Need carb help
 

Hoping someone with a better understanding than I can tell me what's going on. I have a .5mm overbored 5.3 with flat top pistons, LS1 cam, with the edlebrock dualplane and 670 Brawler (rebranded quickfuel from Holley). Using an MSD box.

I have been dealing with a bog that was at one time constant and now it goes away after the motor has been running at temp for some time. Usually I notice it go away after shutting it off for a bit and then going again.

You don't detail RPM's or what MSD your running but,,, couple guesses for you,,
Power valve is the first thing I'd check, the stock ones HATE ethanol. You can buy eth/meth rated ones..

Well,, msd boxes.. When they work they rock when they don't,, they don't.. If its a digital they are pretty good. the old school 6A or 6A/L's I wish I had a buck for every one I had to fix after a DNF.. They don't like heat at all, Jump starts are the kiss of death, luckily the wiring diagram is out there and you can actually repair them on the bench. there are a couple devices on the output end and caps that go bad.. You can usually tell by the overheated circuit board trace at the solder joints. Or you send them back to MSD for repair,, if they feel like fixing them.

You also need to pickup a Alcohol test kit, carbs don't vaporize alcohol like injection, I've bought pump fuel that was north of 20% out of pumps labeled 10% and it seems to raise hell with the tune. Best help I know of is finding someone who tunes dirt cars in one of the carburetor classes .

Where are you located? Fuel quality, in particular gas volatility, changes with winter vs. summer gas and of course air temp affects the ability to meter. Not surprising things run different in the cold.

No mention of a wideband O2 sensor. Probably the single best investment for tuning a carb.

When does the bog occur? Off idle, cruising, WOT? What RPM typically? Does the carb have a choke? Vac secondaries? Does it happen with quick throttle movements or just gradual ones?

There are a number of basics to check, like no leaking gaskets, good fuel pressure, float levels, no vacuum leaks, no blocked carb orifices. Make sure there is zero slop in the accel pump linkage. Even the slightest throttle movement should cause a strong squirt out of the accel pump.

Most guys can tune WOT pretty well, just get the right jets. Tuning part throttle bogs can be the hardest part to get just right. Again an 02 sensor is invaluable to get this right.

Goes then bog or bog then goes ?

MSD box 6014. The bog happens from cruise 1000-1500. First carb setup had a bog too. Upping the power valve from 6.5 to 8.5 helped. Motor is pulling 18-20 in vacuum at cruise. Also increased the pump shot nozzle from .031 to .037 with a more aggressive accelerator cam.

I can get some non-ethanol fuel to test if that is giving me problems. There's a station that carries it in the next town over. I'm still trying to understand why it runs better after the motor has been warmed up for sometime. It doesn't go away as soon as the motor reaches running temp.

Can't comment on fuel quality. Vac secondaries with a choke.Bog happens with a quick aggressive stab of the throttle from cruise around 1000-1500. I can cruise all day without issue. Made all the checks you listed.

Fuel is middle of sight glass
pressure is around 6lbs.

Have a wideband 02 sensor. Cruise sits at 14-15 though sometimes it decides to go to 16. WOT is around 12.

Bog then go.

Bog and go sounds like a possible lean condition on acceleration, A cold engine needs more fuel and less as it warms.
I would look at accelerator pump shot adjustment and size.
How and Why to Adjust a Holley Carburetor Accelerator Pump

OK, and see it has a vacuum secondary carb.

Have you tried any different springs in the diaphragm ?
With your vacuum so high, it sounds to its reacting too fast and needs a stiffer spring...possibly.

https://www.holley.com/products/fuel...ts/parts/20-13

Some good reading here...
https://www.mortec.com/carbtip1.htm

Considered the secondary opening. The carb has an adjustment screw to fine tune the opening rate. Tried closing all the way and still had it occur. Will test again when I get a chance. I bought a spring kit and tried the brown spring but didn't notice any difference. Using the paperclip method I determined the secondaries where not opening till 2000 RPM. I might bolt on a 50cc accelerator pump next. my current squirter is at the limit for the 30cc one and I am already using a very aggressive pump cam.

