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

That's exactly right.

Look at timing like this. More leans a motor... less fattens it up.

I will go out on a limb here in hopes that it will be taken with less salt than usual. You have too big a carb for this smaller motor and the fuel curve will need to be adjusted to compensate for that fact.

So, when tuning this particular carb you MUST keep in mind that it was designed and calibrated for 350+ inch motors(or VERY high winding 300 CID setups) and you'll surely have to tune it down to the fuel curve this smaller motor needs. Holley likely has a chart somewhere if you want to see what I'm saying in a graph format.

Timing is critical with bigger cams and single planes(although your intake is on the smaller side and has less impact). For one moment.. forget all that you've learned based on what the "factory used" and other cookie cutter comments that spread around the net. Here's the reality. Small motors move less air/fuel at low rpm's and the mixture motion is VERY, VERY, slow.. even with tighter/better chambers known to promote quench and high swirl. Timing lead requirement goes up somewhat proportionally when mixture density goes down(nothing to do with A/F ratio). Smaller motor.. bigger cam.. single plane.. and bigger exhaust system = MORE advance to effectively light rediuced density mixtures and build higher cylinder pressures at low rpm(idle and off-idle cruise). The higher it goes.. the higher the vacuum.

So, here's what I propose for you to do as a simple test. Switch the ported vacuum over to full manifold vacuum and retune the idle and mixture screws. I have not much doubt that this combo could be pushed near 16 inches with correct idle/off idle timing curve.

The BIGGEST thing to watch out for during these tests is OVER REVVING UNDER HEAVY/FULL LOAD once you've added more initial timing and connected the disty's vac source over to full manifold vacuum. The vac advance won't do anything at WOT because it falls away completely as the ported source does.. but the added initial will push total advance a bit too high at higher-rpm's. T be safer, keep it @4,000 rpm and below under varying loads. When searching for timing/fueling detonation limits.. lugging hill's in taller gears will tell you what the engine will tolerate from a max timing standpoint and/or if fuel is going too lean. Road testing is essential and you really want to be testing for off-idle/light-cruise performance under varying loads. Ideally, watching a vacuum gauge WHILE driving helps clarify what the engine likes most.. regardless of whether or not your ears or ass-dyno can still detect any change. Keeping track of the various rpm's, loads, and vac readings will also help you build a picture of exactly where gains start and stop as well as.. where exactly the engine dislikes an ignition or fuel curve going + or - from its previous setting. Depending on how many hours are invested the first tuning session.. fuel and timing curves must be tweaked back and forth(because each one affects the setting requirement of the other) over a longer day.. or shorter couple.. just to get it all nailed down. This is life when testing without an O2 sensor.

BTW.. what's total set at right now?

You'll also likely offset the likelihood that the idle bleeds are a bit too small(which richens the idle mixture) on this larger than required carb.. so, timing will be your friend to lean things out to what a smaller motor will like. Slap a 347 bottom end in it and this carb wouldn't need nearly as much work and may even be good enough right out of the box.

I would bump timing to 12-16 degrees(start low).. and reroute the vac advance to a full manifold source on the carb.

Once you see(vac guage).. and FEEL(no dyno required and your ass won't lie).. the effect I'm trying to lead you towards here?.. you won't want to go back to previous settings. Throttle response/tip-in.. and mileage(less throttle angle to achieve the same cruising and highway speeds as previous combo).. can improve dramatically. Once idle vacuum and light cruise is dialed in?.. we can tune the other parts of the timing curve and fuel curves to match.

Generally speaking.. and assuming the timing is off further than the fuel curve is.. adding more initial timing will help an engine pop off easier than just getting the fuel curve "a little closer", ever will. I know it's easier for me to say this because I've fiddled with them for so many years by now.. but the rewards gained from these headaches are well worth the time investment. Plus, you'll get to see what your combo can really do and be overly proud of the fact that you dialed it in yourself and saved yourself hundreds of dollars in the process($500 in dyno tuning time allowance comes and passes by very fast). Also, I don't believe you'd be as willing to spend larger amounts of cash on fuel injection(wet manifold kits are not as good as a dry manifold setup from max power and economy).. if this carb can blow your socks off and still be easy enough to start and maintain. Plus.. you my as well figure on a programmable ignition box while your at it with FI.. or you're still only half way there from an efficiency and easy tuning standpoint. It's always something else, isn't it?

I'm interested in helping you dial it in and you can feel free to PM me with your contact info should you want more real-time type help. Just doing chores in the garage today and I can check back here on occasion if you have more questions. So, I can easily be reached.

