Hot Rods sbc stumble that won't go away

I'de say keep tuning the carb.......It's not one pair of shoes fit's all,you have to find a pair that fit.......YOUR FEET!
Some insane suggestions going on here.....
It's trial and error,but you are getting closer,so don't be discouraged,you cant just bolt on the "standard" 600 cfm holley and go...... You have a specific combo that has certain needs...... I know your not afraid to play,so sometimes= things just fall into place,sometimes it requires a lil tinkering...
Timing with a SBC doesn't require you to be a rocket scientist......
I know it's frustrating at times,and adjustments cost $,and time,but your on the right track,so keep going.
Also- as stated above= a VERY intelligent idea,and SHOULD keep track of #'s,and changes,and results......
But I'm sure you do that anyways. You'll get it solved. It IS an ENGINE,not a freak,nor a one off! Air/Fuel/Spark......When they all dance together= the world is right.........

 

i did notice the car is cold blooded. it really wants to be at full temp. i have a 180 in it now. the 190 does not sound out of line. i want to try to tune it with out the vac advance if possible. i do intend on running this car at the track. the seat of the pants gauge says high 11s so far. i have been a mopar guy all my life and this 327 really impresses me. i can't wait for my next day off to try the timing changes. yes the holly 20 degree

 

I'm more accustom to fuel injected engines with blowers burning nitromethane with locked out distributors and crank triggers. engines that have the distributor up front and the spark plugs on top were you can get at them. these sbc engines are a whole different animal to what I'm used to. i have to say the car is faster now than it has been in the last 31 years. i graduated in 84

 

If you do a 2000rpm cruise what do the plugs look like, with a key cut off, also dont hook you vac adv, to manifold like some one here said hook it to ported vacuum base of carb, its a lean or rich condition a plug check using the throttle chop method will tell you witch, remember each fuel circuit over laps the next one so if you open the idle screws up more than they are now and the condition improves at 2k then the next circuit in turn is to lean. You might set the screws for best idle then add a full turn drive it if better try a little more if better you need to rejet

 

Some holley carburetors come from the factory, with drilled throttle plates.. better call them and tell them they are hacks!

 

Please enlighten me on your opinion of the vacuum advance connection...I am curious..

 

O.k., here we go....
Yes, you may have to drill the primary throttle plates to get correct throttle plate positioning over the transition slots.It is crucial to part throttle "tip-in"(I usually use a #31 drill bit) you can also open secondaries slightly to do the same thing, as long as you don't expose too much of the transition slot.But if it has a huge *** cam, with lots of over lap ,like a 106-108 LSA (lobe separation angle), you may have to drill them for enough idle air flow,and to block off the transition circuit partially, so you can save it for slight acceleration. . The bigger the heads,and camshaft, the slower the air is moving at lower speeds.so the idle,transition circuit suffers from slow moving air..But you mentioned they were good, so move along.....
Sounds like you pretty much covered most of the bases.. but. ,(there's always a but... )
The spring on the power valve controls when the valve opens,not the amount of fuel it adds.the orifices behind the power valve, determines that, you can change the size of those as well. keep this in mind too. That's another tuning tool. As well as air bleeds..
The transition circuit gets it's fuel from the idle fuel orifice. So to cover up a stumble like you describe, you more than likely will have to open up the idle fuel orifice. to probably .036"+/-(all 4 idle circuit restrictors)if it has 4 corner or not, it will have these restrictors in the rear too,as it is for the secondary transition. >>>If you want to be sure it will cover up the stumble before you drill the metering blocks, simply open the idle fuel mixture screws a bunch, like 4 turns. and drive it. It will idle like ****, but that's not what we're trying to fix. so ignore the idle, concentrate on the stumble with the screws out like that.if it's too terrible,but the stumble id gone, turn the c=screws in q turn and try it again, etc.. Don't sweat the things being drilled out, you can chop off excess idle fuel with the mixture screws, but your transition circuit will love you. The transition circuit is your friend.
Also ,i agree with many on here, get all the timing in as fast as you can. The mechanical advance on an engine like that is basically for starting. usually in the 36-38 degree total advance, range is normal.(W/O VACUUM) I like mine all in by the time i let off the starter LOL.. Usually on MSD stuff, black bushing (biggest),and silver(lightest) springs. some times only one spring !
Vacuum advance can be used to lower EGT's as advanced timing under no load situation is good for cooler running combustion chambers,,and lower EGT's . And will not cause detonation if amount of vacuum advance is limited. sometimes, no-load(cruise) timing advance can be as high as 45 + degrees, with no adverse results. But this requires connection the vacuum advance to MANIFOLD VACUUM. then when in cruise state, the vacuum advance will be fully seated,to your setting,and when you roll on the throttle, the vacuum "goes away" (atmosphere comes in) And the timing retards to your mechanical advance setting. It's a win-win, situation., plus it will cover up some light throttle stumbles too. Just the opposite is true if you connect it to ported vacuum. that's for another story.it get really ugly,really fast.
As far as plug gap. .045" = No performance gain over .032"
The only difference is it will fuel foul easier with the bigger gap, .as the spark doesn't want jump the gap,it just goes down the wet ceramic. (path of least resistance,as fuel is semi-conductor) So,, I would close the plugs up to .032" While you're messing with it. I've done this a zillion times on the dyno, to prove it to other engine builders/customers/etc.. trust me...
There ya go, hope that helps!!
o.k. blast away !!

 

Good info...

 

i have a few things im going to try this week. i hope to get it figured out this week. thanks to all for posting their ideas. tuning these old engines is becoming a lost art..today tuners are pimply faced kids holding a lap top.

