need some carb tune advice
#1295820
09/02/12 06:44 PM
09/02/12 06:44 PM
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Joined: Apr 2009
Posts: 190 Communist New York State, Comi...
We The People
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Joined: Apr 2009
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Communist New York State, Comi...
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I need a bit of adjustment advice on this 850 dp model 4781-1 on 493" rb . I cant seem to get the idle circut lean enough it is loading up, fowling plugs, and just doesnt seem adjustable. first I realize this is only a 2 corner circut carb, and might not be the best piece but I got it cheap and am not looking for every .01 of a second anymore. Heres the tune in the carb so far, idle screws out 2.25 turns. .078 transfer slot showing. 5/32" (.1562) holes in primary blades. 86/92 jets. #5 pvs (changing to 4.5 and 3.5) heres the cam, comp custom 108+4, 296/296, [Email]266@.050[/Email],108 lobe sep,.600 lift. timing 24 initial, 34 total @ 2500. 11 to 1 indy sr, max wedge. 93 octane pump gas 8-9 inches of vacum idling @1150-1250, 750 - 850 in gear I cant seem to get the idle any lower or leaner, drilling the holes in the blades has only had a very small effect so far( maybe .010 on the transfers and .25 turns on the idle screws). should I drill a bit more and try to close the transfer slot some more? it always runs best at the same curb idle position. The timing is about there I doubt I can move it much with the fuel, it starts ok now but anymore would be iffy. I feel I may be on the cusp of either just drilling a little more and getting some where, or drilling too much. you would think two big ass holes in the blades would have done something by now. thanks guys
Last edited by We The People; 09/02/12 06:46 PM.
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Re: need some carb tune advice
[Re: We The People]
#1295826
09/02/12 09:50 PM
09/02/12 09:50 PM
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Joined: Jan 2003
Posts: 5,444 Indiana
YO7_A66
master
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master
Joined: Jan 2003
Posts: 5,444
Indiana
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""Dropping timing going from 1200 to 800 rpm? Put a light on it and see. You may be into the mechanical advance at your in park idle speed.""
When the engine is up to full running temp, check the initial timing in P/N and then again in D with brake. Your losing 400rpms and it might be due to your dist curve.
1970 YO7 A66 [Canadian Export] F8 Challenger 340 (Currently in shop for stroker assy.)
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Re: need some carb tune advice
[Re: polyspheric]
#1295828
09/02/12 10:12 PM
09/02/12 10:12 PM
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Joined: Jan 2005
Posts: 4,243 Charlotte, North Carolina
sgcuda
master
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master
Joined: Jan 2005
Posts: 4,243
Charlotte, North Carolina
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Did you check if the idle air bleeds are clear? If they are plugged, they will just pull straight fuel into the idle circuits. Put your hand over the secondaries. If no change (want to stall) then you need to increase the secondary blade angle. That would explain why you need so much air on the primary side.
[image][/image]
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Re: need some carb tune advice
[Re: We The People]
#1295831
09/03/12 01:32 AM
09/03/12 01:32 AM
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Joined: Jul 2004
Posts: 19,318 State of confusion
Thumperdart
I Live Here
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I Live Here
Joined: Jul 2004
Posts: 19,318
State of confusion
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Quote:
I need a bit of adjustment advice on this 850 dp model 4781-1 on 493" rb . I cant seem to get the idle circut lean enough it is loading up, fowling plugs, and just doesnt seem adjustable. first I realize this is only a 2 corner circut carb, and might not be the best piece but I got it cheap and am not looking for every .01 of a second anymore. Heres the tune in the carb so far, idle screws out 2.25 turns. .078 transfer slot showing. 5/32" (.1562) holes in primary blades. 86/92 jets. #5 pvs (changing to 4.5 and 3.5) heres the cam, comp custom 108+4, 296/296, [Email]266@.050[/Email],108 lobe sep,.600 lift. timing 24 initial, 34 total @ 2500. 11 to 1 indy sr, max wedge. 93 octane pump gas 8-9 inches of vacum idling @1150-1250, 750 - 850 in gear I cant seem to get the idle any lower or leaner, drilling the holes in the blades has only had a very small effect so far( maybe .010 on the transfers and .25 turns on the idle screws). should I drill a bit more and try to close the transfer slot some more? it always runs best at the same curb idle position. The timing is about there I doubt I can move it much with the fuel, it starts ok now but anymore would be iffy. I feel I may be on the cusp of either just drilling a little more and getting some where, or drilling too much. you would think two big ass holes in the blades would have done something by now. thanks guys
Best tuning advice I can give is to get a wideband so you can see what`s up initially and make the changes to fix it wheather carb or tune. I can make an idle circuit change and read the gauge. I`ve gone from a rich 10.8-11.4 cruise to 12.8-13.5 and I`m still tuning and the wot`s nuts clean. Check out motorspostsvillage and click on the fuel section.....................some good reading in addition to what`s been posted here.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: need some carb tune advice
[Re: We The People]
#1295833
09/03/12 10:15 AM
09/03/12 10:15 AM
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Joined: May 2005
Posts: 598 NC, USA
davenc
mopar
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mopar
Joined: May 2005
Posts: 598
NC, USA
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"I opened the secondaries till the transfer slots were just exposed. It was awful, I shut them back down and it smoothed out"
Perhaps you just opened the secondaries too far, and had them working mostly on the transition circuit. The motor seems to need more air, and your primaries may currently be open too wide at idle, which makes the mixture screws not very useful. If you open the secondaries just a bit at a time, you should be able to close the primaries a bit at a time. You can then close down the mixture screws a bit to get a slightly leaner idle.
