Topic: Vacuum Sync Issues (And More Carburetor Conundrums)

[LouisTheCat]

So I fixed my carburetors from my previous posting (shop gave me poorly synced carbs so I had to redo) and actually rode my bike some. When I was riding, it felt like my engine was chugging along (felt like I was constantly on a bumpy road) around 1/2 throttle, so i figured my needle jet needed adjusting.

I'm running all stock (1975 CB750 K5), and I saw that the needle clip was in 1st position (mine are 657B so they should be in 4th). I adjusted all the carbs to this spec, but when I pulled the top off carb #2, I saw that the clip was already in the 4th position which is weird because the rest were in the 1st position. No big deal, right? I had previously set everything else to spec. I adjusted valve clearance, cam chain tension, and dialed in my timing with a strobe light.

While setting the timing I hear popping from a pipe on the left, what I'm assuming is the 2nd cylinder. I assume it's lean since I synced the bike before all this and this carb was running rich compared to the others, and now that they're all the same this one is lean, but perhaps my logic is incorrect.

I go to vacuum sync and I just can't get it. Not only am I having trouble getting play in the throttle (which I achieved eventually, but not exactly correctly) but I have run into another problem: the bike is revving high again. No matter how I adjust the slides on each carb the bike idles above 3000 RPM, which wasn't a problem when I started the sync.  When I started the sync, I was having trouble keeping the bike running around normal idle which is 900-1000 RPM. Somehow I escalated to this predicament and I am unsure of what to do next.

[flybox1]

Your jetting is wrong.
The bike cant achieve a low enough idle to vacuum sync because the idle mixture is too far off.
Are you 100% sure all main jets are the same brand and size? What brand? Pilot jets the same brand and size? What brand?
Mixture screws all set the same turns ?
If all is good, warm up the bike, pull the old plugs and install a new set of plugs.  Let it idle for 5 minutes witha fan on the engine...
Pull the plugs, line them up 1-2-3-4, and take a picture of the tips.
Post the picture here.

[harisuluv]

Do you know what the idle adjust screw is?  If so please tell us, otherwise we can't proceed.

[LouisTheCat]

Main and idle jets are all brand new Keihin jets, 105 main and 40 slow. New genuine float valves and floats set to spec as well. New plugs as well. Idle mixture screws all one turn out. I will post pictures of plugs tomorrow.

[LouisTheCat]

Your jetting is wrong.
The bike cant achieve a low enough idle to vacuum sync because the idle mixture is too far off.
Are you 100% sure all main jets are the same brand and size? What brand? Pilot jets the same brand and size? What brand?
Mixture screws all set the same turns ?
If all is good, warm up the bike, pull the old plugs and install a new set of plugs.  Let it idle for 5 minutes witha fan on the engine...
Pull the plugs, line them up 1-2-3-4, and take a picture of the tips.
Post the picture here.

First picture is all four; second picture is one and two; third picture is three and four. This is after my syncing issue. I didn't get a chance to do a proper plug chop so I'm just showing my plugs after my issue. I'll get around to the chop tonight, hopefully.






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[flybox1]

do your chop on new plugs please.   those are too fouled (rich) to get an idea of what idle mixture looks like.
is #3 wet?  oil?

[LouisTheCat]

do your chop on new plugs please.   those are too fouled (rich) to get an idea of what idle mixture looks like.
is #3 wet?  oil?

It was a little bit. I think the sunlight might've made it look wetter than it was. That pipe was never blowing smoke though. My new plugs will be here tomorrow. I'll do the chop and post the results then. I might take the float bowls off tonight and check for anything in there that could be clogging an orifice


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[LouisTheCat]

do your chop on new plugs please.   those are too fouled (rich) to get an idea of what idle mixture looks like.
is #3 wet?  oil?

Here are the new plugs (NGK D8EA) 1-4, respectively, after 5 minutes of idle.




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[harisuluv]

Ok, that's great that we have some plug pics and all, but if you look at it from our side, all we have is SEVEN enormous pictures of plugs.  We don't even know what your bike looks like.  How about pictures of your bike, your carbs, etc.

[Duanob]

There is a protocol for all of these bikes. Start with a 3000 mile tune up. Then make sure the carbs are in spec: needle position, float height, all jets are the same and not tampered with.

Check for vacuum leaks. Then do a vac sync. It sounds like you might be missing something.

[flybox1]

Lean, for sure.
While you are taking pictures of your bike, turn your mixture screws richer by 1 full turn, and retest.

[DH]

I go to vacuum sync and I just can't get it. Not only am I having trouble getting play in the throttle (which I achieved eventually, but not exactly correctly) but I have run into another problem: the bike is revving high again. No matter how I adjust the slides on each carb the bike idles above 3000 RPM, which wasn't a problem when I started the sync.  When I started the sync, I was having trouble keeping the bike running around normal idle which is 900-1000 RPM. Somehow I escalated to this predicament and I am unsure of what to do next.

[/quote]


In addition to what's been stated, check for this condition on each carb. If needle plate is bent (like the upper drawing) the throttle rod will have slop in it. Rod should sit inside of needle plate and spring be lightly loaded. If bent, carefully straighten plate till correct.

[JMS28411 I'm no Spring Chicken]

Prior to 1974 I believe the 750's had 110 main jets, they were changed to 105's in 1974 models to meet emission standards and the bike's did not run as well, I know I had a 1974 750 and was not happy with the lack of performance until I replaced the 105 mains with 110 mains

I don't remember what the pilot/slow jets were prior to 1974.

I do like the early model carbs better than the later versions, being able to drop the float bowl by popping off the spring clip and easily make changes to the jets is nice and you don't have to worry about the 4 little screws.

[LouisTheCat]

Ok, that's great that we have some plug pics and all, but if you look at it from our side, all we have is SEVEN enormous pictures of plugs.  We don't even know what your bike looks like.  How about pictures of your bike, your carbs, etc.

Ok. Here are some pics of my bike.





So in testing and syncing some more, I decided to do another compression test. I did a leak down and compression test over the winter and everything was solid. I hit 120 psi on all jugs on a cold engine.

I warmed up the bike and I should note that when I get it warmed and idling smoke comes out of the crankcase breather. In the test today, #1 got to 130psi; #2 went to 120; #3 90 and #4 about 130. Could it be possible I have a valve stuck open for cylinder number 3? I don't have smoke pouring out of my exhaust and the plug for that cylinder isn't coated in oil, but it looks like it has a tiny bit of oil on it.