Topic: Vintage Motorcycle shop says I don't need to rejet

[mgbgt89]

I have pods on my 650. It runs like poop off idle, especially when it is cold out. Runs hot in the summer, Sputters in the rain. Rejetted the mains, but my local shop didn't carry the idle jets for the VB carbs. I really need those if i'm ever going to get it running right.

My 350 has the stock airbox. It runs much better. Probably 8 out of 10 times i ride i decide to take the 350.

[754]

The first time I went to Sturgis was in 76, when I had just got my 73 750.

 Back in those days and the years that followed there were gobs of 750 streetracing and showing off.. sort of the golden days of 750s.
 After a few days the action shifted to the dragstrips nearby, but tp till 81 or 82, there was plenty of street racing to watch or participate in..

 Dont recall any of them running airboxes.. a dew ran 77/78 carbs, which I tried too.

 Never noticed a real problem in the rain, but definitely avoid getting pods wet at a car wash. I did notice a problem when I went thru Colorado, and went thu 5 or 6 passes that were at 11.000 feet..over a 2 day period.

 I ran the frst series K&N, cylindrical, with rubber ends..

 In the early 80s I started running Webers, and never looked back.. til end Aug of 09 when I started riding this 74  750 .. up till now.. first 4 piper and airbox bike I rode in decades..

[greenjeans]

If you can get a new set of airbox rubbers for $80 then by all means do it.
You'll have to pay at least $50 for pods and labor for the shop etc.
You'll end up spending at least $80 total.

Factory airbox = zero headache.   I've tried both many times.  I always seem to end up with the factory airbox.  just my 2c

[sangyo soichiro]

For what it's worth...

Since your box looks pretty bad, I think I'd do something... anything.

Unless you can get another airbox, you'll probably have to at least consider rejetting, adjusting the needle position, and maybe even adjusting the idle, when switching to pods.

One thing you can do to test these things is to play with the choke while riding throughout different throttle ranges.  By playing with the choke, you are in effect changing the amount of air intake.  By seeing if the bike runs better or worse while choked tells you something about your air to fuel ratio.

The down side: While this may sound simple, it's not.  Not only are you dinking with the choke at high speeds while taking your eyes off the road and only using one hand on the handlebars, but trying to judge whether the bike runs better or worse with different degrees of choke while doing this is tough.


Attached is a figure I got from this forum that gives a rough idea of the relative importance of each functioning part of the carburetor in relation to the throttle position. 

[69modelj]

314-385-3334......A.I.M.S. new and used motorcycle parts
6300 Martin Luther King Dr.
St. Louis, Mo. 63133


Might help, dunno. Worth trying. Open on Sat. till about 2 IIRC.
Ask if they have a bike like yours and if you could look at it in order to identify the parts you need. They're usually pretty cool.

[TwoTired]

But your making my point for me... Why is the vacuum decreased in the carb with pods? Its because the air is moving at a greater velocity(V), V x cm = cm³, cm³ = volume, if you increase the velocity of the air, you increase the amount of air you are getting, therefore if you do not re-jet you are running lean. Proper tuning of an engine will allow you to run perfectly well with pods, and I do agree you wont see the added power unless you are racing, great, fine, but you can, if you like pods, tune your bike to run very well with pod filters.

And you are totally missing (and obfuscating) the point.  You are assuming "greater velocity" as fact, and searching for support of that belief, (which is false), without a full understanding of how the system actually works.
An engine requires a certain volume of air to maintain, say, 1000 RPM, (assuming the proper A/F mixture).  Changing the air filter does not change the volume requirement, which is what determines air speed through a fixed sized orifice, (the intake runner).
Therefore, the air flow through all pods or the single stock air filter is exactly the same.  Same is true for all standard power settings.
What IS different, is that the stock filter membrane is far better and demanding of trapping finer particles.  This causes a pressure drop across the filter membrane and that differential is "reflected" back (in graduated fashion) toward the pressure drop source (the falling piston).  In effect, the pressure drop source extreme differential is moved toward the more demanding filter membrane.  In the case of pods, the vacuum extreme moves farther toward the source and away from the jet outlets.  Which is why you have to enrich the the idle and midrange fuel metering to compensate, as the carbs were originally set up assuming the stock filter membrane pressure drop across that media.

