carb/timing help

[steelworker77]

Still playing with my carb on my BP 347. Came with a Holley 670 Brawler 4 corner carb. No experience with the 4 corner setup at all. Issue I am having is I have a stumble under load right off very light throttle. It easily pushes through it but it bothers me enough to try to get rid of it. Its amplified when its cold until it gets some heat in it. Originally it was pig rich at idle to the point it would foul plugs fairly quickly at idle. Off idle it actually ran fairly clean. I took it to the local "performance" shop to see what they could do with it and was alittle dissapointed with the result. They cleaned up the idle but that slight hesitation was still there under load as well as in neutral if you stab the throttle off idle quickly it goes bahhhhhhhwaaaaaa if that makes sense.

They did the following changes: Drilled primary blades 3/32" and set mixture screws at 1/2 out in rear and 3/4 out in front. Unhooked vac advance, light springs in so I had full timing by about 15-1700 rpms. Intial set at 18 and total at 36. No pinging at all but seemed odd to just disconnect vac advance.

I have tried various changes in timing and idle to try to get rid of that slight hesitation. In order to completely get rid of that bog I end up having my mixtures screws out at 3/4 rear and 1 in the front with the vac advance hooked back up. This made the engine way to rich again at idle but that transition off idle was perfect. I have verified I do not have the primary blades open to far as well as checked my accelerator pump adjustment. Float levels are correct as well as fuel pressure. Not sure what to try next. I have read about issues like this running air gap intakes and cold weather. Right now low is about 15* and high in the mid 20's. Temp is staying right around 175-180 while driving. Any help on this is really appreciated. Its my wifes daily driver so I really need to make it so she can bump the key, fire off and idle for a length of time so its not fouling plugs or stalling as well as just be as smooth as possible. Aside from the off idle stumble it really runs strong. It will melt the 33's fairly easily, just cant shift to quickly with the np435 lol.

Current setup is this and still have that slight stumble.
Timing. Initial 18* with total 36*. Vacuum advance hooked up to ported with 1 heavy 1 light spring so all in at 2800 rpms.
Carb. rear out 1/2 front out 1 1/4 secondary idle screw out 1/4 turn from where it was.

Engine specs are as follows as well as carb specs:

Description:​

• Horsepower & Torque: 415 Horsepower / 415 FT. LBS. Torque
• Compression Ratio: 10.0:1
• Aluminum Heads
• Cast Crankshaft
• Hydraulic Roller Cam

Block:​

• BluePrint Engines High Strength Cast Iron Block
• Increased Deck Thickness and Strength Webbing
• Priority Main Oiling
• 4.000" Bore
• 4-Bolt Main
• 1-Piece Rear Main Seal
• Improved Water Jacket Design

Rotating Assembly:​

• Cast Crankshaft
• 3.400" Stroke
• Forged 5.400" Connecting Rods
• Forged Pistons
• 28oz Externally Balanced Front and Rear

Cylinder Heads:​

• BluePrint Aluminum Cylinder Heads - HP9009
• 60cc Chamber
• 2.02" Intake / 1.60" Exhaust Valves
• 190cc Intake / 60cc Exhaust Runners

Cam Specs:​

• Hydraulic Roller Cam
• .543 Intake / .554 Exhaust Lift
• 218 Intake / 226 Exhaust Duration
• @.050-112 Degree Lobe Separation

Ignition Timing:​

• 10-16 Degrees Initial, 32-34 Total

Carb specs from holleys site:

Barrel Quantity	4
Barrels	4
Billet Color	Diecast- No billet
Body Color	Polished
Body Finish	Polished
Booster	Down Leg
Booster Design	Downleg
Brand	Brawler
Carburetor Use	Street
CFM	670
Choke	Electric
Circuit	2
Condition	New
Cubic Feet Per Minute Rating	670
Emission Code	3
Finish	Shiny
Fuel	Gasoline
Fuel Inlet	Dual
Fuel System	Carbureted
Grade Type	Performance
High Speed Air Bleed	28/28
Idle Air Bleed Size	70;70
Linkage Attached	Yes
Marine Use	No
Material	Aluminum
Model	Brawler Diecast
Primary Main Jet	70
Primary Power Valve	6.5
Primary Pump Nozzle Size	31
Primary Venturii Size	1.300"
Product Type	Carburetor
Sale Category	Carburetors and Components
Secondaries	Vacuum
Secondary Barrel Type	Vacuum
Secondary Main Jet	74
Secondary Venturii Size	1.300"
Supercharged Application	No
Throttle Bore	1 11/16"
Warranty	Limited 90 Day
UPC	090127003657
Part Number	BR-67256

 

[steelworker77]

Just pulled #1 plug after drive to work this morning. Total timing maybe a little to much reading the ground strap?

