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Discussion Topic:
52 Merc. Holley
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mikemerc |
01-02-2020 @ 12:20 PM
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Member
Posts: 23
Joined: Jul 2019
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Hey guys , I’ve been chasing a mid to high RPM multiple cylinder miss. I have done compression check,fuel pressure check ,had distributor rebuilt, new plugs, points ,condenser cap,rotor,checked plug wires with ohm meter,all had good continuity. The carb was rebuilt 10 years ago. It will idle good,it starts to stumble at about half throttle, and completely runs like cr*p anywhere above that RPM. I was thinking the carb is my problem. Also ,sprayed carb cleaner around carb base gasket throttle shaft choke shaft all around intake manifold gasket and intake bolts.cant find any vacuum leaks. Any suggestions? Pulling my hair out here! Any help greatly appreciated.
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trjford8 |
01-02-2020 @ 4:32 PM
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Senior
Posts: 4214
Joined: Oct 2009
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Sounds like it may be starving for fuel. Is there a filter in line anywhere? Have you pulled the tank sending unit and looked in the tank to see if it is clean or not?. Do you see any debris in the glass fuel bowl? The other thing to check is the vacuum advance unit on the distributor. A bad advance unit can give a similar symptoms. Was a new advance unit put on when the distributor was rebuilt?
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carcrazy |
01-02-2020 @ 7:39 PM
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Senior
Posts: 1651
Joined: Oct 2009
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What kind of secondary ignition wires are you using? If you are using radio resistance wires and they check good for resistance when they are cold, warm up the engine to operating temperature and check them again. I have had radio resistance wires that checked good cold but went to infinite resistance when up to operating temperature. As the load on the engine increases, as when accelerating or climbing a hill, the pressure in the combustion chamber increases putting more stress on the ignition system. If the resistance of the ignition wires checks out OK at operating temperature, the next thing to check out is the fuel delivery system. Your fuel pump may be putting out a marginal amount of fuel that is sufficient for low speed operation and idle but insufficient for high speed operation when the demand for fuel is greater. There could be some contaminants in the carburetor which are blocking one of both of the power valve restricters reducing the flow of fuel under high load conditions.
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TomO |
01-03-2020 @ 8:41 AM
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Senior
Posts: 7250
Joined: Oct 2009
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Your problem could be weak spark or insufficient fuel delivery. Check you spark at idle for a blue colored spark that will jump 1/2" from a plug wire to ground. The wire must be removed from the plug for this test. Causes of weak spark are: bad coil, bad or poorly grounded condenser, incorrect dwell, bad rotor or cap and bad wires. To check fuel delivery, you need a graduated 16 ounce or larger container to determine the amount of fuel delivered in a measured time. Disconnect the fuel pump line from the carburetor and direct it into a graduated container. Crank the engine long enough to get 2 ounces of gas in the container and then continue cranking for 15 more strokes of the pump. The pump should have delivered 6 more ounces of gas. Insufficient fuel delivery can be caused by a vacuum leak, plugged flex line, worn pivot in the pump arm, stretched diaphragm, plugged fuel line or tank pickup, plugged tank vent or wrong gas cap or a worn push rod. Good luck and come back here with the results of your tests.
Tom
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mikemerc |
01-03-2020 @ 8:51 AM
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Member
Posts: 23
Joined: Jul 2019
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tjrford8. Thanks for responding, there is no added on filter,fuel bowl had small amount fine debris,screen filter at carb fuel inlet pretty clean. The tank had been boiled and repaired 3 years ago. I recently drained tank found small amount fine debris.refilled with fresh gas. The vacuum advance appears new ,and appears to be working .when timing engine I hooked up vacuum line to distributor and rpms came up to almost normal idle. Thanks mike
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mikemerc |
01-03-2020 @ 8:58 AM
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Member
Posts: 23
Joined: Jul 2019
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car crazy, thanks for helping. The wires appear to be copper strand soldered at both ends of the wire. Fuel pump tested at 3to4 pounds pressure at idle and when at an rpm to make engine miss. I will check volume of pump next .thanks. Mike
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mikemerc |
01-03-2020 @ 9:09 AM
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Member
Posts: 23
Joined: Jul 2019
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TomO. Thanks for responding, I will check spark each cylinder.and check fuel pump as you explained. Thanks mike
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TomO |
01-04-2020 @ 7:32 PM
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Senior
Posts: 7250
Joined: Oct 2009
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Mike, another thing to check is the primary wire inside the distributor. The insulation could be bad and the ignition shorting out when the advance moves the point plate.
Tom
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mikemerc |
01-07-2020 @ 2:19 PM
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Member
Posts: 23
Joined: Jul 2019
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This message is to all who responded to my problem. Coil wire and all 8 plug wires show a good whiteish blue spark at about 1/2 inch from a good ground. Fuel pump delivers a total of 8 ounces of gas as per the instructions given. No kinks or obstructions in line from tank to carb.the one thing I forgot to say about my problem is that this condition happens whether engine is cold or at running temperature. I am thinking now that my problem could be in the vacuum advance and or the source of vacuum at carb, or the carb itself as mentioned in the responses to me. Going to put this mess on hold, going to where it’s warmer for a few months and tackle in the spring .thanks to all who helped out ,it meant a lot to hear from you all. Mike
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56MarkII |
04-13-2020 @ 10:25 AM
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Member
Posts: 63
Joined: Jan 2012
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This message was edited by 56MarkII on 4-13-20 @ 10:29 AM
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