Low vacuum on one cylinder

Have a question. Is there a vacuum line diagram anywhere on line for a 2009 venture. What should the vacuum be on a at idle? what should it be at say 3500 rpm? I have one cylinder that has 5 CmHg lower reading than the other 3 cylinders. At idle 3 are reading 22 CmHg and the other reads 17 CmHg. at around 3500 or so They are all reading around 25 CmHg but the low one at idle is slow to come up to the 25 reading. Could this be a vacuum leak? I thought I read a post somewhere that said the vacuum should be around 37. I noticed this after running a tank of gas with a dose of seamfoam and resyncing the carbs afterwards. Still continue to have problem with popping on deceleration and also has an intermittent miss at crusing speed on a flat road. This is mainly in the lower rpm band. By the way the cyl with low vacuum is the left front. Any ideas anyone?

What tool you using for vacuum ? How many miles ?

Can be several things causing vacuum issues like vacuum line - carb.boots leak - carb. slide sticking - sticky valve or valve to tight not setting good - valve adjustment - carbon build up

Three cylinders good readings one is low but not major

I thought I had a cylinder issues when I was syc.carbs but turned out my meter rod in my carbtune was sticking took unit apart cleaned rods all well now

It's hard to diagnose over the forum

also help do a compression test and see where the reading are compared to the others all readings within each other then you know it's not valve or ring issue

The first thing to check is carburetor sync. After that vacuum is the inverse of compression. If all the cylinders have near equal compression and carbs are synced than look for a leak. Some people spray carb cleaner around the base of the carb. I don't like it because carb cleaner can eat plastic and rubber. So I just mist water from a spray bottle. if you hit the leak vacuum and rpm will rise.

Mike

Most likely you're out of sync. Not sure about the Venture off the top of my head, but many service manuals call for them to be matched to 2" Hg, which is 5.8cm. So you're not way out.

On my gauges a well tuned Venture pulls about 25cm Hg.

2009 venture with 9500 miles. Switched the gauges around so I know it is not the gauges.

Most likely you're out of sync. Not sure about the Venture off the top of my head, but many service manuals call for them to be matched to 2" Hg, which is 5.8cm. So you're not way out.

 

On my gauges a well tuned Venture pulls about 25cm Hg.

The spec for the RSV is 280mm or 11" at idle (there is no spec at any other RPM). The service manual says the difference in vacuum for each carb should not exceed 10mm or .4" Hg of each other. A 2" difference would be HUGE.

As for the original post - not sure where to begin my comments. OP said he "noticed this after running a tank of gas with a dose of seamfoam and resyncing the carbs afterwards." That makes no sense to me, as syncing the carbs by definition makes ALL EQUAL in vacuum. Furthermore, you cannot have a problem with leaking vacuum lines that would show as low vacuum since you must remove the two vacuum lines to measure.

A vacuum leak around the carb boot (intake tube) COULD cause the problem, but that is extremely rare on a 2nd gen of any age.

Here is some general information on RSV vacuum: As stated above, the spec is 11" at idle; however, it is not that common to find one that reaches the full 11" in normal running trim. In general, consider 10" good and anything below 9" a real problem. Of course you cannot take any meaningful reading unless the carbs are synced since the very process of adjusting them changes the vacuum!

There are only two things that affect the actual potential vacuum from the engine at a specific RPM - 1) the condition of the cylinder rings and condition/adjustment of the valves, and 2) any restriction in the intake tract.

The carburetor adjustment screws you use to sync the carbs simply change the vacuum by opening or closing the butterfly valve located at the "base" of the carb. The slides in a CV carb do not move at idle, so their position is effectively the same for all carbs and cannot affect the sync. Having equal vacuum simply means that the air flowing through all carbs is the same - it does NOT mean that the gas flowing through all carbs is the same!!

The biggest reason for low vacuum at idle on these bikes is blocked idle jets in one or more carbs. When any idle jet is even partially blocked, the engine cannot reach minimum idle speed without cranking open the idle speed knob under the right side of the tank - this simply opens all four butterfly valves together just like rolling on the throttle. And as you pull the butterfly valves open, you reduce the intake restriction which lowers the vacuum. In the worst cases, where the idle jets are totally blocked, the engine can only idle when the throttle is opened so much that the carbs start feeding gas through the main jets. It is theoretically possible to have one of these engines in complete sync but only running on a single cylinder (either because of carb problems or even bad ignition).

Finally, if you really think you have a problem with significantly low vacuum on a single cylinder, the first thing to do is to run both a compression check and leak-down check on the engine. If those tests show the correct compression for all cylinders, then you can try to measure the maximum potential vacuum on each carb by following this test:

I know that is a long post, but I hope the information helps. do not hesitate to ask if any clarification is needed.

Goose

Should be impossible to have more than 30" of vacuum.

Should be impossible to have more than 30" of vacuum.

Possible or not, I do not understand the reason for this post - I don't think anyone mentioned a number higher than 30 inches?

Goose

Atmospheric pressure is ~ 30" Hg. A perfect vacuum is 0" Hg. You really don't measure vacuum, you measure difference to atmosphere, so 30" Hg would be the maximum.

We were working in cm Hg though. 30" = 76.2cm.

11" Hg = 27.84 cm Hg.

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Whether one cylinder is running poorly or not the vacuum will be the same on all of them if it's synced.

Great post Goose! Great site too!!

I'm new here and I've been searching for some info to help me get an 89 running right, unfortunately so I can sell it. Your post on engine vacuum explained it perfectly and turned the light bulb over my head on, so to speak. It helped me get the bike running better than ever despite determining that I still have a severe blockage in the pilot circuit for cylinder #3. Thank you!