Service won't work on carbs without the origional air box...

[timblin]

I purchased a 1979 cb750k 10th Anniversary edition bike that has a little over 13,000 miles on it. The bike had been sitting in this guys garage for a couple years, and I think had hardly been started or ridden on. He had the master cylinder replaced, new fork seals put on, and the bikes condition was good cosmetically (no rust, original pipes etc.) The bike starts first try every time, with no issues what-so-ever.

But here's the problem. He had put pod air filters the bike and removed the stock airbox, he said he had the carbs worked on by a personal friend who (supposedly) rebuilt the carbs and put in larger jets (I don't know if this is true, and I also don't know if he stepped the size of the jets up properly). The bike idles and rides great until you input full throttle, where the bike bogs down and has a dramatic decrease of RPM / power. My limited knowledge of carburetors tells me that this is a main jet issue.

I have taken the bike to two different places to try and get the problem fixed. The first place, a local vintage bike mechanic (who has been literally, I'm not kidding, recommended by everyone in Tulsa, OK), told me he could / would not work on the carbs unless the bike had the stock air-box; and the second, was the Honda Power Sports dealership (who recommended the local mechanic when I came to them with the issue), and also said the same thing.

Is this true, that the carbs really can't / shouldn't be worked on without the airbox? I mean, what major differences are faced when working on carbs with and without the stock airbox? Would you not be turning the same screws, and replacing the same parts regardless?

Personally I like the pod filters, and the idea that I would get a slight increase of power, BUT if buying a stock airbox is what it takes to get the bike fixed I'll do that. If I had someone to teach me, or had a "good" carb to take apart and compare my work off of, I would attempt to fix the carbs myself. BUT being that this is my first bike, I'd like it done right especially since it's a special model. I've never worked on a carb before.


Thanks for any input!!

[Lucky 1]

It can be jetted to run right but since you have no experience at all. It would be an uphill battle.
Since this is a first bike for you, don't ruin the whole experience  over this air filter issue.

My advice is put the stock jets and stock air filter back on the bike.
Not only would it need to be rejetted but it also needs the needles
 changed or raised and they may not even be adjustable.

Like I said it can be done but it would take a lot of work by an expert.
Many people who rejet the carbs do not get it right .

You want your bike to run well under most everyday different conditions.
I would put the stock airbox back on . That is what i would do.

The airboxes on these bikes produced at that time were like giant chokes.
The air intake was only about 3.7 square inches and the filter was a paper filter which was restrictive and caused it to be choked further ,meaning a richer mix with that filter box design.

[timblin]

That's pretty much what I'm going to have to do if I ever want to ride the bike. I just don't get how if someone KNOWS how to fix carbs, why they can't do it with pods on, but can do it if I track down an air-box. Every day I have to wait is another day of riding in this great weather down the tubes!

Ah. I have dove into my notes from last summer year when I bought the bike. The last owner said that the carburetors were rebuilt a couple years back, but that "the over-sized jets didn't work, so the normal jets were put back on."

So that's obviously the issue if there are pod filters, a bigger jet needs to be be put on, and the carbs tuned properly. I just bet the guy didn't know what he was doing and decided to sell. The bike works and sounds great EXCEPT at full throttle. I'm bummed I'll have to pay at least $300 to have someone fix the carbs when I KNOW the problem but can't do it myself.

=(

[robs750]

If you go to the other site,the first thing they tell you is to junk the pods and use the stock box and filter.
I guess the box is designed to work with the stock carbs.

[Lucky 1]

It is a 1979 and that is part of the problem.

Those are CV type carbs.
Also they have the accelerator pump function.
That is probably clogged up from sitting too.

[shinyribs]

Hey Timlin,every guy with a DOHC bike like yours will tell you that pods will not work with those carbs. It's the truth. The carbs rely on the vacuum created by the factory air box. It's a necessary evil,I'm afraid.

[TOOLS1]

If, you do a search, for pod filters on the forum here, you will find hundreds of posts about pod air filters, and they all say the same thing about pods, and CV carbs on DOHC CB 750's "Don't do it!"
TOOLS

[timblin]

Thanks everyone

[seestheday]

Another option would be to buy a different set of carbs.  Something from Cycle X or the CR carbs would probably be best.  It's a pricey way to do it, but it would be a way to get the pods to work.

[sgtslag]

Two things you can do to improve performance, for little money:

1)  Put in a UNI oiled, foam filter (use the carcass of a paper filter to make the basic housing to wrap the UNI foam around -- will require some fabricating, but not terribly difficult; you can sew the foam ends together by hand -- basic, easy stitches are all that is needed, or you could use RTV Silicone glue to seal it to the frame, as well as 'welding' the foam ends together; use the K&N Oil on the UNI foam, available at auto parts stores everywhere); spend the $60 for a K&N Air Filter for this bike (I did, before I knew about UNI Foam -- much cheaper, about the same performance improvement!)

2)  Install NGK Iridium Spark Plugs:  will almost never miss; easier, much faster start-up's; good for 50,000+ miles (will last 100,000 miles in a car); cost is around $8/plug, but worth it, in my opinion, as the stock coils are weak, but they will fire the Iridium plugs superbly.

Cheers!

[seestheday]

+1 for the iridium plugs.  Night and day difference on my bike.

[cafeloco]

I am in the process of trying to get this same bike running and was really hoping to use pod filters. is there any way to change the carbs to make them work or would i need new carbs?

[sgtslag]

You will need non-CV carbs, such as what was equipped on pre-1979 models.  Some folks claim to have made CV carbs work with pod filters, but the majority say that it can't be done.  I tend to believe the majority.  Sorry.

[timblin]

Airbox is on it's way, going back to black. I've determined that the '80 fits the '79. I'm pretty excited to get this bike screaming again (she sings, she just doesn't scream, yet). NGK plugs are next, thanks.

http://www.ebay.com/itm/261086305501?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

[Karl H]

Wow..... I really wish I had seen this 6 months ago when I dropped the money on the K&N pods for my 79 project..... poop

[Lucky 1]

The EPA has done everything they can to make you NOT be able
to use the carbs except the way they were manufactured.

At least the later models like the 81-82's were not EPA'd in a hasty manner.
Also the vacuum diaphrams made the carbs responsive to the elevation and weather conditions.

[Lucky 1]

One more thing.

There ARE people that could make those run with pods but it takes
a lot of work and step by step testing, and the people with the DYNOS have a huge advantage.

There are other carbs that will work too BUT it all has to be worked on and tested,tweaked and
done step by step. It is just not worthwhile for the average person.

[Lucky 1]

The majority do not have the know how or experience to do what is needed, and it is NOT just main jets.
So it is no surprise that the majority say it cannot be done.

[timblin]

I located an original air-box a couple months back, installed it, and the bike ran rougher with the air-box than when the pods were on it. I'm thinking maybe the jets are not stock. It's a bit beyond my know how, and the best guy in Tulsa, OK said he doesn't want to do the carb work, so I'm feeling SOL on this one. Might just have to call it a loss and sell, if I can't do it my self by the time summer rolls around. Bummer since I refurbished the speedometer and tach, put on new tires, bought a new chain, replaced a mis-matched side cover (with the appropriate 10th anniversary badge), installed new master cylinder, fork seals, and a couple other things I can't remember. The bike rides well, just not when you giver her full throttle with the pods installed.

[shinyribs]

If the gauges on your bike are anything like mine were...they're a pain. If you can crack open gauges and close them with success then carbs should not be an issue for you. You should be able to get the stock jets/adjustments out of the FSM. Just use those specs with your stock airbox and you should be golden.