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need some carb tune advice
Posted By: We The People
need some carb tune advice - 09/02/12 10:44 PM
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
Posted By: Blown61
Re: need some carb tune advice - 09/02/12 10:53 PM
Blown pv's?? pull the metering blocks and see if there is gas in the chamber that see vacuum.
Posted By: We The People
Re: need some carb tune advice - 09/02/12 11:11 PM
maybe...I did have a couple backfires from the fouled plugs today.
can any degree of back fire blow one out or does it have to be particularly bad? I dont think that carb has a check ball either.
thanks
Posted By: RobX4406
Re: need some carb tune advice - 09/03/12 12:47 AM
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.
Posted By: We The People
Re: need some carb tune advice - 09/03/12 01:31 AM
maybe,.... I'll check. but I dont think that is the issue with the idle quality and control. I dont realy think the PV is gone either (I will check though)cause i stays running and its not like puking fuel rich, just kinda burn your eyes a bit not quite right, and slowly fouling out rich.
I'm thinking if if nothing is blown, maybe a bit larger holes, to shut off the garden hoseish transfer slots. then maybe a good metering block, or drilling the idle feeds, to feed the bigger cubes
any input on those ideas?
thanks guys
Posted By: Mopar-Al
Re: need some carb tune advice - 09/03/12 01:44 AM
You didnt say anything about your plug type heat range, or what kind of ignition you have for your tune.
Posted By: YO7_A66
Re: need some carb tune advice - 09/03/12 01:50 AM
""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.
Posted By: polyspheric
Re: need some carb tune advice *DELETED* - 09/03/12 01:51 AM
Post deleted by polyspheric
Posted By: sgcuda
Re: need some carb tune advice - 09/03/12 02:12 AM
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.
Posted By: We The People
Re: need some carb tune advice - 09/03/12 02:43 AM
its an msd 6 al, msd pro billet, and autolite racing 3934 plugs.
Posted By: We The People
Re: need some carb tune advice - 09/03/12 02:54 AM
all the bleeds are clear. I opened the secondaries till the transfer slots were just exposed. It was awful, I shut them back down and it smoothed out.
I'll try a set of lighter springs to get some more lead at idle speed. I dont want any more broken ring gears from hard starts, I've dealt with that.
thanks everyone
Posted By: Thumperdart
Re: need some carb tune advice - 09/03/12 05:32 AM
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. 
Posted By: Cab_Burge
Re: need some carb tune advice - 09/03/12 05:49 AM
Posted By: davenc
Re: need some carb tune advice - 09/03/12 02:15 PM
"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.
Posted By: We The People
Re: need some carb tune advice - 09/03/12 10:18 PM
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.
Posted By: We The People
Re: need some carb tune advice - 09/04/12 02:59 PM
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
Posted By: turner
Re: need some carb tune advice - 09/05/12 01:18 AM
The best advice on this exact department from here on this board last year for me, was to take it to the local dyno shop and get it dyno tuned,(wich happened to be Jakes) and my idling and driving issues are behind me........ best money spent to date, they found 4 things wrong with my carb set-up immediately and fixed them in 15 minutes...........my car gets more street miles on it now than ever, runs mid 10's on pump gas
Posted By: We The People
Re: need some carb tune advice - 09/05/12 03:18 AM
thanks, thats a great idea. honestly I hadn't thought of that, since I'm in the boondocks. I'll have to search chassis dynos near me, I'll probly have to tow three hours.........or cross over to canada and deal with the border gestapo.
Posted By: We The People
Re: need some carb tune advice - 09/05/12 03:34 AM
I located Jim Fords Dyno service in fulton NY, only 2 hrs from me .....anybody ever use them??
I know him very well....just read your email.
Posted By: Thumperdart
Re: need some carb tune advice - 09/05/12 05:33 PM
Quote:
thanks, thats a great idea. honestly I hadn't thought of that, since I'm in the boondocks. I'll have to search chassis dynos near me, I'll probly have to tow three hours.........or cross over to canada and deal with the border gestapo.
It is a good idea but why not hook up a wide band and have a year round dyno at your finger tips plus, your not moving on the chassis dyno which changes your wot readings...........
Posted By: We The People
Re: need some carb tune advice - 09/05/12 11:30 PM
Thats a good idea too. Right now I'm broke though....so I'm still gonna mess with it myself for a bit.
Heres where I'm at now (for those who care, or maybe just following along giggling) I'm gonna put the 3.5 and 3.0 pvs in to eliminate fuel dribble with wandering vac. I locked the dist. @ 26* initial, so I can experiment slowly at idle increasing the timing. I thought 26* initial would be enough but now I'm thinking low initial may have been affecting things more than I expected. I may also have a custom advance bushing made if I end up with some #s like 30* initial 10* start retard and 2*-5* mech. advance.
