Topic: 1974 CB750 Engine Rebuild

[N30R3L0AD3D]

Well, I've owned my cb750 for about 5 years now. Bought it for 700 bucks, was in kind of rough shape. Needed new tires, and as I found later a lot of engine work. It was worth it. So the story of the rebuild was once I bought it, I had discovered the cam chain had never been adjusted since day 1 on 15k miles. I had re-rung it and did a top end job, and discovered 2 valves were bent on #1 and #4 so I replaced them and slapped it together. Ran fine, but something was still not right with it. The machine shop that I took it in to do a hone for me, took too much off one of my cylinders causing a compression drop in #1 cylinder by almost 20 psi. I ran it this way for a while, ran ok other than fouling the plug once in a while. Last fall, I decided to go through the motor completely. Here's a list of what's been done sofar:

Replaced rod bearings
Replaced crank bearings
New heavy duty cam chain
New cam chain tentioner and rollers
New exhaust valve guides (Cycle X)
New Super flow exhaust valves (Cycle X)
Milled the head (Cycle X)
50 over piston kit (Cycle X)
New web cam #41 (stock one has a weird wear on one lobe from orig owner not ever adjusting cam chain tentioner)
Cam tower modification (Top Secret)
Port polish
All new seals
Oil pressure gauge (coming soon)

Some things I've already done to the bike prior to teardown:

Aftermarket Andrews ignition coils + new wires + plugs
530 chain conversion (18 front, 45 rear)
New rear dual damper shocks
Mac 4 into 1 exhaust (I had a kerker but the baffle disintegrated)
Put 110 jets in the carbs
Veshra racing clutch springs and disks
New Swing arm bushings
New brakes (rear)
Dunlop E3 tires / front and rear


Teardown time. As I said before, other than the cylinder with low compression.. I knew something wasn't right. Split the cases, nothing seemed out of the ordinary. Put the new green crank bearings in, and started putting the new rod bearings in when something became quite apparent..



Yep. I'd been riding this bike for 3 years with a bent rod. Luckily for me, I had a set of stock rods to replace. The fact this motor ran as good as it did, is amazing. Just another reason why I prefer honda.

So how did cycle X do on the headwork? Let's take a look:





50 over kit from cycle X:



Webcam:




Runs absolutely great. Got a little over 200 miles on it sofar, but wait... No build goes without a hitch right? Went to use the kickstart, and the kickstart spring came undone in the case.. A little detail overlooked when putting the cases back together. Yep, had to tear it down. 40 bucks in gaskets later, it's back together. 

Final result, I give you my "not so stock" 1974 CB750 Honda:




Special thanks to Hondaman for all his help and advice.

[N30R3L0AD3D]

Must have done something right to get over 60 views and not one comment

Now the biggest issue I am having is carb related. Cylinder 1 is running way richer than 2,3,4... Can't figure out why. Float's are set correct, jets were swapped, swapped boots, new fuel line ran, switched coils and wires, cleaned carbs twice. Definitely not a compression issue, 140 across the board cold.

How big a difference?



So this is what I am working on now.

[Danno]

It probably is a difference in the jet needles you did not bottom one out at some point and bend it did you does this one have the mix needles on the bottom if so a 1/4 turn to the right on the offending carb looks like it would solve it also are you using one fuel feed line or 2

[N30R3L0AD3D]

I plan on pulling the carbs today and putting a different set of them on to try and pin point the issue. All new fuel line was run, and it's one fuel line that T's off into 2. It's see through, and both lines are getting fuel. It is possible the original owner may have messed with something in the carbs that I overlooked. I know it's not a carb boot leak. I replaced the original clamps with new 2.5" worm screw clamps. No change.



Shot of the #1 carb

I'll report back when I swap carbs.

[N30R3L0AD3D]

Update:

I put together a set of Frankenstein carbs over the weekend. And walla, it's a whole new bike. All cylinders are the same color, and it's running MUCH better. So what's wrong with the stock carbs?

First of all, they were binding. After I tore down #1, it came quite apparent what had happened. When the original owner put a kerker on it, he must have messed with the needles. They were not in factory settings. And more importantly, the needle on #1 was BENT.... Hence the rich plug.

2 and 3 were ok from what I could tell...

4 was a vacuum leak, most likely the top of the carb being the culprit. There was a lot of play and the rubber up top was sorta smashed.

Boy this bike sure had a rough life before I got it, that's for sure. Pics to follow soon.

[Retro Rocket]

Looking good mate, love the bent rod.....

[fastbroshi]

4 was a vacuum leak, most likely the top of the carb being the culprit. There was a lot of play and the rubber up top was sorta smashed.

Was this little o-ring type piece of rubber that goes in the cap?  Love the way the early type carbs go together btw.

[N30R3L0AD3D]

No this was the o ring on top of the cap, and it was smashed. And as for that bent rod, yes I drove on that for 3 years. It would occasionally make a metal slapping sound but not as severe as some that I've heard in videos on the forums. I never had a clue it was bent. Why would I? When you do a top end job, you leave the pistons on the rods. So I slapped it back together thinking things were fine. There was no dmg to the pistons from when valve 1 and 4 hit, and so I just put 2 new valves in and went with it. Other than low compression and a bent rod, yeah it ran alright. Heck I even took it on a 4 mile straight stretch at 8k RPM for a top speed run and it didn't blow up. These engines really are bullet proof, so long as you change the oil and keep up on a few things.

