Topic: Jetting for pods VB750 1977-78

[lucky]

Sorry.... CB not VB

After much experimentation this is where I am now.

First I just richened up the idle jets.
Went from stock #35 to #38 then to #45's then back to #42's.
I thought part of the problem was the midrange controlled by the slide needle like some of the other Hondas I have worked on.
So I added shims to raise the needle. .058 thousandths shims with a #45mm idle jet worked really well but it was very rich- soot on the plugs.
Then I moved down on the shims on the needle to .037 thousandths and #42mm idle jet. Not as rich but ran horrible.
The mistake I made on that move was making two changes at one time.
So I was going BACK to putting the .058 shims under the needles again and then decided to STOP and remove all the shims under the needle and go back to the #45mm idle jet only. No shims.
I was reading an old article on CB750 Choppers and a guy suggested drilling out the #35mm idle jets to #45mm because ,in his words, "cure the annoying problem of the lean idle." So to me it seemed like all of this had been worked on before and now with the "cafe boom" happening again some of this information was forgotten. Many of the people that worked out all the problems are old now and may not even use the internet.

So I thought he does not mention anything about the mid range but I will just go back to the #45mm idle jets and just see what performance the bike has with no shims on the slide needle. But at least I know now the idle jet is a SOLID choice.(#45)
So  I started the bike with this set up:

velocity stacks with screens (Steel Dragon).OR Pods.
4 into 2 into 1 exhaust with baffle.
(4 into 1 with muffler would be about the same).
#120mm main jets.
Stock needles -no shims.
#45mm idle jets.
1 full turn open on the mixture screws.

It started and ran like a champ.
Good throttle response and no back firing.
So I am satisfied that the #45mm idle jet IS BETTER than the #42mm idle jet.

Now the last part. Will the midrange respond and not bog when on the road?

As soon as the rain stops I will report back to you.

I know it has been a long road and a long story, but we will all benefit.

I have put the carbs on and off at least 10 times or more.
I just will not accept the carbs being finished until it runs good.
I found out if you just put a pan under the bike and open all of the float bowl drain screws it is much faster than removing the carbs and then draining each float bowl.
I also installed all new mixture screws and O rings.
The new mixture screws have serrated thumb screws to make adjustments easier.

I hope you are enjoying the story.

[I Zombie]

Looking forward to a report on your progess Lucky

[lucky]

Results on the road test are coming very soon. As soon as it stops raining.
Thanks for your continued interest I Zombie.

[I Zombie]

You're doing what i would be doin, except I'm waitin on parts. So if you nail it before I get em, I'll have a better base line to start my fine tuning. So I thank you.

[lucky]

You're doing what i would be doin, except I'm waitin on parts. So if you nail it before I get em, I'll have a better base line to start my fine tuning. So I thank you.

Wait'n for parts is hard.

[lucky]

Good weather this coming Saturday and Sunday and all next week!!
I will get the riding needed.

[lucky]

I took the bike on the road today for a little ride. The idle was not as good as yesterday.
I did accelerate smoothly in the gears. The idle kept being rough and lopey and I now suspect that the spark plugs may be bad from all of the previous running with experimental overly rich sooty combinations .

I am starting to get the feeling that to really fix the problems, that a new needle with a thinner taper could be needed and do not know if they would even be available.

Honda has done a lot of work to make this carb NON ADJUSTABLE. THe main problem is that the stock air box is so UGLY!!! A BIG BLACK CHEAP LOOKING PLASTIC BOX. The Ko models had a smaller box that was painted to match the bike.
I may go with some chrome vinyl stick on graphic material LOL...lol..lol
If it just looked better I would not be wanting to change it.

Anyway I think I am going to bail on this project. Now one else has solved it all these years since 1978 and I do not want to pay for all of the tapered needles to experiment with.  I am throwing in the towel people.

Sorry.

NON adjustable needle.
NON adjustable push in style idle jets.
Slides controlled by ridiculous mechanical linkages.
All of the machines of the future will be worse.
All electronic NON adjustable and probably turn you in(to the authorities) electronically if certain parts are moved. You just have to drive the ugly crap those rich kids design and manufacture.

Sure it may be possible to overcome, but cheeesssshhhh!!!!!

I'm outta here.

Back to building motorcycles.

[lucky]

One more idea.

I thought back last night about all of my experiences with HONDA carbs all the way back since the first Honda 50 motorcycle up to the present.

When I first started working on Honda carbs Myself and others found out that we could make adjustments to carbs by making changes to the fuel system. The jets and the needles on the slides.
We were controlling the fuel.
The one thing was constant was the air coming into the carb.
Even with the filter box.

