Topic: pod restrictors? Or re-jetting a 400F? rehash of an old post with test.

[Don R]

 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.

[martin99]

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

[Steve_K]

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

[Don R]

 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.

[TwoTired]

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.

[dave500]

make some velocity stacks out of plastic kitchen funnels?

[martin99]

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?

[Don R]

 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.

[dave500]

at least your working on getting the airbox parts.

[Don R]

 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.

[dave500]

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.

[Mydlyfkryzis]

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.



Sent from planet Earth using mysterious electronic devices.

[Jerry Rxman Griffin aka MuthaF'er]

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?!

[Don R]

 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.

[Tugboat]

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.

[dave500]

Yoplait octane booster?

[Retro Rocket]

The best way i've heard to restrict them is to throw them in the ocean....

[Don R]

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.

[Retro Rocket]

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 ....

[scottly]

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.  

[Don R]

 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.

[lucky]

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)

[Retro Rocket]

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...

[martin99]

Here you go Don (hope you're sitting down)

http://www.ebay.com/itm/Air-Box-With-Rubber-Adapters-17300-333-000-Honda-CB350F-CB400F-/171243841805

http://www.ebay.com/itm/1977-Honda-CB400-CB-400-Air-Box-/361100386757

http://www.ebay.com/itm/1975-Honda-CB400F-Air-Box-/161472189996

[Don R]

 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.

[Don R]

 Well here I am years later searching for pod info except now with a KZ1000 with pods. It had unifilters and they were falling apart. My kz guy put on the round pods that he uses on his drag bikes and jetted it with his best guess on a 40 degree day.
   I remembered the yogurt cup baffle post and made a set from a gallon oil jug, I then put my shop vac on one and couldn't suck the baffle out so I'm going to assume it's safe to try.
 I resolved the cb400's pod problem by watching ebay and getting stock parts for them so my own hard learned lesson may be the best route again. In the meantime I'll test these baffles and raise the needles a notch.

[Don R]

 FYI, I made 4 baffles by cutting plastic strips from a 1 gallon oil jug, I made a test pattern from a beer carton to match the cone shape of the filter ID. The curve matched the label on the oil jug so I traced out 4 curved strips that when rolled made a cone. They have a hole punched in each end piece so I can hook them for removal. They roll up and then open after they're in the pod, the strips tuck under the inside edge of the rubber so they're stuck in there. I tried to suck one out with my big shop vac and it stayed in place. They block about an inch of filter area, I assume a little air will flow between the baffle and the pleats of the filter too, then turn back toward the carb.

 Anyway it was a good day for a test, it's about the same temp outside as before. We can argue the reason why, but it ran quite a bit better. It still needs some more midrange fuel but this is an improvement and it pulls like a kz should.
 My thought is it straightens out the air flow near the front of the carbs and allows the air bleeds to see a little more vacuum signal. Everything should apply to a sohc with pods so use your own judgement. It cost nothing to try except a few oz of gas.