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SShink


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RPM drop at idle-Carb issue?
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The Chevelle is running great on the street.  But... as I'm coming to a stop the rpm drops to about 600 rpm instead of the 900 rpm I have set the idle at.  It acts like its running on 7 cylinders, then it will come back to idle right where it should. 

Blipping the throttle doesn't seem to have any effect. 

Sometimes it will stay at 600 rpm for a long time, and other times it is just a few seconds. 

Once I leave the dead stop, it runs fine. 

I double checked the idle circuit adj. screws and the rpm is at the highest I can get plus 1/4 turn out.  I checked it with a vacuum gauge, and no change.

The carb is only a year old (Edelbrock 750 cfm).

I've also checked all the plug wires at the plugs and the disty.  All seems good.

What could be causing the rpm drop?

Sticking float?

Looking for some troubleshooting tips.

Thanks!

Stan



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

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Chris R


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What type of distributor are you running? Does it have mechanical advance weights?

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Chris - Ramsey, MN.

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SShink


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Chris R wrote:

What type of distributor are you running? Does it have mechanical advance weights?



1 year old Summit Racing GM HEI disty.

I have the vacuum advance hooked up to the timed port on the carb.

It ran great last summer and didn't have this issue...

I have had the disty cap off a few times this year.  Are you thinking the weights might be binding?  I will need to check the cap again to make sure it's seated properly.



-- Edited by SShink on Wednesday 21st of July 2010 06:50:30 PM

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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
Enganeer


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Any vacuum leaks?

Try the full vacuum port and see it that make the difference also.

Here is a good write up at TC

http://www.chevelles.com/forums/showthread.php?t=189195

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John E - Rogers, MN

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SShink


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Enganeer wrote:

Any vacuum leaks?

Try the full vacuum port and see it that make the difference also.

Here is a good write up at TC

http://www.chevelles.com/forums/showthread.php?t=189195


John, there shouldn't be any vacuum leaks as everything is new, including all the vacuum hoses.  I'll double check again though. 



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
SShink


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Enganeer wrote:

Try the full vacuum port and see it that make the difference also.

Here is a good write up at TC

http://www.chevelles.com/forums/showthread.php?t=189195


John, great write up.  It sounds like I've had the vacuum advance hooked up to the wrong port since I installed the motor last summer (I used the 'timed' port since that's what the instructions said on the disty from Summit...) and it should be on the full vacuum port!

I'll switch the lines on the ports, recheck the timing, and give it a try.

And, I may need to get a different vacuum advance can since my motor only pulls a max of 13" of water at idle.  The write up says stock cans only pull at 15" or higher.

 



-- Edited by SShink on Thursday 22nd of July 2010 11:47:32 AM

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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
Bungy L-76


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Stan, If you have low vacuum from a big cam or whatever, try this vacuum canister: Echlin part number VC-1810. It's about $10 at NAPA and it works at 8" of vacuum.

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SShink


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Bungy L-76 wrote:

Stan, If you have low vacuum from a big cam or whatever, try this vacuum canister: Echlin part number VC-1810. It's about $10 at NAPA and it works at 8" of vacuum.


Steve, that's the same one mentioned in the TC article.  I'm going to stop by Napa tonight.

 



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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
Bungy L-76


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I didn't even see that above. I got that part number from 348-409.com website infact it's the same "timing and vacuum advance 101" article word for word. However after reading some of the TC posts, it looks like HEI and points have different canisters. I think that VC1810 part number is for points dist. You might want to look at the list of part numbers on that TC thread and find a different number for your HEI.

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1964 Malibu Convert
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SShink


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I think we have a winner!  John Enga!

I bought a new vacuum canister per the post that John E. gave me from TC, and when I pulled out the old one, I noticed that the mounting bracket was bent and twisted enough that the rod was rubbing against the bracket binding the linkage.  See attached pic.

I think this happened when I pulled the motor and reinstalled it this spring during the first part of the trans issues!  cry

The chain we used to pull the motor made contact with the carb linkage and now it looks like it bent the vacuum can as well.  Lesson learned is that no matter how much of a pain it might be, pull the disty when pulling a motor! It took too much abuse banging against the fire wall even though we tried to protect it...

Troubleshooting 101 should have told me to go back and look at what had changed since last year when it ran so well, and this has got to be it.  If the vacuum can linkage was binding, it would cause intermittant vacuum advance at idle, which is what was happening.

I haven't driven it yet, but it explains away the issue.  I'm driving it tomorrow to find out and will post results.

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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
4-door Chevelle


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Is it tomorrow yet? Curious to know if all is well.

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Darren - Crystal, MN
1972 4-door Chevelle driver/racer
2003 Silverado 1500HD Crew Cab
SShink


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Thanks Darren for checking.  Yes, all is well again.  It's running the best it ever has now.

I actually found another issue that may have been the main contributor...  On the passenger side of the carb throttle plate linkage, there is a spring that keeps the blades closed, and the spring had a tang that was hitting the electric choke housing in the idle position.  This would cause the throttle blades to 'hang up' just off idle and the car would lurch ahead when it moved.  A little trimming of the tang and everything was free like it was supposed to be.

Not sure why the tang was hitting other than I may have bent the linkage when the motor was pulled and reinstalled this spring.  Next time I'll take the carb off and the disty out!

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Stan S.-Twin Cities 'South Metro'

1972 Malibu Convertible 2nd time around 

1999 SS Camaro LS1-6 speed

Forum influenced terms: 'Link Paste', 'Stanitized', & 'Revolving garage door...' 

 
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