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rodderbilly 02-13-2014 03:47 PM

flathead idle!

Hi all....i need a little help and input....i have in my flathead 2 holley has a choke the other not...carb with choke is on rear....problem is this...engine sometimes backfire ,the front carb sucks a lot of air.....when i put a hand on front carb leaving only a little space for air the car idles.......i change the power valves from 7.5 to 4.5....but engine not idle yet...what could be the problem?thanks

41ford1 02-13-2014 04:53 PM

Re: flathead idle!

What are size are the main jets and power valves? What manifold are you running? Are you running straight or progressive linkage? Progressive linkage can be hard to set up correctly. #47 jets and 3.5 power valves should be a good start for straight linkage. I know more questions to be able to better answer yours.
You might have a vacuum leak that is some what corrected when your hand is blocking the front carb. If you are running a regular dual manifold (Offy) try blocking the rear position. Then tune each carb on the front position. That should give you a decent starting point and show any carb problems. Good luck and keep us posted.

rodderbilly 02-14-2014 09:42 AM

Re: flathead idle!

41ford1....idle issue is solved!i open the rear carb and i try 2 air holes closed from some sealant,,i clear the holes and a put a new gasket....but i don't solve the front carb issue .....still asking a lot of air....with uni-syn in front carb the ball stays on top.....on rear carb ball stay all down but if i start to close the choke ball starts to go up..... for manifold i'm running a 49-53 weiand intake....i try to change gaskets in the front carb now ...the main jets i think are the original 51.. power valves are 4.5....

super flat 02-14-2014 10:12 AM

Re: flathead idle!

Sounds like your throttle plates are to far open to start with. Back off the idle screws until it idles slow then adjust the mixture with the unisyn.

Terry,OH 02-14-2014 10:28 AM

Re: flathead idle!

1 Attachment(s)
The Uni-Syn indicates the air through the Carb at idle and allows you to set multiple carbs at the same level. You adjust the idle RPM adjustment on the Carb. so all are the same. The adjustment on the Uni-Syn allows the tool to be placed on top of the Carb without bogging down the engine RPM and yet getting a reading on the float. Open the tool adjustment place it over one of the Carbs. and then adjust the tool closed so the float moves to some readable level and the engine RPM has not changed. Once you get the tool set, don't change it when you move the tool to the other Carb.
If you place the tool on another carb and the engine RPM changes then that carb is drawing much more air than the carb you calibrated on and you then need to raise the RPM on the first carb and lower it on the second then you may want to recalibrate the tool and then recheck You can adjust either Carb or both to have both read the same float level on the tool, without moving the tool adjustment.

ken ct 02-14-2014 10:43 AM

Re: flathead idle!

The throttle plate setting is not done with the mixture screws. Its done with the single screw on the throttle lever. I think your jets are too big ,should be around .043-.046 with a 2x2 using straight linkage. Your PV's are in the ball park OMO ken ct.Your mixture screws should be 1 turn out from stop.

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