Topic: 1978 CB750F3 Build/Project

[cmonSTART]

Great thread!  I'm starting a similar project on my '78 F which I'm going to build into a racer. 

Have fun with your project!

[cmonSTART]

Okay, I checked my compression just now and #$%*!  I dont know much about this stuff but I'm pretty sure it's definitely not good.  I did a dry test and wet test on each cylinder.  For the wet test, I put about 1 teaspoon of oil in each cylinder and cranked it slowly 3 times for each cylinder.
#1: Dry- 76,  Wet- 83
#2: Dry- 75,  Wet- 95
#3: Dry- 70,  Wet- 115
#4: Dry- 78,  Wet- 112

So I'm guessing I need new piston and rings, and maybe the head gasket is leaking?

Let me know what ya think

Probably some issues there.  Uh oh.  Were you holding the throttle wide open and using the electric starter?

[Steven.Burns]

Throttle was wide open but not the electric start since my battery was almost dead.    Just kicked it till there was no more pressure building up.  Does that work the same or does it need to be the electric start?

[cmonSTART]

Good hits with the kick starter will work fine.  Sounds like the motor needs a bit of TLC. 

[Steven.Burns]

Yeah I'm thinkin so too! She was running great the other day but I guess I don't really know how this bike is supposed to run since it's my first SOHC4. Felt really powerful and responsive and then all of a sudden it got all sputtery and crap 

[ZanVooden]

I wouldnt be so sure right away. Compression testers are all a little bit different (length of hose, construction of hose, etc.). Now your dry numbers do seems a bit low, but they are all within 10% (except for #3).

As for the wet #, did you put in exactly a tablespoon of oil? More or less can play with the numbers a bit.

First thing I would try if your worried about compression, is get some Seafoam and pour about 1/2-3/4 cup in each cylinder (i usually pour any remaining Seafoam in the top end through a tappet cap) and let it sit for a week. Turn the engine over a couple times with the electric start to get it out (throw a towel over the engine, otherwises you'll shoot seafoam across the garage (or across the street if you're outside) and try your compression check again.

The seafoam will loosen stuck rings. Now I dont believe your compression is a problem. A problem would be if you had one cylinder significantly different than the rest (maybe two).

But I could be completely wrong as I'm not the one working on the bike and experiencing the issues.

While you soak your cylinders, it would be a great time to pull the carbs and disassemble them, clean them, and replace all the rubber bit (remember, unless someone has done it before you, that rubber is 34 years old, and with ethanol fuel they could be weak. Also you can make sure your jets are free and clear).

Another thing that isnt a bad idea, is to buy yourself a point file, and drag it across your points (top and bottom) a couple times. This cleans any corrosion that might have built up on them.

I dont believe your having compression problems, I'm guess your issues stem from ignition or carb issues.

[Freaky1]

I agree with Zan, I don't think your issue is your rings either, to me this sounds like an electrical issue. If everything is good when you start but once you get really warmed up on a good ride things start going funky. My suggestions in the order I'd tackle them, 1st: resolve the issue with the #3 plug they have threads and fit tight for a reason. 2nd: clean your points with the file, reset them, check timing and then check points again. 3rd: check the wiring and your regulator rectifier, it sounds to me like something somewhere is getting hot and creating issues. I've had times where my bike, after a long ride then putting around town, gots freaky with the backfiring and sputtering. If you get off and let it sit for a while does it run better when you get back on? I'm not talking about 5 minutes either, more like 20 min to half an hour. If so then I'd bet the bank you've got heat related issues with your wiring or some electronic item somewhere.

Lastly if you plan on going with Hondaman's ignition I'd suggest ordering and entirely new points plate at the same time, nothing is more frustrating than skipping a simple thing only to have it bit you when you want ride with your new gadget. I chose not to replace my reg/rec with an electronic one when I did my bike the first time and I had fits when I put in my electronic ignition 3 months later only to have to delay the project another week because the regulator crapped out on me.   

[DWS]

Could some answer this for me, i have a 1978 CB750 super sport, and when i was replacing the oil filter and so on, i notice that there is no oil filter washer and or collar.
Is there suppose to be a collar on this bike? If there is, i am having a hard time finding one.
Thanks for your help
Duane

[Steven.Burns]

Try looking on Bike Bandit. Take a look at the diagrams they have to see if you are missing anything. The might even have what you may be looking for

[madmtnmotors]

Could some answer this for me, i have a 1978 CB750 super sport, and when i was replacing the oil filter and so on, i notice that there is no oil filter washer and or collar.
Is there suppose to be a collar on this bike? If there is, i am have a hard time finding one.
Thanks for your help
Duane

Some pictures would help. The washer helps keep the spring from digging into the rubber on the filter. The washer usually sticks to the filter and gets discarded along with the filter during an oil change. They are cheap and still available from Honda. As for the "collar", the only collar I am aware of is the collar that serves as a spacer where the oil filter bolt passes through the "oil cooler" chunk of aluminum heat sink between the oil filter and the lower crankcase. These auxillary heat sink oil coolers are often removed, especially since they require an additional o-ring, and since every other CB750 works fine without them. They can be removed and a shorter oil filter bolt is used. If you have this oil cooler in place, but do not have the spacer collar, then the spring may not be tensioned properly against the oil filter which could allow unfiltered oil to bypass the filter. Is the oil cooler still in place? If not, then you won't need the collar.

[Steven.Burns]

Its been a real long time since an update on here but I've got quite a bit of things done since I posted last.  I decided to head a different direction instead of a cafe racer I wanted more of a scrambler/tracker style for more comfort and control on the dirt roads.  I recently cut the frame and welded a rear loop and made a new seat.  Got rid of the clip-ons and put on some Flanders dirt track bars. Got rid of the gauges and made a little instrument panel out of aluminum for my ignition switch, choke, headlight switch, idiot lights.  Polished fork lowers, stripped paint off tank. Got new dual sport tires. New suspension too. 

[Steven.Burns]

Some more recent photos since the weekend with most of the progress. Also a photo of my bike on the flatbed truck after I had some electrical problems on the way to Born Free 5. Stuck on the side of the 91 freeway in 113 Degree weather. Bummer but I still made it in time to see most of the #$%* there. 
The problem I'm having was from what I think my own fault as always.  I cut the wires running from my kill switch and my starter button.  I re wired the Black/White wire and Black/Red wire to the solid Black wire.  Don't know if that's the reason I blew the main fuse but I put a 30amp fuse in there since that was all i had left in my bag.  I ended up melting a small amount of my fuse box next to the main fuse. Now I cant even get spark. Maybe the coils? My battery is pretty bad too so I gotta replace that while I'm thinking about it.  If anyone can help my stupidity I would appreciate it a whole lot.

[Steven.Burns]

More photos 

[Steven.Burns]

One on the flat bed while I cry