1979 cb650 resto-mod


You can always get it running with time or money.
Hi folks,

As I mentioned in my introduction, I obtained a 1979 Honda cb650 with 8,1xx miles on the clock. It was a little rough as a result of sitting, and there were some electrical gremlins.

I started by rebuilding the carbs - I used the rebuild kits from a '78 cb750, which fit except for the top cover gasket, which I was able to reuse. I also replaced the accelerator pump and bench synced the carbs. I used a mixture of hot water and pine sol to clean the carb bodies with, and that, combined with some thin copper wire strands, cleaned everything up pretty well. Nothing like the color of clean raw aluminum!

I had discovered that the spark units had melted themselves, and both coil packs were cracked. I was getting a spark, but it was a pretty pathetic one. So, upgrade time! :D

I used this schematic and some GM HEI parts to upgrade:

I ended up using coil packs off a mid 90's Saturn, and made custom wires using non-resistor wire, the stock Honda spark plug ends/boots, and the stock Saturn coil pack ends and boots. I cut the plugs off the melted spark units, and wired in the GM HEI ignition modules, heat sinking them and mounting them to some 1/8" aluminum flat stock. Using the stock plugs made it "plug and play." I mounted the Saturn coil packs in roughly the same place as the stock coils, and the tank covers them up just fine. I can fire a spark about 1/2" now, and in a pinch, use the coils as spot welders! ;D

I also had to re-wind the charging system rotor with fresh magnet wire - the resistance of it was less than 2 ohms, and since Voltage = Current x Resistance, at 12.5 volts, the rotor was using more than 6 amps! :eek: Sadly, this high current draw fried the regulator/rectifier, but I was able to use one out of an '83 Nighthawk 650sc.

I used the instructions below as a rough guide for rewinding the rotor, but I used Kapton tape instead of epoxy to secure the windings together. Epoxy was making the wire "donut" too big.


Anyway, I have 12.5 volts at 1500 rpm, and it just goes up when I rev it, so I'm happy for now.

I had all kinds of problems keeping this thing running, but that ended up being due to there being no air filter or air filter cover on the bike. I found a NOS air filter on fleabay, along with some plastic side covers, but I had to make an improvised air filter cover out of cardboard until I can get one ordered. After all that drama, it starts right up and idles happily.

I also shimmed the carb needles ~1mm with some small washers, as this thing was running kind of lean.

That leads into my current problem - the thing won't rev over ~5,000 rpms. She starts popping and spitting and bogging no matter how much throttle I give. She runs great up to 5,000, but then just falls on her face. Any thoughts as to what I should check? I am going to pull the exhaust pipes off and run it open to make sure there's no obstructions in them, but I'm thinking that's kind of a long shot. Suggestions are appreciated!


You can always get it running with time or money.

What the bike looked like when I purchased it:



You can always get it running with time or money.
What the bike looks like today - I also tossed some cheapo Bike Master lower bars on it, and I think that alone cleans up the look of the bike. The fender and grab bar will go back on for now, but it looks pretty mean now ;D



You can always get it running with time or money.
Yea, if I can get the bike to rev over 5k, it will be very nice! The bike wants to run and is sooo smooth, so it's annoying to have the 5k problem.


You can always get it running with time or money.
I can open the throttle all the way, and it will still spit and fight at 5k. I can sneak up on it slowly, still doesn't matter. It just won't rev over 5k :p


You can always get it running with time or money.
YoungBlood said:
Maybe the ignition advance?
Yea, I'm going to check that out. I checked the timing statically when I first obtained the bike, and tested and lubed the advance weights. Everything looked and moved fine, and the inside of the timing cover was immaculate. However, I've only just been able to get the bike to idle consistently, so I will test the timing and advance dynamically. I think full advance occurs at 3,xxx RPMs, so that's well below the 5,xxx RPM cutoff I'm hitting.

You know, all this just reminds me again that low milage old bikes that aren't ridden are a lot worse than high milage bikes that get regular exercise...


You can always get it running with time or money.
So I was poking around on http://www.allballsracing.com and was amazed at all the late model Honda street bike forks I could swap onto the 650 for the price of a couple sets of fork swap bearings. However, the price of a CBR600 front fork is ridiculous on EBay. Anyone have a good source for sport bike front ends that won't cost more than a new CBR?


You can always get it running with time or money.
I did more work and diagnostics on the bike today, and after testing and fooling with the timing, I think I can safely say that the ignition is messed up. I wonder if the GM HEI modules I'm using have a redline around 5000 RPMs? I haven't heard of that happening to anyone else, but who knows.

