Topic: First bike: 1978 CB750 K8. Let's turn this piece into a solid runner.

[therobbstory]

Hi all, first post. Let's jump right in.

After 8 years of living license and car-free in our nations capitol, I decided on a whim to get a bike. After three arduous trips to the DMV, a weekend spent taking the MSF, and many dollars later, I'm the proud recipient of a plastic card with an M on it.

I wasted no time in finding a scoot. I negotiated an unbelievable deal on a CB750 after a few laps around the block and hasty visual inspection. I was concerned about it being a much bigger and heavier bike than the Rebel 250 I rode in the MSF, but ten minutes into my 10 mile ride home from the PO's house, I felt totally comfortable and at ease with the size. Manuvering around a parking spot with the engine off is a total pain, but slow speed handling in 1st and 2nd feels really natural.

So this is my project log. I'm not interested in turning it into a hipster bike. I just want something safe and reliable to ride for a year or so while I build skills and save up for something really impractical like a BMW R90.

Engine

• runs
• starts instantly with starter
• Haven't attempted a kickstart, but I'm told it works
• Oil leaking from head gasket
• Oil leaking from oil tank cap
• Won't run unless choke is fully on. Choke has a tendency to open back up and engine runs rough until closed again.
• Throttle is a bit twitchy. Doesn't spring back to idle unless twisted. Could be the stock cables on the clubman bars.

Brakes

• Rear brake is stronger than the front
• They feel a little soft

Electrical

• All lights are non-stock
• Stupid adhesive strip LEDs for tail light. One strip for running light, two strips for actuated brakes
• Turn signals are very intermittent. Flipping the switch will turn them on, but they only blink sometimes. There's an audible chime when they are working which is cool. Is that a stock feature?
• Headlight seems dim. Not much difference between hi and low beam
• Horn button puts a load on the electrical as evidenced by dimming headlight. Probably just a bad horn. Or a short somewhere.

Fuel

• delivery seems okay, but I haven't had the bike over about 40mph
• It was dark when I looked with a flashlight, but it looked like there was a little bit of rust floating in the full tank.
• No idea what condition the carbs are in.

Cosmetic

• Has clubman bars. Uncomfortable, but they don't hit the tank
• stripped tank has a ton of surface rust, but clear coated. No big dents or scratches, though. I might paint this.
• Some PO was huge Ravens fan and rattle canned the whole bike purple and black. There are still some purple screw heads and brackets.
• some light surface rust on spokes
• original seat, but huge tears towards the front. Might do a tracker seat, or just recover stock seat

And here's some eye candy. She's rough, but she's mine.

[calj737]

Well, the easy part is over; finding and buying a bike. Now, the fun begins!

You would be extremely well-served to buy Mark Parris' (Hondaman) book:
http://forums.sohc4.net/index.php/topic,65293.0.html

It sounds as though the bike needs a complete 3,000 Mile Service interval which will also give you insight into repairing or replacing anything damaged or broken. The "Phillips" screws on the bike are not Phillips, they're JIS and they need a specific type bit to avoid stripping them out. You can buy a set of those in the Parts For Sale section.

It sounds as though you may need a fresh battery too, but at a minimum, a complete service of the harness and connectors (look here) http://forums.sohc4.net/index.php/topic,137351.msg1549191.html#msg1549191

I have one of those impractical R90s, so watch what you say. I live right down the road in Richmond and know where you live! 

Your throttle cable is probably routed improperly and binding, adding to its sticking and twitching.
Drain the tank, flush it with soapy water, and de-rust it thoroughly. Any rust will make its way to the carbs, and cleaning your carbs without cleaning the tank is pointless.

You can also check out the FAQ section for carb rebuilding info, tuning, and some great baseline data. But get Mark's book. It will save you heaps of money and answer most questions you have in the middle of the night with dirty hands.

Welcome and keep the updates coming!

[therobbstory]

Thanks for the advice!

The battery is actually only a month old.

I'll check into the Hondaman book. I've got a Clymer which I've already learned a lot from. Very thankful that resources like this forum exist!

I've owned the bike less than 24 hours. I'll have a lot more time to dig in once the colder temps set in and I move the bike indoors.

[oldhatt45]

therobstory,

+2, no +3 to what calj737 said.
AND DO BUY THE Hondaman Book, as well as a set of JIS screwdrivers (including the Impacta)!!!!!!

Charlie

[therobbstory]

Many thanks, gents!

I took it out for about an hour after work last night to get comfortable riding in city traffic, and to get a better feel for the bike now that my nerves have calmed. Took off down my street feeling like Steve McQueen only to have the bike sputter and stall at the first stop light. Drained the battery trying to restart before realizing I hadn't turned the fuel on. Kicked right over and I was on my way.

My main priority for this weekend is to solve the won't-run-without-full-choke issue. I'm guessing the idle jets are all gummed up starving the motor of fuel. Would that make sense? The PO said he'd never done any carb work, and he owned it for a year, which would include a mid-atlantic winter. I'm going to pull the carbs and air filter on Sunday to see what I'm working with.

