Honda CB750 with low compression.. Will it run?

what's going on guys welcome back to classic octane i am taylor today is going to be a will it run video which i know you guys love so much uh we are actually going to be starting on the next customer project so little update this guy is the one you saw the video on of the m unit it's almost done i just need to paint the gas tank but we're actually holding off on painting that tank until we find a gas cap it's actually proving to be a little bit difficult finding a 7778 cb550 gas cap and i don't want to paint this tank and then have us you know end up not being able to find a good replacement cap so i'm holding off on that which means we have a free day to move on to the next project it was going to be the 78 bmw r100 that's out here in the driveway but we are ordering we've already ordered a bunch of custom parts for it from cognitomoto uh modern gsxr front end all that kind of stuff uh but they are pretty far behind i think it's gonna be like another four weeks before they get here so we're going to try and knock out another build in the meantime so let me go pull it out of storage we'll pull it in put it on the lift see if we can get that thing running here she is this is a 1975 honda cb750 i bought this bike a little over a year ago you guys probably seen it on the channel before it came with a white gas tank but this one's a little bit better condition so we're going to use this one for this build this is going to be similar to the bikes i've built in the past obviously a lot of the bikes i've built have been cb550s it's been over a year since i've built a 750 on the channel so i'm excited to get back into them i personally kind of want to build a 750 for myself i want to have a custom 350 550 and 750 just because i'm a honda order so the main goal today is going to be to do what i call a pre-build inspection and that's going through every system of the bike and judging its conditions seeing what needs to be replaced what's in you know good condition how's the suspension look the frame mainly the health of the engine is what i'm most concerned with electrical most of that's going to be replaced anyway tires are going to be replaced condition of the wheels is important do we need to replace the wheels replace the spokes bearings that kind of stuff just to give us a full kind of health indicator of the bike before we fully dive into it this bike is going to be somewhat of a clone to that moss green cb550 i built a couple of months ago it's going to have a dark moss green tank a brown seat this one will have side covers that are going to be moss green as well as well as the front fender going to be chopped so it's going to be kind of a tracker-ish street it's still going to be a very good street bike it's going to have a little bit of those vintage knobbies on it it's going to be a really cool build i'm excited to dive into it so when i bought this bike i bought it knowing that one of the cylinders has low compression and what i'm hoping for is i've had i don't know five to seven bikes with low compression before almost all of them except for maybe one or two came back to life once i was able to get the bike running we got those compression numbers right back up so what typically happens is one of the cylinders is going to have a valve that's open when the bike is sitting just naturally how four-stroke four-stroke engines work one of the valves is going to be open and it can develop a little bit of rust around that valve seat that can cause the compression readings to turn out low when the bike is running and those valves are going up and down it cleans that rust off within i would say 20 30 seconds and i've seen engines go 40 50 psi higher on compression readings just after the engine running for you know 30 seconds to a minute so i'm going to do a full compression test so we know which one of our cylinders is you know suspect and then we will go through typical will it run style see if we can get this thing running then we'll recheck all the compression fingers crossed it comes back up if not i will start to look through my storage and i have a couple of spare 750 engines we'll find the best condition one and run with that so that's going to be the main goal today what do you say we try to get this pig running the gas tank out of the way so we can work around it went ahead and pulled the spark plugs out when i store bikes i like to make sure to keep spark plugs in them as well as rags in the intake especially if there's no carburetors on there if there's carbs on there really you can just close the choke i just want to keep stuff from being able to crawl in there or you know debris or whatever falling in there somehow just better to keep the inside as clean as possible so i'm going to go ahead and just start off with a compression test right off the bat and what i want to do okay there's no oil in here i want to make sure that we can turn the bike over using the electric start it's a real pain to try and get compression readings with just a kick start because the engine really doesn't turn over that fast so you gotta kick it like eight times to try and get a compression reading so i'd rather be able to use that electric start first and foremost don't want to turn it over too much without oiling it so let me throw some oil in there and then we'll throw a jump box on this old battery and we'll see if our electrical system seems to be up to the task if not we'll have to start there all right got the jump box all hooked up we're going to turn the key low oil pressure light neutral light perfect exactly what we'd expect hit the start button nothing okay stand by let me uh i'm just gonna rig up something simple to get our starter solenoid going then we cannot turn it over that okay just throw a wire on here we should be able to just ground there we go so something with our wiring from the start button to here is disconnected but not a big deal let's go ahead and turn it over for a little while see if our low oil pressure light turns off this turned off see if it comes back on come on yep there it is perfect so we should be ready to hook up the compression tester and get our numbers we're just going to go in numerical order and start with cylinder number one see how we're looking about 130. perfect you get a little notepad so i can write this down move on to number two [Music] about 120. let's do it one more time yeah i'm gonna say 120. slightly lower but not a concerning