How low can I set my vacuum when doing a TBS?

[TimmyTuber2000]

So I decided to sync my throttle bodies. I used an automotive dial-type vacuum gauge from the local auto parts store. I tried to set all throttle bodies to 25cm but number 2 would only adjust from 15 to 19 cm so I gather that I have a vacuum leak, right? I tightened the clamps on both sides of the throttle body. (Both sides seemed tight enough but they could have been tighter, so I tightened them as tight as the others.) I got maybe another cm at best. I sprayed a little WD40 around the throttle body and never detected any change in idle. I think my target is supposed to be 250mm and I can get close to 210mm on number 2 if I run it all the way in, but leaving it all the way in doesn't seem like the right thing to do. Knowing that the accuracy of my relatively cheap gauge may be questionable, I ran number 2 all the way in (about 205 mm) and backed it out 1.5 turns to about 195 mm, then I adjusted the other three throttle bodies nearer to number 2. Now all throttle bodies are adjusted to between 195mm and 210mm and I turned the idle back down to 1050. I know I should find out why number 2 won't come in line but I don't want to get into that until I have a service manual in front of me.

I drove it to the grocery store and back. Wow, it has never felt so smooth. It has always had a goofy feel between 2700 and 2900 rpm, like the engine was fighting against itself, sort of a rattle, it felt like I was lugging the engine. Because it has done that since it was new I just assumed that 2700 was lugging the engine so I kept the rpms above 3 grand anytime the engine was loaded. Well, 2700 rpm may be lugging the engine a little bit but now the engine is smooth and relatively strong even through the previously iffy 2700 to 2900 rpm range.

My question is this, am I doing any harm by running at 200mm of vacuum? This is my daily driver and it will probably be a month or two before I get a chance to work on it again. (I really don't understand how the amplitude of the vacuum relates to the engine.)

Thanks in advance!

- Tim

 

[okmac]

Tim,

I wish someone in the know would answer your question and I would like to know the answer also.

Mac

 

[03HiYoSilver]

Timmy,

Most never hit those exact numbers and nor have I. Just as long as you have them all at the same reference point, you should be fine and they are balanced.

Glad your machine is running smoothly.

 

[Shadow]

Firstly cylinder #3 is the one you work from.

Sitting on the bike #1 is farthest left then 2, 3 & 4

If #3 is say idling at 100 then you adjust the others to match, or get as close as possible.

They will never be all the exact same.

Say 3 is @ 100 then when you adjust 2 #3 may climb to 150 so you then adjust 2 again then do the whole process for all 4 cylinders.

It is time consuming and often a bit frustrating.

There is no low or high number to work with, but generally the higher your vacuum the better.

All bikes are different so the high number on one may be either higher or lower on another.

 

[McRuss]

Tim,
I wish someone in the know would answer your question and I would like to know the answer also.

Mac

The last post was probably the "one in the know" but I use a TwinMax and it has no numbers. I've been syncing throttle bodies for many years with it (BMW twins mostly but the FJR for 60,000 miles now.) I use #3 and then match the others to it one at a time. The TwinMax only shows fluxuation from center and is easy to use without worrying about numbers. Many folks make their own magnometers (I think that's what they are called) from lengths of clear fuel/vacuum hose and ATF. Others use mercury sticks. But the main thing is to just get all four throttle bodies pulling the same amount regardless of what the number is. YMMV.

 

[C&C]

The service manual uses 250 mm/Hg as standard. Barometric pressure as well as elevation will effect whether you can tune to this number. More importantly is to have the throttle bodies in synch (to whatever vacuum you achieve on number three cylinder). And all this adjustment is only going to help at idle and partial throttle, so at highway speeds it almost become moot. Don't obcess over the TBS; just try to equalize to number three at a close to factory spec. vacuum.

 

[TimmyTuber2000]

So I decided to sync my throttle bodies. [...] My question is this, am I doing any harm by running at 200mm of vacuum? This is my daily driver and it will probably be a month or two before I get a chance to work on it again. (I really don't understand how the amplitude of the vacuum relates to the engine.)

