Engine shuts down at 4,000 RPM

[Blain]

Can't find an answer on the other forum. See what you guys can do....

I have a 78 with mild cam, Edlebrock spread bore, Hookers, and no somg crap.

It pulls hard up to about 4300 RPM then it is just like shutting the key off and on. Not just a miss but completely cutting in and out until I let off then runs good again.

Did a search and found the flattening of the fuel line so I installed a fuel pressure gage and have average of 5 lbs even when cutting out.
Have a Hypertech HEI with new silicone wires. Replaced the rotor and condensor but have not changed cap, coil or module.

Check voltage at the coil and have about 12.25 v with key on. Have not monitored voltage on the road. I read a thread about an HEI distributror in a points car having a resistance wire. this car came with HEI so I doubt thats it.

Next post;

Still giving me fits.
Replaced ignition module
Checked carb fuel filter
Pulled carb and checked float level
Checked centrifical and vacuum advance

Being totally frustrated I removed the air filter, left the hood open and put it to the wood. At around 4,000 RPM the secondaries started to open and when they did the engine cut out as before but what I saw was fuel mist being blown back out of the secondaries. Not sure which came first the miss or the mist but seem to be related.

Ideas?

Next post;

Good thought but I changed the whole distributor after the module. Went for the easy swap first.


Did a search for valve float and couldn't find much on he subject.
Could valve float cause this blow back? No valve noise, used new springs for the rebuild about 1,00 miles ago. Can this engine have a saturated push tube?

If it could be valve float would just one cause this?

Running out of ideas.


Next post;

Still don't have anyone explaining what the blowback is. Maybe a better explaination;

I stab it, rpm comes up to about 4,000 the secondaries open and what looks like a mist of fuel blows up and out of the secondaries. When this happens the rpms rapidly drop off until everything stabilizes and it happens again, all within a second or so.

Why would anything be blowing OUT of the carb?



OK, Gurus. I thought I could work it out but this one is baffleing.

Blain

 

[big2bird]

I expect Lars to have a better response than I, but I will start with 3 questions.
What is your total mech advance at what RPM? Is your vacuum advance correct for your idle vacuum? Is your distributor gear indexed properly to your rotor? (Does the dot align with the rotor tip?)

 

[Blain]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

 

[mrvette]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

I would not be too concerned long as timing is correct and the advance works decently ie, no more than 36-8 at 3000 crank rpm....

I think you have that top flapper spring on the Qjet too loose...so dumping it open too easy.....OR maybe you too lean so when it does open you not getting maybe the rods lifted or the mix from the secondaries is WAY too lean....due to incorrect size....

:thumbs:

 

[big2bird]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

The dizzy gear has a dimple on it. It MUST be on the same side as the rotor tip, or the rotor will not align with the cap buttons at the proper time, resulting in crossfire in the cap.

I would go to the tech section, download Lars timing papers and q-jt papers, and follow them to the letter.

Set the mech timing to 36* mech advance at 3,000RPM,(vacuum advance plugged), and adjust your q-jet secondary wind up to 3/4.

Read the papers, and you'll understand, or ask more questions.:beer:

 

[mrvette]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

The dizzy gear has a dimple on it. It MUST be on the same side as the rotor tip, or the rotor will not align with the cap buttons at the proper time, resulting in crossfire in the cap.

I would go to the tech section, download Lars timing papers and q-jt papers, and follow them to the letter.

Set the mech timing to 36* mech advance at 3,000RPM,(vacuum advance plugged), and adjust your q-jet secondary wind up to 3/4.

Read the papers, and you'll understand, or ask more questions.:beer:

Bird, I gotta think about that one a second here, if all we changing is the gear position, we can twist the housing around to get any reasonable timing we want via the crank.....ok...

so on top of the dizzy shaft is either the HEI reluctor or the points block/cam, that never changes with respect to the top bar holding the rotor cap...so how could that firing ever change?? been so long since a points dizzy, I forget that setup.....but with the HEI, that reluctor wheel is fixed on the shaft...no?

:stirpot::sos:

 

[big2bird]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

The dizzy gear has a dimple on it. It MUST be on the same side as the rotor tip, or the rotor will not align with the cap buttons at the proper time, resulting in crossfire in the cap.

I would go to the tech section, download Lars timing papers and q-jt papers, and follow them to the letter.

Set the mech timing to 36* mech advance at 3,000RPM,(vacuum advance plugged), and adjust your q-jet secondary wind up to 3/4.

