Author Topic: very low vacuum reading on all four carbs (Read 1074 times)

samm_j2 · « **on:** March 01, 2020, 04:29:19 pm »

Hello everyone,

I am new to online forums and don't know where or how to post so I hope someone reads this and can help me out...

I am hoping for some help... I am a long time reader of this forum but just became a member as I am beating my head against the wall trying to figure out what is going on with my bike.

I have a 1978 Honda Cb750f

History of the bike: I really wanted to go all out with this build, I'll try to figure out how to post a picture and a vid to show whats up.
I put an 836 kit in it with a mild cam, wiseco pistons, cryo treated chains n what not. Did a full motor tear down and restore with new bearings and what have you. I am running the stock carbs but with an air box from cognito motor.

First time around: when build was finished everything was great, did a break in period and bike was happy and health, very good throttle response and very snappy. Well one day just cruzing around it seemed to have lost a little bit of power, was not as responsive and I could not figure out why. Upon further inspection I had a leak from the head gasket, nothing serious but it was weeping which seemed to be a common thing. Be being a bit of a nut said ok gonna lets tear it down and find out why.

Instead of pulling the entire motor I opted to cut the frame and put one of the weld in kits for the cross bars, since I was only getting in the top end I thought this would be a little easier. So after I get the valve/cam cover off I realize two two things: 1 an oiler for one of the rocker arms was clogged and I had fried the cam, bummer but not the end of the world. I also noticed that some of the studs were loose! well shoot. They are the kibblewhite studs but with regular stud bolts. So I pull the head make sure all the bolts are tight and re-torqued, put a new base gasket on, put the cylinders back on put decided to reuse the piston rings. bike has less that 2k miles on in and thought that would be fine... maybe not??? I reinstall the head but this time with kibblewhite nuts, expensive nuts but hopefully worth it. followed a lot of info from this site and made sure to seal those pucks up. (it also had the drain hole modified and a valve job done the first go around by cycle x). none of this is really the problem but just giving some history.

So get the bike all buttoned back up but the bike is not that happy at idle. its will change and then also suffer form a hanging idle. all of which seem to indicate a vacuum leak. so I take the carbs off and give them a clean, put them back together and no difference. I check with a vacuum gauge and while the needle flutters it stays towards the bottom of the gauge ON ALL FOUR CARBS. I think the gauge works as the first time I synced the carbs it went as expected, they are the alpha moto ones and when I suck on the tube the needle moves. I have hosed the boots down with carb cleaner to find a leak but nothing. What could cause the gauge to be low on all four carbs? did I miss something inside the head? I even changed out carb body one for a spare I had and same thing. just for fun I adjusted one of the slides and sure enough I get more vacuum but I can't base it off of number two. I took them off and bench synced them and same story, very very low vacuum. The boots are fairly new, well new when I did the rebuild, I even pulled out the stock clamps to tighten them down and nothing. I rechecked valve lash and same thing.

The bike does run fine, throttle response is not as crips as the first time I built it. but the bike feels like it makes some good power and can really go. I am not sure what to try next except new boots and clamps, hell at this point maybe just put some mikuni rs34 on it or something but I dont really just wanna throw a part at it and never find out what was really going on...

It has a cycle x four to one stainless steel exhaust.

all symptoms point to a vacuum leak that I can not find to save my life, I have read every post on the forum and others to try to find a solution.
Has this happened to anyone else?

again I am new to forums and a not sure how or where to post stuff.

please let me know if anyone has any insight, thanks
Sammy

tlbranth · « **Reply #1 on:** March 01, 2020, 05:51:19 pm »

Are you sure your camshaft is timed correctly?

samm_j2 · « **Reply #2 on:** March 01, 2020, 06:03:02 pm »

as far as I know yes, line was parallel with keyhole up.

can the bike run without a valve hitting the pistons if it were installed incorrectly?

newday777 · « **Reply #3 on:** March 01, 2020, 06:25:25 pm »

Is the muffler plugged from mouse nest?

samm_j2 · « **Reply #4 on:** March 01, 2020, 06:28:03 pm »

just looked up it with a light and it's as clean as a whistle.

tlbranth · « **Reply #5 on:** March 02, 2020, 04:05:02 pm »

If vacuum is low on all 4 cylinders, look for something common to all 4. All that come to me at the moment are cam timing or bad gauge

samm_j2 · « **Reply #6 on:** March 02, 2020, 04:52:54 pm »

Quote from: tlbranth on March 02, 2020, 04:05:02 pm

If vacuum is low on all 4 cylinders, look for something common to all 4. All that come to me at the moment are cam timing or bad gauge

I'll pull the cover and check the cam tonight, and try to find a second gauge to test it against. is there a way to test the gauge? besides sucking on the tubes?

tlbranth · « **Reply #7 on:** March 02, 2020, 06:00:38 pm »

What type of gauge setup do you have? Got a picture of it?

samm_j2 · « **Reply #8 on:** March 02, 2020, 07:16:13 pm »

it is alpha moto, I'll see if it will upload the pic of them.

