oldsonharmont

Got a freshly rebuilt '71 455 that starts smooth, but then starts to chug and miss after the choke pulls off. The service manual seems to point to carburetor (Q-jet) issues, any other ideas here? The car ran great a few weeks ago, then developed this problem. Thanks.

shiftbyear

Could the colder weather require a choke adjustment? How does it run when fully warmed up?

olds 307 and 403

Take the hose off the choke pull off, manually push in the diaphragm and tightly hold your finger on the vacuum nipple opening. Hold the finger on the nipple and release the plunger, if it stays put, it is good. If it moves back, it is leaking. It may also need the choke plate opening more at start up, more than likely the issue.

droldsmorland

Check for vacuum leaks everywhere even the power brake booster, its check valve and the PCV system distributor advance canister etc.... Take a vacuum reading from the throttle base or intake once the car reaches OP temp. Adjust the A/F screws to achieve the highest vac reading possible.Then check timing both with/without vacuum advance hooked up. Volley between A/F and timing adjustments to achieve the highest possible vac reading. Read the spark plugs, what do they say? Road test. Spark knock back off a few degrees on timing. Always run 91+ non ethanol gas. Still have Op temp rough idle? Look at the float level and the PCV valve. The PCV needs to be for your application not a fits-all. Today's excuse for gas needs a slightly lower float level to prevent burble over (leave float drop setting alone). Your plugs will tell you if the level is too high.

oldsonharmont

Really great advice here, thanks. The rough idle and chug persist even after the engine is warmed up., and the choke pull off seems to work okay. I plan to read the plugs this weekend and look closer for vacuum leaks. I still suspect the carb since it sat for 10 years prior to the rebuild, and would occasionally surge during low speed driving. Engine oil and antifreeze are also clean.

Fun71

Unfortunately, all gasoline in the Phoenix metro area is 10% ethanol due to EPA mandates, and 91 is the highest we have, unless you live near a race track where some pumps have 101 unleaded.

droldsmorland

Dont forget points and dwell (if equipped). I've seen a bad dwell angel (point gap) cause this as well.

shiftbyear

Do you see any fuel dripping out of the primary nozzles at idle? You may need a flashlight to see it. If so, could be too much fuel pressure or dirt under the needle seat in the carb. Moisture in the distributor cap can cause crossfiring also. Good luck.

oldsonharmont

I checked the plugs tonight and so far they look good (light tan). I think droldsmoreland might be on to something with his PCV suggestion. The L.H. valve cover seems to have the wrong PCV set up according to the factory service manual diagram, but I've found 2 PCV valves for my application, one with nipples on the top and bottom, and the other with a nipple on top and a flat bottom which looks like it will fit my grommet. I can see where a poor fit here would cause a big vacuum leak, so it's looking like I should use the flat bottom PCV valve.

oldcutlass

Yes, use the flat bottom.

droldsmorland

Your plugs should show lean with a bad PCV. But not always. Disconnect PCV and cap its vac source off when the rough condition occurs see if that follows the problem.
Did you verify your distributor dwell and carb choke settings?
Before you go off into the weeds lets get some data. If isolating the PCV produced zero change move on to the next simplest step(s). Verify the basic specs....timing, vac reading at idle cold and warm, Optimized A/F mixture with a vac gauge, correct dwell angle, condition of point set, cap, rotor, wires etc...Dont start replacing parts at random let the data lead you to the root cause.

droldsmorland

Yes a poor fit where the vacuum line hooks between the carb and the PCV will indeed cause a vacuum leak. However, a loose fit into the rocker cover grommet will likely only produce excessive blow-by indicated by increased oil leaks/weeps and excessive vapors coming out of both rocker covers. This lose fit into the grommet should not effect idle quality unless the PCV itself is faulty, eg stuck open or has an incorrect spring rate for the application. A 4 or 6 cylinder PCV may fit into the grommet but the application is wrong and can effect idle stability due to incorrect spring/valve rating. Adjustable PCVs are on the market. Expensive but 100% t unable if you can not locate an Olds 350 or 455 PCV.

