Topic: Misfire #2 CB750 1973 ** Solved - Petcock and Condenser related issues **

[tgtint]

I've been working on this 73' CB750 for a few months when I have time, I cannot get it to run on all 4 cylinders.  Here is the breakdown of what I've done.

Carbs rebuilt twice (noticed improvement over first time, I hit the idle circuit much better this time), float height set to 26mm bench synced.  Unknown jet sizes, my guess is large.

Compression tested 130-135 across the board.

Timing is set, points ok.

Pull a plug on #2 and its getting fuel.

Plug is arcing on the head when tested

Thought maybe a bad coil, so I switched the wires going into the coil, one blue with one yellow and the two black/white wires.  When I did this the bike would not fire, in fact I got a wild backfire from the exhaust! switched back, fired up but now its running even worse.  

#2 is without a doubt not running, #3 seems intermittent, outer exhaust pipes much hotter to touch/wet rag test.  

Any help would be greatly appreciated.  

[Gordon]

Well, 2 and 3 share a coil and points set, so you're correct in thinking it could be an ignition related issue, but in order to test that you have to switch the spark plug leads with 1 & 4 as well as switching the wires going to the coils.  Try that again and see if the problem follows the coil. 

[tgtint]

Tried that, coils work just fine it seems. 

Very odd its #2 and #3 acting like this.  I am noticing a vacuum leak so I'm going to replace all the clamps attaching the carbs to the rubber boots. 

Plugs are wet, they are getting fuel, they are getting spark, timing is good, compression is good, float height is set properly, carbs are getting plenty of fuel.  Aside from a vacuum issue I'm lost as to what else could be causing this but I know the symptoms are all leaning towards a carb issue.  As the world turns.... I'll post up once I replace the clamps and eliminate all vacuum issues. 

[Gordon]

You can test for a leak at the carb boots by spraying some wd-40 or carb cleaner around the boots when the engine is running.  If the idle changes then there's a leak. 

On a side note, please tell me your avatar picture is actually a self portrait.

[Spanner 1]

Try new, clean plugs in 2 and 3 and trim 3/8 inch off the plug wires  at the caps and screw 'em back on.....

[tgtint]

You can test for a leak at the carb boots by spraying some wd-40 or carb cleaner around the boots when the engine is running.  If the idle changes then there's a leak. 

On a side note, please tell me your avatar picture is actually a self portrait.

Yep, started spraying we got a leak!  The clamps are just toast no bite at all from the boot to the carb. 

I will also try the suggestion on the plug wires, fresh plugs too.

Dude, I wish my mom would have let me get the laser portraits.  Although I woulda been snugglin' with my dog rockin a Cosby sweater! 

[tgtint]

Couldn't find any clamps that were acceptable yet.

Decided some HD zipties would work for testing this weekend.

Got new plugs, wouldn't start on choke.  Pulled choke and fired right up.  Idles smooth checked for vacuum leaks with carb cleaner and no leaks currently. 

Goes up through the rpms quite nicely but I'm still not getting any heat to the #2 #3 exhaust pipes even after running for several minutes.  I think I'll post a vid of it idling.

Pulled the plugs on #2 and #3 they have a slight hint of a fuel smell but not nearly like before where they were wet and stank of fuel. 

I'm getting closer!

[tgtint]

Here is a video I just uploaded.  Running it through the ranges, seems smooth but did hesitate when I blipped the throttle.

[Spanner 1]

Your bike's running on 2cyls. if 2+3 are cold!..... I think the fuel delivery from the tank is poor and will only 'fill' 2 carbs, in other words the fuel level in 2 and 3 is below the jets = no running.  Check the flow from the petcock first ( let it run a minute) into a clean container.
Then open the drain screws on the carbs and observe the flow ( again over a reasonable time ) . A quick ' I see gas flowing' is not a good test for gas flow, need to let it run for at least 1 min.  Hope that's the fix.

[tgtint]

First of all, I want to say thank for all the input I really appreciate it.

