7041251 tuning...

[Tancuda]

Hi everybody,

I would like your suggestion on what I could do to have better vacuum and idle...

The carb (7041251) is rebuild (stock) by a known rebuilder in the US.
 
What I know: 71 jets, 49b prim.rods, AK sec rod.
                    Choke work well, acceleration and cruise is ok.

Engine: 1970 455 (low comp) with headers
Cam lift is .448 INT, 456EXH, unknown duration.
Compression was 135 to 145.
Base timing 14, dwell 30
Plugs are pretty white with good gap.

What I did:   Did send it back to rebuilder last year and he said that he had enriched the idle a bit
                    but I don't see any improvement.

                    Idle mixture screws are out 7-8 turns
                    If I put my hand over the carb, rpm rise and my vacuum is much better (14 to 18)
                 
                   Checked vacuum leak with carb cleaner around intake and fitting with
                   all hose disconnected (PCV, brake booster, trans modulator, distributor)

                   No leak except a little one around secondary throttle shaft.
                 
                   So I really think the carb is the problem.
                 
Is there something I can try that wouldn't need the carb to be taken apart ?

Do you think that with today's fuel, the stock spec on that carb is wrong ?

Thanks a lot.

           

[lightning boy]

Tancuda, if you can post some pics we can tell if the carb has been rebuilt by the wrong outfit. Hopefully your "rebuilder" is someone who specializes in Qjets. Unfortunately, you need to take it apart and measure a few things to see where it's at. Looks like it needs more idle fuel.

[77cruiser]

With that low of cranking comp. it could use more timing too.

[Tancuda]

Tancuda, if you can post some pics we can tell if the carb has been rebuilt by the wrong outfit. Hopefully your "rebuilder" is someone who specializes in Qjets. Unfortunately, you need to take it apart and measure a few things to see where it's at. Looks like it needs more idle fuel.

Thanks for the reply, yes the rebuilder specializes in Qjets, here some pics on and off the engine.
The carb is 7041251, and also I would like to know if it's the correct baseplate (7029025).

Thanks

[Tancuda]

With that low of cranking comp. it could use more timing too.

Thanks for the reply,
Yes I can bump timing but my total will be too much.
Would have to play with the advance in distributor. I wonder how I can't have it right with the stock setting ?

Thanks

[Kenth]

Problem is likely the cam, too much duration, too tight LSA acts like a vacuum leak at idle/low speed.

You may try at least .042" idle tubes and .095" idle needle holes.

[lightning boy]

It is because you don't have your engine at stock settings. It didn't come from the factory with 130 PSI compression.

[Tancuda]

Problem is likely the cam, too much duration, too tight LSA acts like a vacuum leak at idle/low speed.

You may try at least .042" idle tubes and .095" idle needle holes.

Ok thanks will check on this. Idle needle holes, you mean the holes behind the mixture screw ?

And I just realize that on the picture, my primary throttle blades are drilled !!

I saw a lots of quadrajet on the web, but never saw drilled plates (stock).

Any ideas ?

[Tancuda]

It is because you don't have your engine at stock settings. It didn't come from the factory with 130 PSI compression.

Ok thanks, but what do you think would be good numbers for a 1970 455 low compression engine ?

At least there are no more than 10% variation between cylinders.

[77cruiser]

I'd think about 160 for a stock engine. Maybe the cam was installed retarded.

[Kenth]

Ok thanks will check on this. Idle needle holes, you mean the holes behind the mixture screw ?

And I just realize that on the picture, my primary throttle blades are drilled !!

I saw a lots of quadrajet on the web, but never saw drilled plates (stock).

Any ideas ?

Most, if not all, 1971-74 Olds Qjets has the bypass .110" holes in the primary throttle blades. And, yes i was referring to the holes behind the mixture screws.

[Tancuda]

I'd think about 160 for a stock engine. Maybe the cam was installed retarded.

Good thanks

[Tancuda]

Most, if not all, 1971-74 Olds Qjets has the bypass .110" holes in the primary throttle blades. And, yes i was referring to the holes behind the mixture screws.

Good to know thank you.