Honda CB750 Cafe Racer Race Shift Conversion + Carburettor Adjustment | 33
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- Опубліковано 28 кві 2024
- Honda CB750 Cafe Racer Race Shift Conversion + Carburettor Adjustment | 33
In the last video when I finally got to ride my project cafe racer CB750 I found a few issues. So in this one I try to address them.
Things that I used:
Carburetor Synchronizer Gauges: ebay.us/g2cm5F
Motorcycle Auxiliary Fuel Tank: ebay.us/7aeVud
The first couple of rides on this project bike didn’t go as smoothly as I would have liked. But with any project you always expect some teething troubles. There seemed to be 3 main issues, but in reality they all came back to the one main issue. And that was that the bike would not idle well. It was inconsistent, it either hung, in that when you closed the throttle the revs took a while to fall back, or cut out altogether. This made the other problems seem worse.
The other two problems were centred around the gear shift. The rear set shifter didn’t work as well as it should. Changing up seemed fine, but when changing down the connecting rod seemed to snag and stop it clicking back positively. I had tried adjusting it a few times, but I still couldn’t get it right. The inconsistent idle made this seem much worse, especially trying to find neutral.
I decided that I would try to cut out the linkage altogether. It is quite common practice for race bikes with rear set footrests to put the gear change lever on facing backwards. This makes the shift pattern reversed, but this is quite easy to deal with. I bought a standard lever and fitted it but it was too long to be able to use with the footrests where they are. Rather than trying to move the footrest I decided to shorten the shift lever. Although it wouldn’t give the same leverage, it should still work quite well.
As this was a trial I decided to bolt the two parts of the lever together when I cut them. Overall I took around 5cm out of the lever's length. Eventually I will probably get the shaft welded, but only when I am really happy about the required length.
As the idle seems to be the biggest problem, I decided to synchronise the carburettors again, having the carbs working together makes the biggest difference to the running of multi cylinder bikes, so it is always worth doing. When I started the carbs were out of sync, so that would have been having some effect on the running.
I also decided to have a go at adjusting the air screws. On the CB750 carburetors the air screws adjust how much air is let through to the pilot jet when the bike is idling. I usually set these at the standard setting from the manual, and to be honest I have never managed to find anything changing when I have fiddled with them on other bikes.
Like my previous experiences moving the air screws didn’t seem to make any difference to the idle. In hindsight it may be that the carbs are not as clean as I thought, however I did run carb cleaner through all of the airways that the air screws use and they did seem OK.
However, the bike now idles quite well, but it doesn’t really want to start well when cold, great when warm though. But with the changed gear shift it all works quite well. Neutral is still a little hard to find, but I think this is more an issue of not having a light to confirm when you have selected it. It is quite a comfortable bike to ride and does seem to be becoming more useable.
Everything in the garden isn’t completely rosy. There is a strange phenomenon when riding along. At first I thought it was the clutch slipping, but it isn’t clear cut. Basically the engine note changes, perhaps sounding like the engine is picking up, even though the throttle hasn’t changed. Strangely it doesn’t really make an impact on how you feel the bike is pulling, and the rev counter doesn’t increase. I think I will have to take a few more trips out on it to try to find out what is causing it. Oh the joy of projects :)
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It sounded to me like there might be a little too much tension on the clutch lever and it looked like you were able to change gear without pulling the clutch lever in as far as you would normally. Difficult to tell at that angle though. Anyway, there's nothing quite like a symphony composed of a big 70's four playing a 4 into 1 pipe - just lovely! Great video as always and thanks for sharing the build - it's coming along just great 👍😊👍
Thanks. I am investigating the clutch next. But the engine does sound good, and is already running quite well.
Well, "they" say to use a digital tach to see the very small changes in RPM the air screw adjustments make.
The revving of the motor sounds a slipping clutch to me as I just replaced mine on my 77K. All parts were within specs but I don't know what oil the previous owner used.
