No vacuum in #3 Carb

So I've spent an hour trying to sync my carbs and am pulling my hair out.
I cannot get any vacuum on the #3 carb. I took the carbs off, bench synced them, when I put them on the bike fired up, and I have no problem getting #1, #2, or #4 to have even vacuum, but cannot get any reading from #3, even when I rev it.
YICS blocker is in, valves are in spec.
Advice wise ones?

 

Unless if its just a clogged vacuum port you're connecting to, that's gotta be an internal problem.

I would think its a valve issue, bent, not closing, something along those lines.

 

Have you checked your compresion on number 3? A stuck or bent valve could be the problem.

Kenneth

 

Do you feel any vacuum at the port on the intake boot? Can't think of anything else other than what the others have said.
I would like to see the compression numbers.

 

Thanks I will do a comp test. It had vacuum before I did the valves and #3 intake was in spec I didn't even touch it so I'm stumped.

 

Durk, I just went through a similar problem. My 1950 Ferguson farm tractor blew the top of the radiator off while brush hogging the back field. After putting a new radiator on I thought that I should check the valves and adjust. I found a broken valve spring on one and ended up having to replace valves, springs, guides ect. I put all back together and it would start but in a short time would quit? Checked everything out, couldn't find anything wrong, said the heck with it and went to bed. Next morning a thought came to me, wondered if a mud wasp could have gotten into the fuel line while I had it off? Took the fuel line off-sure enough, half way through it was plugged solid with dried solid mud. I cleaned it out put it back together and WALA! She runs again! Sometimes it's just the little stupid things that we don't think of. grunt007, 81' 750 Seca R, Mi.

 

Yeah it is always something dumb. I'm also going to take my YICS blocker out and make sure the o-rings are still on it. I may try to sync it w/o it and then put it in to fine tune the sync.

 

if you have a carb/cylinder with no vacuum, theres not really a point in syncing. Why not fix the problem, then do the sync? Spend that time you'd be wasting syncing it twice on finding the problem!

 

You may have a bad sync tool. Switch two tubes around and see if the trouble follows the tool.

 

Do a Compression Test -- Dry and Wet, ... and get and idea of what the situation might be.

You could even to a Quick-check.
Hold your finger on the Spark Plug Hole and see if there's enough pressure to move it off on the Compression stroke.

 

I'm going to borrow my dads comp tester and I'll post the numbers tomorrow.
Taking the YICS blocker out did nothing. Just to make sure I'm not losing my mind I popped the valve cover off and #3 is in spec I .15, E .18.
The reason I'm trying to sync is because the bike was running I just did the valves, when I started the bike after the valve job it barely ran, so I took the carbs off and bench synced them. Just for giggles I measured the vacuum before I did the valves and everything was fine w/ all four carbs.
I think I just really messed up the bench sync.
Rick it passed your finger test enough compression to push my finger out.

 

Way out of sync can look like that. You should be able to turn the vacuum lower on the others until this one rises. You'll probably have to adjust the idle stop several times as you work into it..

 

If you have a Vacuum Leak big enough to drastically lower Manifold Vacuum.

The Manifold -to- Head seal might have been broken during removal of the Carbs.
The Original Manifold O-ring may have pulled-away from the Head and lost its resilience; leaking-by under load.

Smear a bead of RTV all around the Manifold to Head joint.
Tightening or loosening the fasteners poses a great risk.

Examine the Manifold Clamps.
Misplaced Clamps will SLICE right through the Manifold making an almost undetectable incision.
Ill-fitting replacement clamps will murder a manifold.

Probe the Manifold Vacuum-Pipe Orifice with a Pipe Cleaner saturated in Carb Cleaner.

 

Use a strip of 3X5 Card or Resume' Paper to Bench Sync.

Start-off knowing you have all the Throttle Plates adjusted so that they'll close.

 

Will the carb cleaner actually affect the idle without gasoline getting pulled in the combustion chamber too though?

 

Probe = Check and clean.
Don't have it running.
Just take a Pipe Cleaner, ... soak it in Carb Cleaner, ... and make sure the Vacuum Nipple doesn't have any foreign matter or some obstruction that's closed it off rather than being able to breathe.

It it doesn't have OEM-style Clamps that fit the narrow reinforced channel where the Carb Intake Neck fits ... and Clamps too wide for the Locating Channel are substituted ...

Remove the Clamps and carefully inspect the Manifold Rubber for a clean incision through the Manifold beyond the reinforced inlet groove.