Read the papers, and you'll understand, or ask more questions.:beer:

Bird, I gotta think about that one a second here, if all we changing is the gear position, we can twist the housing around to get any reasonable timing we want via the crank.....ok...

so on top of the dizzy shaft is either the HEI reluctor or the points block/cam, that never changes with respect to the top bar holding the rotor cap...so how could that firing ever change?? been so long since a points dizzy, I forget that setup.....but with the HEI, that reluctor wheel is fixed on the shaft...no?

:stirpot::sos:

It's about the rotor alignment with the cap buttons. From retarded to advanced, you vary form rotor tip leading edge to trailing edge. If 180* out, the rotor tip will fire when the tip and cap is too far away, resulting in cap crossfire.

 

[mrvette]

Don't have an exact answer but about 10-12 degrees at around 1500 mech, vacuum is another 15 or so.

I don't understand the indexing.

The dizzy gear has a dimple on it. It MUST be on the same side as the rotor tip, or the rotor will not align with the cap buttons at the proper time, resulting in crossfire in the cap.

I would go to the tech section, download Lars timing papers and q-jt papers, and follow them to the letter.

Set the mech timing to 36* mech advance at 3,000RPM,(vacuum advance plugged), and adjust your q-jet secondary wind up to 3/4.

Read the papers, and you'll understand, or ask more questions.:beer:

Bird, I gotta think about that one a second here, if all we changing is the gear position, we can twist the housing around to get any reasonable timing we want via the crank.....ok...

so on top of the dizzy shaft is either the HEI reluctor or the points block/cam, that never changes with respect to the top bar holding the rotor cap...so how could that firing ever change?? been so long since a points dizzy, I forget that setup.....but with the HEI, that reluctor wheel is fixed on the shaft...no?

:stirpot::sos:

It's about the rotor alignment with the cap buttons. From retarded to advanced, you vary form rotor tip leading edge to trailing edge. If 180* out, the rotor tip will fire when the tip and cap is too far away, resulting in cap crossfire.

Since the relucter is fixed in relation to the rotor that makes NO difference...I can't see HOW....now turning the dizzy housing obviously controls advance....when the thing lines up, that's it....NO?? what did I miss?? i'ts the sender(reluctor, points) in relationship to the rotor button output tip, nothing to do with the gear on bottom.....can't see it....

twisting the housing tells the advance, that is the sending coil, with the pointed tips....

 

[Blain]

So, timing issues aside, I clamped the secondaries closed and stomped on it with the same results.

Guess I'm out of my generation or something but never heard the term "Dizzy" but I assume we are talking about the distributor. No, I'm not that dense.

I thought we have excluded the timing but maybe I'm still missing something.

Blain

 

[big2bird]

So, timing issues aside, I clamped the secondaries closed and stomped on it with the same results.

Guess I'm out of my generation or something but never heard the term "Dizzy" but I assume we are talking about the distributor. No, I'm not that dense.

I thought we have excluded the timing but maybe I'm still missing something.

Blain

Read Lars timing paper. Base timing is irrelevent.

 

[bobs77vet]

how about valve float fro mthe valves being adjusted too tight?

 

[lars]

Valve float will cause a distinctive "breakup" sound in the exhaust and should be pretty obvious and detectable.

The fuel mist you're seeing shooting out of the secondaries above the airvalve is normal and correct - that's the Q-Jet's way of producing a secondary "pump shot." It looks even more spectacular in high speed video - the fuel plume initially shoots out of the top of the carb like a small nuclear mushroom cloud before being sucked back down. That's a good sign that you're seeing that.

Before tackling the carb, make sure that timing really is right: You need 36 degrees total timing coming in no later than about 3000 rpm. Make sure the timing stays stable at 36 degrees above 3000. Distributor clocking and vacuum advance is irrelevant. But rev it up through 4000 very briefly and make sure you are seeing stable spark and timing with your timing light.

If all is well, check a few things on the carb. You cannot clamp the airvalve closed on the secondary side as you described - the throttle plates are still opening up below the airvalve, and this will cause a massive flood-out condition. Rather, you have to lock out the secondary throttle plate using the lockout lever on the passenger side to see if the secondaries are an issue. Set your secondary airvalve windup spring to 3/4 to 7/8 turn and make sure the secondary rods are as specified for the carb number. You might also want to verify float level - it is often set too low for a performance application. For a divorced-choke carb (EDL-1901 or equiv), run the float at .375". For integral choke carbs, run the float at about .400". While you're checking float level, you might also want to verify that the power piston is not stuck in the full lean position and that it's popping up into the power enrichment mode as it should.

