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SU's: What could be wrong?


Guest Zpeed

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I've got some SU's that i bought second hand... and i've installed on my engine, and it sounds like it is missing, i have tuned it to the best of my ability using the flow meter, and i've tried to have it lean and in the middle and i've tired to run it rich, but on each case the engine seems to miss every few seconds, and struggles... the carbs do backfire when reved... to around 2500rpm??

 

is it the needles, i don't know what needles are in there, i've got a L26 and i think if it had the bigger needles for a L28 i thought it would be fine only .2ltr differance.. and if it was the stock 2.4ltr needles, i thought it would also be right with .2ltr differance...

 

and i know for sure it is the carbs, due to putting my old flatops onto the engine and it doesn't show the same symptoms when i put the SU's on...

 

and the carbs are 3screw round top 240z ones, and i know that diffinatly...

 

any suggestions... should i just take them to someone who knows SU's or what???

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If the carbs are backfiring then the engine is lean. Have you ever tuned SU carbs before? If not, then get Scott Bruning's (ZTherapy) carb tuning video. In the mean time, richen up each carb about 1/2 turn.

 

Also, check for vacume leaks, make sure the carb pistons are free through their range of travel, make sure there are no air leaks around the pistons or domes, etc.

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  • 2 months later...
Guest Anonymous

Spend extra time making sure the air flow is balanced between your carbs. That will make more of a noticable difference in the way your car runs if it's off a little bit. Then worry about the mixture.

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Also make sure the float bowl levels are high enough and equal. Get a couple of fittings and some clear hose and attach them to the screw holes in the bottom of the float bowls. If you have a mechanical pump, disconnect the distributor wire and turn the engine over to fill the bowls once you have everything hooked up... if you have an electric pump then it is easier... Anyway, you will be able to see whether or not the levels are equal and how high they are in the bowls. If the level is too low in either or both, it won't run properly...

G-luck,

Mike

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