tied vacuum nipples together on 78 std

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

I tied the 2 vacuum nipples together today and Im amazed at the difference it made. Feels like I had the motor balanced. Anyone know why that made such a big difference ?

Modemcyclist

My guess is that one or both of your petcock diaphrams was leaking air.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

Both petcocks are manual, carbs were synched prior. Just tried it after reading XSJohn & Pamcopete talking about it. I like it !

pamcopete · Last edited by pamcopete on April 7, 2009, 9:15 pm; edited 1 time in total

WOODMAN,

Well, regardless of how well you balance (sync) the carbs, there is always a slight difference in their vacuum, so connecting the two barbs together smooths things out. Also, without the crossover, each carb has to respond to the individual vacuum pulses of their respective cylinder. With the crossover, the number of pulses is double, which means that each carb is continously primed. A third reason is that the mixture is never just right at idle. With the crossover, you get a blend of the two mixtures, so the mixture that is "inhaled" is more likely to be correct.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

You guys are sharp, I never would have believed it would make that much difference, thank you for letting us know about it. Before the bike would dance across the garge floor running on the center stand, now it doesnt

HndaTch627

so yeah, i am pretty sure the petcock is leaking vacuum. will the petcock deliver enough fuel to run the bike when in prime? if so i 'll just use it as an on off switch for now

pamcopete

HndaTch627

The petcock will deliver adequate fule in the prime position, but the prime position uses the same fuel pickup as the reserve position. You are effectively always on reserve in the prime position, so you could run out of gas with no warning and no reserve.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

There's almost no end to the evil caused by them stupid, vacuum-operated petcocks. I was around when they were the latest thing. They never did work right. Owners always had trouble with them.

pamcopete

jimmythetrucker

Well, I must be the exception. Both my '78/E and my '81/H have vacuum petcocks and I have not had any trouble with them. In fact, I think they are great...saves having to shut off the fuel manually.

When I had my wreck, the bike was lying on it's side with fuel running out of the carbs just slightly uphill of where my poor battered body lay with one foot trapped under the bike, but the fuel evaporated before it reached me. The vacuum petcocks kept the rest in the tank. There was at least one as*&%$ who came over to see the body with a cigarette in his mouth.....I disapointed him, however, because I was still breathing and wasn't on fire.

HndaTch627

i went ahead and ordered a new petcock from mikesxs, at least that would explain why the bike started having a high idle like the carbs were unbalanced all of a sudden.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

I just put a Pingel on my 95 Harley Fatboy because harley in their infinete wisdom didnt put a prime position on the vacuum petcok, just on & reserve so no vacuum, no run. I figured 14 years was as long as I wanted to run the original vacuum petcock. I also put a Pingel on my 80 Suzuki GS1100E because they didnt even put an off & on handle on the petcock the first year of the 16 valve engine, you have to use a screwdriver. A new original one cost almost what the Pingel does. I used a new Mikes tank on the 78 XS650 std and it came with 2 petcocks, a new gas cap and a really nice paint job.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

Pete -- You know, in my whole life I never thought about the possibility of a fire after laying one down. Now that you mention it, I see it'd be easy enough to have a fire, too, while sliding across the concrete with sparks peeling off the steel pipes. . . .

Still -- the thought of burning myself to death doesn't sway me. I hate vaccuum petcocks.

ABSINTHE · Full Member Joined: Nov 05, 2008 Posts: 53

interesting talk when you guys say tied the niples together you are refering to the vacum niples on the intake manifolds and if so when you say tied together you are talking about joining them with a hose from one niple to the other ? if this is corect can this also be done on a bike fited with mikuni vm 34 roundslides thanx a lot in advance

jayel

either that or it's getting kinky around here.... early models didn't have the vacuum barbs so I drilled a 3/16 hole thru my intake boots where they would fit and JB welded in a short piece of 3/16 brake line, makes carb sync easier and going to try this crossover tube mod, don't know if it will do anything tho' my BS38's already have a balance tube between the carbs but since this is actually in the intake tract it may be different

ABSINTHE · Full Member Joined: Nov 05, 2008 Posts: 53

ill give it a try too

pamcopete

Just remember to sync the carbs without the crossover by blocking the barbs. Use the dead cylinder sync method by removing the block on the barbs, one at a time.

The crossover provides that last bit of vacuum equalization to smooth out the idle. It will also prolong the time between sync sessions.

Another sync "trick" that I use is to adjust the sync screw until both exhausts sound the same. No more that 1/8 turn either way. Do this after the dead cylinder sync and before you cross the barbs.

Here is the sequence I use on my '81/H:

1. Set both mixture screws to stock turns.
2. Block the vacuum barbs.
3. Use the dead cylinder method to sync the carbs by removing one block at a time.
4. Final tune the sync screw so both exhausts sound the same.
5. Connect the vacuum barbs together.

I think that a lot of people spend endless hours screwing with the mixture screws, when in fact, their problem is the carbs are not synced, so that is why you set the mixture screws to factory turns. This may result initially in a rough running engine, but that is because the carbs are not synced. I never have to change the mixture screws from their factory turns when I use the above method to sync the carbs.

On the BS34's, the idle speed may initially surge when you pull the block off of the barb. Just wait a few moments and it will settle down.

yamaman

Bit of an overkill, but if you don't have vacuum barbs, you could do it this way:

www.xs650.org.au/tech1.html

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

I had already balanced them with my mercury sticks and its plenty good for me. If your carbs have the blocked holes for the balancer adaptors you could use 2 of them. They are nipples that screw into the carbs that dont have the manifolds with the nipples on them. Sudco carries them I believe

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