81' CB750 Vacuum Leaks - Carb Boots & Throttle Shafts

[@dco1884]

Hello. I have a 1979 cb750K four. I'm in the middle of rebuilding the bike and I'm glad I watched this video - I should replace the throttle/choke shaft seals. However, no kits are available in my area. I was wondering if you could indicate what size the o-rings should be for the throttle shaft and choke shaft. Thank you.

[@badboybikerdude]

I realize this is an older video and the bike is long gone but I have some experience in replacing felt throttle shaft seals in these Keihin VB carbs. I too bought seals from motorcyclecarbs.com because I was frustrated with vacuum leaks at the throttle shaft on bikes I did carbs jobs on in my own shop. The seals I got were very flimsy and only worked 50% of the time, same as you, and that frustrated me even more. My solution was to manufacture better seals myself. I learned how felt seals are made, scanned the classifieds for a used punch press, paid a tool and dies shop to make custom cutting dies for my press, researched different felts and finally ended up with a seal that is twice as thick as the originals, made from denser felt and sized so it holds the shaft much better that the original seals Honda used. Here is a link to my seal install video showing in detail how the job is done. kzbin.info/www/bejne/n2aydpqblrCEjKM I also sell these seals, and have sold them all over the world. They won't leak if they are installed properly and I have lots of testimonials from satisfied customers.

[@HTMR]

Give me link to your sales page, I'll put it up on my website.

[@badboybikerdude]

@@HTMR Sorry for the delay, I don't get on KZbin very often. Anyone who wants seals can just go to www.dudesgarage.com and contact me there. Thanks for your support, it is truly appreciated. I'm not some big business, just a mechanic who makes a solution for the vintage Hondas I love to keep on the road.

[@HTMR]

Your probably looking at 6-8 labor hours to replace throttle shaft seals. The carbs have to be de-ganged and all butterfly plates removed. Labor rates vary.

[@DanRegalia]

Loved the steps to test the seals on the carb boots... working on one of these now. I have a question tho, that main vacuum line that goes between carbs 2 and 3, where does that reconnect to? I should have taken notes..lol

[@HTMR]

To the fuel petcock.

[@brianelliott6148]

Is there a site that you use to find old parts for all bikes i have a 1980 yamaha xj650 maxim. And I cannot find manifold intake boots .I waswondering if they were all the same I seethere is only like two different kinds but are they "interchangeable " to a degree

[@BodyCountRises]

It's the pods dude. These DOHC 4s have constant velocity carburetors. They're not meant to run pods. Part of your vacuum leak is from the lack of airbox. The pods are letting too much air in. That's why you need the choke. You can TRY rejetting, but it will never run as good as stock.

[@gummitarzan8545]

exactly

[@jakobsinamon1683]

IA23XCV ok for one why would you tape off a air pod ? Why not keep stock airbox then lol

[@HTMR]

The gas leak was coming from the accelerator pump, so I assume the gaskets/seal were bad.

[@AeroRamer]

I have the same model bike. My issue is that it bogs down at top speed around 7000 rpm I have to twist the throttle back and forth to get the speed to accelerate further. Is that a vacuum leak issue or carb jets/pilot?

[@jerodrobinson4040]

I have an 82 Yamaha XJ-750 Maxim, and it will not idle without Choke. It also seems to be super underpowered at higher RPM's. I've replaced the Carb Boot's, the Valve stem seals checked the rings Adjusted Valve's Cleaned the Jet's. And nothing seems to make a difference. Any theroy's on what I'm dear with are welcome..

[@HTMR]

It wants more fuel. Have you adjusted the fuel screws per this video: kzbin.info/www/bejne/en3PlaB4r9aFldU

[@jerodrobinson4040]

@@HTMR No I have not, but I think your on the right track. When I got the Bike the Factory Exhaust was roached, so I got a 4 into 1 for it. I'm wondering if I may need to increase jet size too.

[@jerodrobinson4040]

@@HTMR Your the Man! And that's an excellent Video. The Pilot Screw in my number 3 Carb is seized and stripped out. And I think that explains why my number 3 plug still looks brand new after about 2 hours of accumulated run time.

[@jerodrobinson4040]

@@HTMR my issue now is how I'm gonna break the Pilot screw free. Putting an easy out on it worries me..

[@matts417]

I know I am a little late on this.. but I got an 82 XJ750 myself.. and it doesn't idle unless I give it gas... I know for sure I got a leak in the petcock, but its not terrible, oh and when I give it gas sometimes it sticks open.. aka I pull off the throttle and it stays and climbs until I put load on the engine (pulling off the clutch a little puts the idle back down ) I mean I am guessing that would be an air leak,simply because it doesn't stay high...I tried to retune the carbs but Without the carb tool I did it by ear the best I could .. I am able to ride still, but it is just weird I am sitting at a red light and all of a sudden the idle hypes up and sounds like I am reving.. idk seems like a vacuum problem IMO but just wanted a second opinion .. I might make a video on it later when the weather gets a little warmer

[@HTMR]

+matts417 Matt, first off don't ride the bike until you take care of the fuel leaks. I don't want to see your bike burn to the ground. Have you checked for vac leaks like in the video?