The only thing left is to try n fix the idle issue I am having. After reving down from high rpm, it doesn't want to idle right unless you blip the throttle a little, then it will idle ok. It's most likely due to the webcam #41, and I set my screws out 1 1/8 turn.

Here's a video of it running (somewhat warmed up). The camera video quality is sub par.

MVI_2263.AVI

[N30R3L0AD3D]

And I am back with yet another tear down report. Towards the end of the season last year, I developed an oil leak on the left side of the head with just over 300 miles on the new motor. I thought at first it was one of those damn annoying rubber plugs, but after I pulled the motor out last week my worst fear came to be. The rubber plugs were fine, but my head gasket was soaked in oil, and was blowing out the left side between the jugs and the head. After looking at the pictures of the milling job, I should have looked more closely before slapping this head on. Once I tore the top half of the motor off, I got a piece of glass with some valve grinding compound to see how far it was off, and I was shocked to see that after 20 or so passes it was WAY off. It was only hitting in a few places, which would explain why my head gasket was soaked in oil.

Pictures to follow.

[lucky]

The bike looks great!!!
Will be interested to see how far off the cylinder head was.

[andy750]

Wow you are not having so much luck! Id also be interested to see how much its off. Dont know how I missed this last summer but good write up + photos. Thanks.

How did you like the 18/45 combo? Not familiar with dropping the rear (only the front to 17) to 45. Thanks.

Andy

[N30R3L0AD3D]

The 18/45 around here where I live is perfect, but to me it still winds up a bit down the highway.

Originally when I started this, it was only making contact near the outside edges, but that was about it. After about 2 hours of working this head on the glass, here's where I was at:



As you can see above, towards the center it still was not making good contact. So I kept going for another hour or so and here's the final verdict:



There is still a little area around the center it wasn't quite hitting, but this will have to do. I ran out of valve grinding compound, and elbow grease  I applied copper coat to the new Veshra head gasket, and torque'd down the head per spec. Once I get this running, I'll throw up some more video's. I am hoping that this is the last frikken time I tear this thing down.

[brandEn]

Can you explain what you are doing with the glass and compound? I am not familiar with this.

[MCRider]

Wow you are not having so much luck! Id also be interested to see how much its off. Dont know how I missed this last summer but good write up + photos. Thanks.

How did you like the 18/45 combo? Not familiar with dropping the rear (only the front to 17) to 45. Thanks.

Andy

Remembering the sandcast/K0s came with a 16/45, then a 17/45. Settling on a 18/48, essentially the same ratio as the 17/45, but with larger diameters to hopefully mitigate chain wear/failure. Which was a distraction from the real problem discovered later being battery acid on the chains.

So the 45 is an OEM sprocket readily available.

N3or3: I know you said you think the engine is still spinning too fast for your preference. I would encourage you to not go any higher as you'll tend to lug the motor. Its supposed to spin higher than we're often used to hearing.  Personally, I preferred 17/48 for touring, and all around, with an 888cc engine.

FYI: The Dunstall CB750 conversions used a 19t countershaft sprocket. With an 48 rear.  The CR750 Daytona bikes used at least a 40t on the rear (don't know about the front) to get 150mph out of them. Probably used other rears as well.

[N30R3L0AD3D]

Thanks for the heads up MCRider. I was never sure, just sounded as if the motor was working harder than it should have been. Mine came with a 630 chain 17/48 with an aluminum rear gear that was worn beyond belief.

Can you explain what you are doing with the glass and compound? I am not familiar with this.

What I did, was get a piece of tempered glass and used valve grinding compound and water on a flat surface and kept rubbing the head on the glass. You make passes on the glass, varying the motions. Up, down, diagonal, rotate, left, right, figure 8, etc. You definitely want to make sure your valves are covered, I used gorilla tape, and or just disassemble the head all together. Once your done, wash the head thoroughly to make sure you get all that compound off the head.

[N30R3L0AD3D]

Yet another update.

While I had my motor out, I decided to tackle the dragging front brake and front forks. When I pulled the first fork tubes apart, I didn't run into too much trouble other than some minor rust towards the top. I hit it with some 400 grit sand paper and wet sanded it. When I got to the second fork tube, I was horrified to see this coming out of it:



It took me a long time to get the old seal out, as it was fused in. Cleaned up the fork tubes, and put it back in.

Before:



After:


Next came the front brake. I've replaced everything in the system, yet my front brake kept sticking on me. I hit the rotor with some 150 grit and took that shinny surface off it, then roughed up the pads. Put it all back together and yet again it was still sticking. Frustrated, I pulled it apart again only to discover the front pad caliper side was stuck again. I pulled it out again, and took that goofy ass spring and rubber o-ring out, slapped it back together and what do you know... no more sticking pad. Works fine.

Got the motor in it last night and I am happy to report it's running like a champ. I'll post a video whenever it stops snowing where I live..

[N30R3L0AD3D]

And here it is after a 10 mile warm up ride. The next video I post will be out riding, once I figure out a way to mount my camera.

MVI_2399.AVI

All and all, I am glad this thing is finished. I hope to god that I never have to tear this motor down again. I hope I've helped some of you, or given you something positive to take from all this. Overall, I am happy with all the parts minus the mill job and the head gasket. Everything else is good in my eyes. Again, thanks to Hondaman for all your advice, and everyone else who have helped me through this.