Now with this 1977-78 set of carbs adjustments can no longer be made by altering the idle jets ,main jets or the slide needle.
The carb function IS being controlled by the AIR not the FUEL.
If I put the choke on 3/4 the engine will respond.
If I raise the needle on the slide the engine will respond but only in a narrow range and then the plugs are covered with carbon.

A tapered needle might solve the problem but the air jets may have to be altered as well. The main jet emulsion tube may need to be altered  etc.,.

I see that the only solution now is that the opening in the stock Honda air box must be compared to the total area of the opening of the four carbs total intake area and add in the filter restriction and that whole system is like putting one big choke on the air coming into the engine.

Altering ANY of the jets is not going to get you where you want to go which is to get rid of the ugly black large air box.

So I have decided I may have to chrome the air box or something. Thats all thats left.

If you had a carb airflow bench and many selections of tapered needles with different straight sections and adjustable air jets and carb bodies you could do it.

There is the alternative to just get early model CB750 carbs and put them on the 77-78 engine. That also has its problems. It may be a solution. I just do not know.

[adrianm1972]

Let's take a look at the real root cause here. You are on the right track with the air being the issue and that the air drives how the fuel flows. However, it's not that the carbs are not adjustable. Idle and main jets can be replaced on the 77-78 carbs, needles can be shimmed.

The problem is air FLOW.  There are 2 flow related issues.

1. Consistent vacuum. The air box works well because it creates a consistent vacuum on the intake side of all the carbs. It's this consistent vacuum that leads to consistent fuel draw. This consistent fuel draw leads to even mixtures in all cylinders and smooth running.  This is why air boxes have become crucial part of the induction system in all performance bikes.

2.  The other issues is smooth airflow.  All airboxes incorporate velocity stacks that straighten out the air flow into the carb.  This is crucial.  If there is any turbulence In the venturi where the fuel sucked into the airstream the mixture will be inconsistent.  Think about what tirbulence is, swirling high and low pressure air.  Since carbs use vacuum to meter out the fuel you can see the problem. 

The alternative to air boxes are pods or velocity stacks. Velocity stacks are the next best solution as they straighten out the airflow and this  increase the  likelyhood of the vacuum in the carb Venturi being similar between each carb.  However, the possibility of ingesting debris is high and this will destroy any engine in time.

This take us to pods. The problem with pods is that the air filter creates tremendous turbulence and there is no velocity stack to straighten out the flow.  You can usually tune engines to work ok with pods at high rpm as the increase in air velocity overcomes the turbulence issue. However, for street applications this is a problem.  You can get pods close being tolerable depending on what that means to you but they will never be perfect unless you can mount them on the end of a velocity stack at least as long as the carb ID (longer is better). Unfortunately we don't have the space to do this on most SOHC4's.

I think the best solution is a modified airbox. Keep the front part, toss the rear part and mount high flow filter to the rear (550&650) or under (750) the existing forward airbox. This reduces the ugly factor and lets you retain the benefits of the airbox.



Check the 550 in this thread:  http://forums.sohc4.net/index.php?topic=103836.0

[lucky]

Thanks for your input.
I agree with most of what you say.
Modern sport bike style motorcycles were designed with the velocity stacks and air box units.
33 years ago it was not the same.

But the 1977-78 CB750 DOES have the rubber velocity stacks incorporated into the air box.  It was as they say these days... "a game changer."

I think that air box with the exposed bottom half with the filter exposed is not that attractive either to me.

I will try to do something to the looks of the air box to make me fell better about it.

If I had the extra money i would just get a set of CR carbs. But that costs $800.00!!!!! Or if it was a chopper I would go with the duel carb set up with Mikuni's.

BTW...I like that CR painted on those PD carbs! LOL...olol...

[Retro Rocket]

Thanks for your input.
I agree with most of what you say.
Modern sport bike style motorcycles were designed with the velocity stacks and air box units.
33 years ago it was not the same.

That couldn't be further from the truth lucky, whilst technology has come a long way the original cb750 was carefully designed and engineered,  especially the induction system, it has been covered 100's of times in graphic detail on this site and is the reason why pods {especially without velocity stacks} will NEVER work as well as the standard induction system, especially with the stock carbs, as they were all part of the original design and engineered as a unit to work efficiently with the stock airbox. Everything adrianm1972 said is exactly right.  I would go as far as to say that Honda never "stuck" anything on any piece of machinery they made without thorough engineering, period.

[adrianm1972]

BTW...I like that CR painted on those PD carbs! LOL...olol...

I saw that, cracked me up too. There are better alternatives to the stock filter hung under. I have see the bottom blocked off and a round k&n filter straight back like on a 550 or a different filter below the airbox.

[Retro Rocket]

BTW...I like that CR painted on those PD carbs! LOL...olol...