I reinstalled the rear fender and grab bar, replaced the fuel tank fuel line, and was just generally annoyed at not getting much done on the bike.

However, I was able to tear down the motor on my Dad's Triumph (car), and it's going out to get hot tanked and the crank cut Monday. Please enjoy these thread unrelated pictures! :D



You can always get it running with time or money.

I was able to get two spark units and two coil packs off a cb750, and bolted them up. That fixed my ignition issues - moral of the story is that the GM HEI mod works, but only if you get very good quality HEI modules.

I also solved a charging issue - a poorly routed wire in my re-wound rotor wore through, preventing the bike from charging. My fault, and an easy fix. I also replaced the cooked regulator/rectifier (killed by the shorted windings in the rotor) with a Nighthawk 650 regulator/rectifier. The bike now charges at 13.5 volts until the battery is full. Very nice!

I also repaired the pair of new to me side plastics I scored off ebay. Devcon Plastic Welder epoxy is wonderful stuff! I was able to replace a broken off tab/pin with an appropriate diameter bolt braced and epoxyed in place. I also repaired some cracks and braced some weak points.

All of this led to a couple of test rides...


You can always get it running with time or money.
The bike idles well, though is hard to get running. I understand this is an issue with the CB650s in general due to the head design. I gather than the head flows so well that there are some cold start atomization issues?

Also, there's a suggestion that installing a relay triggered by the coil positive lead to put battery voltage to the coils might help. I'm going to do that next time I get time to work on the bike.

In any case, once it starts up with some cold start in a can, it idles well. It now revs freely up to redline, hurray for a working ignition system!

However, when I hit the road, I encountered an uncomfortable grabbing/shaking phenomenon when accelerating past 6000 rpms / ~50 mph. It took me a while to figure it out, but I finally realized that the clutch was slipping. This thing only has 8,000 miles on it - how is the clutch shot!?!? I adjusted clutch fork preload to factory specs, but no dice.

Some thinking about the issue makes me think that the clutch springs are probably shot. I threw a Hail Mary and ordered up a set of EBC 10% stiffer springs. They're inexpensive, and if I'm opening up the clutch cover, I'm going to replace the springs. I'm hoping that the clutch plates are ok, but will measure them when I tear into it.

More updates and pictures after the clutchectomy!


You can always get it running with time or money.
A quick update - apparently part of the hard starting issues can be attributed to the choke not closing all the way.

This 650 has plastic choke "fingers" on the carb, and they disintegrated long ago. I cobbled together a fix, but just won an ebay auction for 3/4s of a '78 750 carb rack. I can swap the metal "fingers" from the 750 rack to the 650 rack. Then I'll have a nice rack?

Here's a picture if what I said made no sense at all. If you are interested in any of the other parts off this rack, let me know. I'll be breaking down the rack anyway to get the "finger" out, so if you need a carb body or anything else, shoot me a PM.



You can always get it running with time or money.
More progress today - I installed the replacement EBC clutch springs.

I took a chance and put the bike on its side stand and removed the clutch cover without draining the oil. As I suspected, the side stand caused enough of a lean that only a little oil escaped when I removed the clutch cover. The gasket was toast - it was hard as a rock and took air tools to remove from the cover and the block.

I disassembled the clutch and measured the springs and clutch plates for wear. The clutch plates all had plenty of life left in them, and the springs were the proper length. However, I could compress the old springs by hand, whereas the new EBC springs I could not. I suspect the stock springs are shot after being compressed for ~33 years. In the first picture below, you can see me deflecting the spring with my fingers.

I had to make a new gasket, as the old one was pieces and dust, so I used a technique I learned a long time ago. I took the clutch cover and set a piece of gasket paper on top of it. While holding the paper in place on the cover's gasket face, I tapped it down over the edges of the cover with a small hammer. The relatively sharp edges of the cover cut into the paper, either cutting it almost all the way through or leaving enough of an impression that a razor blade can make short work of cutting out the gasket.

I repeated this process on the inside of the cover and soon had a new gasket. I flipped the cover over and used it to press the gasket against the workbench, and then used a drill to drill the gasket holes through the cover holes. Tada, new gasket. Better than spending $15 and waiting who knows how long.

Sadly, the rain prevented a test ride to determine if the clutch will stop slipping with the new springs installed, but the bike did fire up and rev and run fine on the center stand, so that's something.



