Fine tuning carburetors with vacuum gauges

78jotadave

78jotadave

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Location
Palm Desert, California. (near Palm Springs)
I have for many years used vacuum gauges to synchronize the carbs (Dells) on my Jota after adjusting the mixture screws, right, that's what you do. Now after synchronizing the triple carbs with the balance screws on the top/linkages of the carb to get them as close as possible, I leave the vacuum gauges in place spot on fine-tuning the gauge readings is done by fine-tweaking the mixture screws, the result my Jota is running perfect carburation.

Instructions for fine-tuning the Mikunis on my 1200 led me to do the same for the Dells, it worked a treat.
 
Personally, I first put the carbs all on the same adjustment for the mixture screws, then balance them first time. Then go over the mixture screws again and look for the highest rev-point (center between the two positions where the revs drop, maybe a bit on the rich side), then check balance again and adjust accordingly. That gave perfect results where my Jota with a megaphone exhaust and open bellmouthes ran like a swiss clock even at 600 rpm without stalling (just for show, not for permanent).

That all said, it is absolutely mandatory to check all other conditions before balancing: valve gaps, no air-leaks, ignition timing, carb in perfect condition, choke sliders moving freely and - much more important, close completly! etc.

Edit: oh - and floathights need to be perfect, else adjustment will just mask this problem...
 
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About to set up the keihin carbs on a Honda cbx750. Have been right through them and rectified quite a few problems. I will set the mixture screws to 1.5 turns out, then sync with the spindle adjusters, working from the centre out. Correct, if everything is going ok, I should be able to find tune on the mixture screws, but sometimes it's hard to see any change
 
The intercarby linkages on all i worked on had non linear span idle stop to WOT stop, hence my comment to set the idle at 3300rpm. Did solve problems in my experience. Idle was not nearly so fussy about sync.

Not exactly understanding "Restore the linkages" all the same Piet. j
 
iis_iis said:
The intercarby linkages on all i worked on had non linear span idle stop to WOT stop, hence my comment to set the idle at 3300rpm. Did solve problems in my experience. Idle was not nearly so fussy about sync.

Not exactly understanding "Restore the linkages" all the same Piet. j
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I'm guessing Piet means, replace all worn ports of the linkage so that it's better than new...

cheers,

bazzee
 
iis_iis said:
The intercarby linkages on all i worked on had non linear span idle stop to WOT stop, hence my comment to set the idle at 3300rpm. Did solve problems in my experience. Idle was not nearly so fussy about sync.

Not exactly understanding "Restore the linkages" all the same Piet. j
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The slop in the shafts and their bearings will allow the levers to alter their contact points through their travel arc, immediately stuffing any synchronising that may have been performed beforehand. Bent/warped carb brackets and bent levers can cause much the same symptoms. Wise to leave the inlet manifolds slightly loose when fitting the early type clip-fitting carbs, tighten them with the carb assembly half-way on when it has positioned the manifolds optimally to suit the actual carb spacing. But, to get there, the inlet manifolds must also be truely square to the intake port flanges, ie, the insulation plates and manifold flanges must be dead flat.

Place the flange of a used manifold on a glass plate, you'll quickly see what I mean...

piet
 
Piet's commend is spot on, as allways. The brackets that hold the Dell'Orto carbs together is very weak and can be bend easily when popping the carbs back onto the bike. I had a set here where this was bent so badly that when I adjusted the sliders at lower position all to the same hights with a drill-bit after the restauration, they were so heavily out of sync at WOT that the left one was 5mm lower than the right one :oops:. Took the brackets off again, made sure they are straight and alligne the carbs 100% straight and told the owner he shall be VERY cautious when mounting the carbs back onto the bike and check they are still straight.

Can even happen to the bikes with the non-rubber installation (the ones that are clamped directly onto the manifold without rubber)...

