When adjusting a holley carb, what does "stumbling" mean?

It only temporarily "goes away" at very abrupt throttle angle change and/or WOT.

Otherwise.. it's always varying the amount of timing it adds into the mix directly based on engine vacuum. Which in itself is a good indicator of engine load and when an engine can most tolerate it while avoiding pre-ignition.

The fact is that an engines spark requirement changes based on A/F ratio, mixture density, as well as mixture speed because those particular mixtures burn slower and take more time for the flame to travel across the entire combustion space. Hence the additional lead requirements in those situations which improves manifold vacuum production.

So, considering that both density and mixture speed changes with load(throttle angle) and rpm(airflow).. what better way to decide when additional timing can be added for improved efficiency?.. than letting a vac advance pot decide when to best utilize it while under higher manifold vacuum. IE.. at the engines highest vacuum level, under lowest load, and limited throttle angle when pre-ignition is least likely to occur.

In the end.. nearly all engines can benefit from additional advance lead during idle, light throttle/lean fire situations but are limited in what amount may be most effectively implemented due to vehicle weight(changes engine loading and rpm rise/acceleration time), compression ratio, ignition spark strength, cam timing/overlap, and a host of other variables. But.. just because the engine generates the most vacuum at idle to 2,500rpm at say.. 38 degree's of spark lead.. doesn't necessarily mean that the engine/drivetrain/chassis combination will allow it without detonation issues cropping up. It's always a compromise and extra safety margin for fuel quality and ambient temperatures is very smart practice. Especially with cast piston engines like Scoop's that are less tolerant to detonation.

Sorry if that's a bit redundant and scatter-brained response. Trying to get ready for a b-day party and it took me hours to put all that together. I'll have more time to think and respond later on if better explanations are required.

 

This a good read explaining the application of timing advance and the relationships of mechanical and vacuum advance.

http://www.bangshift.com/forum/forum/bangshift/tech-section/32851-ported-vs-manifold-vacuum-advance?p=634637#post634637

 

Hi again Scoop. An 8 inch vac drop is pretty excessive and shows that the engine likes that advance at 900 but cannot produce best vacuum at 550 rpm with similar settings.

As I said before.. adjust the throttle stop to mimic an idle speed you prefer and then tune the disty and carb at THAT SPECIFIC RPM. With that cam.. 700 rpm in gear would be as low as I'd get. Then when the max advance is achieved at say.. 700rpm?.. adjust the throttle stop back up to say 900 to mimic your typical idle speed in neutral.

BTW.. 18 inches for that cam is outstanding and shows what's possible with additional advance. Now the trick is to KEEP that vacuum up as rpm climbs and while under very light throttle angles/light loads.

As I mentioned earlier on the phone.. better to go easy and sneak up on the optimum timing after some road testing than just tune for max vacuum alone. Ultimately.. the specific combo will tell you what amount of timing will be tolerated until detonation starts rearing its ugly head. Be more cautious with cast pistons since heavy load/high speed detonation can break ring lands and crack rings.

Just get a bit closer for now.. and it sounds like you are.. and enjoy it for now. Then when you're more serious about making bigger changes after the mini-meet is over with.. we can start tuning the rest of the advance curve to match the full manifold vacuum sources requirement. At that point.. carb tuning will become a requirement to fully optimize the tune.

 

I was under the assumption that my vacuum drop was from the load in gear, not the drop in rpms...

 

MSmith, what is n AFR kit?

 

AFR = Air/Fuel Ratio gauge.

I had one, but when I got rid of the long tube headers, the bung went with that exhaust setup. So I sold the A/F gauge and kit.

 

stumble means start to miss or quit running

 

Ok, I never heard it referred to as a kit.

 

No reason to be sorry, you just spent hours to answer questions for people on the internet!

 

It's a bit of both actually. If you were to compare vacuum readings between 750rpm in gear to 750rpm out of gear?.. the numbers wouldn't be as far off as they are here.

Here's another way to tune the idle to part throttle transition. It will give you the most conclusive result as to what the motor can tolerate for an initial advance setting because it eliminates the other mechanisms during the testing and tuning process.

1. Put two of the heaviest springs on the mechanical advance mechanism to keep it held tight and to virtually eliminate it from adding any advance for the first 4,000 - 5,000 rpm, or so. You're essentially "locking out the timing" with stiff springs.

