Carb Diagnosis

[Wbulk]

This is a quiz since I finally found the solution. So put your best diagnostic skills to the test and see if you can figure this one out. It took me a couple of weeks off and on with the aid of a wide band O2 sensor.

Engine is a 306 mild build 212 @ .050 comp cam with 9.2 compression.
Total timing not including vacuum advance is 35 deg. 15 for vacuum advance.
Rev. limiter set at 5400.

CARB
New out of the box Quick Fuel 600 VS Slayer series. I like this carb. It is very tunable. I live at 5,000 ft. which complicates the issue. Just about all air bleeds, jets and restrictors where changed more than once. Granted engine is over carbed for the rpm range it runs in.

SETTINGS/SYMTOMS
It likes to idle at 12-12.5 AFR at 16" manifold vacuum. Idle screws at this altitude are 1/2 turn out.
As the engine heats up the AFR at idle seems to wander from 11.9 to 13.5. I have a 1/2" heat spacer under the carb.
Idle transfer circuit restrictor jets were changed to allow for altitude and is running from 13-14 AFR.
Main Metering jets were decreased 3 steps for altitude. 2 steps were also tried.
Power valve restrictors were reduced from .055 to .051.
Primary main metering kicks in at exactly 2,000 rpm. Air Bleeds were increased in size to what Quick Fuel recommended for the altitude.

As the engine gets into main metering sometimes the AFR runs correct from about 13.5 to 14.5 and other times it stays rich at a pretty much consistent 11.2 AFR. At times it appears to be heat related, (cooler rich, hotter good), but not always.
During light acceleration, less than the PV opening vacuum rate, may go lean to 16 or may stay below 15. During WOT AFR is about 12.2 at the beginning and goes to about 13.8 as rpms increase. After this AFR may remain rich or be correct. The plugs look good but were pulled after I arrived back home each time after running in idle transfer for about three minutes to get home.
It took awhile to get to the point were the symptoms were identifiable due to necessary altitude tuning in the idle transfer area, air bleeds and main jets.

Quick fuel was called four times on the issues but only recommended main jet and air bleed changes. They were nice and professional but I could tell they kind of thought I was nuts. I began to think the same thing to.

I also hooked up my laptop to the wideband and recorded a graphed session for 15 minutes when the main metering was running rich. It confirmed the gauge was correct. This was a tough one. Although rare, this condition could exist on most Holley based carbs and you may not know it if you don't have a wide band sensor.

Consider all options. Good luck.

Added: One rule, because carb issues (circuits) can be inter-related you must get the issue dead on, not in the ballpark.

[vector1]

the sometimes good/not good would lead me to believe gasket/sealing issue or something variable, like float level.

like operating over the internet.

[Wbulk]

Interesting thought process. A thinking man! A Quick Fuel tech told me they did have a similar incident with a misaligned gasket, but that is not it. Me thinks I must not give clues this early in the game.

[jhirasak]

Faulty power valve diaphragm with a crack or leak which allows the PV to occasionally open to early resulting in over-rich AFR while you are operating on the main circuit. As the diaphragm moves, the amount of the air leak can vary which can allow it to open earlier than desired, hence your sometimes too rich AFR symptom. Each time the engine is stopped, the PV fully opens because there is no manifold vacuum to keep it closed. The next time you start it up, the manifold vacuum should reseat the valve and depending on the amount of air leak through the diaphragm, the PV could open early or stay closed if there is sufficient manifold vacuum.

[Wbulk]

jhirasak, well thought out and articulate, and reasonably plausible. However that is not the cause of this issue. Perhaps I should have mentioned I have two power valves and tried them both. I also tested the diaphragms of both with my Mighty Vac which I have a special cup just for testing power valves. I also questioned Quick Fuel if they ever had one where the needle was not seating. This is hard to test. They said they had just one.

I want to give clues, but then this would be over too soon. Remember you must be dead on with the issue.

[jhirasak]

Wayne,

There are a few of other scenarios associated with the Power Valve that I could describe that will all give you the symptoms that you have described. These include a failed spring, sticking valve, partial blockage of the manifold vacuum port, or improper seating of the gasket on the power valve seat. With the information provided thus far, I cannot give you a better guess as the actual cause of your problem.

