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'78 CB400 "Urban Scrambler" project

Did a fair bit of wrenching on my day off. Nothing major, just working on lots of the little things here and there.

Replaced the rusty rear shocks/springs with the good set from the parts bike.

Removed the front forks and disassembled them to get ready for paint on the lowers, and swapping in the better stanchions from the parts bike.

Swapped over the good rear drum, now my rear wheel can rotate!

Stared at the rear grab bar thing while holding an angle grinder for a bit. Didn't cut anything yet, but I have an idea for shortening it so I can get closer to that scrambler look I'm after.

Since I got spark, I wanted to see if I could get anything to happen with some starter fluid. Sprayed some in the air box, but got nothing. So I took the carbs off and I'm starting the fun process of cleaning them up! Figured I'd have to anyhow. These things are looking preettttyyyyy rough. I'm going to grab an ultrasonic cleaner on my way home from work today and see what that does.

Here's how they look fresh off the bike.

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Not too bad... ..A hand-polishing of the venturi throats, slide-pistons, and inner-sides of the intake "insulators" before or after the ultrasonic broiling will work wonders. If the needles are a Beryllium color and not a polished brass color - PROCEED CAUTIOUSLY ! - I began with 70% Isopropyl Rubbing Alcohol...
 
LongDistanceRider said:
Penetrating oil and a heat lamp for a day first.
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Oooohhhh a heat lamp, that's an idea I hadn't heard of yet. I've already soaked it in PB Blaster, but I can hit it again and leave it under heat for a bit. Thanks for the advice!

Also, I believe you are just a bit South of where I'm located, so if you have any other insights regarding bikes that have sat out in our climate specifically, I'm all ears!
 
I also used a $100 HarFreight counter-top ultrasonic-cleaner, hair-dryer, Liquid Wrench, and a Snap-On impact driver to loosen any stuck Phillips screws…
..which, is “Old News” on this forum… .. “Search” feature function of this VHT Forum is HIGHLY RECOMMENDED!
 
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Well, they didn't have the ultrasonic cleaner I wanted in stock, so that's gonna be a couple of days, but I did break down one side of the carbs. The insides weren't as bad as I expected, I'm a novice, but I think these are serviceable.

Boosted, the needles weren't the color of beryllium, but there was what I can only imagine is leftover ancient gasoline in one of the bowls that was emerald green 🤢

Some elbow grease and a rebuild kit, and these should 🤞🏻 be in working order. Had to stop short of breaking down the other side and fully cleaning them as I had to make dinner and get the kids to bed. I feel like we're getting there slowly but surely though! Thanks for coming along on this adventure with me, more updates as they come.

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TheVinci said:
Stared at the rear grab bar handle while holding an angle grinder for a bit. Didn't cut anything yet, but I have an idea for shortening it so I can get closer to that scrambler look I'm after.
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I use the grab bar handle every time I pull my 400-pound bike back onto its center stand while holding the handlebars straight with my left hand to keep it upright. Can't imagine it being less challenging without it.

Carbs notice : you'll want to replace that "uni-hoser" drain hose connecting the 2 bowl drains with 2 seperated drain hoses long enough to carry the gas to the ground and away from exhaust

Also, the "insulators" or intake manifold ducts that install between the carbs and cylinder heads are not interchangeable between the HondaMatics and manually-shifted bikes and are Unobtainium for the HondaMatics, so treat them VERY CAREFULLY

That "emerald green" residue in the float chamber bowl looks like 2-stroke oil, which some use to coat the inside of empty gas tanks for long-term storage to keep the tank from rusting

And, that needle showing in your post #51 above does appear to me to be the color of the KeiHin beryllium needles - Any identifiers punch-stamped into the large end of those needles, such as the digits 05E ?

Finally, I see either the tiny spring or the tinier metal washer in the threaded hole of the carb body for the pilot mixture screw of that one carb body in the photo above - They're re-usable but the rubber O-ring beneath both the spring and washer is likely done, judging by the condition of the other rubber in the photos...
 
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Boosted said:
I use the grab bar handle every time I pull my 400-pound bike back onto its center stand while holding the handlebars straight with my left hand to keep it upright. Can't imagine it being less challenging without it.
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The goal isn't to remove it, but to shorten it to match up with how I've shortened the seat. Essentially moved it forward the span of the rear mount to middle mount on the bar. I'll get a picture of the finished result, I think it looks good.

I've been taking my time soaking and cleaning the carbs, just got the rebuild kit in the mail today and I'll probably start reassembling them tomorrow.

Boosted: thanks for the pointers, I'll be sure to fix the drain hoses, and be cautious of the intake boots and the beryllium needles.

Question regarding the springs and whatnot that are part of the throttle assembly that connects the two carbs together: are replacements available? I haven't found anything, but I may be using the wrong search terms, and I didn't see anything in CMSNL that looked to be what I'm after, but again, I may be suffering from noobness. A few of mine seem a little frail and I'm worried about them breaking.

