I have a 425 super rocket motor with the stock rochester quadrajet carb and the stock distributor connected to a Accel super coil. Most of the time when I accelerate (WOT) it goes from first to second to third nicely, but sometimes from first to second or second to third I feel a sudden power loss between 4500 and 5000 rpm. It's like if I'd quickly release the gas.

I know that my base gasket is leaking a bit.

What can be the problem?

(The mechanical advance in) the distributor?
Dirty fuel filter/lines?
Wrong air/fuel ratio?
Vacuum leak carb?
Spark, wires, etc?

Regards,

Robert

 

Change the base gasket and try again. Is it misfiring?

 

I'll have a look at it. Thanks. No misfiring.

 

Fuel Filter or Fuel Pump would be my guess.

 

Few questions to help diagnose the problem:
Did this set-up run ok recently?
Was anything changed or done to the car before the problem occurred?
Does it shut-off or keep running during the power loss?
Is the engine reaching full temperature when the problem occurs?
Was it sitting for an extended period?
Was the car originally a four barrel?

 

What is the fuel pressure when this happens?may have a fuel line leaking in the rear sucking in air.

 

Did this set-up run ok recently? I took me a while to get comfortable with the power of the car. I guess at first it wasn't there, but when I started to do more WOT sprints the problem became more noticeable.
Was anything changed or done to the car before the problem occurred? Changed the air filter, retuned the carb, changed a belt, that's it.
Does it shut-off or keep running during the power loss? It does not shut off. I let go off the throttle when the problem occurs and then quickly press the throttle again.
Is the engine reaching full temperature when the problem occurs? Normal operating temperature.
Was it sitting for an extended period? I only drive it in the weekends.
Was the car originally a four barrel? Not original engine in the car, but engine was/is a four barrel.

I have a video where I rev the engine. When I reach my maximum rpm and let go off the throttle and let it idle agin, you can see, hear and feel that it's revving a bit below idle. With a little blip on the throttle, it idles back normally.

You can notice it at 0:20 seconds in the video.


Let me hear what you guys think.

Regards,

Robert

 

Nice car Robert. My initial thoughts yesterday when I first read your thread is you are running out of fuel in the carb. That means a fuel pump, to me.

After viewing your video today, I still feel the same. You might have a slight carb malfunction, though its hard to tell. It sounds pretty good to me and not too far off from being right.

 

When you say stock distributor, do you mean points? Is your dwell set to 30? What is your timing set to? Is your distributor mechanical advance working?

 

I mean factory original distributor, but I was mistaken. I have a distributor from Mallory.

But, I was driving today and the problem occurred at 3000 RPM. The throttle was half open and I was accelerating. But it wasn't linear, it was like bogging but not stalling. If "+" is fuel and "-" is no fuel, it was like: ++++++--++++++---++++--++++ instead of ++++++++++++++++++++++++++. If that makes sense.

Can it be the filter or fuel pump? I wil upload pictures later.

Regards,

Robert

 

It's starting to sound electrical to me.

Is that Mallory distributor HEI? Could be a bad module.

- Eric

 

Is the distributor points or electronic? Most vacuum gauges can be used to monitor fuel pressure. You can T and clamp it into the fuel line an tape it to the windshield and monitor the pressure when it happens. You can also pull your spark plugs and see what they look like. Check your mechanical advance on the distributor and make sure its working properly. Look down into the carb and see if its dribbling fuel while you operate the throttle by hand.

 

Yes, your problem could very well be fuel pump or fuel filter related. Could also be in tank sock if it is still in place. Possibly electrical but I don't think this one is.

 

Went back and reread the full thread. Your running out of gas in the carb. You need a better fuel pump plain and simple.

 

Although there could be dozens of causes for this.....my money is on a fuel filter, too! AND....easy to replace first, before getting into anything else.....it probably needs it anyway!!

 

Here is a picture of my fuel filter

 

First off get rid of all that rubber hose and get the fuel line away from being in contact with the heater hose. Warm/Hot fuel is a bad thing.
That's a weird looking filter. I'd replace it just because. Get one you can put fittings in the ends of and then run a hard line from pump to filter to carb. It takes a little time to do but it'll be much safer and could cure your issue.

 

What about your other fuel filter?

The one in the carburetor.

If it's still in there, it could be completely gummed up.

