[shaker455]

Originally Posted by TransAm 474

While we wait ... , here are a few pictures of the 383 on the stand before it was installed....

Nice job guys!
I did a 383 awhile back, strong runner!

[TransAm 474]

Originally Posted by shaker455

Nice job guys!
I did a 383 awhile back, strong runner!

Thank You! 😃

[TransAm 474]

We received the Q-Jet intake today, it arrived early ... I have another question, about the Q-Jet base gasket. I see Cliff offers a 1/4" base gasket(purple), and it has 4-holes in it. I was wondering if the same base gasket is available with just the center divider in it? Could we simply just cut out the 4-hole gasket to make it open on both side, but retain the center divider? Below is a couple photos representing what I am talking about ... I am not even sure if it matters or not, but I thought I would ask

First is the Intake Flange....

Second is the gasket Cliff offers....

Third is the gasket Cliff offers that I edited to represent what I am talking about.....

[TransAm 474]

..... or, would it be better to run a 1/4" gasket like this, with the addition of divider cut out between the Secondaries only, since we won't be modifying the divider in the Intake itself?

[shaker455]

Your intake is fully divided so cut your 4-hole gasket to match the intake as the 4-hole will have unsupported sections.
I do prefer the air gap intake over standard RPM

[TransAm 474]

Originally Posted by shaker455

Your intake is fully divided so cut your 4-hole gasket to match the intake as the 4-hole will have unsupported sections.
I do prefer the air gap intake over standard RPM

Thanks Shaker, that is exactly what we were thinking. Would it be beneficial to cut the divider in the gasket between the secondaries, as my last picture I posted above, since we won't be cutting the divider in the intake itself?

[shaker455]

Originally Posted by TransAm 474

Thanks Shaker, that is exactly what we were thinking. Would it be beneficial to cut the divider in the gasket between the secondaries, as my last picture I posted above, since we won't be cutting the divider in the intake itself?

I would say no at this point.
That's something you can test at a later date, for now get it running right.
I found the carb responds better fully divided

[Schurkey]

Originally Posted by Cliff R

How could our race car SLOW DOWN adding a very expensive billet distributor vs a plain old $5 junkyard variety points unit. All I did to the points unit was to brase down the center plate and "lock-out" the advance. The MSD was also locked-out, and with both distributors we set the timing at idle speed at 36 degrees exactly.

Hum? Well it turns out the day we tested the new MSD at the track on "old timer" was pitted next to us...

...He just smiled and told us that you lost your natural timing retard at high RPM's when the dwell falls off slightly. Me and my good friend Mike looked at each other with a "duh" expression on our faces!.......

Not that simple. (but keep in mind that I quit dealing with points distributors about forty years ago, and have zero interest in going back there at least until the EMP blast destroys all of our electronic devices.)

Any magnetic-pulse-triggered ignition will also have "high speed timing retard" of ABOUT 1 degree per thousand RPM (round numbers.) When I measured a particular HEI distributor, it was more like 2/3 or 3/4 degree per thousand RPM. The GM service manuals used to advertise this "feature" in regard to the old, 1960's electronic ignition (Option K66.)

This RPM-related retard is covered-up by the centrifugal advance mechanism. If your mag-pulse distributor gives you an indicated 20 degrees of centrifugal advance at 3000 rpm, if you were to measure the geometry of the thing, you'd find 22-ish degrees of advance, minus a couple of degrees of RPM-related retard. So this RPM-related retard only becomes apparent if the centrifugal advance is either locked-out, or at the limit of it's travel.

Some folks blame this RPM-related retard on the module circuitry--the module reacts at a given, predictable rate, but as RPM increases, that fixed reaction time becomes larger in proportion to the shorter time between firing intervals. Yeah, that may be some tiny part of the overall RPM-related retard. The major factor is the reaction of the pickup coil/reluctor itself. Which is why all mag-pulse triggered ignitions have RPM-related retard, no matter what sort of module they're connected to.


So, in terms of losing performance compared to the points distributor, I'm wondering if the HEI had MORE "high speed retard" than the Points system, and you then had not-enough advance at the "top end". I don't know how much the points-system dwell varies over the RPM range you were running. I thought dwell was pretty stable, so "high speed retard" based on dwell variation on a GM distributor is kinda new to me.

