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Offline Steveola

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cb750 backfies and won't start
« on: August 16, 2012, 06:53:40 PM »
First of all I want to thank all in this forum for all the help I have received.  in trying to sort my 1976 CB750K. I recently cooked the top end. I didn't know that CB750 oil pumps required priming and when I rebuilt the engine it started right up no problem but within 50 miles I had completely cooked the cam and rocker assemblies. Well.... I pulled the engine and replaced the top end. The cam, cam towers and rocker assemblies were ruined. Anyway, When I got it back together I tried to start it. All I get is a freaking sonic boom backfire. I installed the cam per the service manual. I timed the valves with number 1 cylinder in TDC appeared to be in the correct stoke according to the manual. The cam and rockers for number one were both closed and the number 2 and 3 cylinders were in the correct positions. I adjusted the valves in accordance with my shop manual. Is it possible that when I bolted my cam chain sprocket I set the timing180 degrees out? DAMN!!!!!  Do I have to pull the freakin engine again?!!!!
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Offline killersoundz

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Re: cb750 backfies and won't start
« Reply #1 on: August 16, 2012, 07:04:55 PM »
FIRST thing first, make sure you have the spark plug wires coming from the right coils. One coil should be 1 and 4 and the other coil 2 and 3. That will screw up your day. I actually just got an amazing deal on a suzuki gs650 some guys were trying to get running and their main problem is they had the plug wires on wrong LOL

You should have set the cam with the notch on the end facing up, and the spark advancer at the right spot when you set the timing chain. I found it easy as long as I knew what I was doing...
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Offline steam-powered man

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Re: cb750 backfies and won't start
« Reply #2 on: August 16, 2012, 09:13:57 PM »
to find out if you installed the coils 180 out, swap the blue/yellow points wires and try to start.  if it purrs like a kitten, no worries. 
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Offline Steveola

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Re: cb750 backfies and won't start
« Reply #3 on: August 17, 2012, 06:38:10 AM »
I will definitely try the plug and coil wires. That would be great if that's what it is. I REALLY don't have to pull that engine again. I am using a Dyna S ignition but I don't know if that makes a difference. As I say I was very careful to install the cam with the square notch at the top with Cylinders 1 and 4 at the very top of their stroke. After I had everything bolted in, I rotated the engine slowly in the clockwise rotation until cylinder 1 was TDC with both valves closed. I would have to look at the chart in the manual as far as which valves for cylinders 2 and 3 but they corresponded correctly with the manual.
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #4 on: August 17, 2012, 07:35:45 AM »
Quote from: steam-powered man on August 16, 2012, 09:13:57 PM
to find out if you installed the coils 180 out, swap the blue/yellow points wires and try to start.  if it purrs like a kitten, no worries.
I recommended this to a forum member with a similar problem. I was told by several other members it wouldn't change anything. But I was told by the member with the problem it did change things, fixed it!

Essentially the cam is timed wrong, but doesn't matter when you switch the wires.
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #5 on: August 17, 2012, 07:38:57 AM »
Quote from: Don R on August 16, 2012, 10:08:15 PM
If your cam was installed incorrectly it would not have run 50 miles. Have you checked compression? Could a valve be hanging open?
he had it all right when he rode 50miles and cooked the cam. BTW: I don't think it was priming the oil pump that cooked the cam, I think it was debris in the oil orifices. Nevertheless...

Its on the second rebuild that the cam may be in wrong and it doesn't start.
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Ron
1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
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Offline Steveola

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Re: cb750 backfies and won't start
« Reply #6 on: August 17, 2012, 08:00:00 AM »
MCRider, you are probably right, but I don't know how I would have gotten the cam in wrong unless I didn't get the cam chain sprocket in right. It would be worth trying as opposed to taking out the engine, but it's going to bug me that I don't know what I did wrong so I won't do it again. Which wires would you try first, the wires to the plugs or the wires from the ignition module down where the points used to be?
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Offline iron_worker

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Re: cb750 backfies and won't start
« Reply #7 on: August 17, 2012, 08:03:52 AM »
Probably easiest to just the colored wires to the points.

IW
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #8 on: August 17, 2012, 08:12:35 AM »
Well definitely as Killer says, you gotta know your plug wires are correct. But if they are OEM they are hard to mess up due to their lengths. 2 and 3 can be interchanged, and they are too short to reach 1 and 4. The 1 and 4 can be interchanged, but even they are slightly different lengths and may not reach but 1 each. They will reach 2 and 3, which would be wrong. But then you don't have any left to go to 1 and 4.

Anywhere you see a blue and a yellow, probably near the coils, you can swap them out. Dollars to donuts it starts.

You may have a point lead shorting out on the point plate. It may have rotated to where the hard metal part touches the point plate. That will make a loud bang also. IIRC

OOps I just read "where points used to be". Switching the blue/yellow is still in play. But not the pt lead shorting to the point plate.

What Ig do you have again?
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #9 on: August 17, 2012, 08:16:41 AM »
BTW: I think its good practice for those of us posting solutions to indicate whether we are postulating or actually have first hand experience.

