Thread: Car Tune

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Hiya,

I got a timing problem that's just racking my brains out.

I got a 350 small block that's spitting through the carb especially when cold.

The crank timing is 10 BTDC
The cam is a Lunati bare bones , the middle one of a selection of 3 offers hafta find specs but it works between 1500 to 5500 RPM. Sprocket set to stock.
The timing chain is a fresh true double roller.
The distributor is a street fire with moroso centrifugal weights Springs start at 6 degrees at 1500 RPM and max out to 23 degrees at 3000 RPM.
I found by accident that I have an adjustable vacuum advance set loose as possible so it don't max out so quick.
The spark comes from a street fire external box similar to a MSD but cheaper. Multi spark.
The wires are Taylor 8mm silicone and loomed. There is no arcing between wires or anything I checked at night. Besides it fires on all 8.
Carb is a tuned edelbrock 1406. Perfect mixture all the way.
Fuel pump is a regulated Big End mechanical. Pressure set a 6.5 PSI to help feed the nitrous solenoid.
Air cleaner is a K&N with the element top.
Heads are World sportsters with 202,150 valves and 71cc chambers i think.
Pistons are 030 over TRW flat tops. Compression is between 9-10 to 1.
Flow tech headers with 2-12 pipes with flow master knockoff mufflers and crossover pipe.
I had the car om a dyno and it has 236 HP at the wheels at 10 BTDC I gain 1 horsepower at 11 BTDC. Only change was setting the accelerator pump at max squirt.

The spark spits through the carb at 34 throttle and stalls from idle to WOT every time. Idle set at 650 RPM. Cold weather aggravates the problem.

Occasional backfire when drag racing. In fact in 2 years it only did it twice.

Yes, I checked the cap, rotor, weights, wires, plugs, timing chain slop vacuum advance, hoses,
Other than slight rust spot on centrifugal weight post I didn't find anything wrong.

Anyway, i'm out of ideas, does anyone have a clue?

when i had both my 351 stang and my 351 pinto .. the stang was/is a 5 speed and the pinto was an automatic .. a brand new carb on the pinto would spit back every time i stabbed the throttle ..the mustang dint have that problem with an old carb .. as you i tried every tune imanigable to make the pop go away .. then i simply swapped carbs with each other and no more problem .. the mustang with the clutch i could rev past the pop and the pinto never popped again .. if you have access to another carb give it a try ...

I tried a 600 CFM Holley and had to change the accelerator pump size and the primary squirter to make it work. I changed out due to Holley having a few leak problems. The Carter AFB look-alike is a safer alternative. No, it didn't pop with the Holley on but I think it's unrelated. I made lots of tunes and upgrades since then. It's definitely firing through the intake.
PS, I never seen a Pinto do that!

Yep, accelerator pump sounds likely

Originally Posted by Good Wrench

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PS, I never seen a Pinto do that!

you never seen a pinto do this ? .. HA ,, keep playing with the pump stroke .. that`s all it can be ...

Another thing you may want to check--verify top dead center. Balancers have been known to slip and give incorrect readings.

Hiya,

Balancer is spanking new and verified.... Unless it slipped during drag racing. But it is a performance part. I'll check it again. Let me ask the question this way..
What is the total timing curve allowed for a small block? crank, weights, vacuum advance?
Also, how much timing advance does the vacuum advance allow?
I'd just set crank timing back to 8 BTDC if it wasn't such a batch to start in cold weather, And max horsepower is at 11 BTDC, I should be able to set it close.

Generally, 36 degrees total should put you in the ballpark. Between 2500-3000 rpm, you should have reached total advance.

And at W.O.T. the vacuum advance does nothing.

OK 10 at the crank and 23 at the distributor is 33. I still got 3 to spare. There's just got to be something wrong. I just can't find it. Rats!

If it isn't the distributor, it is probably the carburetor. Do you have or can you borrow another carburetor?

Seems to me you already identified the problem being the carb in Post #3, and both Hoss and Firebird pointed you back to it in #4 and #5. It didn't pop with the Holley, you're in good tune across the rpm range which says the jets are right in the Edelbrock, but it pops in the transition. Fix your accelerator pump. You're going lean with the inrush of air at WOT, and you're getting a lean pop on one or two cylinders as the intake is closing.

Hiya,

On the dyno the operator suggested more crank timing and setting the accelerator pump on the furthest adjustment hole possible which I did. On paper it looked fine unless it's just deteriorated. I remember with the Holley that I had to get 2 sizes up on the primary squirter and an aggressive pump cam. I was just shy of getting a Reo pump for it. For the Elelbrock I don't think one can do much with the pump short of what i did with the hole adjustment. Other than rebuild the thing.
I still have the Holley but the last time I used it it worked perfectly without leaking but lacked any kind of power, A pinto was faster. I just got sick of constantly jacking with it.
I must add that this popping is only under load. You can rap the engine all day and it won't pop at all. Light throttle is fine. Midrange power is superb. WOT stalls, No pop but stalls. 34 throttle under load will pop especially when cold.
Fuel is regulated at 6.5 PSI. I was told the Elelbrock is fussy about fuel pressure but I cranked it up to feel the nitrous gas solenoid cause i think it does run lean when applied. I was told 5.5 is max pressure for the Elelbrock 1401.
The plugs are reading a tan color on the electrode porcelain. They're Bosch plugs rated cold for the nitrous.
The manifold is a Weiand dual plane with the heat transfer passage blocked off at the gaskets.
Only other thing I can say is the engine is capable of 6500 RPM sustainable. Which only adds to my confusion.
Incidentally, 236 HP on the dyno was disappointing to hear. Especially after all the work I did on it. But the power curves and mixture checked out fine.
Well, i'm gonna be working on it this next few days so i can appreciate and new ideas. I'm checking everything again.

Pull the accelerator pump and look at it. Today's fuels eat parts quickly, especially if left idle for any period of time. Increasing fuel pressure is a bad idea, as all that can do is force the needle off the seat and flood your carb. The job of the fuel pump is simply to keep the bowls full under all load conditions. If you're starving out at WOT that points to one of two things, a weak fuel pump or undersized fuel line. Increasing pressure at the carb is not going to help either condition.

The Edelbrock web site has a chart and instructions. The calibration kit they sell for the Edelbrocks, along with the chart and instructions, should help you to work out the lean spot in the carb.