Extremely rough idle.

[Custom/20]

Hi everyone, I've been trying to figure out what is wrong with my 69 c/20. I've got the stock 350 that was rebuilt and 3 days ago it was running fine and all of a sudden it started running like crap at idle. I had it idling at 750-800 and its now idling extremely rough at 500-550. It smooths out at about 1000rpm and up but you can hear a contant faint back fire out the exhaust. I've replaced the cap and rotor, coil, wires, and plugs and nothing has changed. I got a timing light and its at 6 degrees at around 800rpm. I did some math and if the timing was set at 12-13 then if I'm correct and if it jumped a tooth it would be right around 6. I just would like to get some opinions before I go diving into it to put on a new chain.

[geezer#99]

Bump the timing up to 14.
Plug off the vac pot when you set it.
Then turn down your idle down and adjust the mixture screws.

[57taskforce]

You can get a decent idea of cam timing by bring #1 up to compression tdc. Pull the distributor cap and look at the rotor. It should be pointing approximately to #1 cylinder. If it's not than you've got issues.

[Custom/20]

Originally Posted by 57taskforce

You can get a decent idea of cam timing by bring #1 up to compression tdc. Pull the distributor cap and look at the rotor. It should be pointing approximately to #1 cylinder. If it's not than you've got issues.

I know, I am planning on doing that next time I go out to the garage. Just going to take a break from if for a little bit. Was going to be putting new wheels and tires on it in a couple days so after that I'll go back at it.

[geezer#99]

Originally Posted by 57taskforce

You can get a decent idea of cam timing by bring #1 up to compression tdc. Pull the distributor cap and look at the rotor. It should be pointing approximately to #1 cylinder. If it's not than you've got issues.

That tells you nothing!
What if the balancer slipped?
And only points at #1 if your timing is zero. If it's advanced or retarded it'll be off quite a bit.

[CaptRMW]

You said you have the stock 350 that was rebuilt which in my opinion means a new chain and gears for sure. Unless you have put lots of miles on it since the rebuild you should look elsewhere for the source of the problem unless no new timing set was used. Just my $.02

[57taskforce]

Originally Posted by geezer#99

That tells you nothing!
What if the balancer slipped?
And only points at #1 if your timing is zero. If it's advanced or retarded it'll be off quite a bit.

I had one jump and I did what I mentioned and the rotor was WAY Off... worked for me guess not for you... tore it down and the plastic timing gear had come apart. Maybe I'm having a brain fart but when I brought to tdc I used a screw driver in plug hole, Not the balancer. if the balancer slipped it wouldn't have had any effect on what I was doing. Not trying to argue just stating what I did.

[geezer#99]

Did you know those uncalibrated screwdrivers can be off 20 degrees.
But it matters not!
All he needs to do is put some timing in it.

[Custom/20]

I'd like to also mention there has always been something that sounded like a rattle in the front of the block when idling above 1200 rpm

[57taskforce]

Originally Posted by geezer#99

Did you know those uncalibrated screwdrivers can be off 20 degrees.
But it matters not!
All he needs to do is put some timing in it.

Ok your right. I'm out of this one.

[LongBox]

You sure you don't just have a vacuum leak somewhere? Came on suddenly--smooths out over 1000 rpm--could be a bad vac hose or fitting, or even bad carb to manifold or manifold to head gasket.

[RichardJ]

In Nov you were working on an idea of changing your HEI to an external coil.

http://67-72chevytrucks.com/vboard/s...&highlight=hei

I suspect your problems originate from the tach and what you were doing there. Even your HEI wiring leaves a lot to be desired.

[Custom/20]

Originally Posted by RichardJ

In Nov you were working on an idea of changing your HEI to an external coil.

http://67-72chevytrucks.com/vboard/s...&highlight=hei

I suspect your problems originate from the tach and what you were doing there. Even your HEI wiring leaves a lot to be desired.