amount i'd say if they're within i don't know 10 10 i think it's fine i think some people say 5 some people say 15 you go for number three [Music] exactly 120 as well all right now it's just down to number four which really this is great news so far um you know ideally number four tests um tests out fine but i kind of remember the seller telling me that that was the low compression one which is why i kept it for last so let's jump on that side see what we got go for four see if i can reach my wire here [Music] 80. it's not zero let's try again [Music] yeah we're looking right about 80 right now um okay so again i'm going to say that that's a good sign for two reasons one it does have some compression meaning there's not a hole in a piston or you know a big broken um ring something like that ring land broken off a piston something like that it's not catastrophic damage two since it's an outside cylinder that's one that's even more prone to have water get into it or some kind of moisture when the bike is sitting which can cause that rust i was talking about around the valve seat so i'm going to continue on that assumption that it's just a little bit of rust and we're going to continue to try and get this bike to run and i'm going to bring you guys along for the ride to see if those numbers come up so we got right at 80 on the compression test so let me write that down so we're looking at 130 121 20 80. so if we can get this up to around that 120 130 mark i'm going to give this guy a stamp of approval next thing i need to dive into is going to be let's just start with the ignition system so i'm just gonna hardwire these coils uh to make sure that we are in fact getting power i guess i could test that first we'll test the coils right now to see if we get power when the kill switch is in the run position if we are getting good power we'll throw some spark plugs on here turn the engine over make sure we're getting spark then we can move on to fuel after that so check power to these coils i'm just looking for a wire that's black with a white stripe all classic honda's are the same that is 12 volt power that goes through our kill switch and into both coils so it's right on this side i'm just going to test it with a good old-fashioned test light so we've got ground on one side positive side of the battery makes it turn on we know our lights good so we'll follow this black with a white stripe okay so it's these two top posts right here kill switch is on key still on try to stick it oh there we go we got power test my theory i should be able to hit this tail switch and this light should turn off boom cool we'll test the right side coil good does it turn off yes on cool so we know we're getting good power to our coils so let's throw some spark plugs in there i'm actually going to bolt this down so that it can have a good ground too it looks like this is just kind of floating around loose in here i want to make sure the coils are grounded properly as well and we'll test that then we can dive into putting spark plugs on checking if we actually have spark at the end of the leads so we're ready to test for spark now got spark plug in a few different leads oh yeah we got power to the coil and everything so let's see what we got kion mission on we'll test right side coil first [Music] nothing see if one of these on on the left side coil does anything [Music] nothing on that either okay pull the points cover off see if we got any corrosion or anything under there apparently it's already missing a screw it's probably because it's blocked by this crash bar we will be removing that but i'm glad it was there because it's definitely seen the ground at some point all right let's see what we got in here everything looks complete yeah there's some some corrosion on the points not terrible so what i like to do is just take a little piece of the sandpaper this is 1000 grit fold it over on itself stick it between the two points and let it close and just work it like that that way you're kind of evenly sanding both sides you'll be able to see better on this side just like that because ideally we have a perfectly flat mating surface between these two so we don't want to introduce some kind of crazy angle or something we also don't want to remove a huge amount of material we're just looking to remove any corrosion buildup so i want to go ahead and test these points and one thing that's pretty common with these is there's a little isolator that basically keeps the arm that moves isolated from the like being grounded through anywhere else because what you want is for that to break the circuit every time that that gap opens so a good way to test that is you can take your multimeter put it on continuity so it'll beep when there's continuity there it would make sense that we have continuity when the points are closed but if we hold it on there and we open it that should lose continuity when the point is open that's constant continuity no matter what test on this side exact same thing let me show you on a bike with known good points and i'll you can spot the difference i can show you on this cb 500 right here exact same points set up so right now we should have connection now if i open the point we lose it that's what actually lets it break that circuit and that's what makes the spark plugs for those two cylinders in this case number one and number four fire is when that opens up it discharges fires the spark plug so that's a good way uh to be able to test them you can also test it on the brand new point itself so what i'm going to do is go ahead and throw in the fresh points fresh condensers because i have them throw that in on the cb750 see if that makes any difference for us on the ignition front fresh points and condensers are in i'm only about 60 sure the points are actually bad i think most of our problem was probably in the condensers but i have the parts so i figured why not throw them in anyway went ahead and set the point gaps as well so we should be ready to test key on take this random spark plug here grab a wire oh yeah we got spark on one let's try the next one spark there can we go three four three [Music] nice let me grab a fourth spark plug and we'll test the last one oh yeah we got spark on all four people ready to move on now that i know i have sparking all four plugs i'm gonna go ahead and actually install them plug the wires in then we can move on to i'm gonna just find a set of carbs stick them on here and i'm gonna start with just spraying