I doubt it has anything to do with its misfire but I think Carb #1 & 4 are swapped because the drain screw is not facing outwards with either.  I'm positive I rebuilt and assembled them as they were removed but regardless to me it makes no sense for those screws to be facing inwards so they must be wrong.

Attacked carb #2 and drained it, decent amount of fuel came out.  Then opened the petcock and watched to see how much of the container it would fill.  With the bike on its kickstand a steady drip drip, bike upright a very steady flow of fuel came in.  I'm wondering if its worth running, then draining that carb and then taking the contents of that and removing the bowl and pouring it in.  See where the fuel line reaches.  About to go hit carb #3 with the same. 

My plan was to hit #1 and see how that one was going but since the drain screw is facing inwards its tough to reach and I think I must now swap #1 and 4 around to make it right. 

[Spanner 1]

I'm sure your aware, but!... you only need to unclip the carb bowls and move them to the correct carb for drain screws to face left or right.

[tgtint]

I'm sure your aware, but!... you only need to unclip the carb bowls and move them to the correct carb for drain screws to face left or right.

Nope I spaced and did not.  I owe this guy a beer.  Won't hide my newb moments.  Sometimes that right side of the brain just doesn't turn over, a little internal misfire you might say 

[tgtint]

swapped em, ironically carb bowl #1 & 2 are being filled the near identical amounts of fuel.

Bowl #2


When the bike is on the stand it doesn't flow well to #2 but upright its very steady.  I'm thinking of a bike leaning into corners and this would be an issue. 

Tank has plenty of fuel in it, I replaced the gaskets on the petcock recently.  New lines, routed as they should be from the factory. 

I'm missing something, I don't know what but I'm going through the list one thing at a time till I get it.

[tgtint]

The other thing I wanted to add was when I first started getting the bike running I know for sure it ran on three, I can see where the paint on the exhaust is flaking on 1, 3 & 4.  Then suddenly it goes to only 1 & 4

Tomorrow I think I am going to tear through the carbs once more.  Or at least triple check the float height etc...

Also testing fuel flow more from the petcock right now. 

[Spanner 1]

Can ya sit the bike on it's mainstand, turn-on the gas, wait a minute, apply full choke and hit the starter..... what happens?

[tgtint]

friggin 4 into 1 wont let me run the center stand.  I'll stand it up and do that tomorrow AM.  Wish I lived in the sticks, neighbors wouldn't like me starting it right now. 

[tgtint]

friggin 4 into 1 wont let me run the center stand.  I'll stand it up and do that tomorrow AM.  Wish I lived in the sticks, neighbors wouldn't like me starting it right now. 

Encountered a stuck float this morning, had fuel spewing out the airbox.  Cleaned up that mess.  Not sure which was stuck but removed the bowls and checked them out and put it back together and that issue is no more. 

After that I stood it upright and ran.  No luck, plugs on #2 and 3 look like new just smell like fuel. 

I'm beyond stumped at this point.  I've eliminated ignition by switching the coils and it still runs just fine on 1 & 4 correct?  Timing is set properly too.

No vacuum leaks, nice new clamps holding all the intake boots in place properly. 

I'm fighting it but I think I need to take these carbs off again and just examine it all. 

Whats really got me thinking is how it goes from running on three 1, 3, 4 poorly while dealing with vacuum issues to just 1, 4

I feel like I'm going in circles, I'm missing something painfully obvious I can almost feel it.

[tgtint]

Removed the carbs, well Spanner you are a wise man.



Went through the petcock and made sure everything was right.  Its flowing better now.  Still not running on 2 and 3 so I'm attacking the ignition.  I'm sure its related to that at this point.  Although it wasn't getting enough fuel either so this issue seems to be two part.

[tgtint]

Big thanks for all who assisted me, it means a lot!

I attacked the condensers, I must have not tested them properly.  I decided to switch them around to see if the problem would move to 1 and 4.

Ironically the bike fired up on all four once this was done.  Not sure if that means a bad connection was the issue but I have a feeling I'll need some new condensers.  Going back to test everything once more and comb through.  Sounds good though, I need a baffle!

Now I must go through tuning the carbs, idle and move on to the things needed to get it road worthy.  Need a chain, front calipers and registration.