&^%&#$# Carb tuning - please tell me where i am going wrong?

[Buffalo]

I cannot get the lean drop method in the FSM to work. Before i turn the FSM into kindling, please can someone either tell me where i am going wrong, or else tell me another way to tune my carb'd 3F??

First up. the carb on my 3F is not the same as that shown in the FSM sketch, because the idle mix screw is on the LHS (when looking at it from front of car) of the carb, under the air intake hose. Everything else is on the back of the carb, nearest the firewall (which makes tuning it a PITA!). It is a Toyota carb though for this engine.

The FSM says to get the fastest idle using the mix screw and then adjust using the idle screw. However, when i get the fastest idle using the mix screw, the revs are higher than 690rpm that the FSM states so i have nothing left on the idle screw to bring it down with; backing off the idle screw makes no difference, the revs will only go up by screwing it in (clockwise).

In which position do i start the turning of each screw? Am i supposed to start with the idle screw all the way in to give me something to back it off to? - because this would result in the engine revs being very high to start... Screwing the mix all the way in results in engine stall, so where is that supposed to start from..?

Hope someone can either tell me how to do this properly, or else show me a better way.

Cheers ~PHIL

 

[2mbb]

Your idle mixture screw sounds like it is in the correct position. It is on the side of the carb next to the valve cover. You probably have a different air cleaner set up than the 2F, though. The drawing below is from the perspective as if you were standing on the distributor side of the engine and peering over the valve cover. The linkage on the left side of the drawing is the linkage that faces the engine compartment firewall

i usually start with the idle mixture screw 1 1/2 turns out (counter clockwise) from being seated. You also need to make sure that you are turning the correct idle speed screw. There are two screws on the firewall side of the carb. one is for the idle and the other is for the fast idle (when the choke is on). The idle mixture is the one closer to the carb. If you're turning the right screw(s) and still can't get your idle speed down, then I would start looking for a vacuum leak, or make sure there is nothing keeping the throttle plate open (choke on, linkage sticking, etc.)

 

[Buffalo]

Thanks for your reply. I reset the mix screw to 1,3/4 turns. The idle is around 650-690rpm. The idle screw is not doing anything but increase the revs by screwing in further, nothing by screwing out further.

Is this acceptable. Idle is not brilliantly smooth - sounds about as low as it will comfortably go...

Cheers ~PHIL

(Will check the vac advance line as that looks a bit brittle...)

 

[Marc311]

x2 on what 2mbb said. The fast idle screw jumps out at you. The idle speed adjusting screw takes a bit more contorting to see.

EDIT: I must have been typing while your were typing. If the idle speed screw is all the way out, your engine should be dying. Are you certain you're not turning the fast idle screw?

 

[Buffalo]

Positive! I identified the idle screw from a combination of the diagrams above and the other manual i have. The fast idle one is easier to work out anyway because of it's position.

I suppose i may not have set something up correctly...

Background: The car has just gone to the auto-electricians and had the alternator repaired and the voltage regulator fiddled with. It is now holding charge correctly. When it came back it was revving too high at idle.

This morning I checked the ign timing and dwell first. They were both roughtly as they were before but the engine was revving about 1100rpm when warm. I have the timing slightly advanced because it runs better at the altitude we are at here like that. Points though, were at 41' dwell, as they were before.

As they were what they were supposed to be, all i did was to adjust the timing a bit, leave the dwell angle alone and then try to move on to the carb and this is where it doesn't seem right.

I realise that given the only work done was electrical it is most likely to be the electrical side causing anomolies but like i said it all seemed to be as was. I therefore thought i could move straight to the carb...

I guess i could go back and adjust the points from scratch?

~PHIL

 

[donny paradise]

points?

 

[Buffalo]

Well... Don't i feel a pillock. After reading Marc's answer, i checked everything again, including teh throttle linkage which i had checked to be tight before. I found i could kill the engine by holding it down hard against the stops and that by adjusting the idle screw at the same time as doing this, keep the engine running...

So i started at the throttle pedal, checking the cable for anything that might be keeping it open, only to see that the hand throttle was open about a 1/4 inch... DOH!!!!!!!

Who knows why that was open as i've never used it, perhaps the garage opened it whilst they were fiddling around and never closed it..? Still, very amateur of me, should have checked that first. Might have even saved myself 4 hours of time spent messing around with the truck this morning trying to get the damned thing running properly.

Still lesson leearnt and it's idling nicely on the drive now.

Thanks ~PHIL

 

[Marc311]

As simple as it is that's pretty good troubleshooting. Not sure you'll find an FSM anywhere to tell you to check the hand throttle first!