I saw that, cracked me up too. There are better alternatives to the stock filter hung under. I have see the bottom blocked off and a round k&n filter straight back like on a 550 or a different filter below the airbox.

I'm going to experiment with enlarging the airbox on my 1000cc build, i know there's not much room but the idea is to have the bottom of the box sitting on the engine case, adding to its height as much as i can to increase volume, then i was toying with the idea of adding some cold air induction by using some flexible conduit over the top of the engine and down to the filter section so that at higher speeds i get a bit more air, my approach may not be very scientific but i would like to try it out and see if it improves {or makes worse} the induction system. Similar systems run on modern sport bikes....

[adrianm1972]

BTW...I like that CR painted on those PD carbs! LOL...olol...

I saw that, cracked me up too. There are better alternatives to the stock filter hung under. I have see the bottom blocked off and a round k&n filter straight back like on a 550 or a different filter below the airbox.

I'm going to experiment with enlarging the airbox on my 1000cc build, i know there's not much room but the idea is to have the bottom of the box sitting on the engine case, adding to its height as much as i can to increase volume, then i was toying with the idea of adding some cold air induction by using some flexible conduit over the top of the engine and down to the filter section so that at higher speeds i get a bit more air, my approach may not be very scientific but i would like to try it out and see if it improves {or makes worse} the induction system. Similar systems run on modern sport bikes....

Be careful, if there is any ram air effect you'll have to equalize the pressure from the air box to the float bowls or the bike will run lean the faster you go.

BTW, I am new to bikes but have been fooling around with cars and carbs since i was a little kid.  Mostly Chevy V8's (Holley's/Rochester Quadrajets) and some VW's (Webers)

[Retro Rocket]

BTW...I like that CR painted on those PD carbs! LOL...olol...

I saw that, cracked me up too. There are better alternatives to the stock filter hung under. I have see the bottom blocked off and a round k&n filter straight back like on a 550 or a different filter below the airbox.

I'm going to experiment with enlarging the airbox on my 1000cc build, i know there's not much room but the idea is to have the bottom of the box sitting on the engine case, adding to its height as much as i can to increase volume, then i was toying with the idea of adding some cold air induction by using some flexible conduit over the top of the engine and down to the filter section so that at higher speeds i get a bit more air, my approach may not be very scientific but i would like to try it out and see if it improves {or makes worse} the induction system. Similar systems run on modern sport bikes....

Be careful, if there is any ram air effect you'll have to equalize the pressure from the air box to the float bowls or the bike will run lean the faster you go.

BTW, I am new to bikes but have been fooling around with cars and carbs since i was a little kid.  Mostly Chevy V8's (Holley's/Rochester Quadrajets) and some VW's (Webers)

I'll be running RS34 Mikuni's and then possibly fuel injection. How does this type of induction {ram effect} work on modern sport bikes..?

[adrianm1972]

Modern bikes use fuel injection. The fuel is metered by a solenoid actuated injector under high pressure. I am new to bikes but I assume that use a mass airflow sensor and/or a manifold air pressure sensor. These let the ECU know the volume and pressure of the air in the airbox (as well as air temp, exhaust O2 levels, throttle opening, engine rpm and if you have traction control the diff between front and rear wheel rotation)  and call up the fuel and ignition timing map corresponding to those conditions.

Ram air is free supercharging and FI is really good at making use of it. On carbureted ram air bikes you would have to route airbox pressure to the float bowls or the pressure in the air box would stop the fuel flow.

[Retro Rocket]

Modern bikes use fuel injection. The fuel is metered by a solenoid actuated injector under high pressure. I am new to bikes but I assume that use a mass airflow sensor and/or a manifold air pressure sensor. These let the ECU know the volume and pressure of the air in the airbox (as well as air temp, exhaust O2 levels, throttle opening, engine rpm and if you have traction control the diff between front and rear wheel rotation)  and call up the fuel and ignition timing map corresponding to those conditions.

Ram air is free supercharging and FI is really good at making use of it. On carbureted ram air bikes you would have to route airbox pressure to the float bowls or the pressure in the air box would stop the fuel flow.

I should have mentioned that the bike i was thinking about when i made that last post was a 1990 Kawasaki ZXR750, they had big ram tubes through the tank and into the airbox, they were carbed bikes, not injection.... 40mm carbs if i recall correctly...

[lucky]

The air opening on the 1978 CB750 is only 40% of the total area of the carb inlets which is 12.56 square inches!!!

So those 1977-78 air boxes are acting like a big choke really.

I think that is why when I tried the stacks - #45 idle jet, 120 main jet, and .058 thickness shims on the slide needles and 3/4 turn open on the mixture screws, it ran so good. It loves more fuel!!!

The problem is that it needs a different taper on the needle.