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Author Topic: 81 CB650 build thread  (Read 2132 times)

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Offline basketcase81

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81 CB650 build thread
« on: April 02, 2016, 08:37:56 AM »
I just typed out a pretty long post and it didn't save so I'm gonna keep it short this time

I got this bike for free. After getting the electrical fixed and cranking over I discovered that I had zero compression. After tearing the head down I discovered that all valve hit the pistons.

I plan on buying a good used head instead of rebuilding. Still debating whether or not to do Rings while head is off (I probably will)

Couple questions followed by some pictures

1 The cam removal was the hardest part because there was no  bolt for the=e tensioner. I'm thinking this may be cause for the damage. Thoughts?

2. What else should I look at before putting it back together?

3. CB650C parts will fit the regular CB650 right?

4. Whats the easiest way to make this a hard tail?

5. Any other input is appreciated. I will probably need more help along the way so if you could follow this threads that would be great
« Last Edit: April 02, 2016, 08:52:05 AM by basketcase81 »
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Offline SOHC4 Cafe Racer Fan

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Re: 81 CB650 build thread
« Reply #1 on: April 02, 2016, 09:06:43 AM »
If you are going to replace the head, you have the option of buying a 80-81 head, which is spaced to use the stock CV carburetors, or to swap in a 79-80 head, which uses differently spaced stock carburetors that use the same spacing as a 750 (giving you more options for performance).
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Offline basketcase81

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Re: 81 CB650 build thread
« Reply #2 on: April 02, 2016, 09:16:19 AM »
Quote from: CB750 Cafe Racer Fan on April 02, 2016, 09:06:43 AM
If you are going to replace the head, you have the option of buying a 80-81 head, which is spaced to use the stock CV carburetors, or to swap in a 79-80 head, which uses differently spaced stock carburetors that use the same spacing as a 750 (giving you more options for performance).

I already bought the head, its from an 81 which is fine. I just wanna get this thing going so I can ride
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Offline calj737

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Re: 81 CB650 build thread
« Reply #3 on: April 02, 2016, 11:30:43 AM »
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The
At way, you'd have open valves during full compression (causing no compression) and this striking the Pistons. You might investigate this before rebuilding any further...
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Offline basketcase81

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Re: 81 CB650 build thread
« Reply #4 on: April 02, 2016, 01:33:19 PM »
Quote from: calj737 on April 02, 2016, 11:30:43 AM
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The
At way, you'd have open valves during full compression (causing no compression) and this striking the Pistons. You might investigate this before rebuilding any further...

i cant really investigate any further because it is already tore down. It wont matter anyway as long as I out the cam in right  when I rebuild, right? Or am I missing your point?
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Offline calj737

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Re: 81 CB650 build thread
« Reply #5 on: April 02, 2016, 01:52:14 PM »
Nope, you're right.
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline scottly

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Re: 81 CB650 build thread
« Reply #6 on: April 02, 2016, 07:31:42 PM »
Quote from: calj737 on April 02, 2016, 11:30:43 AM
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The

Cal, I can't believe you typed this. :o ??? ??? ???
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Offline basketcase81

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Re: 81 CB650 build thread
« Reply #7 on: April 03, 2016, 07:17:42 AM »
Quote from: scottly on April 02, 2016, 07:31:42 PM
Quote from: calj737 on April 02, 2016, 11:30:43 AM
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The

Cal, I can't believe you typed this. :o ??? ??? ???

What do you mean?
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Offline scottly

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Re: 81 CB650 build thread
« Reply #8 on: April 03, 2016, 06:11:50 PM »
Quote from: basketcase81 on April 03, 2016, 07:17:42 AM
Quote from: scottly on April 02, 2016, 07:31:42 PM
Quote from: calj737 on April 02, 2016, 11:30:43 AM
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The

Cal, I can't believe you typed this. :o ??? ??? ???

What do you mean?
The cam can't be installed "180* out" on these motors. ;)
You mentioned a missing bolt for the tensioner; this may have caused the problem?
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Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline basketcase81

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Re: 81 CB650 build thread
« Reply #9 on: April 03, 2016, 06:15:19 PM »
Quote from: scottly on April 02, 2016, 07:31:42 PM

The cam can't be installed "180* out" on these motors. ;)
You mentioned a missing bolt for the tensioner; this may have caused the problem?

Thanks for the info

So, do you agree that just replacing the tensioner and head wouldnt be a bad choice?
« Last Edit: April 03, 2016, 06:19:28 PM by basketcase81 »
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Offline basketcase81

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Re: 81 CB650 build thread
« Reply #10 on: April 05, 2016, 02:48:41 PM »
is there some kind of trick to getting the cam chain tensioner into the slot?
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Offline basketcase81

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Re: 81 CB650 build thread
« Reply #11 on: May 01, 2016, 01:08:01 PM »
Quote from: scottly on April 02, 2016, 07:31:42 PM
Quote from: calj737 on April 02, 2016, 11:30:43 AM
If you had no compression, and the valves were hitting the Pistons, my bet is on the cam being 180* out when installed. The

Cal, I can't believe you typed this. :o ??? ??? ???
Quote from: CB750 Cafe Racer Fan on April 02, 2016, 09:06:43 AM
If you are going to replace the head, you have the option of buying a 80-81 head, which is spaced to use the stock CV carburetors, or to swap in a 79-80 head, which uses differently spaced stock carburetors that use the same spacing as a 750 (giving you more options for performance).
Quote from: calj737 on April 02, 2016, 01:52:14 PM
Nope, you're right.

ok, so finally got the thing most of the way back together, minus oil pan and clutch cover. I did a compression test.....

I got 80 on #1, 90 on #2, and ~110 on #3&4

This bike was sitting for an unknown amount of time and I am in the process of rebuilding. PO didnt put the cam tensioner in correctly and bent all the valves. I got a good used head and these are the numbers i got. Should I keep going or should I look for something else causing those low numbers?
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Offline calj737

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Re: 81 CB650 build thread
« Reply #12 on: May 01, 2016, 04:28:20 PM »
Low compression numbers are not unusual for a motor that's sat a long while. Also, you need to be WOT to run full compression number checks. And of course, your valves need to be adjusted properly...?
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"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline basketcase81

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Re: 81 CB650 build thread
« Reply #13 on: May 01, 2016, 04:49:23 PM »
Quote from: calj737 on May 01, 2016, 04:28:20 PM
Low compression numbers are not unusual for a motor that's sat a long while. Also, you need to be WOT to run full compression number checks. And of course, your valves need to be adjusted properly...?

no carbs so it is WOT, and as far as i know the valves are set
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