Topic: Honda CB750 K4

[Dammerung]

Hello Everyone!
                         I got my hands on a 74 Cb750. Always wanted one. I would like to do all the work on it myself as to me that adds to the whole experience.

Few things i could really use some help on.

1. My Throttle is sticky. I checked the actual wires and they have no resistance. When connected to carbs they stay open and wont shut. I looked at Carbs diagrams and the upper part has throttle valves in them? MAybe they are sticking to the walls?

2. When i start the bike within abt 30 seconds the headers are quite warm. Warm enough I kant keep my hands on them. Is that normal? (Sorry I know very newbie question. Well i am a newbie with the intention to learn ) Want to know if im running rich or lien or normal.

3. The latest issue.
---Upon starting from Cold (Parked Overnight) Starts up no problem but upon feeling the headers, cylinder 1-2-3 are hot but cylinder 4 is totally cold. (Sitting on bike, left hand side being 1st cylinder or Chain and Sprocket side being cylinder 1)
--- Let it warm up start riding, power is very low and hesitant...give her throttle and she feels sluggish. Sounds hesitant as well.
--- After a few kilometers and engine warmed up good, cylinder number 4 kicks in by itself and the bike rides accelerates like it should. Ignition coil? Spark wires?? :S:S:S:S


Thank you!

[phil71]

Welcome. That is a pretty bikeT but it's going to need some going over. The throttle linkage should be pretty easy to get free.  Since you're going to be pulling the carbs off anyway (yes, you are), you can give the shafts and slides a good once over.
download the shop manual from this site, and give it the 6000 mile (maybe it's 3000 mile, can't remember) tune-up. The procedures are fairly simple, even for a newcomer , and will not only solve your problems, but introduce you to the theory of operation of your carbs and ignition systems.

[Dammerung]

Welcome. That is a pretty bikeT but it's going to need some going over. The throttle linkage should be pretty easy to get free.  Since you're going to be pulling the carbs off anyway (yes, you are), you can give the shafts and slides a good once over.
download the shop manual from this site, and give it the 6000 mile (maybe it's 3000 mile, can't remember) tune-up. The procedures are fairly simple, even for a newcomer , and will not only solve your problems, but introduce you to the theory of operation of your carbs and ignition systems.

Hey Phil,
                Thanks for the quick reply . I was worried at first and had the bike in a shop. So the 3000 mile tune up is done. Oil change, points and ignition timing and valve timing adjustments. Carbs tuned and synched. But I believe they only opened the bottom end and not the part where there are valves. Im not sure how to take the top part off and clean them. Im trying to find a manual  The first shop i took it to said yeah no worries we can tc of it. When the bike came back i felt so dissapointed. Dont know wth they did but it wud not rev down even with throttle closed. Then I found a really nice old man who fixed that up for me .

My main concern is the 4th cylinder not kicking in when cold. :S

[ekpent]

If the the throttle valves still have some varnish or other crud on them they can stick and be sluggish. The carbs need to be removed from the mounting plates and the tops screwed off to accress them. You said there fine but check those cables well also. All 4 head pipes should almost be to hot to touch by the engine after 30 seconds. Sounds like you may have a carb and or spark plug combo issue on the #4 cyclinder that will need to be addressed.
  Did the shop that did your work have experience with the old bikes ?

[Dammerung]

If the the throttle valves still have some varnish or other crud on them they can stick and be sluggish. The carbs need to be removed from the mounting plates and the tops screwed off to accress them. You said there fine but check those cables well also. All 4 head pipes should almost be to hot to touch by the engine after 30 seconds. Sounds like you may have a carb and or spark plug combo issue on the #4 cyclinder that will need to be addressed.
  Did the shop that did your work have experience with the old bikes ?

 They claimed to have 35 years of experience. specializing in 4 cylinder Hondas. My guess is it was all BS. I have videos on youtube of the throttle cable actually. Search alizdafamily on youtube and you will see 2 videos title connected/disconnected throttle cables. Please let me know what you think. As far as the cylinders, before all 4 cylinders were fine. Lately i noticed my spark wire number 4 is a bit bent out of shape at the spark plug connection. (the plastic thing with NGK written on it.)

