I wonder if some of you guys could give me a little tutorial on the operation and adjustment of idle bleed jets. Car is 68' with 3.08 & M21. Engine is fresh street 489. Was dyno'd at 528 bhp @ 5950 rpm/565 ft lb @ 3850 rpm. Inlet is Edelbrock RPM Air Gap. Cam is hydraulic flat tappet 236/242. Brodix Race Rite Oval heads, 10.5 cr, 22 degree lead on timing (38 degree total) Carb is Proform 950 DP Idle Bleeds are #71, hi-speed bleeds are #38, primary pumper is #33, secondary is #35. The engine really runs fine from 1200 rpm upwards, secondaries 'pull-in' well with no bogging. There is just a 'hole' (not a stumble or bog!) pulling away from standstill untill engine reaches 1200 rpm or so. I am sure primary squirter delivers enough gas, pumper cam is maximimized for max shot at earliest opening - it just dosen't cover the 'hole'. I am also intrigued re. the relation ship between idle bleed jets and adjustment sensitivity - the Proform has the 'four corner idle' adjustment - it just seems extremely sensitive i.e. even the smallest discernable rotation (say 1/8") it goes from idling a little weak to idlling too rich. Would an increase in idle bleed jet size reduce this sensitivity ? is there a down-side to this? Any suggestions would be most welcome.

 

When you say : (pumper cam is maximimized for max shot at earliest opening) does that mean you have a green cam on the primary side?

 

Is your 950 hp car number 80496-1?

If it is the recommended idle air jets are 74

Is your pump cam on position # 2 for a quick big squirt?

I would also recommend a 4 hole wood spacer and decreasing your total advance to 34 ish with out the vacuum advance hooked up. I should ask if the dyno operators tried lower timing?

 

Thanks for response - its 8-30pm and -6 centigrade outset at the moment. I will go out to garage and have a look tomorrow.

 

I value your posts gkull and was hoping you would post. I will have a look tomorrow (too cold right now) for the part# of the carb & double check cam position. Can't use the wood spacer as there is only 1/4" clearance left under the L88 style hood. Engine was actually dyno'd and made best power/torque with 40 degree total. It was 'knocked back' two degrees for safety.

 

my2cents, the idle air bleeds are primarily for the idle circuit and not so much off idle performance. leave your timing alone and if you can run it up to 40 i would do so after a few mild street pulls in different gears. me thinks you need to play with your accel, pump adjustment. i have seen times when a big initial squirt sometimes leads to the lean hole you are describing. try backing off the pump adjusting screw and spring until you get around .002 clearance AFTER you set the idle screw with a warm engine. this may help and it doesn't take that long to do . i would also check the pump squirter nozzles and be sure they are squirting directly into the boosters. if by setting the pump "looser" and the problem is worse, then you know you must play with the pump and cam etc......good luck sometimes all it takes is a slight tweaking to resolve mushy off idle performance and it sounds like everything else is working properly. it could also be you are too lean on the primary main jets......

 

I too was wondering too about primary jetting as dyno figures are calculated with the engine 'on load' and presumably worked for best HP - so low end 'drivability' isn't really checked out - proof of this was that when I actually ran the engine once installed in car one of the secondary idle bleed jets was totally blocked with a tiny bit of silica gel from the original manufacturers packing! - they obviously didn't show-up on dyno runs as idle quality wasn't a considered part of the 'pulls'!)

 

Leave you air bleeds alone for now. What is your primary jetting ? Also try the 35 squirter in the primary

 

I tried to find some info for you. The holley sight has all the stock specs for each carb. Air bleed changes are not recommended for the novice. But returning to stock gets you in the ball park.

http://www.chevyhiperformance.com/te...ics/index.html

 

With 22 degrees of initial timing, I'd guess that some one setup your distributor without vacuum advance, correct? If so, you have too much timing for hard acceleration off idle and the engine fights itself. One can only do this if you have a performance (high stall) torque converter which allows the engine to get above 1200 rpm, but with a stick M-21 (2.20:1) and 3.08, you're just too high geared and have too much advance.

Add vacuum advance (~15 degrees) and backoff the initial to 12 to 14 degrees and I bet that your problem will go away. (You'll also need to recurve the distributor to have the centrifugal + initial peak at 36 to 38 degrees at ~ 2800 rpm.)

Remember that most carb problems are caused by ignition/timing!

 

Has MSD 6T+distributor etc c/w vacumn advance! There is no indication of engine 'pinging' although it has a tendency to 'run-on' when ignition switched off (which I put down a rich idle causing sooted plugs to ignite off carbon deposits) BTW with my old stock L36 427 I ran stock 7 degree initial, 28 degrees on crank and all-in at 2000 rpm with full vac advance. This never 'pinked' and ran smooth as anything. My engine guy finds that all ally headed BBC's run best with 40 degrees total (but he does built mainly race engines)

 

I build and run motors on the dyno for customer all over the world. I work at a historic racing shop. We are a Ferrari V-12 shop mostly, but we have Cosworth 4's and 8's, Jag 6's and 12's out to 7.2 liter , lotus, maserati, bugati, lambo's, BMW, chevy, ford. none of the spelling is right

All dyno testing is done with 110 leaded race gas. Dyno runs don't even start till the motor can take on full water pressure. So they don't even start till 3000 and up. We build the numbers for clients and test many cars for them on the tracks, So they can jump in and drive without problems. I personally don't have a Chevy motor that comes all in at less than 3000-3200 rpm. If the shop made more power at @40 degrees I would find out what octane the fuel was used for testing, bsfc, and what oil temp was on the dyno sheet.