A distributor machine and an hour would tell that tale...but I suspect it's not worth the effort. Just adjust the initial timing to provide what you need in the important part of the powerband, and call it a day.

But I've been wrong before.

[Cliff R]

I really don't know why our stout little 406 ran quicker with a stock points distributor vs an MSD billet with no other changes made anyplace. The car even left on a Trans-Brake and auto-shifted using a box, etc. So no variable whatsoever from the driver. It was early in my learning curve with these things but why really doesn't matter to me and I'm not going to disect it really hard, just learn from it.

I'm NOT advocating anyone toss their bug-zapping distributors or anything of the sort. Just realize that IF and WHEN you replace something that's working good and timing is set at the same base number, plus the same timiing curve, there are no big advantages one type of system over the other.

More recently (less than 20 years ago) we were on the dyno with my own engine after doing a "refresh" and testing a couple of camshafts. Since the new cam was NOT making the grade and the carb was spot on we swapped out my HEI for an MSD billet distributor with a 6AL box. Not only did it not make one more ounce of power at any RPM there was a very, very slight LOSS with the MSD over the stock HEI (recurved with a positive stop welded in and not other mods). Another lesson in bug-zapping distributors and boxes making more power than stock parts.....FWIW.

[Cliff R]

"Would it be beneficial to cut the divider in the gasket between the secondaries, as my last picture I posted above, since we won't be cutting the divider in the intake itself?"

Call me at the shop or PM when you get a moment and I'll get the right gasket for what you are doing. I've done a LOT of testing in that area over the years and can supply the best set-up the first time in.........

[shaker455]

Originally Posted by TransAm 474

Just wanted to add here, that I found my receipt from where I purchased the carb, and it has the Jet Size, Primary Rod, and Seconadry Rod on the receipt. I am still awaiting the rest of the Specs from Sean Murphy, if he will get back to me.

Q-Jet #17084231 (S.M.I. #36002 800cfm)

Main Jet- "74"
Primary Rod- "52M"
Secondary Rod- "DA"

You get a note back from SMI?

[TransAm 474]

Originally Posted by shaker455

You get a note back from SMI?

Yeah, he finally emailed me back, and said the specifics were "Proprietary" Lol .... so tomorrow, we will probably jerk the Air Horn off and do some measuring so we know what we are working with..... we got the Intake changed out today, over to the new Spreadbore RPM.

[shaker455]

Okay, any snow in MO?

[TransAm 474]

Originally Posted by shaker455

Okay, any snow in MO?

Not as of yet, the temps have been kinda of a rollercoaster though, 40 one week, 70 the next, then back to 30 Lol ....

[TransAm 474]

Ok .... to follow up on this thread, we pulled the new SMI Carb apart and measured everything in it. Here are the specs ....

#17084231 Q-Jet

Jet- 74
Primary Rod- 52M
Secondary Rod- DA
Secondary Rod Hanger- H
Main Air Bleed- .086"
Upper Idle Air Bleed- .067"
Lower Idle Air Bleed- .076"
Idle Tube- .038"
Down Channel Restrictor- .055"
Mixture Screw Holes- .093"
Idle Bypass- Plugged in Main Body with aluminum plugs, .083" holes drilled in Primary throttle blades
Fuel Inlet Needle & Seat- .130" (windowed)

Now.... I have been in conversation with Cliff, and we are making several changes to it with one of Cliff's HP Kits to try to correct it, here is what it will look like after making the changes ....

Jet- 76(was 74)
Primary Rod- 50C (Cliffs custom Smooth Taper) was 52M
Secondary Rod- DA
Secondary Hanger- H
Main Air Bleed- .086"
Upper Idle Air Bleed- .067"
Lower Idle Air Bleed- Reduced to .063"(was .076")
Idle Tube- .038"
Down Channel Restrictor- .055"
Mixture Screw Holes- .089"(was .093)
Idle Bypass- Now .110"(was .083")(changed Baseplate out for another with Solid Primary Blades, Removed Plugs from Main Body, opened up Idle Bypass Holes in baseplate to .110")
Fuel Inlet Needle & Seat- .140" High Flow(Non-Windowed)(was .130" Windowed)

I will report back with the results as soon as we get it back together and running .... Thanks to all for the help and suggestions, it is very much appreciated!