---I wanted to know the proceedure to change the ignition spark system...I dont want to change something that doesnt need to be changed. I was thinking the 2 ignition coils and the spark wires?

[wrenchmuch]

The plug boots thread onto the wires. Take the boot off and test it for resistance with an ohm meter. They could
be 5 k ohm or 10 k ohm boots. Sometimes the end of the wire where it goes into the boot is corroded. Cutting a little off the end of the wire can renew the connection. Be careful . If this has already been done in the past the wire might be too short to cut and still reach the plug . Eliminate electrical causes to problems before turning your attention to carbs.

[Dammerung]

The plug boots thread onto the wires. Take the boot off and test it for resistance with an ohm meter. They could
be 5 k ohm or 10 k ohm boots. Sometimes the end of the wire where it goes into the boot is corroded. Cutting a little off the end of the wire can renew the connection. Be careful . If this has already been done in the past the wire might be too short to cut and still reach the plug . Eliminate electrical causes to problems before turning your attention to carbs.

  Thanks Wrenchmuch I suspect it to be an electrical issue. Once the bike is nice and warm all 4 cylinders run well and the power output is like it shud be. Im not great with electricals but trying to learn. Im sure taking the boot off to check the connection shudnt be too difficult. I noticed right where the wire coming from the ignition coil meets the boot is bent out of shape. Best to start there.

I have another noob question...while using the ohm meter, its just to check how much resistance is betweem--starting of sparl wire to the end of the boot or just the wire itself??

[phil71]

the problem you're describing sounds like fuel.. If you unscrew the spark plug, then put it in its boot and hold the threads against the head, you ought to see spark jumping across the gap.
WHat you mention sounds like fuel, namely the bowl not filling as quickly as the rest. If the needle/seat is dodgey it may be only letting a trickle of fuel in..

[Dammerung]

the problem you're describing sounds like fuel.. If you unscrew the spark plug, then put it in its boot and hold the threads against the head, you ought to see spark jumping across the gap.
WHat you mention sounds like fuel, namely the bowl not filling as quickly as the rest. If the needle/seat is dodgey it may be only letting a trickle of fuel in..

I am about to go test this right now. Ill unscrew spark plug and check for spark. Ill also unscrew the float bowl one to see if all 4 have gas in them.

[Dammerung]

the problem you're describing sounds like fuel.. If you unscrew the spark plug, then put it in its boot and hold the threads against the head, you ought to see spark jumping across the gap.
WHat you mention sounds like fuel, namely the bowl not filling as quickly as the rest. If the needle/seat is dodgey it may be only letting a trickle of fuel in..

((no i mean cylinder #3 is running lean (white plug) after taking it for a ride while the others run fine. I solved the problem of cylinder #3 not running at idle (cold exhaust) and misfiring whilst riding by switching the vent tube and the vacuum tube running to cylinder #3 from the automatic fuel valve. this seemed to solve the problem of the cylinder misfiring/not running but now that cylinder seems to be running lean. here is a photo of the auto fuel valve . i switched hoses labeled #3 and #4 because they were opposite of what they are in this picture on my bike.)))

This is what i got from a another member under 1 cylinder running lean. Apparantly this solved his problem.

I checked a few things. First off, Cyl 4 spark plug had a lot of carbon deposit on it. however, it did spark, 

Cyl 1 Spark plug looks chocolate brown. Am i running lean?

[Dammerung]

Ahh i see the above post is for the dohc ones. i dont think sohc has auto fuel valve? even though i have the exact problem

[phil71]

some of the same principals apply, but yeah, that guy has CV carbs.. you don't.
I am still betting on a lack of fuel in the bowl. you can get it off without taking the rack off... clean out the needle /seat . while apart, set something under it and turn the fuel on for a second. it should flow like crazy.

[Dammerung]

some of the same principals apply, but yeah, that guy has CV carbs.. you don't.
I am still betting on a lack of fuel in the bowl. you can get it off without taking the rack off... clean out the needle /seat . while apart, set something under it and turn the fuel on for a second. it should flow like crazy.

I had two shops do exactly that. the 1st who knows wat they did but the bike wudnt rev down upon throttle. The next older gentlemen fixed the issue. the 2nd guy said something abt my float valve being no good. however he sid he  managed somewhat. i need someone to tear down the carbs and clean them inside out including throttle valves and set the needle valves in the float correctly.