You can always get it running with time or money.
Breakthrough time. When I rebuilt the carbs, I didn't set the floats right. I reset them to 12.5mm / 0.5" and the bike runs much better. I also replaced the broke plastic choke lever with a metal unit from a '78 CB750 carb rack, which ensures that the choke closes all the way now. I noticed the accelerator pump arm was pushing the pump rod down about half way even with the throttle shut, so I bent that to the proper clearance.

I also tested all the spark plug boots again, and discovered that one of them had infinite resistance, instead of the ~5k ohm resistance I was expecting. Fortunately, I had three spare boots that I received with the replacement coils and spark units, so I replaced the dead boot.

For the first time since I started working on the bike I was able to start it without using any starting fluid. Couple twists of the throttle to prime the carbs, and after some cranking she fired up. It is still hard to start from cold, but at least it does start.

With all that work, the bike will run hard up to 7,000 RPM, but then fight itself and misfire, barely getting to 8,000 RPM. It seems to be running out of fuel. I picked up a set of 110 main jets to replace the stock 90 jets, so we'll have to see if that fixes the top end problems. I might give the floats a bit more of a tweak to adjust the fuel level to make sure the idle and main jets aren't getting uncovered.

Any thoughts on the float height tweaking? I don't want to flood the bike, but I don't want it running out of fuel either...


You can always get it running with time or money.
I dropped in the 110 jets, and the thing runs like I shot it out of a cannon! It lifted the front wheel in 2nd and 3rd, which was very exciting on sketchy 33 year old rubber.

However, it is running quite rich, as I still have the needles shimmed up two washers each. I think I'm going to remove both washers and see how it runs then. I wish it wasn't such a PITA to remove the washers, but it is what it is. Trying to keep the idle at a steady RPM is hard with the shims in as well, so hopefully I won't have to mess with the idle adjustment quite so much.

Next steps, now that it is reliably running, is to address the rest of the age related safety concerns. The brake master will be rebuilt, the levers will be replaced, a replacement speedometer cable and good used tach will be fitted, the petcock will be rebuilt, the fork seals and dust seals will be replaced, and I will attempt to fit a CB650SC Nighthawk oil cooler. Also, new rubber (Shinko) is on the way. Turn signals would be a good idea at some point as well.

Next steps after that are new paint, and possibly re-fitting the GM HEI ignition system I built. It appears that fuel was the problem, so I'm interested in how the HEI ignition affects starting and performance. Or, I could just be causing myself more trouble...


You can always get it running with time or money.
More work done...fork seals, new tires, new levers, new mirrors.

Pulled and dismembered the forks:

Old crusty hard as rock seals come out:

New seals go in!

Hey, I've always wanted a hover bike! :D

New Shinko tires to replace cracked and hard 33 year old rubber - street/wet compound 712s:

Re-sealed forks and wheel reinstalled, speedometer gearbox regreased, new speedometer cable installed, fender chrome polished, and (not shown) underside of fender cleaned, de-rusted, and painted:

Rear wheel reinstalled, and new bar end Napoleon style mirrors mounted to replace the one bent Yamaha mirror that came with the bike:

Better shot of mirror and new clutch lever:

I pulled the brake master to rebuild it, as the brakes have been spongy despite bleeding and cleaning of the caliper. However, I received the wrong rebuild kit. The kit I received seems to have been for a dual caliper cb650/cb750, and therefore the piston's diameter is too large. No problem, I think, I'll reuse the old piston. Except it is cracked in half in the middle. As they say, "Thar's yar prablem!" Time to order a replacement kit and send the old one back.

For reference when ordering from Parts N More, the correct kit P/N is 08-0231 NOT 08-0210.

Picture of the wrong stuff:

So there she sits, until I get the replacement rebuild kit. I also received a replacement tachometer (cb750's use the same tach), and an oil cooler assembly off a mid 80's CB650SC Nighthawk. I'm not sure the cooler will be a bolt on affair, but we'll see. The '79 CB650 is reportedly the hottest running SOHC 4 Honda ever made, partially because of the cam and partially because of emissions, but hopefully the extra fuel from the larger jets and the cooler will help prolong the life of the oil and the power train in general, and prevent the idle and drivability issues that start happening when a bike gets hot.



Looking good. I really like the 650's. I don't they get enough respect. I like how your fixing everything as you go, too.
As far as forks go, I've found some on craigslist for $150-$200. Sometimes that includes everything like wheel and tire, brakes, master cyl, ect. Keep your eye out, and I'm sure you'll find one. Also try craigslook (claz.org). Not many people know about that one. You can set your distance to travel, too. Good luck.

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