Topic: Trixie - 1977 CB750K - Project

[BrockSamson]

Hello everyone! 

I recently purchased a used 77 CB750K with 20k on the Odometer.  Despite not being anywhere near as original as I would have liked I am really happy with the purchase. 

My goals before spring are to get the bike titled, tagged as an antique and finally get the bike running perfectly.  Given that this is my first bike I am going to keep it as is for the spring/summer and then look into completely tearing the bike down next winter.

As you can see in the picture it has an aftermarket exhaust and pod filters.  It also has Mikuni carbs.  It obviously also has the typical cafe seat, handle bars and a couple other cosmetic changes.

The bike runs but not perfect.  The PO informed me that the carbs need to be cleaned.  So that is the first order of business.  Also, this is where I would like the first bit of advice about the best route to take.

I purchased Mark Paris' My CB750 Book which deals mostly with the pre-77 CB750, but the differences are so small from what I understand that I saw no reason to not have this book on hand.  I read the carb section of the book in its entirety.  I have also done a bit of research here on the forums.  I kept coming across threads about people putting pod filters on their carbs and for a few reasons produced inconsistencies in A/F ratios across different throttle positions.  Generally it seemed jetting the carbs to 130s fixed the problem but not always.  I also noticed that this occurred mostly on pre-77 CB750s. 

So I need advice on what route to take. I have the original airbox and carbs.  On the bike now are the pod filters and Mikuni carbs.   Should I keep the Mikuni carbs and pod filters or put back on the original airbox and carbs?  I have read in the threads here that the original 77-78 carbs are "terrible" and I have heard that they are great and much easier to tune than pre-77.  Completely contradictory statements  .  I also know they have accelerator pumps, and I am getting ready to do more research on what that actually is and what that means for making this decision.

Any advice on this would very much be appreciated.  My goal is to have the bike be very consistent and run well around town and at freeway speeds.  I have no plans to race.  If the Mikuni carbs and pod filters will provide a performance increase without cost to it running properly and without increased risk of damage to the bike I would like to keep them on. 

Thanks!

[lofton05]

Nice looking bike.  When I did my carbs a few months ago I used the following website.  It has alot of great info.

http://www.salocal.com/sohc/tech/carb/carb.htm

Kevin

[BrockSamson]

Thanks!  Yeah I actually had that exact page open.  I was thinking of taking a bike to a local shop to get their opinion but I feel like I would regret doing that.

[BrockSamson]

For those interested or if you were face palming after seeing another Pod question I have finished going through the forums and I came to a decision.  No matter what those Pods are going to have to go.  TwoTired really laid down the facts and science behind why removing the air box and replacing with pod filters will just be a headache for me in the long run.  If you see this TwoTired, thank you very much!  I did like the look of the pod filters, oh well.

I also have been able to get a general idea of what the accelerator pumps actually do.  From what I understand is that it improves throttle response that was lost because of the leaning out of the A/F ratio in the 77-78 engines. 

So right now I think the best route to take is keep the Mikuni carbs and fit them to the original air box.  It is making me wonder that if the "carb cleaning" that was needed was actually a conclusion he came to because of symptoms of using the pod filters. 

I am going to pull the carbs off the bike later this week and take them apart to get a good idea of the condition they are in.  I will take a lot of pictures.

Question:
Anyone know of a reason that I should go with the original Keihin carbs over the Mikuni?

[mrrch]

Going with stock carbs it will run as it is supposed to. Tuning the Mikuni's if required may cause you some possible headaches (or they may be good to go as is) Go with stock and get it dialed in then when your skills permit maybe try the Mikuni's later.

[BrockSamson]

Thanks for the advice.  I just looked through your build and you have an incredible bike.  I saw you had pods originally and that you switched to the stacks.  You mentioned something about the choke in your build thread, were you having the issues I have read about on other 750s with pods and the stacks resolved that?

[mrrch]

I read about pods and their issues,then I found the velocity stacks and went that route instead before I even ran it(actually the pods were interfering with the choke linkage on the carb). Nothing different about my choke other than a custom cable mount to clear my speedometer. Bike runs great with the stacks (in my opinion) just some minor rejetting and fine tuning required (I'm still running abit rich but I'll sort that out come spring).

[BrockSamson]

Pulled the original PD41A Carbs out and just kinda eyed them... the look like they are in surprisingly good shape.  Ha, but what do I know?  I guess I expected a bunch of dirt and grime to be in there.