I will try to do it myself. fingers crossed at this point.

I checked my spark plugs too. cyl 1-2-4 all had carbon deposit. I sprayed with brake cleaner and polished them. Looked like almost new by the end of it. So im guessing im burning rich?

[phil71]

you shouldn't make too many determinations about rich/lean till you've got it back to a known starting point. Those carbs are pretty simple. I've purchased really rough bikes, and it didn't take a really drastic cleaning to get them right. you'll be fine.. and if you can, try and check it out while assembled.. .it may not be a big deal.

[Dammerung]

you shouldn't make too many determinations about rich/lean till you've got it back to a known starting point. Those carbs are pretty simple. I've purchased really rough bikes, and it didn't take a really drastic cleaning to get them right. you'll be fine.. and if you can, try and check it out while assembled.. .it may not be a big deal.

That sounds about right Ill take the float bowls off and see whats goin on. Ill see if the manual has some step by step proceedures in it on how to set the float valve/needle jets right.

Meanwhile i took some pics of the spark plugs.

Pic 1- Cyl 4
Pic 2- Cyl 2
Pic 1- Cyl 1

Cyl 4 keeps getting carbon deposit even if i clean them and just start for 2 minutes.

Is there a good website or a step by step proceedure on how to clean and set the carbs right?

[kpier883]

Plug #4 looks really sooty like too much gas.  Nevertheless, I would follow the advice already given and make sure all plug wires are correct and connections tight.

If that doesn't solve it, maybe move on to the carbs. 

I also have a K4 CB750 and when I got it it always gas fouled plug #1 or #4.  It has been so long I can't remember which one it was.  I could never run more than a tank or two through it before fouling a plug.  One thing I recall is that it took almost zero choke for starting up, even on a cool morning.  It was not cold natured at all.  It also got very low mpg.  I wound up changing the idle jets and later the mains.  The biggest improvement with respect to the plug fouling issue was the idle jet change. 

Here is a post from a thread from August 2009 where I mentioned the jet sizes that I used:

Try the 125s and go from there,883,you have less restrictive pipes and are going to go down? I would think maybe 110 mains.,Bill

PO (or factory) had 110s  in it when I got it.  I was always fouling out plugs, even the D7.  It laid down black smoke on heavy throttle and would not take full throttle.  The motor would bog at WOT, but pick up if I backed off of full throttle some.  I got terrible mileage - high 20's and low 30's.  So I changed to 105s and smaller pilots.  No more fouled plugs, better response on top, and low 40s for mpg. 

The pipes are probably junker JC Whitney or somesuch.  All I know is that it was too rich before and I still think it may be a little rich. 

The only negative is that now I actually have to use the choke when the engine is cold.  Maybe PO used some Keyster parts somewhere in the carbs to make it behave so...

[kpier883]

One other thing, what brand and heat range plug do you have there?  I run NGK D8EA and have good success.

[phil71]

whoa, those ARE sooty. Ok, looks like you have the stock airbox..and a performance exhaust.
Often, people assume that the free-flowing aftermarket exhaust will warrant upjetting, but that is not always the case.. if someone, in it's history decided 'hey, new pipe, new jets!"
that could be part of it. If the ignition is not in the best state of tune.. it may not be able to burn as efficiently as it should.
  You can unscrew the pilot and main jets very easily from one of the outer carbs, why don't you get a pair out and post some pics, maybe we can help you figure out if someone overshot the runway with upjetting.

[kpier883]

I just took another look at your bike pictures.  I think you have the same aftermarket exhaust that I have.  You may need to run close to the same jetting I run.

[kpier883]

Ok, more searching turned up this from 2007.  The posts are always better at "remembering" than I am.  So actually, I only picked up a few mpg by going to #35 idle jets.  I picked up another 5 mpg or so by going from #110 mains to #105 mains.  From 2007:

I put in a new air filter and changed the idle jets (#40 to #35) to leaner and picked up maybe two or three mpg.  I was getting 29 - 32 mpg before and maybe 31 - 35 mpg now. 

I don't like to change too many things at once because then I wouldn't know which change caused a potential problem. 

