I've noticed the idle on the car had creeped up to about 900 rpm so this afternoon I decided to set it back around 700 rpm. When I blipped the throttle the carb backfired through the secondaries. Just to be sure I did it again. Any ideas on where to start looking. BTW the car runs fine under load once its warmed up. I do get hesitation on the highway if I stomp on it when its cold. Thanks for the help. I have checked the floats and have them set to barely dribble when I remove the sight plug.

Joe
'65 327/350 NOM Holley 2818 carb

 

are you sure you don't mean the backfire is thru the primaries. by just 'blipping' the throttle it would be hard to get the secondaries to open.
anyway, i would check out the accelerator pump operation and setting. that could cause it. check timing and all the obvious stuff...

 

If you have vacuum secondaries "blip" of the throttle will not open the secondaries. Even on mechanical secondaries I doubt they would open from juts a "blip". I'm guessing you have a choke and you see the backfire through the rear side of the carb opening without the choke plate. That side of the carb is open to the same areas of the intake as the primaries. If you have a choke and it is closing the plate, my guess it is still too lean. Open up the mixture screws a little and see what happens. Oh yeah. Make sure your timing is correct and your vacuum and mechanical advance are working properly first.

 

In the first place, the idle will not just creep up in rpm by way of the idle screw adjustment moving on their own. That doesn't happen unless the spring around it is broken or missing.

Something changed to cause your idle speed to go up.

Does that Holley have a choke linkage interlock to keep the secondaries from opening when the choke is on? In any case, you shouldn't have the secondaries open whent the engine is cold anyway.

 

Good point, MikeM. Maybe a vacuum leak too. That'll cause a high idle and possible backfire.

 

Thanks for all your help and speedy reply. After reading through your thoughts things do look somewhat clearer. Sky65 you are probably right in where the backfire came from, I assumed it was from the secondary side because it was towards the middle of the carb; on reading your post you made me rethink what I saw. When I had first got the car I had the front float pull the fulcrum pin from the bowl twice. I was told by the PO that the carb was rebuilt prior to the sale of the car. I'm starting to doubt this. MikeM : After finally getting the float set properly, they were to high, my idle went from about 750 rpm to 900 rpm that was the change. What confused me was where I thought the backfire was from but Sky65 cleared that up. The smartest thing for me to do will be to do it right and start from the beginning and check that the carb is setup properly. I already have a rebuild kit and since it's staring to get cooler here in NJ rebuilding the carb will make a nice weekend project unless of course the little lady adds something to the "honey-do" list.
On a side note; I belong to a number of different forums for various breeds of cars, this forum along with the Healey forum are by far the most normal bunch of people. Both are made up of great car guys with a strong desire to help others enjoy the marque as much as they do. You should all pat yourself on the back and take a bow.

Joe

 

One backfire on a Holley usually toast the power valve. You most likely have a small vacuum leak that caused the idle increase.

 

How can you diagnose if the power valve has blown?

 

Joe, Don't blame the carb just yet......look at your timing. A couple of thoughts.....Is your static timing set right? Is it stable on the timing marker at the harmonic balancer with the vacuum advance disconnected and plugged? If not, you could have a loose timing chain and blipping the throttle will cause a backfire if the chain has too much play in it. Also, check to see that your vacuum advance can holds a vacuum. While looking at the distributor, pull the cap and check the centrifugal weights for freedom of movement. If they stick or are not easily moved, pull the springs and weights and clean them thoroughly with carb cleaner. Then apply a dab of light grease under the weights or on the raised buttons on the weight plate. Make sure they can move freely. Also, try to lift the distributor shaft up and down. If it moves more than a 1/6th to an 1/8 inch, you are probably ready for some shims on the bottom end of the distributor. Good luck.

Mark

 

He claims we're "normal". Never made be feel better. Can't wait to tell my shrink tomorrow. Bill. PS Remember the words of Jimmy Buffet: if we wern't all crazy, we would go insane