Topic: 1978 CB750 F - Barn find restoration (low quality vids added)

[jahmic]

I can't seem to keep myself from wrenching after work.

Yesterday I got lucky and found a trusty craftsman induction timing light on CL for  $15, and promptly put it to use. At idle, I was just barely advanced on both 1-4 and 2-3. I then held it at 2500 rpms to check the spark advance, and was dead on for both 1-4 and 2-3.

I was about to grumble and fidget with the timing when I realized that it was likely advanced at idle since the bike idles poorly under 1500 rpms...and I had it set just above that mark. The carbs have currently only been bench synched, so I'm hoping that once I vacuum synch and recheck the timing things will be in order.

I did double check the accelerator pump, and am getting a solid stream from all 4 posts now. The clearance between the pump rod and the actuator looks good as well...forgot  to grab a feeler gauge to verify it, but I can barely see a gap...it'll get checked tomorrow regardless.

Today I pulled the patched airbox off since I have a replacement on the way. Once that was done, I fired her up out of curiosity. The patched box only secures the halves on one side, so the right side has  a 3" gap going around it. I'm pretty sure that turbulence on one side was causing  problems, because the bike actually idled smoother than it did right before I removed the box. Tomorrow I plan on duct taping the gaping airbox, and throwing some silicone on the poor seals at the airbox side of the boots...the lips are torn in places. I'm convinced that I'll notice a difference if I seal those leaks.

Speaking of air leaks...I temporarily solved the air leak issue at the intake boots. Turns out I forgot that I needed to replace 2 of the clamps since the heads on the screws were rounded out a little. I left them loose...Never did find hose clamps that would work last weekend when I was at the hardware store, and forgot to snug them down.  :/   Got those snug, fired her up...and was able to idle at 1400 smoothly.

Getting close.

Tomorrow I'm going to check the tappet clearances, and patch up  that airbox with duct tape and silicone...just to have something mocked up for now. I'd like to vacuum synch over the weekend so that I can hopefully get the bike to run under 1400 rpms, and then recheck the timing. Once I get  this airbox replaced, I'll go back and try to dial in the carbs. Before I get to do any of that though, I need to source some washers and o-rings for the mixture screws. I'll be on a scavenger hunt around town tomorrow...

I got my fingers crossed that I'll get this bike sorted. So far so good...seems like each step has shown some progress. Sorry for the lack of pics, btw...I'm having computer issues, but will hopefully be back up soon.

[jahmic]

Disclaimer:  I did NOT do this...




I was hoping there would be enough left to get a socket over it, but no such luck. Looks like adjusting the #1 exhaust tappet will be on hold for a while.

[jahmic]

I pulled the carbs this morning...wanted to remove the mixture screws and check the o-rings. When I was initially cleaning the carbs, I had noticed a couple missing washers, and figured the springs tore up the o-rings.

As it turns out, I was only missing one washer...one of the carbs had the washer placed in first, it was hiding behind the o-ring. All the o-rings looked good too, so off I went to find a small washer. I already tried the metric ones at radio shack. 2mm had an ID that was too small, 3mm had an OD that was too large.

The answer came in the form of a #4 lock washer from the RC hobby shop.

Original on left, lockwasher on right.



Bent to shape, and it was a perfect fit.

[jahmic]

Once that was taken care of, I decided to see if taping the airbox up would make a noticeable difference. No change in idling, but the response rolling on the throttle was noticeably much improved. I did get a smoother idle by going back over the clamps on the boots and cranking them down. I also figured it'd be a lot easier to install the intake boots onto the head first,  then attach the carbs.  MUCh easier. I plan on replacing those boots if I can find them, but I'm now  fairly confident that I've limited the vacuum leak to the shot airbox.

I still need to check the tappets, but decided to hook up the manometer and see if any cylinders were pulling significantly more of a vacuum. I figured any drastic differences between cylinders might point out that I still had a leak at the head.

Here's a vid. I think it's evident that I have a leak across the board, as all the levels are low...even after calibrating the manometer and opening up the lines as much as possible (calibration screws on the manometer were turned in all the way).



It will be interesting to see what happens when I replace the airbox. I'm wondering if the leak on that end is bad enough that it's 'hiding' any potential leaks at the intake boots. I should mention that the carbs have only been bench synched; they were that even in the video without adjusting the slides at all...which is why I'm curious to see what happens when I replace the airbox.

I took her out for a quick ride around the neighborhood...running SO much better than when I brought her home. No hesitating, no dying...running pretty well considering the vacuum leak. Running lean though, and the idle hangs when I come to a stop.

I just hope fixing the airbox solves those issues...

Here's another vid. Choke was on slightly here since I didn't have the fan out to let her warm up properly...but you get the idea. Sorry about the crappy phone quality...computer still dead.



Either way, although not perfect yet, I'm confident she.hasnt sound better in years. I really wish I took a before vid...it was just too depressing at the time.

[jahmic]

I can't tell if my attempt to embed the vids worked...so here are the links.

http://static.photobucket.com/player.swf?file=http://vid14.photobucket.com/albums/a333/rmitchell726/VIDEO0038.mp4



http://static.photobucket.com/player.swf?file=http://vid14.photobucket.com/albums/a333/rmitchell726/VIDEO0039.mp4