Yes at the time I thought that it was just the compatibility but when I decided to try a new one I saw on the back of the tachs circuit board that a capacitor actually fried which was the reason for the tach not responding to the signal. The new tach I got from brothers trucks works just fine. Ever since the beginning when I've started the truck up when the choke was on there's been a rattle in the front of the engine that I never thought to mention or really care about but it sounds just like a loose chain. I've checked all the hoses and especially the one for vacuum advance and they are all good. I don't lose coolant or burn oil so I'm really betting its skipped a tooth. But like I said I'm going to get opinions before I just go in and replace parts even though I have a can that needs to be replaced.

[Custom/20]

And I have find of it rodded the engine.... Like going on the freeway going 4k+ rpm a couple times

[RichardJ]

>>And I have find of it rodded the engine.... <<

In English, that would mean?

Try a stethoscope to listen at your noise. Wood dowel, rubber hose or $4 Harbor Freight.


Listen to Alternator bearings, water pump and cam chain cover. Your description of slight backfire might be exhaust leak. Listen along exhaust manifold and valve cover.

Remove distributor cap. Rotate crank until distributor rotor moves, then rotate crank in the opposite direction until rotor moves again.

[randy500]

You didn't skip a tooth unless you have a very old used up nylon timing gear in it. If its rebuilt it then the timing hasn't changed.

Push the secondary throttle closed, I bet its hung open a little bit and pouring gas in the engine (explains the slight backfire sound), also look down the primary and see if fuel is dripping from the carb venture at idle, there should be no drip at all, if its dripping then you need to close the throttle idle screw and add timing.

Add timing for sure...add at least 6 degrees and back off the idle screw.

[garyd1961]

Take the belts off/loose and start it up. If the rattle goes away it's not the timing chain.

[elkniks]

Just thinking aloud here. Did you install a new cam or lifters?

[re1]

Originally Posted by LongBox

You sure you don't just have a vacuum leak somewhere? Came on suddenly--smooths out over 1000 rpm--could be a bad vac hose or fitting, or even bad carb to manifold or manifold to head gasket.

x2

[Custom/20]

Originally Posted by elkniks

Just thinking aloud here. Did you install a new cam or lifters?

No but I was planning on doing it and since its in this situation I guess I should.

[geezer#99]

Did you bump up your timing yet??

[Custom/20]

Hello everyone. I did some more research and pulled the cap and tested how many degrees my crank would turn counter clockwise until the rotor on the distributor would start moving. It took 12 degrees of the crank untill the rotor would move. This is one sloppy chain. A chain that needs replacing should be no more than 7.

[Gromit]

Thanks for reporting back!

It is very good etiquette because as you can see lots of people are racking their brains and most of us have been there trying to chase down these gremlins..

Always appreciate seeing the eventual solution.

I was going to guess the carb hold down bolts needing to be re-torqued 'cause those gaskets are so thick and the torque is only 60 to 80 inch-pounds and they tend to loosen up a lot in the first running

Member Longbox mentioned this already I just noticed..

[dmjlambert]

Originally Posted by Custom/20

Hello everyone. I did some more research and pulled the cap and tested how many degrees my crank would turn counter clockwise until the rotor on the distributor would start moving. It took 12 degrees of the crank untill the rotor would move. This is one sloppy chain. A chain that needs replacing should be no more than 7.

It would be great to know how you figured that out. What did you read or hear about that took you there? And how do you do that test?

I've been watching this thread with interest.

You mentioned in the first post about the 350 being rebuilt 3 days ago, so this has me curious. Is this a failed new component, or missed during examination of the components, something that would have been replaced with the rebuild, other things were focused on during the rebuild, just plain bad luck, or don't know yet?

[Custom/20]

Originally Posted by dmjlambert

It would be great to know how you figured that out. What did you read or hear about that took you there? And how do you do that test?

I've been watching this thread with interest.

You mentioned in the first post about the 350 being rebuilt 3 days ago, so this has me curious. Is this a failed new component, or missed during examination of the components, something that would have been replaced with the rebuild, other things were focused on during the rebuild, just plain bad luck, or don't know yet?

Oh no lol. I should have mentioned that the engine was rebuilt a year and a half ago. It just started doing this 3 days prior to me post in this thread. I was leaving my friends place. It was warming up and when I kicked the electric choke off it just idled really bad. I found out that trick from a video by eric the car guy on YouTube. https://youtu.be/4_f5ukZVri8 Definitely saved me some frustration.