oamhmad

iTrader: (1)

So I finally got a speedometer hooked up and i'm now able to figure out how well I've been doing with tuning a Holley 80457 using an AEM wide band AFR gauge.

Pre AFR gauge tuning: 10 MPG (city)
With AFR gauge tuning: 12 MPG (city)

I'm a little disappointed as I thought I would at least be able to get 15 MPG. The problem I'm running into is that with the carb tuned at around 13.5 at idle on a warmed up motor, if I shut down and restart the engine (after going into a store for example 20 mins) the gauge shows very lean about 15.5 spiking to 16.5-17.

I've turned the idle mixture screws rich to cure this problem and now it runs rich at other times (around 12.5-11.5) until I shut down for 20 minutes or so and restart. Then it leans out to the correct mixture.

So I've tuned my carb rich to compensate for a very specific overly lean condition that only manifests itself after the warmed up engine is shut down and restarted after 20 minutes.

I think what's happening is that while the motor is turned off, the fuel line is getting heat soaked where it runs by the water neck and it alters the density of the fuel and leans out the air fuel mixture when I restart it.

Note, this does not happen if I'm driving around, unless I'm in stop and go traffic for a while then I notice the AFR leaning out, even though the engine is no where close to over heating. I've verified this with an infra red temp sensor. (It's about 168 right at the water neck). Other then stop and go traffic, I can drive around all day and the AFR does not lean out....Until I shut down and restart, then it all starts again. Frustrating.

IROCtheRoad

I agree with you - it sounds like heat soak. Are you running a return line?

oamhmad

iTrader: (1)

Yes there is a retun line. It's the stock plumbing on an carbed 84 TA. I have experienced mild vapor lock before on a really hot day.

Is there a special kind of insulated fuel line I can buy?

IROCtheRoad

Not to my knowledge. I guess you could buy some heat wrap and see if that helps.

Apeiron

Have you tried checking the temperature of the fuel line and float bowls?

oamhmad

iTrader: (1)

Nope. I will check and report back.

Fast355

iTrader: (2)

Try a Q-Jet...My stock Q-Jet on my 5,000 lbs G-Van was getting better than 12 mpg around town despite having the slush box.

Street Lethal

iTrader: (16)

This does in fact sound like vapor lock, although you left out a few details. The 168 at the waterneck leaves me questioning the method used to obtain that number. Putting aside when your driving around because your coolant temps will obviously be lower, what are your coolant temps during stop and go traffic? Are you running a mechanical fan, or an electric fan, and if electric, what is its' trigger point(s)? My LB9 stalled due to vapor lock a few weeks ago on my way back from New York (intank pump was screaming like a banshee from the resistance), and when this occurred my coolant temps were at their highest points (primary fan enables at 180-degrees, but temps reached close to 230-degrees that day). EGT's were excessive, and fuel lines anywhere near the exhaust was effected by it. Come to find out my secondary fan was down due to a bad switch, replaced it, and it never happened again, even on hotter days since then. Don't bother replacing fuel lines, a better cooling system, or targeting a lower temperature with the stock one, will do the trick...

oamhmad

iTrader: (1)

I drove around for a while until the engine was warmed up and then pointed an infra red temp senor at the water neck for a few seconds until the reading settled down. It was 168. I've checked this several times and it's always around there. I've verified the accuracy of the sensor by checking boiling water, ice, under my tongue, etc LOL. It's pretty accurate.

I'm running the stock mechanical-fan cooling system on an 84 TA. I have a new water pump, fan-clutch and coolant hoses. The in car temp gauge also shows around 170 when it's warmed up. The highest I've seen it is 190-200 on a very hot day in stop and go, but it very quickly cools down to around 170 as soon as I'm able to get out of traffic. It used to get up to at 220-230, but an air dam soon fixed that.

I installed a heat shield under the carb and the float bowls are cooler to the touch now. AFR readings have also settle downed somewhat. MPG is still around 11 though.

Maybe I'll just go LS1 and EFI. A quadrajet is at least 200 plus another 150 for a spread bore intake. Might be better off saving the loot towards a real engine.

On a side note... why do these cars have such wildly fluctuating coolant temperatures? My civic warms up to always the same spot on the temp gauge (190 i think) and the needle never ever move a millimeter from there, ac or no ac, traffic or no traffic, even if I beat on it. Must a be a very well engineered system.

Ozz1967

iTrader: (4)

My qjet gives me 14city and 18 highway with a 700R4 and a 320hp 350 under the hood. In every way, a tuned a Qjet will get better MPG than a holley. They're just designed better and designed to do it. Harder to work on maybe, but designed better.

paul_huryk

+2 mpg is nothing to sneeze at - you probably picked up a few more MPG on the highway and probably more power too.

Problem is with carbs (all of them) is the fact that they only have a finite amount of adjustments - they are a mechanical fuel metering device after all.

pancherj

I was getting 13-14MPG with my 750 Mighty Demon. My 350 is around 465HP and I have a 700R4 with a 3600 converter. At the time, I had 3.73 gears. I haven't checked it with the new 4.10's. This was combined driving. I see no reason why you can't do better. It takes "x" amount of fuel mixed with air to have good combustion. The chamber could care less how the fuel gets there (Holley, Quadrajet, injection). I don't know what your engine setup is, but the O2 sensor may not be the thing to look at when refining the low speed and idle circuits. Read the plugs, use the O2 and give the engine what it wants (not what you think it wants). Also, the timing curve will play a big part in the gas mileage and if your timing is retarded from where it needs to be, your mileage will suffer and your car will run warmer.

F-BIRD'88

I would look for a exhaust leak (gasket joint, weld, clamp joint) that opens up when the motor is hot soaked. Causes the AFR to look false lean.