Cylinder 1 and 2 not firing - suggestions

[loxley007]

Hi, full rebuild of a 1979 CB750 but never managed to get all four cylinders firing. The bike runs but just on two cylinders.

I have spark, compression, tested the coils (new and have swapped to check), replaced the spark leads. Carbs have been rebuilt and jets replaced/cleaned (including the slow speed jets which were clogged). Valve clearances have all been checked and shims adjusted so they are all within tolerance. I’ve synced the carbs but it was my first time doing it so that may be an issue. Bike has stock air box.

My thinking now is that I should either:

1) sync carbs again and make sure I do it right
2) pull the carb bank and check the jets again

I haven’t done anything with the timing but figured that’s not an issue as surely it would affect 1 and 3 or 2 and 4 rather than 1 and 2.

Any other thoughts or am I on the right track?

Appreciate the help and advice.

 

[dirtdigger]

carb sync wont affect whether a cylinder fires or not. you can spray some carb clean in the non running carbs and see if they fire then you know your not getting fuel. do you have the spark plug wires going to the right cylinders? 1 and 2 would indicate not an ignition issue uless you have the wrong wires going to the wrong plugs.

 

[bbaart]

I would check timing. Do a static timing check.
Also, further to what dirtdigger said, are the wires going to the right plugs? the wires from the left-hand coil go to cylinders 1 and 4. the wires from the right-hand coil go to cylinders 2 and 3. So the order from left to right when looking from above is 1,4,2,3.

 

[93trident]

Hi did you solve this? Thanks

Hi, full rebuild of a 1979 CB750 but never managed to get all four cylinders firing. The bike runs but just on two cylinders.

I have spark, compression, tested the coils (new and have swapped to check), replaced the spark leads. Carbs have been rebuilt and jets replaced/cleaned (including the slow speed jets which were clogged). Valve clearances have all been checked and shims adjusted so they are all within tolerance. I’ve synced the carbs but it was my first time doing it so that may be an issue. Bike has stock air box.

My thinking now is that I should either:

1) sync carbs again and make sure I do it right
2) pull the carb bank and check the jets again

I haven’t done anything with the timing but figured that’s not an issue as surely it would affect 1 and 3 or 2 and 4 rather than 1 and 2.

Any other thoughts or am I on the right track?

Appreciate the help and advice.

Hi did you solve this problem thanks

 

[plantguy]

Hi, full rebuild of a 1979 CB750 but never managed to get all four cylinders firing. The bike runs but just on two cylinders.

I have spark, compression, tested the coils (new and have swapped to check), replaced the spark leads. Carbs have been rebuilt and jets replaced/cleaned (including the slow speed jets which were clogged). Valve clearances have all been checked and shims adjusted so they are all within tolerance. I’ve synced the carbs but it was my first time doing it so that may be an issue. Bike has stock air box.

My thinking now is that I should either:

1) sync carbs again and make sure I do it right
2) pull the carb bank and check the jets again

I haven’t done anything with the timing but figured that’s not an issue as surely it would affect 1 and 3 or 2 and 4 rather than 1 and 2.

Any other thoughts or am I on the right track?

Appreciate the help and advice.

Been out of this fourm for a while picked up a 25 KTM EXC350f and have been busy with it. Did ride the 750 a couple of weeks ago and after sitting for sevral months it started and ran perfect.
Check my post in this thread. Maybe this could be pinned?

https://www.cb750.com/threads/cdi-ignition-module.9296/