Your run on can be something as simple as the wrong heat range extended tip spark plugs

We have this dyno.

http://www.land-and-sea.com/dyno-tec...using_bsfc.htm

 

I would start by doing a free load test, using the idle screw, with the tranny in park, the engine warmed up to operating temp, increase the rpms from idle to around 2200 rpms and back to idle rpms slowly. If the engine hesitates than more than likely it's lean. By using the idle screw the acc pump will not be active and will not cover up any short comings in the fuel curve. If it doesn't hesitate or stumble than I would look at the pump curve and nozzle size. Without a way to log the afr I would not change the air bleeds. The air bleeds will have different effects on the fuel curve, both rich and lean. A change of any one orifice (IR,IAB,HSAB, PJ, SJ,Pri PVCR and Sec PVCR) will have an effect on the fuel curve. Example- if a reduction of the PJ area by 3% is made than to keep the same AFR at WOT an increase in SJ or PVCR area of 3% will need to be made to keep the WOT AFR near were it was.

I have logged about 30 free load tests of just IAB and HSAB changes and you would be surprised at the changes to the fuel curve it makes. Generally when I increased the air bleed size it leans the very light throttle openings say from 0 to 4% and than it richened the the fuel curve from there till the MJ starts flowing, it caused a large spike of the afr of about 1.5 points which looked to have leaned the afr but really it just leaned a small section of the total fuel curve at the 1 to 4% throttle opening.

If you increase the rpms slowly while holding the brake with the tranny in gear what does it do?

Neal

 

Yes he uses Land Sea dyno. Gas used is Shell V-Power super unleaded (this is generally acknowleged as being the best pump gas available here) it certainly isn't as high a rating as yours - but in UK our rating are not the same as yours and its very difficult to directly compare. When I get a minute I will try and post the dyno sheet. To be fair I really didn't get the time to make any changes before winter set-in.

 

I spoke today with my engine builder. He confirms that replacing idle bleed jets with larger ones will create a bigger 'flat spot' and that in his experience these type of carbs with 4 corner idle system always run rich. Most of his customers engine spec's are race or pro-street and accordingly normally sets-up to maximise power/torque etc on the dyno and (except for low power 4 banger stuff) dosen't pay too much attention to cruise jetting unless really way out. Recons that going-up a jet size or two will cure things. As a bonus he has just bought himself a rolling road and has offered to set my cruise etc up on that if I want.

 

I'm still searching for a definative answer to what function the idle bleed and high speeds jet REALLY have and by changing them up or down what other effects they may cause. Previously I went up a couple of sizes on the primaries which helped the transitional stumble a bit. Last weekend I used a A/F meter (in the tailpipe so perhaps not 100% accurate) to get a better idea of things - idle is too choppy and won't register but on cruise its around 13 - accellerate and it richens to 11.5 or so - bring the secondaries in and it will drop momentarily to 10.5 - I'm happy with all this, it is running beautiful at cruise and anything over 2000 rpm and its recording 15.8 mpg at 70 mph cruise (thats imperial mpg, not US!) The transitional stumble still shows itself at around 1400 - 1800 rpm and is especially noticeable after slowing down and then re-accellerating - the A/F reading dips at this point momentarily between 15 and 18, and then stabilises to 12 or 13. Can I tune this out with the high speed (or idle) bleeds? any thoughts guys - I don't really want to start an overkill by increasing the main jets to try and cover-up the problem.

 

Air bleeds should only be played around with by experts, and there aren't many left period. I did find one when I was having a lean problem on my Race demon. Stock was .039" and he told me to go to .026" I did and with a few other changes I fixed my carb after searching for answers for 2 years.

Like I said not too many left that really know how a carb works when it comes down to air bleeds etc.

These are not my words, actually said much better than I can tell you with regards to high speed air bleeds :

Not only will larger air bleeds lean out the mixture, they also delay the effect of that circuit by weakening its signal.

Conversely, installing smaller air bleeds boosts the signal and speeds up the transition into that circuit.

That said I don't really think that is your problem or you would stay lean, did you try a bigger squirter in the primary like I suggested 8 months ago

 

Yeah I know Bro - Iv'e got to give it a try - its on the list - when I can get around to doing it - last year or so has been rather tumultuous with plenty of HD aggravation Valid point about finding someone who knows anything about the 'bleeds' - years ago we used to just drill some 1/8" holes in the butterflys!

 

lol, I did some drilling in my butterflys!, worked a treat!!

I had to use a bigger squirter to solve my stumble

 

The bigger squirter does the trick no problem Dave on heavier acceleration but not on light usage - even though enlarging it may kinda do the job I don't think that it is still the total answer. (I have changed the cam to give a more prolonged squirt) I am going to borrow some idle & high speed bleed jets from a buddy who went through all this with a street usage 1050 Dominator and see what happens.