 

Solid advice Rick thanks, I'll take look. I guess I have to work backwards and after I verify the compression I'll make sure the boot and nipple are ok and then do another bench sync. I cut up a business card to do it the first time.

 

So are you running with a "permanent YICS eliminator?"

 

I almost typed a serious reply Fitz. :wink:
I re-read the bench sync and I botched it. I didn't start with #3.
Try, try and try again.

 

So my compression test didn't go well. 125 psi in #3. #4 wasn't too hot either, at 135. I took the carbs off, re-bench synced them, and still no vacuum reading in #3. I can feel the vacuum in the tube from #3 to the manometer though.

 

If you feel confident that you have thoroughly checked and double checked everything that was suggested to you; then, ... to make sure you have covered all the bases, ... pull the Cam Cover and check the condition of the Lobes on both Cams for the 3-Hole.

 

Yeah, I went back and re-read, and all I saw was "valves are in spec."

Did YOU put them in spec; or personally check the clearances, or are you taking a PO's word for it? Because it would explain a lot. #4 is fine at 135; but I'd do as Rick suggested in regard to #3.

 

why isn't the vacuum from 1,2,4 reflected to 3 with the yics tool out?

"I can feel the vacuum in the tube from #3 to the manometer though."
something here just don't make sense.
if the vacuum nipple is open and you feel it in the line, what's left?
did you ever see #3 move, or is it stuck?
use your God given vacuum pump, put #3 and #4 hose in your mouth and suck

 

Yes I put the valves in spec.
I also took a tube and blew into the #3 nipple to make sure it wasn't clogged, and it wasn't. I also didn't notice any wear on the cam lobes.
Timeline: All 4 carbs balanced.
Put valves in spec, gapped plugs, changed mixture settings to 3.75 turns, bumped jetting up to 128/41.
Result: No vacuum in #3.

The next thing I'm going to try is setting the mixture screws to their stock 2.5? turns out. I'm getting ready to find a competent bike shop to take a crack at a sync and colortune. At this point I'd rather pay to have someone do it right, I don't want to go on a goose chase for internal engine probs for no reason.

 

Back to the valves for a sec, I'm trying to eliminate (or validate) the possibility of a burnt valve. If you've got a bad valve, all the sync'ing and ColorTuning in the world won't help.

How many miles are on the bike?
What were the numbers like on the valves BEFORE you put them in spec?
You said you didn't mess with intake #3, what's its clearance? What's exhaust 3 at?

Run a dry compression test on all 4 cylinders, post results.
Run a "wet" compression test (only a couple of teaspoons of oil) on all 4 and post results.

Let's see what those tests tell us about #3.

 

#3 Intake was in spec at .15, so I did not touch it.
#3 E was put in spec at .18. The paper that has the before and afters for all the valves is at home. The bike has 28k miles on it.

 

I agree with polock something in your description doesnt make sense.

1.You "balanced" the cabs using what? a manometer works on vacuum, if you had a "signal" in #3 to balance, you had vacuum.

2. Please describe what you are doing thats making you say "I have no vacuum". In other words, are you putting your hand over the intake side of the carb? are you putting a finger over the vacuum nipple on the intake manifold? What condition are you describing as "no vacuum"

because Im confused.

 

The first time I attempted this I had no vacuum reading and didn't even feel anything in the vacuum line going to the manometer.
The 2nd time I bench synced it again and still no reading but I could feel pulsing in the vacuum line, much stronger than the other lines. Both times I got #1,2,4 dead even with nothing for #3.

 

what kind of manometer are you using?

 

I'm not sure of the brand but it is a real one, not homemade.

 

Urk.

I'm concerned about the #3 exhaust valve; I'd like to know what its clearance was before being re-shimmed.

Run dry and wet compression tests; now that the valves are in spec, that should give us some good info to go by.

And just for goops and piggles, after the wet test, turn the motor over slowly by hand and see if you get a "gurgling" sound (like oil-filled air escaping under pressure) from #3.

Have you tried swapping with a different hose on the manometer to be sure it's not just a problem with the instrument?

 

what I meant was a vacuum gauge model (dials) or a fluid tube (mercury or other fluid) model. And does it have restrictors in the lines or on the gauges if its a dial type.

Im only trying to be clear on how you are determining there is "no vacuum" to rule out any chance that you are getting a false negative.


I do agree with fitz though, a dry and then wet compression test would be extremely useful info here.

 

IIRC the #3 E was .12 before I shimmed it, it only went up 1 size.
I'll do the dry and wet compression tests. It's all I can do at the moment since I lost a throttle sync screw.
It's a mercury manometer. I'll check the lines again.
Continued thanks for everyone's help.

