Topic: 1974 CB750 wont start back up! help!

[exf5003]

My apologies in advanced for I am a newbie and this is my first bike    I like the challenge of owning an old bike and want to learn the ins and outs of this bike.  I purchased the bike not so long ago to get it ready for riding season and it was riding like a champ then.  Last week I took it out for a spin and after about an hour of casual riding I came to a stop light and when I pulled out I noticed that the throttle was not picking up normally and I could hear a difference in the engine noise.  I hit another stop light and when I down shifted from 3rd to 2nd the engine cut out on me completely.  I was stranded for about 10 minutes when I kept trying to start it back up with both ignition and kick starter.  I finally managed to get it going again and it felt fine again.  I had to let it sit for a good week due to weather but now I cannot get it started.  She has new battery fully charged, new spark plugs with a Dyna electronic ignition, newly rebuild carbs with new jet and gaskets.  There are no leaks anywhere.  I have tried repeatedly with ignition and kicker and only managed to get it started once with full choke and full throttle but died seconds after if it was not throttled.  Even full throttle read low rpms.   I noticed that #3 and #4 was not hitting when I did get it to start with pipes being cool and other two hot.  I have let it sit for a day before trying again several times with no luck and scared I could have flooded.  It sounds like it wants to turn over but never does.   I have been reading everything I can and searching for similar situations with no luck.  I was told to check for spark by inserting screw driver to spark plug boot and grounding close to the engine to see if it sparks when hitting ignition (any advice on doing this procedure correct??) .  If plug boots all checks out, any ideas what else could cause this or where I should start???

[Bikecrazy]

I have a 74 cb750 I have to keep the fuel tank  half full or it will do the same as yours bike ?

[exf5003]

That doesnt seem to be the problem because it is only 1/4 full right now and nothing... thanks and i'll keep that in mind.

[DJ_AX]

Fuel delivery problem? Check your filter and fuel lines. And the tank vent hole.

More fuel in the tank will help too!

[DJ_AX]

Oh yeah and sometimes when they sit for too long the carbs can get gummy...
Learn to love cleaning the carbs! 

The more you ride it the better!

[exf5003]

Yea I'm going to check filters, fuel lines, and for sparks.  It really confuses me why it shot off after a good ride with plenty of fuel left when the carbs have been recently rebuilt completely.  I've been doing a lot of reading and watching instructional videos for carbs and i'll look forward to the challenge if it comes down to it. Any suggestions if problem persists after that? thanks in advance.

[dylboss]

Could be that some debris got into the jets after passing through the petcock filter.  I had a bunch of problems with this on the fuel intake on the carbs on my CB450.  Had to pull the carbs multiple times to clean the jets.  I would fully flush your tank with water to get all debris out, then rinse with some fuel additive (water absorber, like HEET), then immediately fill with gas. 

Not all would agree with this, but if you have some inner tank rust, install inline filters from your tank to the carbs.  Short term fix, but it will get you going.  Long term, you should use a rust remover and seal the tank.  Por-15 or Kreem are good products.  Do a search on that, you'll get more info than you need.

[m00ntan]

I purchased the bike not so long ago to get it ready for riding season and it was riding like a champ then. 

Last week I took it out for a spin and after about an hour of casual riding I came to a stop light and when I pulled out I noticed that the throttle was not picking up normally and I could hear a difference in the engine noise.  I hit another stop light and when I down shifted from 3rd to 2nd the engine cut out on me completely.  I was stranded for about 10 minutes when I kept trying to start it back up with both ignition and kick starter.  I finally managed to get it going again and it felt fine again.  I had to let it sit for a good week due to weather but now I cannot get it started. 

She has
new battery fully charged,
new spark plugs with a
Dyna electronic ignition,
newly rebuild carbs with new jet and gaskets.  There are no leaks anywhere. 

I have tried repeatedly with ignition and kicker and only managed to get it started once with full choke and full throttle but died seconds after if it was not throttled.  Even full throttle read low rpms.   

I noticed that #3 and #4 was not hitting when I did get it to start with pipes being cool and other two hot.

I have let it sit for a day before trying again several times with no luck and scared I could have flooded.  It sounds like it wants to turn over but never does.   I have been reading everything I can and searching for similar situations with no luck. 

I was told to check for spark by inserting screw driver to spark plug boot and grounding close to the engine to see if it sparks when hitting ignition (any advice on doing this procedure correct??) . 

If plug boots all checks out, any ideas what else could cause this or where I should start???

Welcome to the forum. 

This is the place for for help for your problem.

I cut up your post above to the way it can be diagnosed, I'm not qualified, but you'll find someone who is.

The tank vent is the easiest to check. 

Pull the gas lines off the petcock and open the petcock to see if you get fuel. 

If not, open the gas cap.  If you still don't have fuel, sort the petcock. 

If you have fuel, close the petcock, put the lines back on, open the float bowl drains and the petcock. 

If you still don't have fuel, pull the carbs, clean them, and install  in line fuel filters, high flow.  Then Por 15 your tank. 

The plugged vent cap lets you go a bit, but dies when the carbs are starved.

Is yours Boss Maroon?

[exf5003]

i checked to see if I was getting any fuel. NOPE.  i took apart petcock and cleaned it out. I dont think i was getting any fuel because i didnt realize how long the valve rod is on the petcock.  It seems as if i need to have at least half tank at all time.  How anyone tried to cut down the rod 1/2 or 3/4 inch to try to squeeze more available gas???

[exf5003]

I finally did get her started Thank you all very much for the help.  But I did run into another problem, my battery is not charging I had a full charge on the battery and took it out for a good spin for about 2 hours.  Once i got back  and had her off for 30 minutes the battery wasnt able to start the bike back up and was completely dead. I talked to a guy at my local auto parts shop and he said he had his stator go bad once and to start checking there.  Any suggestions from you guys? or anyone know of another thread that went over this?  The search function isnt working for me.

[phil71]

the petcock reserve is supposed to take gas all the way down to the opening for the valve. if you get nothing on RES, it's time to rebuild or replace that petcock.
  Stators don't fail as often as regulators and rectifiers.. but first ferret around for connections that look stressed.. by that I mean white connectors that have browned or melted from heat, or spade terminals with a similar look. Also, if you see any wires with aftermarket connectors or 'twist-n-tape' repairs, I'd be suspect.

[Duke McDukiedook]

How old is your battery, if you don't know odds are it is time to replace it.

[exf5003]

The battery is brand new and I got it checked to make sure it wasnt the problem.  I will start looking at all connections and wires. Are their any procedures I can follow for my year and model to check the regulator, rectifier and even the stator?

[LesterPiglet]

There are guides in the FAQ section.

http://forums.sohc4.net/index.php?topic=1012.0

[Duke McDukiedook]

Odds are the problem is not the stator electrically speaking, the rectifier or the regulator would be the places to look first.
Dowload the guide Lester poited you to for stepbysteps.

[exf5003]

I have been away all weekend but am getting ready to check what is causing this.  Quick question, if i find that the regulator or rectifier is the cause of this issue, would it be better to replace both the reg and rec with a combined reg/rec similar to the one found in electrosport? http://www.electrosport.com/street-bikes/honda/cb750k1-k8-f1-f2-sohc-71-78.html?pt=regulator-rectifiers