Topic: Senior project for high school - 78 CB750K

[Restoration Fan]

My son is rebuilding a 78 CB750K for his senior project in high school.  This is his first experience with working on a motorcycle, but with the help of people on various forums, YouTube, manuals and friends (and his dad...me...of course), I'm expecting that he'll be able to get it done.

The bike was mostly complete when we bought it and the engine would turn over but we have been accumulating missing parts over the past month and finally got everything I think we'll need so we began tearing down the bike on Monday.  Tonight, we finally got everything down to the bare frame and have the engine up on the workbench, ready to be worked on.

Thought I'd post some photos as we begin the thread before I start posting questions and progress later on in the thread.

[Restoration Fan]

I was able to obtain a 4-into-1 chrome header/exhaust that is in almost flawless condition.  As we get prepare to clean and rebuild the carbs, shouldn't we also plan to upgrade the jets?  We will be using the stock air filter box and filter but if I understand correctly, changing the exhaust should mean larger jets to accommodate the different exhaust?  So what size jets should I get?

Also, we plan to replace the points/condensors with a Pamco ignition system.  On www.vintagecb750.com, there are 3 different Pamco packages:  Basic, Standard, and Ultimate.  Which of these should we get?

Also I forgot to mention that the original plan was to fix up the bike and sell it for him to make a profit.  But I've decided that it would be just too cool to have a bike restored by my son so I'm going to get the bike valued and pay him the difference between the value of the bike and whatever I have in the bike for parts.  Then I'll keep the bike to ride. 

[Stev-o]

Great project.   The K8's were jetted lean and the stock exhaust was very restrictive (and quiet) so, most likely will have to increase the size of the jets. 
There is no chart that has info with x exhaust needs y jet.  What header do you have and does it have a muffler with baffle? What size main jets does it have??

[Restoration Fan]

Great project.   The K8's were jetted lean and the stock exhaust was very restrictive (and quiet) so, most likely will have to increase the size of the jets. 
There is no chart that has info with x exhaust needs y jet.  What header do you have and does it have a muffler with baffle? What size main jets does it have??

The exhaust has a baffle.  I've attached a picture of the exhaust because I'm such a newb that I don't have a clue what brand it is or know anything else other than it fits because we test fitted it into the bike before we bought them.

And we haven't taken the carbs apart yet; the rack is sitting on the shelf ready to be cleaned either tomorrow or Friday so I'll know what size main jets are on there once we take one apart but for now, I'm just working under the assumption that they are still the stock main jets that came with the bike.

[Stev-o]

Nice looking pipe, sorry does not look familiar to me. 

[madmtnmotors]

That pipe is also designed to work with the "spigots" that were used on 76' and earlier engines. They will work on a 78', you just have to source the spigots and clamps from an earlier model.

[70CB750]

Subscribed.

[pamcopete]

Also, we plan to replace the points/condensors with a Pamco ignition system.  On www.vintagecb750.com, there are 3 different Pamco packages:  Basic, Standard, and Ultimate.  Which of these should we get?

The basic kit uses the stock coils. It is recommended that you replace your spark plug caps, however.  This would be my choice for your project at this time as you are probably on a budget. You can always get the high performance coils later.

The standard kit comes with coils that are equivalent to the stock coils.

The "Ultimate" kit comes with high performance coils that will give you a bit more power and better gas mileage.

[Stoli]

Wow. Great project for your son. Excellent learning experience and your plan to keep the bike is spot on. The pride he gets everytime he hears it fire up in the garage will never fade. Looks like the bike is a good candidate as well.

How far do you plan on going with the engine? Clean and paint as is or are you going all the way in to the lower end?

+1 on Stev-o's comments regarding jets.

Main jets: The stock main jets on that bike were 110. With a less restrictive exhaust, you'll probably need to go up to 115-120.

Slow jets: Stock size on that bike is 35. If you keep a stock filter in your airbox, you shouldn't have to touch the slow jets but make sure you clean them good when you have the carbs apart. The slow jets are press fit on your carbs and don't unscrew easily like the earlier models. Grab them gently with a pair of pliers and give them a very slight twist to "crack the seal" then a small back and forth twisting motion to work them out. You have to remove them to get them clean. Don't worry about marring the outside of the jets as that won't hurt anything. Just don't grab them so hard that you crush them and change the opening. To replace, gently tap them back in with the handle of a screw driver or something that won't damage the top of the jet. If you put a less restrictive air filter in your airbox then you might need to up the slow jets to 38-40.