I measured the IFRs wich are .035 P and .037 S ( anyone know if .035 is enough for 2 circut idle on a 266 @.050 493?)
I wish MSDs start retard was more finite in it adjustability, the 5* jumps make it hard.
Posted By: MR_P_BODY
Re: need some carb tune advice - 09/06/12 12:53 AM
You have the timing set right FIRST.... next WHY are
you at 2.25 or more turns out on the mixture screws...
you should be about 1 1/4 turns out
Posted By: We The People
Re: need some carb tune advice - 09/06/12 01:30 AM
yes, the screws are out there cause thats where it would run...I thought the timing was good cause it would run pretty well and start, (getting close) but with out kicking back.
Now... I am thinking it may need more timing to run well..and I'll need to utilize more * in the start retard.
I'll be going back to a bit of a baseline of 26* with 5* start retard, while locked out. Then I'll get it idling and bring the timing up a bit at a time.
I know it will ping in gear @ 35* cause the big 10* advance bushing stuck out once (needed some more oblonging).
I'm thinking I've got about 6* to mess with and look for the sweet spot. Then a more custom bushing may come into play.
I'm learning a lot as I go here, and trying not to break stuff. Kinda fun .........kinda a pain
Posted By: MR_P_BODY
Re: need some carb tune advice - 09/06/12 01:42 AM
Quote:
yes, the screws are out there cause thats where it would run...I thought the timing was good cause it would run pretty well and start, (getting close) but with out kicking back.
Now... I am thinking it may need more timing to run well..and I'll need to utilize more * in the start retard.
I'll be going back to a bit of a baseline of 26* with 5* start retard, while locked out. Then I'll get it idling and bring the timing up a bit at a time.
I know it will ping in gear @ 35* cause the big 10* advance bushing stuck out once (needed some more oblonging).
I'm thinking I've got about 6* to mess with and look for the sweet spot. Then a more custom bushing may come into play.
I'm learning a lot as I go here, and trying not to break stuff. Kinda fun .........kinda a pain
Turn the start retard off and lock the dist... set
the timing at 18*-20* dont worry about total YET...
you need to figure out the basic timing and get the
carb set.... once you know some basic numbers you
can figure the total needed and then use your retard
Posted By: We The People
Re: need some carb tune advice - 09/06/12 03:08 AM
Yeah, thats about where it starts to liven up 21* (no retard) it fires and runs, you can just feel the starter begining to work there. Is it normal for the motor to want more near the kick back point, or is it just an every ones unique type thing?
I know I'm getting closer bit by bit, feels like a science experiment
Posted By: MR_P_BODY
Re: need some carb tune advice - 09/06/12 03:13 AM
Are you SURE the cam is in right... did you degree it
EDIT
Is this MSD ignition... sounds like the dist wires are
backwards
Posted By: We The People
Re: need some carb tune advice - 09/06/12 03:38 AM
yes it was degreed to spec, it is MSD. I've used the dist before in my 440 ran 11.01 @ 121 all wiring is the same, and I took this 493 for a ride. Theres nothing that drastic wrong, just my limited fine tuning experience, and maybe an ill matched carb. Plus its not licensed yet so I cant drive it around freely for testing.
thanks
Posted By: SLOW67
Re: need some carb tune advice - 09/06/12 03:45 AM
Quote:
Quote:
thanks, thats a great idea. honestly I hadn't thought of that, since I'm in the boondocks. I'll have to search chassis dynos near me, I'll probly have to tow three hours.........or cross over to canada and deal with the border gestapo.
It is a good idea but why not hook up a wide band and have a year round dyno at your finger tips plus, your not moving on the chassis dyno which changes your wot readings...........
I will never build another car boosted or not without a wideband. I have saved alot of time pulling my hair out by getting an Innovate that datalogs
Posted By: We The People
Re: need some carb tune advice - 09/06/12 04:07 AM
yeah Guys, NO doubt there great. I do plan on getting a wide band for next year, when I can drive it on the road.
but Right now you don,t even need it to know it is rich as hell idling. I'd be happy with a ball park decent idle, so I can store the car happy.
The problem more lies in the fact that if I had a wide band and it told me I was way rich at Idle, I'd still have to figure out how to cut it back, smooth it out, and keep the car running. Thats what is giving me the fits, the normal procedures dont seem to be netting goals. Maybe I'm asking too much of the old 850dp.