The other big issue is the sticky throttle. It's damn near impossible to get any kind of smooth throttle response off of idle. It's kind of on or off until you get into 2nd and 3rd. I'm hoping new cables (all of them have rotting housings and should be replaced) of the correct length on some normal bars will help.

Read the Clymer on those procedures and feel pretty confident in doing the work myself.

Will get some better photos this weekend. It's dark out by the time I leave work. I picked a great time to buy a bike 

[calj737]

Slow down, Grasshopper! If you take your carbs off the bike (which you really, really should do) they need a full teardown and proper cleaning. Be prepared to need some gaskets and O-rings too. Don't swap the factory brass and NEVER buy Keyster brand carb kits; they're absolute junk!

Locate the specs for your carbs (jet sizes, screw settings, float heights, etc) and clean and configure your carbs to factory specs. Clean your tank too! Check to see if theres an inline fuel filter, if so remove it or at least replace it and be certain the fuel in is oriented downhill the entire route.

Your throttle cables need to be checked for proper routing and length. If the PO swapped to clubman bars but used the stock cables, they're too long and likely binding in their routing. Order shorter cables (or longer). But first get your carbs cleaned up and lube the cables internally. Check for throttle sticking by rotating the front end side-to-side, full lock and notice any rev changes. If none, then the issue is not cabling.

[therobbstory]

Thank you, o wise and merciful master!

I'm going to pull the tank this weekend and clean the inside using one of the myriad methods found on this forum.

I've also ordered some carb kits with stock jets, fuel lines, and some filters.

Quick photo from this AM, which has prompted me to order a cover since she'll be living outside the next month or so.

Suicide Machine by Robb Hohmann, on Flickr

[Desert-SOHC]

Yup, those throttle cables look to be the stockers and to long by the pic.

[calj737]

As I was examining the picture, I thought, "What the hell, is that plasti-dip on the bars???" Only to finally realize it was rain having collected. Phew!

Just bear in mind the counsel about the carb kits...

[SOHC4 Cafe Racer Fan]

Owning a K7, I am not the biggest fan of the bulbous gas tank, but I have seen them work decently with tracker-style seats.  It's a personal taste thing, though.  If you love it (or just don't care to spend to replace it), keep it. 

[therobbstory]

Just bear in mind the counsel about the carb kits...

Went with Honda OE parts from Dime City. Got some pre-drilled bars and a set of replacement cables, too.

[therobbstory]

Owning a K7, I am not the biggest fan of the bulbous gas tank, but I have seen them work decently with tracker-style seats.  It's a personal taste thing, though.  If you love it (or just don't care to spend to replace it), keep it.

I was wondering about this tank. Is it not stock?

I do plan to go with a tracker seat, but cosmetics are a long way away for now.

[SOHC4 Cafe Racer Fan]

Owning a K7, I am not the biggest fan of the bulbous gas tank, but I have seen them work decently with tracker-style seats.  It's a personal taste thing, though.  If you love it (or just don't care to spend to replace it), keep it.

I was wondering about this tank. Is it not stock?

I do plan to go with a tracker seat, but cosmetics are a long way away for now.

The tank is stock for the K7 and K8 (1977-1978) models.  If it works and you are just trying to get the bike into reliable running condition, don't worry about it right now.

For the future, you can also use the earlier model tanks from the 1969-76 CB750K models or the 1975-78 CB750F (Super Sport) models.  Using a different tank will require you to adjust the spacing between the tank and seat -- if you are still planning to use the stock seat on hinges.  Essentially, you will need to unscrew the seat hinge mounts and adjust the mount points to line up with the frame rails (but accommodate the longer or shorter length tanks).  It is not brain surgery.

For me, I really didn't care so much for the K7-K8 tank and used a CB750F tank.  Others have done this too.

[calj737]

Went with Honda OE parts from Dime City. Got some pre-drilled bars and a set of replacement cables, too.

I don't believe they resell Honda OE parts. They resell aftermarket kits to the best of my recollection. Keep your receipt in case they are not true Honda brand.

[therobbstory]

Went with Honda OE parts from Dime City. Got some pre-drilled bars and a set of replacement cables, too.

I don't believe they resell Honda OE parts. They resell aftermarket kits to the best of my recollection. Keep your receipt in case they are not true Honda brand.

Interesting. Says here they're gen. Will see in about a week when they arrive.

http://www.dimecitycycles.com/77-78-vintage-honda-cb750-carburetor-gasket-and-o-ring-kit.html

[Stev-o]

Subscribed, I had a K7 until just recently.  Personally, I dont buy parts from DCC, especially Honda parts.

I like Partzilla and they are cheaper here...

http://www.partzilla.com/parts/detail/honda/HP-16010-405-004.html

[therobbstory]

Subscribed, I had a K7 until just recently.  Personally, I dont buy parts from DCC, especially Honda parts.

