I've used a set of vac gauges to set my carbs up this weekend.
It runs worse now they are evenly balanced than before.
I think the gauges are accurate because I swapped them over and got the same reading from each cylinder using the same gauge.
A mate says he's found this too using my gauges and simply adjusts his carbs by ear after using the gauges to get close.
Can anyone shed any light on this odd situation...
--
These are my own opinions, and not necessarily those of all Toads.
|
|
Did you set your valve clearances, change plugs etc at the same time? In what way does it run worse?
|
Did you set your valve clearances, change plugs etc at the same time? In what way does it run worse?
Did the VC's. Plugs are not old.
It has slight vibration and tickover was lowered slightly. Not what I'd expect from carbs being in *better* tune.
--
These are my own opinions, and not necessarily those of all Toads.
|
|
Sounds like a typical four cylinder, probably has a slightly rattly clutch at idle as well. What's it like out on the road at working revs?
|
Sounds like a typical four cylinder, probably has a slightly rattly clutch at idle as well. What's it like out on the road at working revs?
I know it rattles. I know it's supposed to have vibrations.
What I'm asking is why is it worse after balancing even though the vac gauges are consistant.
Surely it shouldn't be better to adjust carbs by ear? Yet evidence from a mate suggests it is.
This problem specifically relates to carb balancing, I'm not looking for causes of vibrations.
TO put the question another way:
"Why has a mate found it better to balance 4 carbs by ear than by using a set of vac gauges thet *seem* to be ok. [1]"
[1] THe only test that has been done on carb accuracy is to swap the tubes to ensure for the same cylinder they read the same. IYSWIM.
--
These are my own opinions, and not necessarily those of all Toads.
|
|
The carbs are in balance but your engine isn't.
|
The carbs are in balance but your engine isn't.
If that was the case surely it would not be possible to smooth it out ignoring the carb vacuum values and adjusting by ear?
--
These are my own opinions, and not necessarily those of all Toads.
|
Did you tune the car last time, or is this your first attempt ??
What car, what carbs ??
Genaral. These carbs may have been set for a balanced mixture and the vac balance ignored even using a pair of colour tune plugs.
Now you have put the vac right the mixture at tick over in not correct for either carb due to the changed vacuum.
Weaken each mixture until the tickover lifts slightly then richen slightly repeat with other carb then redo mixture again then the vac then the mixture. Also listen to the inlet side of the butterfy with a stethoscope or small plastic pipe you can here the fuel whistling up into the air stream.
Rev the engine to 3k every 20-30 seconds to clear the passage and cool the carbs.
Find the balance on breathing vacuum and mixture and give it a run.
Regards
Peter
|
|
|
|
My first thoughts, Toad, are that it could indicate uneven wear in the spindles of the carbs. What the gauge does is assist you to set the same vacuum at each test point, but if there is an air leak in one carb then it may cause you to adjust something and alter the effective mixture.
Preparing to be shot down in flames!
;-)
Ian Cook
|
I'd have thought adjusting by ear meant balancing the engine as a whole, I remember watching an AA chap balancing the carbs of an MGB by stuffing two tubes down the carbs and listening to the other ends, he reckoned it was the best way to get an even balance.
If one plug is fouled the other running cylinders will create a vacuum in the fourth, which is why I asked if the plugs had been changed. If your rings are a bit below par in one cylinder it will produce a different vacuum, etc, etc. It's not a yes/no question.
|
Thanks for *all* the views guys.
Very helpful.
Shuold I tune to the vacuum or to my ears?
--
These are my own opinions, and not necessarily those of all Toads.
|
|
|
|
Toad
I'm assuming you've balanced the carbs both at idle and when the throttles are open?
ie: Disconnect the throttle linkages, get the idle right (including mixture) with each carb pulling the same flow, then reconnect the linkage and make sure they both open simultaneously? (Check this by ensuring the carbs are still balanced with the engine running at a couple of thousand revs). You can never be sure the throttle linkage hasn't got out of synchronisation.
Have to say I've done them both by ear and using gauges, and there's not a lot to choose between them.
What car is it by the way? I'm assuming it's an older British car with twin SUs in which case there's plenty of scope for linkage wear.
Regards
John S
|
What car is it by the way? I'm assuming it's an older British car with twin SUs in which case there's plenty of scope for linkage wear.
It's two bikes.
A '98 ZZR 600 and a '94 XJ 600 Diversion.
--
These are my own opinions, and not necessarily those of all Toads.
|
toad
Ah, yes, forgot your predeliction for two wheels!
So presumably there's some way of ensuring the two throttles open simultaneously by adjusting the cables.
I've also noted you say the balancing has left the idle slow. That should be correctable by equal adjustment of the individual idle screws. Always get the two carbs correctlty set up and then make surethe two throttles open simultaneously.
Regards
John S
|
So presumably there's some way of ensuring the two throttles open simultaneously by adjusting the cables.
I assume that what the balancing screws do.
I've also noted you say the balancing has left the idle slow. That should be correctable by equal adjustment of the individual idle screws.
Always get the two carbs correctlty set up and then make surethe two throttles open simultaneously.
Hmm. The haynes manual mentions none of this. I assumed the sync screws do everything. Individual tickover; the lot. THere's one central adjuster for the tickover itself.
--
These are my own opinions, and not necessarily those of all Toads.
|
I have tuned and balanced several cars with twin carbs, and reckon to have it down to a fine art. For what it is worth, my method is:
1) Get everything else right - cylinders, valves, plugs, ignition, carb mechanical condition.Check the pistons rise and fall without sticking.
2) Set the mixture for each carb. You could use Colourtune, I use the time-honoured way of lifting the piston a fraction and seeing if the speed rises (rich), falls (weak), or stays the same or rises and then falls again (correct)
3) Disconnect the throttle connecting link - you need to be able to adjust the idling screw on each carb independently. By a combination of increasing one and reducing the other, get an equal vacuum on each carb. Once achieved, adjust both screws by equal amounts to set the correct idling speed. Then recheck the balance.
4) Reconnect the link, give the engine a good rev, then recheck the mixtures, and the balance.
It is worth making absolutely sure that any slack in the linkage is taken up equally by each carb. If necessary use a fine feeler gauge to find the point at which each spindle just starts to move.
|
Cliff
Exactly how I do the job.
Toad, the individual idle screws will enable each carb to be balanced at idle, and the single idle adjuster will then move them in sych, it appears. However, you still need to be sure the throttle moves them exactly together, or the balance will be lost when you move off idle.
Regards
John S
|
|
|
|
John
When WILL people cotton on to stating what vehicle they are asking about at the beginning rather than after a dozen posts.
We've had plenty of cases where the make/model of the car has not been stated, but getting a dozen answers based on a car when it turns out to be a bike really does take the biscuit!
|
Sorry Brian! It was badly worded beginning to end!
I will be more carefull in future!
--
These are my own opinions, and not necessarily those of all Toads.
|
On a bike there is more than one tweak to balance carbs.
The most common one is the three screws (for 4 cylinder) that adjust the relative opening of the butterfly valves.
The other one is the idle adjustment (air bleed, mixture or whatever it's called) This is sometimes a sealed adjustment - especially in the USA where you have to drill out a blanking plate. Myself, I've always found that setting them to the manufacturers recommended setting (2.5 turns out in my case) works well enough.
A final setting that may have an effect a float level - a pain to set up though.
Doing just the butterfly balance on mine has always been enough to make things run smoothly though.
Martin
|
|
|
|
|