 

I put the following phrase into an Internet search engine

msd ignition stumble 2000 rpm

up came a lot of stuff from the MSD forum that you possibly should see

It seems the MSD ignition can be very fussy about component placement ...... according to MSD

www.msdignition.com/forum/

 

I might agree with you on this IF the motor wasn't telling you that's what IT needs, who can say for sure. I think I would save the post from trickysteve, sounds like he's been down this dusty road before. Can't say for certain(CRS) but seems to me that John Lingenfelter was doing this on his econo dragster in the mid 70's. I know for certain it was being done because I bought a brand new 750 (from Holley) for my race car around 1978 that THEY
drilled, carb. came with a dedicated part number.
One thing for sure; if it is decided it was the wrong thing to do, it can always be reversed pretty easily.

 

Listen to Tricky Steve. He pretty much nailed it.

 

Buying a carb from holley or a reputable builder who has done the research & made the needed changes to the idle circuits & the main circuits to compensate for drilled ****erflies is light years away from joe schmoe arbitrarily drilling holes in his ****erflies hoping for a miracle cure... that's what I'm saying , let's compare apples to apples...... what the op is experiencing could well be circuit timing however I'd still reccomend ignition timing adjustments first..
dave

 

roger that

 

What style holley did you get? Does it have the down leg boosters or the straight ones? I have had better drive ability with the down leg booster style carbs they seem to get a better signal from the engine.

 

Have you tried to replace your 30cc accelerator pump with a 50cc, that'll give a longer shot to cover over the lean spot. Might have to play around and put it on the secondaries. Just need one, not two.
1 1/2turns is too much on the idle, with 4 corner I rarely go much over 1 turn.

 

i would need a 50 gallon pump because if i drive down the freeway at 2000 rpm it stumlbes as long as im at that rpm.

 

Second vote for simply upping the squirters
when opening the throttle you are increasing the amount of air - a little too much air and not enough squirt get's you a stumble

 

X for running RFG you should go up 1-2 gets sizes to make it run right , with the RFG we have to tune with a a/f meter ( we stole from a pimpled faced computer car kid it was the only good thing about him) to get it some what in the ball park vs the old days because of the extra oxygen it posesses and leans it out ( the computers fix this for us with EFI ) . you know BP gold is non ALky gas and tunes easier , drain the tank down and try some of it and see if the stumbling goes away if not you might have to play with the powervalves/jets in size to get it right . one way to tell if your jetted right we found is your hand will be real moist if placed over the tailpipe when the engine is warm . if its lean its dry and hot . bring the motor up to 2000 rpm and see if its moist if its hot and dry its lean , ( burn your eyes its real lean )

 

Is that a tell tale sign pointing to ignition? Pete

 

i did go up two jet sizes already. i may go up two more. i think i want to up the initial timing first. then go after the jets again.

 

What do your plugs look like?

 

i will pull a few tomorrow

 

Its 2015... buy/borrow a wide band and actually see whats going on first. Its so much easier than guessing if its rich or lean at that RPM. I used mine a lot... makes life so much easier!!

 

you dont have a carb issue. You have an ign tuning issue. If you do decide to use the formula for success that I have shown you your problems will be over. The vacuum advance does not make a car slower at the strip, but on the street it makes it all work. Whatever you do, never hook ported vacuum to the dizzy. Ported vacuum is 0 at idle and increases with rpm. Manifold vacuum is highest at idle and decreases with rpm. Vacuum advance is not included in the total advance number. Like Tricky Steve said! He obviously knows what he is talking about.
I have recurved so many dizzys I cant count them. Was an engine machinist in a high end speed shop and made a lot of money as a tuner. This engine combo has been built thousands of times and there is an easy way to make it work. Good luck.

 

Agree with previous posts. More initial and minor recurve of dizzy for ~ 36 total. Maybe 16 initial. Then mess with squirter sizes and accelerator pump linkage... make sure there's no slack and the squirters start to drip at the slightest pressure with your finger on the linkage. Might bump floats up just a 1/4 turn and test. FWIW

 

no slack in linkage. floats set were if the fender is lightly bumper fuel drips out of the hole. i picked up a timing tape today. tomorrow i will make the dist mods. a friend of mine works in a dealership. he has been a mechanic for the last 40 years he said after cars started getting alum heads the cars needed a thermostat of 190 degrees to run right. he said it could make all the difference. the air gap intake blocks off the heat p***age. so im going to change the thermostat tomorrow. this was mentioned earlier.

 

ported vacuum increases with throttle opening its in the throttle plate base, you want the vacuum to increase,and advance the timing,with throttle opening, hooking to manifold would do the opposite you would have manifold vacuum at its highest when the engine is at idle, pulling full advance in, and loosing advance as you open the throttle

 

This is wrong. You are missing something in your understanding of how they work. I am not arguing with your logic, cause it makes sense to you, but this is not how they are designed to work. You may be forgetting about the advance weights.
There is a great short article on here somewhere which tells how they actually are designed to work and why. Try this link.
http://api.viglink.com/api/click?format=go&jsonp=vglnk_142859947737110&key=3ddbb42d66675f20d3a5103dae5d4319&libId=i8af7q99010025g1000DAe61pqm6z&loc=http://www.jalopyjournal.com/forum/threads/vacuum-advance-problems.306636/page-2&v=1&out=http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html&ref=http://www.jalopyjournal.com/forum/threads/vacuum-advance-problems.306636/&***le=Vacuum advance problems | Page 2 | The H.A.M.B.&txt=Read what a GM engineer thinks about ported timing.

 

Then I'd use the 50cc on the rear. It still might be working on the front too, that brown cam has quite a range to it.
The 50cc pump is an easy improvement, I'd try that before anything else. It is what it was designed for.