I don't believe you can make pods work as well, over all power settings, as the stock induction ducting, with the stock motor, for reasons I have already described.  Unless perhaps, you intend to ignore the stock red line, which is beyond what the stock inlet duct was designed to operate.
There is far more engineering in that device than POD advocates care to admit, or possibly, even understand.

Can a bike with pod filters and retuned carbs be driven on the street?  Of course.
Will the bike "perform" better at some throttle positions?  Possibly.
Are you going to use only those specific throttle positions?  Probably not.
Will there be other environmental/climate associated side effects?  Most likely.
Will the engine actually be worse at some throttle setting?  Probably.  (I actually rather expect certainty)
Will pod advocates admit that?  Undetermined.  But so far, the concensus seems to be ... no.

It doesn't seem like POD advocate can even agree upon which pod, brand, style, or pod performance envelope is better or best.  Even worse, they don't even know what the pod performance envelope is, only that if they change things enough in the carb, the bike won't run too badly.  There are only a handful of forum members that have dyno tests on their engines.  And, I think those using pods also have some pretty serious engine mods done, as well.  So, it is hardly a valid comparison.   One thing for certain.  If you just put pods on your bike, you won't have the same dyno output graph for your engine as they have.

Why is it so hard for people to understand that Honda engineered the bike as a total working package/system.  You can change the looks of the bike in many areas without effect engine operation.  PODs, though an easy "looks" change for an amateur, effects the operating system of the bike.  Don't be surprised of getting a system error during some phase of it's operation, thereafter.
....or not...  Some people are luckier than others, and chance favors them.  You can't rely on chance during an actual engineering exercise. (Can get you fired... or promoted to management).   

[sangyo soichiro]

Will there be other environmental/climate associated side effects?  Most likely.

Dammit!  I'm going back to the stock box!  (No, not the ex wife...)
 



But seriously, TwoTired makes a good argument.  A lot of noobies do three things:
1. "Bob" the front fender,
2. Mess with the exhaust (drill holes, cut, remove the baffles, etc.), and
3. Put pods on and then ask the forum what jet size they need.

Usually they do #2 and #3 naively thinking that somehow they're enhancing their performance.  Racers are different, but for the general commuter, I have little doubt that the stock box gives the best overall performance throughout the most used RPMs.

People have seen my cheapo pods on the street and comment to me that they "got to get me some of those."  I can understand how it becomes annoying to the experienced SOHCers.  After my rebuild, I'm going to try both my pods and box to compare them.  I have no doubt that the box will be more trouble-free.  But those damn pods do look cool however.   

[Cormac]

TT, so if the necessary pressure drop is created by the factory filter media, then wouldn't changing the filter to , say a K&N, reduce the pressure drop resulting in the carb tuning being off? I can buy into the turbulence arguement, and I can acknowledge proven experience; but some things just don't add up.

I will admit, I don't know bikes all that well. But I do know automobiles in their many flavors. And I am reminded of the snorkel air-filter housings so prevelant in the 60's and 70's. These were engineered systems as well, and the cars ran fine with them installed. But dyno tests and practical experience has shown conclusively that the same cars ran better with the same paper element without the little snorkel. The engineers spent days (probably) to create an engineered system that did nothing more than eliminate that lovely sucking noise big engines create.

I suspect that the airbox is probably similar. I know it works better in the rain, I know that everything is tuned to use it, I understand that it isn't affected by crosswinds and the like. But I suspect that ultimately it was engineered to eliminate "that sucking sound".

Now, the real question is, has anyone tried to make a manifold to create a smooth air flow that looks cool? Perhaps using a single cone filter?

[Retro Rocket]

But I suspect that ultimately it was engineered to eliminate "that sucking sound".

