Topic: Rebuilt CB400F no start sitiuation

[Flyin900]

I just restored a CB400F with a hone job and new OEM piston rings and complete carb rebuild and I am getting a no fire situation where there is no attempt of the motor to fire. The carbs were rebuilt with OEM brass yet aftermarket Japanese fuel valves and seats all set to the factory spec.

This is what I have checked so far and what I have observed is there is no fuel appearing to get into the motor. After copious amounts of cranking and twisting the throttle the new plugs still appear dry. I have tried some quick start in a couple of cylinders and replaced the plugs and still not even a fart. This I find kind of strange, as there should be at least some signs of life.

There is spark at all the plugs and fuel in the bowls when I opened the drain screw on all four carbs. I reset the points to .012 at the highest point on the points lobe and that is the only adjustment the FSM suggests to set them initially. I did a compression test to confirm there is decent compression and they all read about 120, which for a new motor and rings not seated I feel should be good enough to get it firing at least.

It seems carb related and the clear tube method maybe another test I need to do to show if the fuel level is too low. I am going to try some quick start shot into the carb sync screw holes of each manifold tomorrow and see what that yields.

Anything I am missing or other suggestions for getting this bike to start?

[calj737]

The one thing I would double check is the valve adjustments. Sure, your compression readings indicate they should be close enough to get fuel to the cylinders, but an absence of a wet plug says lack of vacuum to me (all other things being “correct”).

[Yoshimatic]

It’s definitely a carb problem, you are right to first check the fuel bowl level. If the fuel bowl level checks out ok then you have a problem with the primary circuit, either the air screw is screwed all the way home or the primary jet gallery is blocked. I assume that you have tried with the choke on? The bowl fuel level too low would be the most likely problem since the carbs shouldn’t have blocked galleries being rebuilt, unless they were not confirmed clear with a shot of air.

[scottly]

I have tried some quick start in a couple of cylinders and replaced the plugs and still not even a fart. This I find kind of strange, as there should be at least some signs of life.

By "quick start" do you mean starting fluid (ether)?? If yes, you don't have a carb problem, since the ether is the fuel. Since you do have spark on all cylinders, I would suspect your ignition timing is 180* out. Did you disassemble the spark advancer unit?

[Yoshimatic]

Also check that the blue wire and the yellow wire from the points plate are going to the correct coils. But you say that there is no fuel on the plugs at all? Can you smell fuel on them? I have never had start aid ether help start anything apart from a Diesel engine.

[Flyin900]

The advancer was off the motor yet it can only go on one way when reassembled I believe, since the alignment tab needs to be in the hole in the crank. Am I missing something else here and can it be put on 180 degrees out?

I will check the air screws since I am not sure I did reset them to spec. The valves were set to spec and when I bring it to 1 cylinder TDC both have loose valve arms, yet I will recheck that too.

Will also check the fuel level in the bowls.

Thanks for the tips

[Flyin900]

Ok this is what I have determined so far with only tests done and no attempt to start the bike again with starting fluid.

The clear tube test method shows two carb levels very close to the top of the float bowl with one probably even the other slightly below. The other two are about 3-4 mm below the top of the carb bowl at the area where the sides of the bowl angle out to the mounting holes, so a little low by a few mm. I checked the air screws and they are set to about 2-1/4 turns out.

The points and electrical need a second opinion as the right point set has the blue wire going to the right points and the yellow wire going to the left points. On the coils the right hand coil when sitting on the bike is the blue wire and the left hand coil is the yellow wire. yet these are colour coded so hard to get wrong. Thoughts here?

[Yoshimatic]

Ok this is what I have determined so far with only tests done and no attempt to start the bike again with starting fluid.

The clear tube test method shows two carb levels very close to the top of the float bowl with one probably even the other slightly below. The other two are about 3-4 mm below the top of the carb bowl at the area where the sides of the bowl angle out to the mounting holes, so a little low by a few mm. I checked the air screws and they are set to about 2-1/4 turns out.

The points and electrical need a second opinion as the right point set has the blue wire going to the right points and the yellow wire going to the left points. On the coils the right hand coil when sitting on the bike is the blue wire and the left hand coil is the yellow wire. yet these are colour coded so hard to get wrong. Thoughts here?

Just to confirm the spark is at the right place at the right time you can have a spark plug in #4 plug cap and rotate the engine slowly with the ignition on and when the 1:4 points open (spark) the plug in #4 cap should spark with it. If the #4 plug cap sparks with the 2:3 points opening then you have to swap the blue wire with the yellow wire.

[calj737]

After copious amounts of cranking and twisting the throttle the new plugs still appear dry.

If the plugs are dry, then the issue is fuel, not ignition. If you have spark but no fuel, even the headers will become hot due to ignition. Can you confirm this please?

If the points/cam firing order were off, you'd be experiencing backfires, not cranking without fire.

Go back to engine vacuum/fuel issues.

[Flyin900]

Thanks Cal.

I think Scotty wins that big stuffed bear though, as it was the timing advancer that I had apart and the metal cylinder that the points block ride on was 180* out. I just learned something today then, as I didn't realize there was a right and wrong way to position that thing.

Cal, I didn't see any really noticeable soaking of the plugs with gas, yet since they are brand new it didn't show the same either as a dirty plug would. The old nose didn't specifically detect a strong gas smell from the plugs or cylinders either.

It started and I am just setting the timing and then need to get out on it for a proper break in of the rings on the cylinder walls.

Thanks again everyone for the tips as sometimes I am just too close to the issue and need this community as a sober second opinion ... hell even drunk will work if your  right! 

[Yoshimatic]

After copious amounts of cranking and twisting the throttle the new plugs still appear dry.

If the plugs are dry, then the issue is fuel, not ignition. If you have spark but no fuel, even the headers will become hot due to ignition. Can you confirm this please?

No apology needed....
If the points/cam firing order were off, you'd be experiencing backfires, not cranking without fire.

Go back to engine vacuum/fuel issues.

[Don R]

Thanks for the report. Now enjoy that 400F.