Thread: 350 running rich

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Hello all!

I'm working on a carbeurated LT1 for a friend and am having a heck of a time figuring out what's wrong with it. It's running way too rich and no matter what I do I can't seem to correct it. It's running a Holley Brawler carb. Timing is set at 32°, fuel pressure is right around 5 psi, spark looks very good. We're in Pueblo, Colorado so the first thing I did was change out the jets to 68 thinking that the altitude was messing with it, but that made no difference. It's pretty much immediately carbon fouling the plugs. Vacuum is way lower than it should be at 1000 rpm idle, we don't get decent vacuum until about 2500 rpm. Adjusting the primary idle mixture screws does absolutely nothing, when screwed all the way in there is no change in how the engine runs, and zero change on the vacuum gauge. When adjusting the secondary screws, it will stall out when they are screwed all the way in, but still virtually no change on the vacuum gauge. Even when adjusting the timing there is no change on the vacuum gauge. I'm using one of the ports in the front bottom of the carb to check vacuum and also run the vacuum advance. I was thinking vacuum leak, but I can't seem to find any anywhere. The fact that the primary idle screw adjustment makes no difference either way makes me think that it's an issue with the carb. I pulled the power valve today to look at it, but it looks fine to me.

Does anyone have any suggestions as to where I might look next to try and solve this issue? If I left any info out that would be helpful, please let me know.

Thank you in advance for any tips/advice!!

Change the power valve it most likely has a bad diaphragm leaking fuel into vacuum ports making it rich and low vacuum.

4,692′!! You're gonna probably have to lean it out a lot.

Thank you for the suggestions!!

Right now there's a 6.5 power valve in it, do you recommend going with the same size? And where would you start as far as jet size to lean it out, is 2 sizes down enough?

Until your vacuum stabilizes we won't know if the 6.5 will work or not, and the vacuum won't come up until the mixture leans (catch 22). Luckily, Power Valves are not real expensive, I would buy a couple different sizes, 8.5 to put in it now and a 6.5 incase it needs it later. Stick with the standard flow rates for now, with your altitude I doubt you’ll need higher flow rates.
8.5 & 6.5 are linked below;

https://www.holley.com/products/fuel...s/parts/125-85
https://www.holley.com/products/fuel...s/parts/125-65

With your altitude 2 Jet sizes smaller may not be enough, I would buy an assortment of #52, 54, 56, 58, 60, 62, 64;

https://www.holley.com/products/fuel...s/parts/230052

Tuning for this altitude is going to take some time and expense, be prepared to spend some bucks. Buy several sets of plugs, it'll probably foul a couple sets before we get it to run decent enough to dial it in.

Change the Power Valve and start with #64 primary jets, don't mess with the secondary jets until you get idle and cruise under control. once you get it running you can start reading the plugs.

No sense in looking at the plugs now, we know it's pig rich and they'll be black and probably wet.

Thank you for the detailed reply!!

He actually has the 60-69 jet kit already. Power valves are on order, I'll update after I work on it this coming weekend.

Is it ok to clean and reuse carbon fouled plugs, and if so, how many times before they just need to be replaced?

Originally Posted by JD323

Is it ok to clean and reuse carbon fouled plugs

Sure, if you don't want to know if your tuning did anything at all.

Seriously, I warned you it's gonna get a little expensive.

No prob! Just making sure. These are what we're running for testing, any objections or suggestions? The engine is a mid 90's LT1, unsure of the internals.

https://shop.advanceautoparts.com/p/...2756/5390532-P

Use a regular non resistor plug. Resistor wires are OK, but resistor plugs tend to give trouble

Keep listening to 36, the poor guy isn't getting any sleep lately, he's helping tune my carb too lol. Stick with his advice, he'll get you fixed up.

Originally Posted by JD323

Adjusting the primary idle mixture screws does absolutely nothing, when screwed all the way in there is no change in how the engine runs, and zero change on the vacuum gauge. When adjusting the secondary screws, it will stall out when they are screwed all the way in, but still virtually no change on the vacuum gauge. Even when adjusting the timing there is no change on the vacuum gauge. I'm using one of the ports in the front bottom of the carb to check vacuum and also run the vacuum advance. I was thinking vacuum leak, but I can't seem to find any anywhere. The fact that the primary idle screw adjustment makes no difference either way makes me think that it's an issue with the carb.

Hi Denny,
Float levels was a good catch, I totally missed it. If the floats are too high it will spill into the carb's venturi and run way rich.

Usually when you can shut the idle mixture off and make no difference it indicates it's getting fuel and air from somewhere else. While a vacuum leak can be a cause, usually it causes a lean condition not rich, this usually leans towards a carburetor issue. High floats or a damaged Power Valve diaphragm or gasket are likely suspects. The vacuum at idle is going to stay low until the overly rich condition is corrected, I suspect a damaged Power Valve issue, and it may be float levels as well.

Mid 90's LT1 usually don't like as much total timing, 32-34° is usually max for their head design.

A couple of caveats about Power Valves:
Power Valves manage (control) the enrichment circuit in this type of carburetor. Their design is to add fuel under loads and WOT (this is the purpose of all carburetor enrichment circuits). With Power Valves the rating number on the PV indicates the vacuum level at which it opens and supplies fuel to the venturi boosters. Manifold vacuum holds the PV closed, as vacuum drops with throttle opening the PV opens and supplies the necessary additional fuel needed.

Power Valves (or any carburetor's enrichment circuit) can be and should be used to tune WOT because the secondaries take a little time to activate before they can supply fuel. While Power valves come in different vacuum ratings, they also come in different flow rates, standard, high flow, 2 corner, 4 corner and stages (single or two stage), with the two stage adding a slight amount of fuel at medium throttle levels (usually not needed). PV enrichment circuit's passage restrictors can also be increased or decreased in size to further tune the circuit (not advised for un-experienced).

Also, you will hear people say "Power Valves can not add fuel at idle because they are a venturi driven circuit (above throttle) and with the throttle closed there is no venturi vacuum to draw fuel up into the boosters". And this is true, EXCEPT, when the PV diaphragm is damaged or housing/gasket leaks. When this condition exist (damaged PV), manifold vacuum pulls the fuel through/past the PV into the manifold and adds fuel EVEN at idle. And JMHO, why it should be one of the first things checked when a rich idle condition exists.

Finally got a chance to work on the truck again. I installed the new 8.5 PV and the 64 jets and new plugs. Still having the same issue though. There's no movement on the vacuum gauge when adjusting the idle mixture screws. I have the idle set right around 1000 rpm. I recorded a video of me adjusting the screws. I screwed them both all the way in and then back out 2 turns. It does seem like it's making more vacuum with the new PV, but I'm assuming that's because it's a larger size? Any ideas what to try next? Here's a pic of one of the plugs after about 10 minutes of running.

https://youtu.be/YZ9MNaWyO1Y

Picture of your vacuum gauge please, it's kinda bury in the video. Drop your idle down to 700rpm and see if it adjusts and check idle timing at 700rpm too.
Leave this set of plugs in it til it's running better

You will never get an 8.5 power valve to close at that altitude- put an 4.5 PV in and go down to 50 jets-------your problem is running rich with low/no vacume that the carbs circuits will work--------Make it LEANNNNNNNNNNNNNNNN and then come back up -----
In artillery-----over, under, adjust----------your over!!!!!!!!!! now get under!!!!!!!!!!!


I've never heard of a Brawler carb-------does it have mechanical seconaries and maybe even 2 power valves???????????