Topic: ** First CB750 - Starting Trouble - help needed **

[Ericcb750]

Hey guys,  My name is Eric and I've just finished a full resto/ rebuild of my first CB 750K . It's a 1978  with 17K miles on it. I cleaned up this bike completely, powder coated frame, polished / painted engine, etc. I've put a lot of work into this bike and I love it. I'm just having trouble starting it. After double checking the carbs, spark, points/condensers, etc. I had one more thing that I thought may be the problem. I had a 50/50 chance and with my Luck I lost. When I installed the camshaft after the rebuild, I lined it up perfectly with the timing marks but unfortunately the darn 4th cylinder was on the compression stroke instead of the 1st cyl. It is my first rebuild so I'm still  learning , but damn really? The crank turns smoothly, no valves are hitting this cylinders or anything.  Now I have to pull the whole engine again and fix it. I dont' think there really is a way to fix this without having to pull the motor again? If anyone knows how , any tips would be a huge help. Thanks everyone, any help is really appreciated .

[rklystron]

The cam is installed correctly by what you have indicated. You might want to check and make sure the ignition advance is not 180 out. This is easy to mess up on and I have do this myself. Also make sure the coils are wired correctly. Blue wire 1-4 yellow 2-3. I should ask if the bike fires at all?

[Ericcb750]

Hi Thanks ryklstron,    Yeah the bike fires, starter turns over great and if I give it a few blips of the throttle it even starts, putts a bit for a few seconds then dies. I appreciate your help as I said I'm just learning how to do most of this stuff. The advance is the plate that the points are on correct? I know the points are gapped to .014 and i static timed them so that they spark when the "F" is matching the index mark for both 1-4 and 2-3. Maybe I have to turn the entire plate and then re-gap them ?

[LesterPiglet]

Try reversing the blue/yellow wires to the points and see if it makes any difference.

[bjbuchanan]

Definitely exhaust all your other options but when the advancer is reversed you usually just get some louder pops and not much else. Check the blue/yellow bullets down near the rear engine mount, they are easily overlooked and pop up as trouble sometimes

[iron_worker]

I agree with above. If your cam was installed as described it's fine. There are a lot people out there saying they installed their cam "180 out" but really you can't install it 180 out. It will work either way due to the wasted spark ignition (cylinder fires at top of exhaust stroke and top of compression stroke so both 1/4 or 2/3 fire every revolution.

If you lined up your "T 1-4" mark at your points and put the cam in with the line facing directly up or down ... it is fine.

Also agree with others that if your points are 180 out that it won't start up at all.

IW

[Vinhead1957]

Pull the plugs and check for proper spark and presence of fuel

[Ericcb750]

Oh alright cool, thanks guys. That makes me feel better about the cam thing. I'll double check the points wires/connections too. In the meantime I'll clean out the carbs again but you guys are probably right it having something to do with the timing because the carbs are really clean. Thanks for all your help as i'm new to this. I'll keep some updates with how it turns out.
   
  Iron Worker - what do you mean by the points being 180 out? Sorry i'm not sure what you guys mean by the ignition advance being 180 out? Thanks again.

[Ericcb750]

 Hi Calj737, thanks for your help. I just want to make sure i understand correctly. The advance is behind the points plate right? So if I just flip this around 180 deg that could be it? I will definitely try that. I just read on one of the other forum a guy who had what sounds like the same issue but somehow his wires for the points were reversed. When he swapped them around the bike fired right up perfectly, so I'll check these things out too. Take care guys, thanks again.

[LesterPiglet]

I mentioned the wires up there ^^^   

[Ericcb750]

Hey Lester, thank you for your help. If the points are wired to the correct Coils right now (blue - blue and yellow-yellow) if I switch them around to check if this is the problem, will it ruin anything in the system?  I apologize I'm a real rookie to this whole point's thing. Thanks for your help.

[MoMo]

Make sure the spark advance is not frozen.  I had a similar problem a couple times and found the spark advance was rusted/seized, so the bike would sort of start but you couldn't give it any throttle without the motor dying...Larry

[LesterPiglet]

Hey Lester, thank you for your help. If the points are wired to the correct Coils right now (blue - blue and yellow-yellow) if I switch them around to check if this is the problem, will it ruin anything in the system?  I apologize I'm a real rookie to this whole point's thing. Thanks for your help.

You won't do any harm.
Just to clarify, plug the yellow to blue and the blue to yellow.