With the current setup, does the motor respond to changes in the mixture screws? Can you close them down enough to make the motor stall? If you notice little or no change with changes in the mixture screws, then it would confirm that the carb is operating on the transfer slots (transition circuit). Changing the transition circuit requires changing Idle Feed Restrictions and Idle Air Bleed, which are not easy to do on a stock Holley.
"I'll try a set of lighter springs to get some more lead at idle speed."
That is not a great approach. As others have noted, if you are relying on mechanical advance at idle, then any RPM drop due to putting the car into gear is going to change things, and make it even harder to get a good tune. My may be able to find springs that seem to work OK in the driveway, but I bet you would find that you will have problems in combination with real world road conditions. My advice would be to not take that approach.
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Re: need some carb tune advice
[Re: davenc]
#1295834
09/03/12 06:18 PM
09/03/12 06:18 PM
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Joined: Apr 2009
Posts: 190 Communist New York State, Comi...
We The People
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OP
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Remember guys this is only front idle circut carb. did some work today on it. The timing is dead on, and does NOT move when you put it in gear. the power valves were fine,and have back fire protection. I did bump it up a bit to 26 degrees initial 36 total, and turned on my start retard for 5 degrees. The Idle speed still drops alot when it is put in gear, and the vacum drops into the power valve open point 5", making it stumble. My primary drilled holes are at 3/16" and I will not go any more. I do have more control with the idle circut screws,and see signs of somewhat leaner mixture on the plugs. I will order some 3.5 power valves.
My feeling at this point is that I may need a good metering block for the idle tuning to change the feed restriction to feed it more fuel with the idle screws (If the new PVs dont smooth it right out). I think that the plate holes are about right, and the power valves dumping makes me open up the curb idle to burn the extra, then it starts to race and repeats till it cascades out of my control. I did have some time where the idle was very stable, seems like after you take the carb off and it airs out it runs well till the pvs start loading it up.
Last edited by We The People; 09/03/12 06:26 PM.
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Re: need some carb tune advice
[Re: We The People]
#1295835
09/04/12 10:59 AM
09/04/12 10:59 AM
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Joined: Apr 2009
Posts: 190 Communist New York State, Comi...
We The People
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I wanted to rewrite my last post to make it clearer, I think the exhaust got to me yesterday . My pvs need to go down so I ordered a 3.5 and 3. that should take care of the dumping and me having to open the plates to burn it off, which in turn exposes the T slots making it rich as well. IF the new power valves stablize it I SHOULD then be able to close the plates more, effectively leaning the mix. If it stablizes and I still need the plates open into the T slots that tells me (I think) that it needs more mixture from the Idle circut to feed 493" so I may have to enlarge the IFRs a bit, then I would be able to close the plates more. Again, its only two idle circut, and Holley says only drill primary plates on two circut carbs, thats why the holes are so big. thanks all
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Re: need some carb tune advice
[Re: We The People]
#1295839
09/04/12 11:51 PM
09/04/12 11:51 PM
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Joined: Nov 2004
Posts: 3,544 Syracuse,NY
CompWedgeEngines
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Joined: Nov 2004
Posts: 3,544
Syracuse,NY
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I know him very well....just read your email.
RIP Monte Smith
Your work is a reflection of yourself, autograph it with quality.
WD for Diamond Pistons,Sidewinder cylinder heads, Wiseco, K1 rods and cranks,BAM lifters, Morel lifters, Molnar Technologies, Harland Sharp, Pro Gear, Cometic, King Engine Bearings and many others.
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