Topic: cb750 no start

[jakec]

So today I'm trying to start my 750 chop... It is a 1978 SS engine running round top carbs. The carbs are recently rebuilt with stock jets and settings. I've tested spark (done static timing) and done other tune up items like valve adjustment etc. The wiring isn't stock, but it should be right as the circuit for the coils is really simple, and I already confirmed spark at the right times while doing the timing.

So I'm trying to start it up and only getting little puffs. Like when you're shoving a bunch of unburnt fuel through the system and it goes puff - puff. I didn't get it to start up for even a second though. I confirmed fuel in all 4 carbs. So right now my idea is to remove the plugs and see if they're all wet, let the cylinders dry out a little bit, and test for spark again to confirm the coils are going off at the right time.

Any other ideas?

[jakec]

One thing I can think of is I spilled a ton of ATF on the connections to the coils last night... I will look into that as well

[jakec]

Ok, confirmed coil wires aren’t crossed or anything. They’re going to the correct coils. Also looked at connections and they’re fine, the headlight connections got oil on them as well and the headlight is working fine.

[jakec]

Also just confirmed spark.

[JerrodR]

Are you Positive about the Valve timing?

[robvangulik]

Ok, confirmed coil wires aren’t crossed or anything. They’re going to the correct coils. Also looked at connections and they’re fine,

Are you sure of that? Describe the order please....

[jakec]

Blue wires are going to the coil that gives spark for 1 & 4 and yellow is going to 2 & 3.

I drained the carbs and put some starting fluid in there and it starts up. So I’m guess it’s a problem with the carbs.

[robvangulik]

Sounds right, first step is then to unscrew the drainplugs to see if fuel is getting to each carb.

[jakec]

That was one of the first things I did. They all had fuel. They are off now so I will check on jets and such

[bryanj]

If it fires on fluid but not on gas suggests to me the bench sync isnt right

[jakec]

Is bench sync enough to make it so it doesn't run at all? Because I never did a bench sync. I totally rebuilt but skipped the bench sync because I was planning to sync with gauges immediately.

[seanbarney41]

Is this a kickstart only situation?  Are you doing anything with the throttle and or choke?

I know that textbook start procedure tells you not to touch the throttle but if you are out of sync than that is not in the textbook.  Every bike has it's own personal starting procedure that may involve different throttle and choke settings.  You may just have to figure out what the bike wants.

[jakec]

Yes I am definitely trying everything there. it has electric start but I like to kick to get a "feel" for it when it isn't starting. so I am trying 60% kick 40% electric start. I am also playing around with throttle. However opening the throttle at all changes the intake sound quite a lot. it goes from high compression to low compression sound if I open the throttle. Idle screw is all the way out currently, but I experimented with kicking on all positions of throttle, from closed to 100%. When I got it started on starter fluid I gave it about 10% throttle.

[seanbarney41]

Then, Maybe try closing down the throttle plates a bit with the idle adjust knob?  Throttle plates open too far for starting the way you have them adjusted after rebuild?

I can't really see even badly out of sync carbs causing a complete no start situation...certainly it may not idle on all 4

[754]

Just look at the slides from aircleaner end, can you get a 1/16 to 1/8 drill bit in there? Has to be part way open to run.
 I use an 1/8  rod  to check slides with idle screw lifting open slightly. . Set all to 1/8 .. gets it close, you can then lower idle on all 4, up or down with idke speed screw..

[jakec]

Right now the idle screw is all the way out out, so it's not opening the slides at all. I think the clearance for the intake side (ellipse shape) is more than 1/8". My plan this morning is to recheck the jets and float level, and then bench sync with small drill bit.

[Medyo Bastos]

Right now the idle screw is all the way out out, so it's not opening the slides at all. I think the clearance for the intake side (ellipse shape) is more than 1/8". My plan this morning is to recheck the jets and float level, and then bench sync with small drill bit.

That’s a good plan. The smaller the better.


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[jakec]

Too far open? This is where it was

[Medyo Bastos]

Look at the other side of the slide


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[jakec]

By eye they are all the same and look like this. Looks good to me. I cleaned all the Jets and found that #2 slow jet was a tiny bit dirty jet and #2 float was a little rich

[754]

What about the other 3 ?
 Make sure the idle screw is lifting rack slightly when bench syncing. That way you can adjust both ways when done.

Need 3 things to run, spark , compression  , fuel.
 If you squirt raw fuel in it  and it tries to run ,  and dies not on its own, then carbs  or fuel deivery are the problem.
 Make sure you do this outside, and use a tiny squirt, or you can get fire..

[jakec]

They are all very close to the same. I have started bikes where they were worse and/or way further open. When I got my 550 I synced using a drill bit at the rear, so the minimum idle was 3,500. But it still ran fine.

[754]

Likely fuel delivery , if it can't get from tank to carb throat , it can't run.
 If you add fuel by squirt in sayb3 holes and it runs, that means crabs are not letting fuel througn. Or  petcock , or airlock.

[Medyo Bastos]

Gas cap?


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[jakec]

I was using an auxiliary tank but I was able to confirm flow with clear filters. Not sure what I did, but doing the stuff I mentioned above, it started. So i am pretty stumped as to why it didn’t start, since I didn’t change much.