[ZanVooden]

I'd personally go back to stock carbs, and get a feel for the way the engine was intended to run. Stock airbox and stock carb are very reliable and pretty easy to tune if they have the correct jets in them. I personally like the later model carb with the accelerator pump. I've never had any issue with either of my bikes. I really like the throttle response with them.

Once you know how the engine was intended to run then play with your Mikunis. That way you have a baseline or control.
Thats my two pennies on the table.

[BrockSamson]

I am leaning more and more towards the stock carbs.  Just opened one up and they have 115 mains.  As I examine them I get the impression that they were rebuilt recently...

[BrockSamson]

ZanVooden, I saw in one of your builds that the the PO had put in sheet metal screws because he stripped it out.  It appears that I have a single stripped hole... what was your solution for this?

[BrockSamson]

Looks like all of the tubes/boots that go into the air box are shot.  One was even cut in half... poorly.  :/ sigh.  New boots are ~$45.  Not to mention whatever crappy foam filter was in there literally turned to dust in my hands. 

[ZanVooden]

ZanVooden, I saw in one of your builds that the the PO had put in sheet metal screws because he stripped it out.  It appears that I have a single stripped hole... what was your solution for this?

My solution was to drill a size larger and tap the holes. I've also heard decent results with guys smearing the inside of the striped hole with JB weld and retapping to the original size.

[BrockSamson]

Thanks ZanVooden.  Not sure why these get stripped so easily.  I am lucky that the PO didn't try to jam sheet metal screws into mine. 

Random thought this morning:

I am beginning to realize why its easy to just suck up the aggravation of tuning for pods or to a lesser extent stacks.  The rubber stacks in my airbox are garbage... maybe one of them is salvagable, one was cut in half for who knows what reason  .  New rubber stacks are ~$45.  Thankfully my intake boots are in amazing shape, but those are ~$100 if you are unlucky enough to have bad rubber.  Then I need to get a new K&N filter for the air box.  Then in my case I already have the damn pod filters which appear to be brand new.  Jets are cheap.  The temptation is really strong.  On the other hand, if I do buy these parts and decide to go for stacks, back to pods or the anti-pod I could sell the original air box with new rubbers and make most of my money back. 

Btw if anyone is reading this and they want to completely take apart the air box for a CB750K7 to clean it, be aware that it doesn't come fully apart. There are two bolts that will not come out and I just do not see a reason that it was designed this way.  It also makes it very difficult to clean prep and paint the metal parts if you were looking to do that sort of thing.  Finally, all the built up crap in the air box scares me away from unfiltered stacks.  Although this was probably decades worth of dirt, it still made me question stacks for a bike that will be driven regularly.

[HonderCB]

my bike had pods on it when i got it, and wouldnt properly idle without choke (and not even then without feathering teh throttle).  After F-ing with the carbs and air screws for 2 nights with my father-in-law with little success i said, "i wonder if i put a stock airbox on if that will help?"  My father-in-law says to me, "lets grab the one off that bike over there and try it!"  Runs fine now...

...Bottom line is PODS ARE THE DEVIL (its too bad they look so damn cool)

I ended up giving him to pods in exchange for the stock airbox (although i could have had it free), he thinks he might try them on his 69' (not sandcast) that he is building.  I suspect he will end up using the stock airbox lol.

[ZanVooden]

Those screws get stripped easily because the carb bodies are Aluminum thus rather soft metal.
If they are tight enough the bowls will leak gas past the gasket (between the bowl and body). So people just tighten them too far to prevent and make the problem worse.
I havent come across a set of carbs that at least one screw has been stripped yet.
 

[BrockSamson]

Well decided to guess at some replacement screws from the hardware store while I was there.  Also picked up a few tools to polish off all the grime on the carb.  Finally, thought someone would get a kick out of the condition of my rubber velocity stacks...really strange how they are all in different stages of decay. 

[BrockSamson]

EDIT:

Retracted my question about carb to engine rubbers.

I resolved it myself already.  As you can see from the pictures I have already posted I HAVE a good set of rubbers from the carb to the engine... just did not realize it.  Really.  For some reason my brain did not process that the air box velocity stacks connect directly to the carb.  Thought the rubbers I had set aside and nearly forgotten about were part of the air box connectors.

I am assuming that the blue rubber tubing on there now was because the carb bore for the Mukini carbs is bigger and the rubbers did not fit.  Ha, its late and its been a long day.

