Hi, I posted a query about a coolant issue I had while testing the air fuel ratio of my carb at WOT. Setting up the carb better than it came out of the box was my aim not chasing coolant issues I guess on a dyno at 10.5/1 I would possibly see black smoke out of the exhaust at. Should I be aiming for a A/F ratio around 12.5 - 13.5 at WOT? I'm guessing I should come down a couple of jet sizes on the secondaries?
The carb is a Quick Fuel BD-1957 fitted to a fairly std smog era (1974)350 SBC. I fitted this carb to replace an ageing Holley 1850 carb that was getting worn in the spindles.
The carb seems to be rich everywhere but I have managed to get the idle AF ratio on a hot engine to the high 13 low 14's. Doesn't seem to matter where I set this though the engine will not idle smoothly (it was the same with the Holley fitted). Timing is currently set at 12 deg BTDC at idle and 36 deg at 2500 rpm. shifting the idle timing doesn't alter the roughness of the idle, I thought a V8 SBC in a mild site of tune would hardly move at idle when running correctly?
To broaden my knowledge about carbs, perhaps someone can tell me why in a four barrel carb such as the above where all of the barrels are the same size and presumably shift the same amount of air the secondaries are jetted way bigger 74's v's 67's in this case.

 

You want 12.5:1 at WOT.

The primary side has a power valve that opens when vacuum drops below a certain point and adds about 6 jet sizes. The secondaries don't have a power valve (at least not the lower cost "regular" carbs) so has to be jetted richer.

 

Yes...10.5 is awful fat. Shhot for high 12's to 13'ish at WOT.

Sure there's no vacuum leaks? Flat cam lobes? What intake is on it?

Usually there are no power valves on rear side so you need a lot more jet to make up for it.

JIM

 

Check for vacuum leaks first, in order to smooth the idle-especially if both carbs cause a rough idle.. Then download a manual for the carb. The manual may have charts as to suggested jet sizes.

 

Hi, no vacuum leaks that I can find, had one in the vac tank - a split now fixed, the system will hold vac sufficient to open the headlights with the engine off. Only place I can think of now would be a leak between the manifold into the lifter valley.
There is no vac can on the dist but there is a MAP sensor to control vac advance remote mounted from the engine, I could block this off to see if there is any change.
Everything seemed to be okay when I fitted the manifold about three years ago, the manifold is a Performer EPS. I reset the valves a couple of weeks ago, it made no difference and everything seemed normal.
Ahh, the power valve, I overlooked its influence on the primary side, it all makes sense now

 

What sort of distributor do you have?? Got pics?? Not understanding how a remote MAP is doing anything to a mechanical distributor...and it doesn't have a vacuum can? How is the MAP controlling vacuum advance?

JIM

 

Hi, the distributor is the standard GM tach drive unit fitted with a Pertronix ignition, the Pertronix feeds into a spark control unit that electronically controls the spark advance, this unit is fitted with the MAP sensor which allows for vac advance. In the pic the black box with the red label is the ignition unit.

 

OK...so the distributor is locked out with no advance? What brand of ignition control is it? Is it programmable?

JIM

 

Inspect the timing at idle and see if it is steady or jumping around. Do they still make that Pertronix controller? How old is it?

 

Hi, yes the dist is locked out, the ignition unit does all of the controlling and yes it is programmable with a laptop. Its an Aldon unit.

 

Hi, I will check the ignition again once I have reassembled the carb and refitted it, I'm fairly sure it is steady but I have not looked at it in a while. The ignition unit is not a Pertronix only the pickup in the dist is. The ignition unit is made by Aldon and it works in conjunction with a conventional coil.

 

Unless you have a really mild cam 12 dgrs is not enough at idle. Do you know what duration your cam has?

 

Hi, sorry I don't know what cam is fitted, the cam has no lope and judging by the rest of the engine, I would guess the engine was rebuilt for a pickup truck and found its way into this vette. The PO "upgraded" the heads so I assume when he got it the truck small valve ones were on it. A local garage fitted reconditioned 882 heads to the engine, I guess they were an upgrade 15 years ago One thing I do know is the harmonic balancer fitted is heavier than what would have been fitted to a '70 base model engine and the pistons are flat top.
What would you suggest as a trial point for the ignition timing at idle?

 

20*+ at idle at least (without vacuum advance). How much advance does the "vacuum" add to the base at idle?

Is the engine smooth off idle and as you rev it up? Any chance there's something mechanically wrong making it rough feeling? Like a terrible balance job during rebuild or mismatched components? Is it possibly an external balance balancer? Is there a chance the timing mark on it doesn't coincide with TDC?