Needles: The needles on your carbs do not have the adjustment clip that earlier models had. Again, if you keep the stock filter setup, you shouldn't have to mess with the needle height but if you find that the bike runs lean in the mid throttle range, you can put a shim or two (small washer) in there to do that. There are a couple of threads on here with some pics and a good step by step on how to do that. If you can't find it, let me know. I have it bookmarked somewhere.

As Stev-o pointed out, there is no chart for jetting, but here is some good theory to get you started thinking about what is going on. Keep in mind that the different jets are related to throttle position and not your rpm or how fast you are going -



http://www.hondachopper.com/garage/carb_info/carb_info.htmll

There is a lot of good info regarding jetting on this forum but some not so good as well. Try not to get confused about the air/pilot adjustment screws (not to be confused with the idle adjust knob on the throttle linkage). On earlier carbs they were called "air" screws and they did just that - let in more or less air. Your PD carbs have "pilot" screws and they let in more or less fuel. So on your PD carbs, in is leaner and out is richer. The pre '77 carbs were the opposite (in richer, out leaner).

Have fun.

[Restoration Fan]

pamcopete,
Thank you.  I think we're going to go with the Ultimate.  Who doesn't want more horsepower when it's available, right?

Stoli,
What a great explanation.  I can't thank you enough for taking the time to post that.  One question though.  How do I tell what size main jet I have right now?  Do you measure the opening?  Is the # stamped on the side?

Regarding how far we plan to take the engine:  we have to crack open the bottom at least a little because there are missing gear teeth on the kick starter gears.  Now that we have the engine off the bike and up on the workbench, I think we're going to use some engine degreaser to knock off some of the oil & grime on the outside first.  Then, we'll take off the cover (I call it a starter gear cover but I don't know the correct term for it) and see if we can determine what parts need to be replaced in there.  The spring is apparently still in place because, although when you kick it and the lever goes straight to the bottom without resistance, it immediately returns back up to the top when you let off with your foot.  When Logan gets home from school today, that's the first order of business so that we can figure out what other parts need to be ordered.

Also, at the very least, we'll replace the gaskets but I thought it might be prudent to take off the oil pan and see what I find in there.  If we find metal filings or other gunk that shouldn't be there, then I think we'll try and tear the whole thing down and do a complete rebuild....but my preference would be to avoid that if we don't have to do so.

[madmtnmotors]

Pretty sure you'll have to split the cases to replace the kickstarter gear.

[Restoration Fan]

Pretty sure you'll have to split the cases to replace the kickstarter gear.

Well, if that's the case, as my buddy says, "At that point, you've already got the skirt lifted up.  You might as well go ahead and seal the deal!"

[70CB750]

Pretty sure you'll have to split the cases to replace the kickstarter gear.

Well, if that's the case, as my buddy says, "At that point, you've already got the skirt lifted up.  You might as well go ahead and seal the deal!"

 

I use regular gasoline to clean the outside of an engine or transmission.  Engine degreasers are nasty stuff, gasoline, rags and old toothbrush works quite well.

[Restoration Fan]

Also forgot to mention:
pamcopete - I forgot to respond to the comment about the project being on a budget.  But in actuality, while I won't say money isn't an object, money isn't of primary concern to me at this point.  I'll probably purchase the ultimate set and just charge the project's base cost for the basic set. 

What I want to do is this:  if there's a situation where we are replacing a part and we have a choice of good, better or best, I'll probably choose the best.  If we were flipping the bike for profit though, we'd choose the good.  So I won't penalize my son's "profitability" for my decision to upgrade the parts.  But in the end, I want the bike to be reliable and something that I'll enjoy riding for many years to come.

[Restoration Fan]

OK, so we took off the oil pan and discovered some little pieces of very thin, brittle debris in there that looks and feels like plastic.  Anyone have any idea what this stuff might be?

Also, am I correct in thinking that finding this stuff means that we should probably plan to take the engine apart and make sure this stuff hasn't made its way through the motor?

[Stoli]

The jet sizes are stamped either on the side or top. You might need a magnifying glass to read them but they will be there.

Those pieces in the pan look like they might be part of a gasket, which is not a big deal. They could also be part of one of your chain tensioners (cam or primary). Either way, those pieces are not reason alone to do a complete rebuild.

