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Missing under load after 1 hard accelleration.

Dave

CB750 Junkie
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Location
Merced, Ca.
Hey guys,

This just happened today. The bike has been running great for a long time. I got a live bee in my helmet while on the freeway so I pulled over to let him out before he got pissed off.

When I took off I ran it up to the redline (7k) in 1st before hitting 2nd gear. (I backed off the throttle and clutched to shift)

Now it's missing under any but the slightest load. It revs fine in nuetral. I tried changing the plugs, no difference.

Going to check the points, maybe compression.

Any ideas?
 
That sounds weird. How do you normally drive it? It's been a couple of days, have you figured anything out?
 
I don't hammer it constantly but I don't putt either. I run the revs up when ever I get an honest chance at it, like getting on the freeway. The clutch is on it's last few thousandths so I keep the shifting smooth and don't push it so hard that it slips, because it will.

The points were sparking, 2-3 real bad, so I replaced points and condensors. (got lucky and found a kit with points and condensors for $25 on Ebay because we are out of condensors here) Idles smoother but still missing under load.

Coils/Plug wires are pretty old. Found the insulation on #3 is actually broken at the coil. Spark was jumping there and #3 pipe isn't as hot as the other 3. Tried some silicone for a temp fix, helped a little. Ordered a pair of new coils from CB750supply. They should show this week.

My compression gauge has ran off but the exhaust smells fine and it sounds fine under no load so I'm hoping the new coils/wires will take care of it.
 
Coils not due till tues but, I had to take the bike on a short ride yesterday, about 30 miles (15 each way). About half way home it suddenly cleared up. I just felt a sudden rush of power and the miss vanished. Then I ran out of gas and switched to reserve and it kept running smooth the rest of the way home.

This morning I fired it up to ride into town and didn't get a 1/4 mile before it suddenly started missing, popping, backfiring, and actually jerking the bike. Running even worse then before.

Update: Sep 8th

Today I pulled the carbs, went through them, #2 had some debris in it from the adhesive "Micheals Reno Powersports" used on the rubber bowl gaskets. Don't ever let these idiots work on your bike.

(I paid these morons to replace all the jets in my carbs soon after I got the bike because it had tons of varnish in the carbs and I had to cram little wires through all the jets to open them up, they didn't even pull the carbs off, took every shortcut they could think of, and the bike ran worse. I had to take it to "Reno Road Riders" afterwards to get the carbs synced.)

Anyways. carbs are cleaner inside and out but it made no difference.
 
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So, the new coils. (Part#17-6823) from CB750 Supply show up. These coils DO NOT fit like the stock coils or match the connections on the stock wiring harness so don't expect Plug & Play but, it wasn't too hard to get them installed.

I mounted them on the bottom of the stock brackets instead of the top (so that the electrical connectors clear the brackets) and cut both the wires from the harness and the new coils, soldered and shrink wrapped them together (since there are connectors on the coils you can unplug) and with a little harness re-working brought the wiring in from the back of the coils instead of in front. This is an improvment as it keeps them clear of the plug wires and out of the way of the gas tank mounts.

I fire it up and it's still missing! The new condensor on 2-3 has totally failed. I put in the old 1-4 condensor and it's back in the happy place.

So, it looks like the coil got so bad it took out the condensor. Then it took out the new condensor in a matter of (runtime) hours.

I went ahead an ordered a Basic Pamco kit and new spark plug caps because dual points are a pain in the ass and if a coil failed, the caps can't be far behind.
 
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As I said above, it turned out that one of the coils got so bad it caused the condensor to fail. The timing was apparently a coincidence. I replaced the condensors 1st, and the bad coil killed another condensor.

Once I got the new coils on and a pair of good condensors it ran fine but, I prefer to plan ahead so after getting it running good again, I went ahead and replaced the plug caps too and then, I pulled the entire points/condensor assembly, stuck it in a box for safe keeping, and replaced it with an electronic unit. (Pamco)

I don't like the wiring on the pamco (it has many uneeded wires) but it wasn't hard to install and works just like it's supposed to.
 
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