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She bangs.. :(

Lasper01

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Hello! I need help! My father's CB 750C has difficulties taking the gas.. It wont rev properly.. sputters and bangs .. Is it false air? Must we clean the carburetor?
Ignition is pretty good, new coils and the timing is set OK.. but there is oil at the pulser generator coils.. should there be oil?

We tried to videofilm it, so you could get a feel for it ..
The night before with would rev up to 6000 pretty fast when we did the timing, but when we went for a ride it almost wouldn't rev.. but if we drove carefully and low rpm it was OK .. And the next day it sounded so awful.. like something is hitting something inside the motor..


Sorry for bad explanation.. Im not that good at english, but I hope you get the picture.. :)

We would be glad for your help! Thanks!!

Lasse and Leif (dad)
 
what is the bikes history? how many miles ,how long have you had it , was it stored ,when did it run last. I am not familiar with this noise its making .damn thing sounds like a tractor .If the bike has not run for some time there are plenty of things you should check . First thing I did when I got mine was clean and rebuild the carbs ,check valve clearance ,cam chain tension,check for cracked rubbers to the carbs etc etc im sure one of the more experienced gents here could give you a complete check list .Maybe even recognize the problem from the video.Good luck and welcome to CB750.com Guys :)
 
Thanks :)

He have had for some years now, but it hasn't been running good for a while.. We found out that the ignition coils were bad so we replaced them the day before the video. That night as I wrote it didn't sound like that tractor..
But you are properly right.. we should check those other things.. Do you know about the oil at the generator pulser coils?
 
Those carbs are notorious for having bad air cut off valves. They are under the round covers on the sides of the carbs. They are a rubber diaphram that are always cracked and leaking. I have an 81 cb750 that hasn't run in years because I can't afford to rebuild the carbs. Cheapest rebuild kit I have found are on ebay for $215 for the set. Cracked intake rubbers are very common also. I would check for air leaks, I use carb clean and spray around the carbs and see if the idle picks up. I think the noise may be coming from gear backlash because of the rough running. As far as the oil on the pulser coils I cant recall exactly off hand but I think they are supposed to be dry. I will do some checking.
 
So your crank seal is leaking? You shouldn't have oil on the pick-ups. How much oil is leaking?

As far as it not running right, I would start with the basics... check compression, spark, check for air leaks, and then tear into the carbs, ect..

Do I see sparking on the inside of the stator cover? Do you have short or bad ground or something? :shrug:
 
Thanks guys!!

Travis.. Im not sure exactly how much its leaking, but its too much! :( Is it possible to change to crank seal without spilling the cases? Im not were to bike is now, so I can go and look.. but can we "just" pull the seal and install a new one?
Thats the first thing I think we should do.. and then as you say, do the basics..

I will keep you updated :)
 
Ok, I think I have good news for you in regards to the crank seal. I wasn't sure how the DOHC 750 was built so I took the side cover off my 1980 750 DOHC motor and it does not use a crank seal. There is supposed to be oil on the pulsor coils as they is oil in that side of the engine so you have no worries there. Does the really bad rattle noise go away or get a lot better if you pull the clutch in while it is running. The clutches have a habbit of rattling at idle but I think you have carb issues.
 
Whoops sorry about that. I thought the DOHC engines had a seal behind the alternator and were dry. :eek: I guess they're wet just like the SOHC engines.

So now onto your other problems!
 
Mine sounded like that when I first started it due to the choke not being hooked up. Could be you are drawing air from the boots and the mixture is weak. Try removing the plug caps one at a time, the one that does not make a difference is giving you problems. Also, are the plug leads connected as per firing order.
 
Thanks again!
The news about the crank seal is making me happy.. one thing less to be concerned about.. I did try to pull the clutch but it didnt change the sound.

Im sorry to say I havent had to time to go to my dad, so I have no news yet.. But I will be posting as soon as possible :)
 
Alright.. A little update..

Now we had time to test some more and we found out that the ignition amplifier (guess its also is called CDI) may be broken. The Haynes manual says it should show 0 V and 12 V when performing the test, but they both are showing 1,7 V and 12 V..
When we testet the ignition timing with stroboscopic light, it blinked very irregularly and almost stopped blinking when we rev'd the engine. So we dont suspect the carbs right now..

Thats it for now.. to be continued :)
 
Thanks.. and you are right.. and we are having a good time doing it :)

As I said we found out that the CDI boxes are broken. They are very expensive (310 dollars a piece) so we would like to know if anybody has a cheaper trick/idea to use something else.. maybe DIY or some other CDIs that can be used?
 
Cycle Xchange has there own ignition system that costs $375. You get a whole new ignition system that is more accurate than the stock stuff. Go to there website and look under the 750 DOHC section and read up on it. I would rather spend $375 on this than $310 x2 on the stock stuff. The only thing I dont know is if it fits under the stock cover? I would call them and ask them some questions. I have dealt with the guys at cycle x many times and they are very helpfull. They are a small company with a lot of work going on right now so be patient with the phone (sometimes don't answer right away). Or they have email that they always get back to me right away. Thought I would mention this ignition system to you as an option.
 
Im sad to say that there aren't much to write about.. We got the new cdi-boxes.. They didn't change much. The bike can idle, but when we put a timing light on the first spark cable it fires uneven. Like: blink, blink, blink, blink, pause, pause, blink.. :confused:
When we put it on the second spark cable it almost never blink.. but the bike is still idling alright.. But when we rev it up it holds back and seems like it can't keep up with the firing.. :(

We are pretty lost right now...
 
Check the plug caps, plug leads, then the coils. The Honda OEM coils are not very good. I believe that coils from the Chevrolet Nova are a popular replacement, need to make up a new bracket though.
 
We have replaced the coils, the cables, the caps, the sparks and the cdi's... its getting frustrating :banghead: How can it run when it fires so inconsistent? :shrug:

we measured the pulser coils.. the ohms is off by 50-60 ohms.. the manual says +/- 30.. could that be the problem? I dont remember the numbers correct, but they were not as the manual says they should be..
 
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