Low vacumn during carb syncing

[Scudrunner]

Took my !st Gen RV to the JAX M&E yesterday and Chicken Hawk was kind enough to bring his Carbtune tool to sync my carbs. When connected to the carbs and with the bike warmed up and idling about 1000 rpms the tool barely showed any vacumn reading until revd up. The bike idles ok and accelerates fine with the only problem being a somewhat slow throttle response but it is just not exactly smooth. I had taken the carbs apart in the past and cleaned out the jets and passages, the diaphrams are free of pinholes but probably original. Tried the Seafoam treatment and some suggested checking the tank for any rust although I recently change the gas filter and didn't see a lot of garbage in the old one. I just don't understand what could cause the low vacumn reading which is preventing me from syncing the carbs which I feel is all it needs to run better. I have no problem driving it like it is but would like to put the plastics back on. Can anyone shed some light on this issue? Oh, and BTW, the bike only has 16,000 miles on it.

[OddStar]

Just sync'd mine, and although I had good vacuum at idle decided to check around the intake manifolds. Sprayed a little carb cleaner and got a bog . Guess I'll be tightening them or using some rtv? I don't think there's any O rings on these like the VMax has?

[YamaDuck]

Hi Bob,

It was a pleasure meeting you in Jacksonville Saturday. Sorry I had to leave early but I needed to get on the road. I know nothing about a 1st gen but here are a couple of things that may be worth checking. Check all the vacuum hoses and you may want to just replace them. I know on auto's they hoses will crack in places that are very hard to see. Also in auto's sometimes the hoses get weak and will try to collapse restricting the vacuum. Also check the 4 rubber boots that the carbs sit on. I know most of the times they just crack and the crack doesn't go all the way through but just maybe??? Anyway I'm through guessing. Good luck.

 

Harry

[dingy]

Took my 1st Gen RV to the JAX M&E yesterday and Chicken Hawk was kind enough to bring his Carbtune tool to sync my carbs. When connected to the carbs and with the bike warmed up and idling about 1000 rpms the tool barely showed any vacumn reading until revd up.

 

This will be interesting to what is causing it.

 

If it runs decent and idles OK its got to have compression. If its got compression & runs good, it has to be sucking air through the carbs to get air fuel mixture. If its doing all that there seems like there has to be vacuum below the throttle plates at idle.

 

 

Gary

[MiCarl]

When I've seen this situation it's always been from a cylinder not firing at idle.

[YamaDuck]

When I've seen this situation it's always been from a cylinder not firing at idle.

If that is the case then it may be a clogged pilot jet. A friend with a 05 had a couple of clogged pilot jets and it ran fine but idling and starting off was a little rough. I put new pilot jets in mine just to prevent anything like that happening. They are not very expensive. I probably did not need them but I had the carb apart anyway. If you do replace the pilot jets make sure they go in the right place. May not make a difference on a 1st gen (don't know) but on a 2nd gen they are different even though they look the same.

[puppy]

Originally Posted by MiCarl

When I've seen this situation it's always been from a cylinder not firing at idle.

Interestingly enough, I just went through a situation on my 86 where 1 cylinder was not getting gas, and was not operating. It ran out pretty decent, but #4 cylinder carburetor was shot, toast, de-nada, zippo, zilch, nothing happening. Carburetor was so bad it would not even let a drop of gas into the cylinder. But, bike still ran well, and drove out well. Only way I could tell was by SKYDOC helping me out on the phone, and finally did the header pipe heat test. Just got the carbs fixed and back on today. runs great!!!!! Thanks SKYDOC!! My point is maybe check if you have a cylinder not working, thus, not giving you the correct vacuum reading.

[Marcarl]

Check to make sure that the hoses for the gauges are not collapsed, and that the restrictors are not plugged. You could remove the restrictors to see if that would give you more vaccum, but don't rev the engine, the higher vaccum can cause some unwanted reactions. If you have vacuum without the restrictors, then they are your problem and you will need the either clean them or replace them before you can do a sync.

