Fixing some of the problems for my CB750 SOHC Cafe Racer Project Bike found after first start | 26

After a lengthy first start, mainly because it seemed that the battery wouldn’t get the engine to turn over fast enough, I also had an issue with how it ran. It seemed that cylinders 1 and 2 weren’t firing. I could tell that the engine wasn’t running well, and a temperature gauge aimed at the exhausts showed that 1 and 2 were a lot colder than 3 and 4. And to add to the fun a day or so later I discovered that it was also leaking oil, not a lot but some!
Things that I used.
YB14L-A2 AGM Battery: ebay.us/cKvfzl
Motorcycle Auxiliary Fuel Tank: ebay.us/7aeVud
Battery Jumper Cables: ebay.us/opZgON
We were concerned that it might be the starter but then thought that it was probably an engine earth, but even then the starter still didn’t turn over as well as I expected.
This bike came with a Lithium battery that is at least 6 or 7 years old, and it seemed a miracle that it held any charge at all. I had only charged it by connecting it to another battery, this works but only to a point. As a charger for the old battery would cost more than a good quality AGM battery I decided on the obvious and ordered a gel battery. At least I would know what I am dealing with. I bought the battery from Tayna (I always say it wrong) and their delivery is so good it almost arrives before you have completed the order. So I had the battery the next day.
Once the new battery was fitted the engine span over really well, so it wasn’t the earth or the starter :) I have since found out that, and correct me if I am wrong, that Lithium batteries give a really good burst of power for the initial crank, but if the charge is low it quickly gives up. This is probably what was causing the issues with the starting, and was affecting the linked batteries when we tried them as well. I am also under the impression that not being used for a long time probably hasn’t damaged the battery, so I may end up seeing if I can use it once the bike is up and running fine.
With the battery/starter issue attended to I next needed to look at the oil leak. I don’t want to wreck the engine straight away, so I wanted to make sure that it would have enough oil. On inspection it seems like the oil may be coming from the two pipes that connect the engine to the oil tank. I used the old method of putting paper towels around different parts of the pipes to see if I could track it down. Oil won't run uphill so I was looking for the highest point that oil was coming out of. It definitely seems to be the pipes themselves, possibly where they are crimped at the top, or the actual tube itself inside the braiding. It definitely wasn’t the connection to the oil tank or anywhere else on the engine. At least it doesn’t seem to be anything I have done wrong. I will need to fix this, but it isn’t such an issue at the moment that it would stop me starting the engine.
Finally I could get to looking at why the two cylinders were not running. The 4 carbs have two different fuel feeds one for 1 and 2, and another for 3 and 4. So it did seem obvious that the issue must be something to do with the fuel feeds. I spent a lot of time looking at these and missed the actual problem. I had already ruled out the ignition, The coils also work on two cylinders each one on 1 and 4 and the other on 2 and 3, so it was unlikely that they were causing the problem. I had also ruled out the spark plugs.
It was only on the second time of stripping the carbs, this time, that I spotted that there was an issue with the pilot jets. Overall the carbs were very clean, and I did think that I had blown them through with compressed air. However when I looked more carefully I could see that number 1 pilot jet was fully blocked and that number 2 was partially blocked. I used a high E string from an electric guitar to clean them out. It shows that you should always look at everything from first principles, not taking anything for granted. Although it is possible that the pilot jets had gunged up after the initial first start, especially with the bike being on its side stand with the jets being one of the lowest points in the float bowls.
Now the pilot jets are clear, the bike starts and runs quite well. I am very pleased overall, still plenty to do before I can even think of riding the bike. I think that I might have a go at using a set of gauges to balance the carbs. Just going off the exhaust temperature doesn’t give a full picture. But that will have to wait for another day..
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[@chrisrynston8749]

Thanks for your honest vids. It restores my faith and the carbs. ‘In and out’ happens to the best of us. Calm, methodical and watching your exploits is sooo much better than watching the news 😀

[@SPANNERRASH]

Thanks for your kind words. Never watch the news, it's too depressing, and there's nothing you can do about it anyway! But a bit of carb fettling never hurt anybody :)

[@10summoners]

Another great video. I'm going through the same process on my '75 F0. I sincerely appreciate rewatching your patient, methodical approach. Loved the guitar string tip! Thanks again!

[@SPANNERRASH]

Thanks. If you look through our previous videos, there are a couple of playlists on my other CB750s, they have a few episodes on carbs. :)

[@wmbepeo]

Great bike! I had one in the 80s... I always used a measuring glass to get all 4 at the same level. Quite a lot of trial and error until you were there though! 🙂

[@SPANNERRASH]

Not sure what you mean, with the measuring glass but I agree it takes a lot of trial and error. I'll get there in the end.

[@BasicPoke]

@@SPANNERRASH Google "clear tube method float height"

[@SPANNERRASH]

Yes I have seen that. Might try it.

[@rickmcconnell6520]

I "assume" he's measuring the amount of fuel in the bowl to determine float level.@@SPANNERRASH

[@travishorton-fq3bg]

Boots on the sparkplugs go bad. Sparkplug boots have given me problems on several bikes I've restored. I can't say it enough sparkplug BOOTS

[@SPANNERRASH]

Thanks for the HUD, yes, always worth checking, and I haven't yet. But to be fair these are the ones with the Dyna coils, so pretty new.