2. disconnect the vac advance.

3. Set initial timing to about 20 degrees BTDC so the starter doesn't get overstressed and/or kick back on you.

4. Once the car is started.. adjust the disty to an initial advance setting of 38 degrees and verify that it does not add any mechanical advance at higher rpm's(between 4,000 - 5,000 rpm).

5. While in neutral.. adjust the idle speed down to your required in gear idle speed(around 700 rpm in your case) and readjust mixture screws to achieve smoothest vacuum levels.

6. Readjust neutral idle speed back to about 900 rpm or until reaching a setting that gives you the desired 700 rpm when dropped back in gear. Do NOT readjust mixture screws for the now higher neutral rpm.

7. Road test and reduce timing as necessary to eliminate pinging. I usually leave the disty just loose enough to avoid moving while driving but still loose enough to allow wrenchless adjustments. Also make note of what rpm and load levels that any pinging occurs for future ignition curve retuning purposes.

Manifold vacuum should again approach the same 18" readings previously found at idle when the disty was set to 38 degrees of combined initial and vac advance. You should also have outstanding throttle response and eliminate that slight flat spot/bog that you were seeing without the ported vacuum connection.. as the vac advance can no longer "fall away" under abrupt or heavy throttle openings when hooked to full manifold vacuum. The tires should more easily shed their rubber.

 

I have been following this thread very closely because I suffered the same stumbling/rough idle issues with my 302 running a Holley 600 carb. Well yesterday I decided to take Groberts' advice and switched over to full manifold vacuum with the suggested adjustments.

Before getting started I dug into the dizzy to confirm the centrifugal advance from the weights on the autocam mechanism would provide 26* advance (13L marked on the dizzy plate) so I set my initial timing to 10* BTDC and confirmed it was running 36* at total advance at about 2,700rpm plus with vacuum plugged. Hooked to manifold vacuum, adjusted mixture for maximum vacuum (16"in neutral, drops to 12" in gear) and set the idle to 900rpm.

I was only able to road test it few miles cause of rainy weather but I gotta admit that this car has come alive! Throttle response is immediate with no hesitation, stumble or pinging and idle is nice and smooth. Can't wait to get it out for some real running and final adjustments (if needed)!!
Just want thank Scooper for bringing it up and Groberts for some great advice on a fix. Manifold vacuum all the way baby!!

 

Way to go!!! Glad I was (sorta) able to help!

Nice vacuum numbers...I can't get above 12 and 8.

So idling in neutral at 900, then drops to 550-600 in gear. But I don't mind that...that is a good car-rocking rpm for sitting at red lights.

 

Huh?.. isn't that's lower than the manifold vac readings you had with the ported vacuum setup?

Last I heard you were at..

What happened to those numbers?

@Rasit. Good deal.. glad to hear it.

See?.. the internet ain't all bad. Sometimes it takes a leap of faith to learn new things though. :Handshake

PS. Not to brag or anything.. but if I lived close enough?.. you guys would be running 20+ degrees of initial timing with an extremely limited centrifugal sweep. Add a tiny bit of vac advance pull(the minimum is about 8* degrees) for those long highway cruises?.. and you'll be amazed at that little motors low end grunt.

PS.PS. make damned sure you guys aren't leaning your mixtures out excessively with the extra timing. Jet changes are quite common and air bleeds help you fine tune it from there. Cooler t-stat's in the summer(like 160's) also allow more aggressive tunes and add safety margin. I'd seriously feel pretty bad if you guys cracked a ring land or popped a gasket because you underestimated the importance of tuning back and forth between the ignition curve and the fuel curve. Sometimes fresh plugs help to get better reads on where the tune is at and end up being cheap insurance in the long run.

 

Speaking of leaning out...I lost my top end when I did all of this. I get pops and misses as i gun it full throttle.

Still fast, but not like it was. Quicker on the low end transition to acceleration, but the bottom falls off when I get over 4000 rpms.

Still...gonna work on it.

 

Holy crap dude!.. sorry.. I just noticed your reply here. You might very well be leaning out your mixture with any added timing up top of the rev range! Not good at all with cast pistons. Head gaskets can get fried very quickly in that environment too.

Did you change the total advance from what settings were used on the dyno?

Can you be more specific as to what rpm ranges and what exactly does.. "bottom falls off when I get over 4000 rpms".. mean?