Topic: 1975 CB750F firing order

[rocks]

Now I'm always getting confused which bike is a sohc and which is a dohc. Is a dohc came after 1979?

I have my firing order as 1324 starting from the lest when seating. I looked into this, but answers didn't really much.

Some are saying don't worry about the order, some are saying the order is a necessity(the bike will run better and smoother, and improves mpg).

But most of those posts are ancient. I've been running it like that for a couple thousand miles. 

[goldarrow]

Go by the manual and you'll be just fine and ride.  Who cares about firing order 1234 4321 1324 1423, 16 combinations.  Pick one.  And ride

[Jerry Rxman Griffin aka MuthaF'er]

DOHC came AFTER 1978. 1 & 3 fire together and 2 & 4 fire together.

[Dream750]

For those SOHC 750 owners trying to visualize the factory correct 1-2-4-3 firing order, this may help:

http://www.salocal.com/sohc/tech/fire4-lcnt.gif

(Java required)

[madmtnmotors]

DOHC came AFTER 1978. 1 &   3   4 fire together and 2 &   4   3 fire together.

FTFY. 1 and 4, 2 and 3  (at least on the 78 and earlier SOHC, not sure about the 79 and later DOHC's). Each pair get a wasted spark on the exhaust stroke. If 1 and 3 fired together they would be on the same set of points. 

Sooooo...  1,2,4,3
orrrr....     1,3,4,2
orrrr....     4,3,1,2
orrrr....     4,2,1,3
orrrr....     2,1,3,4
orrrr....     2,4,3,1
orrrr....     3,4,2,1
orrrr....     3,1,4,2

Actual required firing order would depend upon the cam lobes which determines when each cylinder is under the compression stroke (valves closed). This would only matter if using a distributor setup and a single coil...


What does the manual say? 

Please don't make ME look it up!     

I have my firing order as 1324 starting from the lest when seating.

How do you change the "firing order"? Are you using a distributor with a single coil? Otherwise, both the 69-78 SOHC, as well as the 79-83 (and later?) DOHC used dual coils. One coil fires 1 and 4 while the other coil fires 2 and 3 at the same time (one spark wasted on the exhaust stroke) and again, I am referring to the 78 and earlier SOHC, not sure about the 79 and later DOHC's. How do you alter the firing order with the stock setup (other than connecting the 2/3 coil to 1 or 4 and vise/versa, which would result in a very poorly running engine, if it would run at all)? 

Of course, I'm only guessing that the later DOHC stayed with the same 1/4, 2/3 arrangement (as opposed to changing to 1/3 and 2/4 or 1/2 and 3/4 for instance) since I don't have much "hands on" with the DOHC's.

What year and model are we talking about? 

[Jerry Rxman Griffin aka MuthaF'er]

DOHC came AFTER 1978. 1 & 3 fire together and 2 & 4 fire together.

Nobody caught that earlier?! For the original poster, please disregard my attempt at humor. 1 & 4 fire at the same time from one coil and 2 & 3 likewise but opposite from 1 & 4.

OK Wilbur, now you gotta LOOK IT UP bro..... 

[madmtnmotors]

OK Wilbur, now you gotta LOOK IT UP bro..... 

Alright, here ya go, straight from the SOHC manual:




Which can be downloaded from this very site:

http://www.sohc4.net/cb750k-manuals/

[Don R]

 The wires and cylinders are numbered 1-2-3-4 from the riders left to right. If that's what your question is.

[MCRider]

The DOHC is also 1,2,4,3. But the real question is, what is the question?

http://www.deuceswilddohc.com/tech/specs/general_specs.html

[madmtnmotors]

The DOHC is also 1,2,4,3. But the real question is, what is the question?

http://www.deuceswilddohc.com/tech/specs/general_specs.html

I think Don nailed it.

The wires and cylinders are numbered 1-2-3-4 from the riders left to right. If that's what your question is.

After reviewing the original post, I think the OP is confusing the numbers on the wires with "firing order". rocks indicates a "firing order" of 1,3,2,4 which makes me think that's what the numbers on the plug wires show, when considered from left to right. Of course, this has no bearing on firing order since the 2/3 wires are swapped. Now if 2 or 3 had been swapped with 1 or 4 then things would not be working very well...

Since 2 and 3 fire at the same time, and 1 and 4 fire at the same time then these wires can be swapped back and forth (2 with 3 and 1 with 4) without any ill effects.

[Don R]

My V8 race car has a 4/7 cylinder swap cam. That will cause you to stop and think.

[Joemetal861]

1978 cb750k
1 and 4 - 2 and 3


Sent from my iPhone using Tapatalk

[madmtnmotors]

Soooo.... based on the fact that the cylinders fire in pairs, the actual firing order would be:

1/4, 2/3, 1/4, 2/3 

[70CB750]

I say you go by the actual power stroke - i.e. where the spark ignites air/fuel mixture.  1243

Firing order is actually very interesting topic to research:

http://en.wikipedia.org/wiki/Firing_order

but I remember only my jeeps 153624 (CJ7 and GC) and SOHC4.

