71 Cuda 68' 440 4spd-- some kind of lumpy cam came with the car.. :dunno: Holley 4779-2 750 cfm mechanical secondaries
So this all started just replacing a leaky accelerator pump diaphragm. Well, I thought while I was at it I would take it apart,clean out the old gas varnish and put a Holley Renew kit in it. Ever since then, this car will buck and jerk under light acceleration while trying to just cruise at a steady speed.
I had a heck of a time trying to set the float level ,which I eventually found out was because of a piece of dirt under the needle and seat. The plugs were black and old and i replaced em with Autolite 85's (nobody had Champions at the time).
I have the timing advanced to the highest vacuum without a ping. Have been messing with the idle mixture to no avail.. I pulled the Power valve back out and that holds a vacuum.I am not sure I have the correct Power valve in. I installed a 6.5" hg that came in the kit.I blew out all the holes in the metering block while I had it apart the 2nd time,..no dirt found. Fuel pressure is 4.5psi at idle. Holding the choke part way shut to see if it was too lean did not help. I wanna believe it is something i did or didn't do to the carb and not ignition,because once I step on it,the jerking and bucking goes away and the car takes off. Feeling stupid I cant fix this. Ps.. The plugs now look normal after several short test drives... Any suggestions would be much appreciated
Just a thought, the kits come with an assortment of gaskets. Since you didn't have a problem before, did you match up the new gaskets with the old ones? Maybe you have a hole open or closed where it wasn't before. Did you mess with the accelerator pump adjustment? Just a few ideas.
Thx fotr the reply.
yea ,I did check that when installing new gaskets and have rechecked it several times now, having the carb on and off too many times.I ordered a 4.5 "hg power valve today, since I do remember my old one not quite matching up "by looks " as the (it did not have any #s on it) ones that came with the kit.
What power valve was in the carb before you rebuilt it, a 6.5 or a different one? A good troubleshooting guide here - https://documents.holley.com/9100083.pdf
As I remember (did the carb 6months ago)there were no markings on the old one,but it did look different than the ones in the renew kit.So I went on Holleys site and came up with the conclusion a 6.5 " hg would be good.
What is your vacuum at idle?
Keep in mind that your Holley is running off of different circuits when idling than it is at cruise, so if it is acting up at cruise then trying to troubleshoot it while it is idling may be fruitless. Since you are looking for a Holley Guru, I assume that you aren't one. But a Holley Guru on the internet can only help so much. If you really want to get this carb dialed in right, I'd recommend installing a wide band O2 meter. The wide band meter can provide real time feedback on what your carb is doing at cruise. Typically, when a car bucks or surges at cruise it is running lean. But again, that would only be for the primary circuit. Could be fine or even rich at idle and then leaning out when you get into the primary circuit.
Also, a lumpy cam that provides a low vacuum signal will make it more difficult to tune your carb.
My recommendation would be to get a good book and start watching all of the Holley carb tuning videos on line. Eventually you will be the Holley Guru. :alan2cents:
Get a vacuum reading at idle in gear, with a lumpy cam you probably don`t have a lot and may need a 5.5 or even a 4.5.
We can start with some basic tuning recommendations. For a street car with an automatic transmission, use your vacuum gauge to read the idle vacuum of a fully warm engine at idle in gear. Let's put this idle vacuum at 13 "Hg. If we divide the idle vacuum in half, this results in a 6.5 "Hg power as a great starting place for tuning.
https://www.holley.com/blog/post/how_to_tune_the_power_valve_in_a_holley_carburetor/
Like everyone said get a vacuum reading at idle. Take the reading and divide by 2 which should put you in the ballpark of the correct Powervalve. Most folks run 13-15" of vacuum which makes a 6.5 Powervalve ideal. The idea here is that you come off the idle circuit and go onto the mains. The Powervalve lets allows fuel into the carb when the manifold vacuum drops below it's rating.
The other thing to do is when you have an unknown carb - I like to put it back to stock and work from there. If you play with someone else's guesses it can make it a hell of a time to sort out.
It also seems you have a squirter mismatch. Either it's too much or not enough because you are either putting too much down or not enough when transitioning from idle to main. You can also adjust the amount of squirt from your accelerator pump but that will only go so far.
Are you sure about your fuel pressure? Holley recommends 6.5 to 7 psi. See if reving your engine a bit past idle brings up the fuel pressure when its needed.
Also, here is a trouble shooting guide that may help a bit.
https://www.racingjunk.com/news/troubleshooting-your-holley-carb/
Good luck!
IF auto make sure the vacuum reading is done while in gear !
My bucking/jerking saga continues..Thanks for the responses, much appreciated :)
So before I do anything more, I got another correct Holley Renew Kit. I spoke with the Holley Tech guy and asked him,should the hole(the one my pencil is pointing to,.. be covered by the gasket or no ? They look like dead end holes to me with an air blow gun test. He said he could not answer for sure.The casting # on the carb and primary metering block would not tell him. He said I did have the correct kit.
I will install the new 4.5"hg Power valve i bought and then will retard my timing back to around 4 btdc where it was and reset idle mix for best vacuum and recheck float levels and also recheck fuel pressure while the missing is occuring. Also need to check coil voltage and see if I can hook up my Fluke 88 and see if there is an ignition problem.But if ignition is the problem, why does the car pull away once I get over the "bad spot"
It just bugs me that Holley's exploded parts list shows different #'s for the primary and secondary metering block gaskets,yet they are exactly the same in this kit as they were in the kit I installed last time. Not a pro Holley rebuilder ,can u tell..lol
I suppose that if you are trying to return to the last baseline when the car ran well that it would make sense to back the timing off, but I can't believe that your car would really run well at 4 degrees BTDC timing. I'd guess that most folks timing for performance are running somewhere in the range of 15+ degrees BTDC. :alan2cents:
I agree with u on the timing, but yes ,trying to eliminate 1 possibility at a time
If the carb is still in pieces, take a can of carb cleaner with the red straw in the nozzle and blow the cleaner through every office/passage in the carb, paying special attention to the metering block. Make sure the cleaner is blowing through cleanly. Back when I rebuilt cards for a living, I learned this the hard way. Just because compressed air is blown through the passages doesn't mean there's not some piece of dirt or rust partially plugging it up. Eventually this became part of my routine and it made a difference.
Last carb I rebuilt I cleaned it with a ultrasonic cleaner. Did a very good job. It was a small engine carb but those ultrasonic cleaners clean all the little passageways etc very well.