[BrockSamson]

So I am getting a few repair parts and tune up parts.  Hoping to put my order in tomorrow.  I have done my research and fairly certain I got everything correct.  Still, I would like to post it here and if anyone sees something obviously wrong or missing I would appreciate any advice.

Tune Up:
ASMOIL 20W-50 Advanced Synthetic Motorcycle Oil
NGK D8EA Spark Plugs
K&N KN-401 Oil Filter
K&N HA-0100 Air Filter

Replacement Parts:
Air Box Velocity Stack Rubbers - 17253-405-300.
http://www.ebay.com/itm/Honda-New-CB750-Carburetor-Air-Box-Boot-Carb-750-77-78-/320816003412?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item4ab21fb954#ht_506wt_868

[BrockSamson]

Did a lot of polishing on the carbs.  Had to stop when my neighbor started banging on the wall.  I guess they did not like the drill running for 2 hours  .  I am really impressed at how they cleaned up.  I was going to paint them but I like them as they are.  This weekend I am going to bench sync and get them to OEM specifications. 

[BrockSamson]

Ok I need help.  I am a bit confused about float bowl height and measuring it.  I have searched and searched and blah blah blah and I got conflicting wording on how to properly measure. 

**NOTE** I am using a tape measure that has centimeters on it.  This not by any means accurate, I am still waiting for the more accurate measuring tool to come into the mail.

In the first picture you can see how I am measuring the float.  It is perfectly parallel to the carb.  Also this is where separation from seating on that valve begins to occur.  Right where the red arrow is pointing.  This is how I **think** I am supposed to measure this.  I am supposed to be at 14.5mm and it looks like I will be right on once I have the correct tool... at least in the ball park.

The second picture is me measuring with the float bowl pushed down.  I read somewhere that this is how you are supposed to measure and from the lowest point.  Not only does this not make sense it does not seem to line up with the actuation of the valve.  Also, its damn near impossible to take a picture of with two hands. haha.

Am I doing it right in the first picture?  Also, how do I adjust.  I can not figure out what the hell would adjust that.

Thanks!

[Gonzowerke]

Don't Brock-block me Samson!

Sorry, I LOVE Venture brothers!

What works for me, and what an old Honda tech told me, was turn the carbs upside down. The floats should be level, i.e. straight across, with the bowl flange Both my 78K and my 77F run stock cardbs and airbox. There is only one reason IMHO to go with pods, and that is looks. You don't really get any better performance from deleting your velocity stacks. They were engineered to work with the carbs and airbox for a reason.

[mrrch]

The best way with these carbs is to do it on the bench(carbs upright and hooked to a small fuel bottle) with some clear hose run up alongside the carb body from the drain nipple and open the drain screw and fuel level will be seen in the tube.I set mine about 1 mm below the float bowl seam. Then you can also check that the accellerator jets are squirting (be warned, they will squirt about a foot if working properly)
Should be able to find some pics when doing a search.

[BrockSamson]

Haha yeah obviously my favorite TV show.  They really need to get going with new episodes.

I already made the choice to go with the stock airbox and carbs.  I have no desire to screw around with carbs for a week straight, just want to get these PDs and airbox back on and have the bike running well for now.

So they are good and level and measure correctly when turned upside down.  Well as correctly as they can be using the only tool I have...measuring tape.  I have a more accurate tool coming in the mail soon along with the in box K&N air filter.

Mrrch, I have actually seen that method.  I really didn't want to screw around with gas in my apartment.  Was thinking I could just use distilled water or something else... but then it occurred to me that fuel could easily be light/heavier than distilled water.  I am going to measure out each float when they unseat from the valve... which is actually the same location as where Gonzo suggest I measure.  If I run into issues (which after reading all the posts here seems like at least one thing will go wrong with a carb installation/tune) I will look into doing it the way you suggested and that I have previously read about.

Thank you both for your advice.

One more question, its not a big one... but the air box boots seem offset a little bit.  They all also have a little extra rubber, like a tab.  I am currently doing a search on this to see if they are supposed to sit in the air box a certain way.  The (for lack of a better term) threads on the rubber velocity stacks/boots that go into the airbox also seem to be offset a bit as if they were to be installed a certain way.  Does anyone have any experience with these and know if there is something I am missing.  I have searched and I keep seeing people who have old hard warped boots... that is certainly not my problem. 

Below are the new rubbers next to the old.  Definitely an improvement I would say.   

[cmonSTART]

Wow.  Those carburetors look great!  What did you use to polish them?