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Author Topic: 78 750F head  (Read 790 times)

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Offline KRONUS0100

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78 750F head
« on: November 19, 2012, 10:19:56 AM »
dangit.  my almost no mileage 78 F motor has given me an issue.  as the head was off the motor when i got it, it appears to not be the head that goes to the motor.  Exhaust valve guides got at least .080 wiggle in them.  looks like goona be ordering the hi po kit from Ken at Cyclex.......................................as soon as i'm not spending money on livestock.
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MATT
current bikes:  1976 CB750F, 1981 GS1100E
bikes owned:1981 GL1100I, 1990 GS500E, 1981 GS850, 1977 and 1979 GS750, 1974 CB750, 1975 CB750, and a 1982 GS750E

Offline phil71

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Re: 78 750F head
« Reply #1 on: November 19, 2012, 10:25:36 AM »
Why do you think that it doesn't belong on that block. Certain year F motors had notoriously crappy guides.. Search the forum and you can find out what years that was true.. Or someone with a better memory will chime in.
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Offline KRONUS0100

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Re: 78 750F head
« Reply #2 on: November 19, 2012, 12:04:54 PM »
no way guides wore that bad in under 15 miles, like i said almost no mileage motor.
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MATT
current bikes:  1976 CB750F, 1981 GS1100E
bikes owned:1981 GL1100I, 1990 GS500E, 1981 GS850, 1977 and 1979 GS750, 1974 CB750, 1975 CB750, and a 1982 GS750E

Offline ZanVooden

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Re: 78 750F head
« Reply #3 on: November 19, 2012, 12:52:16 PM »
The 78F heads are know for bad guide wear. From what I've been told they normally need to be replaced between 20 and 30k miles.

I just replaced mine. The exhaust side guides were easily visibly worn. The holes were significantly oblong.

Modern guides hold up much better from what I understand.

From what I've read the reason for this was Honda trying to keep up with the DOHC engines and increased the valve size. The larger valves were at a slightly different angle to accommodate and created significant wear on the guides.
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Offline ZanVooden

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Re: 78 750F head
« Reply #4 on: November 19, 2012, 12:54:58 PM »
Nevermind, I just read your engine only has 15 miles on it.

So was the head reworked before you put it together and 15miles later you took it apart?
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Offline madmtnmotors

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Re: 78 750F head
« Reply #5 on: November 19, 2012, 01:41:07 PM »
Quote from: KRONUS0100 on November 19, 2012, 12:04:54 PM
no way guides wore that bad in under 15 miles

Quote from: KRONUS0100 on November 19, 2012, 10:19:56 AM
the head was off the motor when i got it

So the P.O. drove it for 15 miles then took the head back off? Me thinks there is more to the story...
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Offline madScientist

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Re: 78 750F head
« Reply #6 on: November 19, 2012, 03:35:46 PM »
Sounds like either a new speedometer was put on in the VERY recent past or the speedo cable was unhooked most of it's life.

As for the head, mine had 12k miles on it and the guides were noticeably oblong.
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Offline Thousandson

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Re: 78 750F head
« Reply #7 on: November 19, 2012, 05:15:56 PM »
Would the best answer be? Replace the valve guides. My motor is in pieces, I'm doing a full rebuild. I was thinking of leaving that off the list, but hearing this I should def measure my guides.
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Offline madScientist

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Re: 78 750F head
« Reply #8 on: November 19, 2012, 06:20:20 PM »
You've got the engine apart anyway...why not do it? Added reliability = peace of mind.
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You CAN do great things...with enough beer.


78 cb750f
http://forums.sohc4.net/index.php?topic=113293.0
70 CL350 Street Tracker / Cafe
http://forums.sohc4.net/index.php?topic=125641.0
83 KZ1100ltd
76 GL1000
71 Kaw H2 (to be built)
77 cb750A (sold)
74 cb125s (sold)

Offline KRONUS0100

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Re: 78 750F head
« Reply #9 on: November 20, 2012, 08:41:58 AM »
this is the motor that the original owner was killed on his way hoe.  the dealership that sold it took motor out and set it ion corner of shop.   a guy i know aquired the motor and all paperwork and it sat in his shop and had parts taken from it over the years.  I think at some point the relatively new head from this motor was put on another bike and its head was put with this motor.  I am in the process of putting this motor back together with new rings,seals,gaskets.....and now looks like new guides and possibly a couple valves.
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MATT
current bikes:  1976 CB750F, 1981 GS1100E
bikes owned:1981 GL1100I, 1990 GS500E, 1981 GS850, 1977 and 1979 GS750, 1974 CB750, 1975 CB750, and a 1982 GS750E

Offline phil71

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Re: 78 750F head
« Reply #10 on: November 20, 2012, 01:17:59 PM »
 To change a head on the SOHC 750, you have to pull it from the frame. If someone was going to pillage the head for an OLD MOTOR, wouldn't they just put the whole low mile engine in that bike? Doesn't make sense
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