some card cleaner straight down there we'll hit the key or the wire in this case see if we get any pops and bangs i believe we're ready i put the carbs on i don't have a return spring for these cards i gotta locate one of those so i'll have to manually push the throttle linkage back down let's open it i think we're ready to go kion mission on all right people let's see what happens [Applause] [Applause] hmm run a little low on charge [Music] just open a little bit [Applause] [Applause] hmm that's such luck at the moment i'm a little worried about how low our jump box is let me grab my other jump box and see if that's any better i ended up just charging my jump box for a little while um because my other one was dead too i'm about to try it again one thing i noticed when i was about to spray some more brake clean in there is uh if you're gonna try to use brake clean as you know starting fluid don't uh don't get the non-flammable version it's not gonna work that could be part of the reason we didn't get any pops and bangs the first time so what i'm gonna do now is just a little bit of gas in a squirt bottle i could just squirt some down each card and what i'm looking for is just some pops and bangs enough for us to justify bringing the auxiliary gas tank hooking up the rest i could argue that i should just do that now but anyway key on battery box on ignition on i'm gonna still go no choke let's see c okay there's still enough pops and bangs let's get the auxiliary fuel tank and see if we get this thing to run for a little bit longer we seem to have an oil leak out of the box [Music] and we need to get the fan out let me uh air this place out a little bit okay let it air out a little bit our oil leak was just uh once the engine started to pump oil back into the tank uh our oil was just a little bit over filled so it started to come out of the breather so i just wanted to make sure it wasn't like a an oil line and ruptured or something that could be catastrophic not that just started to come out of the breather i still have the drain pan down here just in case i did remove some of the oil just to try and get the level a little bit more accurate but man that blew so much crap out of the exhaust i mean there's like junk on the floor all the way back to here yeah i want to run it and i want to run it a little bit longer again just to try and heat cycle everything you know get the engine kind of move in some oil circulating then we can recheck that compression so let's fire it up a second time fuels on keys on i'm gonna see if it'll start back up with no choke [Music] okay i'll choke oh it helps if i turn my ignition on too [Music] [Music] foreign [Music] i'm going to turn my fan on let it sit here and idle like this for a little while but i need to get the circulation going let it run for another i don't know a minute or two judging by my thermometer it wasn't really running on number four at least not very consistent uh very consistent which is not necessarily a good sign um but let's see what our compression says we were right at 80 before yeah spark plugs pretty wet which is not necessarily a good sign that means it was getting fuel but not burning at all i'd give us a 40 chance that this worked that might even be generous okay here key on let's make sure our ignition's off so that this thing doesn't try and start while we're doing this should be ready to go everybody cross your fingers 50. well now we got carbs on here so let me and make sure the throttle's all the way open that can actually make a really big difference 75 checks so it looks like this engine it needs to have the top end pulled to see what's really going on see if it's still just really heavy corrosion on the valves or what it's pointing to now is maybe an issue with a piston ring might have a broken one you know it didn't take out a a chunk of the ring land or anything but maybe a piston ring is cracked or broken and that's uh just not build up enough compression let me think about what our options are okay so where we're at right now this engine like i just mentioned is going to be coming out either way you can't do a top end on a 750 while the engine is in the bike so we won't be able to really tear it apart you actually can't even get these valve covers off uh with the engine in the bike one of the things that's nicer about a 500 or 550 you can access all of that without pulling the engine so we're going to be pulling this engine regardless i have two cb750 engines over on the shelf of unknown condition one of them i think is locked up so i don't think that's going to be any better than what we have here the other one may or may not be a good contender i also have another complete cb 750 in storage that i don't have a title for or anything so it's not going to be a bike that i would build to you know sell to a customer because i don't have a title for it so what do you say that i do another will it run video on that bike we can see what the condition of that engine is because it's complete like i said so it should in theory not take much more work than this did to get running i can test all the compression i can make sure that that engine is good to go in that frame then i'll just pull that engine pull this engine do a switch maru and then we'll be in business so i think that's what we're going to do so in the next i don't know probably a couple of days because i think i'm going to do it tomorrow because i really need to get this bike going as quickly as we can i'm going to do another run on another cb750 and we'll cross our fingers that that one uh checks out so stay tuned for that i appreciate you guys watching as always i know you guys love these will it runs and i love making them because making old things work is literally my biggest passion so i appreciate you guys watching as always and i'll see you in the next one you

Channel: Classic Octane

Views: 22,008

Rating: 4.9632277 out of 5

Keywords: Honda, CB550, Cafe, Racer, Build, Project, CB750, CB350, CB360, Yamaha, Motorcycle, Garage. Shop, Update, Triumph, hardtail, bobber, chopper, welding, fab, springer, custom, trail, riding, dual, sport, pit, bike, monkey, Stunt, gloves, cam, corder, burnout, handlebar, stunts, stunting, wheelie, hd, drift, Xv1100, XvS650, Harley, Davidson, motorcycle, review, comparison, scooter, fix, tips, tricks, tools, painting, glueing, soldering, design, fabricating, solution, motovlog

Id: kymH647CCNQ

Length: 29min 55sec (1795 seconds)

Published: Wed Oct 07 2020