 

Here's what I think we need to do now, ...

Using a reliable Vacuum Gauge; measure the Manifold Vacuum on each Intake Manifold.
Use a "Restriction Device", ... (Otherwise the Needle will fluctuate wildly.)

[[ Restriction Device* = Carb Cleaner Red Tube epoxied in a Hose Union Splice.
Reduces the Union to the Inside Diameter of the Red Tube. ]]



The Needle will STILL fluctuate.
The MAX-Vac will be Indicated at the highest number attained as the fluctuations settle-down using the Restrictor Device.

 

Ok so I wanted to update you guys since you were kind enough to lend my your advice.
I'm just terrible at syncing carbs. I bench synced again and also reset the pilots to 2.5 turns out, fired the bike up and saw some life at #3 finally. That gave me some hope. After the bike was warm I closed all 3 sync screws and that finally made my #3 vacuum shoot up. After that I ran out of time as it started to rain. So I think I just had my sync screws turned too far out. #3 probably has a slightly burned valve due to the low compression. I feel confident that the next time I have time to sync I'll be able to get everything right.

 

A burned valve would not be caused by low compression, but may cause low compression.

 

Ah, that's what I meant I had it switched around.
It's frustrating having to more or less relearn all this. My Honda you didn't even have to bench sync, you just turned the screws out 2 1/4 turn and you were in the ballpark.

 

Did you post actual compression numbers?

You could have had the butterfly on #3 closed all the way and a closed or partly clogged pilot circuit would certainly cause low vacuum reading because no air is moving.

If you get the sync right I'd check the compression anyway but the worries probably stop there.

 

Yeah when time allows I'm going to compression test everything.
Time is precious with a 1.5 year old and a wife that thinks my time is better spent on other things.

 

I hear that. I got an 8 y/o part time and a 3 month old.

 

Ok, so I'm laying in bed last night and it hits me.
I've got a 30 year old motor with compression on the low side, and gapped the plugs to the factory specs, when it ran fine before with the plugs right out of the box.
I am betting myself a cold beer that if I take the gapped plugs out and put non gapped plugs I'll pull enough vacuum to sync this sucker up.
Gapped plugs are probably great for a bike that has had its valves done every 5k.

 

...Im betting a cold beer you are wrong.

Just do a compression test. It shouldnt take you more than 15-20 minutes. to do both dry and wet tests.

 

I don't follow the logic in gapped vrs non gapped plugs in relation to vacuum. Are you sure you didn't have a few cold beers before you thought of this 'logic'?

 

Completely unrelated. What's an "ungapped" plug, anyway?

 

I'll have some time this weekend to monkey around with it.
I hope I'm right at least, I'd like to avoid digging into the top end.
By ungapped I mean it's out of the box gap not having the gap set.

My logic is if the engine has low comperssion I just made the spark smaller and things worse by gapping the plugs which might be why it's not running well enough to sync. The larger stock gap on the plugs might have been compensating for that, which is why the bike continued to run halfway decent way after it needed the valves serviced, and why it runs like absolute garbage now that I gapped them to spec.
Besides it takes a minute to change the plugs out it's worth a try.

 

Plugs generally come out of the box set to anywhere within the recommended "range." By just using them straight from the box they could be too wide, too tight or just right; but there's no guarantee that any of them are the same.

A better practice would be to check and gap them all the same. If you want to run at the tight end of the recommended range or the wide end, that's fine. By using them "ungapped" all you're doing is throwing another variable into the mix.

Spark plug gap has nothing to do with how much vacuum the motor is producing when you're trying to sync.

If you genuinely want to solve this; then quit tilting at windmills and take the advice given:

Run both dry and wet compression tests on all 4 cylinders and let's see what those numbers look like. Then you'll at least have an idea of what is going on, and WITHOUT pulling the head (yet.) You don't want to tear into the motor without having run those tests first anyway.

DO THIS FIRST. Otherwise, you may very well be wasting your time on all of these other things; but you just don't know. The "logic" you are using to associate one thing with the other is faulty. Quit guessing. Find out. You can't fix a problem if you don't know what it is; and right now, you don't.

 

Durk:

Before you go get new plugs, ...
Make a pot of coffee.
It doesn't sound like you should be working on the bike at the moment.

 

that's it, how far are you from the turnpike?
this has gone on long enough, i'am ready to make a house call.
i've got tools, you've got coffee, lets get this over with

 

Polock, That's excellent. You are a good man!!

 

Go get 'em, P! Take the Burnarr with you so he can see how frustrating Hitachis can be.

Good man!