Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: blackeagles on July 12, 2020, 01:24:10 PM
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following on from my earlier post about drive chain noise and syncing the carb to help....i have been having a little trouble setting them up and i just noticed, looking at an illustration of the pd42 carb, that the 3 pipes that connect the acc. pump to each carb seem to have a restrictor in each (is it part number 16026-393-004 or maybe that is what joins the carbs fuel supply together). no surprise when i took the old pipes off they didn't have such a restrictor so i have not used one....is that likely to cause an issue with setting the sync or the slow running????
bearing in mind they cannot be obtained (as far as i know)
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I think it has the potential to bleed through and affect mixture when not intended, whether it's doing that I can't immediately think of a competent and easy check.
Ordinarily the system is supposed to work as requires pressure to force the fuel out only when pump breaches the restriction, so you could see a pathway if not there as it should not contribute under any non pumped condition. That's true of any accelerator pump design as you'd not want vacuum pulling fuel through that's essentially unplanned.
We'll need to try to come up with a logical method to isolate or prove the affect and affect a cure if that's the case.
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i have had another close look at a number of diagrams online and the 3 acc. pump rubber hoses between the carbs do not seem to show a restrictor to fit into them.
i got the idea they may need such a restrictor(s) as i came across a post on this site that mentioned there were "3 brass restrictors per pipe" and i am guessing that if they actually exist they will be unobtainable or, of course, i may have misread the post (not that i can find it again anyway).
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They were just open brass pipes in mine not restrictors.
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Have you a local mot station that would be willing to give you a set of exhaust emission readings to see where you are?
Thinking along the lines of setup as best you think you can obtain, then get a test with it warmed up to confirm where it's sitting from analysis.
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an interesting idea and i assume my 'local' has a gas tester although i have not seen one when there.
i just read a post you did back in 2017 where you were quoting 'honda manual' about syncing then adjusting slow running no 2 screw to max rev then back 100rpm, readjust and then do the others likewise. i cant say i have tried this, just set them all the same setting so maybe i will try although i have got to get the syncing right/checked first and find an electronic rev counter to do the final adjusting as i doubt the bike one is accurate enough (or maybe it is goo enought as i'm not looking for the last bhp!)
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The car mot equipment usually has to have a decent calibrated meter so maybe a scource if they are friendly :)
Was my previous post (referred to by you above) the one with a photo from the Honda manual in it?
The routine is their official one and it does indicate that the factory carries out this routine during production to give accurate running at low speed.
They also state not to move the screws in normal service, but only re-established them after carb rebuild. What it's doing is normalising each individual cylinder's combustion process until they are equal, irrespective of the final screw position as it's combustion parity that counts in smooth running.
It's a very neat routine that effectively "micrometer" adjusts the low speed running to refine it, but as you point out, main throttle syncing needs to be correct before this stage.
I think its just as accurate if you sync them first with Bryan's bench method, you don't need gauges for it this way.
Rev counter, if you've a DMM ( digital multi meter) that reads frequency, that will give you an accurate rev counter when connected to one points set as one revolution produces one pulse.
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i don't think it had a pic, just a requote from a post you did earlier but it still hold good today and something i will try.
bike is not fully insured so can not run it down the bypass to test out the changes, or go to the local garage, although i don't think i am that far away as things stand. i am going to reset the gauges by using one cylinder so they should all show the same afterwards (i did do the bench set but it ran badly so may have screwed that up) and anyway i guess they only need to be close not necessarily spot on - i may see if i can borrow carbtune pro or such if i feel it will help.
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The exhaust analysis would be to assess if you've a basic mixture problem in regard to uncontrolled fuel faults. By reading O2 C0 and HC, it would be clear if there's an underlying issue.
It won't be necessary if you can work through and get where you need conventionally though.
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There should be a non return valve in the accelerator pump body (the removable cap) that prevents fuel flow until the diaphragm is pumped, could that possibly be stuck slightly open. They are still available from CMS. https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/diaphragm-pump_16021393004/#.Xw6z8puSmUl (https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/diaphragm-pump_16021393004/#.Xw6z8puSmUl)
There are no restrictors as such in mine just small bore brass tubes inside rubber tubes (I assume to prevent the rubber tubes collapsing). There's a small air vent in the side of the carb bodies, if one or more is blocked it's impossible to get even running. The vent is hard to see and cannot be cleaned with carbs assembled. Also, I have never seen that vent mentioned in any manual.