I like Partzilla and they are cheaper here...

http://www.partzilla.com/parts/detail/honda/HP-16010-405-004.html

Good to know. I just went with DCC because they're in my hometown.

Rode over to my girl's house tonight. Stalled about a block into my ride and walk-of-shamed home. Indicator lights were very dim, and electric start would obviously not work. Threw it on the charger for about an hour, and was able to kickstart it. The bike was idling rough, and stalled out  again at a light on Pennsylvania Ave. I managed to kickstart again but, only with 1/4 throttle. Idle was real rough unless I kept it above 2-3k. I assume that's carbs.

Is this also indicative of a bad charging system, or do these bikes just have ineffective charging in low-RPM city riding? Another system for me to consider refreshing, I guess.

EDIT: Tonight's homework - http://forums.sohc4.net/index.php/topic,1012.0.html

[Stev-o]

Sounds like a charging issue. Should have no issue riding in the City.

[therobbstory]

Had an existential crisis of buyer's remorse on Saturday. After waiting all day for the rain to stop, I got her fired up and planned to ride back to my place. Got halfway around Lincoln Park and she #$%* the bed as soon as I came off the throttle at a red light. Crab-walked back to my girlfriend's place and cracked a beer.

Battery didn't seem to be an issue this time, as it cranked strong but just wouldn't fire for more than a second or two. Tried again the next morning with a cold engine and it fired right up. Idle was really rough, so I dicked with the idle screw which I'd recently discovered. Once the bike was warm, I adjusted the idle screw to about 3k and pushed the choke in (PO told me he could only ever ride it with the choke all the way out). It settled down to about 1k, idled beautifully, and I rode home basking in the beautiful sunlight and stares of douchebag DC Sunday brunchers.

So the charging system works, at least well enough. I'll probably put it on a tender for any extended periods where I'm not going to be starting it.

Is it best to give these 750's a warm-up period? Took her out again today to return the PO's tags, and it idled pretty roughly for the first half of the trip. Once warmed up she sang beautifully.

While idling, it sounds like the engine case has an entire socket set rattling around in there. Once I give it some gas, everything seems to sync up and it sounds like a motorcycle again. Is this normal?

Suspension-wise, I noticed a couple knocks from what we in the bicycle world call 'the headset.' I Read a good thread here recently on upgrading a bunch of suspension bits to All Balls, including the main steering bearings. Something I'm considering once I get the engine kinks figured out.

Also got some goodies in the mail today. Cheap-o $20 Amazon amber turn signals to replace the blue LEDs, and Bates-style LED tail light to replace the LED strips. Super bike bars and new throttle cables should be here tomorrow along with the carb kits.

[calj737]

I'm going to repeat for you what everyone has already said: Perform the 3,000 Mile Service. Simple as that. You can tinker and Dick with it all you want, but it will never run right, run reliably, and stop leaving you walking down the street with your helmet in your hand until you do.

And, until you do perform the maintenance, there's a very real possibility that you're doing damage to the internals (like a cam chain eating a hole in your engine, or bending valves). Or you can continue buying cheap "look good" crap and asking questions that will elicit the same response.

But hey, it's your bike and your legs. I hope you look good walking and that your helmet doesn't get too heavy.

[SOHC4 Cafe Racer Fan]

Performing the full service will give you peace of mind.  It sounds like your carburetors still need ultrasonic cleaning and synching.  New spark plugs and an inspection of your wires -- and a meter test of your electrical harness is a good idea, too.

[evanphi]

Hi from Reddit!

[therobbstory]

Well, sh!t on a biscuit.

Noticed a hearty clatter coming from the head cover on my way home last night. Today, I pulled the tappet covers to re-adjust valve clearances and found the attached image. I can't immediately see where the locknut went, but I'm going to poke around tonight with a flashlight and a magnet.

I'd know pretty quickly if it fell into the cam chain guide, right? What other potential things should I be looking for before I replace the locknut and tappet covers, assuming I find the missing nut?

In other news, I performed all of the 3,000 mile maintenance after acquiring all the requisite tools and test equipment. Carbs weren't too badly out of sync, but I did swap out the points after losing cylinders 1/4 not too long ago. Old points were nasty. I've also got a HondaMan ignition on the way. Couldn't hurt, right?

[evanphi]

You could also drain your oil and drop the oil pan, that way you'll know if it made it all the way down to the bottom of the case!

[therobbstory]

You could also drain your oil and drop the oil pan, that way you'll know if it made it all the way down to the bottom of the case!

Likely what I'll end up doing. Shame, because I did a complete oil  change about 30 miles ago. At least I'll be able to inspect the oil screen. Good time to replace the gasket as well.

Hopefully it's just sitting on top of the head somewhere as other's on the board have reported. Fingers crossed I can find it with a flashlight and magnet. New locknut, adjuster, tappet cover, and seals will be here on Thursday.