Bizarre High RPM Lean... Why?

I am NOT trying to break anyones balls. If you are going to post info in an attempt to help another person on here then the info should be correct or at a minimum stated that it is your opinion. What you originally suggested ,"It seems like you need to try different springs/weights to get the full 48-53* with zero vacuum" and I originally responded to was outrageous.

The remaining posts by me to you were an attempt for you to get a better understanding of timing and what the vacuum advance unit does and when. I obviously failed in my attempt. So be it.

If you are so thin skinned that you are unable to take constructive criticism that is your problem. I guess if I wasn't able to come to a reasonable end and had nothing else to offer then I'd 'cuss too. When all else fails 'cuss 'em out! That will really help! LOL.


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These are some good areas to check. I tend to agree with onovakind67 at this point. Especially the valve springs if they have a lot of runs on them.

I would agree with that thought.

That does seem like a good possibility. I will admit the valvesprings on there may not be the best ones in the world, but they do hold the valve to the seat. I looked at the ignition when I had the cap and rotor off and made sure the magnetic pickups were clean and dust was clear of everything. Also, really noticed how nice the Pro Billet looked on the inside, especially since I am going to school to be a machinist, a gorgeous bit of work, but I digress...

Anyways, I went to see a friend of mine who is the auto shop teacher at the high school I went to. I had the car on a lift looking to see if I had any crushed or mangles fuel lines. I asked him what he would check and he asked if I had tried another carb. I told him I had thought of it, but didn't have another carb to try. He had another Q-jet that he had rebuilt for his Camaro, but it wound up with a holley leaving this perfectly fine Q-jet. Anyways, rapidly approaching race time, I threw it on thinking it would be better than the existing carb if it worked. It worked. No lean spot in high rpm, it is a bit rich and the idle circuits are not set up for my car, but it ran better than the one that was on there. The event got rained out (thankfully) so I have another week to tear my carb apart and go through it very thoroughly. When I took it apart the last time, I was looking for something obvious, and apparently something not so obvious is the culprit. Somehting must be hiding in the corners and making it not run right.

Time for a few hours with some compressed air, carb cleaner and a small brush. Here's hoping it runs like it did before this little gremlin popped up.

Also, about how many runs before a modest hydraulic cam with .500 lift and about 230 @ .050 needs new valvesprings?

Depends on what spring you are using. Its a mild cam, sounds like the same one I am using ,so if they are matched to the cam then they should last a while.

Steve

Tell us a little about your carburetor. Does it use a divorced choke?

The Edelbrock 1910, now out of production, is a 850 cfm Q-jet. It uses an electric choke. I bought the carb used for $25, new was somewhere around $500. It had been used for mainly propane before (figure that one), but had seen some gas through it. I pulled it apart when I got it to make sure it was clean. It was as if it had come right from the factory. I bolted it on the 350 that was in the car at the time, it was too much but it got me away from the electric q-jet that had defaulted to full rich mode. When I did the engine swap, the carb went on the 383 and ran like crazy with minimal adjustment (guess I got lucky!). It has run on the 383 since May of '05 and went that season and this season up until a week ago when it started to surge, you probably have read what I have done in a quest to make it right. I have not had time to dig into the carb today, yard sales, wedding receptions, blah blah blah.

I was talking to a friend of mine who works auto parts. He says he has seen a handfull of the edelbrock q-jets (including the 1910) come back with casting flaws that lead to a surging condition. I have no data to back up a complaint like that but I am searching to see if such a complaint has been documented or is known to exist. Just one more thing to look for when I pull the thing apart. I am guessing that a casting flaw such as that would be on the secondary side because I can run it up to 4500-5000 with no problemon the primaries, and nail the gas and have it surge with the secondaries opened up.

For now I have a borrowed Q-jet that mixes fuel (albeit not set up for my car), but it does not surge. I will let you know of my findings as soon as I can get the thing opened up.

Are you sure your secondaries are opening up? There is a choke lockout pin on the end of the secondary shaft that will keep the secondaries from opening if the choke isn't fully actuated.

I'm pretty sure they are opening up. The car runs 16.9's without the secondaries. It went 15.4 surging. The first thing I looked at was the lockout pin, it was fully away from the throttle shaft (you know what I mean, it was disengaged). I made sure it was disengaged everytime I made a test run of it.

Anyways, I have the carb disassembled and I am waiting for the epoxy to dry on the caps over the primary and secondary wells and one other cap on the underside of the main body. I did this in case it was sucking a little air past the outside of the caps.

Here is a pic of what I have done.

What kind of air cleaner are you running?

Small enough to make the motor go rich rather than lean. It's a 10 or 11 inch round by 2 inches tall k&N with an Xtreme lid. As far as I can tell, it's just big enough for the motor as long as I keep it clean. I clean it about once a month and it had been cleaned about two weeks ago, so it has seen a good night of racing before this surging issue came up. It has been dirtyier than it is and still run like crazy.

Nothing interferes with any of the linkage. I had to get the velocity stack style base so it would clear the accelerator pump. And it seems small because it is all I have room for under the hood with a 1" spacer under the carb.

Needles and seats for Q-jets come in different diameters. In the Doug Roe Q-Jet book there is a section on drag racing where the seats are progressively changed to smaller ones until a lean surge is achieved.

Not sure what Edelbrock is using but at one time four or five different diameter seats were available from GM.

Paul S

I know the needle and seat for the float are a "high flow" at .149". I have made sure that the needle and seat were clear. I will have the carb back together later today and hopefully a test run to see if it is any better.

For those of you still following this stroy and those who want to get to the end of the post to see the answer...

A combination of a thorough rebuild and putting epoxy all over and around the plugs, it runs like crazy now. It works real good now, pulls clean to 5500 then next gear and 4700 to the end of the 1320. I may need to adjust the secondary air door tension a little bit, but it is very close to the tension it was when I pulled it apart.