[@kenkennedy5516]

Pilot jet cleaner……try interdental brushes……sized to suit……👍🚀

[@SPANNERRASH]

That's a great idea, thanks for sharing!

[@philiponsolent7232]

I had a similar misfire problem this year, it took me 6 months and at least six carb cleans, before it ran properly. I still don't know what the problem was, but it runs well now.

[@SPANNERRASH]

Sometimes that is the way it goes. I always prefer to find an actual problem, but as long is it works :)

[@ethanhawkins849]

Great video! I wonder if the idle jet passage is a little blocked up on no2 making it want more vacuum to draw fuel in

[@SPANNERRASH]

Great point! I'm planning to get some bigger mains, so when the carbs are off again, I will give them all another clean.

[@davidallen3050]

THANKS AGAIN BLOKE ,I'M SURE ONCE YOU GO FOR A BURN IT'LL IRON ITSELF OUT !

[@SPANNERRASH]

I'm with you, once it is on the road I am sure it will run pretty well. I think I iwll have ot up the jetting for the different exhaust and air filters, but that shouldn't be too big a mountain to climb. :)

[@cabovermike]

Senior moment on the lose jet "issue" like ive said before check (brick house builds) kill switch robbing power issues , something to do with ohms,. clean kill switch contacts, but lets be honest cb750's never seem to run sweet at tickover, always sound "lumpy" did you check needle heights ? i think an ultrasonic bath has been done at some stage in this build ? air filter may help to, cheers,.

[@SPANNERRASH]

Still quite surprised about the jet, but I noticed a post on FaceBook where 3 or 4 others have done the same, made me feel a lot better :) It doesn't have a standard set of switches so I am pretty sure it isnt the Kill Switch issue, I have had that with an old Triumph in the past. I am pretty sure the jetting will need looking at, the exhaust will be very free flowing and I am going to fit some cone filters. My main concern was that the carb issue may be hiding some deeper issue in the rebuild. But I'm pretty sure it isn't now.

[@skydemon02]

Vacuum sync the carbs. That solved my exact same situation

[@SPANNERRASH]

Great suggestion. I am going to try that.

[@1234567marks]

If you leave fuel in there it doesn’t take long for the primary jets to block, just a couple of weeks can do it in warm temperatures, the good news is you only have to take the cone filters off and squirt carb cleaner down the balance pipes to clear them, (provided it’s just congealed fuel), you’ll thank me for that tip in the summer 🙂👍

[@SPANNERRASH]

Great tip. I tend to turn the fuel off on my carbed bikes shortly before I stop to park them, then let them run out. It doesn't work perfectly but I am mostly OK.

[@garywhitmire8018]

Possible intake manifold vacuum leak on #2 cylinder

[@SPANNERRASH]

Good suggestion, I'll check that out.

[@CarlWickett-yj1jm]

Great to see someone honest makes us all feel a bit more human, we all make mistakes never to happen again lol, keep up the good work, fellow cafe racer rebuilder, enjoy life

[@SPANNERRASH]

Thanks 👍 Solving the problems keeps our minds agile :)

[@onemanhisdub1969]

I wish i lived in your part of the country. The weather is bloody amazing , as it starts piddling down! Looking forward to hearing her running correctly 👍🏻

[@SPANNERRASH]

You and me both! I do enjoy the Summers, but they never last long enough! :)

[@philw4053]

Good to see the progress. The holes in those jets are so fine it's a miracle they don't block on a regular basis! Could I ask - what float height did you use in the end?

[@SPANNERRASH]

The PO had been through the carbs, and they did look pretty good. Shows you can't go by looks, and need to be more diligent. But it could have been dried up fuel from the original time we started the bike. I set the floats to 26mm, I made that gauge in and earlier video.

[@philw4053]

@@SPANNERRASH Many thanks. I did see the reference to 26mm in your previous video but wanted to make sure as you mentioned you "re set the float heights" in this one. I have a problem with my CB 750 after recently doing a complete overhaul on the carbs and I'm thinking it might be float heights. The bike starts fine and runs well at low revs but when riding if I open the throttle the engine literally dies. Until it's back to low revs again. It has electronic ignition fitted and I believe the ignition advance is working OK. So similarly to you I'm left with carbs.....

[@SPANNERRASH]

I'm looking at increasing the main jets, not really because the capacity is bigger, but because the exhaust is a lot more free flowing than the original pipes and it will have cone filters. If you have made these type of changes to your bike then that might be your issue. The mains don't really come in until 1/2 to 3/4 throttle or so, sounds like it would give the effect you are getting, then the needles also open up further. You could try raising the needles a notch or two first. I have seen a couple of forum posts that give calculations for changing the main jets, so I am thinking of trying one of them, but it isn't an issue for me until I can ride the bike.

[@mulliedog1]

I’ve enjoyed it! 😁 Nice to see your progress!!

[@SPANNERRASH]

Awesome! Thank you! I can't wait to ride it :)

[@mikeford5106]

Your blocked "pilot jets" .... are the MAIN jets!

[@SPANNERRASH]

You think? Not in my paradigm :)

[@philw4053]

I think the main jet is the one that unscrewed itself?

[@BasicPoke]

The large jet is called the main jet. The small one is called the slow jet or pilot jet. Be sure to look thru the pilot jets and at least make sure the hole looks exactly round and they all look the same size.