Carburretor - Settings

Find a Haynes or workshop manual. My knowledge is out of date. I thought no one could get away with carburettors any more on account of emmissions/cost. There used to be 'slide' carburettors, which have a slide operated by the throttle cable. The slide 'throttles' the air and is attached to a needle-jet which makes an attempt at dosing the fuel into the airstream in a ratio that might burn in the cylinder. In many conditions they deliver a fuel/air mixture which the engine can do something with but which falls short on exhaust emissions. The slide carb has particular difficulty with transient conditions and generally requires a means of enrichenment if the throttle is opened quickly, otherwise there is a loss of power untill some sort of equilibrium between the flow of the air uel is re-established. This is accomplished on the slide carbs on my Ducati by an 'accelerator pump' which chucks extra fuel in while the slide is moving using a ramp on the slide.

The more elegant 'CV' carburettors in general use on bikes since the '70s actually measure the air flow through the carb body and open the jet accordingly. They can be damped to provide the temporary enrichment needed for transient demand. The one's I had once on a Honda 500 (Keihin?) were pretty similar to the Stromberg diaphragm type. A piston in the airflow is opened (these days against a combination of gravity and a spring) by the vacuum or depression in a venturi, so that the vacuum is somewhere near constant (Constant Vaccum, see). The piston also controls the fuel flow by means of a needle valve, but the conditions in the venturi are far more stable thus the metering of fuel can be more precise and importantly so can it's dispersion. Sports bikes stayed with the slide longest in the belief that they were less restrictive on full-throttle but both fuel consumption and emissions are better using the CV.

Look up the SU (Skinner Union) carburettor on the web, it's oldest and the simplest form of CV, patented 100 years ago or more.

I believe A. Grahame Bell has a good book on bike engines and carbeuretters. Veloce publishing also have special books on tuning carbeuretters for racing.

there is onot alot of difference between a well set up carby and fuel injection for bikes. in fact injection can make them quite temperamental. they differ from cars in being both smaller and have a downward deflection of airflow on cornering due to the lean of the bike.

still own a MGB and Spitfire with Stromberg diaphragm type carbs, playing with them one learns alot

I thought the MGB had SU carbs, or was it just the older ones and the later ones had Strombergs?

I thought CV carbs were constant velocity. In maintaining constant air velocities through the venturi, the fuel metering is more consistent, hence better able to regulate fuel flow for efficiency and emissions. The vacuum in the intakes is a factor of RPM and throttle position ( i.e. slides, whether cable controlled or diaphram controlled) and it changes drastically depending on the load on the engine. My first two bikes a 1971 CL175 and then a 1970 CB750 both had slide throttle carbs. Those carburetors did not do well if you whacked the throttles wide open at low RPM. You learned to downshift to get the revs up if you needed WOT or use more finesse and patience. Increasing the RPM helped increase the air velocities to improve the carburetor response. All my more recent bikes have had the CV carbs (except my '06 Buell Ulysses with MPFI) and they are more tolerant of ham fisted throttle actions.

By the way, off road motorcycles today still use cable actuated slide throttle carburetors in many applications because they are simpler, more rugged, and less likely to be affected by hitting bumps, landing off of jumps, etc.

I thought long about which term to use constant vaccum or constant velocity. Since in the early SU the depression in the veturi was governed by a mass of metal acted upon by gravity, it seemed pretty clearly a case of constant vacuum. The velocity is also pretty constant but density changes mean not entirely. Variations on the theme with springs as well as the mass adapt the airflow-vacuum relationship to, not sure why because the needle profile was supposed to provide the mixture variations.\

Manifold depression varies drastically but that is the point of the throttle(butterfly), the carb venturi is upstream of this.

"The velocity is also pretty constant but density changes mean not entirely"

OK maybe it's the other way round - help anyone?

Constant velocity, refers to the air moving past the main jet. The system is somewhat self regulating. more rpm= more vacumn = larger opening. less density= less rpm.......

if the fuel air ratio is too far off no rpm

Ah

this could be the cause of the lack of power, from your former post.

check for vacumn leaks around the rubber boots [ the 4 separate intake manifolds] by spraying starting fluid if the engine races replace gaskets or rubber parts. The exaust systems should also be checked for leaks, especially the connections into the cylinder head

Set the valves, syncronize the carbs. syncing the carbs requires a specialized vacumn gauge that can give you a comparison between all 4 carbs @ once. The carbs must be set the same or the different cylinders wil try to run @ different speeds, causing a loss of power.

...or use lighter gas sprayed in the area of leaks......works the same or propane/methane from an unlit torch.....its also cleaner!!

Garthh has some good info. Setting up 4 carbs can be very tricky and time consuming. As soon as you get three set up: the last one can screw them all up. It takes a lot of going back and forth to get it right. There are also seperate drains for each bowl that should be drained each riding season. Look for the screw by the carb and drain till they're clear. They have a tendency of accumulating water while in winter storage. This leads to hard starting and poor performance. Best advice would be to get the manual (Haynes). Pictures always help.

Well i have a 4stroke scooter with kehin carburetor it is having a needle jet , jet ideal , jet needle , controls for throttle opperations , it does effect the performance of the vehicle with respect to adjustments done in any part .fine tunning such carburretors is quiet tedious rather than fun.you can increase effeciency to better of factory settings , even CO , NOX , CO2 emmissions can be altered just by adjusting ideal jet , needle valve is used to control needle jet & jet needle simulteneously while accelerating that can be done by setting needle slot , mine is having 5 slots , i have kept it to 2nd slot from top ,ideal jet screw is adjusted to 2 turns anticlockwise from centre , i do get far better fuel efficiency than it was adjusted by manufacturer / workshop

HI Friends,

Thanks a lot for all your feedbacks/Suggestions. The culprit was the carburetor. The choke piston was jammed in the carb, hence not seated properly. This was leading to poor pickup and poor mileage as well......

Thanks once again all my friends...

Mr. Idiotalways

Glad it and rider are happy now :)