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1978 GS750 mid range popping?????

  • Thread starter Thread starter Madmax
  • Start date Start date
M

Madmax

Guest
Just did a new top end on this bike. Everything seemed fine and all of the sudden at aprox 3000 rpm it starts to pop,pop,pop exhaust. When you get through that it will go to red line. Looking for suggestions/ideas.
V&H 4 to 1
Pods
115 main jets
17.5 pilot jets
needles raised one notch as recommended by V&H
I'm thinking it is lean in that mid range. New manifolds. No un-metered air.
What say you?
 
clean the plugs, take it out at the troubled rpm for a few minutes, pull in the clutch and kill it, come to a rolling stop and look at the plugs, it's hopefully jetting.
 
I had this problem, my air mixture screws on the sides of the carbs were turned too far out. Turned them in an 1/8 - 1/4 turn and fixed it. Not saying that is your problem, but might be worth a shot.
 
What slot is the needle set to? If you are in the fourth notch down, you may need to shim it a touch.

Those mains could go up a bit too, but that's not causing this problem at lowish mid-range. At what throttle position does the popping happen? RPMs don't mean anything here.
 
Some 78 and 79 GS750 carbs have the dual taper and very skinny 5DL36 needles, others have the thicker 5F21 single taper. If you have the 5F21's, I'd suggest going to a 5DL35 or 5DL36 in the 2.5 (shimmed) or 3rd notch from the top.

It may be as simple as your fuel screws on the bottom of the carbs upside down need turned out 1/16 to 3/16 of a turn more. Do you have crappy EMGO pods, or do you have quality K&N pods that are properly oiled? not oiled enough and they will run your bike very lean.
 
Plug chop at that throttle position and rpm is the best way to tell what is going on for sure, as previously mentioned. Heck, you could be running so rich you are fouling out a plug or more, and it's misfiring and dumping all that raw fuel vapor into the exhaust where it ignites as the other cylinders are on their exhaust stroke. Lean popping is the same thing, not enough air/fuel mixture to fire the cylinder, dumps it all into the exhaust, vapor then gets ignited from the other cylinders that are firing.

What are your air and fuel screws set at? Have you set your float heights and double checked your real world fuel level with a hose barb adapter and clear hose screwed into the fuel bowl drain port? This is VERY VERY CRITICAL, something I overlooked forever, until I just could not figure out my low rpm carb tuning problems... new seats were needed, actual fuel level check would have showed me that the bench set float height adjustments were not the end of fuel supply setup and tuning.
 
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