Author Topic: 1978 CB750F Nears Completion (Read 4731 times)

dtran · « **on:** May 15, 2018, 05:41:52 PM »

I have been a long time lurker of the forum over the past 6 years and have been working on this bike for about just as long. The bike has reached various points of completion over the years but I was always taking it back apart to change something. Most recently, the engine had to be dismantled again due to reports that the cam chain tensioners from vintagecb750 were defective. Since the engine was already coming apart I decided to go ahead and get a 836 kit and now the bike is back together and almost complete pending some tuning.

Just wanted to share my bike and thank all of the very knowledgeable members of the forum that have helped me throughout the years. The work and parts I have put into the bike are numerous so I will spare y’all listing that out.

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madmtnmotors · « **Reply #1 on:** May 15, 2018, 06:00:19 PM »

Looks good! So how many miles did you put on the suspect cam chain tensioner and what kind of condition was it in when you took it out?

dtran · « **Reply #2 on:** May 15, 2018, 06:44:55 PM »

Quote from: madmtnmotors on May 15, 2018, 06:00:19 PM

Looks good! So how many miles did you put on the suspect cam chain tensioner and what kind of condition was it in when you took it out?

I only had about 700 miles on the bike. It had sat for a little while. The tensioner wheel actually appeared to be in good condition when I pulled it out, but I switched in an OEM part just to be safe. Comparing the vintagecb750 part to a OEM Honda one, the rubber definitely seemed to be stiffer on the Honda part.

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seanbarney41 · « **Reply #3 on:** May 15, 2018, 07:15:14 PM »

EBC rotors?...diggin the fork brace and the rearsets How about some color? Didn't anyone ever tell you the world is not black and white ha ha.

dtran · « **Reply #4 on:** May 15, 2018, 09:03:08 PM »

Quote from: seanbarney41 on May 15, 2018, 07:15:14 PM

EBC rotors?...diggin the fork brace and the rearsets How about some color? Didn't anyone ever tell you the world is not black and white ha ha.

Yes on the EBC rotors I found a great deal on them on eBay. I would have loved to have done some type of two tone design for the paint but I painted it myself and taping off designs is beyond my skills.

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dtran · « **Reply #5 on:** June 09, 2018, 10:27:17 AM »

Update. I was really close on the tuning and the bike was running well so I decided to take a longer test ride. Bike started to run hot after about 45 minutes and I noticed a oil leak in the head area. Put in an order for a frame kit from Scottly and decided to pull the head to see if it might be the cam pucks. Turns out one of the cam pucks had somehow ripped causing the leak and also starving the rockers of oil destroying the rockers and cam.

Now have replacement rocker tower and a CX1 cam here. Was hoping to just drop the new cam in but as I learned from Hondaman, the CX1 cam requires extremely tight valve guide clearances lest the rocker overshoot the valve. Thus the head will soon go out for a valve job and some porting.

Any opinions on cast iron vs bronze guides?

Any final opinions on the Cyclex Superflow valves? Cyclex will likely be doing the head work and the KPMI valves are more expensive.

On another note, also decided to order some Renthal bars and a new Nissin master cylinder as I have grown tired of the clubmans. Will update once the parts are installed.

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seanbarney41 · « **Reply #6 on:** June 09, 2018, 11:27:35 AM »

I don't think a ripped/leaking puck would cause cam/valve train damage. Have you verified the presence of the oil jets and the 4 associated o-rings and made sure the oil passages in the cam towers are clear?

dtran · « **Reply #7 on:** June 09, 2018, 02:10:29 PM »

Looked like the oil passage actually got clogged by bit of the torn cam puck and some of the sealant I used. I had used Permatex 2 but this time will probably try some hylomar.

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70CB750 · « **Reply #8 on:** June 11, 2018, 04:36:05 AM »

When building Red (it has 78 F engine) I went with bronze guides and valves and retainers from CycleX. Also new tappet adjusters.

kixbox · « **Reply #9 on:** June 11, 2018, 07:56:09 AM »

Hi -

I am impressed with the bike - well done bud.

1) Would it be possible to share a few pictures of the under side of be seat and how it is attached to the frame? I like the dimensions and it looks as though you did not cut the frame tail but can’t be certain.

2) did you paint the exhaust? What did you use to get that nice black? Mine is super rusted from the outside and would like to have a similar look as yours.

Thanks in advance.

dtran · « **Reply #10 on:** June 14, 2018, 05:36:46 PM »

Kix,

The seat is a long base seat for a CB750F that you can get from either Carpy’s or Airtech.

I am attaching some pics that should be pretty self explanatory on mounting, but let me know if you need more. Not the cleanest method but it works. Basically you fiberglass in a metal bar in the seat hump that sits on top of the subframe and bolts up to it. For the front I just used two pipe clamps with screws going through them that than go into the seat. The screws are than covered by the seat pan and upholstery. You should put some rubber any where the seat contacts metal on the frame just for some isolation. I did not cut or modify the frame in anyway, in case I ever want to return to a stock look with a stock tank and seat.

Paint I used in the exhaust is in the following link. It is available in semi gloss.
https://www.rustoleum.com/product-catalog/consumer-brands/specialty/high-heat-ultra/

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kixbox · « **Reply #11 on:** June 16, 2018, 03:41:21 AM »

Quote from: dtran on June 14, 2018, 05:36:46 PM

Kix,

The seat is a long base seat for a CB750F that you can get from either Carpy’s or Airtech.

