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Discussion Starter · #1 ·
My bike won't pull past 65. I can get up to 60 in 4th and when I shift into 5th it'll pull right to 65 and if I try to push higher it starts acting like it's going to die.

The bike is a 1971 cb350 bored 1mm over with 16t front & 38t rear sprocket with stock 3.50-18 tire, I have pods on it with pvc extensions to get a more correct velocity profile. My jetting is slightly rich, plugs are on the blacker side of tan and are ngk b9es
 

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^^^add to that timing and the advancer unit operation. and if you're running a bit rich, heat range 9 plugs isn't helping either. you should be running B8ES to start with and could run B7ES if a bit rich, but getting the jetting closer couldn't hurt either
 

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Discussion Starter · #4 ·
Diaphragms age unknown, but do move when vacuum applied, charging system is putting out 13.5-14 volts battery is 8ah but still healthy, the exhaust is stock Honda pipes with a set of Harley mufflers clamped on, engine has 600mi on since rebuild, point have at spec and timing set with timing light, shows full spark advance around 3000rpm. The b9 plugs and Rich mixture were kind of a cautionary choice, engine was rebuilt due to a detonation problem
 

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So you have CB headers and are the Harley tips baffled?

You should take the time to confirm your carb diaphragms don't have a slight hole that would prevent the piston from being raised.

Why b9 plugs? <<<edit, saw the comment about detonation. What gas are you using?
 

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Discussion Starter · #6 ·
Mufflers are glasspack style straight through. What doesn't make sense to me is that I can run up to ~8500rpm in every gear but 5th. In 5th about 6500rpm is what she'll give me.
 

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I'd take a look at the carb diaphragms and the advance unit, and make sure you have a good, fully charged battery with clean cable connections.
 

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Discussion Starter · #9 ·
Mufflers are glasspack style. Using 91 octane.

I did some looking and noticed that I don't have the correct type of petcock on the bike. It has a single outlet feeding both carbs instead of dual, I have ordered a repro petcock just to eliminate that variable. Starting to think I'm not flowing enough fuel at higher speeds.

I can rev the bike with filters off and see the slides raising and lowering with rpm. Of course that doesn't mean a pinhole is impossible.
 

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What is the RPM at your max speed?

I have a 17 Front & 36 Rear at 70 I'm running 7100 rpm.
I can get it to pull more but don't have a good road to do it on.
The max I've had mine up to is 85.
 

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Discussion Starter · #12 ·
60mph in 4th @ ~8500rpm
65mph in 5th @ ~6500-6800rpm
That's as high as she'll cruise at, I can pull to 75 for a few seconds then she starts to cut out and forces me back to 65
 

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That's as high as she'll cruise at, I can pull to 75 for a few seconds then she starts to cut out and forces me back to 65
That says fuel or battery voltage to me. Easy to hook up a cheap digital volt meter (Harbor Freight often gives them away, otherwise very inexpensive) for a quick check on that. Your dual outlet petcock may fix it. Make sure you have a filter in both lines. Also, petcocks often have integral filter that can get plugged.
 

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In line filters on these bike are usually more of a problem than a fix and tend to create more problems then they solve.

Most are designed for a pressure feed system not a gravity feed one like we have on the older twins.

They tend to restrict fuel flow.

Cleaning out and de-rusting your tank, cleaning out the Petcock screen filter is the right way to go.


How much do you weigh?
What is your body position?

Are you running points?
If yes how old is your condenser?

It sole purpose is to suppress arcing that occurs when the points open and close.
When the points are arcing they are not allowing charging current to flow to the coils.

At high rpm high speeds you need all the spark you can get.

Also double check the advance is working correctly with a dynamic timing light.

Full speed requires full advance.
 
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