Topic: CB750 K2 restomod to "originalish" condition // Forkbrace added ! ! !

[AgPete139]

Looks great! Its always nice to see finished bikes and where they started.

[CROMOLY]

Verry nice, must feel good to ride.


Cromoly.

[Viktor.J]

I've never experienced a wobble on my K4 even at high speed.  My brother had a Yamaha XS1100 that would develop a high speed wobble in the turns but was caused by a small aftermarket fairing he installed.
How are you going to mount that?

Im not 100% sureif im going to use it, but I think it looks cool. A must for a "racing look", and if it adds a extra protection thats great
The mounting is a later problem, haha.

I`ve experienced one high sdpeed wobble way back when I had my first K1 . Maybe the first year I owned on the interstate screaming down I20 with my hair on fire. The most scary thing I`ve ever experienced. Turned out to be loose spokes in the rear wheel...............Gary

yeah that sounds scarry. And since im often cruising around in highway speeds 60-90 mph I hope It can add some extra stability.

Looks great! Its always nice to see finished bikes and where they started.

thanks pete, it's not really done yet but im starting to get there, thanks !

Verry nice, must feel good to ride.
Cromoly.

Thanks Cromoly, how is everything going with your bike ?

[Viktor.J]

So, the last weeks I've been away from the forum, mostly since I had a lot to do in school and that the bike was done
So last 2 weeks Ive been driven when I had the time and weather (still Sweden). Ive driven about 300 miles the last 2 week, until yesterday......

It always starts on the first try and runs fine.
But yesterday when I was about to ride something strange happend, after about 2 minutes she said BAM, BOOM BANG (exhaust bangs) very loud and died within about 2 seconds.

Here are things that can affect, as I and my father noted:
* It starts every time but dies repeatedly, sometimes after a few seconds, sometimes 1 minute.
* Often when it dies it produces very powerful exhaust bangs, very sharp before it dies. or it just loses power and die.
* when exhaust banging black soot flies out of exhaust pipes.
*It seems to have gone on all cylinders, but certainly not always. Sometimes it did not really start at all on all cylinders, and then died, sometimes started and it went on all 4.
* When was running OK and i released the clutch in first gear it died
* I felt that I had to have high revolution on the engine so it would't die.
* When I put my hand in front of the exhaust pipes it was a lot more pressure in the left side (cyl 1 & 2) than the right side (cyl 3 & 4)

The only thing I can think of is two possibilities.

1: bad spark, ignition
The probably  talking for this cause is that it was humid out and rained. Which should be sensitive to these bikes, but so sensitive? And I drove in the rain earlier on that day a few minutes. After that ride I parked it in the garage. And she ran as normal for about 2 minutes before she died after some exhaust bang.
And after that she started on almost every (9/10) attempt and went for a while and then die.


2: Carburetor:
 What speaks for the carburators is well that once it died, it sometimes just slowly lost power and died, as when you don't have any gas. . But at the same time you could start it right up a few seconds later again, and again and again (petrol in the tank).



Has anyone experienced something similar? Any one have any suggestions ?

[Viktor.J]

I thought I could add some pics on her from the last days

A picture on the mainland straight out starts the Swedish archipelago found on the west, south and east coast (this is south)



Outside my school after a late night studying (during summer months the sun goes down late, in the north Sweden it never goes down)


Picture from my first "real run" with the bike.

[KRONUS0100]

sweet looking bike. 

[cheftuskey121]

No viktor say it isn't so! I can't offer too much in the way of advice but mine was dying and then not even starting because my 4 wires wires going to the coils were not solid connections and my black/white kill switch wire in the headlight bucket was not solid either. I soldered them up and now mine is running like a champ. I would check you electrical even if it looks fine

Start the bike with the headlight bucket open and wiggle that black/white wire and see if it dies immediately. Make sure all your connections are tight, even if they look good. Then after that see what your plugs look like, maybe check your petcock filter, look in your tank, then I need be clean your carbs. Just some stuff to look at. Maybe the road vibration has caused wires to go loose. Hope you get it resolved. It sure it pretty!

[Stev-o]

Good suggestions from Chef. Also - check for spark after it dies, that will give you a clue if it is indeed ignition or carbs.

[Viktor.J]

sweet looking bike. 

Thanks man ! 

No viktor say it isn't so! I can't offer too much in the way of advice but mine was dying and then not even starting because my 4 wires wires going to the coils were not solid connections and my black/white kill switch wire in the headlight bucket was not solid either. I soldered them up and now mine is running like a champ. I would check you electrical even if it looks fine

Start the bike with the headlight bucket open and wiggle that black/white wire and see if it dies immediately. Make sure all your connections are tight, even if they look good. Then after that see what your plugs look like, maybe check your petcock filter, look in your tank, then I need be clean your carbs. Just some stuff to look at. Maybe the road vibration has caused wires to go loose. Hope you get it resolved. It sure it pretty!

