Velocity stacks tuning

[teazer]

Unfiltered air causes our race bikes to ingest way too much dust etc and on the street it's a recipe for disaster.

Some bikes with CV carbs are very sensitive to set up and removal of the air box is often enough to upset the fueling. It's been a long time since I messed around with a CB350 but I know that CB900's are a complete PIA to jet correctly without the airbox.

On late model GT750's replacement of the airbox with pods has a pretty high correlation to failed pistons at part throttle running when the slide is not where one might expect it to be.

Greg at Ohio cafe racers, what has been your experience with CB350s with stock CV carbs and pods on a stock motor with say gutted exhaust or cheap "mufflers"?

 

[pacreamer]

I am having a similar problem on my CB100. Yes....I know, its in no way, shape or form a high performance race bike: its my fun little cruise around town bike. However, I want to get as fast as possible by tuning it out correctly...


I bought it off someone who put a Chinese, mikuni knockoff. I'm going to put on the stock carb hopefully this week.


For this 100cc beast, would there be a great performance difference between pods vs. stacks?


Obviously the same wear of engine internals would occur with both, correct?

 

[crazypj]

Hi Greg you know I have to disagree with you ;D
As soon as Honda changed to fixed needles, the carbs got much harder to set up with anything other than stock airbox and exhaust pipe at least as long as stock, plus similar volume (internal volume, not loudness)
CB350 was probably the last bike they made where set up was relatively easy no matter what you did to it
Saying that, the later ones (1973) were easier to tune with Mikuni's or AMAL (you could get AMAL's real cheap in 70's)
I've yet to find a mid~late 70's bike that runs OK top end with even a 'stock aftermarket' silencer/pipe fitted.
They run real lean from the factory, any deviation makes them run even leaner.
Fitting bigger main jets opens up a completely new 'can of worms'
As for original post, It's still not easy, even when you know how, just means we spend less time than you would

 

[Rocan]

it will run with stock jetting, sure. But it will run lean. Most people dick around and don't have any idea what there doing, hence why you see so many bikes running like crap even after rejetting.

truth is, the only sure fire way to get a good mixture after making changes to stock is to get it on a dyno with an exhaust gas analyzer. thats not even mentioning that most bikes will have exhaust changes and intake changes before they even run good as a stock bike...

 

[kioskguy]

Well I have my Velocity stacks on my 1971 CL450. I was amazed that they seem pretty good as is, even idled better than before. But then I had rebuilt the carbs with new jets and tuned them with no filter/pod/stack. So the jump to a Velocity stack is not much different.
I also have a strong spark with new coils, plugs, cables, etc. , new petcock, fuel lines, etc. etc. So everything is already running at peak performance.
I think I still need to fine tune the carbs just a bit more to get the mixture juuuusssst right. But at the moment is seems really good. Ask me in a month or so how I feel about them.


So my advice is to jump in and give them a try. The water is warm.


Images of my bike are available here: http://www.flickr.com/photos/kiosks/sets/72157627025891230/detail/

 

[t71ford]

There is so much more to this than whether to jet or not. For instance, here in Bozeman, MT, I can get away with pods and a 4-1 exhaust on stock jetting, and not be lean on the top end. That's what the difference of 5K feet elevation will make for air density. Now, I did have problems with other aspects of the carb mix with that setup, but more due to scavenging effects from the exhaust than from "more air flow". These were resolved by exhaust modifications, and needle shaping. I think to arbitrarily state that jetting will be required is not all that realistic. I think it best to start from the baseline, and making jetting changes as required byt the setup, not by a one-size fits-all demand that jetting would be required. 99% of the time changes will be necessary, but perhaps not as much as one might think. That is why the charts posted on all of the bike specific forums drive me nuts...there is a big difference between the guy in Death Valley to the guy in Denver. And that doesn't even account for temperature!

I say try it first, and then make changes as necessary. If it ain't broke, dont fix it ;D

 

[surffly]

Cheap pods will make almost every bike run like crap
Stacks with screens are better then pods in most applications.
open stacks will actually make a bike perform better then either of the choices

As with everything there are pros and cons.
Any of the SOHC4 hondas that i have been around are jetted lean from the factory and "opening" the intake or exhaust make things even more lean.....Sooooo rejet...

It takes alot of trial and error to get right.
And also "opening" up things will highlight problems in other areas of the bike.
That 45 year old bike that spent the last 40 years in a barn that you put a battery on it and now what pods isnt going to work in most cases as the bike has other issues under the surface
Just running isnt good enough when starting to do performance work

 

[80xs650]

Well I have my Velocity stacks on my 1971 CL450...