Other progress includes disassembling the front forks, and painting the lowers. Fork seal kit going in, and refreshed stanchions will be installed before going back on the bike. I also shortened the seat and made a template for a new seat cover.
 
Gentlemen, we have a runner!

After cleaning and rebuilding the carbs, I got the bike all put back together, and I tell ya, getting those carbs back into the boots is a pain in the ass! Anyway, I grabbed some starter fluid and sprayed it in the air box, gave it some good kicks, and nothing happened. Compression was quite low, ~90psi for both cylinders, with no changes after adding some oil through the spark plug ports. Gave it a rest for a day, as I had a battery being delivered the next day and wanted to see if I had any more luck with the electric start over just kicking it. Battery came the next day, and I got right to it when I got home from work.


Sprayed some starter fluid in, and started cranking it. After some time of cranking and playing with various choke and throttle positions, I got it to turn over and run for a few seconds! Had to leave it at that for the night, but yesterday I put some actual gas in the tank, and got it to start and run for about a minute before it died! So now I'm working on getting it running consistently, and running WELL. First order of business was to check compression again, and it's now sitting at ~145 & 155, so I'm going to check the valves, and see if adjusting those makes a difference. If I need to rebuild the top end, I'm not too stressed about it. Valve seals and piston rings on these seem pretty easy compared to the crap I've had to do on my VWs throughout the years!

I'm also changing out the rusted up exhaust system. The right header was actually rusted all of the way through just on the outside of where it comes out of the head, I'm sure that wasn't helping it. I have an intact set of headers and crossover chamber that I'm going to install this weekend.

I honestly didn't think I'd have it fired up by now, but I'm beyond ecstatic, and looking forward to really starting to make this bike my own.
 
Rusted through headers is for sure not going to help - the lack of back pressure will really mess with your vacuum.
 
EzPete said:
Rusted through headers is for sure not going to help - the lack of back pressure will really mess with your vacuum.
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Absolutely what I was thinking. Unfortunately my left side muffler was also rotted out, so that's going to need a replacement as well.
 
Alrighty, got the headers replaced and looking fresh with some exhaust wrap! Also pulled the valve cover off and checked the clearances, all but one intake valve were in spec, and that one just needed a slight adjustment. Made sure my spark plugs were gapped right too.

Now the bike starts up and runs on gas alone! Dies when I try to turn the choke off though, same thing if I try to give it any throttle. From my limited understanding, this is likely due to the mixture being too lean, correct? So I need to fiddle with the mix screws more. I think I set them to 1 1/4 turn when I put the carbs back together. Any additional advice or guidance here is appreciated. I'm reading all that I can around the web.

Another question: is there supposed to be oil in the stator side (left) cover? I'm getting mixed reports when I try to Google it.
 
Make sure the bike is good and warm before messing with the choke or air mixture. I’m all too often guilty of jumping the gun on that.

If it’ll idle just fine, let it run for a bit the. Slowly start opening the choke. could also be float level as well as fuel air mixture screw. Or that dang lower jet in the VB carbs.
 
It definitely needs some persuasion to stay on idle for the first few minutes, so that's part of what leads me to believe my air mixture needs adjustment. Floats were checked when I cleaned and rebuilt everything, as was the lower jet. I may need to check the intake boots and be sure there's no vacuum leaks.
 
These models are a bit cold blooded so slightly prolonged choke usage is needed.
The turns out in the FSM is a starting point, not final. Need the carbs sync'd first, then a nice several mile ride to get everything hot.
On the center stand adjust the idle to 1200 rpm. Pick a carb and adjust the mixture to the highest idle speed. If it exceeds 1300 reset back to 1200 and adjust the mixture screw again. Once done repeat on the other carb. Go take a 5 mile ride and repeat the setting instructions.
 
TheVinci said:
It definitely needs some persuasion to stay on idle for the first few minutes, so that's part of what leads me to believe my air mixture needs adjustment. Floats were checked when I cleaned and rebuilt everything, as was the lower jet. I may need to check the intake boots and be sure there's no vacuum leaks.
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Usually an intake vacuum leak will cause rapid idle as it leaks out and just kind of takes off on you. Maybe bump the idle screw up a notch and make sure that linkage is sync’d. just getting the washers on the wrong side had my CX acting squirrelly - basically the same carbs.
 
EzPete said:
Usually an intake vacuum leak will cause rapid idle as it leaks out and just kind of takes off on you. Maybe bump the idle screw up a notch and make sure that linkage is sync’d. just getting the washers on the wrong side had my CX acting squirrelly - basically the same carbs.
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Are you referring to the washers under the spring for the mix screw? Should be carb>o ring>washer>spring>screw right?
 
TheVinci said:
Are you referring to the washers under the spring for the mix screw? Should be carb>o ring>washer>spring>screw right?
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That's the correct order for the mixture screws.
However Pete is referring to the sync screw between the carbs, there's 2 washers, one above and one below the aluminum link.
 
TheVinci said:
Are you referring to the washers under the spring for the mix screw? Should be carb>o ring>washer>spring>screw right?
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Yep. Like LDR said it’s this:

If you have the washers out of order, one throttle will open at a slightly later. IMG_0343.jpeg
 
Aha, I see what you're referring to now. I'll double check that, but I'm pretty sure I got that right when I reassembled everything.
 
