Engine misses at higher RPM's

[ramman]

It's a 79 SF with B38's. Stock exhaust and intake. Starts right up, idles nice when warm, runs good through the gears with normal acceleration but when I crack the throttle and the rpm's rise it begins to cut out (like it's starving for fuel) on at least one of the cylinders. Can twist the throttle wide open at lower R's and no problem. I've totally disassembled, cleaned and installed Mike's kits. Seems like carburetion to me but was hoping someone here can point me the right direction. Thanks.

 

[smokedyerstang]

Do you have an inline fuel filter? Have you checked for issues with the petcock?

 

[5twins]

Did you check and set (if need be) the float levels? When you replace the float needle or float needle and seat, you need to re-check the float setting. The new parts may sit in there differently and throw the setting off.

 

[retiredgentleman]

Things to check:
Air filters.............are they clean, or plugged with dust/dirt?
Carb diaphrams...............check for no leakage, with finger covering the oval port.
Spark plugs...............are they clean and gapped correctly?
How old are the ignition coils and plug leads?

 

[TwoManyXS1Bs]

Check condition of points, and if they are arcing at high rpms...

 

[Conrad Russell]

I know that they are different animals but my '81 H ran just like you say yours is.I had plenty of power no pinging or any thing,but would start to miss or break down at higher RPM's.I did the same as you rebuilt the carbs tried spark plugs ect.I got the Pamco ignition and it's like I bought a new engine.My bike runs better than it ever has,I can slow down to 25 mph in 5th, twist the throttle and she pull's right threw no pings,no nock's.I can run 4th up to 55-60 and no missing just horse power.

 

[xjwmx]

If you had a TCI you'd be describing the symptoms that people have fixed by giving attention to the magnet in the rotor.

On your '79 since it only happens at high rpm, what about points issues? Been a long time since I worked on points but I think I remember falling down at high rpm is a symptom of something wrong with the points.

 

[ramman]

Installed new plugs, wires, caps, points, the air filters are clean. Guess I'll be pulling the carbs again. For whatever reason (probably laziness) didn't put inline filters on this one. Gonna do it now. This thing really runs sweet except for this issue.

 

[xjwmx]

Has it done this ever since you're owned it? Another thing to look at with high rpm probs would be the carb diaphragms. You can read on here how to check them for holes and poor sealing. Best to check things you haven't checked already...

 

[retiredgentleman]

If your ignition coils are 34 years old, they are worn out. Time to replace with a new single, dual output coil. You are likely getting a much weaker spark as the rpm's go up. As rpm's go up there is less dwell time, and your coils just can't deal with the shorter dwell.

Also make sure your charging system is producing proper voltage when you rev the engine. 14.1 volts is what you want to see.