Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Arch stanton on April 14, 2019, 05:30:05 PM
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I realise this must have been asked before. So forgive me but.....
I have a 750k1 fitted with a original Marving 4 into 1pipe.
It also has Bell mouths.
The bike appears to ride quite well really but. Is a bit wooly when at low revs. Then pulls OK, ish but runs out of steam in the higher mid range.
What is the recommended starting point for jetting a bike with these mods?
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Recommended starting point is : Put a STD air box on and not sure what the back pressure is on Marving 4 into 1pipe but, these engines love back pressure ;)
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But I don't want a standard airbox.
I used to run my F1 with this set up. And it ran very well indeed for the 50.000 miles I had it.
But I cannot remember the settings I used.
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Original marving had basicaly a tube up the inside with 3/8 holes all along it so basicaly no back pressure!
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But I don't want a standard airbox.
I used to run my F1 with this set up. And it ran very well indeed for the 50.000 miles I had it.
But I cannot remember the settings I used.
A F1 is not a K1 ;)
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Never mind.
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Subtle changes you could use to edge it towards better response would be, close the idle jet screws down to approx half normal setting, raise float height a couple of mm if possible, and lift the main needle jets a notch (move circlip down) to give more fueling across the whole range.
It should at least let you see if any improvement is made to running.
Try it across its rev range, then look at plug colour to see what you're getting.
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Thank you for your intelligent response to my question.
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Thank you for your intelligent response to my question.
My responce was a intelligent one :o If you think a F1 is the same as a K1 then, think again. Different high compression pistons, different cam and so on.
You will never get it to run perfect across the full range because it was never designed to run on pods and a straight through exhaust. You will have to sacrifice the 2000 rpm range to get good top rpm or vice versa and you will always get a flat spot mid range but, what do i know ;D
I have been asked this question a million times over the years and when i have met people who recon they have it spot on, i ask for the keys to the bike to test it for myself and surprisingly they decline ???
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I think generally over the years people just throw bigger jets at it, which heads towards the results described above.
To expand briefly, when Honda set it up it had a vacuum gradient (if that's the right description) with the airbox etc in place. This they matched with carb components to get fueling in the right range. Specifically the throttle slide cutaway and shape of the needle are designed to meter correctly with the prevailing vacuum.
When airbox is removed, the air flow now has a different shape of vacuum "mapping" for the same engine speed and airflow it had before.
Really what it needs is a throttle slide with a different cutaway, plus a change in form of needle to match it. As that's not going to happen in most situations you're left with trying to compensate with what you can change to try and bring back a parity of mixture over the engine response range.
Overriding is that the engine doesn't need to burn more fuel to work, it just pulls less fuel than it did because of altered vacuum. So it mostly runs leaner.
If you just put in bigger idle jets to correct low speed running it may fix that but shift the blend to main jets too rich, so you get a blubbery flat spot as you come up on the throttle, which you struggle to get rid of. If you up the mains, it may correct the top but not link to the lower, hence the steps in response.
You are in effect running into and out of ideal fuel mix ratio. Less than 10 parts air to 1 fuel will give you too rich, this generally gives you those "dead" flat spots as the plug won't reliably fire the mixture. So you can at least feel which way the error is.
More than 13.5 parts air (on conventional carbs) to 1 fuel will give you those lean "waffety" slow responses that just feel lethargic. This is usually because the combustion mix fires but fails to completely burn in its entirety. Mostly, you should be getting this, so trying to push it back towards correct range without overshooting straight out the other side to rich ratio, is what you're trying to get.
The answers usually don't conform to convention (as compared to book values) but by squeezing it with the adjustments you have, it can be reasonably set up.
Out of interest, what plugs and plug caps do you have in it?
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Another small thing to consider when using pods is the internal step when the pod secures around the carb body which cocks up the airflow - I used foam filters that have no step (rubber boot has taper internally to join filter to carb body) - Mark (cr750) used a modified standard carb to aibox rubbers to sort this I think.
Phil
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I thank you for your interest. But I am using spun aluminium bellmouths.
Regarding K2, s question regarding plugs & caps.
I am using NGK R plug caps. & standard NGK plugs. I cannot remember the grade at the moment. And can't look as I am away from home till Friday.