Hi all. Thank you all for your input. I've recently resolved my throttle sync issues. These are the details:

As C&C wrote, the factory spec is 250 mm Hg. I never considered it on my own but his explanation regarding barometric pressure and elevation are perfectly valid. However, I was able to get well over 250 mm on 1, 3, and 4 but 2 maxed out around 200 mm regardless of the vacuum adjustment on the other cylinders. Had my problem been related to atmospheric pressure it would not have been isolated to a single cylinder. The only explanation I could fathom was a vacuum/intake leak on number 2.

I could not get more than about 200 mm on number 2 so I tried adjusting all cylinders to about 200 mm. I rode it that way for about a month. The first time I started the bike cold after syncing the throttle bodies I thought I heard a strange sound. A few days and a few cold starts later I realized that I had created the "knucking" condition described in other posts here. It is a hard misfire or maybe a muffled backfire--it sounds like the engine suddenly decided to turn backwards for a fraction of a stroke--premature detonation to the extreme? It happens one, two, at most four times before the temp gauge shows two marks. After the engine is slightly warm, the "knucking" goes away and all is well. Additionally, my fuel mileage dropped to 40 mpg from about 43 mpg.

Earlier this week I got a little time and grew a pair and decided to find my vacuum leak. I had already tried spraying WD40 and listening for a change in idle but that didn't work this time. I pulled the throttle body assembly out and scratched my head and ogled the little effer as I pondered what part could be a reed valve and the purpose of coolant lines in my fuel system. Well, I don't know much about throttle bodies, but I got the thing back together and kept it clean and didn't lose any parts in the process AND, the good news, I was able to get and keep 250 mm on all four cylinders.

Runs great. Runs like a Feejer!

I guess the little cylindrical plunger-type valve on the bottom of the bodies must have been stuck open on number two.?! Now that I know where it is and what it looks like I think I could have unstuck it (assuming that was the problem) without removing the throttle bodies, but where's the fun in that? I'm not positive how I fixed the vacuum leak but I wanted to write this summary to let people know about a possible source and solution for "knucking". Apparently low vacuum (200 mm or less on several cylinders) can cause knucking. Getting the vacuum near 250 mm was the solution for me.?!

So as usual, more knowledge leads to more questions. (1)Mikuni supplies great downloadable HSR carb info on their website. Is there any good documentation for our Mikuni throttle body assembly? Mikuni tech support line? (2)What is the purpose of the "plunger control unit"--the coolant system component on the throttle body assembly?

Take care.

- Tim

 

[FJRBluesman]

Firstly cylinder #3 is the one you work from.
Sitting on the bike #1 is farthest left then 2, 3 & 4

If #3 is say idling at 100 then you adjust the others to match, or get as close as possible.

They will never be all the exact same.

Say 3 is @ 100 then when you adjust 2 #3 may climb to 150 so you then adjust 2 again then do the whole process for all 4 cylinders.

It is time consuming and often a bit frustrating.

There is no low or high number to work with, but generally the higher your vacuum the better.

All bikes are different so the high number on one may be either higher or lower on another.

Gunny
Mine has no numbers either, just set them all to cylinder number 3.

 

[johnny80s]

Firstly cylinder #3 is the one you work from.
Sitting on the bike #1 is farthest left then 2, 3 & 4

If #3 is say idling at 100 then you adjust the others to match, or get as close as possible.

They will never be all the exact same.

Say 3 is @ 100 then when you adjust 2 #3 may climb to 150 so you then adjust 2 again then do the whole process for all 4 cylinders.

It is time consuming and often a bit frustrating.

There is no low or high number to work with, but generally the higher your vacuum the better.

All bikes are different so the high number on one may be either higher or lower on another.

GUnny
Mine has no numbers either, just set them all to cylinder number 3.

I need to do a TBS. You want to help me and bring your dohicky with you?

