Author Topic: pod restrictors? Or re-jetting a 400F? rehash of an old post with test.  (Read 7226 times)

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Offline Don R

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 My name is Don and I have a honda CB400F with pods.

  I  bought it that way and I know all the reasons to not run them and I've been the guy saying not to do it, buuuut,

 Until I get enough stock air box parts to assemble one I'd like to ride this bike. It runs great with about 1/4 choke.
  I remember someone saying they made some flow restrictors that seemed to help, the K&N's appear to have a place where a plastic disc with a hole in it would fit. Has anyone tried it?

 Is there a math wiz that can make an educated guess at a starting point for the hole on the restrictor?  Lets see, 100cc per cylinder, X % efficency x 10,000 rpm = X cfm = what size air hole.
« Last Edit: November 23, 2017, 10:27:31 am by Don R »
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Offline martin99

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Re: pod restrictors?
« Reply #1 on: October 31, 2014, 01:08:03 pm »
Can't help with the math, but wouldn't adding restrictors amount to the same thing as running with choke? It's just decreasing the size of the inlet and limiting air like a choke flap does?
If you don't fancy embarking on the usual larger mains, raising needles, bigger pilots, trial and error tuning for pods and plan to change it asap anyway, why not just run at 1/4 choke and enjoy?

I'm sure someone will tell me if I've just made a stupid comment ;D
« Last Edit: October 31, 2014, 01:21:39 pm by oldskoolnuts »
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Offline Steve_K

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Re: pod restrictors?
« Reply #2 on: October 31, 2014, 01:51:12 pm »
Why not try wrapping paper around the filters and tape in place?  Adjust to fine tune.  Quick and dirty and may work.  I have also heard that some have found that putting their leg near the filters will help going down the highway.
Note:  I just recall hearing about it,  I have not tried any of that myself.

Good luck Steve
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Offline Don R

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Re: pod restrictors?
« Reply #3 on: October 31, 2014, 03:01:31 pm »
 I'm thinking a restriction further ahead of the choke plate would more closely simulate the factory airbox conditions and be more beneficial to proper fuel mixing than the choke but the choke does work. This little bike is a blast to ride.

 I have 2 of these bikes and almost enough parts for 1 air filter assy. The box and rubber funnels are available as reproduction, we're not so lucky with some of the other parts.
« Last Edit: October 31, 2014, 03:35:07 pm by Don R »
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Offline TwoTired

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Re: pod restrictors?
« Reply #4 on: October 31, 2014, 04:50:10 pm »
Be aware it is not air restriction that is the problem.  It is the change in carb throat pressure that has caused the fuel jets to deliver less fuel volume.  Choke forces the engine vacuum to be greater at the jet outlets, therefore you get more from them.

You either need to restore the vacuum level in the carb throats to that which the carb were tuned to expect (longer duct), or change the fuel metering outlets to provide the same volume of fuel with less pressure differential now in the carb throats.
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Offline dave500

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Re: pod restrictors?
« Reply #5 on: October 31, 2014, 04:58:41 pm »
make some velocity stacks out of plastic kitchen funnels?

Offline martin99

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Re: pod restrictors?
« Reply #6 on: October 31, 2014, 05:04:19 pm »
Rather than insert something into the pods, would it help to set them further back using radiator hose or something else to lengthen the tract?
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Offline Don R

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Re: pod restrictors?
« Reply #7 on: October 31, 2014, 06:14:07 pm »
 Thanks for a good discussion. I'm almost shocked at the complexity and mass of the filter system on a 400F. Maybe It just takes jets and a dyno OR maybe repop stock velocity stacks and air box with a K&N on the inlet? In the 70's I modified these things like everyone did, recently I'm unmodifying them but this red 400F is begging to be a custom.

 On my race car's intake the idea is to avoid the velocity stack in favor of a wider rounded air entry to the carb and a filter to smooth out flow. The velocity stacks ram effect causes turbulence at the air bleeds and inconsistent fuel mixing. Of course there is a K&N scoop up in the relatively clean air grabbing 180 mph air.
« Last Edit: October 31, 2014, 06:22:11 pm by Don R »
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Offline dave500

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Re: pod restrictors?
« Reply #8 on: October 31, 2014, 06:17:44 pm »
at least your working on getting the airbox parts.

Offline Don R

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Re: pod restrictors?
« Reply #9 on: October 31, 2014, 11:54:17 pm »
 How would one go about reproducing the original carb throat pressure? Of course, short of reinstalling all of the original parts.
 I see a carb as only reacting to the air flow and relative pressure it sees at the inlet air bleeds and venturi. If the flow is linear and mostly undisturbed the difference would be in the air pressure. If the stock pressure could be duplicated the carb might be tricked into not knowing the difference.
 I realize it's not easy to do that. I read here the factory airbox works under a lower than ambient air pressure. That's what caused me to think I could simulate that lower air pressure with a restrictor.
  Of course I'm leaving out harmonics and reversion caused by runner length and cam overlap.

