Idle issues
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Idle issues
I have a `67 435hp. After getting it out this spring from the winter sleep here in Michigan. It started and ran fine for two weeks.
Today went for a drive and started fine and warmed up as usual. I drove about 15 miles and it began to struggle at idle i.e stop signs and lights. I can feather the gas to get it back to life. Should idle around 700-750 but it drops below that and then will die if I let it. I checked vacuum at 1000rpm and its at 12" at manifold.
What are some thing I can try or check?
Mike in Michigan
Today went for a drive and started fine and warmed up as usual. I drove about 15 miles and it began to struggle at idle i.e stop signs and lights. I can feather the gas to get it back to life. Should idle around 700-750 but it drops below that and then will die if I let it. I checked vacuum at 1000rpm and its at 12" at manifold.
What are some thing I can try or check?
Mike in Michigan
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First you have to determine if it is too much fuel or not enough.
When it wants to stall try and keep the RPMS above normal idle speed, but not above 1000 RPM, and look down the throat of the carburetors and see if there is any fuel dripping from the boost venturies.
If you see not fuel dripping then using your hand close off the air entering the center carb.
If the engine RPM raises then there is either a vacuum leak or something plugging the idle jets.
Joe
When it wants to stall try and keep the RPMS above normal idle speed, but not above 1000 RPM, and look down the throat of the carburetors and see if there is any fuel dripping from the boost venturies.
If you see not fuel dripping then using your hand close off the air entering the center carb.
If the engine RPM raises then there is either a vacuum leak or something plugging the idle jets.
Joe
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If you do this be very, very careful. Keep your head to the side and back and away from the carb. If you are too close a backfire will singe your eyebrows at best and possibly do a whole lot worse.
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I have refilled gas since it started. Tank and a half has run thru it. Used 50/50 mix 110 race fuel and Mobil 93 octane. I haven't pulled the plug to see if they are fouled. I will check that next.
Seems like a fuel deliver problem at carb when rev down to idle. Above 1000 rpm it runs great.
Seems like a fuel deliver problem at carb when rev down to idle. Above 1000 rpm it runs great.
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Blow the idle passages out with carb cleaner to clear them. Better, pull the center carb off, dump the gas out through the vent and then blow all the passages out with cleaner. Check any filters for gunk.
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What condition is your fuel tank in?? I have seen problems like this pop out of the blue and traced a couple down to fine rust dust from the tank that passes through the fuel filter and in to the carb. Changed the fuel tank out and problem solved permanently. 
Pilot Dan
Pilot Dan
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Pilot Dan, gas tank is bright metal inside. I did run it down it about 1/16 of a tank prior to this happening. Maybe there was some sediment in tank that was sucked up into the carb and is now blocking idle circuit? Might have to pull center carb and go thru it. Anyone have a good procedure for this?
Mike
Mike
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The other issue besides rusty tank debris are reports of fuel lines deteriorating from ethanol fuel and line debris which clogs up fuel filters and carb circuits. I haven't experienced it yet but there seem to be plenty of people who have.
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There is a limited amount of rubber fuel line on the '67 right? I believe only a short piece at tank and another piece by fuel pump. It could make a problem I guess. The car was restored in 05-06 and sat until 2012. I think I will pull center carb and go thru it good. I found that the driver's side idle mixture screw will not go in all the way. Something is blocking it about half way in. That might be the problem.
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I believe I have narrowed down the idle issue to the vacuum advance can. Below are the readings I am getting.
With vacuum can line plugged I get a timing of 5 degrees and 14" Hg @ 800rpm
with the stock "ported" vacuum setup I get 7 degrees and 9" Hg @ 800rpm
with the vacuum advance can connected to full manifold vacuum I get 21 degrees and 17" Hg at 800rpm
It seems like the low vacuum coming from the "ported" connection is not enough to advance the timing at idle.
Has anyone else seen this condition happen?
Mike
With vacuum can line plugged I get a timing of 5 degrees and 14" Hg @ 800rpm
with the stock "ported" vacuum setup I get 7 degrees and 9" Hg @ 800rpm
with the vacuum advance can connected to full manifold vacuum I get 21 degrees and 17" Hg at 800rpm
It seems like the low vacuum coming from the "ported" connection is not enough to advance the timing at idle.
Has anyone else seen this condition happen?
Mike
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Army
That is normal. The ported or 'timed' vacuum port is a throwback to emission control days and provides no vacuum at idle - hence no timing advance. Once the throttle plates are open the vacuum kicks in and the vacuum advance works normally.
You do NOT want ported vacuum for your vacuum advance; you want FULL manifold vacuum. That is what gives you a nice smooth idle and a cooler running motor.
You do NOT want ported vacuum for your vacuum advance; you want FULL manifold vacuum. That is what gives you a nice smooth idle and a cooler running motor.
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The car was original setup with ported vacuum as GM didn't do the A.I.R. System on the '67 L-71. To be able to sell it CA they designed it to use ported vacuum.
I am looking to get the car top flight end at NCRS. I'm wondering what point deduction would be for switching it to manifold vacuum.
I am looking to get the car top flight end at NCRS. I'm wondering what point deduction would be for switching it to manifold vacuum.
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While it could be a lot of things, I would start with the carbs. I got very good at re-building them over the years as that was necessary at least once a year.
When checking for fuel leaking in while idling, check the end carbs as well.
If you haven't rebuilt the carbs lately, you are probably due.
When checking for fuel leaking in while idling, check the end carbs as well.
If you haven't rebuilt the carbs lately, you are probably due.
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Army
The car was original setup with ported vacuum as GM didn't do the A.I.R. System on the '67 L-71. To be able to sell it CA they designed it to use ported vacuum.
I am looking to get the car top flight end at NCRS. I'm wondering what point deduction would be for switching it to manifold vacuum.
I am looking to get the car top flight end at NCRS. I'm wondering what point deduction would be for switching it to manifold vacuum.
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427435, I know that it's not the carbs. It runs smooth as silk when connected to manifold vacuum. How do you have your VAC connected. I am curious to hear what or if other L-71 owners are doing to run ported vacuum. Maybe changing the vac can to one that pulls at a little vacuum?