Are those the Dime City stacks? My son is getting ready to order a set for his 1973 CL450, and was wondering about the fit, and if they seem secure with the setscrews holding them on. Thanks.

 

[whataulooknat]

Pull your spark plugs and check them after a couple miles make sure the bike isn't way too lean. I've seen a pod filter fall off a bike before and lean that side out so much it burnt a hole in the piston. Can't imagine open Vstacks to be much different than an open carb as far as how much it leans it out.

 

[djelliott]

Don't want to speak for Greg (ohio cafe racer) but for anyone questioning his knowledge of the CB350 I can assure you it IS extensive. He's been building and racing these bikes for many years. A lot of what gets past around here and other places has actually been copied from his website. On that note, I recently went back to the stock CV carbs with stacks and did the obligatory "rejet". Ran pretty damn good then after checking the plugs, way to rich. Going to drop back down tonight and go from there.


I don't see the "pain in the ass" of the stock carbs. I guess if they're malfunctioning (bad diaphragms etc) then yeah, but I just dumped my VM30's for them and they're great. I occasionally have to humble myself with the reminder that very smart people in white lab coats did a lot of math 40 years ago to make this bike perform as best they could. Of course I will continue to do everything I can to eek as much out of this and every other bike I own (hell, that's the fun of it) but in this era of instant, and not always correct, information via the internet we need to remind ourselves that there is NO formula for tuning. There is certainly some direction to go but pipes and stacks/pods don't ALWAYS mean rejet. Take the time and do the research for yourself. That's the only way to make your bike run the best it can. This isn't really solely directed at the OP, more of an all of us thing. Happy tuning and toning mates!

 

[kioskguy]

Well said djelliott, I like your statements, and yes Greg at Ohio Cafe Racer knows his stuff.[/size]
[/size]

Are those the Dime City stacks? My son is getting ready to order a set for his 1973 CL450, and was wondering about the fit, and if they seem secure with the setscrews holding them on. Thanks.

Yes, they are DCC stacks. They fit very tight. You could almost run without using the set screws (but I wouldn't). Yes, very secure. They are sweet.


I also recently saw these on Benjie's site and thought they looked good too:
http://www.benjiescaferacer.com/velocity-stacks.php

 

[donald branscom]

yes. And for the record your bike will run worse with velocity stacks all the time, except for the few occasions when you have the throttle cracked wide open.


but they look cool.

If you tune it and set up the jetting correctly it will run perfect.

 

[donald branscom]

No no no no no no no no no NO!

Stacks will make tuning much easier than pods, not quite as easy as the stock airbox, but close. The stock airbox has velocity stacks built into it. When you remove these (and slap on pods) you remove the devices that smooth the air entering the carbs. That's why you will NEVER get the carbs tuned perfectly with pods. Stacks (be it in the airbox or individually) are essential to a solid tune.

This is simply not true.
You can get it to run correctly no matter if you have pods,velocity stacks ,but you have to be wiling to do the work to get the jets and needle position, and other adjustments dialed in. Most people are too lazy. Proof? Where have you read that someone tried larger idle jets or put shims under the needle of a PD carb? Almost no one has. You would think that after all these years it would certainly have all been sorted out. WRONG.

Try to find an article where it WAS figured out and what the full results were. IE., Mainjets,idle jets ,needle position, pilot air screw or fuel enrichment screw position.
Which intake which exhaust and type of muffler?
Usually most posts on the internet just tell you one fact. But they never give you the whole story.

There is a total lack of information.

But check out all the people racing old cb750's with velocity stacks and headers. They must know ...right? You cannot win races if the motorcycle does not run perfect.

 

[tone74]

I don't know jack about working on bikes so after I put pods and open pipes on my cb175 and reading all the stuff about jetting I took my bike to the mechanic just to play it safe and had it jetted. The guy said he was a mechanic for Honda back in the day and that he would make it run better. Biggest mistake I could of made my bike ran like shit after that the guy tried to tweak it a few times and I just gave up. Without the re jetting it was quick and always idled with no problems. Even the front tire would lift some when taking off. Kicking myself in the ass till this day. I basically paid the guy to fuck up my bike. If I could do it again I would leave it alone and just keep checking the plugs from time to time.

 

[Ringo]

My apologies, I should have said *cheap* pods. I have indeed done the work to get both velocity stacks and pods to run correctly. On my cb550 w/stacks? Runs perfect, and was a breeze ironing out the jetting. On my old xs400 with pods? Never could get it right. Decent, but not perfect. Take it for what it's worth. But I will say something different than the next guy, and you better believe everyone's right, including me. :

It's the internet. Use it to get a baseline, then figure out for yourself what works. Because we're all full of shit, to an extent.

