Have Spark, Gas and compression but #2 is not firing?

[bnag0]

My friend just picked up a 1980 CB750c and I am helping him transform it into a cafe bike. We both have little internal motor experience but this is what we are looking at...

The bike has 26k on it.
Bought new plugs
Bought new air filter
Pulled the bowls off the carbs and all the jets are clear (Couldn't get the tops off though) can smell gas on plugs
First compression test (Battery low, engine cold, carbs off) 1-100 2-75 3-90 4-60 (My first compression test ever haha)
We charged the battery and did another comp test and came up with 1-115 2-115 3-115 4-100
put the bike back to running form and ran it for a few minutes #2 still not hitting
another comp test right after it was running 1-120 2-120 3-120 4-115


We have nice blue spark on #2 but maybe while running its not good enough?
What are your thoughts?

 

[Stabler]

I had a similair problem. Turned out my slow jet was drilled out and flooding my 1 cyl. This 'caused it not to fire. Had me chasing my tail for a while since I also had spark, compression and fuel on the plug.
Not saying this is what's wrong with yours, but maybe worth looking at?
Easy way to check this is to keep the throttle half way open while running and see if cyl 2 fires/run (gets as hot as the other 3 cyls).

 

[Stabler]

I guess you've already tried switching the spark plug wires between cyl 2 and 3 to see if the problem follows? As well as moving the spark plugs aroumd and see if the problem follows?

 

[bnag0]

Stabler,
Good call I will look into the jet... maybe switch it out with cyl 1

I did not try switching plugs and wires yet will do that as well

Thanks!

 

[bnag0]

Stabler,
Not sure if this means anything but the pipe on #2 was getting warm just not hot like the rest.. could jsut be heat transfer but the bike was also back firing out of the left side pipes (cyl #1 an #2) when i was reving it a bit... maybe #2 was firing with the correct throttle placement like you stated earlier...

 

[Stabler]

Perhaps.
Try one thing at the time. The easiest first. Switch cables between 2 and 3 and see what happens. After that, switch back and try and switch plugs. If still no difference, take a look at your slow circuit.

Maybe it was heat transfer, depending on for how long you ran your bike. It takes a while to get heat transfer that makes a pipe on a non firing cyl hot. If you just ran it for a few minutes and with half opened throttle and all 4 pipes got equally (or almost) hot, it soumds like the slow circuit. Just try one thing at the time and switch back afterwards if nothing happens, before you try the next thing.

Let us know how it goes.

 

[bnag0]

Thanks a ton man... We are just glad its looking like fuel or spark. We didnt want to touch the motor quite yet $$$$$

If there is good compression could there still be a motor issue?

 

[Stabler]

I don't think a motor issue would cause the symptoms you are describing.

And good compression is a good starting pointon these motors. If you can't hear/feel any bearing noices while running and you got a well working clutch, I'd just make sure you have fresh oil and oil filter and let the motor be. Then I can't see a reason to tear into it.

 

[bnag0]

Well my friend and his cousin swapped the #2 and #3 wires today and that didnt do it so tomorrow they will look into the jets and possibly swapping them with a different carb. It that doesn't do it what should we try next?

 

[Stabler]

Try new plugs as well.

 

[bnag0]

Yep the plugs are new

 

[Stabler]

So, the error stays on cyl 2 when they switch cables between 2 and 3? What happened when switching plugs between 2 and any other cylinder? If cyl 2 still isn't firing then, and you can for certain see/smell gas on the the plug from cyl 2, then it's carb related - most likely flooding that cylinder causing it not to fire. But however, if that plug is bone dry and with no smell of gas, again, carb problem. Most likely a blocked passage/circuit then. This is if the cylinder you now put the old plug from cyl 2 in is firing. If not, start by addressing the ignition problem. Tear into the carbs last - when you are 100 % sure your ignition works as it should.

Plugs foul easily, and are cheap to change out, even though you've got a new set fairly recent. It's just a way to be really sure it's not plug related.

 

[bnag0]

We switched the old plugs around as well as the new along with the cables. It definitely is getting gas. It must be the carb. We have had them off already but we didn't think to check for oversized jets

 

[bnag0]

Is it okay to hook the carbs back up without the air box just as a test? Just wasn't sure with all the vacuum stuff going on between the carb and intake if we should start it without the air box. God I hate that airbox lol. Wish the pods worked better with the cv carbs.

 

[Stabler]

I'd use the airbox if I were you. Or you might get them to act up and masking the actual problem.

Have you checked the float level in that carb?

 

[bnag0]

How do I check the level?

 

[Stabler]

Like this:

http://www.cb750c.com/publicdocs/CB...ru83CB750ServiceManualChapter04FuelSystem.pdf

Page 4-8 (or 54)

 

[Cb75096]

I guess you've already tried switching the spark plug wires between cyl 2 and 3 to see if the problem follows? As well as moving the spark plugs aroumd and see if the problem follows?

I have this problem good compression spark, fuel, no2cyl not firing. Fitted new coils, switched jets with no 3, still same. All exhaust hot but no2 cold. Mmmmmm. I can hear a clicking in rockers, so I'll check tomorrow.