[QUOTE=Bobbed72;20022565]Considered the secondary opening. The carb has an adjustment screw to fine tune the opening rate. Tried closing all the way and still had it occur. Will test again when I get a chance. I bought a spring kit and tried the brown spring but didn't notice any difference. Using the paperclip method I determined the secondaries where not opening till 2000 RPM. I might bolt on a 50cc accelerator pump next. my current squirter is at the limit for the 30cc one and I am already using a very aggressive pump cam.[/

You could also try a bigger squirter nozzle. I would also try jumping up a few main jet sizes because it seems like your having a lean condition by description.

What is your timing at? Is your map timing plot zeroed out and shut off? Shut off the idle control too. May not even be carb related. No way that motor needs 50cc pumps? Doubtfull if it needs 37 squirters either. Is it a new or used carb?

[QUOTE=sccagt1racer;20022802]Bigger Jets will make me way rich at WOT. I'm already rich at WOT. It's lean before the jets come into play.

Timing
16@500rpm
34@3500rpm

I've gone up and down with the initial timing and haven't seen much difference. 15-20 degrees. MAP is at 10 degrees with it dropping out coming off light throttle. I tried it with the map zero'd but it likes it better with some timing. I've also experimented with more/less timing. Idle control is off I read this gives issues.

New carb. Used to have a the bog and hesitation constantly and the bigger squirt and larger pump cam helped that. Now it just happens when the motor is warming up.

[QUOTE=Bobbed72;20022832]Main jets do more than WOT tuning. Ive seen it more than a few times when people keep adding fuel when it actually needed to be leaned out for throttle response.

[QUOTE=sccagt1racer;20022849]Ok. but won't increasing jet size still richen my overly rich WOT? I'm rich at idle also. Have the mixture screws at 1/2 to keep the AFR around 13 and part throttle cruise to 14-15. Much less leans out my part throttle cruise to 15-16.

Start the timing at 14 degrees at 200 rpm. Then ramp it straight to your max timing at 600 rpm. Straight line it all the way across. Get rid of all the dots. Shut off and zero the map. And turn off the idle timing control. It will fire up like a dream and have crazy fast throttle response. You don't have enough timing down low. That's why it won't come out without bogging. We have tried every which way and that is how it works the best for us. Try it and you will see what I mean.

Gave this a shot this morning. Still bogs. At 1500 rpm if I step on it motor dies and then comes to life. I will say it seems to REALLY come to life now. So I will stick with this for now.

I also noticed at 2000rpm to WOT there is no bog. Hmmm. Hope this bit of new data will help solve this problem.

Sounds like you still have something on pulling timing at that rpm. Zero every graph and turn them off. Other than timing of coarse. If that don't work your converter is way too tight and your not getting enough rpm to let it come out.

[quote]

ACCELERATOR PUMP CAMS AND SHOOTERS

Accelerator pump cams come in various sizes and are color coded and number coded by Holley. The cams have different shaped ramps that the arm from the accelerator pump rides on. By changing the size and shape of the arc on the cam, the pump shot can be tailored to start early or later as you go from off idle to full throttle. Changing the cams can have an effect on the way a vehicle leaves the start line in a drag race. If you leave the line off idle or at a higher RPM (while foot braking or when using a trannie brake or when using a clutch with a manual transmission) experimenting with the pump cams can help. There is no set rule for use, you just have to experiment with the different cams and the different cam positioning holes in the throttle linkage of the carb. Holley sells individual cams or you can buy their kit which includes an assortment of cams to choose from.

Pump shooters are another area of experimentation. Holley carbs come with a standard shooter size which differs by carb list#. If you are experiencing a bog or hesitation off idle, you can try a larger, higher # shooter size. The bog or hesitation may be caused by a momentary lean condition when the carb goes from the idle throttle postion to the main metering system. The shooters help richen this momentary condition and eliminate the stumble. Keep going up in shooter size until a puff of black smoke comes out the exhaust, then go back one or two sizes. Playing with the shooter sizes is particularly helpful, when you have an intake with a large plenum area, such as a large open plenum or a tunnel ram. Keep in mind that as you increase the shooter size, you may also need a "hollow" screw to hold the shooters in the carb. At shooter sizes over .039, Holley recommends that you use the "hollow" screw (PN-26-12) which allows more fuel to flow to the shooters.