CB750 Barn find project - First Timer

[Underdog1]

Aha progress!

had the carbs off again, blew all the airways with a compressor and plenty of carb cleaner, reassembled and i think i had the float valves in upside down! although i did reassemble how it was when i took it apart. Cheers previous owner.

Chucked the carbs back on the bike and used a hairdryer this time to soften up the rubbers which made it 100x easier. Tried to kick her over and didn't get anything, checked the battery and it was like 7v. Got a good battery to jump it off and still couldn't get her going and by this point i imagine i had flooded the plugs so i took the choke off and kicked her with a wide open throttle and she sprang into life!!

http://sendvid.com/mw9r6td9 - Vid of her running

Now she wouldn't idle but thats probably due to the carbs not being set up correctly and not having an airbox on but she ran as long as i kept her at about 2000rpm.

Not all good news as cylinder 2 didn't seem to be firing right as the downpipe was only slightly warm compared to the others being too hot to touch, and after i switched her off i could hear a hissing and drips of oil coming from the left hand engine cover.



What do you think the issue could be? I plan on pulling the engine apart but its good if i can't perhaps pre-empt possibilities by doing some research 

Overall a definite step in the right direction!

[MarkCR750]

Great news, put the air box on and it will tickover , makes a huge difference with it off, No2 is either not getting a spark or fuel, check the HT cap is firmly screwed to the HT lead first, if that doesn't work, swap the cap with one of the other caps, by actually unscrewing the caps and swapping them over, don't swap the leads,and see if the problem follows the cap, if that doesn't work check the plug is getting wet with fuel if it isn't you might have a blockage in the carb, but before anything else try with the airbox fitted, try holding revs at 5k and see if the pipe gets hot, if it does your primary jet is blocked on that carb, holding higher revs means it's running on the main jet, regarding the hissing god knows, is the breather in the top of the cylinder head clear?, pull the pipe off for now.
PS video link doesn't work, YouTube is more reliable.

[Underdog1]

Great news, put the air box on and it will tickover , makes a huge difference with it off, No2 is either not getting a spark or fuel, check the HT cap is firmly screwed to the HT lead first, if that doesn't work, swap the cap with one of the other caps, by actually unscrewing the caps and swapping them over, don't swap the leads,and see if the problem follows the cap, if that doesn't work check the plug is getting wet with fuel if it isn't you might have a blockage in the carb, but before anything else try with the airbox fitted, try holding revs at 5k and see if the pipe gets hot, if it does your primary jet is blocked on that carb, holding higher revs means it's running on the main jet, regarding the hissing god knows, is the breather in the top of the cylinder head clear?, pull the pipe off for now.
PS video link doesn't work, YouTube is more reliable.

Good call on the breather, took a fair amount of compressed air through it until it finally spat a glob of something out. The hissing ceased after this. Couldn't actually get the airbox on! It didn't come fitted to the bike and it's like the plastic boots are too small to fit on the carbs, maybe it's just the ancient plastic. In going to go down the pod route (cue eye rolls) so not too fussed anyway. Checked the HT lead for cyl 2 and got a faint click when I pressed down hard on it and the cyclinder seemed to fire properly after that so I think it's just a dodgy cap.

Going to pull the engine out when I get a chance as I'm getting the frame powder coated, even though I'm inexperienced I'm up for opening the lump up and refreshing whatever needs doing to keep the old girl running for another 40 years. What are the typical things that need addressing?

Finally I did notice that the final drive spindle for the front sprocket was spinning when revving the bike in neutral - is this just due to there being no sprocket or chain attached or is it some sort of fault?

Cheers!

[MarkCR750]

If you are going to use pods you'll need bigger main jets, I've used 135's in mine with a 4 into 1 exhaust and the mixture was bang on when dynoed, also check out my thread "how to make pod filters work", (no roll eyes on this forum , it's a league of gentlemen!), spindle spinning in neutral is normal, regarding what's typical when stripping the engine?, it's totally down to the engines previous life, have a search around the threads on this forum and you'll get an idea of the problems you might face,  but you'll probably enter a world of pain, most do , I've rebuilt various engines over the years but when the time comes to rebuild mine I'm giving it to Trigger and paying for his expertise, the cost will be more than offset by me being able to spend my time out riding my other bikes and knowing the jobs been done perfectly (Trig you can knock a fiver off my bill for that 😄).

[Underdog1]

If you are going to use pods you'll need bigger main jets, I've used 135's in mine with a 4 into 1 exhaust and the mixture was bang on when dynoed, also check out my thread "how to make pod filters work", (no roll eyes on this forum , it's a league of gentlemen!), spindle spinning in neutral is normal, regarding what's typical when stripping the engine?, it's totally down to the engines previous life, have a search around the threads on this forum and you'll get an idea of the problems you might face,  but you'll probably enter a world of pain, most do , I've rebuilt various engines over the years but when the time comes to rebuild mine I'm giving it to Trigger and paying for his expertise, the cost will be more than offset by me being able to spend my time out riding my other bikes and knowing the jobs been done perfectly (Trig you can knock a fiver off my bill for that 😄).