The pod filters are harder to tune than the original airbox or an after market "breadbox".
Still it was an improvement .
Thanks for the feedback. Looking back at my carb clean video I did make quite an effort to clean the airways, so I do think they should be clear. I have never had any satisfaction with air/fuel screws and I was surprised this time as I expected to be able to make some difference using them, but I ended up just putting them back to standard. However once running and warmed up the engine seems to run and start quite well. I am going to look a the clutch next, it was an odd symptom that wasn't clear cut clutch slip, but on reflection I suspect that it must be that.
Couple of notes that came to mind and might help to consider:
If you find that your Pilot / Small Jets and passages are in fact thoroughly clean, then your issue with the mixture screws not having much effect are likely do to the Pilot / Small Jets being too small. The stock jets are often at the very leanest the factory could provide the bike with, so that they met with emission standards or maxim mileage efficiency. Combine that with your modifications to the intake and exhaust, (Now providing more air) also the formulation of today's fuel / petrol. (being leaner).
Going up to the next jet size available will likely do the trick for you. Doing this will also likely improve your cold starting and idle stability.
As for comments on Pod filters:
While it is true that MOST pod filters can be problematic for tuning and not really provide much filtering, a big factor of that is in the quality of the pod filter. The inexpensive brands are the biggest culprit, so the best ones currently on the market for the CB750 are provided by K&N brand. (watch out for the Chinese counterfeits) It's also important to clean pod filters much more frequently then a standard air-box style filter. (These things are just the price we pay for the added air and convenience of pods) ;)
Thanks for your very interesting information.
I do think I gave the passages a good clean with carb cleaner clearly going through. I never thought about going up on the pilot jets. they are currently 42s which I think is one size up from standard.
The Pod filters are K&N, but I didn't source them they came with the bike, but I doubt they are conunterfeits
@@SPANNERRASH
Okay, well not sure how the UK fuel differs from here in the US, but for bikes set up like yours, I would typically run 46s for the pilot jets.
As for the pod filters; If they are legitimate K&N's, and you would like additional airflow contort as well as water & UV protection, then you could also add Outerwears Pre-filters.
BTW; This is the first of your Vid's I've seen, but the bike looks to be coming along nicely!
I'm not sure about the difference in fuel, but I am using the E5 99 octane fuel in it. I might see if I can get a set of 46 pilots.
I have never come across the Pre-filter idea before, interesting idea.
Glad you enjoyed the latest vid, why not take a look through our back catalogue. This is the third CB I have done videos on.
Try the gear lever so it's above your foot when riding normally. Set that way the angle is better and you're ready for a quick down change. A quick up change is rarely required.
Good call! I do intend to try the lever in a few positions, having it above the foot should also work a lot better to find neutral.
Great stuff, really interesting.
Thanks for sharing.
Glad you enjoyed it.
Don't Nortons shift this way?
It's so good to see it on the road. My own CB750K was making that very same sound last summer during shifting, and it turned out to be the cable was too tight. I get overzealous about "slop" and over did it.
This has been a really great series to watch. Looking forward to what you do with the tank.
Thanks. I suspect that race Norton's probably did, but I wouldn't really know. I am glad you enjoying the videos. I suspect the clutch issue may just be and adjustment problem. I will be taking a look and showing what I find shortly. Take care.
Getting an OE air box Ida good idea Les, you’ll find it has rubber velocity stacks in it, at present the airflow into your carbs will be tumbling, that not good when your relying on the Venturi effect to draw in fuel, Honda didn’t go to the trouble of having those rubber velocity stacks in the air box for fun, they smooth out the airflow into the carb, as I’ve said before I grafted those rubber stacks onto my pod filters, it ran so much better! 👍
I have got some of the standard rubber velocity stacks but I couldn't really get them to work with the K&N pod filters. I would really like to make a custom filter like the old Triumph Trident filters, with the velocity stacks going into them, but don't think I have the skill to do it, yet!
