QFT 750 Mechanical Secondary on stock 440 help

[69 R/T]

My venerable old #4617S 750 AVS on my stock 69 440 runs great, however it has not so thrilling throttle response. My buddy gave me an Edelbrock CH4B and I bought a new Hot Rod series QFT 750 mechanical secondary carb. I threw on the vacuum gauge and dialed in the idle quality, but that's all I've done. It's essentially out of the box and on the car. The throttle response is awesome but at WOT it feels "held back" which the AVS didn't do. I also did a WOT at 2500 rpm roll and it hesitated a bit which I found odd for a double pumper. The car is a stock 1969 440 with a 4 speed and 4.10 gears, so lagging shouldn't be an issue. My initial timing is 14 BTDC, 37 total mechanical, and 52 at cruise with vacuum advance. Idle vacuum is about 15 in/hg, and it runs flawlessly at other than WOT. Any suggestions on air bleeds, jets, or anything else to tune to this application?

 

[lewtot184]

maybe a fuel pressure/volume issue?

 

[69 R/T]

maybe a fuel pressure/volume issue?

It doesn't do it with the AVS though. Weird.

 

[lewtot184]

i doubt the air bleeds are an issue with hesitation. float level maybe. the 4617 is actually a larger carb venturii wise than the quick fuel which may run better at higher rpm. stock 440's are probably 550cfm engines, so to me cfm rating between the two carbs means nothing. I've done a bunch of work on a 4618 avs and it performs well. the 4617 would be easier than the 4618 because the emissions idle problem isn't as bad. I found some old mopar jetting tips for the avs with a ch4b with the heat blocked. left (throttle side) has .107 jets front and back, right side has .101's front and back. use the stock metering rods.

 

[69 R/T]

Hmmm, that's good to know. I've always been a Carter guy. They're so simple and are very reliable, but they don't seem to have the throttle response as a Holley type carb. My AVS also has an off idle stumble I can't seem to cure. I played with the pump arm setting to no avail. Who knows, maybe I'll go back to the Carter.

 

[Jcharger]

I am sure you can get that to run without a lag and think you should be able to get the same top end WOT. If it were me, I would look into an A/F gauge to see what it is doing. I am sure someone here though can advise on how to set it up. But to start, I would consider calling the people at Holley to ask how they suggest setting up for your application. It might give you a starting point.

 

[69 R/T]

I like that idea. I'm not familiar with Holley type carbs, I've always used or had Carters for the most part.

 

[beanhead]

Getcha a pump cam assortment kit. The lag can likely be tuned out by using a different one. Make certain the pump arm is adjusted properly as well, it should instantaneously begin to squirt gas when the secondaries begin to move...Holley has some video clips on how to check that. An a/f meter is a big help, but you can get it through experimentation too. Get that dialed in before attacking the WOT issue..

 

[Paul_G]

As Jcharger stated, an air fuel ratio gauge would be the ideal tool to get that carb dialed in. Otherwise you have to determine if your problems are from being too rich or too lean. You have not mentioned any back firing, surging, or other signs of being lean. Lazy or sluggish at WOT is usually a sign of an overly rich mixture, while a stumble or hesitation is usually to lean at throttle opening.

You can jet down the primary side 2 jet sizes and see how it feels. But first, check your fuel pressure, then the float levels. Make sure those are right before you start doing any tuning.

 

[beanhead]

Not sure if you got it fixed up yet, but I should have mentioned before...if it feels 'held back' at WOT, open the throttle all the way by hand and look in there and make certain the secondary butterflies are opening all the way. Some adjusting is sometimes in order first..

 

[RJRENTON]

Hmmm, that's good to know. I've always been a Carter guy. They're so simple and are very reliable, but they don't seem to have the throttle response as a Holley type carb. My AVS also has an off idle stumble I can't seem to cure. I played with the pump arm setting to no avail. Who knows, maybe I'll go back to the Carter.

69RT,
I am a Carter carb guy and after many years of tunimg them, I concur that they are simple, reliable and easy to mofify. The AVS carb, used on Mopar cars ftom late '60's to early '70's, had an off idle stumble problem. This is attributable to increasing emission requirements. Carter's fix was to include an adjustable (adjusted at the factory and sealed) that reduced the effect of the accelerator pump fuel shot (which increased the carbon monoxide level in the exhaust). This adjustable air bleed (located under a cap on the air horn directly in ftont of the air cleaner's base). The fix is to eliminate this feature by plugging the air entrance hole with a small diameter split shot fishing sinker (located on the side of the adjustment screw's housing) or with the carb off of the engine and inverted, there are two small passages adjacent to the primary throttle bores that lead to rhe adjustable air bleed passage. Using 2 small split shot sinkers plug these 2 passages. In addition, reduce the diameter of the primary metering rod top step (largest diameter by 0.001" measured with a micrometer). These simple modifications fixed every Carter AVS carb I've worked on from 383's to 440's that had this annoying off idle stumble. PM me if you need any more info.
Bob Renton