Pulled the carbs again to double check floats and the aforementioned washers, everything checked out. Discovered a difference in the cb400t vs cb400a service manuals for setting the initial fuel mix! The "T" variant calls for 1 1/2 turns out, while the "A" variant calls for 1 3/4 turns out! Could be a small thing, but adjusting to that for my starting point got me a lot closer to where I needed to be. I also took the opportunity to replace the o rings behind the intake boots. After getting it all back together, it's MUCH easier to start, and doesn't die when the choke is turned off, or when I try to give it throttle. I still need to fiddle with the mix I think, but it's much better than it was. I can even kick start it now, which was nigh impossible before.

So now that I have it more or less running, I'm starting to think more about what I want to use the bike for and how that will affect the build. The question this brings me to is one of tire choice. I live in Washington State and see myself mainly using this to commute to work (a short distance) and the occasional trip to Capital Forest/Black Hills logging roads. Maybe a trip up the Olympic peninsula with a small amount of camping gear for an overnighter. This means I'll be mostly on pavement, and a high likelihood of that being wet in the rain, with a small amount of dirt/gravel roads. I don't see myself using this as a true "off-road" scrambler, but I don't want to be completely locked out of unpaved roads.

So, with that being said what type of tire would y'all recommend? And is that a reasonable request from this bike? I like the look of some of the 50/50 dual sport tires (looks are important to me, but I don't want form over function), but will they be adequate for mostly road use? In the rain? Is there a 70/30 type? Do I even need "off-road" tires for my use case? Tires are expensive and I want to get it right the first time!
 
Well, it was about time for a setback. Up until this point, I've kept the stator and most other wiring unhooked, since the ignition is separate from everything else on these bikes. Figured it would be good to keep that all isolated until I needed it hooked up.

I decided to start buttoning up some of the electronics and wanted to check things as I went. This started with the alternator windings, which read ~0.9ohm between each of the pins, but then showed continuity from any pin to chassis/ground. Well shoot, that's a short to ground for my coil windings it would appear. All of them show 56 ohms of resistance exactly between pin and ground, whether it's a wire or straight to the chassis/engine.

My first step is going to be pulling the side cover off and seeing if it's as simple as the wiring insulation being damaged. If it's not that though, I'll need to weigh my options.

Option 1. Pay the $250 plus I'm seeing for a used stator, specific to the Hondamatic

Option 2. See how much it'll cost for a custom rewind and go that route

Option 3. Attempt to rewind it myself. I'm handy, electronically and mechanically inclined, and enjoy these kinds of projects, so I'm confident I could do it with the proper research, but feel free to tell me why I shouldn't go this direction

Option 4. Swap out the Hondamatic charging and ignition system for the standard parts. I don't mind this idea, and losing the "safety" features for the side stand and parking brake are of little concern. What bugs me about this is that I JUST traded a perfectly good working manual CDI for a working Hondamatic CDI! I have a standard model engine from my parts bike, so sourcing the stator is no issue. But finding another perfect situation for trading the CDI could take time.

I'm on a tight budget already, so I'll likely end up attempting to rewind the Hondamatic stator, and if that proves too difficult or doesn't work, I'll probably end up switching to the standard charging and ignition system.

I know I know, a "free bike is never free", but I'm pretty proud of how little I've had to spend on this so far, and I'd rather keep it that way, ha!
 
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I'd call Custom Rewind and ask how much before deciding.
Option 1 isn't good because it's a used/abused part of unknown quality
Option 4 is ok but loss of those safety and timing features rates it low
Option 3 is interesting but cost of the special wire may be prohibitive.
 
Just got off the phone with Kirk at Custom Rewind. He has a handful of the cb400a stators in stock, and they get a 1 year guarantee and go for the same as I'd pay for a used one of unknown condition. He said it's a common problem on these bikes, and because of the extra pickup coils they're a little trickier to do. I just need to figure out if mine has a single or a double pickup coil, as there's two versions floating around out there between US and Canadian models.

I'll likely never know the whole story behind where this bike came from, but how much are we willing to bet that this issue is what parked the bike in the first place however many ages ago?

Anyhow, getting the replacement from Kirk appears to be my best option, so I'll be saving up for that over the next few months. These Hondamatic stators look a bit trickier to rewind than a standard stator, which is kinda pushing me away from the idea of rewinding it myself. In the meantime I've plenty of other projects on the bike to keep me occupied!
 
Odds are that's why the bike got parked. Knowledge of the SOHC 400/450 was very limited 10 years ago and only found on the old HT forum. When I had an ignition failure in 2011 at Tail of the Dragon there was virtually nothing online to help.
 
TheVinci said:
Well I'm lucky y'all are around to help keep these around and running. Don't think I'd be as far as this if it weren't for the breadth of knowledge accumulated and shared here.
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Yep, it's what we do, and we're not too bad at it either. :giggle:
 
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