@@SPANNERRASH the way I did mine was simple, I pulled the rubber bungs out of the front of the pod filters, then measured the diameter of the rubber velocity stacks half way along their length and opened up the holes in the front of the pods to that diameter, then warmed up the stacks and bent and pushed the wide end into the holes in the front of the pods, then ran a fillet of Sikaflex around the joints to seal and glue them in position, it was supposed to be temporary, that was years ago and they are still fine, Sikaflex is amazing stuff! 👍🏻👍🏻
I have some ideas about making a shared airbox for the carbs, using the velocity stacks. I will keep your idea in mind though, but I don't really want to start cutting up my K&Ns. :)
Like yourself I've always found adjusting the idle screw to make very little or no difference. Watched a fair few UA-cam vids with professional bike mechanics setting them and can't hear any difference when they set them either.
Having said that, I have seen the adjustment of the idle screw cure the "hanging idle" problem.
I was becoming convinced that I hadn't cleared the airways in the carb. But looking back at my video of cleaning the carbs the second time, I can see that carb cleaner was coming through. I have never had any satisfaction twiddling the air screws, maybe next time.
Try fabricating an airbox to use your std velocity stacks, a shared air source should improve things, also check that the jets are not toooo big. looking and sounding good Les. a while since been to flambrough.
I am thinking of doing that, but once it is warmed up it doesn't run as bad as a feared, so it isn't at the top of my list yet. I suspect that jets will need further looking at, the mains are 125 and pilots are 42s at the moment, but the other issues are in the way of worrying about them. I am going to investigate the clutch next, as that seems to be more likely to be the issue. I'm hoping that I find a simple mistake, but if not a new set of clutch plates won't hurt. It is a nice flat run to Flamborough from my house, just in case I have to push it back :)
Its coming a long nicely! Thinking you will get tired of that exhaust noise! A bit of proper backpressure will likely help the low end too.
I think you are right. But as the bike is more rideable the exhaust become more tollerable :)
Brilliant! Love the progress. Its a pity about the clutch slip or at least what sounded to me like clutch slip.. I would suggest an adjustment or trying another oil before opening up again... keep it up!!
I am going down the same track as you. I haven't had the opportunity to have a look yet, but hopefully this week I will findi out.
@@SPANNERRASH Well, good luck. I'm sure you know a lot about motorcycles so I am sure you'll fix it... I 'm pretty sure its down to adjustment or oil...
Forshadowing, it doesn't seem to be just adjustment! ;)
brilliant ty
Thanks.
I cant remember, still using the points? Or did you go with electronic? That swap cleans up the CB750 firing quite a bit. I bet a nice set of FCR carbs on that bad boy would look nice!
It has a full Dynatech electronic system. On reflection I am not sure about it being some sort of misfire, more likely to be the clutch. Changing the carbs would be some time down the road, I really want to get the standard carbs working if I can.
The HT leads get cooked and crappy on these - has it got new coils and leads?
Hi, Thanks for the infor, this has a full Dyna set up with new coils as well.
Sounds like the clutch is slipping a bit
I am starting to look at that being the most likely.
Your clutch is slipping a little.
And getting worse! :)
Sounded like classic clutch slip in the higher gears to me, have you got ordinary oil in it or synthetic ? or maybe clutch springs,. Best get that chain tightened up to, it resembles the humber bridge,. Some proper mirrors will stop you being racist every time you need to look in them, you reminded me of basil fawlty and his nazi salute, lol, a neutral light isn't fitted to your fancy clock, maybe rig one up, sling some paint at and call it done, try and seal tank from outside, cheers,.
Comprehensive, thanks. I am going to investigate the clutch next. I gave it no attention whatsover once I knew it wasn't sticking, which in hindsight was a mistake. I might get used to the mirrors, but I am thinking of switching them to the top to see if that makes them more comfortable. I can't wait to paint it, but my paint studio is outside, and without some settled weather that isn't going to happen too soon.