[rocks]

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb boots and they were numbered 1324.

thanks for all help

[madmtnmotors]

I checked the carb boots and they were numbered 1324.

Carb boots? You mean "spark plug" boots?

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb (spark plug?) boots and they were numbered 1324.

thanks for all help

If you are indeed talking about the "spark plug" boots, then all you have proven is the #2 and #3 spark plug boots are swapped... which would have no bearing on how the bike runs since 2 and 3 fire at the exact same time.

If you are talking about the "carb" boots, then somebody inadvertently swapped the carb boots as well...

[Airborne 82nd]

I checked the carb boots and they were numbered 1324.

Carb boots? You mean "spark plug" boots?

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb (spark plug?) boots and they were numbered 1324.

thanks for all help

If you are indeed talking about the "spark plug" boots, then all you have proven is the #2 and #3 spark plug boots are swapped... which would have no bearing on how the bike runs since 2 and 3 fire at the exact same time.

If you are talking about the "carb" boots, then somebody inadvertently swapped the carb boots as well...

madmtnmotors is correct.
I think the only guide to go buy would be the people who wrote the service manual for the people who designed and built the bike and that is  Honda and the book clearly states 1243 and I also think you might want to think of the firing order of the power stroke. Not so much the plug that is firing with this system.

[madmtnmotors]

I think rocks is confusing firing order with the labels on the plug wires (or plug boots). The cylinder order doesn't change, and are considered to be numbered from left to right 1,2,3,4. The labels on the plug wires are usually matched up with their corresponding cylinder, although as mentioned several times here, 1 and 4 can be switched (since they fire at the same time) as well as 2 and 3 can be switched (since they fire at the same time). "Firing order" is determined by the eccentric lobe on the spark advancer that opens the points to tell which coil to "fire". When that coil fires, it fires two plugs at the same time. The only one that actually accomplishes any work is the one firing within the cylinder full of compressed fuel and air. The other cylinder (no matter which pair were talking about, either 1/4 or 2/3) will be on the exhaust stroke so this spark is wasted since it accomplishes almost nothing, other than possibly burning off some unburned hydrocarbons left over from the power stroke.

Your SOHC/4 CB750 CANNOT fire "1324" because this would require the 2/3 points to open TWICE followed by the 1/4 points opening TWICE. This is mechanically impossible with this ignition....

but you can certainly CONNECT the number 2 spark plug wire to the number 3 cylinder, and the number 3 spark plug wire to the number 2 cylinder without any ill effects because they FIRE at the same time.

The logic of electrical circuits kicks my butt from time to time. Wait till you get the urge to start wiring up custom relay controlled circuits. One switch controlling another switch, holy crap! 

[AJK]

The firing order is 1,2,4,3. I wrote a little story about it here
http://forums.sohc4.net/index.php?topic=133342.msg1494366#msg1494366

[rocks]

I checked the carb boots and they were numbered 1324.

Carb boots? You mean "spark plug" boots?

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb (spark plug?) boots and they were numbered 1324.

thanks for all help

If you are indeed talking about the "spark plug" boots, then all you have proven is the #2 and #3 spark plug boots are swapped... which would have no bearing on how the bike runs since 2 and 3 fire at the exact same time.

If you are talking about the "carb" boots, then somebody inadvertently swapped the carb boots as well...

Now I'm really confused. Pictures are coming tomorrow.

Well, spark plugs boots are also numbered, but plugs boots #1 and #4 are angled about 120 degrees and they couldn't fit in the middle cylinders.

But also carb boots are numbered #1 #3 #2 #4.

I would say nobody else worked on this bike before me. Last your when I got this bike, and started cleaning the carb it was yellow paint on every nut and bolt on the carb and on the bike. And that paint was the warrantee.

[Don R]

The carb boots don't know or care where someone, even at the factory put them. Has nothing to do with the mechanics of the engine. Just a number on a piece of rubber.

[MCRider]

IF I remember correctly the 2 outside carb boots are labled 1 and 4 each as they are interchangeable. Same for 2 and 3, they both carry both numbers.

As to plug boots, that's what we're saying. 1 and 4 are interchangeable, 2 and 3 are interchangeable.

Firing order on all Honda CB/4s of the era is 1,2,4,3.

[madmtnmotors]

I checked the carb boots and they were numbered 1324.

Carb boots? You mean "spark plug" boots?

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb (spark plug?) boots and they were numbered 1324.

thanks for all help

If you are indeed talking about the "spark plug" boots, then all you have proven is the #2 and #3 spark plug boots are swapped... which would have no bearing on how the bike runs since 2 and 3 fire at the exact same time.

If you are talking about the "carb" boots, then somebody inadvertently swapped the carb boots as well...

Now I'm really confused. Pictures are coming tomorrow.

Well, spark plugs boots are also numbered, but plugs boots #1 and #4 are angled about 120 degrees and they couldn't fit in the middle cylinders.

But also carb boots are numbered #1 #3 #2 #4.

I would say nobody else worked on this bike before me. Last your when I got this bike, and started cleaning the carb it was yellow paint on every nut and bolt on the carb and on the bike. And that paint was the warrantee.

