[dh79]

I'm doing some fine tuning of my carb. It's a 750 double pumper.

I just got a innovate wideband, and I gotta say it is awesome for carb tuning. Here's where the air/fuel mixture is at now:

Idle: 14.5
Cruise: 12.5
WOT: 13.8

The primary jets are 71, secondary jets 85

So I'd like to lean out the cruise and richen the wot. The carb has removable air bleeds, but I haven't experimented with air bleeds before. I figure the simple way to tune this carb would be to jet a bit smaller on the primaries and a bit bigger on the secondaries. Is it possible to have too much difference in jet size between the primaries and secondaries? And if so, would I be better off changing some of the air bleeds also/instead?

My targets af ratios are:
Idle: 14.5
Cruise: 14-15
WOT: 12.5-13
Do these seem reasonable?

thanks,
dh

[crash70rs]

Innovative sells some nice stuff, I just set up the LC-1 on my EFI project.

For Ratios, depending on your setup:

Idle: 13.5-14.0 for something with more cam then stock, but I think it would be better to tune to minimum MAP at idle, using the o2 as reference

Cruise: you might be able to go leaner, around 16:1, watch for ping though

WOT: sounds about right.

Carb tuning: No idea... thats why I went EFI (Dang wiperstappers and there magic boxes)

[dh79]

I guess the other option would be to increase the powervalve circuit flow? Anybody know anything about that? I'm very hesitant to start drilling my metering blocks.

I know a little experimentation with the jets will go a long way. I'm just wondering if there is any problem with having such a difference in jet sizes. Like say primary 66, secondary 90?

[F-BIRD'88]

You'll probabily find that you are for the most part "crusiing" on the idle/idle transfer slot circuit as opposed to the main jets. (the main jets don't come into play untill about 40MPH +/- depending on the rear gear ratio cruising rpm) To trim the cruise try adjusting the size of the idle feed restrictions with small V shaped pieces of mig wire/ guitar string wire.

Take the idle AFR gauge reading with a grain of salt.
A motor with a high overlap cam will cause the AFR meter to show a false lean(er) ratio as the motor's incomplete combustion at idle (rough idle camshaft timing)
allows free oxygen and unburned fuel to pass through the motor to the exhaust.
The o2 sensor does not know the difference. It only senses free left over oxygen in the exhaust. A manifold vacuum gauge is still your best tool for setting the idle. Most carburated engines need a true AFR somewhat richer than 14.7 at idle for good/best idle quality and throttle response.

[F-BIRD'88]

if you want to dial in the WOT jetting you need to get to the track and tune for best MPH in the 1/4 mile. Your AFR gauge is just another tool for you to use.
Then, after finding the best WOT jetting (quarter mile MPH) {while watching your plugs and the AFR gauge}, if there is a big difference between that jet set and the jet set that gives the best overall part throttle AFR, (same day conditions) then you can consider messing with the PVCR's.

You'll need some rear secondary jet extensions.

[oldbogie]

Originally Posted by dh79

I'm doing some fine tuning of my carb. It's a 750 double pumper.

I just got a innovate wideband, and I gotta say it is awesome for carb tuning. Here's where the air/fuel mixture is at now:

Idle: 14.5
Cruise: 12.5
WOT: 13.8

The primary jets are 71, secondary jets 85

So I'd like to lean out the cruise and richen the wot. The carb has removable air bleeds, but I haven't experimented with air bleeds before. I figure the simple way to tune this carb would be to jet a bit smaller on the primaries and a bit bigger on the secondaries. Is it possible to have too much difference in jet size between the primaries and secondaries? And if so, would I be better off changing some of the air bleeds also/instead?

My targets af ratios are:
Idle: 14.5
Cruise: 14-15
WOT: 12.5-13
Do these seem reasonable?

thanks,
dh

Leaning cruise is simply putting in a smaller jet on the primary side, but this really is knowing where the throttle position is against the load. On the flat it's not uncommon to see cruise still on the transfer slots, this is dependent upon vehicle rolling resistance which is made up or bearing and tire resistance added to aerodynamic resistance. Topography also comes to play, it doesn't take too much of an upgrade to require more power which changes the throttle position. You can get somewhat of an idea of power being consumed/throttle position by using a vacuum gauge when testing. This is so much easier to do on a dyno than the street.

Getting into WOT is a more complicated deal as the secondary system is coming into use as well as the power enrichment circuit on the primary. As you already have seen the secondary jetting is much larger than the primary which rather covers the demand for a richer mixture at WOT from that end of the carb. However, along with that is the introduction of the primary system's power circuit. There are two elements to contend with in the power enrichment circuit; first is when or timing, this is managed by the contest between manifold vacuum and a spring in the power valve. The valves are stamped with a number on their side that indicates at what vacuum it turns on. A higher number brings the valve on sooner (with less throttle opening)and a lower number introduces the system later (with relatively more throttle opening). The second concern is amount of fuel which is managed by jets located behind the power valve in the metering block.

Leaning the primary main metering to bring the cruise to stoichiometricly correct 14.7 will also lean WOT. The simplest approach to richening WOT is to increase the secondary jet size. I wouldn't touch the primary's power jets as this is a one way street that if you get wrong is difficult to adequately fix. An old trick on Holleys is to install a primary metering block in the secondary which then provides the secondary with a power circuit. This allows you to back off the main jet sizing a little and still get the proper overall richness at WOT.

