Topic: Carb sync problem?

[Davez134]

I have a 1972 CB750 that I bought about a month ago. Modifications done by PO include pod filters and 4 into 1 exhaust.
It had been sitting for over a year so I cleaned and rebuilt carbs with all new parts including new main jets (130) new slow jets, (40) and cleaning out every air and fuel passage, then bench synched using al the suggestions I have read on here. Bike seems to run ok, but when I went to synch carbs I cannot get equal vacuum.  After getting to operating temperature cylinders 3 and 4 were pulling 15 inches vacuum, but 1 and 2 were showing only 4 and 5 inches respectively, even after attempting to adjust.  (The starts to increase when trying to adjust, but no movement in gauges.)
I am using commercial gauges that i bought online (I checked al vacuum connections,)  and I did a complete tune up beforehand including oil/filter change, cleaning pod filters, adjusting valve clearance (.002intake, .003 exhaust) gapping points (.014) adjusting timing using timing light and checking compression. (cylinders 1234 compression reads 132, 122, 122, 120  respectively with a couple psi changes when tried wet.) I even checked to see if rubber carb mounts were leaking but they aren't. The only abnormal thing I noticed is it seems as if a very small amount of oil is leaking from valve cover gasket between #1 and #2 cylinders after about a 20 mile ride. I'm absolutely stumped and not sure what could be causing the difference in vacuum of if it is something to worry about?

This is my first post on here but I have been looking around as a guest for a while and would like to thank everyone for all the advice and knowledge on this site!

[phil71]

did you switch the gauges around to see if the low vac is still on the same cylinders?

[Davez134]

did you switch the gauges around to see if the low vac is still on the same cylinders?

Yeah I tried that, and I also applied vacuum to all gauges myself (by sucking on tubes and giving myself a headache haha) and that all read the same. Also i forgot to add in my original post that I changes and gapped the plugs according to service manual specs and after reading them after running at WOT then shutting down all appear ok (just #2 looks a little dark but not really bad)

[phil71]

even though the compression test points to everything being okay in this dept, i guess you can double check the valve clearances..
I'm stumped here. Using o-rings on the extender tubes?

[Davez134]

even though the compression test points to everything being okay in this dept, i guess you can double check the valve clearances..
I'm stumped here. Using o-rings on the extender tubes?

Yes, extender tubes are o-ringed and I was thinking the valve clearances might have changed so i double checked them this morning after bike had been sitting all night. I am stumped as well, as nothing makes sense. I'm kinda like Carl from "Slingblade", as I tend to think of the simplest things first, but I'm out of ideas.

[TwoTired]

You do realize that you are not adjusting for the highest vacuum but rather equal vacuum across the bank, whatever that turns out to be, right?

Raising one runner reading by lowering the slide, usually changes engine RPM and lowers others when the idle is reset.

[phil71]

wait.. which gauges are you using?

[Davez134]

You do realize that you are not adjusting for the highest vacuum but rather equal vacuum across the bank, whatever that turns out to be, right?

Raising one runner reading by lowering the slide, usually changes engine RPM and lowers others when the idle is reset.

Yeah, I was not trying to get them all to read 15, but #1 and #2 seemed so low that i didn't try to bring #3 and #4 down. I'll give that  try tomorrow. Its just that when I tried lowering slide on #1 and 2 carbs, there was virtually no change in vacuum reading.

phil71, I am using the gauges in attached file.

[lucky]

They should all be around 10.
Turn your idle down.

15 is too high.

[lucky]

They should all be around 10.
Turn your idle down.

15 is too high.

Those #130 main jets are way too big.
Even #120's are fat for sea level.

The 40 idle jet is perfect.

BTW...what are you turning to adjust the vacuum on cylinders 1,3,and 4?

You cannot adjust #2. So if two says 5 inches Mg (mercury) I would check the valves on that cylinder.

[BobbyR]

I am not sure if yours is like my 78K but #2 is not adjustable and that is the carb you adjust the others to.

I would adjust the gauges by putting one of your extenders on either #1 or 4 since they are easy to get to.  Remove the plastic covers over the faces and you will find a small brass screw. Plug you first gauge onto that extender, let;s say it reads 10. Put the cover back on that one and plug in the next gauge, turn the screw until that gauge reads 10 and so forth. This way all of your gauges read the same. This is easier than sucking on them and a bit more precise. 

Set your idle to the proper speed and try it again, it may run crappy, but should smooth out as the carbs come into alignment with each other.

[Davez134]

Lucky , Thanks for the reply. I will go out and get some smaller mains and try those out.  All 4 carbs on my bike are adjustable. I attached an image of what they look like and they all look exactly the same. To adjust I was loosening up that small upper locknut and turning the larger lower one. I thought this was correct ( I hope so). As far as the valves, I checked them twice today.

BobbyR, thank you too. I will try to calibrate all the gauges that way.

I am done for today, I just needed to step away. I will be away at work for the next 48 hours, so I'll post up some results after I get back home and mess with it some more. I am new to this multiple carburetor thing and I really appreciate all the input!

[BobbyR]

Lucky , Thanks for the reply. I will go out and get some smaller mains and try those out.  All 4 carbs on my bike are adjustable. I attached an image of what they look like and they all look exactly the same. To adjust I was loosening up that small upper locknut and turning the larger lower one. I thought this was correct ( I hope so). As far as the valves, I checked them twice today.

BobbyR, thank you too. I will try to calibrate all the gauges that way.

I am done for today, I just needed to step away. I will be away at work for the next 48 hours, so I'll post up some results after I get back home and mess with it some more. I am new to this multiple carburetor thing and I really appreciate all the input!

It is a process Bro. Step by step. I am running 120s and no issues.

[Rgconner]

All four might be adjustable, but that does not mean you should.

If it were me, I would bench sync using a drill bit as a feeler, then start again, picking one carb, like number 2, to be the one you match against.

Trying to adjust all 4 is going to drive you crazy.

[trueblue]

All four might be adjustable, but that does not mean you should.

If it were me, I would bench sync using a drill bit as a feeler, then start again, picking one carb, like number 2, to be the one you match against.

Trying to adjust all 4 is going to drive you crazy.

+1 do a bench synch and start again chosing one as the master which doesn't get adjusted and bring them all to match, otherwise all you'll be doing is chasing your tail.

[Davez134]

So I ordered some new jets today, comes with different sizes. So I'm gonna try the 120's (since plugs are on the dark side) bench sync, calibrate gauges then go from there when I get home. Ill post up some results.

[Davez134]

Thanks again to everyone for the help.  I ordered rebuild kits for the carbs, bench synched them, change to smaller main jets ( got a variety of sizes and 120 seems to be best and plugs look perfect after running wot). Runs great now with equal vacuum across all carbs. Finally.