Other reason why they can lift unequally is that the rocker under the top cover has become lose. Allways good to check the hex-screw there for being tightened (and maybe fasten it with just a little bit of loctide blue...)
 
sfcpiet said:
The slop in the shafts and their bearings will allow the levers to alter their contact points through their travel arc, immediately stuffing any synchronising that may have been performed beforehand. Bent/warped carb brackets and bent levers can cause much the same symptoms. Wise to leave the inlet manifolds slightly loose when fitting the early type clip-fitting carbs, tighten them with the carb assembly half-way on when it has positioned the manifolds optimally to suit the actual carb spacing. But, to get there, the inlet manifolds must also be truely square to the intake port flanges, ie, the insulation plates and manifold flanges must be dead flat.

Place the flange of a used manifold on a glass plate, you'll quickly see what I mean...

piet
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More clear now, in what you indicated in the short "Restore the linkages" Piet, most of what you indicated was aware of, does no harm being made aware even after many years on the Laverda, tah. Restore can mean "rate" in my fields, hence my request for clarity.

All the engines i had repaired or worked on, had previous owners, ie significant carby issues, from the rider... to the dpo of most all, ageing parts. j
 
I once spent a year trying to work out why my Pantah had suddenly lost all its punch off idle, I thought it was a tuning issue caused by my poor ability with my mercury vacuum gauges so I just kept repeating the same technique at balancing the carbs and getting the same result. I finally dropped it off at a pro and he found the carbs were a good 5mm out of sink on the cables, he used his fingers instead of my Mercury gauge to aline both slides and presdo the toque off the line was found. That episode still confuses my monkey brain about why using vacuum resulted in the mismatched slide heights. Those gauges worked great on my triple.
 
Just saying, but what I really was saying is that after adjusting the mixture screw first, then synching the carbs (worn or slack linkages from 30k plus miles of use) then fine-tuning the mixture screws, and watching the synch on the vacuum gauges the bike came out a treat. 1000rpm idle and wack open the throttle, no bogging just like a pitbull out the gate to the red fuckin line.
 
78jotadave said:
Just saying, but what I really was saying is that after adjusting the mixture screw first, then synching the carbs (worn or slack linkages from 30k plus miles of use) then fine-tuning the mixture screws, and watching the synch on the vacuum gauges the bike came out a treat. 1000rpm idle and wack open the throttle, no bogging just like a pitbull out the gate to the red fuckin line.
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Thanks for bringing this to attention. I’d never heard of this before, I’ll definitely have a go next time I’m messing about with the carbs, although as an amateur I’m not sure I can get it close enough on the first bit… I’ll have a bloody go though 😁
 
Vince said:
I once spent a year trying to work out why my Pantah had suddenly lost all its punch off idle, I thought it was a tuning issue caused by my poor ability with my mercury vacuum gauges so I just kept repeating the same technique at balancing the carbs and getting the same result. I finally dropped it off at a pro and he found the carbs were a good 5mm out of sink on the cables, he used his fingers instead of my Mercury gauge to aline both slides and presdo the toque off the line was found. That episode still confuses my monkey brain about why using vacuum resulted in the mismatched slide heights. Those gauges worked great on my triple.
Click to expand...
I never use vac gauges on a twin. If you can hear properly that both sides idle equally on the stops and mixture, I just ensure that the slides lift exactly the same from shut. If they are half a mil out at wot I don't care, but they are usually spot on. I do not want it picking up on one side first all the time from closed throttle.
With vac gauges equal on my 270 motor it idles on one. An even idle has it way off on the gauges.
On a multi with a rack you have to use gauges.
 
Tippie said:
I never use vac gauges on a twin. If you can hear properly that both sides idle equally on the stops and mixture, I just ensure that the slides lift exactly the same from shut. If they are half a mil out at wot I don't care, but they are usually spot on. I do not want it picking up on one side first all the time from closed throttle.
With vac gauges equal on my 270 motor it idles on one. An even idle has it way off on the gauges.
On a multi with a rack you have to use gauges.
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I never use vacuum gauges on a twin either though I go for equal lift at the top, I'm not too worried about idle.
Paul
 
I learned that lesson in the real world, but I still wonder why fingers and the click of the slide hitting the stop on twins and more than 2 cylinders measuring vacuum is best. Lack of sufficient finger spread seems a bit less scientific a reason.
 
