'72 skylark Qjet

I finally got arount to checking the Qjet number for my '72 Skylark custom 350-4 which is 7043244 LD.

When I looked up the number I found out it goes to a 1973 century 350.

The motor is mostly stock.

I've removed most smog items, smog pump, EGR valve, T.V.S - T.C.S switch.

I left the PVC valve to the carb vac hose and the charcoal canister hose to the air cleaner snorkel and the vacuum hose from the thermostatic air cleaner thingy( the round vacuum diaphragm that sits on top of the air cleaner snorkel) to a vacuum tee that that ties into the I think it some kind of senser underside of the air cleaner to the carb.

All A/C equipment is gone.

The car runs and starts great, after it's warmed up there's no acceleration lag when I go WOT it just goes. As of now it's timed to factory specs with a stock distributor which will change after I get my distributor rebuilt and power timed.

My questions is the engine idles a little rough, it'll varies I would say 25 to 30 RPMs up and down which makes it hard to dial in the Qjet idle mixture screws. I started by screwing in the screws till they gentley bottom out then backed the out 2 full turns for my starting point. I started the motor put it park then turned the screws in to see if the engines RPM would drop which it would but not until the screws were almost bottomed out.

Anyway I tryed to adjust the carb using the instructions found on this forum but found when using a tac
( old Sears engine analyzer) it's very diffacult to find that sweet spot with the rough idle the engine has now.

The car does use some oil, I would say more then a quart every 1,000 miles. There some smoke in the morning ( kinda gray) when I start the car but no smoke when driving during the day so I was thinking valve seals?

The car uses no coolant and the coolant is clean.

One more thing I think I may have a vacuum leak but I can not find anything. I spent 40 minutesthe other day hunting for it. I tried carb cleaner and sprayed every where with no change in engine RPM and also used held a 3/8 hose to my ear and checked every where I could think of including around the intake manifold and could not find a thing. I gave up and thought "maybe that's what the engine just sound like". But still it idles rough.

Could it be because it's not the correct Qjet for the car? The Skylark is a '72 and the carb is for a '73 century 350. May be the guts just won't mate well with my car, or do you think I my need a valve job?

If I did a compression check would the help find out what's going on?

The car starts and runs Great! I kinda hate to mess with that but the rough idle bugs me!

Thanks,

Brad

 

If the carburetor is built correctly, it will be good for your 72 350.
I would get a vacuum gauge and see what the readings look like. That can tell you a lot about your engine.
You could have a vacuum leak many places in that car, since all of the climate control is operated by vacuum. Can also be power brake booster.

Work with your timing. Follow Larry's thread and advance your timing. Change your distributor advance vacuum to manifold and see if your engine likes that better for idle.

 

wavy idle can be caused by

nozzle drip

and

being in the mechanical advance curve at idle

among other things

 

If the carb was ever commercially "remanufactured" it will not work well for what you are doing. This may account for the idle issues as well.

Weak springs or aftermarket distributor parts will also cause idle issues and fluctuations in idle speed as mentioned above.

I just ran into a "fresh" 350 SBC rebuild couple of days ago with a brand new Summit HEI in it. The owner complained of idle speed never being the same any two days he drove it. One of the culprits we found quickly were the POS springs used in the distributor were too weak and allowing some of the mechanical curve to come in early as 650rpm's. The advance mechanism would not pull the weights back to zero advance until you lowered the idle speed screw below 650. Then when you revved up the engine it refused to return to zero advance until you lowered the speed screw some. Then when you set the idle speed back up where it needed to be, the first time you revved up the engine it would not return to idle and you'd have to do it all over again (sound familiar?).

Stretched out timing chain and retarded cam timing can cause issues at idle speed, which include low vacuum.

Any sort of aftermarket cam with more duration than the stock cam or tighter LSA will cause low vacuum at idle and inconstant idle, nozzle drip, varying engine rpms at idle speed, etc.

Worn cam lobe(s) can also play a pretty big role in idle quality but not have a major impact on how it runs at higher rpm's.

Same thing with worn piston rings and lower compression on one or more cylinders. When the load isn't evenly distributed we see the negative results more at idle speed than anywhere else........Cliff

 

Larry I think the cam is stock, I have not install the 1112109 distributor yet since I think the bottom bushing has to much play so I ordered some new bushings from Ken at Everyday Performance which I just recived. Thanks Ken for the fast service! I plan the install the new bottom bushing this weekend. the top bushing seems fine.

I just picked up a vacuum gage two weeks ago so I'll put it to use after I install the new distributor

 

Thanks Cliff for all the info, I think what I'll do I pick up a compression gage and check just what the cylinders are doing. I have a '72 Buick service manual and I saw the have a nice compression chart in it to help you figure out if your engine is sick. AT least that way I'll know what going on.

Can I buy a Qjet rebuild kit that'll match the engine closer then the '73 Qjet thats on there right now?

Looks like I'll have some diagnosing to do over the next few days.

Thanks Guys!

 

When doing a compression test make sure the engine is fully heat soaked and remove all the spark plugs so it spins faster. Also lock the throttle wide open.

You are looking more for uniformity than anything else, so basically all readings should be pretty close. The later low compression engines don't have a lot of squeeze so numbers may be on the low side, but still they need to be consistent.......Cliff

 

Thanks Cliff for info for the compression check, I wanted to do it this weekend but things at work are so that I'll be working the next two weekends. I am going to buy a compression checker tomorrow after work though. I'm looking forward to check the compression, that's sound more fun the watching TV!

Thanks,

Brad