I have some #105 mains that will be replacing the #110 currently in it soon, and hope to improve my mileage some more.  I have noticed that when I really give it some throttle, it will billow black smoke.  Already lowered the needles and it helped but didn't eliminate the smoke.  Also notice that at wot I get less acceleration than at 7/8 throttle and I think it is due to being rich.

So, the point is, check your jet sizes.

[ekpent]

Also a spark plug may still fire/spark when tested outside the engine but will not fire correctly under compression or load in the engine. Cleaning them with cleaner is no good usually,they are fouled.Always have some new correct plugs around when dealing with these issues to eliminate that variable.Some of those symptoms sound related to that along with getting those carbs dialed in.

[Dammerung]

whoa, those ARE sooty. Ok, looks like you have the stock airbox..and a performance exhaust.
Often, people assume that the free-flowing aftermarket exhaust will warrant upjetting, but that is not always the case.. if someone, in it's history decided 'hey, new pipe, new jets!"
that could be part of it. If the ignition is not in the best state of tune.. it may not be able to burn as efficiently as it should.
  You can unscrew the pilot and main jets very easily from one of the outer carbs, why don't you get a pair out and post some pics, maybe we can help you figure out if someone overshot the runway with upjetting.

Damn your good at this diagnostic The hey new pipes new jets is exactly the situation from PO. He put in 120 for mains and im not sure about the idle. Will POD air filters be better for it then??

Aslo where can i find different jets for main and idle?

[Dammerung]

I just took another look at your bike pictures.  I think you have the same aftermarket exhaust that I have.  You may need to run close to the same jetting I run.

I have the one very similar to yours all done by PO. 4-2 exhaust. When i took the carbs apart i think the mains were 120 and the idle was 40. Im not sure abt the idle size but im sure the mains were 120.

I have the exact same symptoms as yours. You know i havent had it on the road yet. Just start up and drive around the parking lot IT EATS GAS RIGHT UP!!! I HAD TO FILL THE TANK TWICE ALREADY :O:O:O
aas for the spark plugs, cyl 4 gets crazy sooty from clean in 2 minutes of start.

So you think its best i run 105mains and 35 idle? Im using NGK D8EA sparks as well. I have stock air box and PODs, i dont want to change to PODs as i like the older look but will do if it means better performance.

[Dammerung]

Also a spark plug may still fire/spark when tested outside the engine but will not fire correctly under compression or load in the engine. Cleaning them with cleaner is no good usually,they are fouled.Always have some new correct plugs around when dealing with these issues to eliminate that variable.Some of those symptoms sound related to that along with getting those carbs dialed in.

Im heading to the store to buy some fresh ones

[kpier883]

I have the one very similar to yours all done by PO. 4-2 exhaust. When i took the carbs apart i think the mains were 120 and the idle was 40. Im not sure abt the idle size but im sure the mains were 120.

I have the exact same symptoms as yours. You know i havent had it on the road yet. Just start up and drive around the parking lot IT EATS GAS RIGHT UP!!! I HAD TO FILL THE TANK TWICE ALREADY :O:O:O
aas for the spark plugs, cyl 4 gets crazy sooty from clean in 2 minutes of start.

So you think its best i run 105mains and 35 idle? Im using NGK D8EA sparks as well. I have stock air box and PODs, i dont want to change to PODs as i like the older look but will do if it means better performance.

Personally I would stick with the stock airbox.  There are many opinions on pods vs stock airbox, but that is mine fwiw.  It is my recollection that #110 main is stock on the '74 and #40 is the stock idle jet.  According to Hondaman all came from the factory jetted rich - see this thread for a collectoin of the writings of Hondaman as gathered into one thread by forum member SteveD CB500F (thanks Steve!):

http://forums.sohc4.net/index.php?topic=7401.0
In particular, this:
4. Jetting. ALL of the inline fours came rich from the factory. At sea level, we started with a 10 size less than what it came with.

I am probably running a tad lean with my combination.  My recommendation would be to switch to #105 mains with fresh plugs and ride a bit then check your plugs to see how they look.  If you still have problems then maybe change the idle jets.  Most people don't recommend the idle jet change I did.  Use it at your own discretion.

Plug #1 afer thousands of miles including long trips and riding to work and back, plus lots of sitting unused: