Under carbed?

[Titleist16]

Would under carbing an engine cause it to get poor gas mileage? I just got my car back with the rebuilt 200 and I am still getting about 8mpg. I was expecting like 20mpg. I have a 1bbl Motorcraft carb on it with headers, dual exhaust, and a 264 110* cam. It also seems to run really rough when I start it up. Even if I try to give it some gas to warm it up it seems to run badly. I think the timing might still be off. It runs smoothly after I drive it for a couple of miles though. Could having too small of a carb give you poor gas mileage?

 

[gtm1086]

You may want to make sure that your choke is not set to heavy. Also has the carb. been rebuilt. If not it maybe time to think about updating.

 

[Titleist16]

Yeah it was rebuilt 1 year ago. The mechanic said that he took the idle screw out as far as he could to keep the car running or else it dies.

 

[MustangSix]

Turning the idle screw out richens the mixture, adding more fuel. I suspect you have a vacuum leak somewhere.

 

[Titleist16]

Is there any way of checking for a vacuum leak?

 

[Anonymous]

sure spray an aerosol can liquid of some sort around any vaccum ports like pcv brake booster egr things like that. If it bubbles up you know you have a leek. It's just like checking a tire for an air leak.

If you would like a more technical approach then buy a vaccum gauge or rent 1 and see whatyourreadings are compares to stock.

 

[Lazy JW]

sure spray an aerosol can liquid of some sort around any vaccum ports like pcv brake booster egr things like that. If it bubbles up you know you have a leek. It's just like checking a tire for an air leak.....

Ummmmm......checking for vacuum leaks is exactly OPPOSITE from checking for air leaks in a tire. Think of vacuum as a NEGATIVE pressure. When you pump up a tire, you are squeezing more air INTO the tire, this causes positive pressure which wants to push the air OUT. This is what causes the bubbles to to escape through a leaky tire.

A vacuum such as inside the intake manifold is removing the air, this is what causes the air to rush INTO the engine. Therefore, it will suck the liquid IN.

The purpose of spraying aerosol is that the engine will pull a flammable liquid in and will rev up faster. Some folks use starting fluid. It can be dangerous so if you choose this method take safety precautions.

To answer your original question, no, undercarbing won't cause poor mileage.
Joe

 

[Guest]

Sounds like the float needs to be adjusted.

 

[Anonymous]

hmmm k i've been proven wrong before but, it worked for me lol. IT does still seem to bubble though. Either way your right i'm wrong t/y for the explanation.

 

[Titleist16]

I checked my spark plugs to see if they are fouled and this is what I saw. The white insulators are white and the metal arms are brown. The metal circle below the threads is like a brown/black color. What does this tell me?

 

[Guest]

I would say check all three issues, the vacuum issues first, the choke also, the throttle body pull back spring (or buy a new one), and also at last ditch, have the carb rebuilt with the float adjusted.

JW, I didnt doubt your knowledge (as were both knowledgeable on troubleshooting on farm machinery and cars), I just thought yet another thing could be checked, in the last of the sequence. The more things you check, the more chance the trouble would be corrected. Starting with the easiest and cheapest, and working your way down to the more complex and costly.