Help me get rid of the bog! 72 cb750

[Bugteeth]

First off, i tried to look through as many of the posts as possible before posting but didn't see anything that i was confident was the problem. So sorry if someone asked this already.

I bought a 72 CB750 this winter that seemingly ran almost perfect. So now that spring is here and i've had it out a few times, i noticed that once it's completely warmed up (15 min or so) is starts to bog off the line. When i give it the first throttle twist it almost dies, usually doesn't but close. Once i am going through the gears it does fine.

So it does have some black smoke when you give it a good rev. I changed the plugs and after checking them after a short ride they were definitely on the black side. I barely have to choke it at all when starting so i would assume its getting too much gas. Which would make sense why it runs awesome until its fully warmed up. It already had a 4 into 1 when i bought it. Most everything else is stock, including the air box. I never really got to speak to the owner (odd story) before purchasing so i didn't get to ask about much about the bike. I dropped the rear shocks from 14" to 12.5". Could that effect anything since the bike sits so much lower?

I'm not a mechanic by any means and a lot of this stuff is all new to me. From chatting with random people i get the impression that its jetting related. Wanted to get some good advice first before taking anything apart.


Anyone have this issue before or have any suggestions? I would appreciate any help.

[barelycompetent]

It does indeed sound like a jet issue.  Check out this website http://www.6sigmajetkit.com/ for jet kits.  They run about 80 bucks, tell them what all has been done to the bike and they will send you several different jet sizes to try.  The instructions with them are very straightforward and easy to follow.  Hopefully that helps.

[Bugteeth]

Thank you! On a scale from 1 to 10, how difficult is it to change them?

[TOOLS1]

It sounds like the pilot jets have been changed, and are too rich. They probably did this because they thought it needed to be richer, for it to run right with the 4 into 1 exhaust. As, for them being easy to change on a scale of 1 being easy, and 10 being hard, I would say a 0.5. You do not even need to remove the carbs from the bike. Just slide the spring clip off the float bowels, pop them off, and use a short screwdriver to remove/replace the pilot jets. Stock should be a #40. However sometimes people drill out the jets to a larger size. So don't count on the number being correct.
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[Bugteeth]

Even better! Thank you.

[Bugteeth]

Doesn't sound like you need to mess with your intermediate jets, just the main? I heard someone mention them in a convo as well.

[Lucky 1]

The shocks will have no effect on the carbs.

If the airbox and filter are stock then all you need to do to accomodate the 4 into 1 exhaust
is to richen the mixture screws about 1/4 turn out. Thats it.

You can use a #120 main just for insurance at WOT to make sure it is not too lean.
Take it for a ride after each change that you make.
Make one change at a time.

The 657 and 7A in 1972 used #120 main jets stock. Needle clip position #3 from the top.

[shinyribs]

I run a 4-1 with a very open muffler situation. I also have the factory air box. I was previously running pods and had to rejet to accommodate the factory airbox going back on.

My previous jetting(for pods) was 125 mains and 42 pilots. My idle mixture screw is at 3/4 turns out. But not all idle mixture screws are the same! Some are solid tipped,others have a hole drilled through the end of them. Mine have the holes.

With the factory airbox back on the bike I was getting an extremely rich bog at WOT with the 125 mains. I dropped down to 120's (all else being the same) and it was no better. I didn't have any 115's to try so I dropped again to 110's. The WOT bog is gone but the bike feels like it may need a bit of fuel at WOT now. I'll probably raise a needle instead of going to larger jets.  

Long story short,I think 120 mains might still be too rich for a stock airbox and a 4-1. JMO.

[TOOLS1]

The "bog" is coming off idle, so that means it is the pilot jets.
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[Bugteeth]

Update:

So this is interesting. I took the bowls off to see what size jetting i was running and find there are NO bowl gaskets. That has to be part of the problem right?

[TOOLS1]

That would be an entirely different problem. Why were they not leaking?
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[Bugteeth]

I wondered that myself.

[shinyribs]

Hmmm,interesting. That may be an indicator that the float levels are set very,very low. Which could possibly cause starvation issues to the pilot jets<-- your bog issue.

[Lucky 1]

Sounds like it could be a fire hazard too.

[Bugteeth]

Thank you! So i ordered gaskets. After I get them on and see how it runs...adjusting the floats and mixture screws is next if its still not running right?

[trapper]

Adjust the floats to make sure they're sitting right. You'll want them to go up enough to fill the bowl and to push the needle gently (not jammed) into the seat to stop fuel flow. As everyone said, this does sound like a pilot jet issue. But, if it did run fine last year, I would remove them and give them a good cleaning before you try to upsize them. Remove the jet and tube it mounts in. Clean it and check all the little holes for any clog. When you put them back, just screw them in snug. Don't wrench down on them... Common sense I'm sure but I have gotten into some ugly carbs before cuz the previous owner wrenched everything in TIGHT....  All this is easy to do and can be done with the carbs on. But taking them off and giving them a good bath never hurts.

As for the mixture screw. Do the cleaning first and adjust the floats. Once there fire it up and see how you are. If you need to work on the mixture, each one can be a little different. How I do it is, I set them all to about 3/4 to 1.25 turns out depending on the bike (mine is 1.25 but I'm jetted). Clean your plugs then take it for a short ride. Then remove each plug and inspect. Adjust each carb richer or leaner based on how it's respective plug looks.

*Edit- Also, when screwing in the mixture screws, again only turn them till they stop. Don't over tighten or you will strip out the inside. Honestly finger tight in fine. Especially cuz you're not staying there.

[trapper]

Another little advice that has come in handy for me. A lot of these old bikes have gasket issues. Either just due to age or from previous owners not replacing them. Hit up your local auto store and get a roll of gasket material. You'll save a bundle in the long run. It's only like $5 and change at AutoZone. If you can keep a steady hand and have a sharp razor, it will only take you a few minutes to make a gasket and will save you boat loads of cash in the long run.
Here's an example of one I just had to make for my 81 750c

[Bugteeth]

Appreciate it! Keep you all posted.

[Bugteeth]

Update: So I did come to realize I did have bowl gaskets, however very disintegrated and pretty much junk. So those are replaced and overall didn't effect my bog situation. Its still there when fully warmed up.

 I may have more problems than just my jetting though, if thats even the problem. Not sure whats all related or not so sorry for not starting another post.

I finally had a chance to get out on good stretch and ran up over 55mph. When i stopped I noticed i had oil all over my pipe and rear tire, etc. Its coming from my breather hose that goes to the top of the oil tank. From reading other posts, i did make sure i do NOT have too much oil in it. Doesn't seem to do it when putting around town though. So far its only at higher speeds over 45-50. Again, fully warmed up.

Also, as this was my first good ride I also now notice what seems to be a vapor/whitish smoke coming from the other breather (crankcase?) after returning home. It doesn't smell burnt or anything. Maybe a bit gas-like, I'll have to look closer. Bad Rings? Seems too much to be normal.

Starting to think im gonna have to have a experienced mechanic familiar with old bikes take a look. This is quickly going beyond my abilities.

[Lucky 1]

You keep jumping around.

You need to be more disiplined about the order of things.

First check the float levels and if you have not replaced the needles
 you could do it now and not wonder about it later.

Next install stock idle jets.
Check the needle clip position. They should be in the stock position.
Main jets 125 just for insurance.

Do not skip any steps.