I just installed a rebuilt Qjet on my 69 WT 400 w/Ram Air IV cam. When you get on the gas and the secondaries open, it backfires. If you gently roll into the secondaries, I get the typical Qjet "moan", but the car doesn’t feel like it should acceleration-wise. I removed a Holley 850 vacuum secondary to install the Qjet and had none of these problems before. The Qjet came off Ebay, and it looked to not have been installed after the rebuild. I am not sure who the rebuilder was. The carb idles perfectly. Any help would be appreciated.
Thanks, Bill.

I just installed a rebuilt Qjet on my 69 WT 400 w/Ram Air IV cam. When you get on the gas and the secondaries open, it backfires. If you gently roll into the secondaries, I get the typical Qjet "moan", but the car doesn’t feel like it should acceleration-wise. I removed a Holley 850 vacuum secondary to install the Qjet and had none of these problems before. The Qjet came off Ebay, and it looked to not have been installed after the rebuild. I am not sure who the rebuilder was. The carb idles perfectly. Any help would be appreciated.
Thanks, Bill.

Look where the linkage from the vacuum pull off attatches to the air valve. There's a small hex head set screw where the shaft comes out. Above that you should see a little adj. screw. Stick a small screw driver in the adj. screw while you loosening the hex screw. Back off the adj. screw (<span class="ev_code_RED">counter clockwise</span>)until all tension on air valve is gone. Now, tighten the adj. screw about 3/4 turn and tighten set screw. Go for a ride and see if bog has gone. If it has gone you can fine tune it by slowly backing the adj. screw until bog comes back and then tighten it back a little. I find that if you just tighten it as much as it will take (<span class="ev_code_RED">3/4 turn</span>)it's OK.

I reset the secondary flap as suggested. I did not test drive, but in the garage, I still have the bakfire when the secondaries kick in. Does this carb need to be gone through? I have checked the timing and it was OK.

Backfire indicates lean, try raising the rods and/or a secondary rod with a richer taper.....Cliff

Thanks for the replies Doug & Cliff R. I have limited knowledge of Qjet carbs.

Cliff R - what would the cost be to go through this carb and make it right?

I sent you an email this morning to the address in your profile and haven't heard anything bak from you yet.

Thanks, Bill

If the backfire is through the carb, check the ignition and/or valve timing before throwing rocks at the carb.

Jon.

I have checked the ignition timing and it is right on. I don't think its a valve timing issue because the engine ran perfectly with a Holly 4150 on it. I am slowly returning the car back to stock appearing and the carb was the one of the last things I needed to change under the hood.

Timing could very well be the issue here. You had a Holley 850 before. Are you using the timed port on the carb for vacuum advance? If so then something to think about. The timed port vacuum is not the same on a Qjet as it is on a Holley or any other carb. The timed ports create their vacuum signal by the velocity of air flow through the primaries. The port in the primaries act as an eductor to create vacuum. Bigger primaries, less signal, smaller primaries, more signal.
Just some thing to think about.

Dryseals, it sounds like you´re descibing the source for the signal to the "vacuum-can" on Holley using "vacuum" operated secondaries, wich are indeed controlled by the airflow through the primaries, unlike the Q-jet´s secondary airvalves wich are pushed open by atmospheric pressure when the secondary throttle plates are opened.
The ported source for ignition vacuum advance (or emission system) works the same on most carbs, though located differently on different carbs.

Thanks guys for all the insight. I have a very basic question...which port should I hook the vacuum advance to? I am using the port off the middle of the passenger side of the carb. I have blocked off the port that should be controlling the modulator valve (my car is a 4 spd.)on the passenger side of the front of the carb. There is another port on the driver's side front that dosen't see to have any vaccum @ idle. What should this port be used for?

Thanks, Bill

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Kenth:
Dryseals, it sounds like you´re descibing the source for the signal to the "vacuum-can" on Holley using "vacuum" operated secondaries, wich are indeed controlled by the airflow through the primaries, unlike the Q-jet´s secondary airvalves wich are pushed open by atmospheric pressure when .
The ported source for ignition vacuum advance (or emission system) works the same on most carbs, though located differently on different carbs. <HR></BLOCKQUOTE>
Kenth, the timed port for the vacuum advance canister works the same way. Yes the vacuum for the Holley secondaries is also a eductor port. The timed port signal for the vacuum advance will vary from carb to carb. And will have an effect on the timing. GM used seveeral different vacuum advance modules depending on the application, same Qjet port and carb. This is the main reason they always said to disconnect the timed port during tuning, to get a base line siganl. Most after market carbs have larger primaries than the Qjet, and thus the singal is weaker. Many will register near zero at idle. And for this reason many people will pump up the base timing at idle to compensate for the signal loss. Then they get that ping at higher RPMs. This maynot be the source of the backfire, but its something that I have seen cause problems many times.

Can someone explain what "a eductor port" is? It is not in any dictionary I have read, nor is it mentioned in the Holley book that I have. Thanks.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Geoff:
Can someone explain what "a eductor port" is? It is not in any dictionary I have read, nor is it mentioned in the Holley book that I have. Thanks. <HR></BLOCKQUOTE>

Gd'ay mate, spent some time on the NW Cape.

An eductor is nothing more than a hole in a pipe. As the air flow moves through it creates a vacuum at the hole. Too much air flow and the pressure will push out the hole. Not enough and the vacuum will not be created. Velocity of the air flow is the key.

Many of the Japanese cars did not have air pumps for the exhaust. Instead they used eductors, the velocity of the exhaust flow in the exhaust pipe would create vacuum at a check valve in the exhaust system, and suck in air.

If this does not get you going, post again and I'll give some better detail.

Had a few glasses tonight, do the pissed up routine, having a hard time typing.

Long live Swan and Emu Export, stuff Fosters, give me a XXXX anyday. Good beer and a yank tank are a good thing for life, just watch out for 10-40.

Ta