[Sleeperhawk]

Check to make sure that the hoses for the gauges are not collapsed, and that the restrictors are not plugged. You could remove the restrictors to see if that would give you more vaccum, but don't rev the engine, the higher vaccum can cause some unwanted reactions. If you have vacuum without the restrictors, then they are your problem and you will need the either clean them or replace them before you can do a sync.

 

 

The hoses on the Carbtune were not collapsed and the restrictors were not plugged.

According to Scudrunner, he was getting the exact same readings on his gauges, why I brought mine to verify his readings.

 

I did smell gas though when we tried the sync job.

[RandyR]

Just a comment from an observer. All 4 carbs seemed to behave exactly the same way. If it was cracked diaphrams or hoses one would expect some variation in behavior from carb to carb, I would think (not knowing First gens specifically).

 

Is there a common point for all 4 carbs that could cause this?

[bkuhr]

Just a comment from an observer. All 4 carbs seemed to behave exactly the same way. If it was cracked diaphrams or hoses one would expect some variation in behavior from carb to carb, I would think (not knowing First gens specifically).

 

Is there a common point for all 4 carbs that could cause this?

 

is YICS still installed?

[Condor]

I had low vacuum at idle with my '83. Concerning at first, but like your's it ran great. So I just followed CarbTune's suggestion, turned the gauge upside down, and sync'd it.

[kbran]

I just got thru dealing with the same problem on my 86 that was purchased from Squidly a few years ago. It had gotten so bad all I could get was the bars shaking on the bottom of the carb tune. Could barley see them, no vaccum. Never could figure it out. Took the carbs off and contacted Dano and Skydoc and after looking at the pilot needles, they looked to be a little scored. Earl's suggestion was to purchase new needles so I did ( the seats didn't appear bad), and while waiting on them I filled up the bowls on the bench with seafoam. I don't know if it was dirty pilot jets or bad needles but after i put it all back together and bench set the adjustments where all butterflys were closed was able to make the adjusments and the vaccum was back, way up. I know after searching the net and looking at how venturi carbs work that the pilot jets can affect the vaccum as can other things.

[RSTDdog]

is YICS still installed?

 

Not 100% on this but I think YICS was only on the inline 4 cylinder yamaha's. And in order to sync those carbs you needed a YICS block off tool.

 

The YICS on the inline 4's was a long port machined through the head effectively tying all four intake manifolds together.

 

If the Venture has a version of this YICS, you will need to block it off to properly sync the carbs.

 

I heard his bike at the M&E and the rpms would hang for a while when revving before dropping back off (indication of leaning out). He also indicated that turning the mixture screws didn't seem to affect anything.

 

Scud- Try the heat test (you are looking for a cold exhaust pipe when the bike is running or one that is not as hot as the other's. This will indicate the problem cylinder. More than likely you will be "Venturing" back into the carburetors.

 

RSTDdog

[Scudrunner]

Thanks for all the ideas guys. I'm going to try the least invasive things first before I start pulling the carbs off. I'll try pulling one plug wire off while idling to see if I have a problem in that area. I used to do that with cars to isolate a bad cylinder and then check from there wheater it's a plug, wire, etc. Will also spray around the carb boots with carb cleaner and a fire extinguisher! Since the bike sat for awhile a few years ago it was also suggested to pull the seat and tank sending unit to check for rust in the tank which might be clogging up the jets in the carb. I"ll report on what I find and thanks again for all the suggestion.

[Venturous Randy]

Yes, the 84 does have the YICS and no, you should not have to block it off to sync the carbs. If you have to block it off to sync the carbs, you have a problem with the YICS as you would want to sync the carbs with the setup that the bike will see. Given all that, I suspect it may be your YICS as it could be the box that is leaking and that may be why all 4 carbs have low vacuum. My suggestion is to do what I did, take the YICS off and with good plug caps, block the YICS ports at the engine block. I did this to mine a couple of years ago and wish I had done it sooner.

With the YICS blocked off and my needle jets shimmed to about .093 from about .125 and new diaphragms, I am now getting up to 48mpg while cruising around on mountain roads at 40 to 55 mph. And, the bike has as much power as it has ever had with the 109,000 miles I have put on it.

One other suggestion, put the year of your bike up in your name area and we won't have to look it up in your profile to try to help you.

