Topic: It has a hard time starting, and when it finally starts it dies

[leesfer]

I'm no good at diagnosing problems, here are two videos of what is going on. The first shows me trying to start it and how it usually acts, then the second is what happens when it finally does start up

1975 Honda CB750 having starting trouble

1975 Honda CB750 dying when running

Details on the bike:

1975 CB750
velocity stacks
120 main jets, 42 pilot jets
newly rebuilt carbs
air/fuel screw is 1.5 turns out
pamco ignition

[LesterPiglet]

Sounds like its running on three possibly two cylinders. Check the headers for temperature differences. Check all your plugs for sparks.

[leesfer]

Ah crap, okay I will head out tomorrow after work and check for spark on each cyl and the temps

[lucky]

Jetting is ok for velocity stacks.
Did you check the float heights?

Check the spark on the cylinders with a timing light clipped onto each one.

[pamcopete]

leesfer,

1. Drain each carb bowl to be sure they are all getting fuel.
2. Compare the amount of fuel from each drainage to see that they are all the same. That's a quick check on float height without having to drop the bowls.
3. Do a continuity check from 1 to 4 spark plug caps and from 2 to 3 spark plug caps to be sure all the wires are making contact. If you have 5K caps, then you should read about 25K Ohms between each cap to cap.
3a. Verify correct pairing of the coils: 1 and 4, 2 and 3
4. Change the spark plugs.

[Don R]

Have you changed anything, if yes did it run before?
 Plugged exhaust will cause that, mice will plug an exhaust with their nests or a dumb PO might have wrapped fiberglass over the end of the pipe inside. That's what I found.

[leesfer]

Okay I have tested the wires using a timing light and 1/4 are fine but 2/3 flash intermittently. I didn't get to test for long because the starter killed the battery so I will take a better look when its charged

I will check for continuity tomorrow when I get a chance

[leesfer]

Have you changed anything, if yes did it run before?
 Plugged exhaust will cause that, mice will plug an exhaust with their nests or a dumb PO might have wrapped fiberglass over the end of the pipe inside. That's what I found.

I've actually never had the bike running properly, I picked it up as a non runner and have replaced literally everything on it. Now I am finding little things like this

[LesterPiglet]

Check your points as cylinders 2/3 use one set of points and 1/4 use the other set. It may also be the coil or leads from the coil.

[pamcopete]

Okay I have tested the wires using a timing light and 1/4 are fine but 2/3 flash intermittently. I didn't get to test for long because the starter killed the battery so I will take a better look when its charged

I will check for continuity tomorrow when I get a chance

Dual output coils produce a positive voltage on one plug wire and a negative on the other. The timing light works best on the negative wire, but there is no easy way to determine which that is, so just try the other plug wire in that pair to see if you get better results.

[leesfer]

Well that makes sense.

I will pull the wires and check if each one is really sparking tonight and I'll compare the fuel from each bowl

[leesfer]

I should also add that the bike puffs a bit of black smoke and sometimes white smoke when starting up but seems to go away when it's warmed up (if I can get it to run that long)

Black smoke I assume from being rich while trying to crank it and white smoke from... oil? I know there is some oil seeping from the head right around the #1 cylinder exhaust outlet

I will pull the plugs and take pictures tonight, too

[leesfer]

OKAY I finally got out to the garage and I pulled the plugs to find them all black:



But I connected them all to the wires and everything sparks very strongly

I also did a compression test while I was out there and I got:

Cyl #1: 160
Cyl #2: 145
Cyl #3: 150
Cyl #4: 155

So they are all still within 10% of each other.

Are the plugs okay to keep using? I assume they are since they are still sparking strong

I need to check if all the floats are okay now...

[leesfer]

Pulled the carbs and checked the floats, they are all 26mm from the base point

Angle of the camera makes it look a bit off though, but they're definitely in the right place and move freely

[mrbreeze]

Did you do what Pete said and drain the float bowls to see if they each held equal amount of gas?

[leesfer]

I just connected the tank and let the bowls fill for a sec and checked them right now...

bowl #1 had slightly more than the others, #2 and #3 were identical, and #4 had slightly less than the rest... do they need to be EXACTLY the same amount of fuel? They are all carrying a good amount

[mrbreeze]

What I would do is make sure bike is level. Drain 1 bowl into container. Take that bowl off and pour the gas back into the bowl. You can then see where the level is. Drain the others and pour each into the bowl to see what levels they are at. It doesn't take long to make this check and if you find that one is on the low side .....it could cause a fuel delivery prob. to that carb. If one is too high it could cause a flooding condition. The fuel level should be just a tad below the top of the bowl. I did my carbs a few years back and had low fuel in #3. I bent the float with carbs on bike until the fuel level matched the other three and haven't had a problem with it since.

[mrbreeze]

I just ran back through this thread and read where you said the bike never has ran good and came to you as a non runner. There is no mention of doing a complete tune up. I would start with that before doing much with the carbs. New properly gapped plugs,dwell & ignition timing,check advance, adjust valves & camchain. Clean fuel with good delivery,clean air filter. I don't recall if you said you went through the carbs but if you did.....did you blow all the body passages out real good and all jets & emulsion tubes. You want to make sure they are as spotless as possible. The last step and most important is a vacuum carb sync. If all this has been done, it should be running alot better. Make sure all coil/pointsplate wiring is sound also.

[leesfer]

I bought it as a non runner but I pulled the motor, changed the piston rings, changed the valve seats, replaced the points with a pamco ignition, replaced the coils, replaced the wires, replaced the plugs, replaced the harness, replaced all the electrical... literally everything on this bike is brand new. The carbs were done by a member on this site. And I've ran fuel through them plenty of times to confirm everything is good and flowing

I would love to do a vacuum carb sync but the bike wont even idle and I cannot do the timing properly because it wont idle. I have it set at the starting point for the pamco which should allow it to run....

[leesfer]

After I put it all back together, I am getting backfiring... This is at 1500 RPM, if I go higher it will start throwing fire... it will not idle below this

https://www.youtube.com/watch?v=yRqSSf5ljlo

[leesfer]

I will go through the valves again and adjust them tomorrow, I am dead for the night with this thing

[mrbreeze]

Well if everything tune up wise is good I think I would just static time it and go for the sync. It should idle after the sync and then just go back and time it properly. Then you will have to go back and re-check the sync. You'll get it.

[leesfer]

What RPM do you guys do the vacuum sync at?

[LesterPiglet]

If I was you I would disconnect that tacho. You have too much faith in it. Use your ears. The revs will rise and fall as you sync the carbs.
Syncing is the last thing on your todo list.

[leesfer]

I don't feel confident to use my ears, this is the first bike I've ever worked on and it's a learning experience for me...

What else is on my to-do list? I've already done every suggestion in this thread, this bike is killing me