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I thank you for your interest. But I am using spun aluminium bellmouths.
Regarding K2, s question regarding plugs & caps.
I am using NGK R plug caps. & standard NGK plugs. I cannot remember the grade at the moment. And can't look as I am away from home till Friday.
Sorry - should have read post properly - senior moment
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I know it's all a little experimental to find a decent setup, but if you're interested in addition to jetting you could try it with resistance plugs as well as plug caps.
I know that's counter to generally accepted views, but there's a possible benefit in how it ignites the mixture and could help with what you have here.
NGK 8 would also be a good start point.
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Find somebody with a dyno and an interest in classic bikes and book a couple of hours dyno time.... well worth it for a non std set up, the only way to get it right. You wont find anybody who can tell you what jets to use over the internet, maybe some can give you a rough starting point but thats all. Good luck with it.
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Find somebody with a dyno and an interest in classic bikes and book a couple of hours dyno time.... well worth it for a non std set up, the only way to get it right. You wont find anybody who can tell you what jets to use over the internet, maybe some can give you a rough starting point but thats all. Good luck with it.
If anywhere near Grantham Roy at RP Motorsport is excellent. Dyno, tons of knowledge and old school too.
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I'm not near Lincolnshire. But I am 1 mile away from Frank Wrathall. So maybe I'll see if I can book it in with him.
But ultimately. All I want is the bike to be as pleasant as possible to ride, in its present form.
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I'm not near Lincolnshire. But I am 1 mile away from Frank Wrathall. So maybe I'll see if I can book it in with him.
But ultimately. All I want is the bike to be as pleasant as possible to ride, in its present form.
Frank is excellent and really knows his stuff. Tunes some of the fastest production bikes in the world.
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Indeed he does.
At the moment the bike is on the bench of one of the best Honda mechanics there is. John Carr. Formerly of Bill Head Motorcycles.
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The answer to my original question is..
130 mains & 1&1/2 turns of the air screw.
I don't have a standard K1 to compare it with. But it feels fine to me. Fine enough to go to the TT on anyway.
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So to clarify, you’ve fitted 130 mains, adjusted the idle screws and all is well on your K1 running open bell-mouths? Did you get someone to set the bike up for you on the dyno in the end..?
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No. I just got my Mechanic John Carr to go through everything. In order to give it a final set up after building the bike over winter. I didn't open the engine as it appears to be a very good one.
I was pleasantly surprised how easy to set up it was.
The differences between the F1 & K1 have been pointed out to me.
But from what I remember. The K1 is riding quite similar the the F1 I did 50,000 miles on many years ago. With the very same set up. Though the F had a 3/4 Rickman fairing.
I think I will be quite satisfied with it.
By the way.
John Carr built the Honda that Alan Jackson won the first 3
World formula 2 World Championships on. + was awarded the Shell oils Mechanic of the year award at the TT around 1982. He knows his stuff.
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I had my 750 sohc dynoed a couple of years back, K6 engine we think!, fuelling was good above 3k revs but a bit lean below, I have a 4 into one (piper pipes and generic reverse cone silencer) and pod filters that I grafted onto the original velocity stacks from the Honda air box, I fitted 135 main jets and left the original pilot jets in, the dyno guy (Sam Ermolenko, google him) said at the time that the silencer was stifling it a bit, at the weekend during a bored moment I drilled some bigger holes in the baffle plate, the result?, it’s gone very lean, like white plug tips lean , oops!, so I just ordered some #40 pilots, this is with PD41A carbs by the way, now I know when I fit them that everything is going to be absolutely perfect and anyone saying different is just plain wrong :), one thing I noticed when setting the float heights is that even if you set them carefully you get variance in the fuel levels across the carbs so I will be using the clear pipe on the drains method to make sure I get them all correct and the same, I have never removed pressed in pilot jets before, does anyone have any tips?, bit of heat?, pull with pliers?, new ones in the freezer before fitting?, all advice greatfully received:)
Here’s mine on the dyno
https://youtu.be/dzyp3A1TQBY
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I hope you had a set of ear defenders on as well!
