79 CB750 spark question and engine miss at speeds.

[Hippie459MN]

Well, I got my CB running ok after a mess of stuff this srping but now I am having another (possibly two) issue. When I am under a mid to hard acceleration the bike runs like a champ but when I am at a constant cruising speed it feel like its missing and only running on 2 cylinders but if I give it gas it will smooth right out. It does have the stock CV carbs on it with pods but all of the jets are factory and I have not changed them. I had to give the carb a really good cleaning this spring and had to readjust the pilot jets to the 1 1/2 turns out that the service manual said they should be at and that seemed to help a lot but now when I am at a constant speed it seems like the bike is missing/not running on all cylinder and when I let off the gas, slowing down, it sounds like I am getting some faint popping out of the exhaust and sometimes at night you can see some flame out of them and its usually the number 1 and number 4 pipes. It does have short straight pipes but I have put in some short (4 inch if I remember right) baffles in each pipe. Thinking a bench sync might be my next try. Going to be ordering a jet kit this week and then I can give that a try too. All of my plugs are kind of a light brown color.

This is the number 1 (right) and number 2 (left) plug. 3 and 4 look pretty much the same.



Now I was checking the spark on each cylinder by pulling all the plugs and turning it over and the no. 1 and no. 4 plug were doing something I think is very odd. Its like there was left over charger after I was done cranking the motor and the plugs would spark a bit a second or two after I was done cranking the motor. I have never seen this and pretty sure that is not supposed to happen. The video is below....

https://www.youtube.com/watch?v=mZ7dVMb0lD8

Also, I did ohm out the coils and was getting 2.5 ohms on the one coil (cylinder 1 and 3) and 2.6 (cylinder 2 and 4)on the other but can not find in the manual what it should be and the secondary side I wasnt able to get anything. I did not do the coil test yet like the manual says but everywhere I have kind of been looking it says that my coils should ohm out more in the 3.5-5 ohm range and anything lower would need replacing. I hate to just start throwing parts at this thing and not have it do anything. This thing ran fine all last year (except it idled like crap but was liveable) until the alt rotor went out and then I let it site full of gas for 6 months gumming up the carbs. Stupid me....

Edit: Oh yeah, I had disconnected the plugs for the spark units and they may have gotten switched around but I am not 100% so I am going to maybe switch them around here tonight and see if it runs any different. I am not sure if that makes a difference or not. Maybe someone else knows... Anyone? lol

Any help or input would be great before I throw this bike in the dumpster. LoL Its been one nightmare after another this year and I cant even go riding without having issues with it running right.

Here is a picture of my CB750 I am working on...

[Truck]

Pods and factory Jets don't mix. Do a search on here for Shineys thread to get pods to work. Takes some experimenting, but it does work.

[Re-run]

Well, the first mistake is pods, especially if they are cheap-o pods. The cv carbs you have carry a natural dislike for pods.

If you need to use pods, then get K&N pods for best results.

As mentioned, there is a thread by shinyribs that explains how he got them to work. I will say your idle mix screw adjustment is just factory start point. It is only a ballpark figure and you will need further tuning from that.

Your plugs are a good color though, these bikes are meant to be lean and so brown plugs are not what a person wants.

[TOOLS1]

The vacuum situation, or lack of it caused by the pods is creating havoc in your vacuum controlled CV carbs. That is why they are not operating correctly at a constant throttle. When you are accelerating, and decelerating the carbs are getting a big change in vacuum so they have no choice but to work, however when at a constant throttle they cannot balance because the pods are not creating enough vacuum for them to even out. A lot of people think you can get CV carbs to work with jet changes, and they might get them to run decent, but they are not running correctly as they were designed to do. To really make them run correctly with pods, you must make changes to the slides.
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[Hippie459MN]

Thank you for the replies. I know last year the bike ran fairly decent and didnt have this stupid miss issue but was a pain to get to really idle at all. I let it sit all winter with gas in the carbs (I know, stupid me) and it gummed them all up so after giving them a complete cleaning and replacing the accelerator pump and when I removed the pilot screws, one was out 3 1/2 turns, one was out 2 and another 2 1/2 and another 2 turns so I put them back to what the service manual said for the 42-A carbs (1 1/2 turn out) and it seemed to run a lot better then it did last year but once I got it out on the road and up to cruising speed thats when I noticed the horrible miss in it but like I said, under any decent acceleration it would run just fine. No last year is when I got the bike and it looked nothing like it does now but when I did get it the prev owner had put tape over half of the pods and starting to think that was why it was working better as you stated with the whole vacuum thing. Just thinking out loud here. This is my first bike that has had these stupid CV carbs on them so its a learning curve. I might try putting some electrical tape back on them until I can get some better filters and re-jet the bike and see if that helps.

Also, I tried looking but couldnt find it. Maybe I am missing it or something but I can find the topic by shinyribs on setting up CVs for pods. Anyone has the link that would be great. And again, huge thanks for the help so far everyone.

[Re-run]

It was slownugly007: http://honda-cb750-s.456789.n3.nabble.com/I-GOT-PODS-TO-WORK-td4039223.html#a4051529

[TOOLS1]

Yea, making pods work is a slow, and ugly process.
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[cblover]

I agree it should be done precised and well made to avoid any mistake upon installing and tuning the engine.

[Hippie459MN]

Well, I did get it to run a bit better buy covering half of the filters with electrical tape. I ordered a jet kit for it and am going to try turning the pilot screws out another turn and see if that helps also. I turned them back in according to the service manual (1.5 turns out). I will be tearing into it again this week. Wish I had a carb sync tool also as im sure that's also part of my problem. One thing at a time but at least its rideable.

Again, thank you everyone that has responded. Ill take every bit of info I can get on these carbs.

[Hippie459MN]

[DoubleMal]

Hey there Hippie459MN - just to comment on your question about the coils my original stock set ('82 CB750c. Not sure if the two bikes used exactly the same coils) measured ~2.4 ohms and a brand new after-market set I bought measured about ~2.3 ohms each....

[LukeM]

FYI, what is happening with the electrical tape on the pod filters, is restricting the airflow on the input of the carb.  The factory airbox does the same thing by design. That restriction is the "vacuum" if you will that the carbs are looking for, for normal operation.  Naturally, there is a balance point between how much tape covers the filter vs. how much the carb needs to work properly, vs. how much air is getting to/through the carbs.

Without the tape, there is less restriction, hence less perceived "vacuum" at the input, hence the carb(s) freaking out because that "vacuum" isn't there.

Luke M