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Power loss at 3500/4000 rpm

F

Frank66

Just got it firing!
Joined
Nov 26, 2012
Messages
14
Location
Richmond, VA
Hello,
My 1974 V7 Sport is suffering from a radical loss of power at 3500/4000 rpms while accelerating through each gear. After a brief interval of this loss, slow acceleration begins and it can get up to speed and feels normal again. It' feels and sounds like it is starved for gas. I have checked for blockage and everything is fine, the air filters are fine. Has anyone had this problem, any ideas, suggestions?
Frank Kostek
 
Are you sure it is lean? How is your economy?
My guess it is running too fat. Why? Because since they removed lead from gasoline it no longer has the same lubrication properties.

Guaranteed your needle jets (atomizers) are ovalized and the needles are worn thinner right at your normal cruise throttle position.
 
Is this a recent condition? Is this bike new to you? It would help to know how it is currently jetted (include atomizer and needle sizes if you can). It would also help to know how fast you are trying to open the throttle.
 
Hi Frank,

We need a bit more info.
Is this a new bike for you or is this a new symptom?

Does it rev when not under load or does it only do this under load? Does it do this if you open the throttle slowly or only when you give it a fistful? Does it rev above this range?

In my experience people are quick to blame carbs when it is more likely to be an ignition problem. Often these problems occur when the engine comes on the cam as the increase BMEP requires more spark energy to light the mixture.

is an electronic ignition fitted or is it the standard points and coil? Are the coils the originals? Are the metal bosch plug caps still fitted? Are resistive plugs fitted? Do the caps have resistors (NGK plugs which I recommend let you fit whatever you wish - I go for none and pick a resistor plug). I have seen a number of cases where the owner fitted. Are the wires carbon or proper wire?

One simple test is to run the bike on one cylinder with a test plug grounded to the cylinder on the inoperative cylinder (Carb fuel off) and check for spark through the affected rev range.

Chris R
 
The clue to your problem could be in the fact that it is fitted with filters whereas it originally wouldn't have had them......the jetting would need to be adjusted from standard to compensate.
 
Hey Frank,

Have you fixed this yet?
My 73 V7 Sport now has a big flat spot on initial excelleration off the line.
I plan on addressing the issue in the comming week or so.

Alex
 
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