[Schurkey]

The idle air bypass was plugged, and the throttle plates drilled instead?

I don't understand that AT ALL.

I have seen Q-Jet throttle plates with drilled holes--almost certainly OEM, although I didn't own the carb from new. But drilling the throttle plates is a third-rate "fix" compared to the idle air bypass.

[TransAm 474]

Originally Posted by Schurkey

The idle air bypass was plugged, and the throttle plates drilled instead?

I don't understand that AT ALL.

I have seen Q-Jet throttle plates with drilled holes--almost certainly OEM, although I didn't own the carb from new. But drilling the throttle plates is a third-rate "fix" compared to the idle air bypass.

Yep! Carb was built by Sean Murphy at S.M.I for this engine .... he had aluminum plugs driven into both passages in the main body, blocking off the Idle Bypass completely, but then had an .083" hole drilled in each Primary Blade for Idle Bypass. Makes ZERO sense to me. Cliff told me he has seen that a LOT through the years, with not only SMI carbs, but other commercial "custom" Q-Jets he has had to fix as well .....

[Cliff R]

The Quadrajet's idle bypass air system is a very precisely controlled vacuum leak. The system routes idle air into two slots at the main body to airhorn gasket surface, over to two voids in the main casting, then down to the baseplate and comes out under the primary throttle plates. To this day I'm not sure why anyone would block off that system right to start with or use drilled throttle plates instead.

Not all Q-jets used this system however. It wasn't incorporated into many of the early units so you either have to add it or drill the throttle plates when using big cams with a lot of overlap or anytime you want to lower the throttle plate angle and expose less transfer slot.

I'll add here that idle bypass air is a tuning tool and there are steps you can take during the tuning process to decide if you need to use it, or add more of it........

[TransAm 474]

Ok, UPDATE ..... We got this thing back togther and fired up finally, and its running fairly decent, it doesn't die anymore when you place it into gear, so that part has been SOLVED! But we have to have the throttle blades cranked open far enough, that the mixture screws have no response to get it to idle around 740-760 RPM. We can pull the PCV valve out of the valve cover and it RPM speeds up about 30-40 RPM.... is this indicating that it needs more Bypass Air? Its currently at .110" .....we are able to close down the primary blades enough, to get it to idle really slow, all the way down to around 650-660 RPM and it will stay running, but the mixture screws still have no response in either direction, and I believe it is due to the Primary Blades still being opened up too far, uncovering the Idle Transfer Slots, and I can't close them down far enough to get it to idle on it's idle circuit to regain control of the mixture screws, without it stalling, seems that it wants more Idle Bypass Air? I didn't check for nozzle drip, but it almost cetainly has to have. Initial Timing is set at 12°, and Vacuum Advance is unplugged while doing the test. I can tip in the choke flap at idle and it will further Decrease Idle RPM. Unhooking the PCV from the valve cover will Increase Idle RPM by about 30-40 RPM. Other than that, it seems to be doing pretty good. I want to be careful about increasing the Idle Bypass Air so much, that we can't get the idle speed down where we want it, around (750-800 RPM). Our Idle Bypass is currently at .110", but I have read people that have as much as .140-.150" .... what do you'll think?

[steve25]

There’s no denying that if your blades are still open passed the transfer slots that you need more idle air to at least get the blades centered over the slots..

I have a larger number drill set and when I need to this I drill the throttle blades.

I start off with a number 53 bit which is about .060”.
If my blades are above the slots I drill two holes in each blade right off the bat.

Most times 3 holes drilled gets the job very well in the ball park, if not then I drill 4.

The nice thing about drilling the blades is if you go too far it’s easy to fill them in.

Since I don’t know what cam you are running if you want to be on the really safe side start off with one number # 60 hole in each blade, this is .040”.

This all assumes that you have at least 8 degrees of base timing in the motor .