| mikemerc | -- 01-02-2020 @ 12:20 PM |
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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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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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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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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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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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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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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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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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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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This message was edited by 56MarkII on 4-13-20 @ 10:29 AM
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| mikemerc | -- 04-15-2020 @ 8:47 AM |
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Hey guys ,back again. With all the things I’ve checked I’m thinking the carburetor might be my problem. Maybe a clogged fuel passage or I’ve heard it may be a power valve? Any ideas on where to get a rebuild kit ? I probably should have mentioned this before, this problem happens when engine is hot or cold,it will rev right up but breaks up when you try to hold the rpms steady. Thanks for any and all help mike.
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| ford38v8 | -- 04-15-2020 @ 9:55 AM |
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Mike, What hasn't been addressed by anyone so far is the flex line from firewall to pump. They tend to collapse internally, causing a restriction that impedes flow, sometimes just enough to cause a miss like yours. Bob Shewman has fuel resistant flex lines: 610-993-6637 Alan
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| mikemerc | -- 04-15-2020 @ 1:00 PM |
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Alan, I did replace that hose before checking the fuel pump pressure and volume.thank you for your input. Mike
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| TomO | -- 04-16-2020 @ 7:47 PM |
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Daytona parts has a kit for your carburetor. That is not an easy carb to rebuild. Be very careful to tighten the screws just enough to close the lock washer and alternate sides when doing this. You do not want to warp the dicast. Set your choke to the rich side and see if that helps. If it does, you have a fuel delivery problem. Watch for fuel dripping from the upper chamber, if you see some the carb needs rebuilding or replacement. Check your dwell at all speeds it should not vary more than 1 degree. If it does, the bushings in the distributor will have to be replaced. Do a cylinder balance test at about 15oo rpm. Short out each plug and watch for a rpm drop. All cylinders should give the same rpm drop. If one or more cylinders have a smaller drop, check the plugs and compression. Dribble some MMO down the carb, you may have sticky valves. Vacuum gauge readings would also help diagnose the problem. Tom
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| mikemerc | -- 04-17-2020 @ 6:28 AM |
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TomO , Thanks for the info . Guess I’ll start with the easiest.gettting some mystery oil today. Thanks mike
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| Barney | -- 04-17-2020 @ 7:44 AM |
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Hi Mike, Here's my 2-cents' worth (maybe only a penny's worth). Let's assume the stumbling starts to rear it's ugly head at 2500 RPM. I certainly don't disagree that it appears to be a fuel starvation issue, or an ignition issue. But I would ask: Does this issue occur during a no-load condition the same as it does during a heavy-load condition? Obviously, significantly more fuel is required to maintain 2500 RPM at heavy-load vs. no-load. If the engine starts to stumble at 2500 under either of the aforementioned conditions I'd say it's safe to declare fuel is not the issue. Of course, it could still be an ignition issue. And I know you don't want to hear this, and I hope I'm wrong, but have you thought that you may have a couple, or a few, weak valve springs? Good luck. Barney PS Has your machine been tested for the C-19 virus?
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| mikemerc | -- 04-17-2020 @ 10:15 AM |
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O.K. Turns out that I hadn’t looked down the throats of the carb deep enough to see that a good amount of gas was dribbling down into the engine with the throttle at 1/4 open .More throttle Above 1/4 the worse it ran. I’m looking into a rebuild kit from Daytona parts. Thanks to all you guys who helped out .Thanks to TomO Who gave me the idea to check a little harder for the drips down into the carb. Feel like a dope for not doing it a while ago. Does the rebuild require special tools or can you get away with regular tools .the manual makes it look pretty intense. Thanks to all ,hope I can return the favor one day. Mike
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| mikemerc | -- 04-17-2020 @ 11:15 AM |
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Barney, It doesn’t affect any different from being in park at high revs or out on the road at 40-60 mph. It has been mentioned about weak valve springs ,but a buddy did a few checks with a vacuum gauge and didn’t see anything during his test. I think I found the problem,it’s in the carb . Too much gas. Gas is dripping gas down into the throats when throttle above an idle and on up. And yes , I tried to get it tested for the virus but they couldn’t get those Q tips past the air breather!
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| mikemerc | -- 04-17-2020 @ 11:31 AM |
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Any ideas on why gas would drip down from upper bowl or possibly out of the nozzle tubes that are connected to throttle plates. It looks more like out of those tubes than up above. It appears that it doesn’t drip at an idle,and it runs really good at idle.
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| carcrazy | -- 04-17-2020 @ 1:52 PM |
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Fuel is supposed to flow into the air stream as it passes through the venturi region of a carburetor. This is the basic principle upon which all carburetion systems work. With carburetors that have a fixed venturi there are several systems that work together to provide the correct amount of fuel for each operating condition. The idle system provides the correct air/fuel (A/F) ratio for idle speed. The transition system provides the correct A/F ratio for speeds between idle and about 1,500 RPM when the main system takes over and fuel begins to flow through the venturis. It is normal for you to see fuel flowing from the venturis once the engine speed gets to 1,500 RPM or so. To solve your problem, look into the ignition system. An old rule of thumb when diagnosing engine running problems on a spark ignition engine is that 10% of the time its a fuel problem and 90% of the time it is an ignition problem. I suspect that you have something causing weak spark at the higher engine speeds. The dwell may be incorrect at the speed where the problem occurs or the contact breaker points spring tension may not be sufficient allowing point bounce or other problems.