I would say it was engineered to supply a stable, more controlled and less turbulent airflow rather than just eliminating induction howl, just as they are with most other bikes.

Mick

[Gordon]

But I suspect that ultimately it was engineered to eliminate "that sucking sound".

I would say it was engineered to supply a stable, more controlled and less turbulent airflow rather than just eliminating induction howl, just as they are with most other bikes.

Mick

+1

Considering the sound that comes out of a set of HM300's I don't think they would have been too worried about the intake noise.

[TwoTired]

If it looks like a fish and swims like a fish, it must be a fish, right?
Except porpoises aren't fish.
Why do you think the SOHC4 induction system works the same as on a 60s/70s car?  Did they have an RPM band that extended from 1000 to 10,000 RPM?  Did those snorkels also have heat controlled air diverter valves in-letting air warmed over the exhaust?  (They weren't all just for noise.)

Anyway, there are far more nuances to the stock system than just the filter media type.  And yes, changing the media type, can effect mixture in the stock system, though it isn't as drastic on most SOHC4s.  The exception may be the lean burn engines of 77-78.

Compare the filter area of 4 pods to the unfolded element of the stock one.  Less area would mean a higher pressure drop, >> IF the media type was the same<<.  Bona fide filter media usually has a rating chart the gives pressure drop vs, CFM traversed.  Found any PODs that give that rating?  Please post.

There is also the action of the inlet runner length.  For simplicity, lets compare two theoretical tubes (because we have no real data for the systems we use now.)

1 - A 12 inch runner, where exists on one end 30 inches of vacuum, and the other zero (as referenced to local atmospheric pressure.)
Lets locate a fuel metering jet exit port 1/3 the way from the vacuum source  (or Four inches).
 The inside of the tube will have a gradient difference of vacuum within it.  For the purpose of illustration, let's say that it is a strictly linear relationship.
One third away from the vacuum source would therefore be  2/3 of the full 30 inch difference, or 20 inches of mercury which the pressure difference pulls the fuel into the intake runner tube.

2. Cut the intake runner tube in half to 6 inches.  It has the same gradient differential end to end as the longer one (30 inches Hg at one end, 0 inches Hg at the air inlet), but over a shorter distance.  The fuel metering exit is not moved, it is still 4 inches from the source.  But, is now 2/3 along the length of the entire runner.  Assuming the same linear gradient relationship along the tube, the fuel metering jet exit port now sees only 10 inches of vacuum pulling on the fuel jet exit, as it is nearer to the pressure equalizing inlet.

The tube size is he same for both examples.  The air volume transferred through both tubes is the same in both examples.
But, I can guarantee that if you reduce the pressure differential across a fuel metering jet by half, less fuel will flow though it.  Given the same volume of air running through the tube in both case, the air/ fuel mixture ratio will be effected.
In these examples, you can compensate by increasing the jet orifice size, which allows more fuel to flow through at a lower pressure differential to get back to the proper A/F ratio for combustion.

For the SOHC4, the inlet duct is unlikely to have a constant linear differential end to end, and variable air speeds (from different RPMs) make this analysis better done with graphs and charts.  The stuff that engineers are more comfortable with using.  These types can look at a chart and graph and say, "cool".  They may, or may not, look a pod filter and say the same thing.  Moreso, after looking at a dyno output chart on an engine, and THEN noticing pods.  

Now, the real question is, has anyone tried to make a manifold to create a smooth air flow that looks cool? Perhaps using a single cone filter?

K&N used to make a filter for the 550 as you describe.  It mounted in place of the air filter box and kept the air plenum and flow straightening couplers ahead of the carbs.  They even had a tube for connection to the engine breather vent.  I have one.  But, the media looks to be way too dirty, and I have yet to find a replacement.  It still has the problem of stray water ingestion (which may be what ruined this one).  But, it appears the media itself has more pleated area, and the original membrane may have a lower pressure drop than stock.  If you had a big bore/camed 550 and intended to wind it well past stock red line, this would be of benefit on the track where weather isn't usually an issue.

