Topic: First Build, '77 CB750F2 SuperSport

[OnLettingGo]

Hey Guys. Quick introduction. I'm new to the forums and this is my first cycle build / restoration. I recently picked up a 1977 CB750 SuperSport, which the PO had stored in his shed for the past few years(~3, AFAIK). Seemed like he tinkered on it every now and then, never got it road-ready.

The bike will start but does not Idle well at all. It basically wont stay running without the throttle open. Its also fouling plugs (cyl 1 & 4) and spitting black sooty liquid out of the exhaust (which I believe is fuel). My biggest hurdle so far has been trying to get that issue figured out, which I have not been able to do yet. Hopefully with the help of some of the more experienced people here, I can solve the issue.

The bike had some mods on it when I bought it, such as; Cycle-X Twin Carb Kit (biked never tuned with these, AFAIK), Handlebars, Rear shocks (goldwing?), MAC exhaust, rear tail-light & seat cowl, I'm probably forgetting a few things. New seat is needed.

Since I've had the bike I've done the following:
Fresh gas
Stripped the tank of paint, clear coated the alloy
Fully disassembled / cleaned both Mikuni Carbs
Jetted down the pilots from #35 to #25 (advised by Ken from CycleX)
Replaced all fuel tubing + added in-line filter
Points and Condensors
Plugs
Coils, Wires & Caps
Also purchased the PAMCO electronic ignition, to install in the future

I thinks thats it for now. I looking forward to meeting and learning from everyone here.

-Joe 

[70CB750]

The side cover looks familiar, was it on the local DC CL?

Nice bike, does it run?

And welcome to the forum, where in MD are you?

[OnLettingGo]

The side cover looks familiar, was it on the local DC CL?

Nice bike, does it run?

And welcome to the forum, where in MD are you?

I'm on the Eastern Shore of MD, the town is Centreville. Mostly fields and farm-land out here.

The bike will start and run, but not a 'healthy' kinda of run, haha. I describe some of the issues in the first post.  I found the bike on Craigslist MD, so theres a good chance you could find it on DC as well.

[70CB750]

My apologies, I just looked at pictures 

I keep an eye on CL just out of curiosity what is on the market.

[Davidov]

Cool F model!

I'm in the middle of rebuilding my 78 750F engine. There is lots to learn from this forum!

[OnLettingGo]

Cool F model!

I'm in the middle of rebuilding my 78 750F engine. There is lots to learn from this forum!

Thanks, man. I've got a lot of work ahead of me, so the knowledge here will be invaluable.

Trying to get a vid uploaded of the bike starting/running, but having trouble. I'd like to get some opinions on what you guys suspect the issues may be. Some Harley guys that I know said that based on a vid i may need a top-end rebuild or at least new valve seals. They seem to think that oil getting into the cylinders is whats fouling the plugs on #1 & #4, based on some exhaust smoke on video. Hopefully I can get the video up soon! Thanks in advance for any advice.

In the pics, cylinders are in order 1-4 from top to bottom. 

[OnLettingGo]

Any chance you can do a compression test to assist in the diagnosis? Perhaps a Harley bud has a motorcycle test kit they could lend?

Typically, if smoke is present at start-ups or after idling, then a good throttle blip, leak is in the head. If it's somewhat constantly puffing blue smoke, you'd look towards rings. But a compression and/or leakdown test is a better analysis. Have you check the valve adjustments too? It shouldn't be off enough to allow oil to seep, but be a good maintenance item to perform anyway.

I have performed a compression test recently. All cyls were within 10% of each other, around 100-105 psi.

What you described, with the smoke at start-up and throttle blip, is whats occuring. My buddies were thinking that I may have to replace Valve seals & guides to stop the oil seeping. I've never taken the head off the bike before, so Im not sure if thats actually whats required here. I havent done any valve adjustments yet, I will have to look into that.

Is it possible to get the valve cover off with the engine in the frame? I've read that the head cannot be removed with the engine in the frame.

Thanks a lot!

[Davidov]

I would address your carbs first. Check float levels, make sure they are adjusted properly. Then decide if you want to dig into the engine.

I personally decided against the frame kit, since it eliminates the triangular brace on that part of the frame.
Removing the engine from the right side of the frame isn't that difficult.

[OnLettingGo]

The overall compression is a bit low, but the wear seems quite even. Has the motor sat a while proof to you owning it, or is it a high mileage motor? If output can re-run the compression test, add a little oil directly to the cylinders when performing the test and see if the compression comes up. If so, you've got ring wear too.

The reason I raise this point is, if you're going to service the head, and still have lower end wear, you're doubling you're work and may still have some fouling occur afterwards. It's less desirable, but not a great deal less work to pull the head and jugs and bore and re-ring new pistons at the same time. Also, if you pull the head, with a frame kit, you can then re-torque the head after a run-in to insure a leak-free top end.

In the custom Google search bar, type "frame kit" and you'll find the product I referenced.

AFAIK the bike sat for at least 3 years, prior to me purchasing it. It has 18k miles. I will definitely re-try the compression test, as you mentioned.
Not sure if I want to cut the bike frame, I'll probably just pull the motor. I'm sure there is a good step-by-step somewhere on the site. Any help would be great.

I would address your carbs first. Check float levels, make sure they are adjusted properly. Then decide if you want to dig into the engine.

I personally decided against the frame kit, since it eliminates the triangular brace on that part of the frame.
Removing the engine from the right side of the frame isn't that difficult.

Carbs are good to go. One of the first things I did after purchase was cleaning the carbs and resetting the floats. I even went to a smaller pilot jet, thinking that too much fuel was being metered in at idle (fouling the plugs)... no improvement after any of these things.
 
