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Jmark 05-18-2005 01:06 PM

Drilling a Qjet
My son's 400 failed emissions 3 times last Saturday. We fiddled and fiddled and could never get the HC below 600 at idle. Max is 450. CO is .7,,,,way too lean, but the mix screws don't have any affect since he built this engine.

First pass the HC was 740 and CO was .3. We backed out the mix screws another 2 turns and backed the timing back to 14 at idle and got the HC down to the 600 and CO came up to .7, but its basically in a lean roll the whole time its idleing in gear.

The cam is the Comp roller 286 and World cast heads. Idle vacuum in drive is barely 8" due to the overlap.
To keep it running, I have the base idle screw cranked in pretty good compaired to where it was when I ran the carb on my 350 after going through it. Now the primary plates are open too far to engage the idle circuit.

Question its, how big and where do I need to drill the hole in each primany throttle blade so I can get the blades back down closer to the idle slot and get some idle mix back? I previously drilled the idle tubes and transition tubes so I know I have the capacity for more idle mix, just can't get the primary blades to close far enough and stay running.

Its already 2 months expired and he's dragging his feet on fixing it, so I guess its dads job!!! LOL


johnsongrass1 05-18-2005 03:02 PM

1/8 to 5/16.

Try opening the secondary's first. You can bend the tab.

DoubleVision 05-18-2005 03:05 PM

As Mojo hand Dr. Johnson said, but honestly, I don`t think you`ll get it to pass emissions. Not saying it`s impossible, but I don`t think it`ll be that easy either.

ratlover 05-18-2005 03:14 PM

Here's what ya got to do....
back off rockers 3/4 turn, more if you dare(I know, That's a lot of lash, don't over rev engine on the way to the test) That will reduce overlap.
Retard timing at least 15 degrees.. more if possible
there is a round plug on top of the Q-jet that can be drilled out for access to a main metering adjusting screw.. lean that out as much as possible
bring idle up to 1000 rpm min. and lean out idle screws.
I have got a few cars through with that approach. But this is to Canadian standards. gooood luck!

Jmark 05-18-2005 05:07 PM

I've thought about loosening the rockers, but the truck will be run at 55 on the testing dyno for the cruise test. (410 gears, over 3 grand on the rpm) Kinda scary with sloppy rockers.

We have the timing at 14 or 15 now. Its already running very hot now when he drives it to work, hate to push it back any more.

My sons shop has a 4 gas machine and maybe we can spend some time after work doing some idle work.

As far as leaning it out more, that will just push it into a lean miss condition and blow the HC readings through the roof.

I'll try the secondaires and see what happens. I may be able to tweek them a few thousands open and see if I can lower the primaries back down some. I have a spare Qjet if I bugger this one up. What really needs to happen is pull the cam and send it back to Comp for a re-grind to a milder profile.

It passed last year after some work but come to find out that when the driver got done with the cruise test, he brought it back down to idle and put it in neutral! That was the ticket last year, but no such luck this time. Supposed to stay in drive during the idle test. I suppose a couple of used cats would do the trick too. Maybe I'll have him call the muffler shop they use and see if they have a couple laying around.

Thanks for the replies.


smallblocker2 05-18-2005 08:39 PM

Drill the holes on the idle discharge port/transition slot side of the shaft in the middle, Start with a 1/16" bit and go up slow in 1/32" increments

Jmark 05-20-2005 10:08 AM

Well, tomorrow is drilling day. Talked to a friend at a different shop, he offered to set it up on the 5 gas after i'm done drilling. He's a pro stock engine builder here in Phx and my son used to work at the same shop. Nice of him to offer the help. Hopefully, it will atleast get the darn thing to pass if I can get the plates back in position. I'll post the results if anyone is interested.


smallblocker2 05-21-2005 02:18 AM

Right on man, good for you. Anyone who can build prostock motors can't be too dumb eh! I'd be listenin' to him, at least.

Jzarczyn 05-21-2005 11:30 AM

Just curious, where abouts in Canada they do emissions tests?

Jzarczyn 05-21-2005 11:36 AM

This may sound like a shot in the dark - h@ll, it is - Try mixing (rubbing)alcohol in with your gasoline when the tank is neer empty, that should bring down those HC levels for the test.
Let me know if it works if you try it.

ratlover 05-21-2005 01:11 PM

anywhere in the lower mainland as we call it in British Columbia. This would be Greater Vancouver which includes Richmond,Surrey,Langley,Delta,Burnaby,New Westminster. Not too sure where the boundary is.... Just have to deal with it! :pain:

Jmark 05-21-2005 01:19 PM

Well, I had partial success. I drilled the primaries with a 5/64" bit and checked everything over one last time. When I started it up, the idle was about 200 rpm higher than before. That was a good sign. I wasn't able to get access to the analyzer this morning so I set it up the best I could by ear and headed to the emission station. The previous HC was just over 600 and CO was .6, way too lean. After the drilling, the HC was 472 and CO was 1.9% Soooo close but still too high on the HC. Limit on HC is 350. CO limit is 4%.
I'm reluctant to back the timing off any more without an analyzer for fear of overheating it.

The thing that sucks was when I was getting out of his truck, my foot snagged the loose weather stripping around the door and sent me to the ground bent over. Man I got some sore ribs. The truck has a 6" lift and 35" tires so it was a long fall! So, i'm sitting here in pain waiting for my friend to call so we can try one last time on his 5 gas machine before going through the testing again.
If it fails this time, either the cam comes out or 2 cats are going on. The machinist that suggested this came was an idiot in my opinion. He knew what my son drove and what he was going to do with it. Oh well, such is life.

ratlover 05-22-2005 09:45 AM

Nothing reduces hC's like ignition retard. I had a problem with a ford once, hooked it up to a scope, had tried to lean it on the screws to no avail. watched the hC's drop from 1300 to 250 while the mech rotated the distributer. I would do whatever it takes to get through the test. pop the hood and crank back the timing before you go through. also if there is a line up do not let your engine idle for a long period of time as it will load up the exhaust system.

Jmark 05-22-2005 06:20 PM

Ratlover, you are correct.
I knew that bumping back the timing would help, but until I got the 5 gas hooked up, I was just peeing in the wind.

Today, I got access to the analyzer. Found one very small intake leak on #2 cyl. intake runner. We were able to tweek the mixture some to compensate for that until we can fix it. Infared thermometer showed that cyl running 55 degrees cooler on the exhaust too.
Then we slowly bumped the timing back 2 degrees at a time and slowly the HC dropped. The max limit is 350 in gear, we finally stopped when it hit 276 and held steady. CO was kept right at 1% at idle. Tomorrow I'm off for one more official test. It idles fine, but is a total mutt off the line. Don't want to drive it any more than necessary like this so I kept it here again tonight and let my son take my '86 home. Hope I get it back!

ratlover 05-23-2005 09:12 AM

best of luck to you. I hope you dont get some jerk driving the crap out of it for the test. I had a test center jockey having trouble finding 1st gear, repeatedly grinding the freshly rebuilt gears.....arrrgh!

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