Or is it definitely a "skip/miss" feeling to the tune?

JIM

 

Unfortunately the Aldon unit is set up to imitate ported vacuum, when the RPM's drop below 1500 it progressively pulls the "vacuum" advance out leaving only the initial timing. When I bought the unit they were in the process of changing from simulating full manifold vacuum to ported and the one I got is the latter. I have been meaning to chase them up to see if it can be reflashed to the older configuration.
It is easy enough to plug in the laptop and advance the initial timing, 20 deg sounds like a lot though?
When you rev the engine above idle it is smooth. The original balancer spun and I replaced it, it is not an externally balanced one. I checked TDC when I did this and it was correct. I'm with you though and suspect a mechanical issue. When you rev it there is sometimes a miss I put this down to the plugs getting fouled because of the mixture so I will address the mixture issue before I get back to the roughness issue.
FWIW I put the car in the local garage to see if they could tune it as I was getting sick of it, this garage specialise in classic cars and the owner has a C3 or two. When I got it back it ran a little better but I found that the mixture screws were set unevenly (1 at 1 turn out the other at 3/4 turn any more than one turn and the engine runs pig rich). The problem was still there but less so. They would not say what else they did other than fit new plugs. I won't say here what else I think they did but lets just say the car will never go back there as long as I own it.

 

For stuff with a lot of cam....I'll run 30* plus all day. My 555" has 32* at idle. Think about a full vacuum setup...it would have 20-25* for sure at idle even if you only had 12* static. High initial helps throttle response,runs cooler etc. Not too many downsides as far as performance.

Have you checked float levels? That carb has removable air bleeds....you could swap them around temporarily to see what happens. Like a larger bleed on idle circuit would lean it a lot and allow you to get better control with idle screws. Maybe they're mixed up and the main bleeds are too small making it go rich up top?

Can you control the "vacuum" rate and amount? Maybe bump up initial and remove some vacuum?

JIM

 

Hi, can't go full vacuum without binning the control module and starting again. I intend to try advancing the static timing to see what happens though.
Float levels were set as per a Holley (just to the bottom of the window) but depending on what you read a Quick fuel is supposed to be set halfway up the sight glass so I have set the primary there but not the secondary yet.
According to the paperwork that came with the carb, the idle air bleed was supposed to be a .73 but I found it was fitted with a .70 I changed this to a .75 to see what would happen, so far I can't tell the difference hence my thought that the fuel side needed leaning out.
Yes I can adjust the "vacuum" so I will do this when I advance the initial timing.
Smaller main jets should be here on Monday so I will fit these and take it for another run then see where I'm at.

 

Chasing stuff like this can be tough, have you done a compression check? had the valve covers off and had a look at the rockers to check for flat cam lobes? I would ensure the fundamentals if you have not done so yet. Best of luck, I had a similar situation-lot of tuning work, compression was good...took the covers off and saw several rockers hardly moving. New cam and timing chain and it ran great

 

Thought I would update on progress so far. I changed the primary main jets from 67 to 65 and the secondaries from 74 to 72. The car feels a lot better as you drive it and WOT A/F ratio has improved to 11.8 to 1 from 10.5 to 1. Part throttle its in the 13's. Still deciding but will most likely try a 71 main jet in the secondaries and hopefully this will get me to 12.5 WOT.
I have not found a supplier of idle fuel restrictors (idle jets) here so I tried the old hot rodders trick of putting a strand of wire in the jets. This leaned out the idle mixture and gave better adjustability on the idle mixture screw but as you start to ruin on the transition slot the mixture is too lean, so I have to decide whether to decrease the idle air bleed size or remove the wire from the IFR's
The engine is idling smoother now so I'm reluctant to open up the IFR's again.
FWIW the Quick fuel carb is much more sensitive to small turns of the idle screw v's the Holley I had fitted. I wondered whether they used a steeper taper on the end of the screw v's the Holley?

 

Idle stability problem was caused by carb settings, changing jets and air bleeds have made an improvement but still need to lean out the WOT jetting some as it is 11.88 to 1.
However, I noticed that there has been an increase of valve noise from the LH cylinder bank and according to my "screwdriver stethoscope" it is from the 3 and 5 cylinders. I pulled the valve cover and ran the engine, valves were opening and closing as normal but I then spotted the glitter of metal particles in the oil. Seems the cam is wiped, I will start a new thread about this as I may need some advice as to where to go from here.