Also, though many have gotten away with rubber hose on the pressure side of the pump, it is considered to be at least a bit unwise, since if the rubber hose is damaged, gasoline will spray all over your engine while you drive, with predictable results, a situation which has befallen members here.

- Eric

 

On a tangent, that carb might work better with some of the choke pieces in place and functioning. For instance, the choke pulloff canister which also serves to limit the speed at which the secodary air valve can open. With this part malfunctioning- and missing is definitely a malfunction- it can cause bog as the secondaries open too quickly.

 

[QUOTE=MDchanic;676255]What about your other fuel filter?

The one in the carburetor.

If it's still in there, it could be completely gummed up.




x2, also Im wondering about debris in one of the jets or in the bowl...

 

Good pickup, Octania.

- Eric

 

I have taken a look at the carburetor fuel inlet housing, but it turned out that there was no filter. Is that usual or does that mean that the previous owner removed it with a reason? Should I put a new filter in the housing?

Can this be the problem of my engine surging under load?

 

The previous owner did the logical thing and removed the small filter from the carburetor when he put a big filter before the carburetor.

There is no need for a filter there, as you have a larger filter before it.

You have now eliminated one possible cause for your problem.

Might as well check fuel pump output volume and pressure at the end of the hose where it attaches to the carburetor, now that you have it apart, to eliminate another.

- Eric

 

I went to a mechanic today and he recommend that I should replace my rubber fuel line with an aluminum fuel line with a filter in the carburetor. I tried to place a filter inside my inlet housing but it won't go in. Something is blocking. It looks like some kind of metal pin. I have highlighted it in the picture.

My questions are:

Why is it there?
What does it do?
Can I remove it?

Regards,

Robert

 

Are you putting the filter in backwards?

- Eric

 

No, not at all. I tried both ways to see if it would make a difference but it did not.

 

Is it a Chebby filter?

I ask because you have a Chebby carburetor, and I'm not certain whether the filters for the two are completely identical.

- Eric

 

I don't know if it's a Chebby filter. What is a Chebby? Chevrolet? How can you tell if I have a Chebby carburetor?

 

Your carb has a side fuel entry=chevy. Olds fuel inlets go straight towards the front.

I've never seen before that thing you have pictured. My only guess its the inlet valve of an old filter. Have you tried to remove it?

 

The number on the carb is 7026250. That tells me it's an Olds carb. Perhaps I am mistaken.
I have not tried to remove it yet, but I will give it a try. Thanks.

 

No, I think you are correct. That number comes back as a 1966 425 carb. I guess they all didn't have front fuel inlets.

 

That shows to be a 66-67 400/425 carb, I believe that stem in there belongs there don't remove it. This should be your filter:


http://www.oreillyauto.com/site/c/de...2515&ppt=C0359

 

Good catch Eric. Its the sintered bronze filter, (however that's spelled), NOT the paper filter used in most other carbs.

 

I didn't know they still made those filters.

As for the carb, yes, you have one of the original run of QuadraJets, which must be original to that 425, which is unusual.
After the first year in '66 (maybe some '67s), Olds, Pont., & Buick used a straight inlet, while Chebby kept the side inlet.
Olds have two different filter and sealing configurations, depending on year, and I'm not sure if these early Chebby-type carbs use the same filter as the early straight-inlet Olds, or a different one.
I suspect that one way or another, you've got the wrong filter.

- Eric

 

I placed a short paper filter in the inlet housing and removed the filter between the pump and carb. I changed the base gasket aswell. The car idles better and I don't have the surging problem anymore. The engine spins to redline wonderfully. The only thing to do next is to adjust the idle mixture again.

 

Glad you got it fixed.

 

So the sintered brass filter was the wrong one?

And we're figuring the cause of the poor running was an obstruction in the add-on filter?

Sounds plausible. Glad you got 'er goin'!

- Eric

 

The 2 inch paper filter did not fit, but a 1 inch paper filter did. I couldn't get my hands on a sintered one.

 

The short paper filter has the same dimensions as the brass filter. I used to sell a lot of those back in my auto parts store counter clerk days.

 

There's also at least one company that makes a short Qjet fuel filter using a fine mesh screen, if you don't like paper or sintered bronze. It's sold for marine applications. On my list of things to do is to fabricate a spacer so that I can use one of these short filters inside the long snout of a later Olds Qjet.