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CB750 K1 Valve Timing Question
K2-K6:
Thought it may be worth adding something about carb setup and how they derive the "correct" idle jet size.
The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heared that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.
Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.
After that, if you put vacuum guages on and run at the above setting it should now target 20/22 cmHg as ideal tickover vacuum. If this reading is too high, that indicates that you need the throttle slides too low (causing too much resistance) to control the engine with the jets you've used. It follows that these are too small a jet, giving lean mixture and needing extra air control to stop idle running away.
Equally, if Hg reading is too low it would mean the idle jets are too large and causing rich mixture, then too high a slide position to compensate with additional air to prevent stalling.
It's a circular loop, but should give you the correct fuel metering to run competently.
cbxman:
--- Quote from: K2-K6 on May 08, 2022, 07:38:06 PM ---Thought it may be worth adding something about carb setup and how they derive the "correct" idle jet size.
The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heared that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.
Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.
After that, if you put vacuum guages on and run at the above setting it should now target 20/22 cmHg as ideal tickover vacuum. If this reading is too high, that indicates that you need the throttle slides too low (causing too much resistance) to control the engine with the jets you've used. It follows that these are too small a jet, giving lean mixture and needing extra air control to stop idle running away.
Equally, if Hg reading is too low it would mean the idle jets are too large and causing rich mixture, then too high a slide position to compensate with additional air to prevent stalling.
It's a circular loop, but should give you the correct fuel metering to run competently.
--- End quote ---
Nigel,
Thank you for the info.
I have seen that sequence, but not noticed the 20/22 Hg figure. I have been wondering whether there is a balance between the slide opening height and the airscrew and whether it needs to be raised a little more. Lifting the slide inevitably increases the revs, then having to reduce the revs by slowing it down with the airscrew seems counter intuitive, but maybe that is where I'm going wrong. The fact that the other cylinders are trying their best to spoil your fun does not help. If the plugs were taken out of the other three cylinders so only the one carb is influencing the revs would help, but I expect that would not work due to all the drag from the non-working parts...
I will have another go and try with higher/lower slide settings.
As for the camshaft. The one from twowheelspares was very nice. Having installed it seems that it might be from a later bike, as the valve timing was different. However, these readings were done after I had re-faced the tappets. This must have been influencing the timing readings from the old cam. The tappets from cylinder 4 were quite badly deformed. It is much easier to set the correct lash now. I will probably replace all the tappets if they start going out of spec soon. I'm not sure how much hardening those faces have.
I have also ordered four new brass floats from CMS, so I have all the same floats. I can then start off from the WSM setting of 26mm and work from there. The current floats are not from this bike, I'm sure. Plastic floats came in with the K2...I think. These are from two different bikes as the valve tangs have to set at different heights. I have tried to compensate for these differences by using the "clear tube" method. However there seem to be differing settings depending on who you listen to. Some say 1 to 2mm and some say 3 to 4mm, some say from the gasket face...which one, body or bowl.....some say from the bowl top. I have managed to get them the same. So hence the new floats.
Another point...I believe the 7A carbs were from the early K1 production (this bike November 1970) and it should have the brass floats and the hollow tipped airscrews. Mine had the solid tipped ones. I do have some hollow tipped airscrews from a CB500-4 which I had many years ago, so I will put the carbs back as Mr Honda decided they should be......
Probably another false assumption.
Cheers,
Jerry
cbxman:
Hi,
Just an update on my struggle with this 750.
After fooling around with jets, settings, advancers, camshafts, needle heights etc………
I did install some new brass floats. I set them at 26mm (some say 24mm). The clear tube method indicate about 4-5mm below gasket. I barely had to adjust the float tangs at all. I don’t think this has any bearing on the problem though as the fuel height was no different. It could be an issue with the angle the tang presents to the float needle, just a guess.
I believe that I may have sorted the problem….
Although there were some camshaft issues, I don’t believe that it was bad enough to cause the fuelling problems I was having. I always felt it was a mostly a carb issue.
Along the way, I have found that the condition of the tappet ends were bad enough to give erroneous cam timing results. I refaced the tappet screws and it made the cam timing results much closer to the spec. The main thing is that it was so much easier to adjust the clearances. However all the minor improvements I have done may have influenced the final result a little, but to me, It was something that Nigel (K2-K6) mentioned to me in a PM…. He pointed me to the Honda carburation setting section of the WSM, which loosely described the set up procedure. It was not very specific on the exact process. Nigel suggested in Reply #25
“The manual has routine for starting with airscrew at a baseline then running it and shifting the airscrew further outward to make it leaner, this until it's heard that the cylinder you are working on starts to falter as it goes too lean. Then bringing the idle speed back downward by 100 rpm when turning the airscrew inwards to make it run slightly richer than that peak rpm just as it falters.
