Thread: Starvation backfire??

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Hello everyone... I'm relatively new to the forum. I'm looking for a 2nd, 3rd, 4th opinion of my diagnoisis.

Just a brief rundown of my engine...
SBC 350ci 2 bolt, bored .060.
mild street cam >.500 lift at the valves.
Edlebrock Preformer heads 70cc
Edlebrock Preformer EPS intake
Edlebrock/Carter AVS 650cfm carb
Jacobs ignition w/ crane distributor
Mallory comp140 fuel pump
Mallory return style fuel reg.

My issue (we don't have problems, we have issues ) the engine stumbles and backfires at WOT above 3000rpm. Huge power loss obviously. After tinkering with things and changing jets and metering rods and ruling out as many other possible "issues" as I can think of, I have come to the conclusion that I am simply running out of fuel in the bowls. Currently my fuel pressure is at 6.5 psi (but after reading a few other threads I'm not sure I trust my liquid filled fuel psi guage).

The remedies I have come up with are...
A) increase fuel pressure
B) install a larger needle and seat assembly in the carb

Any input from my fellow motor heads would be greatly appreciated.

P.S. - other than the problems at WOT above 3000rpm, the engine runs good and snappy.

Could be a valve adjustment issure or other valve train issue. Is is backfireing out the carb or exhaust?

welcome to the forum. i dont know what your problem is, but ill bet you what it ant. i dont belive your running that carb dry. if you are you got a float or fuel reg. issue . how about trash in carb, water in gas, float set to low, valves floating, timing, the fuel pressure dont drop off at all, that dont sound right. sure most of the fuel is not returning back to the tank?

I'd be thinking springs first, then the other things Mike said. Did you use the old springs when you changed heads?

you haven't done something silly ,,,,,like set your rev limiter to low have you???
If not i'd be looking at the valve train.

Thanks for the responces. To answer a few questions...

Heads have new valves, springs etc. assembled at Edlebrock. I was pretty meticulous about setting the lash being sure not to crush the lifters. I will keep that in the back of my mind though.

The crane distributor does not have springs... it has 9 pre-set curves that are selectable with a turn of a switch. I have been playing with that trying to find the "right" curve for my engine, but that doesn't seem to affect the backfire issue.

I just had the Jacobs ignition box tested at the factory and I havn't changed the rev limiter. I should be factory set @ 10,000 rpm. A rev limiter seems a little pointless for a street vehicle with an automatic trans.

I agree that I do need larger squirters for the accelerator pump... I do get a small single pop right when I mash the gas, but that is a different issue. This backfiring occurs well beyond the use of the accelerator pump after I've had the pedal mashed to the floor for a few seconds and the engine has already been accelerating. By this time the engine has to have used up the pump shot.

The backfire comes ou the exhaust. When I lift the throtlle a bit and keep it in 2nd gear, the engine will still continue to accelerate so I'm pretty sure it is not a rev limit issue.

I know the seconardies are set properly. They look almost like a mirror image of the primaries at WOT and are set at what I'd call 95%- 97% open... not quite vertical. They never go vertical and start to close again.

This carb also has the Air Valve Secondary on the top of the carb, which only opens when there is enough vacuum present to open it. I really like this feature because it combines the elements of a vacuum secondary with a mechanical secondary. The AVS is spring-loaded and adjustable. I have it set a little tighter than factory so it opens later when the engine demands the added air which is also recomended for a heavier vehicle (5,000lbs). I can actually hear it open up through the carb and through the exhaust.

This is why I think I'm running out of gas in the bowl because I hear the air being pulled through the seconaries, everything is lighting-up and starting to wind-up and about 2 seconds later.... da dut da dut popita popita pop pop ( <-- me making the engine noise ) It feels like when I lift just enough to let that Air Valve close there is enough gas to feed the primaries again and the engine keeps accelerating, but at a much slower pace of course.

Anyway, just a little more thought about what I am experiencing.

you might be running out of fuel, but its not because the carb. bowl ant holding enough fuel, its gonna be because, the floats are to low, the pressure reg. hooked up wrong, fuel pump problem or fuel line sucking togother, but we cant see all of that. you seem to know a lot about those carb. yourself, its only so much we can tell you without looking at it. im not so sure your not floating the valves. can you run it just on the primaries, cut the secondaries off. and see what happens.