 

[Brush Hog]

I’ll probably start a huge debate but maybe try hooking vacuum advance to the manifold vacuum and re-tune carb and readjust timing. I don’t know much about how each works and that it is an on going debate every where but with the manifold vacuum the way I understand it is you will have timing advance at idle and with your engine depending on cam specs maybe it will make a difference. Easy to try and free. I have a tired old 302 with an edelbrock avs 2 and I have the opposite problem. Stumble on manifold vacuum off idle but not on ported.

 

[73azbronco]

That plug is rich.

Are you using plugs recommended by BP? Might try next step or two hotter see what that does.

so when you took it to the speed shop, did they give you a printout of fuel air ratio? Sounds like they just tuned it using their ear, as in, they don't work with carbs.

All the printout numbers are neat, but not really a thing. I think you need to find a tuner who works with carbs, who knows carbs. Did BP say it was tuned and ready to run, because if it came as a complete setup, and runs like that, they owe you an explanation and a carb set to work with your setup.

I just read up on that 670 carb, seems to me to be the absolute cheapest carb they could possibly make. 4 corner idle adjustment??? why? I recommend after you complain to BP, you call a known carb shop and ask what they can do for you.

Lastly, I can't say enough good things about carburetor unlimited in PHX area. http://carburetorsunlimited.com/ call them and he will answer

 

[BOBS 2 68S]

Have you changed the accelerator pump cam? There are different cam profiles.

 

[Torkman66]

I have the BP 302. Came with Holley Warrior. Mine was doing exactly the same thing. After all the same adjustments you describe, I went back and loosened the accelerator pump nut a turn or so. Used the feeler gauge to ensure cam not touching at idle. That fixed the bog at initial acceleration. Might try that.

 

[steelworker77]

I have the BP 302. Came with Holley Warrior. Mine was doing exactly the same thing. After all the same adjustments you describe, I went back and loosened the accelerator pump nut a turn or so. Used the feeler gauge to ensure cam not touching at idle. That fixed the bog at initial acceleration. Might try that.

I have played with it some. I havent tried different cams or squirters. I was actually thinking of going to a 35 from the 31 to see if that helps?

Did you adjust the linkage while the engine was hot or cold? There is a decent difference with the same adjustment cold vs hot. How it is current is while the engine is hot there is no play at idle. As soon as the throttle moves it squirts fuel.

I will try and go the other way and give it some play while hot.

That air gap manifold I think it causing a bigger difference than I thought. I noticed it I drive it for awhile then shut it off for a few minutes then go drive again the hesitation is not quite as bad until im driving again for alittle bit. Thinking the manifold is heat sinking after shutting it down then cooling off again while driving. This morning driving it was a whopping 13*.

How do you have your timing setup ported or manifold as well as initial and total?

 

[Torkman66]

My vac is ported. I've run it both ways and not much difference. Blueprint suggested ported off mid/front section of carb. I'm at 15 or 16 initial and 32 to 34 all in at around 3200rpm. At 18 your timing may be a bit too advanced...pinging does not always work. Just a thought. Might back it to 16. Also, did you adjust idle mixture using vacuum gauge? My carb only has the two idle/mix screws in front bowl. I also adjusted both bowl floats but not sure that had anything to do with it.

 

[steelworker77]

My vac is ported. I've run it both ways and not much difference. Blueprint suggested ported off mid/front section of carb. I'm at 15 or 16 initial and 32 to 34 all in at around 3200rpm. At 18 your timing may be a bit too advanced...pinging does not always work. Just a thought. Might back it to 16. Also, did you adjust idle mixture using vacuum gauge? My carb only has the two idle/mix screws in front bowl. I also adjusted both bowl floats but not sure that had anything to do with it.

It actually likes even more timing but the 18* bushing is the biggest they sent so im not comfortable going over 36 total so 18 initial is where I landed.

I have used a vac gauge and that does clean my plug up some but makes the off idle hesitation worse. I am basically band aiding with running my mixture alittle rich to help.
Just wasnt sure exactly where to start making changes. I have a message into BP so I should get a call in a day or 2.

I do have it set up with a different shop that I do trust and have used before to dyno tune it. I will need to trailer it out and drop it for a few days. Thats 3 weeks out. I was just trying to see if I could get it figured out myself before then.

 

[73azbronco]

air gap is a nice manifold, but does result in poorer cold running issues, because well, its designed that way.

Did you try calling BP? sounds like they throw any old carb they have on and wish you luck.

 

[steelworker77]

air gap is a nice manifold, but does result in poorer cold running issues, because well, its designed that way.