Cool I'll check that thread out, I'm also looking at a 4 into 1 so hopefully 135s will be in the right ballpark for me too.

Im expecting to have to get my hands dirty with the engine and could easily get someone else on the job for me but as I've never done this before I feel like if I don't do it now I'll never be able to. I'm off to the states for 11 weeks on the 25th with work so will have to put things on hold soon, and in not sure if I want the engine in bits for that long as when I come back ill probably never work out how it goes back together again ahah. On the flip side there's lots of cool bike shops where I'm going and my return luggage allowance is 50kg!

[MarkCR750]

Unfortunately the CB750 engines weighs 93kg otherwise you could have bought a replacement , lots of cheap spares out there though so make yourself a shopping list.

[Underdog1]

Unfortunately the CB750 engines weighs 93kg otherwise you could have bought a replacement , lots of cheap spares out there though so make yourself a shopping list.

And not just spares, aftermarket parts are way cheaper too. Got a long list of stuff I'll be picking up from 4into1 as I'll be popping into San Fran for a day or 2. Basically I'm heading all around the south west so Vegas, San Fran, LA, San Diego and wherever else I feel like going to.. I think Carpy is somewhere around those parts but if anyone knows of any other decent supplies let me know and I'll check em out

[Underdog1]

I need some guidance on batteries,

The spare I had knocking around from an old CBR isn't holding a charge so I'll need to get something for this bike, I'm currently trying to quickly spend my wage on bike parts before my other half makes me be sensible.

I want something that's smallish and will have the juice for electric starts. I'm not going to run indicators and my headlight and tail light will probably be LED. Not sure whether it's best to go for AGM or Lithium and what amps/hour I'll need.

If anyone has any brand recommendations then please fire them over would be good to short list some likely contenders and see which gives the best bang for the buck 

[MarkCR750]

I use a motorbatt gel, tipped on its side under the tail fairing, sterling service for the last 18 months, can't remember the ID number but if you EBay "CB750 sohc battery" you'll find em, think it was about £50 from a seller called busters , lithium are ok but don't like shorts so don't fit one until your electrics are sorted, about £130.

[Underdog1]

I use a motorbatt gel, tipped on its side under the tail fairing, sterling service for the last 18 months, can't remember the ID number but if you EBay "CB750 sohc battery" you'll find em, think it was about £50 from a seller called busters , lithium are ok but don't like shorts so don't fit one until your electrics are sorted, about £130.

Found them in eBay, MandP are selling them. Are physically smaller than the original battery or just lighter? Hasn't got any dimensions on there

[MarkCR750]

Not sure, about the same size and weight I think but the advantage is you can lie them on their side so they'll fit under race rep tail fairings, if you want light lithium is the way to go, so light they feel like an empty box.

[Underdog1]

Not sure, about the same size and weight I think but the advantage is you can lie them on their side so they'll fit under race rep tail fairings, if you want light lithium is the way to go, so light they feel like an empty box.

Cool, after doing some more research I think if I went for a lithium is just be doing it for the sake of it and effectively wasting money so will go for an AGM.

In trying to source a 4 into 1 header, MAC performance do some full 4 into 1 exhaust systems but I don't like the silencers. In going to grab a stubby cone engineering reverse cone when I'm out in the states and it seems a waste to buy a full system to not use part of it. Any recommendations on where I can buy just the downpipes?

[yozzer74]

I think I've seen some on cycle x website.

[Underdog1]

Managed to get a bit of time working on the bike today, fully stripped electrics, oil tank, and pretty much everything else bar the front end.



This bad boy was the biggest pain in the arse, fully seized and came pre-rounded by the previous owner



Literally tried everything to get it off, cutting a slot and using an impact hammer, cutting new flats and trying to get a spanner on it, using this plumbers tool thing, heat, and finally going medieval and cutting it to bits and smashing it with a chisel.



unfortunately I've damaged the filter housing, not too much but enough that i know its going to annoy me.

[Underdog1]

Dropped the lump out, was a bit of a wrestle even doing it the recommended lay-on-the-right-side-lift-off-the-frame method. Not looking forward to getting it back in when its all nicely painted..




Stripped the front end down, the fork stanchions were a right bastard to get out the lower triple tree, took a fair bit of brute force but they came out in the end. Currently mega struggling with getting the fork seals out the lowers, according to my haynes they should just lift out at this point but i can't get them to shift. Even tried cutting into them with a dremel but still couldn't get them to shift. I hope i haven't unwittingly destroyed something that isnt actually part of the oil seal