[jonda500]

I've never had a 750 but with the smaller fours the only way to end up needing to switch the yellow and blue wires is to either install the cam 90 degrees out; or assemble the advance mechanism with the points cam 180 degrees out. If you have it wrong it will not start at all - just pop&bang or if you're really unlucky start a fire around your carbs(it happened to me!)

[Ericcb750]

I'm not sure if am misunderstanding this, But If I switched the Blue and yellow wires wouldn't the 2-3 plugs fire when the 1-4 plugs are supposed to a vice versa? I saw something about the Wasted spark system so that the 1-4 fire on both the compression and exhaust strokes. I just cleaned the carbs like a surgeon last night so I will hook them back up and see if that helps. But I do think it's something to do with the timing.   I can't get the engine to run long enough to strobe time it but if I  advance or retard the timing by turning the whole points plate would that help or probably not ?

[Ericcb750]

Ohh alright I see now. Thanks  you. So if it is off, I can remove the points plate, take off the sprark advancer unit behind the points plate and flip it around 180 degrees ? The plate that you're referring to is the Plate that the points attach to or the one behind it with the "T" "F" 1-4  & " marks ?

[LesterPiglet]

And swapping the wires would verify this without taking anything apart.

[MikeKato]

Pull the plugs and check for proper spark and presence of fuel

Eric never said if he tried that but yes,that would definitely be the first thing to do before he starts repeatedly pulling carbs for another cleaning or switching anything other than the coil wires around. Only takes a few minutes and a spare plug to see if he's getting spark. If that don't work,then deal with why. I always file points and make sure plug wire ends at the boots aren't corroded and the plugs are clean before I even try to start a bike that's been sitting. I usually have to clean a plug or two a couple times at first start-up because I put a teaspoon of oil in each cylinder. I give each rocker a good tablespoon of oil also.  Unbelievable how corroded it sometimes gets up in those plug boots.

[Ericcb750]

Thanks everybody for your help. I'll check this out after work. I put new plugs in also clipped the plug wires back and put brand new boots on as well. I'll make sure the wires aren't switched either. This is my first time doing all this so i'm learning as I go.

[Ericcb750]

Hi Guys,  Alright so I got to do some work on the bike this weekend. I did try to switch the coil wires (normally blue-blue for 1-4 and yellow-yellow for 2-3) which is how they originally were. I switched them around and got nothing so i'm pretty sure that is not it. I'm wondering if it may have to do with the way the valves are adjusted? I tried playing with the points and everything but still got the same issue, Starts runs (doesn't sound quite right) and then dies.  I'm going to go through again and make sure all my valves are adjusted correctly. I know the plugs are getting fuel because they're wet with little drips of gas and they are getting a tiny bit of color, they also smell like gas. I'm getting overly frustrated with this thing but i'm sure it's somthing little.  Has anyone ever adjusted their valves incorrectly? If so, what were the symptoms ?   thanks for your help guys.

[Stev-o]

Plugs are wet, so not firing?   Have you gapped your points and checked plugs for spark?

[Ericcb750]

Hey Stev-o , thanks for the reply. I have gapped my points and checked for the spark. The 2-3 plugs have a nice fat blue spark and the 1-4 seems to have spark intermittenly, they will spark when turning the crank with the ignition on and then sometimes the spark will be between the actual points themselves, it's strange. The points are a little burned (greyish) so maybe I should put in a fresh set of points and new condensers and that might help? I'm new to all this so i'm not sure if that will help.  I haven't tried removing the points plate , advance cam and rotating the crank 180 degrees and then re-installing the advance cam. that's the advice i've received from a few people but haven't gotten a chance to try it yet.

[MoMo]

I mentioned earlier to remove the advance mechanism to make sure it is not seized.  I had a couple bikes that would not fire or barely run at idle that had seized advancers...Larry

[Ericcb750]

Yeah I did remove the points plate and pull off the advance mechanism , the springs and everything work great it opens and closed perfectly, no rust or corrosion at all. Forgot to mention that.

[MikeKato]

Did you pull a points file through the point contacts that you said looked discolored? Did you try using a plug in the caps (one at a time) while the plug is resting on the motor to see if you have spark? You said you have fuel to all jugs. Might want to also try turning it over in the dark(garage lights off) with the kill switch on to see if your coil wires are arcing out before the spark reaches the plug.