Did you try calling BP? sounds like they throw any old carb they have on and wish you luck.

waiting on a call back. Seems the 4 corner adjustment carbs are more for track use with the added adjustability after talking to a few people. I wouldnt be surprised if there was actually something up with the carb. Holley's QC seems to be really lacking the last few years or more.

I am thinking about just picking up a 670 truck avenger as we do plan on doing some offroad stuff and I have actually had great luck with that carb in the past.

 

[73azbronco]

670 not shabby, not the best, but should work with some fiddling, and yes, 4 corner idle is not a street or offroad thing.

I'm still standing that BP needs to pony up a carb that works...

 

[pcf_mark]

More opinions may not help but I'll jump in.

18 degrees sounds like a lot but it more like the top of the scale than off the scale. Looking at your ground strap I would say you are right on the number. I am running 18 n my SBF with EFI and I think it runs and idles amazing. Plenty of vacuum and strong off idle pull. I am running an Explorer 4R70W and stock converter. I would leave the timing and go to the carb settings. You have ported vacuum which may be fine. If you had manifold vacuum it can make a stumble worse - the rpm starts to die, vacuum is reduced AND the timing is reduced because the vac can is seeing less vacuum. Now a small carb stumble turns into a flat feeling engine.

Before propagate a discussion on rich or lean from a spark plug that was in an idling engine I would just forget about it. You can't read a plug well enough unless you remove it from the engine / kill the engine at the moment the bad behavior happened. That carb is a street style carb with the four corner idle added in the 90s to give big cams some idle help. Using a vacuum gage at idle adjust each screw to get max vacuum. Then engine off go back and check that they are all very close to being out the same amount. Unless your idle screws are way off (or one is set way in and the others way out) this is not causing your drivability concern.

What may be causing it:

1) Accelerator pump tuning: lots of advice above that may get you there. Heavy truck, tall tires, not enough gear usually means you need a sizable pump shot (nozzle size) and a decent cam on the arm. This is hard to pick out you just have to try different parts. This usually shows itself as a lean pop you can literally hear if you keep adding throttle. I have drilled nozzles open with pin drills but there is no going back on that unless you have spare parts.
2) Too slow of an opening for the power valve. Depending on how much throttle you are giving it you could be transitioning from main jets to power valve. Most Holley's come with a PV rated at 8.5-9" of vacuum. You do not get the extra fuel until the vacuum drops below that. If you have strong vacuum it stays closed too long and it feel sluggish. Going to a 10.5 or 11 coudl get the fuel you need earlier.
3) Just jetted too lean. Holley ships carbs rich stock I would be shocked if it was too lean. Look at the jets in the front bowl they should be around 64-68.


The area most modern Holley's are poorly tunes is cruise. I find them all to be very rich at cruise rpm but this help to keep the engine cool and makes the transition smoother when you roll on the gas.

If you really want to learn something buy a wide band O2. You may think and feel like your dead spot is lean but it could also be rich. If you watch the O2 when it feels like and the mixture goes to 20:1 you know you have a lean spot.

Good luck and do not be surprised if you just swap to a different carb your problems go away. Later Holley's are not so great.

 

[DirtDonk]

I didn’t notice if you had mentioned it already, but have you verified that there are no vacuum leaks?
I know it’s a new engine, so that would normally be very low on the list of suspects. But it’s totally worth checking.
Also, is there a PCV valve? If so, how is it connected?
What about vacuum brake booster? If so, how is it connected?

If you have a PCV valve, try a quick experiment by plugging it temporarily. Maybe a little bit of tape over the opening.
Drive it around the block and see if the bog has gone away. Or at least lessens a bit.

 

[steelworker77]

More opinions may not help but I'll jump in.

18 degrees sounds like a lot but it more like the top of the scale than off the scale. Looking at your ground strap I would say you are right on the number. I am running 18 n my SBF with EFI and I think it runs and idles amazing. Plenty of vacuum and strong off idle pull. I am running an Explorer 4R70W and stock converter. I would leave the timing and go to the carb settings. You have ported vacuum which may be fine. If you had manifold vacuum it can make a stumble worse - the rpm starts to die, vacuum is reduced AND the timing is reduced because the vac can is seeing less vacuum. Now a small carb stumble turns into a flat feeling engine.

Before propagate a discussion on rich or lean from a spark plug that was in an idling engine I would just forget about it. You can't read a plug well enough unless you remove it from the engine / kill the engine at the moment the bad behavior happened. That carb is a street style carb with the four corner idle added in the 90s to give big cams some idle help. Using a vacuum gage at idle adjust each screw to get max vacuum. Then engine off go back and check that they are all very close to being out the same amount. Unless your idle screws are way off (or one is set way in and the others way out) this is not causing your drivability concern.

