Electrical or Carb Issue?

R

RebelSkyRot

New Member
So my '74 CB750 is nearly done. It starts and idles, but sometimes it'll have fits when I give it some revs. The first time I rode it after rebuild it ran fine, however my mid-range was nonexistent. From maybe 1/4 throttle to a little past 1/2 it would bog, but anywhere past the 1/2 throttle it would open right up. So I figured I needed some bigger idle jets. I think they're the idle jets at least... The ones that are cylindrical, have threads halfway down, then reduce to a tiny cylinder with a few holes in them. They're installed right in the center of the carb. I've got a 4-1 exhaust with velocity stacks, so I've rejetted to one or two sizes bigger than mains, and 142 idles. Stock is 140. Now when I try to ride as soon as I start letting off the clutch it bogs and dies. Once I got two blocks, came to a stop sign, then it died while trying to start again from a stop. I'm confused as to whether this could be electrical or carb related. When this stop sign incident happened, I couldn't get the bike started again no matter what I tried. Then after kicking it over a few times all the electrical went out, lights, horn, everything. I gave up for the day and the next day I tested voltage across the terminals. 12V. I let the engine warm up and revved it and got 14V. Any thoughts? I'm thinking either I've got a ground issue somewhere on the bike that caused all my electrical to fail, or my carbs, probably my needles adjusted.
 
I would assume the "upgrade" of stacks is giving you a good part of the problem.

Zero magic bullet for carb tuning.
You will need a few sets of jets and spend the time tuning it.
Or pay someone that knows a good engine and tune yours.
 
Surffly, I thought stacks were supposed to be an upgrade?! I used to have pods on there, think those would work better?
 
Nope pods are the worst.
Poorly tuned stacks are a close second.

Always best to get a good handle on keeping a bike running 100% stock first.
Just buy a ton of brass and give it to someone with a dyno.
 
i think you have ignition woes, stacks on slide valve carbs should not make it run so bad it wont start in fact it should run pretty well needing only minor jetting tweaks
but like said above always start with a perfect running stock set up and do one mod at a time
 
Thanks for the replies. From all the troubles I've had I would love to have a stock setup, unfortunately I bought the bike with pods and the 4-1, so I've just been trying to make it run the best I can with what I've got, especially since I've hit the bottom of the barrel on this build. xb33bsa, as far as ignition goes, what would you suggest I do? I've got new plugs gapped to spec, and I've set, checked, and rechecked my timing. I'm just most stumped on why it doesn't start up after having been warmed up and riden it a couple blocks
 
have you ever had it running without these issues?
when it dies is all power lost ?
are you sure the tank is venting?
try a test with gas cap loose
carbs are clean ?
coils are good?
you will just have to go thru one thing at a time but if it dies after a short ride it could be coils heating up and breaking down

are the plug caps still old original ? i have seen them fail from the resistor going bad
check the fuse holders
gas tank is supplying a good flow of clean fuel ?
 
I rode it home from the PO's house just fine, well except for all the smoke I was blowing caused by a gouged cylinder (hence the rebuild). The most recent time this happened it was total power loss, but the next day all the electrical worked again (I hadn't touched anything, just pushed it home), carbs are cleaned and bench synced, tank and fuel flow is good, only thing I'm not sure on are the plug caps and coils. I should mention that when that total electrical failure happened I checked the fuses and none were blown. How would I go about checking my coils?
 
get a good quality digital ohm meter(radio shak) it need not be the big one the lil shirt pocket job is a good choice in my opinion
and the test procedure is pretty simple and retset as soon as you get the failure while they are still hot and compare readings
there are other ways i am sure better ones but the idear is to get a baseline reading then one imediately at the time of failure
at the time of failur wiggle the fuses and see if they are hot
 
Cracked coil?

Carbs are not the bottle neck on these motors.
Don't understand why people think the air box is restrictive or that pods/stacks are what the motor needs.

Money spent on the head, the actual weak link, is better spent and will yield better gains.
 
As it turns out, the problem is that I've only been running on 2 cylinders this whole time! While trying to check my spark plugs after the bike was idling I burnt myself on #4 exhaust, then touched #3 to see if it was hot. Cold! So was #1. This to me explains why the bike had such a hard time getting me going. Thought I definitely had either a coil or points problem but then found out #1 & 4 are paired and #2 & 3. Checked the plug caps by switching 4 with 1 and wouldn't ya know it, my 1 & 3 plugs are kaputt. On to ordering some new NGKs and hopefully wrapping this problem up!

Sent from my XT907 using Tapatalk
 
Someone please correct me if I'm wrong, but aren't cylinders 1 and 4 and then 2 and 3 sharing coils. If you're getting cold cylinders on 1 and 3, it might just be that the plug wires are in the incorrect spots?
 
You're spot on. Sorry if I didn't explain that clearly. All this time I thought the pairs were 1-3 2-4, but after looking at my points plate and coils I realized it's actually 1-4 2-3. I was stumped for a little bit, but then decided to switch my #4 plug cap with my #1, since a) they're the same angle of cap and b) since #4 was hot and #1 cold, if I switched them and then #1 was hot and #4 cold, I would know it was because of a bad plug cap. I did the switch and that's exactly what happened, #1 hot, #4 cold.
 
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