Weber Carb Problem?
Discussion
I have a rebuilt Essex 3.0 Stage 1 with a stock DGAS 38 Weber carb. I'm having some problems getting the idle circuit set up. Turning out the mixture screws seems to have no effect. What seems odd is that the engine hunts when both idle jets are properly seated but when I unscrew only the small idle jet on the passenger side of the carb out about a half turn the hunting disappears. That seems odd to me, since it suggests starvation on one side of the carb. I've done my best to blow out all the pathways from main jet through emulsion tube to idle jet to mixture screws but that didn't seem to help.
In any case, even with mixture screws at five turns out the engine seems to be running very lean. I based that on the fact that there is substantial hesitation when I try to pop the throttle.
Any guidelines or tips would be greatly appreciated.
Thanks.
In any case, even with mixture screws at five turns out the engine seems to be running very lean. I based that on the fact that there is substantial hesitation when I try to pop the throttle.
Any guidelines or tips would be greatly appreciated.
Thanks.
janscheit said:
I have a rebuilt Essex 3.0 Stage 1 with a stock DGAS 38 Weber carb.Thanks.
What do you mean by "Stage 1"? Modified engine with different cam perhaps? If so then the standard jetting could be miles out for what the engine actually wants.It would help if you knew what the various screws actually did. There are two idle "mixture" screws on the short flanges of the carb directly opposite each other and one idle "speed" screw on the long side of the carb above the throttle spindle quadrants which adjusts the butterflies. Watch the butterflies through the top of the carb and back the idle speed screw out until it no longer touches the linkage i.e. the butterflies are fully closed. Then screw it in 1/2 a turn to open the butterflies a tad.
Screw both idle mixture screws right in gently and then back out 1 full turn.
Start the engine. Adjust the idle speed screw until it just idles as slowly as possible. 650 rpm or less.
Screw both idle mixture screws in a 1/4 turn at a time until the engine hunts or dies and then back out a bit until it idles again at the fastest speed. Now reset the idle speed screw to get a good idling speed.
If the idle mixture screws still make no difference to the idle quality then the idle jets are too small for the modifications and will need changing. At that stage you probably need to get the car to a rolling road for a proper setup with new jetting.
If you are sure that there are no blockages in the carb, the cause of the problem will be poor inlet manifold vacuum or an air leak (this causes you to have to set the throttle butterflys too far open to get the engine to idle, which causes insufficient fuel to be pulled through the idle drillings adjacent to the butterflys)
Poor vacuum can be caused by many things, valve or ignition timing, tight tappet, air leak or a cam with radical timing.
If it's only stage one it should idle with standard jets.
ETA Could also be a leaking valve, worth checking the compressions.
Poor vacuum can be caused by many things, valve or ignition timing, tight tappet, air leak or a cam with radical timing.
If it's only stage one it should idle with standard jets.
ETA Could also be a leaking valve, worth checking the compressions.
Edited by oakdale on Thursday 28th October 20:20
Might be able to help on this one, I just went through a similar issue with a 32/36 on a Formula Ford. Assuming all the points noted above regarding air leaks, timing ect are spot on.
Firstly check the float level, not point doing anything on a weber until you know the fuel level is correct in the bowl. Also make sure the fuel pressure is not too high. Should be 2.5 psi or so no more than 4psi.
You need to make sure that both throttles can close completely.
Ideally remove the carburettor and turn upside down, using the idle screws open the butterfly's until the idle port is uncovered, maybe 75%. If the 38/38 is similar to the 32/36 then the progression port is very close to the idle port. If the progression port is even slightely uncovered then the mixture screw has no effect and the motor always seems too rich at idle.
Refit the carburettor, set the mixture screws 1 1/12 turns out then start the engine and allow it to warm up.
Once warm return to idle and adjust the mixture screws to achieve the highest rpm.
Now adjust the idle screw to bring the idle to about 900rpm.
Adjust the mixture screw again.
If the mixture screws are 2 or more turns off their seat then you need larger idle jets. If the mixture screw is closer than 1/2 turn to its seat then you need smaller idle jets.
The key to the above is to ensure that the carburettor is set so that fuel is only being drawn through the idle circuit.
Firstly check the float level, not point doing anything on a weber until you know the fuel level is correct in the bowl. Also make sure the fuel pressure is not too high. Should be 2.5 psi or so no more than 4psi.
You need to make sure that both throttles can close completely.
Ideally remove the carburettor and turn upside down, using the idle screws open the butterfly's until the idle port is uncovered, maybe 75%. If the 38/38 is similar to the 32/36 then the progression port is very close to the idle port. If the progression port is even slightely uncovered then the mixture screw has no effect and the motor always seems too rich at idle.
Refit the carburettor, set the mixture screws 1 1/12 turns out then start the engine and allow it to warm up.
Once warm return to idle and adjust the mixture screws to achieve the highest rpm.
Now adjust the idle screw to bring the idle to about 900rpm.
Adjust the mixture screw again.
If the mixture screws are 2 or more turns off their seat then you need larger idle jets. If the mixture screw is closer than 1/2 turn to its seat then you need smaller idle jets.
The key to the above is to ensure that the carburettor is set so that fuel is only being drawn through the idle circuit.
Thanks for the suggestions so far. Here's an update. I replaced the intake manifold using gasgacinch around the intake ports and a bit of silicone around the water ports. Started the engine and once warmed up it still hunts very noticeably favouring the same side as it did before the replacement. Slightly unscrewing the idle jet on the off-side (or passenger side for an english car) results in almost the exact same behaviour as before, the engine rpm increases and the hunting stops though the idle is still a bit rough.
The engine builder suggests that the only jet that could use replacement is the main jet but the problem is only on one side so the main jet is not the problem.
Stage one is simply polishing of the intake ports and 0.30 overbore. Nothing too dramatic, still has a stock cam. I've also checked and rechecked the valve clearances and the timing.
At this point I can't help thinking it's an obstruction in the carb. It's a rebuilt engine that has yet to rev more than 2500 rpm so has not been stressed at all so I can't see anything being wrong with the engine. On the other hand the inlet manifold is a bit crappy but didn't give me any problems on the former engine.
The engine builder suggests that the only jet that could use replacement is the main jet but the problem is only on one side so the main jet is not the problem.
Stage one is simply polishing of the intake ports and 0.30 overbore. Nothing too dramatic, still has a stock cam. I've also checked and rechecked the valve clearances and the timing.
At this point I can't help thinking it's an obstruction in the carb. It's a rebuilt engine that has yet to rev more than 2500 rpm so has not been stressed at all so I can't see anything being wrong with the engine. On the other hand the inlet manifold is a bit crappy but didn't give me any problems on the former engine.
The engine builder is clearly an idiot but no great surprise there.
If the idle mixture screws need to be unscrewed by differing amounts then so be it. In a perfect world both they and the idle jets would be identical but wear and tear takes its toll. As long as you get a decent idle on both sides at some setting or other then live with it.
If the idle mixture screws need to be unscrewed by differing amounts then so be it. In a perfect world both they and the idle jets would be identical but wear and tear takes its toll. As long as you get a decent idle on both sides at some setting or other then live with it.
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