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Loading up at full throttle (sorry so long)

Fuel System: Gas (Petrol) tanks, Carburators
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michaeljulan Member
Posts: 1
Joined: Fri Sep 27, 2019 8:48 am

Loading up at full throttle (sorry so long)

Post by michaeljulan » Wed Jul 08, 2020 10:41 am

I have a '65 CB77 that I am have a rich issue at full throttle.

Compression is @ 145 cold on both cylinders. Carbs ultrasonic cleaned and all jets and needles are oem and oem size. 135 main 42 slow and 24231 needle in the 2nd slot from the top. New floats set to 22.5mm. 2mm throttle valve cutaways with the R installed in the right and the L installed in the left. Throttle cable set with 1/8 play and both carb slides synced. New plugs. New plug wires and ends. New oem style paper air filters. Idle air really likes to be 1 turn from closed. Both mufflers puffing the same pressure (feeling by hand). All lifters set at .004" cold and points gaped correctly. Timing set both statically and then dynamically with timing light. Advance looks to be coming on @ close to 2500 and tops out right at the two marks on the flywheel.

The bike runs really good through 1st, 2nd and 3rd through full rpm range and throttle (this thing is a rocket ship off the line) but I have an issue with loading up when I try to go to full throttle in 4th gear. It just won't go. It will hit 70 and bog down. As soon as I start to back off the throttle it picks back up. If I add even a little choke (rise the lever 1/4") it makes it worse instantly. If I ride around for 20 minutes and don't bog it down at full throttle the plugs look great. They have a very light mocha color and no soot. If I ride it for 20 minutes and try full throttle a few times the plugs are black with soot. Not sure where to go from here. Tried the top needle groove but that killed the bike at 1/4 to half throttle.

Is it possible that I need to go to 130 mains?? I have yet to find anyone running 130's. Most I talk to are going the other direction and using 140's

I have built several CB160 bikes and have fought lean conditions before but never anything like this.

Any help would be appreciated. This one has got me stumped

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brewsky Member
Posts: 1816
Joined: Wed Jul 23, 2008 5:21 am
Location: Princeton, WV

Re: Loading up at full throttle (sorry so long)

Post by brewsky » Thu Jul 09, 2020 5:29 am

Maybe air cleaners too restrictive?
Try running without them and see what happens.
A dyno sniffer would tell for sure.
66 dream, 78 cb750k, 02fz1, 09 wing

mike in idaho Member
Posts: 411
Joined: Sun Jan 29, 2012 9:18 pm
Location: orofino, idaho

Re: Loading up at full throttle (sorry so long)

Post by mike in idaho » Thu Jul 09, 2020 7:06 pm

What rear sprocket size?, sounds like it's pulling too high a gear. Maybe add a few teeth on the rear or drop one on the front. These things have to run at 6,000-7,000 rpm to be in the fat part of the torque curve.
'65 YG1
'65 CB160
'66 CL160
'66 CL77
'78 XS650
'79 GL1000
'69 T100R
'68 TR6
'69 T120
'72 750 Commando
my company car is a Kenworth

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