Cheers,

[Cormac]

Never claimed they were the same, even stated that they weren't. And the warm air intake was a cheap emissions fix that was detrimental to performance. Performance being maximum volumetric efficiency.

Good example, kinda ignores the venturi principle though. So the question becomes, is the fuel drawn due to the vacuum created by the piston, or is it drawn by the velocity increase (and subsequent pressure drop) created by the venturi of the carb throat?
Porpoise time, every Edelbrock, Holley, Carter, Solex, and Weber I've ever tuned, used a venturi to create fuel draw (accelerator pump notwithstanding) and they all have run just fine with the least restrictive filter I could find, or no filter. Except the solexes, they suffered from a phenomenon known as fuel standoff, very weird...

None the less, I will most assuredly concede that pods leave a lot to be desired, and I am currently entertaining ideas on a better filter set up. Just do not want that black box.

Have any pic's of the K&N set up? And unless the filter is burned, those K&N's are easy to clean and re-oil. Just make sure to get the aerosol oil can, not the spritzer type. The aerosol is much easier to apply evenly.

[Noel]

I am going to try to find a compressed air bottle filled in the early 70s. I'll bet TT would just kill to get a hold of some original air for his tires. 

[JS550]

Will there be other environmental/climate associated side effects?  Most likely.

Dammit!  I'm going back to the stock box!  (No, not the ex wife...)
 



But seriously, TwoTired makes a good argument.  A lot of noobies do three things:
1. "Bob" the front fender,
2. Mess with the exhaust (drill holes, cut, remove the baffles, etc.), and
3. Put pods on and then ask the forum what jet size they need.

Usually they do #2 and #3 naively thinking that somehow they're enhancing their performance.  Racers are different, but for the general commuter, I have little doubt that the stock box gives the best overall performance throughout the most used RPMs.

People have seen my cheapo pods on the street and comment to me that they "got to get me some of those."  I can understand how it becomes annoying to the experienced SOHCers.  After my rebuild, I'm going to try both my pods and box to compare them.  I have no doubt that the box will be more trouble-free.  But those damn pods do look cool however.   

Stock box, thats funny.

[edbikerii]

Don't take your bike to that shop anymore.  They don't know the first thing about engines or carburetors.

Pods are a great improvement in power if you take the time to jet the bike properly.  If you don't, then they suck.  That's the bottom line.

I've had no issues with light rain, but I'll readily admit that if it is raining so hard that my pods would be sucking up a lot of water, then I don't ride.  Sorry, I'm too old for that #$%*.

After several thousand miles, I've had no noticeable problems with side-winds.

If you are interested in reading about my experiences re-jetting with pods and a 4-1 pipe, then follow the link in my sig.  Of course, your mileage may vary.

[jamesb]

i don't beleive my 74 cb550 was rejetted i read where you didn't have to rejet the inline 4s on my sons 350 twin it needed it done so stuff the pods with fleece and wha la it works just fine.

[754]

You cant make me run that damn black box.. nope.. not gonna do it..

 I dont think riding in the rain is a problem.. leaving it in the rain uncovered, or washing it is a problem...

[TwoTired]

Good example, kinda ignores the venturi principle though.

I never said what I have written so far is a complete treatise on all the working interaction, and functional workings of the the stock Honda induction system.  There's more, a lot more, including the venturi principle.  But, that works in conjunction with the fluid dynamics of a tube.  We also haven't talked about, the mass of the air charge and how it's fluidic effects change with velocity.  Simply put, the faster it goes the more it begins to behave like a more viscous or denser material.

Have any pic's of the K&N set up? And unless the filter is burned, those K&N's are easy to clean and re-oil. Just make sure to get the aerosol oil can, not the spritzer type. The aerosol is much easier to apply evenly.

Here's a pic of the K&N below.  I think I saw a picture of it installed on a bike in the forum.  But, I don't know where.
I believe the filter media fibers have actually collapsed in this one, I suspect water damage.

