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Ok, heres the deal. The motor is rebuilt, idles fine, but backfires terribly when under load, and bogs down over half throttle. I tried the timing, and adjusting the high speed jet, but it has no effect at all. The carb is an HR37A. I also tried a HR40A but it does the same thing. I would like to know what the stock carb settings and ignition timing settings. Also what would any of you guys recogmend doing. It has great compression, starts right up, idles great, but wont rev. I rebuilt both HRs.
 

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T said:
Ok, heres the deal. The motor is rebuilt, idles fine, but backfires terribly when under load, and bogs down over half throttle. I tried the timing, and adjusting the high speed jet, but it has no effect at all. The carb is an HR37A. I also tried a HR40A but it does the same thing. I would like to know what the stock carb settings and ignition timing settings. Also what would any of you guys recogmend doing. It has great compression, starts right up, idles great, but wont rev. I rebuilt both HRs.
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Have you checked that your cylinder is not loose to the crankcase? i have a 12/3 that was having same problem ,turns out that the cylinder/crankcase nuts where loose and it was sucking air making it run lean thus backfire under load or any more than idle, also check yuor plug for colour , it should be brown/tan i usually run my engines alittle rich chocolate brown plug insulator"better to foul plugs than sieze pistons"
if not the above try your fuel lines , they could be stretched at the fittings causing air to leak in making air bubbles go int the carb making it run lean, or your carb is not tight to the cylinder hope that might help ,Dave
 
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