Hi Guys, it has been a while since my last post. I recently removed my 830dp 4150hp series carb. As I have had a very rich idle and over fuelling issues when hot. To give you some specs I run a 460cuin BB, Auto Trans idle speed of 850rpm at idle in park. In Drive at idle it is 650rpm and a vacuum of 7hg. All 4 butterflies have a 3/16” hole in them.

(Question 1) On stripping the carb I found both primary and secondary PV’s were 6.5hg, I would of thought that this was too high for my engine with a idle vacuum of 9hg at 850rpm

(2) I fitted 2no 3.5hg PV in both metering blocks as I have read some guidance saying that you should fit a PV half of your vacuum at idle?

(3) I reassembled the carb after checking everything else was working fine, i.e. float valves. The transfer slot was set at about 20 thou visible from the underside of the Carb, and then installed it on the car.

(4) I need help with this one. What is the purpose of the transfer slot? Is it a guide for butterfly setting point to allow for air flow at idle. Does this not become obsolete when you have holes drilled in the butterflies anyway?

(5) I had set all 4 corner idle screws out 1and1/2 turns. On initial start up after feathering the throttle a bit it was idleing at about 1400rpm and didn’t change much even once it was warm, so I backed off the throttle stop screw to lower the idle but it would not drop below 1000rpm, with the throttle blades closed completely. I started to adjust the 4 corner idle screws and all that did was increased the revs back up to around 1400rpm again and I was wondering why. The only way I could drop the engine revs was to nearly close off the idle screws in the secondary metering block. But what is the point of that, when they should all be the same

I have watched numerous youtube videos videos on how to tune a carb with 4 corner idling and have done my carb before I changed the PV’s, with no problem at all, but now I’m stumped, as to why I can’t find a way to lower the idle speed.

Will the holes in the butterflies draw fuel from anywhere else in the system with the throttle blades closed or is it only possible through the idle circuit? No fuel is coming through the primary or secondary boosters at all when idling, so its not from there. With a vacuum of 7in hg it is possible to draw fuel through the PV or not if the PV is a 3.5in hg. I have not changed any jets at all everything else is as it came out of the box 10 years ago.

The Car did run ok before but just idled very rich, and had a problem with no secondary squirter, that caused a slight bog when the secondary’s opened, which was why I took it off.


Thanks

Peter

 

Jebby

 

The transfer slot adds fuel as the butterfly begins to open to help with transition before the main boosters take over on fuel. Under the throttle blades for idle there are fuel ports and a bit of the transfer slot is also adding fuel
then when the throttle blade begins to open more of the fuel form the transfer slot happens. Basically that's why it called transfer.

 

really big cams can cause the idle adjustment to be adjusted so high trying to get more air in at idle ti make up for the exhaust polluted overlap of the cam and the transfer slot then ends up opened to much causing more ti much fuel. So one if the fixes for that situation is to drill holes in the butterfly's to allow more air at idle.

 

The throttle stop screw is backed right off it is not even touching the throttle linkage. But it idled fine before and all I have done is changed the PV.

 

Something has changed.....check for vacuum leaks and if the secondary throttle valves are shut all the way too.

Jebby

 

Whem was the last time you checked your timing curve? It may not have changed but you should verify that first before making carb adjustments.

 

oh I see. Do you think changing both the PV’s has caused the issue I have now as they both should be closed at idle? What else can cause the high idle speed at idle with the throttle blades shut? Is it worth plugging the holes in throttle blades and start again with the correct settings for the opening of the transfer slot to the throttle blade?

Thanks

 

no the holes in the throttle blades wo
nt cause any issues. how much initial timing do you have. Good amount there is a good place to start. You are right the fuel enrichment valves dont play any part in anything till the carb gets into the main booster operation.

 

Only 830 holley I remember came on an L/88 cammed engine.

 

Initial timing is 14 degrees btdc at idle and all in (36 deg) by 3000rpm.

Is it worth backing the timing off to 8deg and start again there? I only put more advance in the initial timing as it improved pick up from idle.

