Topic: Carbs, Isolators, Air Box re-install tips please

[Scootch]

Will you experienced wrenches please give me re-install tips. I knew this was going to be the hardest part and it hasn't disappointed.

[RAFster122s]

Heat the insulators with a heat gun, wear gloves if you need to. Old insulators get pretty hard. If you can spring for a new set from Honda, not aftermarket, then do so. Less likely to have leaks and the rubber is softer and more compliant.
A few have put a board across the back of the carb rack on the throat and a rope cinched around the motor, or ratchet strap to pull the rack back on. Crude but effective.

Lube on the rubber insulators helps them slip on too.

[newday777]

Exactly as David has said. The old rubber is hardened and not pliable as it should be. 
Silicone grease or red rubber safe grease helps and a tie-down strap around the head and carbs works
Heating up with a heat gun helps(don't over heat, move the gun around lots)
And yes it is very advisable to get new rubber insulators and air horns from Honda. Get new clamps and screws too if yours are buggered.
This is on your 1970?
David Silver has the rubbers.

Insulators are #8 & #9 $21 & $22, (2) each
https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB750KA_(76)_750_FOUR%2c_JPN%2c_VIN%23_CB750-2540001_TO_CB750-2575894/CYLINDER_HEAD/b3d05c73-22e7-4620-b018-132301d96b6e/58a86ed9-04d7-422c-b0e8-d2356946658a/y


Airborne rubbers are $9.54 each #12
https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB750KA_(76)_750_FOUR%2c_JPN%2c_VIN%23_CB750-2540001_TO_CB750-2575894/AIR_CLEANER/b3d05c73-22e7-4620-b018-132301d96b6e/cd1f4c2f-35dc-443c-bfcf-1fd784080a67/y

[newday777]

Oops forgot to add I've used the ratchet strap against the carbs. No board though some do.

[newday777]

This guy used 2 boards and 2 threaded rods with wing nuts

[newday777]

David Silver Spares UK has them in stock. Get the Genuine as the aftermarket has leakage problems and harden faster.

https://www.davidsilverspares.co.uk/CB750K0-FOUR-1969-1970/

[ekpent]

Be more careful with a K0 then the later model as the bracket that holds the 4 carbs in place is much more fragile.  They want the break in the center. +1 on the heat and a little film of grease and pushing in with an up and down wiggle movement making sure the bike does not roll forward.

[Scootch]

Thank you all very much for taking the time to respond and include links etc ! Yes this is for the 1970. I ordered an air box also from David Silver as well. As I got into this I discovered that the one that was on the bike had been totally destroyed by PO.

[rluser]

Heat the insulators with a heat gun, wear gloves if you need to.

That sounds like a good use case for an oven.

[newday777]

Heat the insulators with a heat gun, wear gloves if you need to.

That sounds like a good use case for an oven.

The heat gun is actually the easiest tool for getting the rubber insulators and air horns soft enough to get the carbs and air boxes back in place.

[ekpent]

I would not want to 'cook' my brass floats (in this case) in an oven as they can swell and distort. Heat gun puts the heat where it is needed.  When I use a heat gun to melt varnish when taking apart super nasty carbs I use thick foil or other methods to protect the floats.

[cooldrum]

I just replaced the carb and air box insulators on my '74 CB750. All the carbs got a thorough cleaning with new float bowl gaskets too.  Putting the carbs back on I used the suggestions of a heat gun and some lube.  This worked well during installation.  New clamps were used as well.  I've always kept up with the tune ups and maintenance.  What a difference in the bike running by replacing these worn parts!  Only 34K miles and I did use the genuine Honda insulators.  Well worth it. Thanks to all for sharing tips on the installation process.  Cheers!

[dave500]

i boil the old ones in water,bit of rubber grease then work fast,still not real easy but gets it done.

[pjlogue]

Make sure your rubber isolators are on the cylinder head nipples correctly.  There are two shapes.  One shape is for cyl's 1&4, the other is for cyl's 2&3.  It takes a bit of tweaking to get them lined up properly.  I use a flat board to make sure the rear edges isolators all are in the same plane.  I then lube the inside edges of the isolators with Dow Corning high vacuum grease.  I have used a ratchet strap to wrap around the cylinder head and around the intake side of the carb bank and tighten it up until the carb nipples slip into the isolators.  I have also used a rubber mallet and block of wood to drive the carb nipples into the rubber isolators, alternating side to side. 

Don't try and mount the carb bank with the airbox parts attached.

I also use cork gasket strips glued to the inside of the clamp rungs that tighten up around the rubber isolators.  The 40+ year old clamps become stretched and the rubber isolators become compressed where the clamps go.  The cork makes a good tight seal.

-P.

[70CB750]

Be more careful with a K0 then the later model as the bracket that holds the 4 carbs in place is much more fragile.  They want the break in the center. +1 on the heat and a little film of grease and pushing in with an up and down wiggle movement making sure the bike does not roll forward.

You are so right, I broke mine from Dorothy.  I used flat bar 6061 aluminum to make replacement.

The original is a cast, I still have the pieces somewhere. 

[Dreadnought]

Silicone spray works wonders

[Slow2]

Just an alignment tip. I retired over 8 years ago and I believe I’ve been away from logical “mechanics “ too long.  To align the air box horns, find something to wedge the carbs down a little. The rubber isolators, especially new ones, will give a lot. Make sure the carb to head clamps are tight. I was able to install the air box in 30 minutes, just putting down pressure on the carbs. Hope this helps someone.

Ed

[Scootch]

On my reassembly I learned that by rocking the isolators that are between carbs and head a little bit with a twisting motion and keeping forward pressure on the carb rack the isolators will seat on the carbs and head. The air horn trumpets seat pretty easily if you rotate them to the orientation they want/need to be at to engage the carbs. I also found out that using genuine OEM parts for the isolators and trumpets work a lot better than other stuff. Genuine OEM parts are more precise and pliable. To rock the isolators I used channel locks with rubber "boots" on the jaws. It doesn't take much force but there is no other way to seat those isolators IMHO.

[Stev-o]

I have my "special tools" for installing carbs...........




Really not bad, had the carbs on in 15 min.   Then cleaned up and installed the airbox...




Obviously this is on a later model than you have.

[fizzlebottom]

This might be a wildly different experience than all you folks with PD carbs, but I've never had an easier time removing and reinstalling my CV carbs than when I have new-ish isolators. They're soft and pliable, and I can pop the carbs in just by pulling each side into the cylinder head.

I did this 3 separate times in the last 2 weeks and it took me a whole 5 mins each time I think. I do remember the first time I did it years ago with old crusty isolators and it was a NIGHTMARE. Brand new Honda isolators are a dream to work with. I would hope this is all the same with other SOHC/4 models, but I only have experience with mine.

In short, brand new isolators make the job about as easy as it gets (for me). No extra tools necessary.