Updated: fixed Holley Carb question, need advice from Holley folks.

[Curly]

Edit: Updated post, see below. Problem seems to be fixed.

This post is long, but I am trying to get all the info out there that might help with the diagnosis. I already know I am going to rebuild the carb, but I don't have much experience with Holley carbs. This is a Holley Street avenger #80670, about ~600cfm or so. Are there any problems/pattern failures to watch out for when rebuilding one of these carbs?
More info below to hopefully help you help me.

I am experiencing what I suspect is a lean backfire on my 77 Cherk w/360. The problem popped up a few weeks ago, I get intake backfires under moderate load, any speed above 35 or so. Level cruising, it seems ok, but add throttle for a slight hill or grade and it begins to slightly surge. Add more throttle, it will backfire multiple times. NOTE: This isn't an accelerator pump problem, the jeep is fine off the line. A visual check shows plenty of squirt from the pump.

At first I thought it was ignition (HEI conversion) so I replaced the cap, rotor, ignition coil and plugs with new.(Napa parts- Echlin and Autolite) Old cap was worn, rotor was in ok condition. The jeep ran better, not a big improvement, but I could feel an improvement. Note: Module was replaced a few months ago, Echlin brand.

Old plugs were in good shape, but showed lean. I had reduced the jet size in a quest for better MPG, so I reinstalled the jets that it came with. (#65, stock for that carb #, didn't touch the secondary jets, primary only) No noticeable improvement.
Other info: Float level checked and correct, float bowl clean, new fuel filter (with return, oriented properly ) Timing is 10*BTDC, Ported vacuum to advance (runs the same with manifold vac) Power Balance test with Snap on Scope shows all cylinder firing evenly, KV readings range from10-12 kv. New MSD wires installed and removed (misfire at idle with wires) Fuel pump volume and psi checked, in specs. (Pump was replaced about a year ago) Fuel lines are new, tank is very clean internally.
Engine runs smooth at idle and low rpm, starts right up every time, hot or cold.

So, any thing I missed? Suggestions? What should I watch out for when rebuilding the carb?
Thanks
Curly

[4bz]

What you describe happened to me once; my engine started and ran just fine, until it was put under a significant load. It turned out to be one of the cam lobes that had worn nearly flat. I recommend that you connect a vacuum gauge so that you can see it while you are driving, and watch the gauge when the problem starts. A normal valve train will show low vacuum with a steady needle under partial or moderate load. If the needle jumps erratically, you may have a valve train issue. When it happened to me, it was an exhaust valve lobe on the cam that failed. Another telling sign was the amount of metal that was present in the oil. If you drain off some (or all) of the engine oil and shine a flashlight into it, you will be able to see if your oil is loaded with metal particles.

If your Holley carb was assembled with the old style paper gaskets, sometimes the metering block will be stuck on the carburetor body. A large C-clamp can be screwed down on the sides of the metering block for extra leverage to remove it without damaging the metering block or the carburetor body.

[Tatsadasayago]

Despite Holley's claim that the power valves are backfire resistant, I would submit that yours is either weak or perforated.
Your altitude and idle vacuum level will dictate which PV you should be using.
And no, your engine will run totally different when going from ported to manifold vacuum. If there is no noticeable change, something else is forked up.

[will e]

The engine will idle different going from ported to manifold but at any other time the vacuum is the same.

[440sixpack]

A blown power valve will result in a rich condition not lean . if it used to run good with this carb I'd look elsewhere first.

Most of the time a non smogger engine will run best on manifold vacuum. what is your mechanical all in at 2500 RPM ? and then hook up the vac advance and what is it? this is the ONLY way to check ignition timing. if you don't have a dial back you don't have a timing light.


A vacuum leak would cause the symptoms you're talking about.

[Curly]

Update: No Change!
Today I took the carb off the engine, took it apart and gave it a good cleaning. Aerosol carb cleaner and compressed air. The carb was/is very clean inside and I doubt that there were any internal problems, at least I couldn't see any. I checked the power valve for proper operation, it holds vacuum and the plunger moves when vacuum is applied. The passages are clear. Power valve is a 6.5 and is the same one that was in it last year when I had no problems.

I increased the primary jet size to #67 (from #65). Put it back together.

I also sprayed down the intake and carb mounting surfaces with brake clean, looking for vacuum leaks, none found. I also double checked the intake bolt torque, also correct.

440: I don't have a dial back light, I guess that is the next step. I think I know a shop with one.
4bz: I really don't think it is a bad cam lobe, I know how those sound. The engine is smooth right up until the time it begins to surge/backfire.

If I can't find a dial back timing light I'll try retarding the timing 5 degrees. (its at 10* B right now)

[will e]

How did you check the fuel pump and when you say 'in specs' what value did you have?

Also, are you running a return line from the filter?

[Curly]

Update #2: Fixed! (I hope!!))
I decided to go back to ignition and see if the problem was there. I tried running without vacuum advance at all to help narrow down whether it was timing. No change. I then swapped out the Echlin module with a spare duralast I had and the problem seems to be gone. Wound the engine up to 65 in 2nd gear, did some light throttle accels up some hills, etc. No backfires at all. Engine pulls well under hard acceleration, so I think I got it whipped. I believe my problem was too fold: 1) Carb was too lean - my fault- trying to improve mpg. 2) Module was defective, breaking down at higher rpm.

well e : I used a vacuum/fuel gauge on the pump outlet. 4.5 psi and 1 pint of fuel in 20 seconds. Yes on the return line from the filter.

Thanks to all who responded.
Curly

[Tatsadasayago]

Good to hear you found and fixed the problem.

[will e]

Yeah that's good pressure. You might check to see what kind of pressure you get AFTER the filter. I hooked up a cheap mechanical gauge and later went to a sensor and in cab gauge. I love having a fuel pressure gauge in the cab.

Sounds like you found the current issue. Good for you! That's always exciting when you are trying to chase down this kind of issue.

[az chip]

Update #2: Fixed! (I hope!!))
I decided to go back to ignition and see if the problem was there. I tried running without vacuum advance at all to help narrow down whether it was timing. No change. I then swapped out the Echlin module with a spare duralast I had and the problem seems to be gone. Wound the engine up to 65 in 2nd gear, did some light throttle accels up some hills, etc. No backfires at all. Engine pulls well under hard acceleration, so I think I got it whipped. I believe my problem was too fold: 1) Carb was too lean - my fault- trying to improve mpg. 2) Module was defective, breaking down at higher rpm.

well e : I used a vacuum/fuel gauge on the pump outlet. 4.5 psi and 1 pint of fuel in 20 seconds. Yes on the return line from the filter.

Thanks to all who responded.
Curly

Tim, MSD.

[Curly]

[/quote]
Tim, MSD.[/quote]

Chip, I have MSD already........Misfires, Severe, Daily!!