More carb tuning problems.

[Humbucker]

Well, I'm just about at the end of my upgrade project. Now that all is done, I just have some minor tuning issues. The car is running like a scalded dog, but I do have one little issue.

It seems to happen at lower RPM, perhaps 1800-2500. The throttle response curve doesn't seem as linear as it used to. The problem seems to be more prominent in 1st, 2nd, and 3rd gears. I can apply the throttle and feather it the best I can and it seems to accelerate somewhat and then, *bam*, the flood gates open. This causes me to break the tires loose in 1st about 5% of the time. The best I can figure would be a transition problem. The following excerpt from http://www.nastyz28.com/perftune.html seems to apply.

"Transition

The system controlled by the main jets does not want
to come in early. The idle system is at the end of its
delivery and the main jets do not want to feed because
they're not getting enough signal. There are many ways
to fix the problem. Less high-speed air bleed will help.
You can go a little richer on the mains. Any air bleeds
above the fuel level will slow down the initiation of
the main jets. This goes for the small air bleed in the
dogleg of the high-speed air channel in the metering block.
The drawback of small high-speed airbleeds is that the
system goes rich under full power. Now you have the
information to go play."

Does this sound like it could be my problem? Also, before the new heads and intake, I was running 71's in the front & rear. I rejetted to 76 in the primary (thinking I was initially at 72's). Could this be the problem as well? I was thinking of backing down to 73's in the primaries and trying again. Opinions?

Thanks all! I'm just about to the point where I can start giving advise now. It's been a great learning experience.

[Tongue Pirate]

you're still running the single holley?

If you maintain that rpm(1800-2500) for a few seconds does it go away?

[Humbucker]

Originally Posted by Tongue Pirate

you're still running the single holley?

It's a single Holley 750 DP (a 4150) modified by Jet Performance.

If you maintain that rpm(1800-2500) for a few seconds does it go away?

Hmmm... Interesting. I haven't noticed for sure. In recalling the last couple days driving I believe the answer would be no. The best way I can describe the effect would be if you can imagine the throttle linkage set up on a cam type rig rather than a smooth linear one. If I feather it VERY gently, I can make it work fine, but I can tell it's wearing on the clutch.

[scottj]

I question 4 sizes up on the mains...that is a big jump. A stock Holley HP is about 2 sizes too rich on the mains. As I recall, this is a Holley HP from a custom carb shop, so it should be right-on as far as the mains. If this is a custom carb then you should go back to who ever built it. They know how it's calibrated and if they are good they know what needs to be changed to correct it to your engine mods...it might be something that needs to go back to them to recalibrate it.
Scott

[Humbucker]

Originally Posted by scottj

I question 4 sizes up on the mains...that is a big jump. A stock Holley HP is about 2 sizes too rich on the mains. As I recall, this is a Holley HP from a custom carb shop, so it should be right-on as far as the mains. If this is a custom carb then you should go back to who ever built it. They know how it's calibrated and if they are good they know what needs to be changed to correct it to your engine mods...it might be something that needs to go back to them to recalibrate it.
Scott

Yeah, that may be the route I'll go. I just hate to send my carb cross country and be without for 3 weeks, but I'm sure it'd be right when it got back.

Actually, it's 5 sizes up.

To fill you in better, I had the Ford crate 392, 430hp engine. I put AFR 185 heads on and a Victor Jr. intake. Those are the only changes I made.

Before that, the car ran perfectly. Very strong and streetable. This was on 71's in the front and rear of the carb. I'm thinking 73's in front and maybe 74's in rear to start with??

Perhaps the 76's I put in on Wednesday was waaaay too much.

[davidlee]

Mine did the same thing except it was consistant at 2100 2200 RPM. I finally found out the problem was in the transition circuit of the carb. I replaced the metering block and the problem went away. I actually had a defective block as no fuel was getting to the engine during the transition. It went from the idle circuit to the mains. The car would take off ok then stumble them bam blast off like a rocket. It was kind of embarassing in traffic, looked like I was trying to race every time I took off.
David

[Humbucker]

Originally Posted by davidlee

Mine did the same thing except it was consistant at 2100 2200 RPM. I finally found out the problem was in the transition circuit of the carb. I replaced the metering block and the problem went away. I actually had a defective block as no fuel was getting to the engine during the transition. It went from the idle circuit to the mains. The car would take off ok then stumble them bam blast off like a rocket. It was kind of embarassing in traffic, looked like I was trying to race every time I took off.
David

Wow... You hit the nail right on the head with describing my problem. I'm going to check it out today and see at exactly what rpm this is happening.

Could a bad metering block gasket cause this to happen as well? I was told by a friend mine could be reused.

What makes this kind of strange is it ran fine for years before these upgrades. I hope it's not a defective block.