Bogie

[dh79]

When I say cruise I mean 3000 rpm on the freeway (100 km/hr). At that point I'm pretty sure I'm on the primary main jets.

I'm using this wideband as another tuning tool in the toolbox. I'll be taking it to the track soon, but I'd like to get my wot down to 12.5-13 first, and experiment from there.

I've been using the wideband and vacuum gauge to set the idle mixture. I think it's nailed down pretty well now. I'm amazed how such a small change in the mixture screws affects the afr.

When are the extended secondary jets recommended? Last time at the track it ran 12.6, but I think low 12's are realistic. It 60' 1.74 last time. Any idea what the air/fuel datalog will look like when the secondary jets go dry? I guess it would lean out for a while, then come back?

thanks for the help,
dh

[dh79]

The question I'm most interested in, which hasn't been answered yet: can there be too much difference between the primary and secondary jet sizes? If not, then I'll just lean out the primaries and richen the secondaries until I get my desired ratios.

dh

[oldbogie]

Originally Posted by dh79

When I say cruise I mean 3000 rpm on the freeway (100 km/hr). At that point I'm pretty sure I'm on the primary main jets.

I'm using this wideband as another tuning tool in the toolbox. I'll be taking it to the track soon, but I'd like to get my wot down to 12.5-13 first, and experiment from there.

I've been using the wideband and vacuum gauge to set the idle mixture. I think it's nailed down pretty well now. I'm amazed how such a small change in the mixture screws affects the afr.

When are the extended secondary jets recommended? Last time at the track it ran 12.6, but I think low 12's are realistic. It 60' 1.74 last time. Any idea what the air/fuel datalog will look like when the secondary jets go dry? I guess it would lean out for a while, then come back?

thanks for the help,
dh

You used a term that got my attention which is "secondary jets go dry"! If the jets were to go dry you'd have a serious leaning of the mixture, either a top end back fire and or detonation would be the first indicators, roasted pistons would be next. The Holley (actually any carb in drag racing) does have a situation from vehicle dynamics where the fuel rushes toward the primary jets and away from the secondary jets. The secondary jets typically being larger will show a greater amount of leaning than the smaller primaries will show richening. This is where jet extensions can be of help, especially on the secondaries because the fuel rushing away from the jets can cause a severe lean drop in the mixture. With jet extensions it keeps the jet connected with the pool of fuel now at the rear of the float bowl. On the primary side I like a bit shorter jet extension to average out the depth of fuel that is now a puddle against the metering block, although, I'm a lot less concerned about the primary side as possible engine destruction isn't an issue here, this is just a tuning issue to keep the mixture in a tight range around what works best for the engine.

Bogie

[oldbogie]

Originally Posted by dh79

The question I'm most interested in, which hasn't been answered yet: can there be too much difference between the primary and secondary jet sizes? If not, then I'll just lean out the primaries and richen the secondaries until I get my desired ratios.

dh

Your in the tuning circle, there isn't any exact ratio between primary and secondary jets, you have to tinker your way to a solution the engine likes for the installation of the vehicle its in and your local environment.

Get a bowl fuel drain kit, keep a roll of aluminum foil around to catch dumped fuel and some rags to mop up with, a stack of bowl and metering block gaskets and a bunch jets about 5 ranges up and 5 ranges down from what you've got, . Don't smoke, but gasoline flavored coffee is OK.

Bogie

[dh79]

Originally Posted by oldbogie

You used a term that got my attention which is "secondary jets go dry"! If the jets were to go dry you'd have a serious leaning of the mixture, either a top end back fire and or detonation would be the first indicators, roasted pistons would be next. The Holley (actually any carb in drag racing) does have a situation from vehicle dynamics where the fuel rushes toward the primary jets and away from the secondary jets. The secondary jets typically being larger will show a greater amount of leaning than the smaller primaries will show richening. This is where jet extensions can be of help, especially on the secondaries because the fuel rushing away from the jets can cause a severe lean drop in the mixture. With jet extensions it keeps the jet connected with the pool of fuel now at the rear of the float bowl. On the primary side I like a bit shorter jet extension to average out the depth of fuel that is now a puddle against the metering block, although, I'm a lot less concerned about the primary side as possible engine destruction isn't an issue here, this is just a tuning issue to keep the mixture in a tight range around what works best for the engine.

Bogie

I don't think the secondary jets are going dry. The only reason I asked is because F-Bird88 mentioned the extensions. I have no reason to believe this is a problem at this time.

I certainly have no problem changing jets. Carb tuning isn't new to me; I was just curious about that one question and the removeable air bleeds which are new to me. It seems to me I could've answered this question by just going out and doing it.

I appreciate the replies; I think it's time for less conversation, more action

dh

[oldbogie]

Originally Posted by dh79

I don't think the secondary jets are going dry. The only reason I asked is because F-Bird88 mentioned the extensions. I have no reason to believe this is a problem at this time.

I certainly have no problem changing jets. Carb tuning isn't new to me; I was just curious about that one question and the removeable air bleeds which are new to me. It seems to me I could've answered this question by just going out and doing it.

I appreciate the replies; I think it's time for less conversation, more action

dh

I was going to write another dissertation sized article, but remembered this link to an engine not too different from yours.

http://www.carcraft.com/techarticles...eds/index.html

bogie

[dh79]

Originally Posted by oldbogie

I was going to write another dissertation sized article, but remembered this link to an engine not too different from yours.

http://www.carcraft.com/techarticles...eds/index.html

bogie

Yeah, that's a decent article.
thanks,
dh