 And yes I am shopping for stock parts, but sometimes I just need a challenge. There is a theory that most dragsters are just as fast with their scoops turned backwards. You lose the ram effect but gain it back in aerodynamics.  I haven't tested that one yet either. K&N made a dragster scoop with a tiny looking hole for the inlet, they worked great but nobody would buy them. lol.
« Last Edit: November 01, 2014, 12:00:05 am by Don R »
No matter how many times you paint over a shadow, it's still there.
If you love it, set it free, if it stays it's probably one of my 750's.
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Offline dave500

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Re: pod restrictors?
« Reply #10 on: November 01, 2014, 02:34:07 am »
you already know the dual or single plenum intake on v8s works like an extractor exhaust system but in reverse,the Honda four has a single intake on each cylinder that is effectively starting and stopping each cycle independantly,thats part of the problem,the rest is witchcraft.

Offline Mydlyfkryzis

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Re: pod restrictors?
« Reply #11 on: November 01, 2014, 10:25:01 am »
The other issue with pods on a 4 cylinder is the the 2 inner cylinders have warmer air and lower air pressure around them.   The ideal jetting for the inner are going to different then the outers.



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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: pod restrictors?
« Reply #12 on: November 01, 2014, 11:16:40 am »
Hi Don, my name is Jerry.

Are you putting the horse before the cart or the other way around? Have you tried it yet or are you planning on having problems before they occur?

ps while I've haven't been able to put many miles on my restomod my pods seem fine without even touching the new carbs. But here's a new conspiracy theory - my pods are oblong shaped. Perhaps it's just round pods?!
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Offline Don R

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Re: pod restrictors?
« Reply #13 on: November 02, 2014, 08:08:13 pm »
 It runs lean as heck Jerry, a little choke and it goes pretty good but that doesn't seem like the thing to do. These are genuine K&N's. The po thought they look cool and work well. I'm not sure how much he really rode it. My thought was restore the one bike to original and mod the other one. That plan co-incides with the parts I have too.
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Offline Tugboat

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Re: pod restrictors?
« Reply #14 on: November 03, 2014, 10:40:36 am »
Buddy of mine here in Nashville cut up some Yoplait yogurt containers and stuck them inside his pods and that seemed to work okay. They're kinda cone-shaped, like pods.
If it's worth doing, it's worth doing twice.

Offline dave500

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Re: pod restrictors?
« Reply #15 on: November 03, 2014, 11:32:03 am »
Yoplait octane booster?

Offline Retro Rocket

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Re: pod restrictors?
« Reply #16 on: November 03, 2014, 12:49:52 pm »
The best way i've heard to restrict them is to throw them in the ocean.... ;D ;)
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Offline Don R

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Re: pod restrictors?
« Reply #17 on: November 03, 2014, 01:43:58 pm »
Yeah but have you seen the filter setup on a 400F? Looks like a government contractor designed it. There's fewer parts in the engine.
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Offline Retro Rocket

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Re: pod restrictors?
« Reply #18 on: November 03, 2014, 02:23:53 pm »
Yeah but have you seen the filter setup on a 400F? Looks like a government contractor designed it. There's fewer parts in the engine.

You want to check out some of the newer bikes Don, you need a degree in engineering .... :o
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Offline scottly

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Re: pod restrictors?
« Reply #19 on: November 03, 2014, 05:57:46 pm »
It runs lean as heck Jerry, a little choke and it goes pretty good but that doesn't seem like the thing to do.
Applying the choke has the same effect as restricting the flow through the pods. The right thing to do is to re-jet the carbs, so that the motor can benefit from the increased airflow. ;) 
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Offline Don R

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Re: pod restrictors?
« Reply #20 on: November 04, 2014, 08:30:45 am »
 I know, I was secretly hoping someone would say they successfully rejetted one and had a baseline. This bike is going stock (really nice) but then I'm back on Ebay searching for parts for the red one.
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Offline lucky

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Re: pod restrictors? Or re-jetting a 400F? Or who's got stock parts? lol.
« Reply #21 on: November 04, 2014, 01:38:35 pm »
This is my advice.
It can run ok on pods.
First find out what all of the stock settings were.
Idle jet size.
Main jet size.
Slide needle position.
And mixture screw adjustment.


Increase the main jet size. If it was 120 go to a 125 .
Increase the idle jet size. Unfortunately I cannot tell you on this engine the right size, but if it had 35's go to 40's etc.,.
Raise up the slide needles  by dropping the slide needles one notch. If there are no notches then you will have to use small washers. You want to raise the needles about .030 thousands of an inch.

This will get you in the ball park.

When you put on pods it leans the mixture .
The slide needle position is critical because it controls 1/4-3/4 throttle. (Most of your driving)

Offline Retro Rocket

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Re: pod restrictors? Or re-jetting a 400F? Or who's got stock parts? lol.
« Reply #22 on: November 04, 2014, 02:33:46 pm »
Don, you could PM member ttr400 {Kevin} , he owns a business that caters only for the 400's in South Africa, and if he doesn't know about it, it probably doesn't exist... ;)
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Offline Don R

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Re: pod restrictors? Or re-jetting a 400F? Or who's got stock parts? lol.
« Reply #24 on: November 04, 2014, 03:11:21 pm »
 Yeow! I believe I found a used one with good rubbers. Now I need the sliding metal funnel thingy, it's gasket and whatever goes inside the air box.

  I won the E bay auction, may have some K&N's for sale. lol. Now I just need the one more. Or some yogurt.
« Last Edit: November 04, 2014, 09:18:26 pm by Don R »
No matter how many times you paint over a shadow, it's still there.
If you love it, set it free, if it stays it's probably one of my 750's.
 CEO at the no kill motorcycle shop.