Idle Problem

[runnerguy]

Hello,

This is my first post, but have visited this forum many times. The problem with my Q-jet is when I accelerate hard enough to activate the secondaries and then slow down and come to a stop. The engine will idle @ around 1400 RPM. I can turn off the engine and back on immediately and the engine will idle normally around 750 -800 RPM. I am very stumped with this problem. I bought an SR kit for this last year. And Cliff's book. The carb is stamped 07029207. I don't know the history this carb. or the background of the carb number. I bought it off ebay last year.

This Q-jet has 68 main jets, 37B primary rods, power piston spring measures 1.030" in length with 10 coils, BG secondary rods, K hanger. Tonight I replaced the secondary air bleed tubes (the originals were bent slightly at the tips) and drilled them out to .036. I have not measured the other restrictions.

Addition background is my car is a 58 Chevy Delray, 40 over 327, with flat top pistons, 882 heads, Comp Cam 265 DEH, Edelbrock Performer RPM Q-Jet intake, 1 5/8 headers, M-20 Muncie, 3:55 open rear end, and 235-60- 15's. It also has a point ignition, Total timing is 38, 14 base, 12 Mech., 12 Vac.

I have narrowed this problem down to the carb. My 600 Holley never had this problem. I really like the performance of this quadrajet except for this problem. Any advise would be greatly appreciated.

Thanks.
Dave

[TommyK]

It sounds like either the primary or secondary butterflies may be hanging up for some reason. The next time it happens and before turning the ignition off, check to see if the there is a gap between the curb idle screw and the stop on the primary shaft. If the primaries are returning to the correct idle position push on the secondary linkage to see if the secondary butterflies are hung up.

[runnerguy]

I don't believe they are hanging up, but I'll check. Because, I can turn the engine off, then on without touching the throttle and the engine will be back to normal idle.

I am under the impression that the carb is getting excess fuel. Because last week when this happened I was able to pull out on the choke pull off, allowing me to manually open the secondary air flaps, while the engine was idling high. When I opened the air flaps, allowing more air into the engine, the RPM had dropped. So, that's when I decided yesterday to change out the secondary air bleed tubes and enlarge them as indicated in the book to .036. with no luck on this problem. The bent tubes were the only thing I can tell was wrong this this quadrajet.

Dave

[Shark Racer]

Engine vacuum can take a throttle blade that feels normal with the vacuum off and make it hang with the vacuum on.

Will the primary throttle return to completely closed via the spring action of the accelerator pump alone?

[runnerguy]

Thanks Shark racer

I will have to get the engine to do this and look at the throttle position more closely later this afternoon. When I had the carburetor apart the throttle shafts felt tight, by that I mean not loose and worn out. When the engine idles high I have stepped on the throttle quickly to see if they are stuck but it doesn't idle down only turning off and back on the engine will make it idle normally. I have also checked the vacuum advance to see if it is getting a signal and it is not, I have checked and made sure the mechanical advance weights are moving free and not sticking.

Thanks,
Dave

[Shark Racer]

Advance at idle will raise the idle 50-100 rpm at most, not 700.

Disconnect the throttle return spring and lightly pull the throttle back, maybe 25%. Can it return under the power of the accel pump spring?

[runnerguy]

OK, Took car for a spin got the carb to act up as normal (unfortunately).  I disconnected the return spring and the throttle returns to the stop on it own, but still will have a idle high same as before, no change. The throttle is hitting the stop when this occurs. I have tried to wiggle the throttles to see if they are hung up this also made no difference. Also I checked to make sure the power pistion was down and it was.

[Shark Racer]

Did you play with the secondaries as well?

Any binding at the secondary air valves?

[runnerguy]

Yes, I played with the secondarys too. Nothing appears to be binding. I also realized that I don't have to be driving the car for this to occur (or under the engine being under load). I can rap the throttle almost or near wide open just sitting in the garage for this problem to happen. Anyway it is a little baffling. Maybe Cliff can give some insight if he has time.

Dave

[Cliff Ruggles]

I would loosen the secondary throttle plate screws and gently tap them into the bores, bump the shaft side to side and put it in the middle of it's travel and tighten the screws.  The throttle plates should fully seal the bores exactly the same on both sides.

Also make sure than there is some clearance at the linkage where it attached to the secondary lever, and not hanging them open every so slightly.

We also sell new returns springs, it may be weak, they come in our HP kits......Cliff

[runnerguy]

Thanks for the advise Cliff. I will work on that in the next couple of days when I have time.  I will post again and let you know how it turned out. Your Quadrajet book is great, always learning something new every time I read through it.

Dave

[runnerguy]

I had some time today to check out the Secondary throttle plates and they were misaligned slightly. I have about a .026 clearance between the linkage and the secondary lever. After I made the adjustments, the idle quality is better, however I am still having the same problem, high idle after the secondaries are activated.

Out of a little frustration and curiosity I tried a differant set of secondary metering rods I have. With the original BG rods I get high smooth idle around 1200 rpm now. I put in a pair of CP rods and the idle is lower around 950rpm and a little radical.

At this point I am thinking I will have to take the carburetor apart and measure all the restrictions to get a better understanding of what is going on.

[Cliff Ruggles]

If secondary metering rods have any effect on idle quality at all, then the secondaries are open far enough to pull in air, and fuel from the nozzles.

Go back and make sure thay are FULLY seated.  Most likely the shaft is twisted slightly, and/or they are out of alignment, or both......Cliff

[runnerguy]

Hello, Just wanted to give an update on my idle problem, or lack of one now. After my last post I checked again to make sure secondaries were fully seated, and they appeared to be. But I did notice the the bores and throttle plates were slighty rough and not as smooth as I thought they should be. So I polished both the secondary bores and the edges of the throttle plates with 1000 grit sand paper, and realigned the plates in the bores. I have no more high idle problems now. Just want to thank all you guys for your help.

Dave

[Cliff Ruggles]

Good news!....Cliff