What may be causing it:

1) Accelerator pump tuning: lots of advice above that may get you there. Heavy truck, tall tires, not enough gear usually means you need a sizable pump shot (nozzle size) and a decent cam on the arm. This is hard to pick out you just have to try different parts. This usually shows itself as a lean pop you can literally hear if you keep adding throttle. I have drilled nozzles open with pin drills but there is no going back on that unless you have spare parts.
2) Too slow of an opening for the power valve. Depending on how much throttle you are giving it you could be transitioning from main jets to power valve. Most Holley's come with a PV rated at 8.5-9" of vacuum. You do not get the extra fuel until the vacuum drops below that. If you have strong vacuum it stays closed too long and it feel sluggish. Going to a 10.5 or 11 coudl get the fuel you need earlier.
3) Just jetted too lean. Holley ships carbs rich stock I would be shocked if it was too lean. Look at the jets in the front bowl they should be around 64-68.


The area most modern Holley's are poorly tunes is cruise. I find them all to be very rich at cruise rpm but this help to keep the engine cool and makes the transition smoother when you roll on the gas.

If you really want to learn something buy a wide band O2. You may think and feel like your dead spot is lean but it could also be rich. If you watch the O2 when it feels like and the mixture goes to 20:1 you know you have a lean spot.

Good luck and do not be surprised if you just swap to a different carb your problems go away. Later Holley's are not so great.

Appreciate the help. I did go ahead and order a 35 pump nozzle. Currently has a 31. Also ordered a cam kit.

Primaries are 70 secondaries are 74's. I did go out and just gave the accel pump linkage some slack to make it come in just slightly later to see if that would help and the hesitation got much worse. Hoping just bumping up the nozzle size helps.

 

[DirtDonk]

That seemed a little counterintuitive to me.
Making it come in earlier and stronger would be the help to a lean bog. Bringing it in later would sound, at least to me, to make it worse.

 

[steelworker77]

I didn’t notice if you had mentioned it already, but have you verified that there are no vacuum leaks?
I know it’s a new engine, so that would normally be very low on the list of suspects. But it’s totally worth checking.
Also, is there a PCV valve? If so, how is it connected?
What about vacuum brake booster? If so, how is it connected?

If you have a PCV valve, try a quick experiment by plugging it temporarily. Maybe a little bit of tape over the opening.
Drive it around the block and see if the bog has gone away. Or at least lessens a bit.

Checked for vacuum leaks multiple times and havent found anything. Running hydroboost so no booster. I do have a pcv valve and I was planning on trying this exact thing when I get home today. Read various stories of people swapping valves and magically all is good.

Someone had recommended this one awhile back to me. It could certainly be the wrong "flow" but not sure.

ACDelco PCV Valve - 19313320​

 

[pcf_mark]

What is your idle vacuum and vacuum at cruise? Vacuum at the time the lean or dead spot happens? This can be a good info (or buy a wide band O2).

Adding gap would definitely make it worse. This video is ok to get it adjusted.

 

[steelworker77]

Idle vacuum is 13. I don't have a reading while driving. May try and rig up a longer hose this weekend. I think im going to invest in a decent wide band setup. Any suggestions

 

[DirtDonk]

That 13 is pretty low, but in line with that cam and HP rating I would think.
Stock is 19 on the low end and 22 on the high end usually. I am a little bit surprised though, that it's that low even with your 18 degrees of advance.
Hmm, I wonder if there is a mis-matched timing mark and pointer situation. Very common with home built Ford engines, but not so much with pro-built stuff like BP.

Just for giggles, you could advance the timing at idle while watching the vacuum. See what it takes to get it up to, say, about 16 or 17 inches. That's about as much as I'd ever expect from a cam that big, but it might be possible to get there.
If it starts running worse while you're getting more vacuum, it's just too much. But if you can get a little more vacuum signal out of it (15 would be a good start!) with some more advance and not lose any drivability, it'd be worth it.
The builder says that 36 degrees total is fine, but I think you said you have it at 34. Am I remembering that correctly? If so, it sounds like you can throw at least a couple more degrees at it just to see if the vacuum goes up.
Only reason I'm on about the vacuum at all, is that higher vacuum signals usually benefit carburetor metering and adjustability.

Speaking of vacuum, is your current power valve the correct rating for that idle vacuum? Earlier it was mentioned that power valves are a common sore spot for being the wrong one for a particular engine build. If you have not done so already then, you might re-visit this aspect and compare your actual reading to what valve is installed.

Paul