Cant remember exactly but I think my Camshaft, Hydraulic Flat Tappet, Duration 292/292, Lift .550/.550,
TC 2400 stall.

Thanks

 

yes they are all shut.

 

it was about 18 months ago I think but I haven’t used it a lot since then (200 miles) no leaks before I took the carb off. And it was running fine.

 

[QUOTE=knodty;1601292485]Initial timing is 14 degrees btdc at idle and all in (36 deg) by 3000rpm.

Is it worth backing the timing off to 8deg and start again there? I only put more advance in the initial timing as it improved pick up from idle.

Cant remember exactly but I think my Camshaft, Hydraulic Flat Tappet, Duration 292/292, Lift .550/.550,
TC 2400 not sure what cam you have but even if hydraulic adjust things where you see more like 18 at idle and if you have big cam you need even more then that at idle. BBC has big cylinders and big diameter head chambers.

 

A 292 hydraulic is more like a 300 in a mechanical cam so you have a pretty big cam
put more timing in it. Then start looking into the carb for what other problem may have happened. Good chance with that cam you need some more total timing as well. I'm thinking you need like 20 at the idle and 38 to 40 total. 3000 rpm is a wise move on pump gas dont go down lower then that.

 

4 x 3/16" is a pretty good bit of air for a relatively mild engine that holds good vacuum. Check that the throttle plates are seated properly (hold the carb up to a light to check..they can be closed but still not actually touching all the way around. Might have to loosen plate/shaft screws to re-center them/ Worn throttle bushings/bores/shafts will let in a lot of air to. Heavy throttle springs or linkage that bottoms out hard can hurt the shafts/bores.

Is this an older 830 or is it one with a idle air screw hidden down in the air cleaner stud hole maybe?

You've got to find where it's getting air. It should be able to overcome the holes drilled...but they aren't helping on this combo for sure.

JIM

 

I saw several posters recommend drilling holes in the throttle plates. Drilling holes in the throttle blades is NOT the way to fix a carb. The idle feed restrictions in the metering blocks are too large by design, which is why Holleys always run rich and require you to open the throllte blades up just to get it to idle. The better way to do this is to cut and strip a piece stranded wire. Bend 4 strands (one for each idle air bleed hole/2 holes per metering block) into an "L" shape and put a single strand (these should be roughly .010"-.012" dia.) in the idle feed restrictions holes. With them bent like an L, the gasket will hold them in place after assembly. Doing this allows the idle screws to become functional and highly tunable. Preferably, use of a wideband A/F gauge will allow you to set it up right.

 

i used to have initial timing of 18-20 degs but I found it hard to start and it used to kickback sometimes or even backfire out of the carb.

 

Sorry Jebby I missed your highlighted comments.

 

The timing is good.....may need a little more depending on what cylinder head you are running but you are in the ballpark. Your initial should be a touch higher at 16-18.
Most likely, this is your cam:
https://www.summitracing.com/parts/cca-11-213-3

This is not terribly big for a 460 inch BBC........you should have more than 7 inches vacuum at idle. I have the small block version of that cam in my 406 and it pulls 10-11 inches.
I feel this is where the problem lies.......I feel you have a vacuum leak somewhere.
Also....could you post a pic of the holes in the plates? 3/16" is huge.....and this in itself could be the issue. If they are actually .1875 in diameter.....you need to solder them up. Four .1875 holes is a LOT of air to pass at idle.....I have built plenty of setups that had that much cam and ran standard plates with no issues.
I just do not feel that that cam needs holes in the plates.......factory L-88's had more cam in them stock and they idled with no holes in the plates......if you had a solid roller with, say, 260 @ .050...then yes....that would work.
Easy thing to do would be to buy a new throttle plate and try it......or solder them up.....soldering them up is easy and free...the cost of a gasket to remove the throttle plate.
Another thing......very few carbs have two power valves in them anymore and they are usually for boat applications.....or road course where you are modulating more than 1/2 throttle to WOT in and out.....which is rare. If it was mine I would block the rear one off and add 8 jet sizes. One less thing to fail....and not needed once the engine is on the cam and accelerating.

Jebby