Interesting thoughts... I'm pretty sure I could disconnect the secondary linkage for a test run. Also, the last time I had the airhorn assembly removed, I thought the fuel level looked a little bit low in the bowls... a little more than half full. I guess I figured that the floats would displace a bit of fuel when assembled. I set the floats according to Edlebrock's recomendations, but I'll try raising them up 1/8" or so after I do the deactivated secondary test.

I had the other Edlebrock/Carter style carb before, the AFB 600cfm, and tinkered with it alot. But I never had to mess with the floats on that one

I'm sure there is no problem with the fuel pump. I've sat there and pumped all the gas out of the tank before I dropped the tank and fuel shot out pretty fast. The regulator is a no brainer to hook up. There is a choice of four ports around the side for input and output and the return comes out the bottom. It may be possible that my pressure guage is inaccurate and I'm starving the carb at the regulator because of an improper setting.

Thanks for your help. I'll try disconecting the secondaries. That should give a good benchmark to rule out other possibilities.

Hey lt1s10,
I tried your idea of cutting the secondaries today and running on just the primaries and guess what... engine ran smooth as silk. I guess that eliminates any question about valves flosting, timing issues, etc. This tells me, and please let me know if you agree, that I am sucking air through the secondary jets. Let me know what you guys think?

Thanks

Originally posted by Hawmps
Hey lt1s10,
I tried your idea of cutting the secondaries today and running on just the primaries and guess what... engine ran smooth as silk. I guess that eliminates any question about valves flosting, timing issues, etc. This tells me, and please let me know if you agree, that I am sucking air through the secondary jets. Let me know what you guys think?

Thanks

that tells you the problem is in the carb. it sounds like it getting all air and no gas, but it could be to much gas also. i think the carb. secd. is opening to soon and fluiding the motor. you think it lean, from here i cant tell. is the rear floats metal or cork?ive seen the cork floats get heavy with fuel and when you get into it they would sink, and when you get out of it they would come back up. maybe your'er way off on your rear jets and metering rods. i dont know what you got. except its in the carb. secondaries. if you clamp off your return hose from the reg. how much pressure do you have? if its not to much try that.

You did all that work to the engine and...

Did you do any work to the fuel system to ensure adequate fuel flow?

You have to remember the sock filter in the tank can become dirty over time and inhibit fuel delivery...you might be getting pressure but not enough volume at higher rpms.... You might also have a crack in one of the rubber lines and be drawing in air?

You are running a lean condition and need to remedy it or you'll be hurting the engine....

I would suggest blowing out the fuel line with commpressed air and dropping the tank and replacing the sock filter....Doc

This carb has 2 floats, left and right, and they are brass. The secondaries operate on jets only... the metering rods affect only the primary curcuit. I also have an Air/Fuel ratio monitor to help with tuning; seems to run better a little on the rich side. Doing this little test with seconbdaries disconnected really helped me dial in the primaries independantly. Brilliant Idea.

As far as the fuel system... it is all new from the tank forward including a new sock filter in the tank (it is the stock tank but I re-plumbed the inlet, return and vent), Mallory Comp140 pump, and Mallory return style regulator. All the lines are 3/8" including the return and are new except for a length of 3/8" hardline running the length of the chassis from the tank to the pump. I didn't think that this engine would warant 1/2" lines. In setting up the fuel system, I just plugged the line to the carb and let the fuel circulate through the return and adjusted the pressure to see what kind of range there was. I ended up being able to adjust the reg. from 3lbs. up to over 12lbs of pressure.

I think my next step will be to adjust the floats and drill out the seats a little. Edlebrock's "high flow" needle and seat kit is basicly the same needles in a .110 seat. A 7/64 drill bit is .1094... should have the same affect without buying new parts. Probably a little more fuel pressure too... but I will make one adjustment at a time of course.

if it was the lines,sock,whatever it would do it on the primaries also. you got to do something so go for it. its been a long time sense ive been into one of those carb, so what does it have for a power valve. i thought all AFB/ Edlebrock carb. had metering rods in the rear. i need to look at one.got a pic. of the internal parts?

It doesn't have a power valve, it just draws fuel on demand from the secondary boosters through the secondary jets.

Today I drilled the seats to .1094 (7/64"). They were just a hair smaller than 3/32. This almost solved the problem... just a couple little pops near the top end. Then I kicked the fuel pressure up to 7psi, from 6. And all poping went away. It might like another 1/2 lb, but I'm going to get a new guage... I just don't trust this one

Still needs some fine tuning though. Now my idle circuit is a little too rich, so I will have to start over with that. I will probably end up putting a larger jet in the seconaries, so I will take some pictures for you when I remove the air horn to do that.

Thanks for your help