[@matts417]

Yeah I did, I used brake cleaner and it did stutter a little so that is probably the issue(had to do it after a half hour ride for the idle to actually stable without me giving it constant throttle), but after further investigation I have reason to believe it is probably the headers, it is an old bike too that about half the bolts are rusted off and I gotta re tap them to make it work, and as for the fuel leak it only drops down onto the carbuator boots and qickly dries before getting anywhere hot... that is something I will fix got a petcock online a while back made for my bike but I remmebered someone put a different tank on it so I got to find a different valve that will actually fit.. Thanks for the reply matt! I have been working a lot lately so I havent had much time to do a lot, Sry for the mistakes in this comment I am sleepy XD

[@HTMR]

+matts417 Whatever you do, take care of the fuel leak first! Trust me on this one. I don't want to see you burn your bike to the ground.

[@HTMR]

It's all part of diagnosing problems.

[@daveco1270]

Couldn't you use any liquid that can be sprayed? Why not use a spray bottle with water? I get nervous spraying carb cleaner on my bike, I don't have compressed air to blow it off afterward.

[@hudg8007]

Water would damage your engine. It doesn't ignite and will cause misfires.

[@HTMR]

Google "randakks dohc kit" Contact them and make sure the o-rings for throttle shaft are included, if you have a leak there as well. If not, I can give you a good resource for sizing o-rings.

[@JeremyBorumComposer]

Thanks for this. It's not the problem I have, but it's always good to cross one more thing off the list.

[@DxhlzK]

Did u change ur caburetor

[@williamschristopher12]

what was the name of that shop for the master over haul kit?.."brand X".com?..having the same problems..thanks

[@TheCPerla]

good vid. What would you estimate this would cost to fix if it was brought into a shop?

[@HTMR]

+Carlos Perla To seal all vacuum leaks including throttle shaft locations (which will take the most time) factor in 6hrs labor at least. Not sure where you live, labor is $80+/hr where I live.

[@HTMR]

No, the owner sold it as is instead of fixing it.

[@scal760]

You mentioned putting oil on the pod filter. Will that help and why?

[@HTMR]

The oil will not help with vacuum leaks. The purpose of the oil is to attract dirt and prevent it from getting into engine.

[@scal760]

MatthewMCRepair Ok so o should dip my pod in oil

[@HTMR]

scal760 No. The oil I use is K&N air filter oil that comes in an aerosol can. You just spray a light coat on. I'm referring to the fabric style pods.

[@littlebigpuff]

hey mate did you sorce boots for the bike ? im just looking for some for mine but no joy

[@HTMR]

Google "1981 cb750 carb holder". The first hit is on amazon.

[@williamschristopher12]

thanks, i appreciate the info..the bike i have is a 78 hondamatic, and is sucking WAY too much air,won't run without the clutch up..in your personal/professional opinion, how much would it cost to have a shop fix the problem?

[@jakobsinamon1683]

Christopher Williams probably a kick stand sensor

[@DerelictDan69]

Carbs need jetted

[@hondacustom]

the cam chains are dieing., adjust them

[@HTMR]

The owner decided not to fix the bike and sold it as is. You might be right about the cam chain, however I noticed that carbs that are not tuned and out of synch can make the engine sound like it is rattling quite a bit.

[@PlisPlassuply]

muy buenos todos los consejos...

[@mikkelskafte8717]

Hey mate! currently looking for a replacement for the standard carb on my 750 81'. What carbs have you got on that bike? :-) and cheers for the great videos!

[@HTMR]

Mikkel Skafte They are stock carbs. I'd search ebay or forum classified.

[@santiagomoscoso7636]

hola tengo km a moto cb 750 y sólo me funcionan dos pistones el 2 y el 3 pero el 1 y el 4 no me funcionan. será o alguien puede ayudarme con un consejo

[@HTMR]

+Santiago Moscoso Not sure of your question exactly, however do you have good compression in all 4 cylinders? Watch this video: kzbin.info/www/bejne/qprdkmuQmdOpn80

[@juntjoonunya9216]

that frame looks beastly compared to my 82 nighthawk 650

[@PlisPlassuply]

thanks a lot, very good advises,

[@santiagomoscoso7636]

la moto es del año 1980

[@rivermetal]

Great VideoThanks

[@georgelucasweiss-tripleB]

there is some things that will only show up if you riding the bike!

[@apathetik11]

Yell at it to get back in the accelerator pump.

[@bobdavis2011]

Sounds a wee bit like my 83 cb 1000c, only yours sounds better.

[@dudesgaragedotcom]

I also had bad luck with replacing the stock seals and since I work on lots of Keihins I decided to manufacture a much improved replacement seal. Here is my vid showing how to do the seal replacement. kzbin.info/www/bejne/n2aydpqblrCEjKM

[@HTMR]

Thanks for that info. If I ever have to go through a set of these carbs, I'll order the seals from you. You should post this on SOHC forum.

[@dudesgaragedotcom]

Glad to help, I love keeping the old iron on the road!