Carb Syncing and Jetting

[mluk1]

I have a 1977 Cb750F2 and it's gone through some heavy mods but nothing really with the fuel system. I am running the stock exhaust but I have a custom Airbox. I am 99% sure that I have set the timing correctly but my bike has had problems starting. I am running kick start only and sometimes I will kick it and I can hear the engine pop or the kick start itself will kick back at me or stop half way down.  

I adjusted the valve according to the specs in the Manual, I set my cam tension and couldn't get it right so I ran the bike and loosened the bolt and tightened it back up that way, then I used a timing light to set the timing for my plugs.

I had everything set up to sync the carbs, the gauges are reading and what not but when I turn 3 of the 4 air screws and blip the throttle there is no change in the position of the needle on the gauge. I turned the screws all the way in and out and they all still read the same. Except carb 1. So I took off the air screws and checked them all. The one that was working you could hear the spring crack as I turned it, the second one sort of and the three and four there was nothing. Is this because I am missing O rings underneath the springs? Is this why I can't sync my carbs?

So my question is, how to I get a proper reading on my Carb Sync Gauge?

Oh and attached is a picture of the airbox I am running, does it justify me changing the main jet? I think the ones in there currently are 108? And the filter in there is a lawn mower filter. And my Spark plugs were carbon fouled and I think one was fuel fouled??

[mluk1]

Ohhh and when I pulled out the air screw for carb 4 the tip of it was bent! They don't sell these things anymore! I just checked and Honda Canada only has 1 available!!!

[TOOLS1]

The air screws come in rebuild kits. I got mine from vintagecb750.com . I would also bet your accelerator pump is not working also. The two balls in it get stuck very easily.
To sync the carbs. First you adjust the air screw one at a time, for maximum idle, adjusting the idle back down after each one with the main idle adjuster. then you remove the tops off the No#1,3, and 4 carbs (No#2 is the base, and can not be adjusted). Then loosen the lock nut, and adjust with the little screw.
TOOLS  

[mluk1]

I've looked at the vintagecb750.com and they sell only a carb packing kit for the Cb750F2. All the other rebuild kits with the air screws are for earlier models.

Can someone tell me if it's the air screws are the problem or am I just adjusting it wrong??

[TOOLS1]

You were adjusting wrong. The air screws do not have anything to do with syncing the carbs. Just adjusting the idle.
Also I'm sorry I got my accelerator pump kit from Vintage, and my carb kits from Jerry's http://www.jerryscycles.com/
TOOLS

[mluk1]

Hahahah that's me being an idiot again! For some reason watching videos of people doing it on Youtube all I ever saw were people adjusting the idle screws when the title was Carb Syncing. I will give it another go then. So should I clean out my accelerator pump and try it again? Oh and on the website it seems like they don't have a kit for the F2 model! Everyone hates this model!

So now how do I know what maximum idle is and what not? How do I set that??

[TOOLS1]

Jerry does not have a kit? They should be the same as the K, and A models.
To set the idle. Adjust the air screw one 1 carb at a time. Adjust it to where you have maximum idle speed, then if needed back off the main idle adjuster (on No#4 carb) to bring it back down. Then do another carb the same way, until all 4 have been done.
TOOLS

[mluk1]

Okay so I sync my carbs up last night. All the gauges were balanced with each other, I'm not sure if the idle screws are correct but the bike is running now. My accelerator pump is down, there was caked on rust in the inside. I cleaned it out and will be buying all those new parts off of Vintage cb750.

I did the thing where you crank the bike all the way up to 5th gear and hit the kill switch. I pulled my plugs and 1 -3 three had barely any discoloration (These are all brand new plugs) but number 4 already was pretty black. My guess is that I didn't adjust the valve correctly on that one because there were about 1 or 2 big pop and bang sounds when I tried to kick it over. Any thoughts? Because after this I have no idea what could be wrong with it.

[mluk1]

Well I tried to test everything out again and I think I need to rejet my carbs, I'm at 108 right now. And I realized I am missing parts in my diaphragm. The shitty thing is, all the CB sites are out of stock on all this stuff so it looks like I'm screwed for a while.

