Author Topic: Bogging down.  (Read 381 times)

Offline philward

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Re: Bogging down.
« Reply #15 on: May 12, 2020, 09:17:12 PM »
I have messed with jetting etc for some time on the CR750 Rep (road use) and now got it pretty much spot on across the range. I can quote what my settings are but there are so many variables on each build. I had no choice when building a CR Rep as you can't have a CR Rep with std airbox and exhausts but on the K6 Cafe racer project I'm building I've re-fitted the airbox and std exhausts (which was a pain as the previous owner cut the airbox and exhaust brackets off and I had to weld them back on!) to make the carb set up easy - ie, standard..
One of the main issues MarkCR750 pointed out was the step from the pod to the carb body that the pods create - ruining the smooth airflow. Mark adopted the standard airbox rubbers to the pods to maintain the airflow I think (there's a post on here somewhere). Marks build uses PD carbs though I think.
My CR runs standard carb with foam pods (no step to carb bodies) and ended up with 135 mains, 45 primary jets (I had to drill 40's out to 45 as no longer available), air screws 1 3/8's out and needle on middle groove. I'm running 4 into 4 race exhausts with K0 baffles in and standard compression 836 kit. This may not help as you will be running a diiferent combination but will give you an idea - took about 10 changes of combination to arrive at the right setting that gives a reasonable tickover, smooth step free mid range and good top end.
Hope that helps
Current Bikes:-
Kawasaki ZZR1100D3
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Offline brodsord

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Re: Bogging down.
« Reply #16 on: May 12, 2020, 09:30:22 PM »
As the others have already posted,  you can really chase your tail on this sort of stuff.

Basic problem is not that the engine will burn more fuel,  but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls.  Going to pods gives you less vacuum in the carb venturi for the same engine speed,  and consequently pulls less fuel up from the float bowls,  hence the weak mixture. 

As a test / see,  you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher,  turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.

It would push it toward where you need to be and at least see if it improves without significant cost.

Edit;- also make sure the plugs are at their minimum specified gap too.
Thanks so much for this info K2.
I’m running just the standard plugs for a k5 engine from DSS


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Offline brodsord

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Re: Bogging down.
« Reply #17 on: May 12, 2020, 09:32:40 PM »
I have messed with jetting etc for some time on the CR750 Rep (road use) and now got it pretty much spot on across the range. I can quote what my settings are but there are so many variables on each build. I had no choice when building a CR Rep as you can't have a CR Rep with std airbox and exhausts but on the K6 Cafe racer project I'm building I've re-fitted the airbox and std exhausts (which was a pain as the previous owner cut the airbox and exhaust brackets off and I had to weld them back on!) to make the carb set up easy - ie, standard..
One of the main issues MarkCR750 pointed out was the step from the pod to the carb body that the pods create - ruining the smooth airflow. Mark adopted the standard airbox rubbers to the pods to maintain the airflow I think (there's a post on here somewhere). Marks build uses PD carbs though I think.
My CR runs standard carb with foam pods (no step to carb bodies) and ended up with 135 mains, 45 primary jets (I had to drill 40's out to 45 as no longer available), air screws 1 3/8's out and needle on middle groove. I'm running 4 into 4 race exhausts with K0 baffles in and standard compression 836 kit. This may not help as you will be running a diiferent combination but will give you an idea - took about 10 changes of combination to arrive at the right setting that gives a reasonable tickover, smooth step free mid range and good top end.
Hope that helps
Thanks so much Phil, very helpful


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Offline brodsord

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Re: Bogging down.
« Reply #18 on: May 13, 2020, 11:06:22 AM »
As the others have already posted,  you can really chase your tail on this sort of stuff.

Basic problem is not that the engine will burn more fuel,  but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls.  Going to pods gives you less vacuum in the carb venturi for the same engine speed,  and consequently pulls less fuel up from the float bowls,  hence the weak mixture. 

As a test / see,  you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher,  turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.

It would push it toward where you need to be and at least see if it improves without significant cost.

Edit;- also make sure the plugs are at their minimum specified gap too.

If you use resisted plugs and resister caps that with give a weak spark  ;)
Thanks for this Phil, yes I think I’m going to do exactly what you have said here to see if it improves any, and move on from here if needed.


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Offline brodsord

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Re: Bogging down.
« Reply #19 on: May 13, 2020, 11:07:48 AM »
Choke lever*
F@cking iPhone


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Hate predictive text Julie


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Offline K2-K6

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Re: Bogging down.
« Reply #20 on: May 13, 2020, 02:09:05 PM »
DS have DR8ES listed as NGK plug spec for the K5.

It's worth fitting the equivalent in higher heat range while you work on the jetting.  So DR9ES as they are more resistant to the plug tip going over temperature during your mixture exploration phase.  It reduces the risk of pre-ignition that could cause damage to the engine.

Offline brodsord

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Re: Bogging down.
« Reply #21 on: May 13, 2020, 02:25:05 PM »
DS have DR8ES listed as NGK plug spec for the K5.

It's worth fitting the equivalent in higher heat range while you work on the jetting.  So DR9ES as they are more resistant to the plug tip going over temperature during your mixture exploration phase.  It reduces the risk of pre-ignition that could cause damage to the engine.
Thanks K2 that’s a very good point.
I’m just taking the carbs of now to see what jets are in there.


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Offline Nurse Julie

  • 1973 CB750 UK K2.1977 CB550/4 Mongrel Brat. 1978 CB400/4.
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Re: Bogging down.
« Reply #22 on: May 13, 2020, 03:14:19 PM »
DS have DR8ES listed as NGK plug spec for the K5.