Slides hanging on a rack is very hard without gauges. Multis with cable to each slide can be done like a twin. TZ750 etc used to use a drillbit to synchronise. Cannot use Dave's mixture screw technique though.
 
"Synchronising" carbs by eye or feel is only half-way there imho. It takes a lot of intimate knowledge of the bike and understanding of what's going on to do it half-decently. It'll do OK for a quick roadside fix, no worries, but for regular maintenance, it's no more than a bodge, sorry. Fine to do it first and then have the guages confirm you weren't far off... ;)

Vacuum guages will show up any untoward circumstances like uneven lifting, varying and excessive slide/body leakage. Synching at WOT is nowhere near as important as idle to midrange, where most riding takes place (one the road at least).

It should be common knowledge that valve clearances must be adjusted evenly before even attempting to synchronise.

Just my $0.02 worth,
piet
 
I have used gauges on both twins and triples, with the twins I could never previously get an idle that would sustain for more than about 20 secs (oil pump! I know, I know) and using a "bank" of gauges is now old hat since there can be inequalities across the gauges. I was lucky enough to get a Yamaha factpory supplied vac gauge kit from a closing dealership sale a good few years ago now, it has one very high quality alcohol filled gauge, and a 6 way tap with six hoses (some Yamaha outboards are V6 and two stroke as well) since then the twin (that I no longer own) idled perfectly and realy did rev out as well, on the triples much the same applies, but when done I lockwire the adjusting screws and here some will chime in and say its not neccesarry, but who cares, it works for me and the first sign of out of sync is the loss of steady idle, and that doesnt happen since doing this.

CABLES on the vast majority of triples there is only one (external) cable from the cross shaft and its mechanism to the twistgrip so if you fit the tank clumsilly and it moves or traps the cable, there is no real drama other than an increased idle speed, but with twins with a top cable, two bottom cables and a splitter, out of sink can occur by the tank "moving" the cables into a different ark, my own choice on twins was allways two cables and a twistrgrip that accomodates those two cables, (regardless of model type) just my preferred choice but cable routing and retaining became a lot more important in order that when refitting the tank nothing changed, ever sat on the bike in the garage and simply turned the handlebars, only to find that the revs increase???? guess why that is!

the pilot adjusting screws should be the same, over all carbs fitted to any bike and in all cases and not used to sync the carbs.
CLEM
 
CLEMTOG said:
I have used gauges on both twins and triples, with the twins I could never previously get an idle that would sustain for more than about 20 secs (oil pump! I know, I know) and using a "bank" of gauges is now old hat since there can be inequalities across the gauges. I was lucky enough to get a Yamaha factpory supplied vac gauge kit from a closing dealership sale a good few years ago now, it has one very high quality alcohol filled gauge, and a 6 way tap with six hoses (some Yamaha outboards are V6 and two stroke as well) since then the twin (that I no longer own) idled perfectly and realy did rev out as well, on the triples much the same applies, but when done I lockwire the adjusting screws and here some will chime in and say its not neccesarry, but who cares, it works for me and the first sign of out of sync is the loss of steady idle, and that doesnt happen since doing this.

CABLES on the vast majority of triples there is only one (external) cable from the cross shaft and its mechanism to the twistgrip so if you fit the tank clumsilly and it moves or traps the cable, there is no real drama other than an increased idle speed, but with twins with a top cable, two bottom cables and a splitter, out of sink can occur by the tank "moving" the cables into a different ark, my own choice on twins was allways two cables and a twistrgrip that accomodates those two cables, (regardless of model type) just my preferred choice but cable routing and retaining became a lot more important in order that when refitting the tank nothing changed, ever sat on the bike in the garage and simply turned the handlebars, only to find that the revs increase???? guess why that is!

the pilot adjusting screws should be the same, over all carbs fitted to any bike and in all cases and not used to sync the carbs.
CLEM
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Putting twin throttle cables on my S is on my list of winter chores. Twin cable Tomaselli throttle sitting on the bench.
I've had quite enough of it going out of synch due to cables moving around in the cable splitter.
No problem on my SF2 however.
Paul
 
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