vaccuum synch genius needed

[yamtom]

Hi All,

As some already know, I rebuilt the carbs on my MNS last spring. Since my honda vacuum gauges were lost at the time, it was bench synched only until now. It ran great all summer, and now since I have found my gauge set I want to do the vac synch. I purchased all new vac lines to link the carbs and gauges. The gauges seem to be in good working order since when I plugged the line into number one, and rotated thru the gauge set, they all acted about the same., giving the same readings or very close.

I set the bike up, warmed up, and started, at first, I had all readings within about two increments, two inches mercury of each other, so I started with the synch matching readings all to number three. Number four came right in with just a bit of adjustment, by the way, the readings at that time were about ten inches, number one was down to eight, and two and four were about nine. I tried to do number one next, also readjusted the pilot fuel screws but it made no difference, all are set at the prescribed one and one quarter turns out. In adjusting number one, something happened, and all of a sudden, numbers three and four suddenly showed a reading of twenty inches of mercury on the gauges, numbers one and two stayed the same, if number one is adjusted in one direction cant remember which, the engine dies after about threee eighths of a turn, the other direction doesnt make any difference in the vac reading on that cylinder. Number two adjustment, does about the same, at no time can the adjustment be made to raise the gauge reading above ten inches. So we have this engine, pulling twenty inches of merc on three and four, and about ten on one and two, obviously this aint working out. I did take off the vac advance, and plugged it, makes no difference. I also sprayed the all the carb boots down with carb cleaner, and it made no difference at all. I havent done compression readings on this thing yet, but last spring , they ranged between 130 and 140 and the bike puts out about the same power now so dont think something has changed with the compression.
Keep in mind, that this bike, started and ran just fine all summer, the vac synch was being done due to the fact that it hadnt been done yet since the carb rebuild.
I am totally open to suggestions here, as I have no idea why three and four suddenly started pulling big numbers, and one and two stayed similar and seem to be unable to be adjusted much at all.
Hit me with suggestions oh you geniuses of XS, I am stumped and have the carbs off now, and am going to bench synch back to where I started out before making any other adjustments.

[GARTHXS]

Don't mess with pilots until the first sync is done it goes in order sync pilots sync done!

[yamtom]

ok, but no fuel screws were changed at all

All the pilot fuel screws were put back to exactly one and a quarter turns, and the problem persisted, but I will do that when I get it bench synched, anyone else have any ideas??

[Larrym]

Calibration???

Verify that the gauge is accurate/reliable:

Connect all four lines together with Tees and then hook it up to a single vacuum source. All readings should be the same. If they're not all the same then find out what's wonky with the gauge.

Actually...I don't use the "sticks":



same "calibration" principle applies though.

[DGXSER]

Hey Tom,

First off, lets be sure your after the correct results. Unless I misread your description (quite possible) it sounded like your trying to match #1 to #3 and then try to match #2 to #3. This will not happen. The synch screw between #1 and #2 bring those two together, while adjusting this screw, ignore #3 and #4 completly.

After #3 & #4 are synched together, same vac, and #1 & #2 are synched together, THEN you want to use the synch screw between 2 and 3 to bring those pairs together. It is typical they would be off by a few inches as in 3 & 4 would match up at say 10" and 1 and 2 maybe at 7" or even 13".

Jumping to 20" seems like something went screwy in your gages perhaps. Although I typically get all the adjustment needed in less than a 1/4 of a turn of the synch screws. Trying to get #1 and #3 to match up using the synch screw between #1 and #2 you could easily have thrown it way the heck out.

As Garth suggested, I typically do not mess with the mix screws while synching. Synch first, then tune, then synch is a pretty common procedure.

[yamtom]

will do,, any others??

I did check them with one cylinder, just putting on the different vac line/gauges to one cylinder, and the only one that was more than one half increment out, or about one quarter inch mercury, was number four gauge. I am using the honda four set of gauges with the damper associated with the gauge. These read in inches mercury , but are not mercury based at all, just vac gauges, I will also check them against my two other freestanding vac gauges.

Also, during this fiasco, I swapped the one side lines with the four side lines, and the gauges all read exactly the way they should have if they were working. In other words, I just transferred the readings from one set of gauges to the other ones. I do have tees, will check it your way too, thanks Larry, anyone else want to weigh in?????????

[CatatonicBug]

You mentioned a change of "3/8 of a turn". Was that on the sync screws? Seems like quite a large turn for a sync screw... Just touching a screwdriver to the screw will have an effect on the sync, so you shouldn't have that much of an adjustment.

Do you have a colortune plug for adjusting the mixture screws? When I first got my LH, all the screws were colortuned at over 4 turns out. Bigfoot had the same settings on his MNS too. Don't assume that the "starting point" is the correct setting.

[mro]

Does sound like you may not be getting good ignition on all 4
Which can be caused by a few different problems and sometimes a combination.

Check the headers to see if they are all close to the same temp.
I use the scientific method (my hand) but don’t recommend it if you have issues or are challenged in some way. An image of some guy with his hand stuck to a header and smoke coming out between his fingers might be funny to some but think it would make me sorta cringe. Spray bottle with water and you can see if they sizzle the same or not might be a better option if you don’t have an infrared reader.

