83 air cooled gas mileage decrease

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83AC
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83 air cooled gas mileage decrease

Post by 83AC » Tue Oct 24, 2017 7:08 pm

Okay friends - our 83 air cooled with engine rebuilt about 15,000 miles ago recently had a change in gas mileage from an average of 18.5 mpg to about 16. The change happened after about an 80 degree day in the Washington DC area where we sat in traffic that was craaaawling for a long time with no way out of it. Our oil temp gauge was reading up to 260ish, but I didn’t stress it much because it has tended to read high almost always, sometimes by 40 or so degrees. Anyway, the fill up prior was averaging 18.5 and since then on the same stretches of straight highway driving 55 we now only get 16.5 at best. I’m wondering where I should start looking first. No change in gas type. Was thinking I should do a valve adjustment. Oil is new and not different from what was in before (Castrol 20W50). Thinking temp II sender, but beyond that I’m not positive what to check. Thought maybe timing but that doesn’t make sense. Temps in the engine have seemed to remain stable as well. Not hearing alarming noises from the engine. Love some ideas.
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Randy in Maine
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Re: 83 air cooled gas mileage decrease

Post by Randy in Maine » Wed Oct 25, 2017 7:44 am

If it were me, I would look back at the engine first to make sure that no hoses have fallen off anything. I would probably then check the dwell (if you are running points) and the timing to confirm that the vacuum advance is still indeed working correctly.

Then I would check the tire pressure to confirm they are what they should be.

Nobody gets great mileage if they are stuck in traffic. Might take it for a country ride to see if it performs better.
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SlowLane
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Re: 83 air cooled gas mileage decrease

Post by SlowLane » Wed Oct 25, 2017 8:10 am

Even though you are putting in the "same type" of gas, keep in mind that fuel composition varies over the year, with gasoline producers tweaking the chemistry to accommodate changes in temperature, humidity, etc. It could be that your most recent tankfuls are of the winter formulation. I have no idea whether this would fully explain such an increase in consumption, but just trying to point out that there are external influences at work, over which you have no control. Does the engine performance seem any different?

But yeah, check your tire pressures.

Oh, wait. Yours is a California model, isn't it? If so, try checking/replacing the oxygen sensor. Also if so, check for plugged-up cat.
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Amskeptic
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Re: 83 air cooled gas mileage decrease

Post by Amskeptic » Thu Oct 26, 2017 8:32 am

83AC wrote:
Tue Oct 24, 2017 7:08 pm
Okay friends - our 83 air cooled with engine rebuilt about 15,000 miles ago recently had a change in gas mileage from an average of 18.5 mpg to about 16. The change happened after about an 80 degree day in the Washington DC area where we sat in traffic that was craaaawling for a long time with no way out of it.Love some ideas.
Inescapable is the richening that both temp sensors provide when cold. Your gas mileage is likely to be less as the outside temps drop. The intake air sensor reads the day's ambient temperatures at all times. If it is cool outside, then you are getting more fuel.

Yes, check timing if you have points. It does retard over time as the gap closes.
Colin
BobD - 1978 Bus . . . . . . . . . . .110,350 miles
Chloe - 1970 bus . . . . . . . . . . . 206,775 miles
Naranja - 1977 Westfalia . . . . 72,350 miles
Pluck - 1973 Squareback . . . . . 55,478 miles
Alexus - 1991 Lexus LS400 . . . 64,425 miles

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