Since I rode it like 500 km last two weeks, it starts right up and runs for a while before it dies the sparks should be good right ? I don't really know I'm just thinking logic
I could check the connections and there's a possibility that there's moist inside of the headlight bucket, but all the connectors have rubber sealing so that seems like a long shot.

Im also thinking of the petcock filter I will have to look into that. If the carbs where dirty wouldn't it run poorly all the time and isnt it a verry small possibility that all 4 breaks down at the same time ??

Dont get me wrong, i'm just trying to "think loudly" about your thought
To me, all of the "signs" indicates to different problems, and the chance that all these problem occurs within 2 seconds after 500 km seems strange. thats why I believe (and hope) that its ONE failure somewhere, causing this.

Good suggestions from Chef. Also - check for spark after it dies, that will give you a clue if it is indeed ignition or carbs.

What do you mean ignition after it dies ? when its dead, its dead haha



Does anyone know how the gasoline is distributed down to the carburetors ? there's 2 lines from the petcock is on line for nbr 1&2 and the other one for nbr 3/4 ?

[edhaeuser]

Unhook fuel line from petcock and see if you have good flow from tank?  Maybe the screen is plugged.

[Viktor.J]

Unhook fuel line from petcock and see if you have good flow from tank?  Maybe the screen is plugged.

I will try it ! thanks for the input Ed !

[cheftuskey121]

What do you mean ignition after it dies ? when its dead, its dead haha

he means take the spark plugs out of the engine but put them in the spark plug wires, lay the spark plug on the engine so the nut is grounded, and turn the bike on killswitch on and try to start it with button or with kick. see if your plugs are actually sparking. I did that with mine and confirmed spark after I soldered my loose connections.

dont rule out your battery. I hope you have a float (trickle) charger. make sure if you get it going again you are testing its voltage output with a multimeter. make sure it goes up to around 14v when you rev the throttle. I have found again and again there are no "stupid" questions and answers. it can be the thing you most suspect, the thing you least suspect, or the thing the most middle suspect

I am always learning something new with my bike everyday, and I don't know what I'd do without this site! good luck Viktor

[Viktor.J]

What do you mean ignition after it dies ? when its dead, its dead haha

he means take the spark plugs out of the engine but put them in the spark plug wires, lay the spark plug on the engine so the nut is grounded, and turn the bike on killswitch on and try to start it with button or with kick. see if your plugs are actually sparking. I did that with mine and confirmed spark after I soldered my loose connections.

dont rule out your battery. I hope you have a float (trickle) charger. make sure if you get it going again you are testing its voltage output with a multimeter. make sure it goes up to around 14v when you rev the throttle. I have found again and again there are no "stupid" questions and answers. it can be the thing you most suspect, the thing you least suspect, or the thing the most middle suspect

I am always learning something new with my bike everyday, and I don't know what I'd do without this site! good luck Viktor

Haha OK, then I understand totally fine
 Is it normal that Batteries does this BOOM just like that ? I was under the impression that they slowly dies/get bad ?
To be clear, i always use the electric start, and it starts every time, the problem is if its running for 5 seconds or 50 seconds
I totally agree with the "don't rule out anything"-philosophy. My replying question though is to make a "qualified" guess since it was running great up until yesterday I don't want to start changing and wrenching everything I find, causing more trouble

Yeah this site rocks, im running parallel with the Swedish "classic honda club" which is a more "hardcore cb750"-club and the members is really great and full of knowledge but few (and old) But these two forums provides a great mix !

[Viktor.J]

Apparently I put on the gasket for the point covers so that shouldn't be a problem.

Since The bike (when not working) is at my dads house he is the trouble shooter
He told be the bike started up nice and ran good. He planes to run it for a turn and then return with some info .

IF it runs good then I could rule out the carburetors right ?

[Viktor.J]

So the bike ran great for about 30 minutes after spending 2 days in the garage.

Then I can rule out the carburetors right ?

Thought I should do the trick I was given here, spraying water on critical parts while the bike is running.
Thought I should do it in the order., because I suspect that its the plug or coils that gets damaged by the water. 

1 Kill switch
2 headlight bucket (Area)
3 point cover
4 plugs
5 coils

[Stev-o]

I wouldn't spray water, that's just asking for corrosion. Check all your connections. 

[Viktor.J]

Ive done that, everything looks altight.
If there's so sensetive to water and corrosion howecome they survived 40 years ?
I can't really see why a little simulated rain should be such a big problem. What do I do if it starts to rain while driving ?