Again the quickest way to tell if the PCV is a problem is to cap off its source and see if it changes anything

Just thought Id clarify that a loose fit in the grommet is different than a source vac leak between carb/intake and PCV.

oldsonharmont

I went through the plugs and vacuum lines last night, and found the PCV wasn't fully seated in the grommet in the valve cover. It probably didn't help that I reused the old PCV valve prior to the engine rebuild. I'll pick up a new one today and see if I can get enough of an idle to put a dwell meter on it and check the other things. The only thing that changed from when it ran better a few weeks ago was pulling the compressor and a/c hoses to be rebuilt. I'm thinking either a vacuum hose got knocked loose, or the carb issues are getting worse. I'll stil check the items you recommended here, thanks.

droldsmorland

Recently installed carb may need a re-torque?

oldsonharmont

Everything else gets checked this weekend, otherwise it's back to the shop where the engine was installed.

oldsonharmont

Glad to report the newly rebuilt 455 is purring like a kitten again. It turned out the distributor wasn't quite snug and the timing had slipped a bit. I also had some expert trouble shooting from my engine builder who suspected either a bad condenser and/or burnt points. Turns out he was right on both counts, so I installed a quality set of points and a new condenser and it's running great after a few adjustments. Now if I could only clone all the knowledge these older guys have I'd be set for life.

droldsmorland

If more performance intrigues you read on:
Even a willing young buck can tackle an old-school problem with basic troubleshooting 101, always start with the simple and work towards the complex. The old school or analog stuff is so simple compared to today's digital age, from .my point as I grew up with it. A well-versed youngster or an up-to-date oldster well equipped with digital tuning tools would think the opposite. Sometimes I do too. An electronic distributor with 2 of the 3 timing events adjusted by turning a small pot is the hot ticket IMO. Bottom line is anyone with the ability or desire to explore basic tbl shooting 101 can handle any problem old or new, automotive or not.

Glad its fixed. My bet is you could wring even more performance out of it with some detailed super tunning. A dialed-in fuel and spark curve will yield real-world improvements including gas mileage.

You will need to look at your points every 5-10K to keep it running at optimal performance. Buy a tach/dwell meter. Corvette Blue Streak points and condenser are the only way to go. Verify all engine grounds(and others) are clean and tight.

You can super-tune yourself with a vacuum gauge, adjustable timing light and a few distributor parts(weights, stops, springs and an adjustable vac advance can). An additional 5-15 smoother horses (or more) is not unheard of with a custom advance spark and fuel curve.
Step up to some good ignition parts. Brass tower dist cap and rotor, high-quality wires and Autolite plugs, get crazy with the plugs and cut the electrodes back.

oldsonharmont

It was amazing to watch the 'old pro' at work, re-clocking the distributor, shifting the plug wires one direction or the other, looking at the color and length of spark from the coil. He didn't even check the factory manual, just dialed it in a half-turn here, and bump the distributor there. As a former farm mechanic turned drag racer then speed shop owner and finally an engine builder he's probably dialed in thousands of engines in his lifetime. I can honestly say I've never heard my engine run so smooth, and start on the first crank. I usually make a practice of buying 'the good stuff' for my car, but was surprised the points in a re-manufactured distributor didn't even last 500 miles.

droldsmorland

Well I dont want to keep you up at night, just want to pass along some experience:
Thats the difference between the offshore chineasium junk and high quality USA made parts. Do it once for a little more or do it cheaply repeatedly until cost actually exceeds the cost of the quality unit. Live n learn.
I'd pull that distributor and check shaft, bushings, cam gear and roll pin. Or just step up to a USA made high quality unit either HEI or points.A quality re-manufactured unit would have come with a good point set. That speaks to the overall quality to me.
At minimum keep an eye on it by periodically checking for excessive side to side play by wiggling the rotor. Id still be leery of the shaft gear, especially in a higher perf application. I've seen the junk eat a good cam mating (drive) gear.
Cost of a new high qual dist verse another cam????