Carb turning with a standard vacuum gauge rather than "mercury" style gauge?

[wired00]

Hi Guys

I recently rebuilt a GSX 250 and the last thing outstanding before rego is the carby giving issues with tuning.

I figure I should try syncing both carbies to rule that out as an issue. Until now I've only "Sync'd" each carby's butterfly by eye.

Process of elimination I guess

Anyway my question, I have a standard "car" Vacuum gauge but its missing the little adapters which screw into the manifold inlet holes so I need to order these. But before ordering I wanted to make sure a standard vacuum gauge will be accurate enough for a motorbike. I read the mercury or "Carbtune" brand sync tools are far more accurate for this application (http://www.carbtune.com/). Thoughts?

Cheers for any help

Also, here's the current issue:
It will generally start fine, then after a minute or two after warming up and idling ok without choke it will start running rough and usually die. It seems to be so drastic as the valves being set wrong and once it warms and they expand they're jamming open/closed(!?) but we've doubled checked the tappet clearances and they are all spot on.

We've checked the timing, checked the spark plugs (they were once really black although now seem pretty good) and checked the tappets & valves. We've checked and fuel is running from the tap, floats are filling, all jets in the carb are perfectly clean and the engine is not overheating.

[vifferman]

Because you're synching the carbs (as opposed to un-synching them on my mental V4), absolute accuracy isn't required. Usually, using more'n one gauge, you can just swap 'em over to make sure the readings are consistent.
BTW - the gauges you meantioned aren't that accurate. My VFR needs the gauges to be accurate to +/- 5 or 10mm/Hg, and only mercury ones or expensive factory ones have that kind of accuracy. My dial-type vacuum gauges were perfect for my VTR, but don't cut it with my VFR.

It will generally start fine, then after a minute or two after warming up and idling ok without choke it will start running rough and usually die. It seems to be so drastic as the valves being set wrong and once it warms and they expand they're jamming open/closed(!?) but we've doubled checked the tappet clearances and they are all spot on.

Sounds like either the mixture is wrong, or you've got a fuel delivery problem. The symptoms are the same as what you'll get if the fuel tank vent tube is blocked or partly blocked, or the idle mixture is way off. An air leak could do this too - doesn't matter when the choke's on, but when it's off the extra air will make the idle mixture too lean. Check the carb boots and exhaust flanges (yes, they can make a difference too!)

[wired00]

Because you're synching the carbs (as opposed to un-synching them on my mental V4), absolute accuracy isn't required. Usually, using more'n one gauge, you can just swap 'em over to make sure the readings are consistent.
BTW - the gauges you meantioned aren't that accurate. My VFR needs the gauges to be accurate to +/- 5 or 10mm/Hg, and only mercury ones or expensive factory ones have that kind of accuracy. My dial-type vacuum gauges were perfect for my VTR, but don't cut it with my VFR.

Sounds like either the mixture is wrong, or you've got a fuel delivery problem. The symptoms are the same as what you'll get if the fuel tank vent tube is blocked or partly blocked, or the idle mixture is way off. An air leak could do this too - doesn't matter when the choke's on, but when it's off the extra air will make the idle mixture too lean. Check the carb boots and exhaust flanges (yes, they can make a difference too!)

Cheers for the advice

Great I'll just go ahead and order the little nylon inlet adapters to check the carby sync with the vacuum guage I already have (even though its probably something more drastic...but its just one more thing out of the way).

Sounds like either the mixture is wrong, or you've got a fuel delivery problem. The symptoms are the same as what you'll get if the fuel tank vent tube is blocked or partly blocked, or the idle mixture is way off. An air leak could do this too - doesn't matter when the choke's on, but when it's off the extra air will make the idle mixture too lean. Check the carb boots and exhaust flanges (yes, they can make a difference too!)

Hmm a lot of what you mention sounds like it could be the issue.

The fuel tank vent tube, I actually cut/sacrificed a short length of it to use as a vacuum hose (cheap I know) and noticed it had all sorts of junk inside (dirt, spider web etc) I should really have thought to clean it out or replace it! Although, that might be the over flow tube? I'll clear the tube and blow through it, i assume it should vent into the top of the tank if its a breather hose

I also suspect the idle mixture however I've followed the Haynes guide exactly (and doubled checked). Pretty sure it said screw the idle screw in all the way then out 1.5 turns out. I'll have to check this again...
Edit: I've just now re-read the Haynes manual and it actually says to keep unscrewing the pilot screw until the engine starts idling at its highest...I have completely missed this step. Until now I'd only unscrewed it 1.5 turns and left it. I never tried unscrewing to see how many turns it takes before the engine is running at fastest idle...

Regarding being too lean, I'm thinking its more likely too rich. At one point the bike wouldn't start ... we noticed the left cylinder wasn't firing at all. Took the brand new spark plug out and sure enough its VERY black...cleaned it, started first time. Also, the intake boots are brand new so they should be fine but I'll check anyway. I'll also check the exhaust outlet

thanks for your help