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

Blown pv's?? pull the metering blocks and see if there is gas in the chamber that see vacuum.

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

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.

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

You didnt say anything about your plug type heat range, or what kind of ignition you have for your tune.

""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.

Post deleted by polyspheric

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.

its an msd 6 al, msd pro billet, and autolite racing 3934 plugs.

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

Did you drill all four throttle plates or just the front two? Try opening the secondary throttle shaft so it is close to what the primary is open now, if the primary is open to the transfer slot it is opened to far 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 You can fix it They (the early Holley 850 DP)will idle nice in gear and not load up, but it takes some work and analyzing what the problems are

"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.

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.

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

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

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.

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.