/thread

 

[crazypj]

The main issue is the type of carb.
CB175, mechanical slide carb- very easy fix, adjustable needles
CB550, mechanical slide carb- not quite as easy but has adjustable needles
XS400, weird CV carb, VERY difficult to get working properly (I haven't spent any time with them so can't help)
CB350 early (crude) CV carb- easy jetting/adjusting
CB360 later CV carb, less adjustments available, easy to jet for max rpm, very hard to get running right mid-range
I've spent way too much time messing with 360 carbs and know a lot about them plus how/where to modify

 

[Ringo]

XS400, weird CV carb, VERY difficult to get working properly (I haven't spent any time with them so can't help)

I do not miss these carbs at all. So finicky.

 

[donald branscom]

Many mechanics at motorcycle shops do not know anything about carbs.
Some do.
A lot of 60 year old guys know all about them, But i doubt you would talk to those old fuddy duddy's.

Anyway there is a lot of confusion and misinformation on this forum and others about jetting and carbs.
If you put pods(I don't not care which kind), or velocity stacks on your engine it WILL need to be re jetted. The reason why is that you will have altered how much air can enter the engine.
If you have a stock motorcycle and you put aftermarket exhausts on and do NOT alter the air intake system you will not have to rejet usually and just a mixture adjustment can richen it up enough to keep it from backfiring on deceleration with the increased flow of the performance exhaust.

NEXT scenario, You then decide to alter the air intake. Then you will have to re jet the carbs because now more air can come into the engine and the mixture will be lean.
In all of my years of experience I have noticed that at least one step up on the idle jet will be needed and one or two steps up on the mainjet will be needed.
On a lot of these post 1976 motorcycles they were so muffled down with air filter systems with boxes that have as many as 9 bends in the box slowing the air down and "fuel enrichment" exhaust valves added to keep it from backfiring from extremely lean mixtures that A lot more fuel will be needed to get things evened out.
Almost all of the engines from that era operate on 14 parts air and one part gas.
Modern engines with direct injection into the combustion chamber are using 65 :1 ratios!!!

So if you have a honda CB750 with a 4 into 1 header and of coarse some kind of noise lowering device you will need a bigger mainjet.

I just read that one person at least did things right. They tried a larger idle jet with velocity stacks and went from a stock#35 to a #45 (.013 thousandths to .016 thousandths) and the dyno told them that a #46(.018) size would be better. an increase of almost .006 thousandths from stock.
So all the people that put on velocity stacks on with a 4 into one header and no increase in the idle jet, are kidding themselves that it is running right.
If the snap the throttle open suddenly it will probably stumble or quit running.

 

[crazypj]

Honda didn't use air cut valves on CB750 until 1977, they also fitted an accelerator pump same year.
Airflow on pilot circuit isn't restricted by airbox so stock pilots should be fine in almost all cases, particularly with a stock engine .
Jetting for top end causes problems in mid-range, particularly on Kei-Hin 3 jet carbs.
'Conventional' slide Kei-Hin are real easy to jet as they have adjustable needles and only pilot and main jets

 

[donald branscom]

Honda didn't use air cut valves on CB750 until 1977, they also fitted an accelerator pump same year.
Airflow on pilot circuit isn't restricted by airbox so stock pilots should be fine in almost all cases, particularly with a stock engine .
Jetting for top end causes problems in mid-range, particularly on Kei-Hin 3 jet carbs.
'Conventional' slide Kei-Hin are real easy to jet as they have adjustable needles and only pilot and main jets

I basically agree with you.
Airflow is slowed down when it has to go around bends in its path on the way to the engine. The airbox does help to have a undisturbed volume of air once it is inside the box.
The jetting of any particular part of the throttle range will overlap into another part of the throttle movement.
You say "conventional" slide Kehien carbs have adjustable needles. I like that classification because I feel like these 1977-1978 PD carbs and others that have no needle adjustment really was the end of an era as far as the manufacturers were concerned, of tuners doing any modifications.

If motorcycles have exhausts that look good and sound good there is not much reason to change them
Many good designs go "unmolested" because they were good from the very beginning.
I believe a motorcycle SHOULD make some sound.
As you know many more people were killed in the last couple of years by hybrid and electric cars because they made no noise. It is a fact you can look up.

Matter of fact some laws have now been passed to get manufacturers to have some kind of warning or noise so that children,old people ,disabled can hear these new quiet cars.

I have been at the supermarket and turned around, there was a car right in back of me and I never heard it approach.

Now all the electric vehicle people will attack me.
Don't bother.