Sent from my iPhone using Tapatalk
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Pardon?
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I had my 750 sohc dynoed a couple of years back, K6 engine we think!, fuelling was good above 3k revs but a bit lean below, I have a 4 into one (piper pipes and generic reverse cone silencer) and pod filters that I grafted onto the original velocity stacks from the Honda air box, I fitted 135 main jets and left the original pilot jets in, the dyno guy (Sam Ermolenko, google him) said at the time that the silencer was stifling it a bit, at the weekend during a bored moment I drilled some bigger holes in the baffle plate, the result?, it’s gone very lean, like white plug tips lean , oops!, so I just ordered some #40 pilots, this is with PD41A carbs by the way, now I know when I fit them that everything is going to be absolutely perfect and anyone saying different is just plain wrong :), one thing I noticed when setting the float heights is that even if you set them carefully you get variance in the fuel levels across the carbs so I will be using the clear pipe on the drains method to make sure I get them all correct and the same, I have never removed pressed in pilot jets before, does anyone have any tips?, bit of heat?, pull with pliers?, new ones in the freezer before fitting?, all advice greatfully received:)
Here’s mine on the dyno
https://youtu.be/dzyp3A1TQBY
Your CR sounds great Mark!
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Cheers Phil, it actually sounds even better when you’re on the bike, a really addictive sound that usually leads to travelling too fast!, there’s something about an 8v 4 that’s hard to beat.
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Good luck with finding your bike's sweet spot. You're always at a compromise between power at full bananas and tractability, even at factory settings.
As a dyno related aside, at Donington once I was asked to take Michael Rutter's RC45, which had just been fitted with a new factory engine, over to a mobile dyno in the paddock. I can't remember the hp, and at the time I wouldn't have been allowed to disclose it, but the sight and sound of an RC45 race bike at full tilt on the rollers certainly made the hairs stand up on the back of the neck. No ear defenders of course, by then it was too late, I'd already seen Quo and Black Sabbath live.
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The old thing is quite nice to ride in most scenarios.
But its an old bike. If I wanted to go quick I'd ride something
More recent. So I'll treat it with a bit of respect.
I have become Too old to die young. So I've decided to stop trying.
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Good luck with finding your bike's sweet spot. You're always at a compromise between power at full bananas and tractability, even at factory settings.
As a dyno related aside, at Donington once I was asked to take Michael Rutter's RC45, which had just been fitted with a new factory engine, over to a mobile dyno in the paddock. I can't remember the hp, and at the time I wouldn't have been allowed to disclose it, but the sight and sound of an RC45 race bike at full tilt on the rollers certainly made the hairs stand up on the back of the neck. No ear defenders of course, by then it was too late, I'd already seen Quo and Black Sabbath live.
I saw him on that at Brands Hatch long circuit, seem to remember thrashing the others when it was wet that race.
They are quite scary when flat out under load like that aren't they, probably a good job you can't hear so much when riding them.
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I saw your bike at Knott End Cafe tonight Arch looks great in the flesh ;), cheers Mick.
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Hello Mick.
Thank you for your kind words.
I saw another two there. Common as muck they are!
Am I to take it. Yours was the very shiny one in Gold?
Regards.
Sean.
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It seems I am the only one that is curious about the result of the Dynorun.... ;)
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Hello Rob.
If you are referring to my bike setup.
I haven't had time to have it ona Dyno.
But after lifting the needles. I feel this may not be necessary.
Sean.
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O sorry, now i read it back it was Mark CR750 who posted that youtube link of the Dyno.
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Rob, I asked Sam to treat the bike gently on the Dyno, I wasn’t interested in how much power it was making just was the fuelling safe, he took it up to 7750 revs, max power was 54bhp if I remember correctly, he said it would have made more with a less restrictive silencer but who knows, I figured that Honda would have been lying about the original claimed 67bhp and it would have been measured at the crank, so in reality CB750’s probably make just over 60bhp at the crank, add in a few bhp to my results to compensate for it being measured as RWHP and it wasn’t far off what it should have been making.
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That is not bad at all, I've seen lots having about 50 hp at the wheel, so taken into consideration that chain and gearbox will consume their fair share of power 54 is a nice number.