? Vacuum leak
Thread Starter
Racer
Joined: Nov 2020
Posts: 330
Likes: 112
? Vacuum leak
Chasing down poor cold start. Full choke and need to constantly blip the throttle to keep it running. If stop motor dies, pump gas a couple times and starts right up, but still needs blipping. Timing is correct , replaced pvc. With all vacuums blocked, still the same.if I raise the idle enough it will keep running. After it warms up I can reduce idle to around 900 but it is not stable. Car is new to me and acted this way when trying to load on trailer. I think I have an internal vacuum leak but want to eliminate everything before taking intake off. After engine is warm it will start right up. Carb is fresh rebuild, 600 Holley manual choke. When carb was off bottom of manifold was wet with either oil or carbon. Idle circuit on left bank is not very responsive. Is there a diagnostic? If intake manifold gasket will there be whiteness marks?
Team Owner
Joined: Feb 2003
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From: Sitting in his Nowhere land Hanover Pa
2024 C2 of the Year Finalist - Unmodified
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use a UN-lit propane torch and move it around the intake to head area and carb base and wait for the motor to change idle
Race Director
Joined: Apr 2005
Posts: 11,290
Likes: 2,148
From: Bowling Green, Kentucky
I think you have a choke issue. I would pay close attention to how manual choke is setup and controlled with corresponding engine RPM. Is heat crossover passage in intake blocked?? This is often the reason for installing a manual choke.
Also put vacuum gage on engine and see what this can tell you. The various service manuals and u-tube videos discuss how to use gage for diagnostics if you need help/advice.
Not everyone rebuilds carbs correctly. You may have an issue remaining also from the rebuild.
Larry
Also put vacuum gage on engine and see what this can tell you. The various service manuals and u-tube videos discuss how to use gage for diagnostics if you need help/advice.
Not everyone rebuilds carbs correctly. You may have an issue remaining also from the rebuild.
Larry
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,201
Likes: 4,295
From: US-PNW
Chasing down poor cold start. Full choke and need to constantly blip the throttle to keep it running. If stop motor dies, pump gas a couple times and starts right up, but still needs blipping. Timing is correct , replaced pvc. With all vacuums blocked, still the same.if I raise the idle enough it will keep running. After it warms up I can reduce idle to around 900 but it is not stable. Car is new to me and acted this way when trying to load on trailer. I think I have an internal vacuum leak but want to eliminate everything before taking intake off. After engine is warm it will start right up. Carb is fresh rebuild, 600 Holley manual choke. When carb was off bottom of manifold was wet with either oil or carbon. Idle circuit on left bank is not very responsive. Is there a diagnostic? If intake manifold gasket will there be whiteness marks?
An oily film probably indicates blow-by sucked out of the crankcase by the PCV system. The carbon is likely from the exhaust cross-over passage in the intake.
A non-responsive idle circuit is something you should diagnose and fix before anything else because it's straight up not performing as it should. Additionally, 900 rpm at idle is pretty high... what engine is this (small block, big block, modified, & which HP option if known)? My '67 idles at ~600 rpm.
A couple pics without the air cleaner may help folks spot something.
Thread Starter
Racer
Joined: Nov 2020
Posts: 330
Likes: 112
Car is new to me. 327 dressed as 350hp Holley 600 double pumper with manual choke.went back and forth with Lars to get timing straight. It had a 750 cam carb that I thought was causing issues and put a 600 I had on it. It has a solid lifter roller cam that has about 11” vacuum at 900 with noticeable lope. Only has fiber gasket between manifold and carb. Choke cable is set to raise idle speed a little before closing butterfly. When fully pulled in accelerator pump gives a shot.weather here is supposed to break tomorrow. I can put a carb from another car and try (327/300). Oh and when I suspected the pvc I replaced it with a Wagner adjustable pvc. Same issues arg
Drifting
Joined: Jun 2006
Posts: 1,608
Likes: 182
From: Durham ON
" It has a solid lifter roller cam"
What are the cam specs?
Is this a street car or a race car?
I would not use a double pump carb on the street car. I always use a single pump, vaccum secondary carb. It's easyer to tune for street driven car.
What are the cam specs?
Is this a street car or a race car?
I would not use a double pump carb on the street car. I always use a single pump, vaccum secondary carb. It's easyer to tune for street driven car.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,201
Likes: 4,295
From: US-PNW
Car is new to me. 327 dressed as 350hp Holley 600 double pumper with manual choke.went back and forth with Lars to get timing straight. It had a 750 cam carb that I thought was causing issues and put a 600 I had on it. It has a solid lifter roller cam that has about 11” vacuum at 900 with noticeable lope. Only has fiber gasket between manifold and carb. Choke cable is set to raise idle speed a little before closing butterfly. When fully pulled in accelerator pump gives a shot.weather here is supposed to break tomorrow. I can put a carb from another car and try (327/300). Oh and when I suspected the pvc I replaced it with a Wagner adjustable pvc. Same issues arg
The cam detail changes my comment about idle speed -- 900 may be as low as you can go depending on the lobe profiles. Hard to tell. If you raise it to 1000 or 1100, does it have a stable idle when fully warmed up?
That the choke cable raises "idle speed a little before closing butterfly" begs further questions: When the choke is fully closed, what is the fast idle speed? I'm thinking it should be about 1500 rpm. How long are you allowing it to run off of full choke before opening it up a bit? How long until you open it fully? What is your measure to determine when to open the choke?
Are the mixture screws "balanced" so they are turned out equally?
Thread Starter
Racer
Joined: Nov 2020
Posts: 330
Likes: 112
Car has the came from PO, specs are unknown. I can measure lift with a dial indicator,but will need degree wheel for duration.. transfer slots showing .040. Street car not race car. It doesn’t have stable idle cold or normal temperature the mixture screws are balanced but drivers side has no effect. I will try a 600 cam vac secondaries tomorrow. Thanks for all the help
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Race Director
Joined: Apr 2005
Posts: 11,290
Likes: 2,148
From: Bowling Green, Kentucky
Do you have intake manifold heat crossover passages blocked or open (as factory OEM)?? Measure this crossover area temperature with heat gun or by hand (carefully as can be either warm or very very hot). This can make a BIG difference in engine warm-up. See my Post #3.
Larry
Larry
Thread Starter
Racer
Joined: Nov 2020
Posts: 330
Likes: 112
Well it would take a novel to pass on all the info. And I flunked English in high school! I am not sure about cross over passage. 3 different carbs acted the same way. More info, replaced gas tank and sending unit. Tank had 1/4” rust and sediment, sides had rust. The sock on the sending unit was split. I blew out rear to front steel line, replaced in-line filter with a clear one to monitor any more contaminates. Next was carb list number says it’s a 750 cfm ( way too big) took bowls off and they had 1/2” reddish brown sediment. I cut the filter housing and filter wasFULL of sediment. Put a rebuilt 600cfm carb on and still same symptoms. Nay one thing left, fuel pump! It has a Carter m6900, took it off and apart. Rust and sediment around the check valves. New pump is on its way! Thanks all Will let you know. Chuck
Drifting
Joined: Jun 2006
Posts: 1,608
Likes: 182
From: Durham ON
It's hard to get the dirt out of the fuel lines. After changing out my gas tank, blowing out the gas lines and taking my carb apart to clean it. I had to put on 800 miles with many in line and carb filter changes before my dirt problem went away.