I am attaching some pics that should be pretty self explanatory on mounting, but let me know if you need more. Not the cleanest method but it works. Basically you fiberglass in a metal bar in the seat hump that sits on top of the subframe and bolts up to it. For the front I just used two pipe clamps with screws going through them that than go into the seat. The screws are than covered by the seat pan and upholstery. You should put some rubber any where the seat contacts metal on the frame just for some isolation. I did not cut or modify the frame in anyway, in case I ever want to return to a stock look with a stock tank and seat.

Paint I used in the exhaust is in the following link. It is available in semi gloss.
https://www.rustoleum.com/product-catalog/consumer-brands/specialty/high-heat-ultra/

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Thanks for that.

The seat you have is well done and seems to me that it has all the right things for it. I may borrow a few inspirations from your approach.

Thanks again

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dtran · « **Reply #12 on:** June 22, 2018, 10:46:14 PM »

Update on the bike. Just got my head back with new guides and porting courtesy of Ken at Cyclex. Going to give it a final clean and slap in the new valves and reassemble the engine.

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754 · « **Reply #13 on:** June 23, 2018, 07:03:58 PM »

The head you just showed is 72 or earlier.
Might mean cutting your knock pins or modifying them.
,it probably had a 410 head on it, bigger ports and valves, different chambers so your compression will change. Easier to get parts for that one.
Check everything while assembling.

dtran · « **Reply #14 on:** June 23, 2018, 11:18:56 PM »

I was already running a K4 head and cylinders before. Cyclex did an exchange on the head and I did notice that this one is an early year head. Will check on the knock pins. Thanks.

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dtran · « **Reply #15 on:** June 24, 2018, 12:39:43 AM »

Knock pins and everything look to be ok. Thanks.

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dtran · « **Reply #16 on:** July 15, 2018, 12:41:00 AM »

Finally had some free time and got the head assembled and back on with new valves and Ape Springs. Next up is to install the cam and degree it in. Just waiting on another set of rockers to come in. The set I got with the towers I bought ended up with a chip in the hard chrome on the edges so I didn’t want to run them.

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dtran · « **Reply #17 on:** November 06, 2018, 03:05:10 PM »

So life got in the way so completion of the bike stalled but it is now starting back up. Due to repeated issues with the flush style gas cap on my old gas tank causing paint bubbling I decided to get a new tank and seat. The tank this time has a built in filler neck so I am hoping paint bubbling will be less of an issue. Just finished fitting the new tank and seat to the bike before sending off for paint while I finish buttoning up the engine.

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754 · « **Reply #18 on:** November 06, 2018, 06:38:51 PM »

Looks good..
I spy a full stack of Kennedy chests in the background.. what line of work are you in ?

SOHC4 Cafe Racer Fan · « **Reply #19 on:** November 07, 2018, 08:27:44 AM »

Quote from: dtran on November 06, 2018, 03:05:10 PM

So life got in the way so completion of the bike stalled but it is now starting back up. Due to repeated issues with the flush style gas cap on my old gas tank causing paint bubbling I decided to get a new tank and seat. The tank this time has a built in filler neck so I am hoping paint bubbling will be less of an issue. Just finished fitting the new tank and seat to the bike before sending off for paint while I finish buttoning up the engine.

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Treat the tank with Caswell or a comparable stout epoxy liner. Otherwise, the ethanol in pump gas will eat through the resin in your fiberglass gas tank.

dtran · « **Reply #20 on:** November 07, 2018, 10:15:39 AM »

Thanks for the tip. I had the tank coated by Airtech while it was made. Last tank was also coated but the issue with the bubbling was caused by gas vapors coming up the flush mount gas cap’s screw holes. Hopefully with the filler neck being glassed in to the tank this time that won’t be an issue.

Those tool chest are just harbor freight specials. I am an attorney by day but do a lot of wrenching on cars and bikes. That and the home remodeling task my wife frequently requests of me.

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MCRider · « **Reply #21 on:** November 07, 2018, 10:42:47 AM »

Quote from: dtran on June 09, 2018, 02:10:29 PM

Looked like the oil passage actually got clogged by bit of the torn cam puck and some of the sealant I used. I had used Permatex 2 but this time will probably try some hylomar.

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From personal experience and observation, the most likely path of problems re your cam and rocker wear, would be, chipping off of the gaslet sealer the squeezed to the inside of the seam, then travled up the oil passages at the rear of the head studs, turning the corner at the head, and sticking in the oil orifices under the cam bearings. That would starve the cam and rockers. OFten the cam will actually seize to the cam bearing block and break in half.

The gasket sealer should be "non-hardening/high tac/ low,no silicone". The torn puck was just a coincidence. Any leakage from there would just make the external top of the head mucky.

OSIT

dtran · « **Reply #22 on:** November 07, 2018, 11:54:46 AM »

That’s probably true. The Permatex 2 I used last time is suppose to be a non-hardening and silicone free sealant, but this time I will be using hylomar and being very careful about excess sealant.

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MCRider · « **Reply #23 on:** November 07, 2018, 01:51:12 PM »

Quote from: dtran on November 07, 2018, 11:54:46 AM

That’s probably true. The Permatex 2 I used last time is suppose to be a non-hardening and silicone free sealant, but this time I will be using hylomar and being very careful about excess sealant.

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I wish I'd kept the cam I twisted in half. I could have made a nice little display wish some whacked pistons and a bent rod.

dtran · « **Reply #24 on:** May 03, 2019, 05:58:09 PM »

Months later my tank and seat are finally painted and I finally have time to continue work on the bike.

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Author Topic: 1978 CB750F Nears Completion (Read 4731 times)