My F2 runs spot on after the second carb rebuild to clean the vents, where previously it would not balance up at all. And don't forget, the air screws work in the opposite direction to earlier 750's.
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There should be a non return valve in the accelerator pump body (the removable cap) that prevents fuel flow until the diaphragm is pumped, could that possibly be stuck slightly open. They are still available from CMS. https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/diaphragm-pump_16021393004/#.Xw6z8puSmUl (https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/diaphragm-pump_16021393004/#.Xw6z8puSmUl)
There are no restrictors as such in mine just small bore brass tubes inside rubber tubes (I assume to prevent the rubber tubes collapsing). There's a small air vent in the side of the carb bodies, if one or more is blocked it's impossible to get even running. The vent is hard to see and cannot be cleaned with carbs assembled. Also, I have never seen that vent mentioned in any manual.
My F2 runs spot on after the second carb rebuild to clean the vents, where previously it would not balance up at all. And don't forget, the air screws work in the opposite direction to earlier 750's.
as far as i can see the acc. pump non return valve is ok.
you say "just small bore brass tubes inside rubber tubes" if you are talking about the 3 rubber tubes connecting the acc. pump supply to each carb then i don't have any of these bass tubes in my rubber tubes and these may be the 'parts' i saw mentioned. they do not show in any diagrams i have seen - are they likely to allow too much fuel into the carb each time the acc.pump works??
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I think they are just there to cut down the amount of fuel held in the pipes and stop the pipe collapsing. The diaphragm will only pump it's volume whatever the setup.
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ah, i should have known that!
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Correction, stops the pipes expanding under diaphragm pressure not contracting. Senior moment.
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it comes to us all!!
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You're right Roy, in 14 pages of official F2 manual they don't even mention breathers as far as I can see.
Pictures/ schematic show nothing more than external drainage from float bowl vents, but no specific data about maintenance of the intricacy of anything else.
I worked much more on them when contemporary, so no real need to strip and replace components just setup.
I can see that opinions of these PD series are based on things like this in justification for an awkward reputation, but they are a very good carb design and certainly address their predecessors (which are just essentially run rich to cover design limitations) shortcomings in accurate fuel metering with increased delivery for acceleration phase. They are in reality how an engine should be run efficiently and with power when needed.
Just need very diligent cleaning and setting to fully optimise their metering accuracy.
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There's a small air vent in the side of the carb bodies, if one or more is blocked it's impossible to get even running. The vent is hard to see and cannot be cleaned with carbs assembled. Also, I have never seen that vent mentioned in any manual.
My F2 runs spot on after the second carb rebuild to clean the vents, where previously it would not balance up at all. And don't forget, the air screws work in the opposite direction to earlier 750's.
ok, something i didn't clean when they are off. can you indicate where the 'vent(s) are ?
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What we found in dealer terms was that from the F2 onwards the carbs were set for california emmissions rules and if stood on showroom floor for more than 3-4 weeks without being run they were a swine to start and pilots blocked for a passtime.
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Depends on your type of PD carbs, there appears to be several different types with no information on them at all. The main bowl vents are between the outside pairs of carbs and are vented through a tee piece and tube to behind the swingarm. Not all 750 PD carbs have these vent tubes. Some carbs also have tiny vents on the sides that need to be clear. Could only find this grainy image as my bike isn't here at the moment for a photo.
[attachimg=1]
This one belongs to someone on the forum but I cant remember who.
[attachimg=2]
It's also absolutely vital that the idle jets are 100% clean, 95% is not good enough as it will prevent correct setup. These carbs are a pain because they are a very accurate metering device and any dirt or blockages of any kind upset the delicate balance. The older K model carbs are much more forgiving on setup.
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ok thanks. mine are different - pd42b - but i have found what believe is them and if i get a pic of the location i will put it up.
this is a pd42a but it shows the bit i'm talking about:
[attach=1]
left side of carb 1 has one but its a blank, the right side of 4 has none, the two facing each other on the right/left of 1/2 and the right/left for 3/4 have what looks like a blank in the hole but a bit of probing with a calor gas pricker found a hole in the centre of which one was blocked and the other 3 clear.