Hi Mike,

 

As mentioned by others, improper fuel ratio is likely the cause.  In my case, I once had to abort a takeoff early in my project when I still used an aftermarket EFI.  You could only set the values for rpm higher than static after take off and the rpm started to increase.  I had just lifted off with the engine rpm inching past static rpm when the engine began a radical surge – would swing from 4800 – 5500 rpm.  Not knowing the cause at the time, I elected to abort the take off and put her back on the 2200 ft runway.  Touched down 300 ft from the end doing approx 70-80 mph and managed to get most stopped on the runway (did go off the end about 10-12 ft).

 

The problem was when the engine started to using the part of the fuel map that I had to extrapolate the values for (rpm higher than static), it was too lean.  As a consequence when the engine hit this region and found less fuel than needed to maintain the rpm, the engine being ‘leaned out” loss power and the rpm dropped back until it found a region of the fuel map that was “rich” enough to support it, it would then start to run back up the fuel map into regions of increasing richness until it again hit the lean region, would drop back, etc.  If I had been more knowledgeable, had more time and thought of it,  I would have simply reached over and increase the manual mixture control – would have certainly been less interesting than my panic 300 ft track of tire rubber left on the tarmac {:>)-

 

As was noted by Mike Wills, not having a visual representation of the fuel map (EM2/3 or my EFISM) and where the engine is operating makes it harder (but not impossible) to identify and fix these problems.  My EFISM shows the fuel map, the engine operating point on the map, an Air/Fuel ratio indicator, as well as the values for each bar of the fuel map all on one screen at one time. Tracy’s EM2/3 provides similar information.   So if your engine hits a fuel bin and the A/F goes lean and you get a hesitation in the engine – then the responsible fuel bin probably needs adjustment.

 

Also, make certain that your EC2/3 manifold pressure sensors are reading manifold pressure.  If they have a leak then they will see ambient pressure and not manifold.  This generally will make the mixture way too rich at idle and low rpm, and then if you adjust the EC2/3 to compensate for this overly richness at idle it becomes too lean at high power settings. 

 

Ed

 

 

Tracy,

>>Large changes in the MAP table values can do that but you did not say at what point in the tuning procedure this turned up.  What have you done or tried so far?<<

I was not specifically tuning, and have not started that seriously. My question was a general one about why surging happens in something like my GEO. I'm hoping to tie the trike down on Sunday and really start tuning (if the weather holds out).

Thanks,

--
Mike

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Mike Fontenot
Apex Consulting & Services LLC
Lakewood, Colorado
303 / 731-6645
mikef AT apexconsultingservices DOT com
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