Q-Jet Throttlebody & 425

Propane, Butane, LPG, GPL, C3H8, C4H10
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kuchen
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Q-Jet Throttlebody & 425

Post by kuchen » Mon May 01, 2017 8:39 am

Hi,

I just came back from Detroit and brought along a Q-jet throttle body from a local junkyard. The plan is to upgrade my dual-fuel setup to dedicated propane, as I was not using the petrol system in like 6 years.

Some holes on the adapter have no corresponding tap in the baseplate, see markings below:
q-jet_base.jpg
Q1:Is it advisable (esp. for the center screw) to machine the baseplate ?
Q2: Does it make sense to build a side adapter for the hot-air choke, or will a designated system run cold without the need of a higher idle ?

Thank you!

Ingo


storm
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Location: NSW, Australia

Re: Q-Jet Throttlebody & 425

Post by storm » Mon May 01, 2017 10:20 pm

kuchen wrote:Q1:Is it advisable (esp. for the center screw) to machine the baseplate ?
I've never done a Qjet conversion but I doubt you'll need/want to machine the baseplate. The outer holes on the Qjet are the long screws that go right up to the Qjet's airhorn. Just gets some short screws or studs and nuts to go in their place. If you got a full kit you should have the required screws anyway. The centre hole doesn't line up with anything in a Qjet so I wonder if it may be for a vacuum port in the adaptor.
kuchen wrote:Q2: Does it make sense to build a side adapter for the hot-air choke, or will a designated system run cold without the need of a higher idle ?
My Holley baseplate's fast idle isn't operational with a 425 so I don't see a need to go to alot of trouble with the Qjet. Chokes, and fast idle, are generally required with petrol because it doesn't mix in with the airflow very well when the engine is cold. LPG already being a gas mixes in immediately.
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kikkegek
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Re: Q-Jet Throttlebody & 425

Post by kikkegek » Mon May 22, 2017 8:33 am

I did a conversion once.

You dont need those three screws.

to mount is to the baseplate I used the two long big screws that screw it into the baseplate and intake and the two small screws in the front.

I'll see if I can find some photos when Im home 2night
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

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Steptoe
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Re: Q-Jet Throttlebody & 425

Post by Steptoe » Mon May 22, 2017 5:13 pm

I have been running a Q jet base plate for over 30 yrs very successfully...over 200,000 miles ago
From memory...
1/ have a spread bore to square bore adaptor, milled off a few mm to clear the hood
2/ the butterfly return spring ,added a spring between the levers on the opposite of the throttle.
3/ added a stop to delay the opening of the secondary.
4/ blanked off the ports on the front.. rubber blanking tubes
5/The extra .. or was it needed ??? a small modification in the gasket, required blanking or cutting the4 gasket, a port
Nothing , other than milling the adapter required any major work.

Oh and careful milling I rem something about could cause a butterfly to clip the bore on the inlet manifold ???

Its all covered in an old post... most properly several many yrs ago in the forums here.
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kikkegek
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Re: Q-Jet Throttlebody & 425

Post by kikkegek » Mon May 22, 2017 5:29 pm

This is the adapter on the baseplate.

I did no modifications at all.
Link to photo
https://goo.gl/photos/3ydyA8XD9XLLEfzMA
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

storm
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Location: NSW, Australia

Re: Q-Jet Throttlebody & 425

Post by storm » Tue May 23, 2017 2:10 am

I have heard that the Qjet baseplate causes distribution issues compared to a Holley baseplate because of the rear opening throttle plates (compared to the Holley which opens towards the centre). Does anyone have any evidence that this is true?
Every job is a self-portrait of the person who did it. Autograph your work with excellence.

Fuel flow requirements calculations viewtopic.php?f=5&t=1638

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kikkegek
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Re: Q-Jet Throttlebody & 425

Post by kikkegek » Tue May 23, 2017 2:39 am

storm wrote:I have heard that the Qjet baseplate causes distribution issues compared to a Holley baseplate because of the rear opening throttle plates (compared to the Holley which opens towards the centre). Does anyone have any evidence that this is true?
No experience yet...I guess in a single plane intake AND high performance it could...but in a dual plane intake I cant imagine any issues.

I run a Edelbrock Torker II intake on my small block chevy and had a quadrajet baseplate for a while with no issues. swapped to a holley baseplate but didnt notice any improvements. But then again I hardly ever go over 3000rpm and just cruise all the time.
daily: 1974 GMC 25HUNDRED Suburban - fresh rebuild 350 small block - TH350-NP203 - 4.1 end gears - IMPCO425 mixer - IMPCO model E vaporizer
project: 1977 GMC C15 Suburban - 454 big block - TH400 - gasoline
sold: 1986 Chevrolet Suburban K20 Silverado

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Steptoe
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Re: Q-Jet Throttlebody & 425

Post by Steptoe » Wed May 24, 2017 1:27 am

Mine is a dual plane.... no issues right out to leaning out high 4K / low 5K rpms sb 355 chev... which is around where it should be.. if anything a little lower.
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C3H8
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Re: Q-Jet Throttlebody & 425

Post by C3H8 » Fri May 26, 2017 12:54 am

The part number you are looking for is AA3-86. Typically they were available from Gann Products in California. They are online. This adaptor was used for decades on the quadrajets with virtually no problems as long as the carburetor base plate was in good condition. power was generally excellent and typically quadrajet base plates had progressive linkage so the primaries opened first to about the 70 or 75% mark and then the secondaries began to open with all four throttle plates open 100% at the same time. Only four or possible six screws as was previously mentioned are used to mount the adaptor which then allows a 425 mixer to be mounted. This set up only increased overall height by 3/4 to 1" higher then the stock set up. No one has ever complained to me in the 20 years or so we used it about any problems with airflow on this set up. The main issue is finding an adaptor as they are almost obsolete. Checking with installers that have been around for some time may yield some used ones.

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