I just tried adjusting the slide screw and then the slide raised up on carb 1 the vacuum gauge reading went up, which I thing is a good thing. I bench synced them off of carb two which is the non adjustable... so if it is just that slide 2 is too low how do I fix that?

Jerry Rxman Griffin aka MuthaF'er · « **Reply #9 on:** March 02, 2020, 08:14:34 pm »

Quote from: samm_j2 on March 02, 2020, 07:16:13 pm

it is alpha moto, I'll see if it will upload the pic of them.

I just tried adjusting the slide screw and then the slide raised up on carb 1 the vacuum gauge reading went up, which I thing is a good thing. I bench synced them off of carb two which is the non adjustable... so if it is just that slide 2 is too low how do I fix that?

Sync the other 3 to the one that does not adjust. That's pretty much all there is to it. You are trying too damn hard here! #2 does not adjust. That is how it's supposed to be. Forget the amount of vacuum. Synching means to get all the carbs adjusted or synchronized to the #2 and does not mean to attempt to raise vacuum.

seanbarney41 · « **Reply #10 on:** March 03, 2020, 02:33:53 am »

Turn up the idle speed screw, then turn down 1, 3, 4 to match vacuum

samm_j2 · « **Reply #11 on:** March 03, 2020, 12:40:18 pm »

hey seanbarney41, thanks, I did try try that however it seems it is when I lower the slides that the vacuum increases and even with the idle screw backed all the way out which puts it at its lowest point it does not improve the vacuum gauge.

jerry muth'Fer, I hear ya. I also feel I am trying too dame hard here but cant felt inhave over looked something... just becuase when I had this gauge to these carbs it was reading around the 20 and now it flutters between 0 and 5.

is there something I did wrong with the reassembly of the carbs that is not letting the carbs sit lower?

seanbarney41 · « **Reply #12 on:** March 03, 2020, 01:02:35 pm »

how does the bike run?

samm_j2 · « **Reply #13 on:** March 03, 2020, 01:40:08 pm »

I gave the full break down above, it runs really good when already on the throttle, the was a bit of a delay when opening the throttle, it's not as snappy as it was when I first built it up. and then there was a hanging idle which led me to belive it was a air leak hence the reexamination of the carbs and then the vacuum gauge belong low has me thingink something is up.

to sum up great but not off the line amd when coming to a stop it'll stay revving high...

seanbarney41 · « **Reply #14 on:** March 03, 2020, 01:42:12 pm »

something is still wrong

samm_j2 · « **Reply #15 on:** March 03, 2020, 02:57:49 pm »

update:

I noticed when I loosen the arm on slide two the slide drops low enough to get vacuum... so gonna pull them apart and take a closer look at the through rod for the slide arms... maybe ita up side down... I'm really considering to take another adjuster arm from a spare carb and make slide 2 adjustable as well.... maybe not the best, maybe just a bandaid... any body got any thoughts on that? I feel likes its blasphemous...

enwri · « **Reply #16 on:** March 03, 2020, 05:23:42 pm »

If you have almost no vacuum and it's not revving its head off, it's most likely late valve and or ignition timing. No valve clearance or jumped tooth. Not just cb engines, all 4 stroke engines.
No vacuum and no revs means open throttles, and it's barely able to run.

Or it's only running on one cylinder and that one is open so far to keep it turning that it has no vacuum and the others don't matter what the opening is.

editing again to add. As member dunnp2 found, incorrectly calibrated gauges have been throwing him off, gauges calibrated to each other and giving the same values when used are more important than actual values. Like the many differing compression values from different model comp. testers.. The relationship between cylinders being the important bit.

samm_j2 · « **Reply #17 on:** March 04, 2020, 07:59:37 pm »

Welp tlbranth and enwri you were absolutely correct... My camshaft was off by just one tooth... they need an emoji of a face palm... I don't know how that happened... newb mistake on my end, I am just glad that a valve didn't hit a piston or anything like that.

so I re-timed the camshaft and it is running as good as when I buttoned it up the first time. its pulling vacuum and everything.

I should have listened to my father from the start as he thought the timing was off as well... live and learn.

Thanks so much for everyone who contributed to helping me figure this out

enwri · « **Reply #18 on:** March 05, 2020, 02:38:43 am »

Good to hear it's running again...In 1979, 13 year old me first encountered a jumped timing chain on my friends XR75, with no money, manuals or concept of buying parts, we stuffed strips of leather in the rocker cover to guide the chain and keep it from slipping. Worked, he eventually sold it on. Back when the tool kit was a flat blade screwdriver and a rock. Didn't have to carry a rock, usually found them wherever you break down. Can also adjust points and remove spark plugs with that toolkit. I've done terrible things...

tlbranth · « **Reply #19 on:** March 05, 2020, 08:07:12 am »

Good for you Sammy

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Author Topic: very low vacuum reading on all four carbs (Read 1074 times)