There are a slew of weird problems that can occur on these older carbs (Edelbrock hasn't built Q-Jets for at least 5 years now, so chances are good your carb has been screwed with a bit). Examples that I have run into related to your problem are:

• Broken secondary rod hanger cam
• Damaged or modified secondary rods
• Missing secondary air baffle
• Altered secondary linkage geometry
• Warped airhorn not sealing the secondary fuel passages at the airhorn interface
• Incorrect needle/seat installed
• Incorrect float
• Choke not opening fully
• Defective choke pulloff

...just to mention a few. Visual inspection of the carb is the only way to identify these problems - troubleshooting cannot be done via e-mail or written correspondance.

Lars

 

[Arthur Brown]

That's a detailed and educating response. I'm learning a lot here. Thank you for taking the time to post all this information, it is highly appreciated.

 

[lars]

Of course, the easy way to see if you have a carb problem at all is to just swap out the carb for another one and see if the problem remains the same... it'll take you about 15 minutes to do that little excercise.

 

[Blain]

Allright guys-time for an update. I had to step back for a couple of weeks before I doused it with gas and... well, you know.

Good carb info. But I bought it new so it wasn't violated. Swapped with a proven Holley and did exactly the same thing.

New cap, rotor, condensor and coil. Ran an electric fuel pump from a jerry can direct to the carb. Jumper wire from the battery to the B+ on the HEI. Ditto with the ground wire.

Scoped the ignition and all are firing.

Every friggin' time it shuts down at the same RPM.

This is frustrating on so many levels. I'm embarrased to even admit my occupation. Suffice it to say that I should be able to diagnose this.

Whatever this turns out to be it will defy all odds. 2 bum carbs or 2 defective distributors or etc, etc.

Blain

 

[Cherry]

Allright guys-time for an update. I had to step back for a couple of weeks before I doused it with gas and... well, you know.

Good carb info. But I bought it new so it wasn't violated. Swapped with a proven Holley and did exactly the same thing.

New cap, rotor, condensor and coil. Ran an electric fuel pump from a jerry can direct to the carb. Jumper wire from the battery to the B+ on the HEI. Ditto with the ground wire.

Scoped the ignition and all are firing.

Every friggin' time it shuts down at the same RPM.

This is frustrating on so many levels. I'm embarrased to even admit my occupation. Suffice it to say that I should be able to diagnose this.

Whatever this turns out to be it will defy all odds. 2 bum carbs or 2 defective distributors or etc, etc.

Blain

There a flat black module mounted in the dizzy. Take it out and see if the heat sink grease has dried and hardened. If the grease has hardened "replace the module",, The leads coming to that module " some times the wire will brake right at the terminal lug connector. check those wires with a multimeter for good conduction,, the pickup black module may very well be braking down in that RPM range. Other than that,, put a voltmeter on the B+ lead and watch it while throttling up. Other than those Items, buy one that runs LOL????

 

[mrvette]

I"d pull that bitch dizzy outta there and change out that sending coil below the rotor plate, under a clamp with 3 screws to hold it ....yes on the sending unit chip, and solder the crimped on wires in the primary of the spark coil....they are crimped over the formvar, and I have caught one dead OPEN once, the coil looked normal....so I soldered the ends, and let the lead/flux burn off that damn formvar.....the stupid coil ran untill I sold the car years later....no reason it could not be intermittent, but first time for everything....UH, when the engine quits, what does the tach do?? stick at ~4300 rpm and stay there while engine struggles??? or tends to drop off when it happens??


:smash:

 

[Blain]

New module coming tomorrow. I read somewhere that a battery charger on "crank" mode would spike the module. And I have done that several times. But that would also mean that the distributor I swapped with would have had the same problem since there was no change between the two.

I did test with vacuum removed from the advance to eliminate the movement/broken wire theory.

 

[Blain]

Forgot to answer MRVETTE;
This happens in a fraction of a second from off to back on. The dampening of the tach barely shows it drop but it does some. It happens so fast that the engine may only lose a couple of hundred before it comes back.

I like the idea of "buy one that runs" Problem is this motor/trans goes in my '74 frame off. The '78 is just a rolling engine stand.

 

[Blain]

Another thought- I connected a timing light and went for a burn. Strobe fired even when engine was cutting out. Would that eliminate the HEI or could it be triggering some weird form of voltage spike that the timing light would recognize but not jump the plug gap? Or way off timing?

Am I reaching too far?