Cheers,

John

[vref120]

I'm no dog in this fight but I did have this in my primary on my QF 780. Not sure where it came from (probably tank) but worked its way into the seat. Caused all kinds of issues.

[Wbulk]

vref120, that looks like Teflon tape. Most carb companies say don't use it for fuel lines now for that exact reason.

[Wbulk]

Jhiarsak, you are so close that I have to l give it to you. Winner, winner, winner!!!!!

Here is what I found. The last day I drove it the problem was still there so I took the carb apart for the umpteenth time. I decided I was going to study every single detail of the gaskets, the metering block, and the power valves and circuits. I checked the power valves again and they tested good. But that was the logical area. Somehow it had to be staying open at times. So, I next followed the vacuum passage and tried to blow air through it. A little air, but there was the anti-backfire valve. So I removed the throttle blade plate from the main body to get to it and tried again. Not conclusive, so I put a hose over the bottom vacuum port for the Mighty Vac and I could get about 5-7" of vacuum as I pumped the handle and it would drop slowly. I then would shake the plate and test it again. This time I was only able to get a few inches of vacuum and it would drop right off. I should be getting no vacuum with my gauge so the port would have no restriction to the power valve.

I then got on the phone with Quick Fuel tech support and discussed the issue. It was suggested I try and push the ball down. I have some micro drill bits so I found one small enough to go in the hole. It was solid. There was no spring against the ball. With no spring the vacuum pulls the ball down to an un machined end of the hole that cuts off most or not all of the vacuum. No or not enough vacuum and the spring pressure holds the power valve open. I am thinking since the bottom of the hole is just drilled and not intended to seal vacuum, over a period of steady state cruise it would build up enough vacuum from closed to fully open and all points in between, causing the AFR to go back and forth.

So, Jhirasak you were right with the restriction in the vacuum port. Good job.

Quick Fuel is sending me a new anti-backfire valve and gaskets. Who would have thought the valve would be installed without a spring during the manufacture. Consider if you have any carb with a anti-backfire valve and it fails to work and you have no wide band hooked up you may not know you have this problem for some time.

[ffr 1137]

Wayne

Kudos on your diagnostics. You say without a wideband a person would never know this was happening. Just curious, what kind of driving issues were you having? And also, had any job offers from those guys? I bet if you had sent that carb back it would have been scrapped.

Mike

[Wbulk]

It was just not very responsive and varied on performance the point of confusing. But there was no radical identifiable issues. I seriously began to think it was me. Without a wide band I would have been lost.

In contrast I also had a brand new Edelbrock 1406 I had in a box for years. I tuned that before I started working on the Quick Fuel. It was very responsive and stable. The only down side was there was no way to adjust the transfer circuit for altitude. I knew that would effect mileage so I moved on to the Quick Fuel. However I must say the primary main and power circuits on the Edelbrocks in my opinion is superior. I am sure this carb will be great once I get the parts and fix this.

[Wbulk]

I drove the car yesterday after installing the new anti-backfire valve that Quick Fuel sent me. Everything is great now and as it should be. The kit is different than what came with the carb. You drill out the hole bigger with the supplied drill bit with drill stop. The seat, ball, and spring are bigger. Possibly an improved design. I took pictures but for some reason I can't upload them, the button is not showing to upload.

Just to summarize this situation:
Quick Fuel left out the spring in the anti-backfire valve. Without the spring the vacuum sucks the ball down to the bottom of the hole and blocks off the vacuum port. No vacuum to the power valve and the power valve stays open. As soon as you get into the primary main metering circuit you will go rich. This was not easy to find and was confusing to say the least. It costs about $100 in jets, air bleeds and restrictor jets and two tanks of gas to get to the problem. About half the jets costs, I would have had to pay anyway to tune this due to altitude. I still really like this carb.

I just have some minor fine tuning to do. I put on the zip tie to try Patrickt's secondary opening check and do the adjustment.

[Jerry Clayton]

those anti back fire valves cause more troubles than actually caused by blown power valves----------KISS