could Mix setting drastically affect vacuum sync?

[garyS-NJ]

I have a '79 gs1000 with K&N knock off Oval pods, 4:1 header and open exhaust (3" pipe with two anti reversion cones and a short 2" reducing tail pipe. It's running pretty well (not perfectly crisp off idle but it does idle and it does rev without limit or stumble) and it sounds nice but I'm getting some weird reading on my Motion Pro Mercury Sync.

just cleaned carbs, new intake manifold O-rings, jetting from PO is 112.5 mains and #20 Pilots (stock is #95 and #15). I set my fuel screws at 1 ture out and air screws at 2 turns out.

vac sync... #1 = 27", #2 = 29", #3 = 11", and #4 = 22". Compression #1= 135#, #2 = 110#, #3 & 4 are 115#. Here's the kicker, exhaust pipe temps #1 = 460f, #2 = 360f, #3 = 660f, and #4 = 500f. I think these mix settings are a baseline start point so must be close. No air leaks (new manifold o rings and I checked with propane). I keep reading don't mess with the vm sync after bench sync but I didn't do bench sync (gonna try tomorrow with carbs on bike). And syncing I imagine means just bring any low one up and high ones down.. (Not like a cv carb set). Could my mix be so far off to make #3 way lean (I know my orifices and jets are clean) and that make the vacume that diff?

Does anyone have a good link for vacuum syncing the VM series carbs? I looked and found nothing.. Oh, and I did read the sticky on pods, pipes... hunting around here and with one previous post, seems I should be starting with 122.5 mains but that can't explain but weird #3 reading..

 

[garyS-NJ]

and do I need to spring for a color tune to hit the mix or are my settings close enough?? I can't imagine seeing an idle change changing one carb at a time.... and it seems that #3 is was off ... maybe I buy that color tune, my birthday just passed.

 

[Guest]

open exhaust (3" pipe with two anti reversion cones and a short 2" reducing tail pipe.

Can you post pics of this pipe? I'm curious to see if it looks like I imagine.

It's running pretty well (not perfectly crisp off idle but it does idle and it does rev without limit or stumble)

Are you talking revving while sitting in neutral or while riding? From what I have seen almost any jetting will let the engine rev without a load on it so that is no guide to go on at all. If this is while riding then it means you aren't horrendously wrong on at least the pilot and needle settings.

I can't say on the sync for VM carbs, have never played with them. You shouldn't need a Colortune, people have been jetting carbs for decades based on how it responds to throttle and choke combined with reading the plugs.


Mark

 

[Big T]

Did you cap the vacuum port on #3 during the sync?

 

[Steve]

Big T has a good point there, make sure the vacuum port is plugged. That would be a HUGE air leak, causing a lean condition that would run hot.

In your first post, you said " I keep reading don't mess with the vm sync after bench sync but I didn't do bench sync (gonna try tomorrow with carbs on bike)." That really intrigued me. First of all, I'm not sure what you mean by the "vm sync", but please explain how you are going to do a BENCH sync when the carbs are on the bike. It's called a "bench sync" because you sync the carbs while they are on the BENCH. THEN you put them on the bike. A decent bench sync just gets them close enough so they will start and run so you can do a VACUUM sync on the bike.

The only real difference between synchronizing the VM carbs and the CV-type carbs is that the VM carbs have individual adjustments off the one and only throttle rail, while the CV-type carbs are all slaved off the #3 carb. The principle remains the same: turn the adjusters to get the vacuum readings all the same.

Personally, I will do a vacuum sync BEFORE doing a mixture adjustment. Doing a vacuum sync first will ensure that all cylinders are pulling the same amount of air, THEN you can play with how much fuel is added to the air. If you play with the mixtures while the carbs are wildly out of sync, you might be trying to listen for a change in engine speed while adjusting the mixture on a carb that is so far out of sync that it is completely closed. No amount of mixture adjustment will change engine speed in that situation, so I do the vacuum sync first.

In fact, if you have trouble hearing subtle changes in engine speed, leave the vacuum gauges connected to do the mixture adjustment. When a mixture change affects the engine speed, the vacuum level will change in all four cylinders, which makes it easy to see.

.

 

[garyS-NJ]

Yes, revving in neutral, good point about load (but that is task 2). And yes, #3 is capped. I even fit back that hose and the sync tool hoses to get snug fit on nipples and checked with propane for leaks. And finally yes, I think a sync would need to be close to check mix. I can check bench sync with the pods off and otherwise will just adjust the sync screws running. And great point on mix changes to idleobserved on vac gauge! I've done that before and forgot. Duh.

 

[garyS-NJ]

Having trouble uploading a pic with my phone as well as adding files.

 

[Steve]

To show pictures, the best way is to upload them to a hosting site like Photobucket, Imgur or others. Then, copy the IMG link from that site and paste it here. You can then put up to 10 pictures per post.

Not sure what files you are trying to add, but forget it, it's not possible. You either type your message or paste links to pictures or videos.

.

 

[garyS-NJ]

Thanks Steve!

Inside the 3" pipe I put an anti reversion cone at the 2-3" conversion (used an exhaust pipe adapter).

http://i.imgur.com/19r398R.jpg
http://i.imgur.com/ZOjhnLe.jpg
http://i.imgur.com/WAEneYy.jpg
http://i.imgur.com/72fQPFg.jpg

 

[Steve]

I have not used Imgur, so don't know if they have IMG links or not, so you might have to use the "Insert Image" icon (looks like a picture hanging on the wall).

I just tried that with one of your pictures, it is too large. There is a maximum size allowed, you would have to see the forum rules to see what they are.
When I upload my pictures to Photobucket, I have them automagically re-sized to 800x600. This is a comfortable size to view and also saves space on the Photobucket server.

.

 

[garyS-NJ]

I just tried the IMGUR links that I posted here from my phone and the links didn't work and then I went on my phone IMGUR and saw the title but none of the pics. who knows. some day when I'm bored and have more energy I'll figure it all out.. (how to post a picture from my phone onto this website_.).

Meanwhile, I got the carbs synced with the mercury gauge and it ran good and reved in neutral and pulled hard off the line but seemed to starve when I opened it up. gonna start a new thread on main jet selection.

 

[garyS-NJ]

Oh, and the exhaust pipe temps evened out pretty close. I bet they would be much closer if I had the inclination to fine tune the fuel and air screws (but screw that for now).