You are mixing up two different concepts here. The cylinders are numbered from left to right:

1,2,3,4

This is for reference only, so that when someone describes "firing order" or "top dead center" or "timing" you can relate the term to the physical location of the cylinder in question. The numbers on the spark plug boots, or sometimes even the spark plug wires, should correspond to the physical location of the cylinder being served. The points plate and spark advancer (that carries the "points cam") determine the correct distribution and timing of the spark (i.e. "firing order"). By putting the number 4 wire on the spark plug of the number 3 cylinder (and the number 3 wire on the number 4 cylinder) would not change the "firing order" because the ignition (points assembly) would still be "firing" the number 4 plug wire in the correct "order", you would have merely connected it to the wrong cylinder. I can verify that this orientation of the plug wires does not work very well. Forget about trying to place the wires in their "firing order" sequence from left to right, that's not what the identifying numbers on the spark plug boots are for. The numbers on the spark plug boots are for identification purposes only and are supposed to be connected to their corresponding cylinder, i.e. 1,2,3,4 from left to right (while sitting on the bike). The ignition system, i.e. points plate, determines "firing order" and distributes the spark accordingly....


As long as you have the spark plug wires connected to their corresponding cylinders....

1,2,3,4 from left to right.

Trust me.


Look at it this way, if you were to connect the wires from left to right, as you obviously think they should be oriented when you are looking at the "firing order" as being described as "1243", you would be connecting the number 4 spark plug wire to the number 3 cylinder and the number 3 wire to the number 4 cylinder. Now this orientation of spark plug wires would confuse the ignition because when it "fires" the number 4 spark plug wire it would be sending a spark to the number 3 cylinder! This usually results in pretty flame out the tailpipe or back through the carb throat. The "order" of the wires doesn't change the sequence in which they "fire". The ignition always "fires" the plug wires in the same "order" no matter where you put them. Whether you decide to connect them to the correct cylinders or not is entirely up to you.

[rocks]

I checked the carb boots and they were numbered 1324.

Carb boots? You mean "spark plug" boots?

Actually the firing order for 1975 CB750F Super Sport is 1324. This is based on an original bike. Mine was set like that, plus I checked the carb (spark plug?) boots and they were numbered 1324.

thanks for all help

If you are indeed talking about the "spark plug" boots, then all you have proven is the #2 and #3 spark plug boots are swapped... which would have no bearing on how the bike runs since 2 and 3 fire at the exact same time.

If you are talking about the "carb" boots, then somebody inadvertently swapped the carb boots as well...

Now I'm really confused. Pictures are coming tomorrow.

Well, spark plugs boots are also numbered, but plugs boots #1 and #4 are angled about 120 degrees and they couldn't fit in the middle cylinders.

But also carb boots are numbered #1 #3 #2 #4.

I would say nobody else worked on this bike before me. Last your when I got this bike, and started cleaning the carb it was yellow paint on every nut and bolt on the carb and on the bike. And that paint was the warrantee.

You are mixing up two different concepts here. The cylinders are numbered from left to right:

1,2,3,4

This is for reference only, so that when someone describes "firing order" or "top dead center" or "timing" you can relate the term to the physical location of the cylinder in question. The numbers on the spark plug boots, or sometimes even the spark plug wires, should correspond to the physical location of the cylinder being served. The points plate and spark advancer (that carries the "points cam") determine the correct distribution and timing of the spark (i.e. "firing order"). By putting the number 4 wire on the spark plug of the number 3 cylinder (and the number 3 wire on the number 4 cylinder) would not change the "firing order" because the ignition (points assembly) would still be "firing" the number 4 plug wire in the correct "order", you would have merely connected it to the wrong cylinder. I can verify that this orientation of the plug wires does not work very well. Forget about trying to place the wires in their "firing order" sequence from left to right, that's not what the identifying numbers on the spark plug boots are for. The numbers on the spark plug boots are for identification purposes only and are supposed to be connected to their corresponding cylinder, i.e. 1,2,3,4 from left to right (while sitting on the bike). The ignition system, i.e. points plate, determines "firing order" and distributes the spark accordingly....


As long as you have the spark plug wires connected to their corresponding cylinders....

1,2,3,4 from left to right.

Trust me.


Look at it this way, if you were to connect the wires from left to right, as you obviously think they should be oriented when you are looking at the "firing order" as being described as "1243", you would be connecting the number 4 spark plug wire to the number 3 cylinder and the number 3 wire to the number 4 cylinder. Now this orientation of spark plug wires would confuse the ignition because when it "fires" the number 4 spark plug wire it would be sending a spark to the number 3 cylinder! This usually results in pretty flame out the tailpipe or back through the carb throat. The "order" of the wires doesn't change the sequence in which they "fire". The ignition always "fires" the plug wires in the same "order" no matter where you put them. Whether you decide to connect them to the correct cylinders or not is entirely up to you.

So basically the the bile has two ignition coils (left and right). On my CB750F the left coil distributes to cylinder #1 and  #4 the right coil distributes to cylinder #2 and #3.  Is this correct?

[70CB750]

Yes, that is correct.  It can be used to an advantage when troubleshooting by making left coil serve 2,3 and right coil 1,4.