Then you reset the main idle speed control (master adjust for carb rack) to bring rpm in reasonable range overall. Repeated for each cylinder, you'll then have set the mixture for those idle jets in use.â€
It made me think that I may have been starting the procedure with too much idle set, rather than starting low and adjusting the airscrews to improve the rpm. I went round the carbs with this in mind and ended up with airscrews out further that I had managed before. I knew that with the airscrews set out would allow the throttle response to good. So with a good idle with a hot engine, I took it out for a ride……really good throttle response on-off the gas and when I stopped it would idle nicely….Result.
So it seems to have the been the mental approach to adjusting the carbs rather assuming only one way to do it.
Could say that the 750 WSM is geared for qualified Honda Service guys, rather than amateurs like me. Maybe if I had the Haynes manual.it might have helped.
I would not say I had any real experience with carbs, in spite of quite happily stripping and refurbishing a six-pack of carbs from the CBX and getting it to run perfectly with just following the set up procedure…
The CBX1000 WSM is very specific regarding tolerances and torque settings, etc making it really useful to the enthusiast….
Big thank you to Nigel for his guidance.
Cheers,
Jerry
Bryanj:
Haynes is worse as its basically the Honda one with bits missing
K2-K6:
Good results from your work Jerry, and always nice to feel progress with the path you take to optimise setup.
Interesting when you can put a couple of minds to work on a problem that it may bring a different facet to the overview, along with generating a potentially different route through to explore and evaluate.
The manuals are originally fairly sparse in content and may be a little influenced by naive translation that may miss some of the original nuances of the factory intention when first written.
Experience, in a technically unrelated field, of working with instruction brought me an approach in which we could question the manufacturer about what the manual was asking us to do (not the accuracy but the intention of what the process was trying to accomplish) and looking at my view of these bike manuals originally this has influenced my reading of them more recently.
Some parts being outright setting in absolute terms for obvious reason, this section being more method to achieve rather than absolute.
I think that section on carburettor setting is a very, very underestimated little gem of a routine that's often not even considered in many discussion of these bikes.
Effectively it's (with no more than a screwdriver as only tool) getting the user to bring each cylinder to a stoichiometric (all fuel burning all oxygen) place to determine the running characteristics of that cylinder, then bringing it back from this point to set a reference mixture for that cylinder. In other words it's calibrating the system you have in place, but without any expensive equipment.
The idle jet size is selected on altitude/barometric conditions first, then the airscrew is used to dial in this choice.
The Hg reading you end up with will actually tell you if the jet size is wrong to start with as too high or low will support a shift in that jet to normalize the range you are working with.
A sort of "squaring the circle" type equality in which you are after a perfect circle of settings, but maybe end up with it egg shaped if you didn't conclude all of the steps accurately. In that scenario, it works reasonably but still with niggling compromise. When its fully resolved, all the characteristics fall into a good place.
It's this setup that ultimately controls slow running refinement and not the "carb balance" routine as often given on internet discussion.
The point of overall Hg figures is to place the slides in the correct position, too low to control runaway idle speed, and they'll give to much vacuum when you close the throttle especially on overrun. The effect of this is;- as you roll it of for example coming into a corner it will start to hold the slides hard against the carb bodies, making it difficult to cleanly and very gently bleed the throttle back in again (those comments about stiff throttle activation have their basis in here) as it's both hard to move and runs slightly lean giving rise to a flat spot hesitancy just as you try to bring the torque back on.
Getting it set as intended brings a smooth linearity to this response in setting it much more like the factory would have done for a new bike.
Each time I read through that section of manual it adds something to my understanding of it, so very simple in appearance but with some very astute logic built into it.
A difference from early to later Honda wsm are little statments that this routine is giving you a accurate setting for your carbs and jets to be recorded as part of bike setup especially after a carburettor rebuild to correct metering at that condition.
The method is a bit like a course of antibiotics in that you needed to complete the entire process to bring correct resolution. As above, I think it's a very neat and forward thing routine contained in there.
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