[Stev-o]

Who said it was a big problem? I said I wouldn't do it and won't.
 I seem to remember a thread about a guy who's bike would not start after he sprayed it with a water hose, ring a bell?

[strynboen]

mine 550 dies in rain. vith aftermarked coils
.have put on,, at old set coils on from a 400 four..now it only runs on 3 cyll in rain...better but not perfekt
a vater sprayer can shov veakness..vitaut to drovn the hole bike


but think it are a fuel problem..thek the bovls ..are there fuel in the caburatore vhen it dies..it vill slow fills up after stop...så thek kvik after it stopps
are there fine rust in the tank..kaburators  normal overflovs..

[andy750]

I had the same issue many years ago with my CB750F2 - it never ran well in the rain - would cut out and die as soon as water hit the bike. I poured water all over the bike at idle (there is no issues with these bikes seeing rain - had plenty of miles in heavy drenching rain storms on both my K4 and K2 ) and could never replicate the condition. I changed spark plug boots, coils, HT leads all the usual suspects. Never solved it. Thinking back now it was probably leaking at the points cover and getting water/moisture in there. Your description of your backfires out the exhaust sound like another problem I had on the F2 -faulty condensors. They would cause misfire/backfire and I couldnt run the bike over 50 mph.

Other possible causes not ignition related (which it sounds like it is) - vapor lock on your fuel tank - this would allow no gas to get to your carbs and give you that "oh dear I have run out of gas" symptom -hard to idle/hard to start.  Check the little vent hole in the gas cap is clear.

Good luck in figuring it out and let us know when you fix it

cheers
Andy

[Viktor.J]

Who said it was a big problem? I said I wouldn't do it and won't.
 I seem to remember a thread about a guy who's bike would not start after he sprayed it with a water hose, ring a bell?

Yup, and I thought that problem was gone but apparently not. Thats why I really want to go through this problem once and for all, I simply don't want to be standing on the roadside eventhough I changed everything that "could" be wrong.

I appreciate your help Steve, Which connections do you believe could cause this problem ? I mean it has to be something that affects the whole bike, doesn't it ? 

mine 550 dies in rain. vith aftermarked coils
.have put on,, at old set coils on from a 400 four..now it only runs on 3 cyll in rain...better but not perfekt
a vater sprayer can shov veakness..vitaut to drovn the hole bike


but think it are a fuel problem..thek the bovls ..are there fuel in the caburatore vhen it dies..it vill slow fills up after stop...så thek kvik after it stopps
are there fine rust in the tank..kaburators  normal overflovs..

What do you think that it might be a fuel problem ? Is it possible that it occurs all of a sudden and then goes away ?
You mean that I should run the bike, and then when it dies check the bowls to see if there's gas there ?

The tank is nice, but I might check the petcock to be sure !

I had the same issue many years ago with my CB750F2 - it never ran well in the rain - would cut out and die as soon as water hit the bike. I poured water all over the bike at idle (there is no issues with these bikes seeing rain - had plenty of miles in heavy drenching rain storms on both my K4 and K2 ) and could never replicate the condition. I changed spark plug boots, coils, HT leads all the usual suspects. Never solved it. Thinking back now it was probably leaking at the points cover and getting water/moisture in there. Your description of your backfires out the exhaust sound like another problem I had on the F2 -faulty condensors. They would cause misfire/backfire and I couldnt run the bike over 50 mph.

Other possible causes not ignition related (which it sounds like it is) - vapor lock on your fuel tank - this would allow no gas to get to your carbs and give you that "oh dear I have run out of gas" symptom -hard to idle/hard to start.  Check the little vent hole in the gas cap is clear.

Good luck in figuring it out and let us know when you fix it

cheers
Andy

Yeah that's why im planning on spray some water on the point cover to see if that might be the problem beafore bying stuff. Im already under kind of a limited money situation as a student
To be clear, the exhaust bangs ONLY occurs when the bike wasn't running, before it died. Now when it's working again there's no exhaust bangs..

Will defenetly check the gas vent. never heard of that Thanks !

[iron_worker]

If it's a fuel starvation problem the bike will usually start to run badly and then gradually get worse until it sputters and dies completely.

If it's an electrical problem then usually the problem will come and go quickly or instantly.

IW

[Viktor.J]

Im thinking of mounting the clip-ons. Never seen these exact ones on a cb750. I Test mounted them today and they felt OK. At least good enough to trying to find a new pair of forks caps to drill/thread and mount them solidly.

But Im not 100% sold, what do you think  ?

[Stev-o]

Looks like they will hit the tank

[edhaeuser]

I like it either way, but I think I like the stock bars a little better.  Did you ever get the engine problem sorted out?