported vacuum from the carb and you are seeing some vacuum from this tap due to exposure of the port from inaccurate throttle adjustment.
Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
The 36 degrees at 3000rpm is with the vacuum can disconnected. With the can connected I have 56 degrees total advance (even with rpms above 3k). It seems to run a lot better with that much timing. Certainly idles a whole lot better.
73'Vette, the surge was worse before the additional advance. I will try disconnecting the vacuum advance (and plugging the line) and see if that makes a difference. The can ads about 20 degrees of advance)
Gerry72 is correct that I am running the vacuum can off a ported supply. Manifold vacuum is a fairly steady 15inHg at idle.
Thanks for all the insights. If advance does not turn out to be the problem does anyone have carb adjustment ideas?
73'Vette, the surge was worse before the additional advance. I will try disconnecting the vacuum advance (and plugging the line) and see if that makes a difference. The can ads about 20 degrees of advance)
Gerry72 is correct that I am running the vacuum can off a ported supply. Manifold vacuum is a fairly steady 15inHg at idle.
Thanks for all the insights. If advance does not turn out to be the problem does anyone have carb adjustment ideas?
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
If it's not the advance (easy to test by just disconnecting the vacuum advance line and trying it out) then you would have a lean surge. This means your idle circuit (and maybe the main jet. But it's hard to say without knowing if you are on your main circuit) is too lean. Under a light load at that rpm, you could still be operating under your idle circuit. You don't say what Holley you are running, but if it's one of the 4165 (mech secondary) or 4175 (vacuum secondary) spreadbores, then the idle circuit should be fine for a 454. It's a tad rich for the smaller engines, but not too far out of the fuel window. If testing the timing does not help, then up the jet size by two. A surge, rather than a misfire, is missing the main circuit by a sliver and two jet sizes will normally fix this without terribly upsetting the rest of the fuel curve. The only address if it's in the idle circuit is to re-size the idle fuel channel restrictor, but this is a bit beyond the abilities of the DIYer.
Bud2
Joined: Jul 2005
Posts: 1,237
Likes: 4
From: Warrnambool Victoria
More info please....
I find the description to be to sparse to be able to try to see what is happening. You seem to be reasonably sure that the surge is not electrical (I think you may be right) but it would help if we knew if the surge is during light acceleration or are you sitting at a constant RPM? How often is the surge? How far between surges? Is it different going up a hill as opposed to a flat? ETC...
Bud.
Bud.
Burning Brakes
Joined: Jul 2006
Posts: 1,108
Likes: 0
From: Sebastian Florida
Mine has been doing this since I changed the weights and springs in the dist. and had the timing set to 36 degrees also. Runs fine under 2K and over 3K but not between. I've been trying to find a vacuum leak, but haven't found any.
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Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
You don't say what Holley you are running
The numbers on the carb (by the fuel inlet, not on the choke horn which is blank) are L7454 and 0331. I called Holley and they said it is a 4360, 450 cfm. Not much flow for a 454.
I find the description to be to sparse to be able to try to see what is happening. You seem to be reasonably sure that the surge is not electrical (I think you may be right) but it would help if we knew if the surge is during light acceleration or are you sitting at a constant RPM? How often is the surge? How far between surges? Is it different going up a hill as opposed to a flat?
The surge is when I am trying to run a constant rpm. It surges non-stop, though it is not terrible I still feel it. Road surface or topography does not make a difference.
I tried running it without the vacuum advance connected, and it still does the surge thing. Just to be clear, by surge I mean it has a hard time holding a steady rpm.
Thanks for all the great input.
The numbers on the carb (by the fuel inlet, not on the choke horn which is blank) are L7454 and 0331. I called Holley and they said it is a 4360, 450 cfm. Not much flow for a 454.
I find the description to be to sparse to be able to try to see what is happening. You seem to be reasonably sure that the surge is not electrical (I think you may be right) but it would help if we knew if the surge is during light acceleration or are you sitting at a constant RPM? How often is the surge? How far between surges? Is it different going up a hill as opposed to a flat?
The surge is when I am trying to run a constant rpm. It surges non-stop, though it is not terrible I still feel it. Road surface or topography does not make a difference.
I tried running it without the vacuum advance connected, and it still does the surge thing. Just to be clear, by surge I mean it has a hard time holding a steady rpm.
Thanks for all the great input.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
There is a reason Holley stopped making that carb many years ago. It was never a very reliable carb. If it was mechanically sound, it would run fine up to its capacity even though it was never intended for a 454. You could try a rebuild on it or just round up a proper spreadbore, either one of the Holley 4165/4175 or get a Q-jet.
Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
Yes, I was a little disappointed when the Holley guy told me it only flowed 450cfm. Seems like the heads would be enough of a bottle neck on that engine that a low flow carb would not be needed.
I will do some post searching, but what cfm do you recommend? Any particular part numbers for the Q-Jet or 4165? There is a swap meet this weekend where I might be able to get a deal.
Eventually I plan on getting the heads ported, but that is many dollars down the road, so I need a carb that will be good now and in the future.
Thanks.
I will do some post searching, but what cfm do you recommend? Any particular part numbers for the Q-Jet or 4165? There is a swap meet this weekend where I might be able to get a deal.
Eventually I plan on getting the heads ported, but that is many dollars down the road, so I need a carb that will be good now and in the future.
Thanks.
Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
There is a reason Holley stopped making that carb many years ago. It was never a very reliable carb. If it was mechanically sound, it would run fine up to its capacity even though it was never intended for a 454. You could try a rebuild on it or just round up a proper spreadbore, either one of the Holley 4165/4175 or get a Q-jet.
Thoughts?
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Apparently both those carbs are 650cfm, which would be more than adequate for the factory heads. From what I have found the next size up spread-bore Holley is 800cfm, which seems like too much for the factory heads, but would be good when I get the warmed over.
Thoughts?
Thoughts?
As far as the 800 being "too much for the factory heads", its not. It is likely larger than what you need for your combo, but it isnt because of your heads.
Thread Starter
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
A rebuilder on ebay has the 800cfm for sale. Based on your post it sounds like you would recommend the 650cfm?
What is the factor making the 800cfm excessive?
I am also getting information from another post, but I have a Qjet from a different project, the numbers are 7042240 M3 and 1362 below that. Checking online, it appears the carb came out of a '72 455 Buick with an auto trans. Any thoughts on how this would run on my '74 454? I think it is also a 800cfm carb.
Thanks.
What is the factor making the 800cfm excessive?
I am also getting information from another post, but I have a Qjet from a different project, the numbers are 7042240 M3 and 1362 below that. Checking online, it appears the carb came out of a '72 455 Buick with an auto trans. Any thoughts on how this would run on my '74 454? I think it is also a 800cfm carb.
Thanks.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
I would work fine. The metering will be close enough that you won't notice a difference. One small problem is the gas line comes in from the side on a Chevy carb, while Buick, Pontiac, and Olds carbs have the inlet facing the front. This can create a clearance problem with the thermostat housing, but you can by a 90-degree adapter from one of the Q-jet suppliers to resolve this.
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
I wouldn't say its excessive, just unnecessary.
You said that your car idles nicely at 800 rpm, so I assume that it is a fairly mild engine
that probably doesn't turn enough rpm to require 800 cfm.
It would probably work fine though, as long as its a vacuum secondary model.
You said that your car idles nicely at 800 rpm, so I assume that it is a fairly mild engine
that probably doesn't turn enough rpm to require 800 cfm.
It would probably work fine though, as long as its a vacuum secondary model.
Bud2
Joined: Jul 2005
Posts: 1,237
Likes: 4
From: Warrnambool Victoria
Well...
While a different carb might cure the problem, at 2000-3000 RPM, 450 cfm should be enough for your 454 that isn't under load. I don't think size is the surging problem. Try that lean test that Matt suggested.
Bud.
Bud.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
You definitely need more carb! On the Holley Q-J replacements:
Many would run the vac. sec. for street.
If you drive hard you might want the mech. sec. one.
800 cfm is not an issue in a spreadbore like Q-J for 383+, the primary is just larger than on the 750 model Q-J.
Post the sec. rods & hanger letters from the '72 455 Buick Q-J & that will give an idea of the jetting.
Many would run the vac. sec. for street.
If you drive hard you might want the mech. sec. one.
800 cfm is not an issue in a spreadbore like Q-J for 383+, the primary is just larger than on the 750 model Q-J.
Post the sec. rods & hanger letters from the '72 455 Buick Q-J & that will give an idea of the jetting.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
It's not really a question of airflow, Bud. It's the same problem many experience by running too big of a carb. It's the fuel metering that's not right. When that Holley came out with this 4360-style carb more than 30 years ago, it was intended for a 350-sized engine to maximize fuel economy. Not a high performance carb by any means. In that regard, the idle circuit will be on the lean side for a 454 and the main circuit will also be too lean. And given the carb's age, it may have been fiddled with by previous owners...you know...trying to make it run better...which often ruins a perfectly good carb. And we're assuming the carb is in otherwise functioning condition, which may be far from true. The airflow is a limitation on the engine but with proper fuel metering, it would fun fine in most normal driving. The engine would probably nose over hard after 4k rpm, but you'd kind of expect that.