[Tongue Pirate]

If you can run up to 2000 rpm & maintain it smoothly yet when you goose it it goes flat for a moment then it's likely in your acc pump system. If you just have a stumble in that area that doesn't go away even when maintaining that rpm you likely have a lean condition & it may be that you need a lower powervalve to fill in in your low vacuum condition. Or it could be something else completely like clogged passages or failing gaskets. I helped a friend rebuild a carburetor yesterday that had holes in the acc pump diaphram. It had a huge flat spot on acceleration, can't imagine why...

[Humbucker]

Originally Posted by Tongue Pirate

If you can run up to 2000 rpm & maintain it smoothly yet when you goose it it goes flat for a moment then it's likely in your acc pump system. If you just have a stumble in that area that doesn't go away even when maintaining that rpm you likely have a lean condition & it may be that you need a lower powervalve to fill in in your low vacuum condition. Or it could be something else completely like clogged passages or failing gaskets. I helped a friend rebuild a carburetor yesterday that had holes in the acc pump diaphram. It had a huge flat spot on acceleration, can't imagine why...

I'll look into this as well. Funny enough, no problems seem to occur when sitting still and adjusting the throttle by hand. It seems to be present only under load.

[scottj]

Originally Posted by Humbucker

Actually, it's 5 sizes up.

To fill you in better, I had the Ford crate 392, 430hp engine. I put AFR 185 heads on and a Victor Jr. intake. Those are the only changes I made.

Before that, the car ran perfectly. Very strong and streetable. This was on 71's in the front and rear of the carb. I'm thinking 73's in front and maybe 74's in rear to start with??

Perhaps the 76's I put in on Wednesday was waaaay too much.

I keep coming back to this major jetting change. If the carb was correct with 71s before then it should be correct with 71s now. Drivability may not be perfect because of changes to the signal strength but, unless you modified the carb the jet requirments wouldn't have changed. I would put it back to what JET Perf had in it and call them.

[Humbucker]

I was under the (perhaps misguided) impression that making a significant upgrade in power and airflow would require one to jet up a bit.

Don't get me wrong. I'll certainly and happily submit to superior knowledge, but was I just mistaken? You're not the first to mention this. A couple local guys suggested I stay the same and see what happens, while a couple others suggested upjetting.

I just figured morre airflow would require more fuel, thus more jet. What am I missing? Thanks for taking the time with this, too. Much appreciated!

[scottj]

Originally Posted by Humbucker

I just figured morre airflow would require more fuel, thus more jet. What am I missing? Thanks for taking the time with this, too. Much appreciated!

You are correct- more airflow does require more fuel and that is exactly what a carb does. The more air that goes through the carb the more fuel that air draws with it. Once the ratio (jetting) is correct the carb will pull the correct amount of fuel based on airflow.
For what it is worth, I switch carbs between 600 hp and a 800 hp engines and I don't have to change anything.

[JLC800]

What size shooters are you useing?
If they are 32s, your OK there. Then try an 8.5 Power valve.
Sounds like you are experiencing a bog or hesitation off idle.
This should eliminate the momentary lean condition from idle to the mains.
Jim

[Rick Parker]

The manifold will have created the biggest impact on the carb since the signal to the mains is reduced until about 2000-2500 when it begins to work, below that it will not create enough vacuum to initiate the mains. If you check with Edlebrock these manifolds make their power from about 2500-8000 RPM. I think this is part of what you are experiencing, but I would agee that your current jetting may be a little large. These manifolds also do not have the best distribution at part throttle consequently the plug readings may not be consistant, specifically #1 & 5. They make VERY good power but in the mid to upper RPM range. I use one on my 289 and really have no regrets.

[Zoom This]

Jake...I had my Holley 850 massaged by Jet 2 years ago for my 418W and what I got back was an extremely rich carb from idle to WOT. Just from what I am hearing, it "sounds" like your power valve may be too low in number. Jet put in a 3.5. I swapped it out for a 6.5 since my vacuum at idle was 14". The idle screws were way too rich and the motor would lightly puff black smoke at idle. Mixture screws way too far out. After rejetting from an A/F ratio of 10.5:1 at WOT to 13:1 at WOT, the car made better HP, +35, and less black smoke. Changed my squirters from 35's to 28's, changed power valve to 6.5, changed jetting from 78/87 to 73/75, turned in idle mixture screws from 2 1/2 turns out to 1 full turn. You would think Jet could do a better job of redoing a carb, knowing all the motor specs you give them, better.

