Flat spots at 4k and 5k

I need some help. Car was running like a Swiss watch till I changed the air filters. My RU-4410 K&N filters were down right filthy and I swapped them out with a new identical set so I could clean them without having the car down for a few days waiting for the filters to dry. However after swapping them out I've developed a flat spot around 4k in 2nd-4th when using partial throttle. When I pull the choke out this goes away instantly or if I go to full throttle the engine pulls like a freight train. With wide open throttle it would pull like a freight train till 5k when it would slow down for 1 to 200 rpm then take off again.

What I've done so far

Car has had a full tune timing, valves, carbs synched and topped off with atf. I played with the mixture today and with the mixture set full rich the symptoms go away and it runs great at partial/full throttle all the way through the rpm range.

Should I be looking at going to a slightly richer needle?

Car setup is
40 over 8.7 pistons
270 cam
hif4's with abd needles
RU-4410 filters
dizzy by advance distributors

Drastic weather changes?

Yeah it was balmy today at 65f

Stewart,

I had a similar problem years ago and tinkered for a while to make her run with new air filters. I THINK I solved it when I went to regular engine oil (10-40) in the dashpots instead of the lighter weight oil I had been using in them. I did some other minor things at the time, but noticed that she would again crank right up to 6000 rpm without a stumble...it is worth a try IMHO.

Bruce

Well I'm now completely stumped pulled the carbs front was perfect rear carb had a bit of junk in the jet that was binding it up. Cleaned that up and its working freely reset the jets to the correct starting point and its the same deal. Went ahead and checked for vacuum leaks and found none and just in case installed new vacuum hoses.

Started out with atf same flat spot, switched over to 30wt and then 20/50 same thing

Observations

When setting the mixture to get it to the point where the rpm climbs just a wee bit before falling off the car ends up way lean under load and the jet on the carbs are above the bridge. However the car will idle all day long at 600rpm

When revving the engine a fine mist can be seen coming out of both carbs.

Did ya change out the back plates of the original filters too?

Where I'm goin' is perhaps the vent holes are different with the new filters.

A WAG for sure as you've been down this road before, but just a thought. If it ran fine with one set and you didn't change anything then the new filter config HAS to be the cause.

Don't "over-tech" this and start hunting for some phantom reason!

Just a quick on and off. The filters fit over the stock air filter base. This really has me baffeled. Ran 4k miles since october driven in temps down to the mid negitive teens and now that its nice poof problems go figure.

Maybe check all the plugs and fittings on that HIF Intake, possibly one of the rubber vac plugs has gotten brittle and leaking,or one of the aux vac ports is coming loose, IIRC mine did that at one time and it affected the running.
Mike

Do you still have the old filters to put back in and see if that is the cause?

Progress at last and I think I have an idea of what's going on. I pulled everything off down to the manifold and went through everything. Pulled both carbs off and cleaned them again. Front was perfect rear the jet was hanging up again and I noticed a good scratch down it's length. I gave it a very light polish and its working smooth again. Reassembled everything and fired up and set the carbs and nope same thing as before with some nice backfiring out the exhaust to scatter the kids in the neighborhood. Pulled the bells and the rear get was well above the bridge while the front was about where you would expect it. Reset the rear carb to match the front and while rich at idle runs perfectly through the gears.

So I am thinking that the scratch and the polish is now allowing extra gas to seep through and messing with the mixture.

It's possible... despite what a lot of people like to say about Brit Engineering those dashpots are engineering marvels. The tolerance int eh fitment of the dashpot and piston is normally so tight you can't fit a cat whisker in the gap. If there's excessive gap then the piston won't lift fast enough, leading to richness.

Another thing you might want to check are the damper caps. If they're not the same it could lead to differing performance between the two carbs. OR, if you have excessive gap on one of the carbs, you could try a damper cap with a smaller vent/no vent in an attempt to even out the performance.

If you have spare springs, you can try playing around with different weights... then again, that might be diverging a little too far...

Stewart said:

Progress at last and I think I have an idea of what's going on. I pulled everything off down to the manifold and went through everything. Pulled both carbs off and cleaned them again. Front was perfect rear the jet was hanging up again and I noticed a good scratch down it's length.

Click to expand...

Down the length of what? Surely not the jet as you can't see the inside of that. If it is the needle, you shouldn't be polishing that as you will change the diameter and get a rich condition unless you adjust it out. If you are really talking about the piston flange that slides inside the dashpot, you need to be very carefull not to remove material there too. Which were you talking about?

374-550 was binding up and the outside had a good scratch on it. I should have a new one tomarrow and hopefully that will allow me to set a proper mixture.

Well its fixed the new jet did the trick and the mixture dialed right in and the car is running like a swiss watch again.