PaulJ2
Northwest Oregon
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Joined: 12/23/2005
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On a problem like this, the first thing I would do is reinstall the original jets. You need a starting point. It may be running too rich instead. This will either make it better or worse (starting point).
Increasing the jet size on a Q jet increases the fuel in the whole range of the primary throttle including the power piston lift portion.
The small tapered portion of the rods lets in much more fuel, maybe too much with the increased jet size. It may be better to reduce the rod size instead for better part throttle responce. The small tapered part is the same size on all metering rod sizes.
Used to rebuild lots of these in a tune-up shop.
Does the engine have EGR? A hanging EGR valve can also cause this.
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timmac
Las Vegas
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Joined: 03/20/2006
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Bag those carb blues and install a Holley 2D TBI fuel inject system, your mpg can also go up and burns cleaner...
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weasel4
Alberta
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Joined: 07/06/2003
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lollypop
If it is HEI Ignition the coil is in the dist cap. Take the wire retainer ring off the top and there sits the coil.
BTC
Everything in moderation, including Sobriety.
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rast
Mascotte Fl.
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Joined: 03/03/2007
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Make sure the secondary air valve (the plate in the top of the secondaries) is not trying to open too soon. It should not move till the secondary butterflies are opening. The spring tension on it is adjustable. Most of the times it is not changed during a rebuild. Just a thought due to it being rebuilt. If it is fluttering, can cause a lean condition. Usually too loose causes a bog at transition however.
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BlackMax
California
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Joined: 10/03/2005
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See if you can borrow a different ignition distributor. I fought a big block once for days that had lost magnetism in the induction pick up. It acted similar to your description of symptoms. Misfire under a specific load. If you can't borrow a distributor, take it out and rotate the shaft with your fingers. You should feel definite "notches" in the spinning from the magnets "grabbing". Don't mess with the carb yet. I've seen too many ruined by "tweeking".
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BlackMax
California
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Joined: 10/03/2005
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Hold the fort! We need some more info. Can you fill us in?
Why was the carb rebuilt?
Is the engine catalyst equipped?
What is the condition of the engine?
Plugs, wire, cap all in good shape?
Vacuum hoses connected correctly?
ERG overactive (Plug off to test)?
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bbsound
Chesapeake, VA
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Joined: 01/07/2007
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I had the same problem on a 91' turned out to be a worn out distributor (bushings)
It would hesitate on a slight increase of load ie: small hill or picking up a little speed on the interstate.
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Lollypop
San Diego
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Joined: 11/28/2007
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You guys are all being absolutely terrific with your help. Let me fill you in on some background. Initially, engine hesitated and lost power going up a slight gradient. I checked the mechanical pump on the engine and was told it was fine. After checking the electrical pump (Carter) at the tank, I noticed no noise - so I checked that pump and found it to be dead. So I replaced that with the Holley Red. This improved the problem tremendously but there was still some hesitation. So I attacked the carb next and decided to rebuild. This was not a bad idea since I noticed totally unbalanced mixture settings for the idle circuit - probably caused by smog check idiot who leaned out the one side to satisfy good ol CA emissions.
So, right now the carb has new gaskets, original jets (75), original metering rods, just installed a new vacuum break assembly as it just failed, electric choke works fine, very clean, no damage and functions nicely under zero load.
However, I have just noticed that
1) Holley pump is mounted at the same height as the top of the fuel tank. The instructions specifically state that the pump does not suck very well, needs gravity to feed and should be mounted level with the bottom of the fuel tank. Thanks to one response above alerting me to this issue. The current configuration may be contributing to a lack of fuel flow going up hill under heavy load.
2) Switching on the ignition (12V only) I hear the pump and also I can hear gurgling of fuel coming through at the carb. Since I also have the mechanical pump in series, I am told that fuel should not flow through the mechanical pump while the engine is NOT running even with 7 psi of pressure at the intake port. So, going full circle, perhaps I do have a malfunctioning mechanical pump as I originally thought. Perhaps someone can shed some light on this issue.
3) I am installing a Holley pressure regulator to reduce the pressure to 5 psi as suggested by Chevy.
To answer some questions by BlackMax
1) Carb rebuilt per above - I probably should have left it alone in hindisght
2) I am sure there is a catalytic converter on the exhaust otherwise it would fail smog
3) Engine is in excellent condition. 50K miles and always purred at idle , ran very smooth at cruise, lots of good power but tended to heat up going up the Grapevine north of LA towing 3000lb. Not extraordinary for these engines.
4) All electrical systems powering the plugs are fine
5) Vacuum hoses, where split, were changed so all fine now, correctly connected and I feel good vacuum at idle at the carb.
So, right now I am going to lower the electric fuel pump to be level with the fuel tank bottom, add the pressure regulator to 5 psi from 7, ensure fuel hoses have not collapsed or kinked, replace the mechanical fuel pump (if anyone suggests I should from the described symptoms above) and try again. I really cannot see anything else wrong with the carb, electrical power or fuel delivery system. I might also wrap reeflective heat shielding fibreglass tape around the metal fuel pipe from the mech pump to the carb inlet to minimise and fuel preheating. If none of this works, perhaps it will be time for half a doz Miller Lights and hit the drawing board again. I never give up but I am sure getting tired.
Once again, thank you all for your help. It really works !
Lolly
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427435
Rochester, Mn
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Joined: 11/12/2005
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Actually, why replace the mechanical fuel pump? Just bypass it------with the electric pump and a pressure regulator, why bother.
Mark
2000 Itasca Suncruiser 35U on a Ford chassis
2003 Ford Explorer toad with US Gear brakes,
ReadyBrute tow bar, and Demco base plate.
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Lollypop
San Diego
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Joined: 11/28/2007
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Having 2 fuel pumps can be a good idea if one fails. When the original Carter electric failed, the mechanical kept the engine and motorhome running that while severely problematic, I still had brakes, steering etc going down a 7% grade and nursed the thing back home. Very thankful.
I suspect fuel will flow through a failed mechanical pump from electric pump pressure if the diaphram tears. Also fuel will probably enter the crankcase. So that could be a good reason not to have the mechanical pump. Is this true?
Also, Can fuel under pressure flow though a good mechanical pump to the carb with the engine off?
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