what am i doing wrong?
Thread Starter
Racer
Joined: Jul 2009
Posts: 350
Likes: 7
From: springstead wi
what am i doing wrong?
I installed my new carb have everything running pretty good trying to fine tune it . at idle 700 rpm im running 8 inch lbs of vacuum , 10 at 1000 and just over 15 at 1500 rpm ( there's very little bonce in the gauge at the different rpms ) . the carb came with the 4 idle mixture screws at 1 1/2 turn out .when trying to get the most vacuum I have to turn the screws to almost bottomed out ( can't be another 1/16 turn left )before anything changes the rpm goes up a little and so does the vacuum . runs ruff going the other way .
Drifting
Joined: Dec 2009
Posts: 1,349
Likes: 34
From: Porterville Ca
1) What was the vacuum before changing carbs ?
It either has a vacuum leak(most likely, base of carb) or the timing is off. Did you change the timing at all? You should have 19 inches of vacuum at idle.
It either has a vacuum leak(most likely, base of carb) or the timing is off. Did you change the timing at all? You should have 19 inches of vacuum at idle.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
I'm going to guess this is a large cam engine with little vacuum available? If not disregard.
If you have already made the base adjustments and still find that you need to have the throttle plates open pretty far to idle I would say you have insufficient idle bypass air.
Big cams with little vacuum need a way to get fuel and air without having to open the throttle blades. Bypass air is the ticket. The stock bypass circuit which is part of the idle circuit may be too small. One crude way it used (and sometimes still is) to be done is by drilling holes in the throttle plates.
Read this article it may help. Then see if you can locate the bypass circuit on your carb.
http://www.hotrod.com/how-to/engine/...rberutor-tune/
If you have already made the base adjustments and still find that you need to have the throttle plates open pretty far to idle I would say you have insufficient idle bypass air.
Big cams with little vacuum need a way to get fuel and air without having to open the throttle blades. Bypass air is the ticket. The stock bypass circuit which is part of the idle circuit may be too small. One crude way it used (and sometimes still is) to be done is by drilling holes in the throttle plates.
Read this article it may help. Then see if you can locate the bypass circuit on your carb.
http://www.hotrod.com/how-to/engine/...rberutor-tune/
Last edited by REELAV8R; Aug 17, 2015 at 02:51 PM.
Thread Starter
Racer
Joined: Jul 2009
Posts: 350
Likes: 7
From: springstead wi
pretty sure I don't have any vacuum leaks , it had low readings with the other carbs I have had on it .haven't ran a double pumper since the 80s . its a quick fuel hr-680-vs . talked to two different guys there first one said its running to rich and go from 70 to 73 on the idle air bleeders the other said to go to 76 im going with the 76 . the one thing I cant remember is what size cam I have . pretty much know the rest . speedpro 11 to 1 pistons 30 over , engines decked 40 , world sport 2 heads 64cc 202 160 with angle plugs , roller rockers . had the whole thing balanced . I use to work in the auto industry so I could get parts at cost . also use to go over to where they built the Kodak car In Virgina. so im had a little inside info but that was many years ago . one thing i can't remember is what size cam . was going go get a chassis dyno run ,but after watching the dyno failures on youtube don't think its ready for that to many old parts in back end .
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Drifting
Joined: Jul 2002
Posts: 1,995
Likes: 110
From: Garland Texas
Prior to going to FI Ford used Holley 4 bbl carbs on trucks and vans. They had 4 corner mixture screws. Toobroke is wrong..... again.
And my double pumper works just fine on my 355. With no lag between gears, which is the whole reason behind double pumpers.
And my double pumper works just fine on my 355. With no lag between gears, which is the whole reason behind double pumpers.
Pro
Joined: Oct 2009
Posts: 664
Likes: 89
From: Marysville Ohio
TOOBROKE - He said, and I quote "its a quick fuel hr-680-vs " - that says vacuum secondary to me, not a double pumper. We all understand you don't subscribe to their use - fair enough, but let's ease off - no need to talk down to someone asking for help.
Upnorth - how are the transition slots set up? I would start with that and float levels as suggested. Outside of that, I would not hesitate to seek advice. The top of my list are people like Lars and Cliff Ruggles, but there are many other knowledgeable folks to draw on here - if you are patient you can pull out the good advice.
Knowing your engine specs might help us.
Upnorth - how are the transition slots set up? I would start with that and float levels as suggested. Outside of that, I would not hesitate to seek advice. The top of my list are people like Lars and Cliff Ruggles, but there are many other knowledgeable folks to draw on here - if you are patient you can pull out the good advice.
Knowing your engine specs might help us.
