"Poor Kid" 1981 CB750K - first build

+1 to HD. I rode the GL today and had full choke for 2-3 minutes, then it gives you a little hiccup and you know to dial it back a bit to half, another 2-3 minutes and the temp guage starts to work and I shut it off. The CM450 I had was the same, full choke to start till idle jumped, then you could cut it to half and start riding and after 1-2 mins shut off choke after warm enough, cold days took longer. If tuning and you can't ride you need to idle with choke on to get the bike to full operating tempurature, then adjust idle, check floats etc. These old honda's are all typically cold blooded and unless its 90+ deg out, need choke when cold i.e. sat overnight or 12+ hours. When I leave at 5 today I will be able to just fire the GL with some throttle and leave choke off because it is 75 and the bike will still be warm from the lunch ride. Hope that helps a bit. You can't expect to fire with the choke and shut it right off after, minimum you'll have to leave it half on until bike is warm, reason is, if you set everything for the bike to run and idle right cold without choke it will be way rich when it gets warm.
 
I'm reading all the above and will respond to each in turn. A good night's sleep and some deep breathing (>.O) did me okay, so I'm giving it another crack.

Just started it again with filters on and screws at 2 turns out. It started strong but got worse after two minutes, with backfires occurring after it got warmer. Letting off the choke murdered it with a shot of smoke out of the #1 cyl/#3 carb air filter and a pop.

No air leak detected at the boots. Fuel flow doesn't appear to be obstructed in the line, I know the fuel inlets and floats are clean.

Based solely on the above, what is going on?
 
Any chance you could post a short video of how it's running so we can hear what's going on?
 
LBC said:
The easiest way is probably to upload to youtube then paste a link here.
Click to expand...


That is correct! If you go to You Tube and copy the link at the top of the page the video will show up here.

http://www.youtube.com/watch?v=twpypAI_wkg
 
Here's a few seconds. If need be I can make another clip with different throttle openings isolated.


http://youtu.be/Ab-PEB8VwRc
 
Sounded like it wasn't running on all four cylinders in the video. You say 2 & 4 headers are hotter. Are 1 & 3 just warm? If 2 & 4 are hot it would lead me away from thinking it was an ignition problem unless you have a couple of duff spark plugs or you have the HT leads on the wrong plugs, as 3 is fed by the right coil and 4 by the left coil so it would prove the coils are working.

In the video it does sound distinctly like my current CB900 project did when I had two bad plugs.
 
The plugs were brand new NGK d9ea I believe, that was quite a few poorly tuned revolutions ago so I should probably pull them and see how they read.

I still feel that this is strongly an air/fuel problem but perhaps that is a confounding element.
 
Also, I ran the coil wires, right coil is only for cyl 3/4 and 1/2 are on the left. They're XS performance coils... Green... And not stock. NGK caps and MSD Ignition wires with solid connections on a new harness. I want to check timing but now I'm at work :(
 
kraptastic k said:
Also, I ran the coil wires, right coil is only for cyl 3/4 and 1/2 are on the left. They're XS performance coils... Green... And not stock. NGK caps and MSD Ignition wires with solid connections on a new harness. I want to check timing but now I'm at work :(
Click to expand...

Ht leads from the coils should be when sat on bike, left coil 1 &4 and right coil 2 & 3 if you have them as above that could be your problem and why you have the backfire.

Should add something to that it also depends on the blue and the yellow wires being on the correct coils blue on the left coil and yellow on the right.
 
Gotcha.

That seems like a simple thing you shouldn't have to tell someone. I had multiple people in the garage taking the tank on and off and somehow never learned that I had done these incorrectly. We'll see how that affects it, probably tonight or tomorrow. I feel real dumb if that's actually causing issues
 
Dont feel dumb. We all do shit like that. I know i have looked right at wires connected wrong and would swear they were right

Sent from my MB525 using Tapatalk 2
 
I have had to fix wiring that was actually on fire while roadside. It's part of, um, the fun.
 
Nah. One of my roommates dropped out on bills in the last month and I had to pay, so I picked up shifts, fewer days off, July 4 with family, etc. Totally stagnated. I contacted a local DTT member and mechanic to consult in the next couple days.

Being broke doesn't limit me from changing screws positions and jets. I'm just pretty lost and not certain of my relative position in relation to a well-tuned bike. I don't really know what a healthy bike engine should sound like. Probably because I built this using the internet!

So for now I wait for payday and a mechanic. I did snatch up a helmet though, not a keeper but it will work for now.
 
Thanks Koon. The new CX500 build is gonna be ultra low budget! I'll link it in here when I get the CB riding.

1981 CB750K
1981 CX500C
 
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