Jumpy timing mark

[igasper10]

Ok... I just rebuilt and installed my original Q-Jet for my 74 L48. I got all my idle speeds set for their initial adjustment (1600 rpm fast idle, 600 rpm in drive curb idle with the screw NOT the solenoid). Its starts up great and runs ok but its only getting about 16 in. of vacuum. So I hooked up my timing light and the mark is jumping all over the place between 8 and 12 degrees! I had a Holley 4175 carb on the car while I was rebuilding the stock carb, and when I set it all up I had no problems. The timing mark was steady and I was getting 19-20 in. of vacuum. What's happened? What causes the timing mark to jump like that? Keep in mind I have moved to Colorado to an elevation of about 6000 ft. Would that have anything to do with it?

PLEASE HELP!!!

ian

 

[Vettehead Mikey]

Sounds like a worn distributor plate or points to me. Is this with the vacuum advance hooked up or not? Does it stabilize with it disconnected?

Why are you adjusting the idle speed with the screw and not the solenoid?

 

[72Mako]

Ian,
1. Do you have both idle mixture screws adjusted correctly?
2. Check for vacuum leaks.
3. You're checking the timing with dist vac advance hose disconnected and plugged.
4. Q-jet throttle shaft bores aren't hogged out from wear (common problem on hi-milage carbs).
Get back with us, I'm sure with more feed back the members will zero in on your problem. Charles

 

[igasper10]

Sounds like a worn distributor plate or points to me. Is this with the vacuum advance hooked up or not? Does it stabilize with it disconnected?

Why are you adjusting the idle speed with the screw and not the solenoid?

Vacuum advance is disconnected...

How do you adjust the solenoid? If its supposed to thread in or out like the screw then I have a bad solenoid. It won't turn at all. It is being energized though.

I started with my mixture screws at 1/2 a turn out. I turned them out a full 2 more times with no result or change in vacuum pressure or rpms. Its hard to adjust these when I know something else is wrong, right?

 

[wishuwerehere82]

Try pulling the distributor cap off and turning the rotor by hand. The mechanical advance will allow it to turn to the flywieght stops and the two little springs under the rotor should snap it back to the unadvanced position. If it doesn't,and feels sloppy, the springs may need to be replaced.

If the distributor shaft goes in and out, the shaft will need to be re shimmed by pulling the timing gear off and shimming to factory specs. It shouldn't screw in and out as you rotate the rotor.

 

[Vettehead Mikey]

Vacuum advance is disconnected...

How do you adjust the solenoid? If its supposed to thread in or out like the screw then I have a bad solenoid. It won't turn at all. It is being energized though.

You should be able to fit a wrench on the hex head of the solenoid plunger and rotate it counterclockwise until it touches the throttle linkage . Adjust your idle with this and not your carb screws. This won't help your timing probelm, that sounds like a bad distributor to me.

 

[cruzer99]

Check your timing chain. That's a typ sym of a bad or worn chain.

 

[vette-dude]

I feel you might have something wrong in the carb. It seems you have a hole (sucking in to much air) in the induction and the carb is trying to compensate which is speeding up and slowing down the idle and this will move the timing marks --(mechanical advance in dist). Just for starters I would put the Holley back on to see if this solves the problem and then you will know for sure if it is in the Qjet or the dist. I think the Qjet has an internal leakage problem in the vacuum operated needle positioning area. I am no expert on Qjets but I have built a few and it is real easy to miss one of the little items internally. Why I suspect this is that you can not adjust the idle mixture screws. They should either slow down the idle or speed it up and when you go too far in or out it should make the engine run rough. Just a suggestion to help you troubleshoot the problem.


Randy

 

[igasper10]

You should be able to fit a wrench on the hex head of the solenoid plunger and rotate it counterclockwise until it touches the throttle linkage . Adjust your idle with this and not your carb screws. This won't help your timing probelm, that sounds like a bad distributor to me.

What's the purpose of the screw I'm using on the throttle side then? (not the mixture screws... sorry if it sounded like I was trying to adjust the idle with them)

 

[Vettehead Mikey]

On emission carbs like ours, the purpose of the solenoid is to allow the throttle to close almost 100% when the ignition key is turned off. This cuts air and fuel flow and helps prevent engine dieseling or run-on , which the early smog motors were famous for. The manual screw is a vestige from the past and has no real purpose, but GM suggests that you disconnect the solenoid during a tune up and adjust the screw to give you an idle of IIRC around 400-500 rpm. Someone once suggested that this was done to serve as a backup if the solenoid failed but I have my doubts.