RandyA

[RSTDdog]

Yes, the 84 does have the YICS and no, you should not have to block it off to sync the carbs. If you have to block it off to sync the carbs, you have a problem with the YICS as you would want to sync the carbs with the setup that the bike will see. Given all that, I suspect it may be your YICS as it could be the box that is leaking and that may be why all 4 carbs have low vacuum. RandyA

 

While I agree he may have a leak or other problem with the YICS causing or contributing to low vacuum, IF the YICS in the 1st gen venture is similar to the inline 4 YICS, in that the vacuum side of all four intakes is tied together, you would have to block of the YICS in order to accurately Sync the carbs (throttle plates) on the bike.

 

On edit: it would appear that the Venture YICS is a series of hoses and a plastic chamber that ties the vacuum side of the intakes together. This is different than the inline 4 blocks with a 3/8" diameter port tying the intakes together.

 

If you don't have the YICS blocked off while you are syncing the carbs, all four intakes are tied together and the vacuum readings will be an average of all 4 cylinders. Changing the sync screw on one will change the readings on all the carbs.

 

Yamaha makes a block off tool that goes into the in line 4 bikes.

I had a 650 maxim with YICS and had to borrow the block off tool to accurately sync the carbs.

 

I still think the OP has other issues, but there is a procedure for Syncing carbs on YICS models. I would "Venture" a guess its in the tech section. (on edit: The tech section doesn't say anything about blocking of YICS for Sync on a Ist GenVenture, but doesn't say what year the SM pages are from either. Later first gens don't have YICS)

 

RSTDdog

Edited May 11, 2010 by RSTDdog
added some stuff after looking around

[bkuhr]

While I agree he may have a leak or other problem with the YICS causing or contributing to low vacuum, IF the YICS in the 1st gen venture is similar to the inline 4 YICS, in that the vacuum side of all four intakes is tied together, you would have to block of the YICS in order to accurately Sync the carbs (throttle plates) on the bike.

 

If you don't have the YICS blocked off while you are syncing the carbs, all four intakes are tied together and the vacuum readings will be an average of all 4 cylinders. Changing the sync screw on one will change the readings on all the carbs.

 

Yamaha makes a block off tool that goes into the in line 4 bikes.

I had a 650 maxim with YICS and had to borrow the block off tool to accurately sync the carbs.

 

I still think the OP has other issues, but there is a procedure for Syncing carbs on YICS models. I would "Venture" a guess its in the tech section.

 

RSTDdog

 

I have 82 xj750 inline4 with YICS that does require block off tool, I made one from directions found on net.

Also have 83 xvz12 venture with YICS, It is not the same setup. YICS here has 4 individual chambers hooked to ports on intake boots below carbs. designed to draw in extra air/fuel charge and store in chamber for next cycle. supposed to help milage.

These 4 chambers should be isolated from each other, but formed from 2 pieces of plastic hydrstic glued together, and many have had leaks between chambers,or to atmosphere. many here say complete removal of YICS chambers, and plug ports on carb boots with sturdy cap plugs

 

suggest at next sync attempt

1. pull any YICS hose and plug port, if vacum in that chamber increases :banana:you know problem, plug the rest.

 

2. pull aircleaner and look into carb bodies, push sliders over and look at throttle plates to verify they completely close, touch in both side of bore, if not adjust sync screws until they do. some here recommend completly back off sync screws, but found completely closed is not fully backed off, link springs start opening again as screws back off. If cant completely close, possible bend linkage-don't ask how I know this!

[jasonm.]

Just sync'd mine, and although I had good vacuum at idle decided to check around the intake manifolds. Sprayed a little carb cleaner and got a bog . Guess I'll be tightening them or using some rtv? I don't think there's any O rings on these like the VMax has?

Yes there are orings. These engines share 95% parts with the 1200 Vmax. Other than the oring leak. It could be clogged pilot gas jets clogged and someone turned up the idle to make up for it. You get HIGHER vacuum on the carbtune the more CLOSED the throttle is. Because you are measuring BELOW the throttle plate. Open plate = less vacuum on the gauge. OR YICS leaking.

[MikesBike]

What was the final solution for this problem?