I'm also leaning against cutting the frame. Any tips on engine removal? Will be a first for me and most likely will be doing it alone..

[70CB750]

I installed engine by laying the frame over it, thats the best way, but not practical if you are just taking engine out.

From standing motorcycle you can use engine hoist and adapter in the shape of C - you can find it on the forum if you try.

Or just go at it if you are strong enough:)



Some reading:

http://forums.sohc4.net/index.php?topic=113347.0

It is also recommended to grind down flanges of the right front mount while the engine is out - makes it much easier to go in.

[70CB750]

One more thread, this is pretty smart, actually:

http://forums.sohc4.net/index.php?topic=102658.0

[OnLettingGo]

Very helpful. Thanks!

[Bankerdanny]

I'd be very tempted to sell off the twin carb kit and track down a set of '76 F model carbs (The '76 carbs are easier to get kits for than the later model PD carbs). The twin carb conversion seems like more trouble than they are worth.

[pamcopete]

I'd be very tempted to sell off the twin carb kit and track down a set of '76 F model carbs (The '76 carbs are easier to get kits for than the later model PD carbs). The twin carb conversion seems like more trouble than they are worth.

I'm with BankerDanny on this one. I think you are getting uneven fuel / air distribution from that two carb setup. There is a lot more to making a two carb manifold than just forming some pipe.

Your compression readings are low. Did you open the carb throttles completely when you did the tests?

Trying a set of four carbs can be done without removing the engine as well! 

We should also not overlook the fact that the uneven spark plug condition matches the points arrangement of 1-4 and 2-3 so it might be a good time to install that PAMCO. As a minimum you should check the spark plug caps because the circuit for each pair of cylinders is a series circuit meaning that a bad spark plug cap could affect both cylinders in that pair. Check the caps first, followed by the PAMCO before you change the carbs just in case the carbs are OK.

[OnLettingGo]

I'd be very tempted to sell off the twin carb kit and track down a set of '76 F model carbs (The '76 carbs are easier to get kits for than the later model PD carbs). The twin carb conversion seems like more trouble than they are worth.

I'm with BankerDanny on this one. I think you are getting uneven fuel / air distribution from that two carb setup. There is a lot more to making a two carb manifold than just forming some pipe.

Your compression readings are low. Did you open the carb throttles completely when you did the tests?

Trying a set of four carbs can be done without removing the engine as well! 

We should also not overlook the fact that the uneven spark plug condition matches the points arrangement of 1-4 and 2-3 so it might be a good time to install that PAMCO. As a minimum you should check the spark plug caps because the circuit for each pair of cylinders is a series circuit meaning that a bad spark plug cap could affect both cylinders in that pair. Check the caps first, followed by the PAMCO before you change the carbs just in case the carbs are OK.

If it comes down to it, I could put the factory carbs back on. But I'd much rather not do that. We'll see.

I did not have the throttle open during the comp. test, which I also recently read in my clymer manual, so Im going to do the test over again. Will post those results asap as well as the video of it running/idling.

I agree with you on the fouled-plugs matching the points arrangement, which is what led me to replacing the coils as well as new points and condensors.  Testing each plug individually, grounded against the fins, all 4 of them fire just fine. So the Ignition seems to be good to go. Each time I pull the plugs I also check the gap on them. Theyre all to spec according to the clymer manual.

Thanks again guys.

[pamcopete]

OnLettingGo,

OK. I think that you will get some very good compression readings, and if you are satisfied that the points are working OK, that just leaves the carbs. You probably have a basically good engine otherwise, so I would do the carbs before removing and overhauling the engine.

[OnLettingGo]

Here is the video of the idling & smoke. Any input greatly appreciated. Like I said, I've gotten opinions from others that I know, which were that I likely need new valve seals & guides.

[Davidov]

Are you plugs still coming out black and fuel fouled? That would definitely cause smoke.

However, F2/3s are known for wearing valve guides faster than earlier 750 engines. The max clearance is .08mm (some manuals say .10mm), I had over .50mm of "wiggle" on the exhaust side, no bueno! The engine ran great though.

[OnLettingGo]

The picture of the fouled plugs (above) was taken just after the video. Pulled the plugs right away and took the photo.

I guess Im not sure if the plugs are fuel or oil fouled?

Wasnt you exhaust valve-guide leaking oil into the cylinder at that point? Thats whats been suggested is the case with mine. Idk how to verify it though.

[madScientist]

How many miles does it have on it?

As several have stated before the F's were notorious for chewing through valve guides. My engine had about 14k miles on it and the guides were starting to turn kind of oval shaped. I've read in other threads that they are pretty well gone by 20k miles.

[OnLettingGo]

The odo has 18k on it, so it has at least that many.   

[Davidov]

Fuel fouling will be wet and smell like raw gasoline. Oil burning will have a more cake-on appearance with some while flakes.

I still agree with the others, seems like too much of a coincidence that its only cylinders 1-4.

[martin99]

I personally decided against the frame kit, since it eliminates the triangular brace on that part of the frame.

Sorry but that's not quite correct. 754's kit comes with plates to enable you to join the cut portions of the triangle and retain the integrity of the brace

[quebec_monstro]

All those information are really helpfull thank you ver much and keep up the good work

[madScientist]

As for the frame kit. I have the 'gordons' variety on my bike. I haven't felt much difference in rigidity between this bike and an F my friend has without the kit...and I ride real hard.

Having the frame kit has saved my bacon several times in the last year as well. I'm super pleased with it.