I tried riding it around but when I came to a stop the bike just ended up dying. I started it again and it died again. I pulled the plugs and 1 -3 I think were carbon fouled and the 4th one I have no idea what is wrong with it.

Here are the pictures of my carbs, they are in order of 1 -4 from left to right.


 

The 1 - 3 carbs I think are telling me to rejet but I have no idea what to do with carb 4?? I am missing the O ring and spacer in the air screw for this one, could that be what's affecting it? It's strange how all this stuff is happening considering all I did was put on a new airbox, adjust the valve, ignition and cam chain timing. But I need some serious help, I have no idea where to go now!!!!!

[TOOLS1]

I have not had much experience with jetting, but the first 3 plugs look good to me. I can't tell on the No#4. Is it carbon, or oil? Have you ran a compression test?
TOOLS

[mluk1]

So it's been a long route, I've checked my Cam Chain, valve and ignition timing and it is all correct. I am unable to start my CB at all now. I removed the starter so it's kick only. When I turn everything on and try to kick it over, it makes deep wind noises, the kick start sometimes kicks back at me and it won't go down anymore, there is some blueish black smoke coming from the 4th carb into the airbox, and most importantly there are huge bang sounds and massive pops.

I checked the the compression and these are the results I got.

Dry Test:

C1 - 110
C2 - 90
C3 - 90
C4 - 60-90

Wet Test:

C1 - 130
C2 - 120
C3 - 125
C4 - 90-120

It was hard to get a proper reading for the 4th Cylinder. So there is a range I guess. What does this mean now, from the book it says that it should be a ring problem. But could it be a bent valve stem because of the symptoms?

[TOOLS1]

Sounds like a ring, and a timing problem. Even with a bad ring it should start. The back firing is probably timing.
TOOLS

[ArtRobin]

I had a '76 CB750 while in Germany and never had any problems -
I just picked up a '77 CB750A - and this has been fun -
Since I don't like adjusting and replacing Points - I replaced the Ignition with a PAMCO Hall effect kit - Including the coils. Vintage CB750.com sells the kits as does CB750 Supply.com and Outwestmotorcycles.com

Prices vary but around $180-200 is in the ball park. The instructions for installation are on the internet and are easy to follow.

After starting and setting the timing - this old thing starts up right away. ( I had also rebuilt the carbs - including replacing the accelerator pump - Outwest had all the parts necessary and shipped quickly.

I suspect that your main problem on the backfiring is timing - unless you REALLY messed up the valves.
OH- btw I was able to open the spark plug gap from .025 to .035 and it runs much better than the point system ever did - and NO MORE BURNT POINTS.

[Lucky 1]

You said you have a "custom" airbox.
If the airbox has less restriction it will want to run leaner and that can make popping and spitting back
when trying to open the throttle. It can make backfire also when trying to advance the throttle.

Usually with a performance exhaust(less restriction) and less air filter resistance you would have to increase the main jet ,raise the needles  and also richen the air pilot screws. ANY combination thereof.

If you just had after market exhausts and NO air box modifications then you can get by with just a air pilot screw adjustment. No extra air in - no extra gas needed.

The fact that you found a bent pilot airscrew makes me think other things could be not right by that same heavy handed person that did that.

One more thing...The cam chain tension must be performed BEFORE valve adjustments.
The cam chain adjustment MUST be done when the crank is in the correct position(about 45º) ATDC on some HONDA's BUT CHECK THE BOOK!!!
Also Valves have to be adjusted with the crankshaft in the correct position.
CHECK THE BOOK.

You cannot do any of this correctly without the workshop manual.

I am a ex dealer mechanic from those days.

[TOOLS1]

My Honda manual says to adjust the cam chain with the engine idling, by loosening the adjuster, and then locking it back down. What are your thoughts on this?
TOOLS

[Lucky 1]

Sounds right.
The CB350's were a different story.
Each model was different.
Whatever the book says.

[Lucky 1]

Which engine ,what year?

[TOOLS1]

It's my Honda manual, for the automatics. 76-78.
TOOLS