In this case: I have the experience with another forum member as I mentioned. Also, I have a reverse experience. I couldn't get mine to start after a rebuild. Only got loud backfires. I had reversed the point wires, blue to yellow and yellow to blue. Once I made them straight it started right up.

So its a little reverse logic I'm using.
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Ron
1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
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Offline Steveola

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Re: cb750 backfies and won't start
« Reply #10 on: August 17, 2012, 08:21:12 AM »
I'm using a Dyna S ignition with the green Dyna coils...I don't recall right off the top of my head whether they are 3 or 5 ohm.
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #11 on: August 17, 2012, 08:36:21 AM »
Quote from: Steveola on August 17, 2012, 08:21:12 AM
I'm using a Dyna S ignition with the green Dyna coils...I don't recall right off the top of my head whether they are 3 or 5 ohm.
Oho! So you made your own plug wires?

Care to have them correct as killer says.

Doesn't matter which. Just pick one and its the 1-4. And the other is the 2-3. Once confirmed, sort your blue yellow wires.
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1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline Steveola

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Re: cb750 backfies and won't start
« Reply #12 on: August 17, 2012, 12:00:16 PM »
Yep, made my own. I'll get that sorted as soon as I get home from work. Man!!! that will make my weekend if it works. Thanks everybody, especially MCRider.
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Offline steam-powered man

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Re: cb750 backfies and won't start
« Reply #13 on: August 17, 2012, 02:38:59 PM »
it's me that installed the cam in my cb750 precisely 180 degrees out.  was dreading yanking the engine and asked for advice here, and it was postulated that maybe i could simply switch the blue/yellow points wires (the connectors under the airbox/near the brake light connectors).  in under 10 seconds i had a perfectly running bike.  yowza!   ;)

that's why i suggested the test.       
« Last Edit: August 17, 2012, 02:41:17 PM by steam-powered man »
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #14 on: August 17, 2012, 02:43:24 PM »
Quote from: steam-powered man on August 17, 2012, 02:38:59 PM
it's me that installed the cam in my cb750 precisely 180 degrees out.  was dreading yanking the engine and asked for advice here, and it was postulated that maybe i could simply switch the blue/yellow points wires (the connectors under the airbox/near the brake light connectors).  in under 10 seconds i had a perfectly running bike.  yowza!   ;)

that's why i suggested the test.     
I thought so but didn't want to look it up.  ;D
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Offline Steveola

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Re: cb750 backfies and won't start
« Reply #15 on: August 17, 2012, 03:41:23 PM »
IT WORKED!!!!!! Runs great. Thanks very much everybody. I switched the wires and it fired right up.
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Offline MCRider

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Re: cb750 backfies and won't start
« Reply #16 on: August 17, 2012, 07:12:38 PM »
Quote from: Steveola on August 17, 2012, 03:41:23 PM
IT WORKED!!!!!! Runs great. Thanks very much everybody. I switched the wires and it fired right up.
Yay! That's two!!
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Ron
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"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline killersoundz

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Re: cb750 backfies and won't start
« Reply #17 on: August 18, 2012, 12:07:56 AM »
Quote from: Steveola on August 17, 2012, 03:41:23 PM
IT WORKED!!!!!! Runs great. Thanks very much everybody. I switched the wires and it fired right up.

Sweet. So I want to know if you just had the coils backwards or had the cam in 180 out?
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Offline Flandango

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Re: cb750 backfies and won't start
« Reply #18 on: September 23, 2014, 04:16:44 PM »
Oh comrades you guys are geniuses!! I've been kick starting the right arse cheek off myself all day and a simple switch of wires has sorted me out! Thanks a million.   :D  This engine has not run in 15 years so how do I make sure that oil is getting up top to the cam, rockers etc?
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Offline Don R

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Re: cb750 backfies and won't start
« Reply #19 on: September 23, 2014, 08:05:35 PM »
Pull the valve adjuster caps with some luck the front ones will toss oil out onto the headers and make a mess. I always use some assembly lube to let the parts get to know each other again until the oil gets there.
 I've used an oil cylinder that's made for adding oil to automotive AC, put a qt in hold the rubber plug against the oil gallery and add air to the cylinder. I made the oil light go out for quite a while. When it sputters all the oil is in. When it starts the scavenge pump will pump it to the oil tank so allow for the total amount.  pre-lubed engine.
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Offline PeWe

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Re: cb750 backfies and won't start
« Reply #20 on: September 24, 2014, 03:10:26 AM »
I have done that twice this year. Rotor 180 degrees wrong on DYNA S ignition
With points: cam in advancer unit mounted wrong, 50% chance when just doing it without check.
Worked direct when switching blue-yellow the first thing I tried. Rotor vs cam in correct position so the elctrical could be correctly connected.
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Offline Oldtech

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Re: cb750 backfies and won't start
« Reply #21 on: September 24, 2014, 07:10:33 AM »
Is it possible that the oil jets got left out? This would drastically lower oil pressure to the head and would trash your top end.
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