Author Topic: 1976 CB750f new boots new clamps still have a vacuum leak at the head?!  (Read 2696 times)

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Offline Mpace

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Ok. I've been banging my head, read every vacuum leak post I can find. I have replaced my carb boots, bought hd clamps from napa that are called constant torque hose clamps and cranked those suckers down. I am still idling high when she warms up and it responds to the vacuum leak test at the head on the #2&3 intake spigots. Could the spigots themselves be leaking? I haven't found much about any issues with them. But I am at a loss. Oh and it's got an 836cc kit with an RC295 camshaft, head has been ported and polished. The carbs have had the EPA port on the idle screw modified as per honda chopper.com, running 115 mains, 40 idle jets, 2nd clip on my needle. Kerker 4-1 exhaust with no baffle and a butt plug. I replaced the head gasket and the carb boots about 6 weeks ago, ever since then it would idle great until it got hot and then the idle would hang at 2500-3k after a rev and also on its own. I thought it was the sync first so I bought a gauge and noticed that my levels were almost balanced across the board but I would get fluctuation of just one cylinder when it started to rev. I found a repair to spigot #4 yesterday and what i thought was a pinhole leak. I repaired that with some jb weld and i am not leaking at #4 anymore just #'s 2&3 at the head side of the intake boot. Again, I have new boots, new clamps and have verified where my leak is, I just can't get it to stop. HELP ME Please! Thanks in advance guys.

Online HondaMan

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Does this '75 have the PD type of carbs? Most '75 bikes did not.

I am not familiar with the EPA port mod you're describing, either?
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline Mpace

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The casting numbers on my carbs are 069A from a 75 cb750f. No accelerator pump or diaphragm in these. I pulled them off and indeed the #3 and #1 spigots were very easy removed from their threads in the head. No thread damage but I'm guessing they've been out before since #2 wouldn't budge. I'm going to get a 37 mm socket today since all I had was a 1 1/2 inch. Any suggestions on how to seal the threads again? I've seen mention of a high temp honda glue. I was thinking either anti seize to prevent galling on reinstallation or using permatex had thread sealer for aluminum. What would you suggest? I'm certain this was where I was leaking but I am going to double check my boots to make sure I haven't inadvertently damaged the new ones. It won't let me post an external link so it's in the garage section of honda chopper dot com under carb tuning and idle screw modification.

Offline ekpent

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Make a few more posts and then you can post up some pics,think you may need 5.  Here I will help if you answer. What day is it today  ;)

Offline Mpace

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It's just another manic Monday. For sure. I tried posting pics but it tells me it's too large.

Offline Mpace

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Trying with pic

Offline Mpace

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Here's the spigot out

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Yeow!
Is there enough thread there to still tighten it back up?

You can use the "Gas Tank Sealer" available at most auto parts stores. After it dries (2 days) it won't burn anymore. It is a clear substance, turns milky after it gets hot a few times. It comes in a squeeze tube. It is also an adhesive, so make sure everything is clean first and it should really seal it up well.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline kghost

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Yeow!
Is there enough thread there to still tighten it back up?


That's what I thought when I saw it.

Hope it holds
Stranger in a strange land

Offline Mpace

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Haha! Oh ye of little faith! Just kidding guys. No not too much thread left and the ports have been heavily polished. Which means to have the spigots indexed correctly they sit loosely in the head. Awesome point 1. So I tried high temp thread lock. No dice. Aviation gasket maker #3 from permatex? Hahaha said my honda. Well heck. I bit the bullet and mixed up my least favorite thing. The original jb weld. If it doesn't work I'm looking for a tig welder bc I'm never getting them back out alive. Wish me luck

Offline Mpace

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Re: 1976 CB750f new boots new clamps still have a vacuum leak at the head?!
« Reply #10 on: April 19, 2013, 06:12:16 PM »
On a more serious note, it will be a terrible shame for this build to go to waste. I am the third owner and if you search RC295 you will find a link on this forum to the build for this engine. This is not to say the article written is about my bike, but rather a supe enthusiast read that article and built that engine. It's not an F2 but it's got an 836 kit, a RC295 cam, I confirmed this by micrometer but its etched on the cam, a kerker 4-1 exhaust, k and n filter, no pods, and the same drive tooth set up. When I discovered all of this replacing the head gasket I knew I had found a little piece of history. Love my cb

Offline Vinhead1957

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Re: 1976 CB750f new boots new clamps still have a vacuum leak at the head?!
« Reply #11 on: April 19, 2013, 07:24:23 PM »
Can't you find a new intake spigot

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: 1976 CB750f new boots new clamps still have a vacuum leak at the head?!
« Reply #12 on: April 19, 2013, 09:24:13 PM »
yeah but then the porting would not match
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)