| tubetype | -- 09-30-2015 @ 11:44 AM |
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starts and idles good. starts missing at 35 in 2nd gear. Plugs and plug wires good. Carb rebuilt since this problem developed. Vacuume steady at 18. cylinder pressures - 1,2,3,4, 90psi. 5,6,7,8, 60PSI. does this sound like a blown head gasket or crack in block or head?? Comments please. BUD
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| MG | -- 09-30-2015 @ 12:15 PM |
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@Tube - Next time this happens, pull over to the side of the road and check to see if your carburetor is overflowing....
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| ken ct. | -- 09-30-2015 @ 1:16 PM |
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Compression kinda low on drivers side. ormally you would not get all cyl on either side to read all the same ? all 90 and all 60.Dont think your carb is at fault. Got to be something else.Did you take compression test the correct way??? ken ct.
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| len47merc | -- 09-30-2015 @ 2:36 PM |
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Side-to-side compression variance and consistency across each respective side does seem odd. Assume no water in the oil. Suggest checking the torque of all head nuts to spec to eliminate or validate one of the obvious potential issues for starters. Steve This message was edited by len47merc on 9-30-15 @ 2:37 PM
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| carcrazy | -- 09-30-2015 @ 2:39 PM |
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Look for a problem in the fuel supply system. Missing at higher speeds and/or loads can indicate a weak or marginal fuel pump. You may have a partially plugged fuel line or plugged fuel tank vent.
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| ken ct. | -- 09-30-2015 @ 3:55 PM |
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A fuel supply problem would not cause low compression on 1 side of the engine which is not very good could be valves ?? ken ct.
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| len47merc | -- 09-30-2015 @ 4:35 PM |
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To ken's point, even if some vermin built a nest or a rag was dropped in one side of the carb/manifold and created a partial blockage it would only affect 2 of the 4 cylinders on each side of the motor. While the engine missing @ rpm is agreed to be a potential air/fuel delivery mix concern carcrazy (&/or an electrical coil/dist/wire(s) breaking down) why all driver's side cylinders are low on compression would be my first concern and order of business. During pursuit of that problem the root cause of the missing-at-rpm may be revealed. If not, at least one serious issue can be addressed and then a standard review of plugs, spark, fuel delivery, etc., can be undertaken. Perhaps I am missing something more obvious but 60 psi across one full side is so far from spec and standard that the motor must be out of balance. Have personally never seen or heard of such a variance. I'm curious here and am all ears nonetheless - someone give us the smoking gun... Steve This message was edited by len47merc on 9-30-15 @ 4:36 PM
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| tubetype | -- 09-30-2015 @ 6:39 PM |
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forgot to say that I have 5 lbs fuel pump pressure. have blown down all fuel lines. fuel tank vent working OK. 5/6 lbs pressure varies on left and right bank. At idle and slight above idle engine is rock steady, vacuume reading also very steady at 18 inches. will check torque of heads later. Thanks you all for you comments. ?? would a leak in intake manifold cause one side to give less cyl pressure.?? BUD
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| carcrazy | -- 09-30-2015 @ 7:35 PM |
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What is the history of the engine? Was it recently rebuilt or assembled? