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Author Topic: 78 750F with 74 750K motor  (Read 1510 times)

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Offline UnCrash

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78 750F with 74 750K motor
« on: July 30, 2007, 07:19:14 AM »
Hi,

I've got a 78 750F and have some concerns about the engine.  It is burning some oil, has 21k miles on it, wasn't taken care of by PO, and is an F3 model which I hear many anecdotes on here about the top end...

It's not dead yet but I'm loving the SOHC and want to prepare for whatever may come my way.  Today I spotted a 74 750K engine in Craigs List about an hour north of me.  It can be had for cheap, but of course would need to test compression, etc.

So a couple questions for the veterans. 

1. If I were to marry a top end to my F bottom, what is a good year to do so?

2. Is it more economical to just address the top end of my engine and give it the tlc it needs on a regular basis?

3. Does it make more sense to just put a complete 750 engine in, and K or F?


I know these are weighty questions.  I'm on the Don't spend more money than you have to and annoy your wife and the "Isn't this thing costing you more than it should all ready" budget.

I've never rebuilt an engine, don't have a welder, but have managed to do many things on this bike that require you to take things apart and put them back together...

Any thoughts or suggestions would be appreciated.

Many thanks,

Ben
« Last Edit: July 30, 2007, 07:21:52 AM by UnCrash »
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Offline KB02

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Re: 78 750F with 74 750K motor
« Reply #1 on: July 30, 2007, 01:06:06 PM »
If you use the top end from the 74 engine, make sure you keep the whole top end together,jugs and cylinders as the oil passages are different. It should bolt right up to the 78 bottom end.

The latest engines had reworked top ends with bigger valves and what not so, in theory, they are the better engine. BUT, that's up for debate... and has been debated... many, many, many times...  ;)

I'd say if the 74 is a runner (or will be with little work) go ahead and get it and run in while you take your time and rebuild the 78 engine. There's always a first time for everything, right?
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Offline UnCrash

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Re: 78 750F with 74 750K motor
« Reply #2 on: July 30, 2007, 06:51:26 PM »
Thanks for the reply KB02, I really appreciate it.

I received the following email today from the seller.  The tone of the email tells me he's a pretty upright guy.


Quote
Hello Ben  I don't know much about the engine.  I bought it in a parts lot.  It has the top end off of it.  I believe all of the parts are there minus the rocker cover.  I checked the numbers and that's how I determined the year.  The bottom end seems fine, all of the connecting rods felt good but is missing a piston.  I also have a crate full of engine parts and the top half of a set of cases from another 750 engine,   I will take 100 bucks for the lot and I'm going to be firm as I ebay a lot of parts and can make much more that way, I just usually deal with British andAmerican bikes.  thanks for your interest.

Any other thoughts on this issue would be greatly appreciated.

Many thanks,

Ben
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Offline UnCrash

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Re: 78 750F with 74 750K motor
« Reply #3 on: July 31, 2007, 04:42:21 AM »
Bump... ?
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Offline KB02

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Re: 78 750F with 74 750K motor
« Reply #4 on: July 31, 2007, 04:50:49 AM »
Actually, I was just about to post a reply when you bumped it.   :D

From reading the description, the 74 engine sounds a little rough. How much Oil is the stock engine burning? Is it a quart every thousand miles, or are you leaving a smoke trail behind you everywhere you go? If it's not that bad, and you want a project, grab the 74 engine and and an overbore kit and have some fun with it. OR, just wait until winter and do a top end rebuild on yours.

That's what I would say.
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Offline UnCrash

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Re: 78 750F with 74 750K motor
« Reply #5 on: July 31, 2007, 06:30:36 AM »
Thanks for the reply mate  :D

I'm burning about a quart every 1000 miles, no discernable smoke trail. 

I like your thinking about rebuilding the top end this winter.  That's feeling like it makes a little more sense to me.

Just spotted this rough 78 on craigslist in Boston for $100. ---> http://boston.craigslist.org/gbs/mcy/385136285.html I just might pick that one up instead of the 74 motor.  I emailed the seller to see if he still has it a day later.  That will go fast!

I don't mean to sound panicked about my bike or anything.  It seems that I spend an inordinate amount of time thinking about it.  If I would have known that getting an SOHC was like taking crack I probably would have stayed away!  I can't stop thinking about it when I'm not on it.  ;D ;D ;D
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Offline scondon

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Re: 78 750F with 74 750K motor
« Reply #6 on: July 31, 2007, 09:07:53 AM »
    If you're burning a quart every 1000 miles, and not leaving a smoke trail, then I would think you are not that bad off considering how much oil these black motors can end up burning and still run well.

 The first engine lot does not sound like it comes with enough pistons to make a complete top end and you would have to pull the exhaust studs from your '78 motor to put in the '74 head in order to mount your exhaust. The second bike is a steal at $100 and would be nice to have for all the spares, but the motor may end up being the same, or worse, as the one your riding.

  If I had room in the garage I would jump on both deals in order to have a plethora of spares to chose from when putting the bike/engine together. I would, however, keep searching for a complete(fully assembled, unmolested) 75-76 F/ 75-78 K motor as the top ends are a straight swap for the '78 motor and, if running, can be installed whole with no additional work so you can keep riding while the '78 motor is rebuilt. That would be the simplest route, in my opinion.
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Offline UnCrash

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Re: 78 750F with 74 750K motor
« Reply #7 on: July 31, 2007, 09:32:14 AM »
Quote
I would, however, keep searching for a complete(fully assembled, unmolested) 75-76 F/ 75-78 K motor as the top ends are a straight swap for the '78 motor and, if running, can be installed whole with no additional work so you can keep riding while the '78 motor is rebuilt.

That's a great tip, thanks!  It's awesome that the top ends from 75-78 will bolt right up to the F bottom.

I'm trying to figure out how to get a truck (which I don't have) to pick up that parts bike in Boston.  I may just rent a uhaul box truck with a ramp and go get it.

Cheers,

Ben
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Offline scondon

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Re: 78 750F with 74 750K motor
« Reply #8 on: July 31, 2007, 10:19:00 AM »
     as KB02 points out, the '74 and under top ends will bolt up to your motor as well. You just need to install exhaust studs in the head. There are some internal changes in the later motors that may be desirable(dowels and seals added to the oil returns, heavier valve springs on the 77-78 K motors, domed pistons in the 75-76 F motor, etc..). But for me, it just comes down to having one less step to bugger up in having to pull exhaust studs from one motor to use in another ;)
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Offline Bodain

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Re: 78 750F with 74 750K motor
« Reply #9 on: July 31, 2007, 05:35:16 PM »
I guess I could tell you my tale of wooo....  78F engine just like yours.. Trying to find a way to make it all work... 78F cylinders and a 75 K head. Modified pistons to accept K rings.  Ah never mind.... It gets real messy.....

I'm still trying to find a K engine myself. Fortunately I have other bikes to work while I patiently wait to find that K.
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Offline scondon

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Re: 78 750F with 74 750K motor
« Reply #10 on: July 31, 2007, 11:39:34 PM »
  Bodain, last I read you had that engine installed in the bike. Any news on how it fared?
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