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My new hobby car (Fiat 126p, 2 cyls air cooled, rear mounted) is not doing correct fuel economy. Spark plugs looks ok. Fumes, at cold start are black but as it warms up, it looks ok. The only suspicious thing here is that one of the intake valves doesn't hold calibration. The rocker arm has some lateral displacement (about 3mm) because the center support for the rockers' rod, which is aluminum and is the oil feeder to up there, got worn by the side of that rocker arm. So if I put the rocker arm in position, I can calibrate, but by just sliding it the clearance will get much bigger. I know the solution is to simply add a spring steel calibration washer between the parts, to obligate the rocker arm into the right position. Obviously, this makes the engine rattle and I should take care of that or the arm tip will get damaged, etc.

However I wonder, and that's the meat of this question: would having such damage, which prevents that valve to maintain a correct clearance, be enough to influence fuel consumption? It should be doing about 15 km/L but it is doing about 10 km/L

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  • This errs on the side of being loose, right? – Pᴀᴜʟsᴛᴇʀ2 Jan 6 '18 at 1:42
  • Correct, I put it in place but then it slides until it rubs the center support element, making the arm it half of the valve's tip and get say the clearance x4. If I calibrate it right there in that position it wouldn't slide to the correct place, but also the pushing rod starts to rub the pipe where it goes within. – Aram Alvarez Jan 6 '18 at 2:00
  • I don't know this for certain ... it may have some effect on fuel efficiency, but I doubt you'd see it reduced by a full 1/3. That's a lot of difference, especially if the engine continues to run at least respectably. The black smoke on start up seems to me to be a fueling issue, like the carb is dropping too much fuel. While the rocker clearance is an issue, I think the carb is where I'd spend on time, at least for this issue. – Pᴀᴜʟsᴛᴇʀ2 Jan 6 '18 at 2:03
  • The fuel consumption is probably down to not driving it like an old granny going shopping... – Solar Mike Jan 6 '18 at 9:47
  • @SolarMike Well I found this little thing quite fun to drive and enjoy its bad attitude :) so yes, I gas well to make it happy, however, I still drive doing tricks I'm used to, to save gas and parts, like evolving gears up as fast as possible and stabilize then, so it cruises and then needs less fuel, run in neutral gear and in idle if road angle let me, go down hills in neutral and idle, shift gears without clutching, ie, "by ear", engage 2nd in idle, so the car "drags", very good for parking or slow rides, etc. – Aram Alvarez Jan 6 '18 at 17:02

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