Author Topic: Bessie, my first rebuild: 78 CB750F3 (Back on the road 2016-05-12) (Read 70780 times)

grcamna2 · « **Reply #225 on:** April 24, 2015, 05:45:23 PM »

That's a tough negative challenge that's been put on you wohali.I hope you are able to salvage #1 & 4 pistons also. Do you have any pics ?

wohali · « **Reply #226 on:** April 24, 2015, 08:18:58 PM »

Will get pics uploaded tomorrow. left the camera in the workshop and am too upset to want to go back in there just now...

wohali · « **Reply #227 on:** April 25, 2015, 02:55:57 PM »

Good news - everything's back together and the head is sitting for 24h before I torque it again.

Big thanks as always to Chris, my partner of 10 years -- he always has a cool head and helps calm me down. The damage was minor and he was able to file the edges smooth on the sides so there won't be any catching there.

Photos after dinner.

grcamna2 · « **Reply #228 on:** April 25, 2015, 08:34:47 PM »

Quote from: wohali on April 25, 2015, 02:55:57 PM

Good news - everything's back together and the head is sitting for 24h before I torque it again.

Big thanks as always to Chris, my partner of 10 years -- he always has a cool head and helps calm me down. The damage was minor and he was able to file the edges smooth on the sides so there won't be any catching there.

Photos after dinner.

That is great news wohali,it was good Chris helped you over that nasty hurdle;you didn't expect that one,huh? !

wohali · « **Reply #229 on:** April 25, 2015, 09:15:10 PM »

I didn't actually...I'm a derp when it comes to these things. I'm so determined to do it myself, I'm afraid to ask for help. I really shouldn't be - without all of you great people here I wouldn't have even started this project!

Safety in numbers.

wohali · « **Reply #230 on:** April 28, 2015, 02:22:37 PM »

Here's the piston damage. Similar damage exists on the #4 piston. Again Chris was able to file the edges down (and protect the rings with some masking tape.

What happened was that I attempted to apply shrink tubing to the 4 exposed studs so that they would be protected. This made the studs too thick to easily pass through the passages in the cylinder, and I had to apply too much force to get any motion. Eventually the shrink tubing decided to shift and roll up at the bottom of the stud, which caused the cylinders themselves to fall down and dent the 1/4 pistons, which were of course at an angle and not ready for insertion.

I ended up removing the shrink tubing, and everything fits together nicely again.

Tomorrow I expect to get back out there and reassemble the cam; will probably not have time to get the engine back into the frame until Friday at the earliest.

Tintop · « **Reply #231 on:** April 28, 2015, 04:11:11 PM »

Nice save.

bytio · « **Reply #232 on:** April 30, 2015, 08:58:30 AM »

Hi I had exactly same oil burning issue as you on my F2, re did everything guides, valves Pistons ring etc, and still burnt oil. I went up to 1.5 mm oversize (German manufacture for F2). I then discussed with Hondaman and as per his book I insisted on a piston clearance of 0008" inch, the machine shop said it was too tight and would seize, but I insisted, and now, in 1000 miles not had to put any oil at all, so zero smoke and zero oil consumption. Nd did not seize, make sure you insist on the clearance!

bytio · « **Reply #233 on:** April 30, 2015, 09:00:30 AM »

Here are the 1.5 mm oversize Pistons

grcamna2 · « **Reply #234 on:** April 30, 2015, 11:49:16 AM »

bytio,those look like nice cast pistons;I thought they were Wiseco's at first.Who's the manufacturer ?

bytio · « **Reply #235 on:** April 30, 2015, 11:52:29 AM »

http://www.scheuerlein-motorentechnik.de/motorrad/Honda/700-799-ccm-h7/CB750-F2--OHC--886/kolbenkit-cb750f2-1-uem-2398.html

wohali · « **Reply #236 on:** May 04, 2015, 11:33:49 AM »

The machine shop definitely did the tight clearances per Hondaman's recommendation. The shop owner used to race CB750s in the 1970s so I have complete faith in his abilities.

My 0.25mm oversize pistons are NOS Honda ones.

The engine is now fully back together. I hope to get it back into the frame tomorrow with help. Rain and responsibilities will probably keep me off the road until the weekend.

wohali · « **Reply #237 on:** May 08, 2015, 11:50:34 AM »

OK, I've got everything back together. Tried turning her over and....no dice, she won't. Checked for spark at the plug, #1 and #4 look fine. Kickstart and the electric starter motor both sound fine, no loud scraping or jangling or clanking. She just won't start.

I noticed a bunch of fuel leaking out of #3 carb after I tried - from way up at the joint of the bowl and the rest of the body, so I am guessing the float in there is stuck. I guess next step is to pull the carbs and run them down again. With any luck that's all it is - a bad fuel/air mixture preventing ignition.