It's worth fitting the equivalent in higher heat range while you work on the jetting.  So DR9ES as they are more resistant to the plug tip going over temperature during your mixture exploration phase.  It reduces the risk of pre-ignition that could cause damage to the engine.
David Silver has 'R' plugs suggested for many of our bikes but thats totally the opposite to what the shop manuals / Haynes says. Its a known fact that the original metal shroud HT leads needed resisted plugs but as many of those caps were binned on PDI and rubber (or maybe they are plastic 🤔) resisted caps fitted, non resisted plugs have been fitted as standard since.
« Last Edit: May 13, 2020, 03:18:29 PM by Nurse Julie »
Im a Nurse, trust me, this won't hurt.....much !!!

LINK TO MY CB400/4 ENGINE STRIP / ASSESSMENT AND REBUILD...NOW COMPLETE
http://www.sohc.co.uk/index.php/topic,14049.msg112691/topicseen.html#new

Offline brodsord

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Re: Bogging down.
« Reply #23 on: May 14, 2020, 09:56:47 PM »
As the others have already posted,  you can really chase your tail on this sort of stuff.

Basic problem is not that the engine will burn more fuel,  but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls.  Going to pods gives you less vacuum in the carb venturi for the same engine speed,  and consequently pulls less fuel up from the float bowls,  hence the weak mixture. 

As a test / see,  you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher,  turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.

It would push it toward where you need to be and at least see if it improves without significant cost.

Edit;- also make sure the plugs are at their minimum specified gap too.
Hi K2,

So I took my carbs off and stripped them down and gave them an overhaul, they were in a shitty condition.
Carb one had a blocked fuel bowl vent so it was probably struggling filling up with fuel, carb four had a broken float pillar.
Sorted both issues and renewed the various seals orings etc as I had some carb seal packs a bought a while back from DSS
Polished up jets and needles  with some 600 wet paper.
Fitted needles with clip retainers on the bottom setting as you suggested.
Just did basic eye up setting with slides as I’ll sync them tomorrow.
Pilot jets are 1.5 turns out and pods are on.
So,
I took her for a test ride and she goes like a rocket, no spluttering or bogging down, just goes like the clappers!!
The pilot jet is 40 and main is 105.
I’ve set the spark plugs at .024 and I’ve raised the floats by a couple of millimetres.

I did about 25 miles earlier and I’ve taken the plugs out to see how they look, seem ok
Thanks for your advice


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Offline Nurse Julie

  • 1973 CB750 UK K2.1977 CB550/4 Mongrel Brat. 1978 CB400/4.
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Re: Bogging down.
« Reply #24 on: May 14, 2020, 10:00:16 PM »
Blimey, that's running lean. You will burn a hole in your pistons and burn the face of the valves if left like that.
Im a Nurse, trust me, this won't hurt.....much !!!

LINK TO MY CB400/4 ENGINE STRIP / ASSESSMENT AND REBUILD...NOW COMPLETE
http://www.sohc.co.uk/index.php/topic,14049.msg112691/topicseen.html#new

Offline K2-K6

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Re: Bogging down.
« Reply #25 on: May 14, 2020, 10:59:30 PM »
Good news that you've got it running more as you want it.

I'd agree that it's a little lean,  but not drastically as the earth bar is "milky coffee" tone.

Definitely fit ngk 9 grade plugs to bring the tip temperature down. They have wider heat transmission path to more easily get the heat out of and away from the electrode.  It's this that ultimately causes a holed piston as if it gets too hot then it ignites the mixture prior to timing point. Classic "pre-ignition" that you can't feel until it's too late.

With the needles up at maximum you've no more to add on mains so it would look wise to go up to 110 jet size that would give you more safety with the needles set as they are.


That should give you a decent baseline for longer assessments to be made along with reducing risks.

Offline Oddjob

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Re: Bogging down.
« Reply #26 on: May 14, 2020, 11:02:56 PM »
Does the bike have red or black NGK plug caps fitted?

If it does those are the wrong type plugs.
Don't play stupid with me, I'm better at it

Offline K2-K6

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Re: Bogging down.
« Reply #27 on: May 14, 2020, 11:18:41 PM »
Does the bike have red or black NGK plug caps fitted?

If it does those are the wrong type plugs.

I feel it's beneficial to run both cap and plug as resisted to help burn a leaner mixture and gives more margin against the effects of detonation. 

As it's non standard intake tract then setup is not necessarily going to follow std manual specification.

Offline brodsord

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Re: Bogging down.
« Reply #28 on: May 14, 2020, 11:35:10 PM »
Good news that you've got it running more as you want it.

I'd agree that it's a little lean,  but not drastically as the earth bar is "milky coffee" tone.

Definitely fit ngk 9 grade plugs to bring the tip temperature down. They have wider heat transmission path to more easily get the heat out of and away from the electrode.  It's this that ultimately causes a holed piston as if it gets too hot then it ignites the mixture prior to timing point. Classic "pre-ignition" that you can't feel until it's too late.

With the needles up at maximum you've no more to add on mains so it would look wise to go up to 110 jet size that would give you more safety with the needles set as they are.


That should give you a decent baseline for longer assessments to be made along with reducing risks.
Thanks K2,

Yes I’m going to order the jets tomorrow and the 9 grade plugs you’ve suggested.
I’m happy that it’s on the right track anyway


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Offline brodsord

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Re: Bogging down.
« Reply #29 on: May 14, 2020, 11:36:44 PM »
Does the bike have red or black NGK plug caps fitted?

If it does those are the wrong type plugs.
Yes it has the black plastic ngk caps Oddjob.


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