Diagnosis round 2

by wunderlich_33 Fri Apr 24, 2015 6:50 pm

So it breaks my heart to say it but you will probably see a few of these topics from me.
Its a little discouraging to have so many issues with the car and so few mechanics who know whats going on.

my previous post stated a lag in acceleration, which turned out to be a plugged pcv valve and some bad timing. both of these issues were corrected and the car ran great for awhile.

the new problem: similar to the old, when the car is "cold " and you put your foot to the floor the car tries to die but only till the car is very warm.
then while your driving and she hasnt warmed up completely when you go to accelerate there is stil a minor lag but only till you "double tap" the gas then it goes away for a bit.

best way i can describe it is that its reminds me of when your trying to get a steady flow from something but theres a bubble/s in the way and then once its gone it flows fine.

Also after starting and waiting for the engine revs to kick down when you go from park to reverse or drive she dies on you unless you give her a good shot of gas to keep it going. but this cant be a problem with the idle as its fairly high when it first starts up.

please help if you can. not being overly proficient in the specific mechnics of cars its very frustrating.
but my thoughts so far are it could be the timing still, it could be the distributor, it could be a misfiring/burnt out spark plug from before, could it be a bent push rod?

by ant7377 Fri Apr 24, 2015 9:10 pm

Do you have a Holley carb on this car? It sounds like your accelerator pump is out of adjustment or if you have a quadrajet you might need a new accelerator pump cup. You have to be patient with these old cars you never know who had their hands in it before you. Also many "young" mechanics simply dont know how to work on them.

by ant7377 Fri Apr 24, 2015 9:28 pm

Heres something to hrlp you if you have a holley

http://forums.holley.com/entry.php?428-How-To-Cure-Carburetor-Stumble

Do you go to a lot of local car shows? If so there may be someone another car guy right in your neighborhood that would help you if you cant find a good mechanic in your town

by bigredlaguna Sat Apr 25, 2015 1:56 am

Sounds like you need to adjust the choke a little tighter to me. If it drives fine after it is completely warmed up, then this would be what I would adjust first. If it stumbles on throttle tip-in all the time, then it may be the accelerator pump.

It could even be a combination of the two. The choke adjustment is easiest to do, so start there.

by pila Sat Apr 25, 2015 6:42 am

I seem to read more that usual about lag in acceleration these days. If you have a Q-jet & need any parts, I've been using this place for rebuild kits and odds & ends.
You mentioned the distributor. I suppose the mechanical advance weights could be sticking when the distributor is cold (?) It may be worth looking at them. I've found a lot of them to be rusty after some years of not looking at them......

Just some more ideas for ya ! Smile

by switchbiker Sat Apr 25, 2015 7:47 am

possibly a vacuum leak somewhere. Check all your connections and inspect vacuum lines for cracks/splits/ect. If that doesn't reveal anything, unplug all the lines from carb/intake and plug all the nipples (even the brake booster).

Start the engine and observe and listen for leaks around the intake gaskets, carb base etc..., if you think you see a bad area mist water from a spray bottle onto it to see if it pulls it in. If everything seems fine, shut off engine and move to the next.

block the tires, hold the brake start the engine and see if you still have the in-gear stumble. If it goes away start plugging hoses back on (starting with brake booster) until the stumble comes back, then you've found the problem.

by wunderlich_33 Sat Apr 25, 2015 10:18 am

ant7377 wrote:Do you have a Holley carb on this car? It sounds like your accelerator pump is out of adjustment or if you have a quadrajet you might need a new accelerator pump cup. You have to be patient with these old cars you never know who had their hands in it before you. Also many "young" mechanics simply dont know how to work on them.

I have a brand new Holley 600 vacuum secondary.
Yeah thats what scares me, i know the hands that have been on that engine. Was built for me last summer but i didnt know the guy untill we got into it and it was all paid for. And sad to say i dont trust going back to him.

by wunderlich_33 Sat Apr 25, 2015 10:21 am

bigredlaguna wrote:Sounds like you need to adjust the choke a little tighter to me. If it drives fine after it is completely warmed up, then this would be what I would adjust first. If it stumbles on throttle tip-in all the time, then it may be the accelerator pump.

It could even be a combination of the two. The choke adjustment is easiest to do, so start there.