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| mikemerc | -- 04-17-2020 @ 2:37 PM |
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Thanks carcrazy,I’ll give that a try Mike
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| TomO | -- 04-18-2020 @ 8:14 AM |
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I should have been more specific about the fuel dripping. These carburetors sometimes develop a leak from the fuel bowl due to cracks in the bowl, look for fuel on the bottom of the bowl. The floats on the teapot are hard to set and you could have fuel dripping from the top cover. Fuel coming out of the discharge tubes at engine speed above idle are normal, this is the main fuel supply. Check your spark plugs against this chart to determine if you have a fuel problem. http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm The stumble could be caused by the distributor advance. Disconnect the and plug the vacuum hose to the vacuum advance and check the timing. If initial timing is correct, slowly increase the engine while watching the timing, it should advance evenly as you increase the engine speed. If it passes this test repeat the advance test with the vacuum advance connected. Did you do the MMO procedure? dribble some down with the engine at a fast idle until the engine smooths out, then dump enough to kill the engine. Let it sit overnight and then check to see if it is better. Tom
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| mikemerc | -- 04-19-2020 @ 6:24 AM |
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Tom When looking down the carb ,the gas is not dripping it’s much much more than a drip. Not in a mist by a long shot. I will check the distributor you and carcrazy have suggested. Also the mystery oil. Thanks mike
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| TomO | -- 04-19-2020 @ 7:52 AM |
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Mike, I'll start up my Lincoln today and refresh my memory about the flow of fuel. The carb on it is similar to your Merc carb, just a lot more complicated. It will not be a mist until it passes the venturi and is mixed with air. Here is a link to the Holley 1901 manual for the carb on your car. http://www.tocmp.com/manuals/Carbs/Holley/H1901-1952/MCarbHoll19011952.html Tom
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| 37RAGTOPMAN | -- 04-19-2020 @ 10:42 AM |
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HI was wondering how it ran before you Tuned it up,??? I suspect the condenser . try the old one,or a made in USA. and see if that make a difference, I did the same thing and when on the road and applied the throttle it started to miss BAD found out the new Chinese TAIWAN made condenser was at fault after going over the fuel system it had maybe 20 miles on it, so who would suspect it, I replace the condenser with a NOS Delco, on my 57CHEV, LET HEAR how you made OUT,,,,,,,, 37Ragtopman
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| mikemerc | -- 04-19-2020 @ 5:09 PM |
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37ragtopman I have checked changed , and put back condenser it ran the same. Thanks mike
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| TomO | -- 04-20-2020 @ 4:28 PM |
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Mike, I looked at the flow of fuel from the main supply on my Lincoln. At idle there is no flow to the main body from the main jets. When I raise the rpm, there is a flow that is not a stream but more like when you hold your finger over the end of a hose. Fuel comes out of the accelerator pump jets when changing speed. How old is the fuel in your tank? Does it have ethanol in it? Did you add a stabilizer to the fuel when it was fresh? Tom
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| mikemerc | -- 04-21-2020 @ 3:26 PM |
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Tom The fuel is4months old and it was stabilized when I topped it off. When I give more throttle above an idle the gas comes down out of the nozzle tubes,which are connected to the center of the choke plates. When it comes out it is in irregular size droplets. I ordered a rebuild kit ,I’m hoping it will do the trick. If not I guess I’ll pull the distributor and send it to someone to rebuild it again. Mike
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| TomO | -- 04-22-2020 @ 7:48 AM |
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That sound pretty normal. Unless your plugs are covered with a black soot, I don't think that your problem is the carburetor. Be aware that if you fix something that is not broken, you could introduce more problems. Did you do the cylinder balance test of shorting out the plugs one at a time and watch for a rpm drop? Tom
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| mikemerc | -- 04-22-2020 @ 5:53 PM |
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Tom Haven’t done that yet ,guess it couldn’t hurt. I’ll try tomorrow Mike
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| mikemerc | -- 04-22-2020 @ 6:03 PM |
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I tried the mmo ,didn’t work. Mike
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| mikemerc | -- 05-01-2020 @ 7:07 AM |
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Hey guys ,between not having the right test equipment, not being a mechanic, and slowly losing my mind I’m going to find someone on Long Island who can work on my car . I want to say thanks to all who tried to help me. Your a good bunch of guys. Mike
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| TomO | -- 05-01-2020 @ 9:06 AM |
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Mike, you did a good job trying to fix it without having the tools to do it. Please let us know what the fix was, I am very curious. Tom
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| sarahcecelia | -- 05-12-2020 @ 9:45 AM |
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Sounds like maybe the acceleration tubes (2) may be clogged! With the motor stopped; Look into the carb and pull the throttle by hand. You should see (2) nice streams flowing STRAIGHT down the barrels. If they "wander" or don't produce a nice stream- their dirty!! Another cause can be a bad accelerator pump in the carb! It feeds those (2) tubes extra fuel on acceleration!!!!!( because at that point, the vacuum drops, so it needs fuel that that low vacuum can't provide!) Sometimes they can cause a "Flat spot" when you rev the motor; when the car is running and standing still in neutral, and you rev it, and that's why! If that's all good, it is very likely ignition; likely culprit- spark plug high tension wires. Regards, Steve Lee
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| stanzuser | -- 09-01-2020 @ 3:41 PM |
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A float adjustment might help also. A float that is out of adjustment could let enough fuel at low RPMs, but starve the the engine at higher RPMs. You might check that before you rebuild the carb just to answer the question.
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