Last edited by cooper9811; Aug 17, 2015 at 10:34 PM. Reason: sp.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Le Mans Master
Joined: May 2007
Posts: 5,009
Likes: 1,116
From: North tonawanda NY
TOOBROKE - He said, and I quote "its a quick fuel hr-680-vs " - that says vacuum secondary to me, not a double pumper. We all understand you don't subscribe to their use - fair enough, but let's ease off - no need to talk down to someone asking for help.
Upnorth - how are the transition slots set up? I would start with that and float levels as suggested. Outside of that, I would not hesitate to seek advice. The top of my list are people like Lars and Cliff Ruggles, but there are many other knowledgeable folks to draw on here - if you are patient you can pull out the good advice.
Knowing your engine specs might help us.
Upnorth - how are the transition slots set up? I would start with that and float levels as suggested. Outside of that, I would not hesitate to seek advice. The top of my list are people like Lars and Cliff Ruggles, but there are many other knowledgeable folks to draw on here - if you are patient you can pull out the good advice.
Knowing your engine specs might help us.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Okay, I missed the part about the "VS" (vacuum secondary) because it wasn't mentioned in the original post so I admit I made a mistake and I apologize for that error. We all make mistakes from time to time and I made one there.
But that still doesn't change the "need" for 4-corner idle mixture screws for a small engine like a 350" that draws very little air at idle. I suggest closing off the secondary butterflies to almost stuck and bottom the mixture screws so all of the idle air is passing thru the primaries.
And street engines perform better throughout the entire rpm and load range of street driving with vacuum or AVS carburetors which is why the factories always used them.
But that still doesn't change the "need" for 4-corner idle mixture screws for a small engine like a 350" that draws very little air at idle. I suggest closing off the secondary butterflies to almost stuck and bottom the mixture screws so all of the idle air is passing thru the primaries.
And street engines perform better throughout the entire rpm and load range of street driving with vacuum or AVS carburetors which is why the factories always used them.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
Okay, I missed the part about the "VS" (vacuum secondary) because it wasn't mentioned in the original post so I admit I made a mistake and I apologize for that error. We all make mistakes from time to time and I made one there.
But that still doesn't change the "need" for 4-corner idle mixture screws for a small engine like a 350" that draws very little air at idle. I suggest closing off the secondary butterflies to almost stuck and bottom the mixture screws so all of the idle air is passing thru the primaries.
And street engines perform better throughout the entire rpm and load range of street driving with vacuum or AVS carburetors which is why the factories always used them.
But that still doesn't change the "need" for 4-corner idle mixture screws for a small engine like a 350" that draws very little air at idle. I suggest closing off the secondary butterflies to almost stuck and bottom the mixture screws so all of the idle air is passing thru the primaries.
And street engines perform better throughout the entire rpm and load range of street driving with vacuum or AVS carburetors which is why the factories always used them.
Factories have warrantys, and they limited performance in many places.
Even new Vettes have the computers detuned.
Old Vettes had the 1 turn preload on the liftersto limit the top end and reduce warranty work..Trust me on this one.
I HIGHLY suggest you read Lars papers on carburetors. It "just" may change your way of thinking on just how they really work.
Even on a vacuum secondary like a Holly, you HAVE to set the transition slot square, and set the primary and secondary throttle plates equally open for it to function correctly. Not BS theory, just the facts.
Thread Starter
Racer
Joined: Jul 2009
Posts: 350
Likes: 7
From: springstead wi
little more info . when I first installed the carb and started it . it ran real rough and missed as it warmed up was real rich smoked up the garage . but after it warmed up it sounded and ran fine . all I did was adjust the idle speed ,the fuel bowl windows show its just a little over 1/2 the window on both . put a regulator and fuel pressure gauge on it . said it was adjusted at the factory for 6 1/2 lbs its right at it according to the gauge . when I took it out it ran better right out of the box then my holley , had it up to around 70 real good response from right off the line and anywhere in between .the big problem is when starting cold, adjustusting the four screws ( turning out it went lower )turning in they made very little difference until bottomed out and that was maybe 1/2 lb of vacuum and not enough gain in rpm to notice . I should get the new idle bleeders in the next couple days .
Le Mans Master
Joined: Aug 2005
Posts: 6,513
Likes: 1,949
From: Temple Terrace Florida
Cam and lifters
How many miles on the cam? Low vacuum can be from all of the above but it also can be caused by a problem with the valve train. Are the valves properly adjusted? Is it possible that the lifters are bad or the cam is wiped? My big block Chevelle had a similar problem that drove me crazy. I replaced the cam and lifters and all is good. The cam and lifter set I took out had zero miles on them. The engine did sit for 7 plus years before I got it. Good luck. Jerry
Thread Starter
Racer
Joined: Jul 2009
Posts: 350
Likes: 7
From: springstead wi
maybe 10,000 . when built I had problems I would blow out the front intake gaskets and from new rebuilt the car always used oil . now this year I have run it more and harder and hardly ever have to add oil and little if no smoke out the back .