Dohc Cb750f: possibly a valve issue

SquidHunter

Active Member
Ok guys, long time no update. I finally bit the bullet and buckled down on the Honda. Completely reworked the wiring. Took my time and did it right. Soldered all the connections and everything. Bike fires up first try and rides amazing. Here’s the kicker: same exact problem! After the motor gets good and heat soaked, the ignition fails and I’m getting no spark. After it cools down, fires back up and runs good. I’ve been riding with a laser thermostat in my pocket because of this. It seems to fail when the ignition model reaches 145 degrees. Then restarts when it cools down by 20 degrees.

I called Gary at power arc, and explained the situation. I’ve literally tried everything from diodes, colder plugs, even a 200 dollar mosfet regulator. Always the same problem, and even weak spark on cylinder one most of the time.

Today I shipped it to him for further testing. He says he can make it right. So far I’m super disappointed with the whole situation. I’ve spent almost as much on this as I paid for the bike


Sent from my iPhone using DO THE TON
 

crazypj

Split personality, I fake being smart
There is something wrong with that ignition. 145f is pretty hot but if built properly it should still be working. If it's got a thermal overload cut out it's set wrong I guess. There has to be a reason it's getting warm though, 120f isn't particularly hot for most motorcycle (or automotive) components inside engine bay area. I used non contact thermometer on the tarmac out side the house last week. Air temp about 89f tarmac temp 154f (and 214f where a truck had dropped some oil earlier) Even Intel chips are ok to 100C or more (had a Xeon running quite happy at 98 C) Just went back through some posts, I said last October to have it checked at 220F but it's failing way before that. Did you tell ignition people you've been online looking for help?
 

SquidHunter

Active Member
crazypj said:
There is something wrong with that ignition. 145f is pretty hot but if built properly it should still be working. If it's got a thermal overload cut out it's set wrong I guess. There has to be a reason it's getting warm though, 120f isn't particularly hot for most motorcycle (or automotive) components inside engine bay area. I used non contact thermometer on the tarmac out side the house last week. Air temp about 89f tarmac temp 154f (and 214f where a truck had dropped some oil earlier) Even Intel chips are ok to 100C or more (had a Xeon running quite happy at 98 C) Just went back through some posts, I said last October to have it checked at 220F but it's failing way before that. Did you tell ignition people you've been online looking for help?
I didn’t tell him i had been looking for help online. I agree 140 isn’t hot for that motorcycle. The jugs get well past 200. It will be interesting to see what he comes back with. It’s like I told him: I need this thing to be a beast on the interstate. A dependable beast. I put close to 30,000 miles on it already, and ultimately I want to run coast to coast on it. I’ll throw away all hopes of it looking good, if I can have dependability and power.

He claims I can have that. We just have to work out the issue. As long as he works with me I’ll try to be patient, but if push comes to shove I’ll go another route.

I like having the simplicity of not having that Honda wire harness


Sent from my iPhone using DO THE TON
 

clem

static fluff
DTT BOTM WINNER
Re: Dohc Cb750f: possibly a valve issue

I know that you don't have the ignition in your hands but something I've done with troubleshooting electronic overheating is to use freeze spray. I don't remember the exact name of the product though it should be common. I had an amp that would overheat and begin to fail. I went down the line and sprayed components with this until the amp would come to life again. Once I found the suspect component, I was able to sit there and keep freezing the chip every several seconds and the amp would stay on and function normally. A similar approach could have worked in your situation. I hope they get it sorted out for you.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app
 

SquidHunter

Active Member
That’s a pretty good idea. I should hear something back next week. In the mean time, I’ll work on collecting a few odds and ends. I need to sort out the cockpit area. I would really like to shorten some cables and the front brake lines.


Sent from my iPhone using DO THE TON
 

CarbsAndCylinders

Careful With That Axe Eugene
Re: Dohc Cb750f: possibly a valve issue

clem said:
I know that you don't have the ignition in your hands but something I've done with troubleshooting electronic overheating is to use freeze spray. I don't remember the exact name of the product though it should be common. I had an amp that would overheat and begin to fail. I went down the line and sprayed components with this until the amp would come to life again. Once I found the suspect component, I was able to sit there and keep freezing the chip every several seconds and the amp would stay on and function normally. A similar approach could have worked in your situation. I hope they get it sorted out for you.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app
Would that product be CO2? It exits an extinguisher at -66 deg C and does not harm electrical components.
 

irk miller

You've been mostly-dead all day.
DTT BOTM WINNER
Re: Dohc Cb750f: possibly a valve issue

CrabsAndCylinders said:
Would that product be CO2? It exits an extinguisher at -66 deg C and does not harm electrical components.
Tetrafluoroethane. Most brands label it as Freeze Spray. Very similar to R12.
 