 

[FJRBluesman]

Firstly cylinder #3 is the one you work from.Sitting on the bike #1 is farthest left then 2, 3 & 4

If #3 is say idling at 100 then you adjust the others to match, or get as close as possible.

They will never be all the exact same.

Say 3 is @ 100 then when you adjust 2 #3 may climb to 150 so you then adjust 2 again then do the whole process for all 4 cylinders.

It is time consuming and often a bit frustrating.

There is no low or high number to work with, but generally the higher your vacuum the better.

All bikes are different so the high number on one may be either higher or lower on another.

GUnnyMine has no numbers either, just set them all to cylinder number 3.

I need to do a TBS. You want to help me and bring your dohicky with you?

I always have my doohickey with me, thank you.
No problem, lets do it. I think you may need the little adapters for the TB. I have male fittings on each cylinder that my hose fits on (shut up pervs). I believe some bikes have a receptacle for a fitting, that you'll need to purchase separately. Make sure the threads are correct. Not sure if the Gen2 FeeJ falls into this category.

Can we wait until the temp gets below 90º?

 

[johnny80s]

Firstly cylinder #3 is the one you work from.Sitting on the bike #1 is farthest left then 2, 3 & 4

If #3 is say idling at 100 then you adjust the others to match, or get as close as possible.

They will never be all the exact same.

Say 3 is @ 100 then when you adjust 2 #3 may climb to 150 so you then adjust 2 again then do the whole process for all 4 cylinders.

It is time consuming and often a bit frustrating.

There is no low or high number to work with, but generally the higher your vacuum the better.

All bikes are different so the high number on one may be either higher or lower on another.

GUnnyMine has no numbers either, just set them all to cylinder number 3.

I need to do a TBS. You want to help me and bring your dohicky with you?

I always have my doohickey with me thank you.
No problem, lets do it. I think you may need the little adapters for the TB. I have male fittings on each cylinder that my hose fits on (shut up pervs). I believe some bikes have a receptacle for a fitting, that you'll need to purchase separately. Make sure the threads are correct. Not sure if the Gen2 FeeJ falls into this category.

Can we wait until the temp gets below 90º?

Yes lets wait for the temp to drop. Can you send me a picture of what I need to get or direct me to a how to TB thread that shows it?

Thanks Simon.

 

[FJRBluesman]

John - Try these.

Forum Gen1 TBS, by broodwich.

Morgan Carb Tune site.

Read up thoroughly on setting up the mangometer and how to do a TBS per Morgan. It sounds all crazy, but it's really easy.... at least on my Gen1.

Also do a search on the web. Youtube has numerous how to vids on this.

 

[RadioHowie]

I pondered what part could be a reed valve and the purpose of coolant lines in my fuel system.

The reed valve isn't in the throttle body assembly. It's in the cylinder head underneath the valve cover and acts as an Exhaust Gas Recirculation device.

The coolant lines running to the fuel system control the "Fast Idle" device when the bike is started cold. It controls the the fast idle valves on each throttle body. It is very likely the fast idle valve on your #2 TB was sticking in the fast idle mode, which would definitely screw up your vacuum readings, since it may have been "open" after your bike warmed up, and bypassing more air to the #2 TB, compared to the other 3, causing dsparate Hg readings.

Good catch on your part.

 

[TimmyTuber2000]

I pondered what part could be a reed valve and the purpose of coolant lines in my fuel system.

The reed valve isn't in the throttle body assembly. It's in the cylinder head underneath the valve cover and acts as an Exhaust Gas Recirculation device.

The coolant lines running to the fuel system control the "Fast Idle" device when the bike is started cold. It controls the the fast idle valves on each throttle body. It is very likely the fast idle valve on your #2 TB was sticking in the fast idle mode, which would definitely screw up your vacuum readings, since it may have been "open" after your bike warmed up, and bypassing more air to the #2 TB, compared to the other 3, causing dsparate Hg readings.

Good catch on your part.

Thanks boss. That coolant line has bothered me ever since I found it. I could see it attached to some kind of sensing device or control device but I couldn't figure what it was. Many thanks.