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Author Topic: 78 cb750k surges and won’t pass 6 grand sometimes  (Read 1047 times)

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Offline Mopar_or_no_car

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78 cb750k surges and won’t pass 6 grand sometimes
« on: July 05, 2018, 11:55:30 AM »
I just got my 1978 cb750 going and it does some odd things I don’t understand when I get up to about 70 after a while it starts to surge/buck and some times it goes away other times it will bog and kill the bike. Most recently it began to surge and wouldn’t let me get any higher than 6000 rpm I’m not sure what would cause it to do this because I only does it after I’ve been cruising for a few minutes I know it’s gettin fuel what causes this because I’ll shut the bike off and fire it back up and it will do just fine till I get back up to around 70?
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Offline lowriderbud

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #1 on: July 06, 2018, 05:09:24 AM »
My '78 750F did something very similar - found it was the gas cap not venting - drilled through the vent hole and the problem went away.

Later, Bud...
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Offline flybox1

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #2 on: July 06, 2018, 05:56:58 AM »
When was the last time all periodic maintenance items were completed?
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Mopar_or_no_car

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #3 on: July 06, 2018, 04:04:15 PM »
I just changed the oil and cleaned the carbs throughly all jets removed that is all I’ve done te bike sat for 10 years before I got it and I’ve put 400 Miles on it
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Offline Mopar_or_no_car

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #4 on: July 06, 2018, 04:08:10 PM »
It just did it again today surged then wouldn’t rev up in gear then died and I shut the bike off for 45 seconds fired it back up and ran fine
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Offline madmtnmotors

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #5 on: July 06, 2018, 05:43:55 PM »
What about the in-tank fuel filter?
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Offline flybox1

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #6 on: July 06, 2018, 07:44:19 PM »
Quote from: Mopar_or_no_car on July 06, 2018, 04:04:15 PM
I just changed the oil and cleaned the carbs throughly all jets removed that is all I’ve done te bike sat for 10 years before I got it and I’ve put 400 Miles on it
how about a valve adjustment, timing and advance....carb vacuum sync....the really important stuff!Q
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Mopar_or_no_car

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #7 on: July 06, 2018, 08:44:33 PM »
The in tank filter is clear and flows fuel as it should this was the first thing I checked when it started but no I have never messed with valves or anything like that internally
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Offline MoMo

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #8 on: July 06, 2018, 09:18:48 PM »
reply 1 is a distinct possibility
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Offline TwoTired

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #9 on: July 06, 2018, 09:41:49 PM »
Look at the spark plug deposits to see if the center insulator is coated with conductive carbon.
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Lloyd... (SOHC4 #11 Original Mail List)
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Offline Mopar_or_no_car

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #10 on: July 07, 2018, 06:56:48 AM »
Today I’m gonna run it with the gas cap off slightly and see if that solves the problem and I actually have a new set of plugs for it I’ve just never put them in so I will do that today as well and hope for the best
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Offline flybox1

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #11 on: July 09, 2018, 08:41:46 AM »
Why would you not complete all the scheduled maintenance/tune-up items before trying to troubleshoot an issue?
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Mopar_or_no_car

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Re: 78 cb750k surges and won’t pass 6 grand sometimes
« Reply #12 on: July 13, 2018, 07:14:57 AM »
I belive I figured it out it must have been something in how the tank vents because I found the petcocks little rubber o ring was ripped and only allowed flow from one setting I put in a new one and bike has run fine for the past 3 days
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