No such thing as "cracking open the bottom just a little". The cover by the kickstart lever is the clutch cover.  If you are missing teeth on your kick starter gear, then you will need to crack the case but that really isn't as big of a deal as it sounds. I'm not positive but I think you can do that without pulling the top end apart. I've always gone all in with the engines I've rebuilt so not sure. As far as rebuilding the top end goes - that will be a judgment call on your part. It would be a good learning experience for your son but your cost and time obviously go up even if you only replace the rings and do a quick cylinder hone. The two things that would help decide whether or not to go into the top end would be low compression and or a leaky head gasket.  How many miles are on this bike?

[SOHC4 Cafe Racer Fan]

I agree with Stoli. Those pieces look like bits of cam chain tensioner. Or maybe gasket.

Great project and YOU get to keep it as a rider. Win win.

[Restoration Fan]

There are just over 19,000 miles on the bike.  We began doing a compression test on the first cylinder and were kicking it over to do the test.

When we bought the bike, the young man we bought it from told us that "every now and then, when you kickstart it, it'll miss and go straight to the pegs...like there's a missing tooth or something".  Since the electric starter wasn't working on the bike, the kickstart was the only way for us to test compression at the time. 

We got it up to right at 110 pounds in cylinder #1 with the the kickstarter missing every now and then.  The compression was still going up when it got to the point that the missing tooth/teeth were making it such that it wouldn't turn over anymore.

[Stoli]

Curious to hear what you find when you remove the clutch cover. The clutch basket has a gear on it that meshes with the gear on the kick starter. So, if you remove the clutch and clutch basket, you'll be able to see the kickstart gear. I think it would be unlikely that the kickstart gear is missing teeth. More likely something awry with the linkage that engages the kickstart shaft with the gear. Might just be something sticking from the bike sitting around for a while?

Still haven't heard any compelling reason to crack the case or the top end. Keep us posted on what you find behind the clutch cover. One of the few special tools required for these bikes is a socket with 4 tabs on it to remove a nut that holds the clutch pack together. I've seen someone use a chisel to remove it but that is up to you. I think I found the socket for about $10 on ebay although some places get $25 for it. One thing you will find out out as you go is the disparity between part prices out there. There is no "one stop shopping" for CB750 parts and some vendors sell some items at very low prices and other items significantly more than other vendors. I know you mentioned that you will pay for quality and that is good but do your homework when searching for parts and tools. You don't always have to pay top dollar for the right part.

[jcop550]

Very cool! Im doing a 78 cb550 cafe/restoration build for my senior project too! Very close to being complete

[ekpent]

That pipe is also designed to work with the "spigots" that were used on 76' and earlier engines. They will work on a 78', you just have to source the spigots and clamps from an earlier model.

Don't forget to buy the spigots if you plan on running that pipe. May want to mock it up when the bike is back together more to see how it fits the frame points as it was designed for a 1976 or older machine or 'old style'. Good luck and better get your son a black t-shirt shows lot less grime than a white one.   

[jrich44]

I have extra spigots and clamps for sale if you're interested.

[Restoration Fan]

I have extra spigots and clamps for sale if you're interested.

Very much so. 
I sent you a PM after Stoli correctly noted that I should probably remove private contact information from a public forum.  (Thanks, Stoli!)

[pamcopete]

Restoration Fan,

I would suggest that you do a leak down test for more accurate and useful information and also to see if there is any significant valve issue. You can rent the tester at some auto supply houses or buy one at Harbor Freight on sale now for $39.99. You would need a compressor to go with the leak down tester, but I can't imagine doing an engine overhaul without both.

As for cleaning the engine, it may be too late now, but I use an engine degreaser at the local DIY car wash.

[Davidov]

I think it would be unlikely that the kickstart gear is missing teeth. More likely something awry with the linkage that engages the kickstart shaft with the gear. Might just be something sticking from the bike sitting around for a while?

There is no "one stop shopping" for CB750 parts and some vendors sell some items at very low prices and other items significantly more than other vendors.

Are you sure the splines on kick shaft arent simply stripped?

+1 for Pamco, Pete gives rapid results on customer service questions. My 750F runs great with it.

http://www.dynoman.net/bikepages/sohc/index.html
CB750Supply.com VintageCB750.com BikeBandit.com should get you rolling.

Showing off a little now, here's by recent engine rebuild thread. http://forums.sohc4.net/index.php?topic=93209.msg1603613#msg1603613