CarbsAndCylinders

Careful With That Axe Eugene
Re: Dohc Cb750f: possibly a valve issue

irk miller said:
Tetrafluoroethane. Most brands label it as Freeze Spray. Very similar to R12.
Cool! I know that some electronic guys do use CO2 but the product you mention might be more suitable.
 

SquidHunter

Active Member
The good thing is, while the bike has been down I’ve had time to think about what I really want to do. I keep coming back back to the same conclusion: cosmetically, I want to change the tail and tank and after that all I care about is function. My problem with the tank is only that the stock one is worn out. It needs a lot of work. The tail is just unappealing to me. The good thing is, the bike actually rides really good.

I’m really needing to focus in on cleaning up the electrical. I feel like lowering the battery down would be better weight distribution, but I’m limited on how much fabrication I can do at the moment. Aside from that, I’d like to introduce an m-unit. I like the idea anyway. It would be nice to have blinkers and a horn again.

I’m interested in you guys opinions, and experience etc. I’m trying to make the right decisions with this one. Not too many f models available here


Sent from my iPhone using DO THE TON
 

SquidHunter

Active Member
Talked with power arc yesterday. He still wasn’t able to replicate the failure I was experiencing. It has to have something to do with the optical pick up. He’s replacing the optical pick up. He’s going to send me the old unit, and a new prototype.

After I get them, and test them I’ll post another update. I feel like it’s something simple and I don’t want to give up on it


Sent from my iPhone using DO THE TON
 

pidjones

Well-Known Member
The "whale tail" on the F is kinda awkward. (as well as the tank) But, without it you have that HUGE azz tail light hanging out there with gnarly wires running around. The rat bike I did over the winter was missing the piece under the tail that filled it in (and had the HONDA lettering). They are real hard to find, so don't trash yours! I filled mine in by gluing together pieces of ABS from a wrecked GL1800 that the Honda service department saved for me. So, if you come up with an idea for what you'd like to do with the area behind the seat (changing the seat will increase options), using busted-up ABS is an option for putting something together. ABS plumbing cement glues that stuff together real well.
 

SquidHunter

Active Member
The tail is pretty displeasing, but it’s a bit iconic as well. It’s all there. The bike is very much complete, it’s just really showing its age. Im fairly certain I can make a 77 tank fit. I would love to know if anyone has ever fit a 450 or 350 tank on one..... a 70s model. I would like to narrow the top a little more so the motor is more prominent. After all, the motor is what makes that bike special. The good thing (in my opinion) about the f model is the 19 front and 18 rear wheel instead of the 16.
It’s going to be tough to chop it up tho. Everything still functions well. I wish it were an easier decision


Sent from my iPhone using DO THE TON
 

pidjones

Well-Known Member
To make the engine stand out more, just pull the valve cover off and polish the crap out of it, then polish the timing and alternator covers. But yeah, it is kind of hidden tucked under that big tank. A smaller tank is going to dictate a lot more frequent fuel stops, though.
 

SquidHunter

Active Member
I’ve thought about the frequent fuel stops. The truth is after 100-125 miles, I need to stop any way. The bike currently gets about 250 miles to a tank of gas


Sent from my iPhone using DO THE TON
 

SquidHunter

Active Member
I got my ignition back. Gary replaced the module. He said it looked like a pinched wire, which is totally possible. I’m not too crazy about the placement of the exit shaft which was milled by cycle x. But anyway..... Gary sent some new coils too, which I’m pretty excited about. I’ve almost got everything wired up. Should know something by the end of the week. Fingers crossed. I hope this is it


Sent from my iPhone using DO THE TON
 

crazypj

Split personality, I fake being smart
Safer to wire them through a 30A relay, that way you can turn them off when ignition is off. Easier than disconnecting every time
 
Top Bottom