Cylinder header 3 and 4 not hot/firing?

If it's broken or just needs tweaked

Moderators: Volker_P, tidd650

AndrewCB650
Posts: 32
Joined: Wed Mar 11, 2015 8:18 pm

Cylinder header 3 and 4 not hot/firing?

Postby AndrewCB650 » Sun Apr 09, 2017 4:00 pm

I recently did a ton of work to my 81 cb, including:
Top end rebuild ( new piston rings, honed cylinders, new gaskets, lapped valves,etc.)
New spark plugs
New charging system (reg/rec, rotor, stator, cdi units)
new battery
carbs rebuilt and cleaned, cleaned, cleaned.
cleaned and sealed tank
fork seals and other misc work..

After I did a rebuild, I went for a test ride and everything seemed good except for the cam chain and clutch needed to be adjusted.
Unfortunately, I didn't make any adjustments right after my ride, I ended up letting the bike sit for about a month until I had time to do the maintenance. So today I started the bike up, it started first turn, but the bike was running poor and was dying with the choke. So I checked that all cylinders were firing and when I touched the headers, 1-2 were hot and 3-4 were cold. I assumed that the # 3-4 carbs were not passing fuel, so pulled the carbs and check floats, jets and cleaned them again. I'm still having the same issues, but here's what i tried so far..

carbs cleaned/checked floats and jets
synced carbs
check valve adjustments
cam chain adjusted
made sure all plug wires are in the correct positions
checked timing(static)
checked spark on all cylinders and noticed 1 and 4 coil has spark but looks weak, 2 and 3 nice blue spark, but again 3 and 4 are not getting hot.

apologies for this long winded post, just want to be thorough. All advice is appreciated! Thanks

User avatar
Folsoml
Posts: 1634
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Cylinder header 3 and 4 not hot/firing?

Postby Folsoml » Sun Apr 09, 2017 6:30 pm

Don't take this the wrong way, but what is the process you are using to clean your carbs? I ask this because I knew one guy who kept saying he "cleaned and cleaned" his carbs, when in reality, he was soaking the entire assembled carb bank in pinesol and water.

Also, you say the plugs are hooked up correctly, and I'm guessing since you mention coil 1 and 4, and 2 and 3, that you do have them hooked up right, but just so I feel better, the left coil (as you sit on the bike) runs 1 and 4, and the right coil runs 2 and 3. Make sure that the wires going to the coils are correct also. Blue to the left on and yellow to the right.

Have you changed out the plug wires? If not, try trimming back about 1/4 inch from each end. Sometimes the ends get corroded and they do not make good contact anymore.

What is you compression like?
Have a problem with your CB650? Have a technical question? Click here!


My Current Bikes: 2005 HD FLHTCUI Electra Glide Ultra Classic, 2007 Yamaha Vino,

AndrewCB650
Posts: 32
Joined: Wed Mar 11, 2015 8:18 pm

Re: Cylinder header 3 and 4 not hot/firing?

Postby AndrewCB650 » Mon Apr 10, 2017 5:15 pm

Thanks, I appreciate any kind of advice I can get! I ended up swapping the 1&4 coil out with a spare I had laying around and I also cut back the plug wires(the connections looked bad), i'm now getting good spark on 1&4. I don't think the coil was bad, just the plug wires but I replaced it before I fixed the wires so I'm not sure. I know, I should have looked at the wire connections first! :shock: I always overlook simple steps!

As for cylinder 3, I found a crack in the carb boot and a vacuum leak. I'm not sure how that went unnoticed when I synced the carbs but it was there.

Regarding the carbs; I like to soak all the metal parts in B-12 parts cleaner, usually 24-48 hours.

Compression is good, 130 psi on #2&4, 120psi on #1&3

I went ahead and ordered new plug wires and carb boots.

Thanks!

User avatar
Folsoml
Posts: 1634
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Cylinder header 3 and 4 not hot/firing?

Postby Folsoml » Tue Apr 11, 2017 12:51 am

AndrewCB650 wrote:
Regarding the carbs; I like to soak all the metal parts in B-12 parts cleaner, usually 24-48 hours.




How far are you taking them down? Are you completely disassembling them? Here is a manual on carb cleaning that I feel is the best there is. The carbs are not exactly like yours, but they techniques are the same:

http://www.cb750c.com/publicdocs/SeanG/ ... l_revG.pdf
Have a problem with your CB650? Have a technical question? Click here!


My Current Bikes: 2005 HD FLHTCUI Electra Glide Ultra Classic, 2007 Yamaha Vino,

AndrewCB650
Posts: 32
Joined: Wed Mar 11, 2015 8:18 pm

Re: Cylinder header 3 and 4 not hot/firing?

Postby AndrewCB650 » Tue Apr 11, 2017 9:50 pm

Good info, thanks man!


Return to “Technical Discussion”

Who is online

Users browsing this forum: Google [Bot] and 39 guests