[JAM1775]

Im trying to help so hopefully you can follow my twists and turns here because I often confuse myself !
But, I am confused why a street car carb would have square jets. I'd call your carb guy and ask about that. A Holley may normally come with ie: 72F/80R but the power valve in the front opens up at a given vacuum to equal approx. 8 jets...therefore square carb when you need the power. 72 +8 +80=160. If your carb was set as square all around at 71F/71R - 142 it would run very rough anywhere down low without an open throttle plate to compensate more air for the extra fuel. Unless, for some "custom" reason they put the PV in the rear ? Perhaps one of the race guys can qualify this but I thought a carb jetted dead square is usually for off the trailer WOT drag racing. Further, all motors being different but jetting for 392's we've seen in our area usually adds up to the 160-168 range for example , 74F/83R + the PV (8)= 165. I think a square setting of 71F/71R with no PV is very lean...the carb guy had to get you more fuel in the curve somehow in that custom build. If your carb guy put the PV in the rear no matter how good the carb/intake is that's still a very heavy dump (uneven 71+8) dump of fuel to the rear four cylinders. A carb can only equally atomize so much of a variance. Holley once told me 10-11 spread front/rear with the front having being the low yet having the extra fuel via the PV , to get closest to square jetting, would be about the biggest spread in main jets before one might have to go the xtra tuning level to adjusting air/fuel better using the air bleeds.
At the very least if YOU WERE square on all 4 the main jets you should then bump up front/rear equally. Better heads may want a bit more jet but 5 on one side is very heavy.

What bigger heads and intake might change is vacuum pull. With a normal carb again would be effect operation of the Power Valve which should be rated in the area of 1/2 your idle vacuum. Sorry for the rant but I think you DO have a Power Valve and your problem may simply be you need to install a lower or higher rated PV to give your motor that extra 1/2 second to rev and match with anticipated fuel dump from the PV. Two things...if you need a lower PV (your idle vacumm lessened) that means your carb is dumping fuel too early as vacuum drops fro your foot going down. The motor now needs to catch up.....dead spot then surge. OR...if your vacuum increased the motor might bog due to LACK OF FUEL until the vac drops lower to open the PV suddenly introduce a surge of fuel.

All you changed were heads and intake. In that regard I would normall expect a possibility of a slightly higher jet change and possible PV change. I'd call the carb guy and see if you can juts tune this out with one of those changes.

[mrmustang]

The thing that comes to mind is too much carb for the engine. When it comes to carb'd engines, bigger is not always better. I suggest a Holley 04777C 650DP to replace your exisiting carb, then play and tweak from there. Just my two cents worth on this particular subject.


Bill S.

[Humbucker]

Holy cow!! Thanks for all that info. VERY informative.

Two things I should mention. First, I fixed the problem. Second, I have another.

The initial problem was found to be in the linkage. I had mounted the "throttle rod" about an inch off, and it apparently made a huge difference in the ratio of the throttle. I fixed that today, and the throttle is now much more smooth and linear.

I am still having tuning issues though, but I think I may have it figured out soon. I am now at 73F, and 78R, and the plugs still look lean. I do have a power valve on the primary side. It's a 6.5. The power valve in the secondary side has been plugged. As well, is it normal to have a small hole in each of the throttle plates? It's hard to determine what all Jet Performance has done to this carb.

Also, I have a 4 corner idle, and my understanding is that I need to have all four screws set the same. My best settings seem to be about 1.25 turns out on the primary side, and .5 turn on the secondary side. Is this a problem?

Thanks again!

[Humbucker]

Originally Posted by mrmustang

The thing that comes to mind is too much carb for the engine. When it comes to carb'd engines, bigger is not always better. I suggest a Holley 04777C 650DP to replace your exisiting carb, then play and tweak from there.

The carb was smooth as silk and pulled like a rocket before adding the heads and intake. Throttle response was crisp and perfect. Now that an upgrade was done that probably added somewhere between 60-80 hp, seems a little awkward to go backwards. Maybe I just don't understand though... Care to elaborate?

[mrmustang]

Jake,

You take the engine cubic inch and multiply by the maximum RPM and divide by 3456. If you have an automatic transmission you will need a vacuum sec. carb, a standard transmission can use a Double Pumper carb. For this to work, you need to get yourself up on a chassis dyno. Sure you can make a larger carb work for maximum full throttle performance, but if you want a carb that is going to handle every day driving, along with your performance needs, then your going to need to drop down in size. Again, get yourself up on a chassis dyno that offers at least HP, torque, and air/fuel ratio and you'll then see what I mean.

Now, with that said, in my case I have a 427 side oiler, solid lifters, 9:7:1 compression, and a Lunati cam, single low riser intake and originally a 780cfm Holley DB (438hp, 468ft/lbs of torque at the rear wheels). After two hours fiddling around with this car on the chassis dyno I took the advice of the owner and used the formula above, dropped down to a 600CFM Holley 04776C DP (some people called me crazy telling me I needed at least a 750DP but the vormular above does not lie) and stepped up with a larger PV and some jetting of the carb. While I lost 3hp and 7ft/lbs of torque at the rear wheels, I gained more performance out of the carb, sharper throttle response, smoother air/fuel ratio across the power band, and a driveability that I had not had before. While the numbers went slightly down in my case, I'll bet performance wise I'll actually be a few seconds faster in the 1/4 mile next time I'm at the strip. So again, when I stated above that bigger is not always better when it comes to the carb you select for your engine.

Hope you find this helpful.

Sincerely,

Bill S.