[TwoTired]

I am going to try to find a compressed air bottle filled in the early 70s. I'll bet TT would just kill to get a hold of some original air for his tires. 

Funny.
But, better you be breathing that 70's air yourself.  'Cause whatever you've been breathing recently has you posting things that don't make any sense.
 

This pod thing kinda reminds me of the guys who removed that big ugly scoop on the bottom of the P-51 airplane in hopes of making it go faster.  In every case the plane got slower.  They just didn't understand that the original design actually added thrust to the plane, as that big scoop housed the engine radiator.  Scooping cold air in and ejecting it hot out the back actually made more thrust than the aerodynamic drag that the scoop consumed.
This airplane was designed in 120 days during 1940, by engineers.   They were smarter then, than a lot of people today.

[TwoTired]

You cant make me run that damn black box.. nope.. not gonna do it..

 I dont think riding in the rain is a problem.. leaving it in the rain uncovered, or washing it is a problem...

No way I would ever hope to do that.  Then your bike wouldn't start in freezing cold with a 6 volt battery.  And, Babe the Blue Ox might abandon you.
Plus, I don't like seein' a growed man cry.


If the pods are the paper types, any water will pretty much change their pressure drop characteristics permanently by collapsing the fiber mesh.

[Noel]

Well, TT, next time I'm up Santa Barbara way I'll look you up. We can do some roll-ons and get to the bottom of this little air box tiff. I'll even spot you a few MPH on account of 50 pounds of useless "Honda engineered" trash I took off the bike.

You're not allowed to bend over, though. I expect you to maintain the Honda approved "You meet the nicest people" posture.  

[TwoTired]

Well, TT, next time I'm up Santa Barbara way I'll look you up. We can do some roll-ons and get to the bottom of this little air box tiff. I'll even spot you a few MPH on account of 50 pounds of useless "Honda engineered" trash I took off the bike.

You're not allowed to bend over, though. I expect you to maintain the Honda approved "You meet the nicest people" posture.  

Care to post some track times, 1/4 mile ET slips.  Measured top Speeds?  Dyno charts?  You know, real test data?  Or, is alcohol fueled bravado the best you can do in your postings?

Will you bring ear plugs for me, too?  You mind if we do the roll on tests, past the police station?
I don't need to bend over, btw, I have a windshield.  I can ride cross country for hours that way.

How about this, we both fill up at the same station and then ride till one of us runs out of gas?  I'll be happy to bring you some gas so you won't have to walk so far.

(Jeez, in the last 30 years I've ADDED 50 useless lbs to ME!) 

[dave500]

i like the standard box,im gathering bits for a not so standard but not so hot next bike,,ill be using the factory air box,,to fit pods for looks is one thing,,if thats someones thing,so be it,,,im still meeting the nicest people on hondas!!

[Frankenkit]

just adding my $.019 cents... (not really worth that whole 2nd cent)
Citizen has said he intends to go on a really, very long tour with this bike this summer.

Being trapped at school and dying to go on a ride like that, I've read probably every trip report here, and it almost seems like everyone who's gone on a trip with pods has expressed some regret or disappointment at least in their performance on a long haul like that.

Pods are probably great on 750s for racing applications or 'round town commuting, etc... but I need to agree with TT on his observations on the super-lean PD carbs, and the idea that pods may not be the way Citizen wants to go with this bike, at least, maybe not until after his trip.  

From my understanding, the stock configurations tend to be more reliable in general on long trips like that, due to the richer mixture (cooler running temps) and more leeway in mixture variance due to altitude changes, etc.

I'm no expert, by any means, but I'm trying to condense opinions and input for our OP.

[dave500]

for all intents and purposes pods were/are a performance thing,,ala put a holley on it,,so if joe blow has a holley it has to be good ,,right?yeah and no,,i think pods fall into this hot cam/racers use it,,look what i got thing,,again,,if its for hot looks ,on an otherwise stock engine,and you can live with and know how to tune,,and dont need to go to a shop for no end of tinkering,sure,i feel the stock set up is easier to live with on an ordinary bike though.