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Discussion Starter #1
Hello again, you may remember me with the fuel problem....

http://www.vn750.com/forum/showthread.php?t=2325

To continue the quest for a better performing bike, I need to know what others have experienced with their vacuum guage.

Mine will wildly swing at idle but will smooth out as throttle is increased.

I syncronized the carbs today and they will sync at 4000 rpm but at idle the rear cylinder has a lot higher vacuum than the front.

I am using Marty's yard stick manometer

http://www.powerchutes.com/manometer.asp

any sugestions? I know from a vacuum check on my car, that the needle should be steady at idle, otherwise you have a vacuum leak or the carbs are too rich or too lean. Which is it for these bikes? Do I need to enrichen the jets? (Oh yeah, I forgot to tell you I only have one vacuum line from the front carb, all the others are gone, and I replaced that hose so probably not a leak.....will the idle adjust effect the sync?

The bike seems to run ..OK, not super, but ok. Maybe I am too pickey and expect this 2 cylinder to run like my '71 honda 750 4cyl.

Thanks for any assistance you can provide.

I will be ear shaving in the very near future and need access to a set of jets and shims, I have tried the links in the verses, but none acctually tell me which jets and shimms are for the VN750.....

Thanks again.
 

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Once Banned
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Me either. Although some people say that the vaccum between my ears is good. I don't know how they know my bike has ears...
 

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When the bike is at idle "I think 1100 RPM" you will need to restrict the tube to get a steady reading. Most carb sync. tools come with little wire restricters. If yours doesn't you will need to find a way to pinch off the tube a bit. Sometimes a small pair of loccking pliers or small c-clamps. Hope this helps!
 

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Discussion Starter #8
tinocker, can you please explain in as much detail as you can, I cannot find any info on this. I can see restriction, but I cannot see how it will help with on side having more vacuum than the other at idle....There needs to be some way to adjust each side????

Thanks.
 

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and the Adventure Cycle
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dta116 said:
There needs to be some way to adjust each side????
If you need to set the carbs so one matches (or is in-sync with) the other, I don't see why you'd need to be able to adjust both.
Just adjust the one 'til it is the same (or very nearly the same) as the other.
Or am I missing something else here?
All I could find on a search were instrucions which say "Adjust the dampening valve to reduce the studder of the needle, or mercury"
There were no pictures to go along with it.
 

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All the carb sync stuff basically says to go with the one screw - between the carbs - and try to even up the pressure (with the screw adjusting the vacuum in the rear carb). But will out o' sync carbs affect the rpms at idle? "Dampening valve" would seem to be some other beast...

Maybe I'll go to the Kihen (Kiehn?) link and see if they have any better pics.
 

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and the Adventure Cycle
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Crobins365 said:
"Dampening valve" would seem to be some other beast...
Sorry Cindy, that was for where dta116 was asking about the 'restrictor' that tinocker had mentioned.

I wouldn't think being out of sync would effect rpms @ idle too much, but, not having done a carb sync, I don't know for sure.
 

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Discussion Starter #14
I have been making numerous mods since I first posted, Still not sure it is not normal for the fluctuations at idle, I did sync the cards at idle and checked the vacuum while increasing the throttle and it only takes very little throttle to smooth the needle. It may be that two cylinders may be on the edge for standard vacuum gauges to perform steadily.

The bike is running flawlessly, and much more powerful after the carb work and tune ups of all things "Bike".

Just completed an "Ear Shave" with K&N's and it will be a while before I complete the proper jetting. Still studying all the idiosyncracies of this "magic" art of jetting.

Anyway, after syncing the carbs at idle the vacuum gauge is still the same, but I'm almost sure everyone should have the same results.

I will keep the forum up to date.
 

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Thanks for the update. I did a "by ear" sync of the carbs over the weekend, and it runs more smoothly, but I'm still not sure about the idle rpms. Plan to take the bike out for a longer ride this evening, so I'll have a better sense of what I've done for it...or to it.

BTW, when you did the carb sync...can you tell me how you hooked up the fuel lines? There's a Verse that talks about using a T link from the petcock, etc., but I'm not totally understanding what I need to do to ensure that there's fuel flowing. Got the $2.00 manometer all set up - just need to finish understanding the process.

Any help you can provide from your experience would be greatly appreciated!!

Thanks.
 

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Cindy,

The T fitting is used because the left carb supplies the vacuum to the petcock. The vacuum is what allows fuel to flow thru the petcock to the carbs so that we don't have to turn the thing off everytime we shutdown the bike. It helps prevent flooding. The T fitting allows you to hook up the manometer while keeping the vacuum to the petcock.
If you look at the petcock you will see three lines leading to it. 2 are fuel feeds, one main and one reserve, the smaller third line that hooks to the vacuum port on the carb is the one that supplies the vacuum to the petcock. This is the line that needs the T fitting unless you are lucky enough to get your carbs sync'd before you run the gas out of the float bowl. I believe you may get erroneous readings though as the fuel level decreases.
I'll try and shoot some pictures in the near future as I will have to pull the carbs on Ursula and clean them out. Seafoam is no longer working. I'll resync on reinstall and document the procedure here.
Hope this helps for now.
 

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Sky Rider said:
Cindy,

The T fitting is used because the left carb supplies the vacuum to the petcock. The vacuum is what allows fuel to flow thru the petcock to the carbs so that we don't have to turn the thing off everytime we shutdown the bike. It helps prevent flooding. The T fitting allows you to hook up the manometer while keeping the vacuum to the petcock.
If you look at the petcock you will see three lines leading to it. 2 are fuel feeds, one main and one reserve, the smaller third line that hooks to the vacuum port on the carb is the one that supplies the vacuum to the petcock. This is the line that needs the T fitting unless you are lucky enough to get your carbs sync'd before you run the gas out of the float bowl. I believe you may get erroneous readings though as the fuel level decreases.
I'll try and shoot some pictures in the near future as I will have to pull the carbs on Ursula and clean them out. Seafoam is no longer working. I'll resync on reinstall and document the procedure here.
Hope this helps for now.

Thank you, thank you, thank you! That helps a ton - and if you have pics of how you hooked it up, that'd be awesome. I'm bound and determined to do this myself, but just want to make sure that I'm not missing any critical steps along the way.

Thanks again!!
 

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Quick question about the t-fitting: my partner used to be a veterinary nurse, and said that they used manometer-type deals all the time to check....hmmm...lung pressure in horses? Anyway, she said that the t-fitting has a valve so that the flow can be closed or open, depending upon what one is trying to do.

So I'm thinking: close the valve so that it's just a vacuum check while reading the manometer; then open the valve (as the fuel gets a little lower) to let some more fuel into the carbs; then close it again to run the vacuum check. Is that about right?

Also - what size t-fitting? 1/8" ID?

Thanks!

BTW, I took my baby into town last night, and my "ear sync" was a disaster. Apparently I am tone-deaf - poor thing ran like I'd bought it sight-unseen off of eBay.:doh:
 

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Cindy, I was wondering about that. I've sync'd both my kz's with 4 cyl and I don't think I could "hear" it sync. It was purely visual. I'm sure it's different on a 2 cyl engine, but still I'd think you need the meters. I've got to give you credit for trying it though! I probably wouldn't have tried that, just too chicken.

I'll bet once one of these 2 cyl v-twins are sync'd, it will feel a lot better on the road.
 
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