Author Topic: Jetting?  (Read 283 times)

Offline Arch stanton

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Jetting?
« on: April 14, 2019, 05:30:05 PM »
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?

Offline Trigger

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Re: Jetting?
« Reply #1 on: April 14, 2019, 06:49:31 PM »
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 ;)

Offline Arch stanton

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Re: Jetting?
« Reply #2 on: April 14, 2019, 07:13:56 PM »
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.

Offline Bryanj

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Re: Jetting?
« Reply #3 on: April 14, 2019, 07:15:15 PM »
Original marving had basicaly a tube up the inside with 3/8 holes all along it so basicaly no back pressure!

Offline Trigger

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Re: Jetting?
« Reply #4 on: April 14, 2019, 08:15:12 PM »
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  ;)

Offline Arch stanton

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Re: Jetting?
« Reply #5 on: April 14, 2019, 08:57:29 PM »
Never mind.

Online K2-K6

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Re: Jetting?
« Reply #6 on: April 14, 2019, 09:16:51 PM »
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.

Offline Arch stanton

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Re: Jetting?
« Reply #7 on: April 14, 2019, 09:21:21 PM »
Thank you for your intelligent response to my question.

Offline Trigger

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Re: Jetting?
« Reply #8 on: April 14, 2019, 10:13:36 PM »
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  ???   

Online K2-K6

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Re: Jetting?
« Reply #9 on: April 15, 2019, 08:58:58 AM »
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?

Offline philward

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Re: Jetting?
« Reply #10 on: April 16, 2019, 08:11:09 PM »
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
Current Bikes:-
Kawasaki ZZR1100D3
Honda CB750K2
Honda CR750 Replica
Honda CB750K6 Cafe Racer Project
Kawasaki ZZR250 (1990)

Offline Arch stanton

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Re: Jetting?
« Reply #11 on: April 16, 2019, 08:18:38 PM »
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.

Offline philward

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Re: Jetting?
« Reply #12 on: April 16, 2019, 08:37:22 PM »
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
Current Bikes:-
Kawasaki ZZR1100D3
Honda CB750K2
Honda CR750 Replica
Honda CB750K6 Cafe Racer Project
Kawasaki ZZR250 (1990)

Online K2-K6

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Re: Jetting?
« Reply #13 on: April 16, 2019, 08:51:59 PM »
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.

Offline Rob62

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Re: Jetting?
« Reply #14 on: April 16, 2019, 09:22:53 PM »
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.