Seams like the easiest place to start with. the last carb i was constantly adjusting th choke cause it was garbage and was waaay to powerful for the car and not even made for a chev.

by wunderlich_33 Sat Apr 25, 2015 10:22 am

pila wrote:I seem to read more that usual about lag in acceleration these days. If you have a Q-jet & need any parts, I've been using this place for rebuild kits and odds & ends.
You mentioned the distributor. I suppose the mechanical advance weights could be sticking when the distributor is cold (?) It may be worth looking at them. I've found a lot of them to be rusty after some years of not looking at them......

Just some more ideas for ya !

Brand new msd distributor , bought last summer with the engine.

by wunderlich_33 Sat Apr 25, 2015 10:24 am

switchbiker wrote:possibly a vacuum leak somewhere. Check all your connections and inspect vacuum lines for cracks/splits/ect. If that doesn't reveal anything, unplug all the lines from carb/intake and plug all the nipples (even the brake booster).

Start the engine and observe and listen for leaks around the intake gaskets, carb base etc..., if you think you see a bad area mist water from a spray bottle onto it to see if it pulls it in. If everything seems fine, shut off engine and move to the next.

block the tires, hold the brake start the engine and see if you still have the in-gear stumble. If it goes away start plugging hoses back on (starting with brake booster) until the stumble comes back, then you've found the problem.

hell of an elimination process lol. but its definately worth a shot.
all i can do is work my way through every possible problem one at at time till i finally get this thing running right. but if its a vacuum problem hopefully it presents itself quickly.

by bigredlaguna Sat Apr 25, 2015 11:43 am

Sometimes it is best to verify the basics are good before doing anything to the carburetor. A timing problem or a vacuum leak can mimic a carburetor problem and if those two things aren't squared away, you will never get the carburetor tuned right.

However, you indicated this to be a cold engine problem and a nuisance as the engine is warming up. Does it go away when the engine is at operating temp? If so, it is simply a choke adjustment.

by wunderlich_33 Sat Apr 25, 2015 11:53 am

bigredlaguna wrote:Sometimes it is best to verify the basics are good before doing anything to the carburetor. A timing problem or a vacuum leak can mimic a carburetor problem and if those two things aren't squared away, you will never get the carburetor tuned right.

However, you indicated this to be a cold engine problem and a nuisance as the engine is warming up. Does it go away when the engine is at operating temp? If so, it is simply a choke adjustment.

Yes, once the engine is warm it works great. Put the foot down and you"ll leave a stip of rubber for 80 feet easy and turning the key and putting it in gear after coming out of a store theres no issue. But uf i go to work at the end of the day its the same thing again.
Its annoying cause it takes 10 min to get to operating temp.

The choke is set at the standard 3/32 or 1/8 gap when cold. You think it needs more air ?

by bigredlaguna Sat Apr 25, 2015 12:27 pm

If you look at the choke housing that contains the spring that opens/closes the choke plate, you should see markings that indicate <- leaner and richer ->. You want to adjust that richer until the problem goes away.

When I set up my chokes, I don't worry about measuring the gap or angle etc. I turn the spring (cold) until the flap is closed all the way, then a little more. The vacuum break will open the choke up enough for the engine to run once there is a vacuum signal.

by 77mali Sat Apr 25, 2015 12:45 pm

In addition to all the good advice already given- check your spark plugs for anything funky like oil or deterioration on the tips themselves to rule out any leaks from the valve seals or intake manifold/head gaskets, etc... Hopefully it's just a choke or maybe even just a dirty carb, in which case try & run some carb cleaner & fuel additive to see if it helps.

Diagnosis round 2 Spark_plug_test

by ant7377 Sat Apr 25, 2015 1:24 pm

If it runs fine warm it probably the choke. What kind of rpm is it at when the fast idle is on? Should be about 1100.

by wunderlich_33 Sat Apr 25, 2015 4:02 pm

The previous owner installed the tack wrong so it doesnt work.
I adjusted the choke. But mine seems to be reversed. I have to adjust it <------- toclose the choke and richen it.

The bog is much less but still present and shifting from park to in gear presents no issues.

However this could be because my chokr does not physically close all the way. Even when its rotated fully back. There seems to be something preventing it.

by bigredlaguna Sat Apr 25, 2015 5:35 pm

That is your fast idle cam stopping it. Hold the throttle open just enough to allow the linkage to move. As far as the direction to move the spring, I was relying on memory so I was wrong about the direction. Sounds like you just about got this whipped.

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