#2 sync screw moves both carb one and two
So #1 is synced to #2
#4 sync screw is used to synk it to #3
#3 had no sync screw
So you sync #4 to #3 then #1 to #2, then #2 to #3

Readjust RPM if it changes between each adjustment
I like to keep it at 1,000 RPM for syncing

After sync I like to adjust the pilot screws
I start by lowering the RPM as low as the engine will allow and keep running.
RPM normally will increase as adjustments made so I reset it as needed.

Once the pilots are adjusted then I recheck the sync.

mro

[natemoen]

So one and two are at 10ish and 3 and 4 are at 20ish, correct? did you then adjust the third sync screw in-between carb 2 and 3 that is for balancing 1/2 to 3/4?

[yamtom]

Don, aha, thats it, much to my chagrin !!

Ok, Don, thats the problem then prolly, I had no idea that they had to be synched as pairs, then matched, although , I think I read that sometime along with a million other things, ha.

Ive definitely been trying to synch them all as individual adjustments ,, all to the number three. Am going to go bench synch them, and am gonna make a special midnight run to the shop to assemble the works and get my answer tonight. Thanks, I will report back before the sun comes up.

[DGXSER]

Hopefully that resolves it Tom. Good luck!

[Larrym]

Ain't Having 4 Carbs Just Way Too Much Fun??

#2 sync screw moves both carb one and two
So #1 is synced to #2
#4 sync screw is used to synk it to #3
#3 had no sync screw
So you sync #4 to #3 then #1 to #2, then #2 to #3

Readjust RPM if it changes between each adjustment
I like to keep it at 1,000 RPM for syncing

After sync I like to adjust the pilot screws
I start by lowering the RPM as low as the engine will allow and keep running.
RPM normally will increase as adjustments made so I reset it as needed.

Once the pilots are adjusted then I recheck the sync.

mro

Yah...What he said. (Official Piggybacking Alert!!!)

Sounds like that if the gauges were reading accurately then the carbs really were that "off". If they were close at the beginning of your syncing process and then ended up getting way outta wack: You turned the wrong screws the wrong way at the wrong time???

Easy to get out of sequence on a "sequentially sensitive" operation like a carb sync. (Way too Xciting, eh???)

Playground Rules: Official "DO-OVER" applies.

Bench sync and then try again.

50/50 chance that it'll go right this time. If not then it's not the gauges, it's not you, it's something else and we'll zero in further on it if it comes to that.

[yamtom]

color tune eh?

Hi Cbug,
Maybe I ought to break down and buy a colortune, I kind of bypassed that since some guys hold them in less esteem than others. Will start to shop for one unless you have an idea for the best vendor for it. Who knows, maybe when I am done, this beast will have the fabled roll on wheelies in three gears that some boast of, ha.

You mentioned a change of "3/8 of a turn". Was that on the sync screws? Seems like quite a large turn for a sync screw... Just touching a screwdriver to the screw will have an effect on the sync, so you shouldn't have that much of an adjustment.

Do you have a colortune plug for adjusting the mixture screws? When I first got my LH, all the screws were colortuned at over 4 turns out. Bigfoot had the same settings on his MNS too. Don't assume that the "starting point" is the correct setting.

[yamtom]

MRO, thanks, will look into an infrared reader too

Thnks MRO,

I cant believe the kind of support that this site has, just plain awesome, think I am gonna donate a few bucks to cover expenses.

Does sound like you may not be getting good ignition on all 4
Which can be caused by a few different problems and sometimes a combination.

Check the headers to see if they are all close to the same temp.
I use the scientific method (my hand) but don’t recommend it if you have issues or are challenged in some way. An image of some guy with his hand stuck to a header and smoke coming out between his fingers might be funny to some but think it would make me sorta cringe. Spray bottle with water and you can see if they sizzle the same or not might be a better option if you don’t have an infrared reader.

#2 sync screw moves both carb one and two
So #1 is synced to #2
#4 sync screw is used to synk it to #3
#3 had no sync screw
So you sync #4 to #3 then #1 to #2, then #2 to #3

Readjust RPM if it changes between each adjustment
I like to keep it at 1,000 RPM for syncing

After sync I like to adjust the pilot screws
I start by lowering the RPM as low as the engine will allow and keep running.
RPM normally will increase as adjustments made so I reset it as needed.

Once the pilots are adjusted then I recheck the sync.

mro

[CatatonicBug]

Hi Cbug,
Maybe I ought to break down and buy a colortune, I kind of bypassed that since some guys hold them in less esteem than others. Will start to shop for one unless you have an idea for the best vendor for it. Who knows, maybe when I am done, this beast will have the fabled roll on wheelies in three gears that some boast of, ha.

They are nice, but yes, not everyone likes them. There is really only one vendor for them, unless you want to go the ebay route and find one that's NOS. Remember you need the 14mm version. The place to get them is www.carbtune.co.uk the same place that sells the carbtune gauges.