I've been stumped for a good couple weeks on this one. 1980 Honda CB750F would absolutely not rev past 5500 rpm. Felt like fuel starvation but power would come back after backing off the throttle. None of the things listed below fixed this issue:
-valves adjusted to looser end of spec (not tight Clymer manual clearances which we all know are too tight)
-intake boots replaced with OEMs and tested for leaks. Great seal no leaks.
-carbs cleaned twice
-carb diaphragms cleaned and inspected
-accelerator pump cleaned and inspected
-fuel replaced with fresh ethanol free 92
-bad grounds checked. All good.
-fuses checked. All good.
-spark plug wires checked. All good.
-spark plugs checked. Looked perfect and consistent between all 4.
-spark plug gaps all checked. In spec.
-picture of the spark plugs for reference (
https://imgur.com/a/n0gSwlg)
After doing all of the above, the bike runs better than it ever has but still only under 5500 rpm...until I took some random poster's suggestion and
removed the air filter cover (?!). Now the thing runs perfectly all throughout the rev range. My analysis: Main jet is likely too rich. I bought the bike from an area at about 1300 FT ASL and am now riding it at about 3500 FT ASL. What is strange is that the bike ran perfectly for months before this 5500 rpm stutter happened. I believe that the ambient air temperature had a role in this too. I was riding the bike in pretty cold weather for the whole time (anywhere from 40º F to 50º F). Springtime came around which happened to coincide with the 5500 rpm issue. The weather the bike now gets ridden in is almost always 50º F+. Warm air is less dense than cold air so put 2 and 2 together and now you have thinner air going into the motor which in and of itself richens the fuel mixture a bit more in addition to the probably oversized main jets. It is so strange because the plugs tell a different story which is that the bike is running perfectly. This is probably because most of my riding is on the pilot jets under 5000 rpm anyways. I have never witnessed this kind of sensitivity on a carb'd bike but I keep hearing about the struggles involved with getting the CV carbs to cooperate and assume this is pretty in line with everybody else's problems...
Will dig into the carb and see what main jet size is currently in there for anyone else having a similar issue...
Also curious to hear from the gurus whether my analysis sounds correct. Has anybody else tried this air filter cover trick? If so what is the proper course of action for making this bike run right? Would like to put the cover back on.