Restoration Fan · « **Reply #238 on:** May 08, 2015, 05:11:08 PM »

If you have a spare plug, instead of pulling each plug, you can just pull the plug cap, insert your extra plug and confirm that you have spark at that plug by holding the plug to the engine case as you try to crank it. If you have spark at all four, then spark is probably not your issue. So, next check your gas.

Check your float bowls next to make sure there's gas in each one. If #3 is overflowing, then you either need to remeasure the float height there or, as you said, check to see if the float is stuck.

If you determine you're getting gas, then remove the air box and confirm that pulling the cable to full choke is actually completely closing the butterfly valves. If they aren't, you can actually manually close them fully with your fingers just so you can try starting them again.

If all of that doesn't work, try posting a video of it trying to start and maybe someone will notice something that you don't.

grcamna2 · « **Reply #239 on:** May 09, 2015, 06:20:01 AM »

wohali · « **Reply #240 on:** May 11, 2015, 02:54:39 PM »

OK, I pulled the carbs today and the main jets on #1, #3, and #4 were just loose inside the bowls

There was also a small shard of metal inside #3 that I need to investigate still. Either way, that or the jets were probably holding the floats up and causing overflow.

Should I consider blue loctite-ing the main jets in place? Or were these just not fully tightened down from when I rebuilt them a few years ago?

grcamna2 · « **Reply #241 on:** May 11, 2015, 04:13:51 PM »

wohali,
You shouldn't use Loctite on there,just tighten them back up( w/ the brass washer underneath them) up well and re-check the floats/needles & float height.

wohali · « **Reply #242 on:** May 11, 2015, 05:02:06 PM »

No brass washer on '78 PDs is there? At least I don't see one on the diagram.

wohali · « **Reply #243 on:** May 18, 2015, 03:05:40 PM »

OK, put the jets back together and got the bike back together today...still no joy, and still some sort of overflow on #3.

I think I will have to set up the carbs off of the bike with the tank and "pump" manually to ensure they're doing the right thing.

So much for a holiday ride today

SOHC Digger · « **Reply #244 on:** June 08, 2015, 01:07:17 PM »

What parts did you replace when you rebuilt your carbs?

wohali · « **Reply #245 on:** June 14, 2015, 03:00:43 PM »

I used the Honda standard carb rebuild kit, which is all the gaskets but not the needles/etc. I have been ill and unable to get back out into the shop to get the carbs off and start looking at them. They ran fine for 700 miles, then they were taken off of the bike and stored sideways. I'm going to guess that stretched out the float springs somewhat and that they need to be recalibrated. Alternately there is crud that got into the needles and they're not closing properly. The final possibility is that the accel pump is stuck open, maybe?

I'm going to (when I feel up to it) get the carbs running correctly off of the bike, which means building some sort of little jig so that they can stand up without being held up, and then some sort of little shelf for my fuel tank and some new tubing so I can have it all running "on the bench." I should be able to pull the accel with a pair of pliers or similar to see if it squirts correctly.

grcamna2 · « **Reply #246 on:** June 14, 2015, 03:44:00 PM »

Quote from: wohali on June 14, 2015, 03:00:43 PM

I used the Honda standard carb rebuild kit, which is all the gaskets but not the needles/etc. I have been ill and unable to get back out into the shop to get the carbs off and start looking at them. They ran fine for 700 miles, then they were taken off of the bike and stored sideways. I'm going to guess that stretched out the float springs somewhat and that they need to be recalibrated. Alternately there is crud that got into the needles and they're not closing properly. The final possibility is that the accel pump is stuck open, maybe?

I'm going to (when I feel up to it) get the carbs running correctly off of the bike, which means building some sort of little jig so that they can stand up without being held up, and then some sort of little shelf for my fuel tank and some new tubing so I can have it all running "on the bench." I should be able to pull the accel with a pair of pliers or similar to see if it squirts correctly.

Good plan on the bench testing of your carbs.

SOHC Digger · « **Reply #247 on:** June 24, 2015, 08:47:43 AM »

You don't need to make a test stand or anything. Just leave the airbox off your bike. You can see the accelerator pump nozzles in each carb and observe them functioning. Your overflow on #3 is most likely due to the float valve not closing completely.

h0tr0d18 · « **Reply #248 on:** January 22, 2016, 01:54:22 PM »

Has there been any progress on this? I just read the whole thread and I always hope to see these things worked out.

grcamna2 · « **Reply #249 on:** January 22, 2016, 07:51:25 PM »

Quote from: h0tr0d18 on January 22, 2016, 01:54:22 PM

Has there been any progress on this? I just read the whole thread and I always hope to see these things worked out.

+1 what's up wohali ?!

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Author Topic: Bessie, my first rebuild: 78 CB750F3 (Back on the road 2016-05-12) (Read 70780 times)