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Discussion Starter #1
So I know that Im gonna have to listen to someone tell me how dumb i am and how im a half rate hack and that if i wasnt such a piece of doodoo id fix my bike on my own without any help. But im gonna post this any way.

Bike is a 73' CB750, with pods and a 4-1.

So.... I put new points on the bike and timed them. Replaced the plugs, added some oil, and put gas in tank. The bike started and sounded ok idling. I started riding it and it ran like donkey balls. Stutters and has very little power. Seems like its running on only 2 or 3 cylinders, if thats possible.

Im calling cylinders 1,2,3,4 from left to right.

When the bike is running you can feel that only cylinders 1 and 4 are getting hot. Cylinders 2 and 3 are hardly warm. The plugs in 2 and 3 will foul black quickly. With new plugs it'll run fine for a little and then start to foul. The mixture on the carbs is real light, so i dont think its just running rich.

I cant tell if its maybe a timing problem or if I have a bad coil. All plugs are getting spark but i dont know how to make sure 2 and 3 are getting enough.

One thing im curious about is the petcock. If the petcock is turned to on gas will only flow from the left nipple freely. If i plug the left side gas will come out of the right. I thought maybe this was part of the problem, but the left nipple runs to carbs 1 and 2, and the right nipple runs to 3 and 4. If gas wasnt coming out of the right nipple when running then I wouldnt have cylinder 4 running fine like it is, so I dont think thats the problem.

I think its a coil problem, i think one is not firing hot enough. That would explain why 1 and 4 run fine and 2 and 3 foul, i think. Also one of the coils looks to be newer than the other. How can I tell if its factory?

Any help would be appreciated.

My CB750 Project:
http://www.caferacer.net/forum/topic.asp?TOPIC_ID=4739
 

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Hey, I'm giving you full credit for trying. My only advice when diagnosing these things is that you really need to start from the beginning, meaning that you need to have a service manual, and follow the procedure for a complete "tune-up". Checking valve adjustment, ignition timing, cam chain tension, compression test etc. Once you've ensured that all the basic settings are acceptable, then you can proceed with trouble shooting running problems.
Having said that, it sounds like your middle cylinders may have low compression. If they're fouling the plugs, and not coming up to temp as quickly as the outer cyls, I don't think it's a fuel problem.
But others on this forum know more about these engines than I do. I just don't recommend trying to skip some of the neccesary diagnostics steps.

FR
 

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Start swapping the parts over, put the coil from 1 and 4 onto 23 and vice versa, also switch the required wiring to ensure the coils fire at the correct time. The points may be set incorrectly, or maby it is a petcock issue. I am not too familiar with your bike but am just going by what I would check first.

I am assuming that the bike was running fine before, right?

Hey someone pas me the wrench........ Ahh screw it where is the torch
 

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Look at the wiring diagram for the points. Assuming two points, two cylinders will be firing in tandem. Swap the wiring for the points. Then swap the wires to the plugs, paying attention that you put the correct cylinder pair to the opposite point (as in, if left point fires 1 and 4, swap the point wires at the earliest connector from the points plate and plug wire 1 into cyl 2, 2 to 1, 4 to 3 and 3 to 4. Do not swap 1-4 or 2-3 as nothing will have changed electrically.) Make sense?

A
 

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Discussion Starter #6
So, I swapped the coils. Just switched them around, and still had the same problem. Running like shit, etc.. Obviously this meant that there wasnt a coil problem b/c it was doing the same thing on 2-3 with the coil that was firing 1-4 fine.

So.. my buddy and I decided to pull the carbs just to double check them. I had them broken down and cleaned when I first got it a month o so ago, but we thought we would check. We pull them off and pull the bowl off on the second carb. As he pulled the bowl off the main jet just falls out and hits the ground. This is obviously the problem so we put it back in tight and the bike starts up and runs fine. Why three had fouled along with two so bad I dont know.

Apparently when the carbs were cleaned the main jet wasnt tightened down enough and it slowly unscrewed itself and eventually fell into the bowl.



My CB750 Project:
http://www.caferacer.net/forum/topic.asp?TOPIC_ID=4739
 
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