 

Sorry about the long post above since it leads to information overload. I need to learn to hit the send button more often rather than keep adding more and more to the same one.

Here's a chart from Holley showing where your 0-82751 fits into the mix.



Even if your cam was much bigger.. you'd need much higher compression to better utilize this carb's out of the box calibration. It can be fixed.

PS.. I forgot to also mention that tailoring in-gear idle speeds can be calibrated fairly closely by turning the idle-speed screw out to whatever rpm that may be while in nuetral. Loaded engine vacuum in-gear at 750 rpm is barely any different than unloaded vacuum off-gear at that same 750 rpm.

IOW.. if idle speed in gear is 750 rpm?.. that's what you tune for. Then simply readjust the off-gear idle speed back up enough so that it again drops down to that previously desired and calibrated 750 rpm when back in-gear. The difference in loading the engine at that same 750 rpm will likely only require very slight(1/8 turn or less) mixture screw settings to dial it all the way back into the previously optimized unloaded off-idle 750 rpm setting. Much better than just adjusting carbs metering to a free spin rpm of 900 and then wondering why the 750rpm in gear setting isn't as nice. Who cares if it's not perfect in an unloaded state?.. we want the best in-gear characteristics we can get.

That also leads to the motor not "pulling down" in rpm as much in-gear.. when compared to its higher unloaded state idle rpm. Heavier initial/vac adv timing has a major impact here as well.

 

No, it was a good read and lots of good information.

Gives me some things to think about and experiment with

 

BTW...what is Y axis on that chart?

Is that engine size?

 

it's based on engine CID

see?.. now that you bought that carb.. you "need" a stroker kit and more compression! hehe.. horsepower is expensive.

here's the online catalog page for your carb
http://www.holley.com/HolleyCatalog/Catalog.asp?pageNumber=28

 

Do you know how to tune your idle air bleed size? That's why you bought that more expensive "tuners" carb in the first place, right?

Inserting really thin wires inserted into the low speed air bleed holes to help displace airflow is the usual method to find trends. In your case.. re-drilling with a pin-vice and small bits will lean it out. Possibly make the carb more responsive too.

PS. do NOT underestimate the affect of increased timings ability to clean this motor up at idle. That's why you smell gas. :Handshake

 

or in other words advance the distributor till it runs good up a hill without pinging and starts good and it doesn't keep running when you shut it off then adjust the idle up in gear with out the engine missing pull the plugs if they are black the carbs too rich if the tips are starting to get very white too lean shoot for a tan color sometimes hard to do with unleaded gas - shade tree mechanics measure with tape mark with chalk cut with torch

 

Honestly, when I purchased the carb, I called Holley and told them my setup, gears, shifting point, etc. and they recommended this carb for me.

It wasn't like I just figured bigger is better.

When I dynoed with Richard Lelsz at Strictly Performance, he agreed that it was the appropriate carb for my setup. He agreed that I would be sacrificing some low-end and low speed street manners, but this carb would work well when I get up in my power band.

 

Any forward progress with tuning?

 

Back to work, haven't had a chance to mess with it. If I work hard this week, I might get friday off to mess with it.

 

ahh.. I see.

Maybe just do a quick and dirty test at idle/high idle speeds by swapping the disty over to full vacuum source. Then readjust the mixture screws to see what manifold vacuum rises to. You should be able to surpass your other high water mark.

 

I work as a counselor for emotionally disturbed kids at a private school, and we had last week off for spring break. This saturday is a MCCI mini meet here in Houston, so I spent the week getting my car running decent, aligned, new tires, and street legal so I could make the meet.

This week, back to school.

Thinking about driving it to work today, because the weather is great, and my kids would love to see the car. Might get a chance to mess with it a little on the way home.

 

OK, timing advanced to 16, then rerouted the tube so it is idling with about 35 degrees of advanced timing.

Idling around 900 in neutral with around 18" vacuum, but drops to 550 rpms with 10" when I put it in gear.

I would rather have it idling down around 750-800 and staying around 600 in gear.

What next?

 

Groberts, one thing I can't wrap my mind around is why connect the vacuum advance to manifold vacuum. The vacuum and also the advance just going to go away as soon as he steps on it which is the opposite of what you want. Can you explain what I'm missing?

 

Because it's a bit more complicated than that, and ported vacuum + Holley carbs is a recipe for a carb that never runs right.

Tuning a Holley carb is a game, you just keep adjusting it until you are happy with it.

I never felt like mine was right until I got that AFR kit installed on it. Then I KNEW it was right.