Does any one no if a 2 circut idle with .035 IFRs will run a 493 with this 266 @.050 cam?
I was looking at the 850 mighty demon........looks good
"I'm making a drink and getting in my hottub"
Me
thanks everyone
Posted By: rbj
Re: need some carb tune advice - 09/06/12 04:28 AM
Try another carb that you know works well, (friends, or off another ride),to see if it reacts the same. If it corrects then it s carb if not you have other issues but will tell you if it is carb tuning. Air screws should only be out , what p body said. Turn them for most vacuum in gear at idle. Power valve should be half of max vacuum. Just my
Posted By: Thumperdart
Re: need some carb tune advice - 09/06/12 05:03 AM
Quote:
You have the timing set right FIRST.... next WHY are
you at 2.25 or more turns out on the mixture screws...
you should be about 1 1/4 turns out
This is where the t-slot mod comes into play 
.........why not try a smaller(richer)idle bleed to get the screws closer to 1-1 1/2 turns out........ 
Posted By: @#$%&*!
Re: need some carb tune advice - 09/06/12 05:16 AM
It sounds to me like you need to open the idle feed jets a couple of sizes. I know you said it's rich but it behaves like it's lean at idle. If it's lean enough to cause a misfire you WILL get fuel out of the exhaust because of the misfire. This can even throw off those tuning with a wideband.
Posted By: We The People
Re: need some carb tune advice - 09/06/12 05:26 AM
yes, I believe it may be lean as far as the idle circuts go. which forces me to open the blades into the t slot to get fuel to it. BUT the t slot then makes it Too rich, then the vac drops and makes the pv open and worsens the problem.
the reason I think this way is, cause no matter how big I drill the plate holes I still have to open the blades into the t slots the same amount. I think it may need the t slot fuel cause the IFRs are too small.
Does this make sense to anyone else?
Posted By: MR_P_BODY
Re: need some carb tune advice - 09/06/12 05:35 AM
Quote:
yes, I believe it may be lean as far as the idle circuts go. which forces me to open the blades into the t slot to get fuel to it. BUT the t slot then makes it Too rich, then the vac drops and makes the pv open and worsens the problem.
the reason I think this way is, cause no matter how big I drill the plate holes I still have to open the blades into the t slots the same amount. I think it may need the t slot fuel cause the IFRs are too small.
Does this make sense to anyone else?
If you drilled the plates I think you screwed them
up... did you pull the mixture screws out and blow
air through the holes
Posted By: We The People
Re: need some carb tune advice - 09/06/12 01:11 PM
the holes may be too big now I agree. I'll just buy a couple blades. I,m pretty sure the idle holes are clear we held the shop vac right up to the bit while drilling. and Ive inspected and ran it about 15 times, and it always had about the same adjustability.
its apart now, I will disassemble and blow it out just in case.
Posted By: MR_P_BODY
Re: need some carb tune advice - 09/06/12 01:36 PM
Quote:
the holes may be too big now I agree. I'll just buy a couple blades. I,m pretty sure the idle holes are clear we held the shop vac right up to the bit while drilling. and Ive inspected and ran it about 15 times, and it always had about the same adjustability.
its apart now, I will disassemble and blow it out just in case.
With the holes you move more air... you said you were
lean on idle so the holes will make it worse
Posted By: rbj
Re: need some carb tune advice - 09/06/12 03:13 PM
no need to buy new ones use a rivit in them.
Posted By: We The People
Re: need some carb tune advice - 09/06/12 05:08 PM
thanks guys,
Posted By: We The People
Re: need some carb tune advice - 09/30/12 04:44 AM
ressurecting Cause of my success in the fight of this problem.
The issue was due to TOO SMALL IFRs.
The throttle plate holes are plugged, the timing is now locked @32 with 5 degrees start retard,(which is the only setting it likes) The ifrs were .035 to start with and ended up @ .041. Now it starts up great cold or hot, idles nice @ 1000 and @ 800 in gear, with 8-9 inches of vac. the transfer slots are right where they should be, the idle mixture screws are out 2.25 turns (a little far but its cold out right now). I did some brake tests (no stumbles) and took it out for a quick rip. It will blaze the slicks from a jog with a quick 1/3 throttle stab.
Thanks Competition Wedge.....great cam
cant wait to get a wide band for summer time testing
Posted By: @#$%&*!
Re: need some carb tune advice - 09/30/12 08:03 PM
Quote:
The issue was due to TOO SMALL IFRs.
Glad to hear it's fixed, what did I win? 
Check the power valves as suggested and verify the vacume in gear at or below 1000 RPM in gear and verify the timing at that RPM also 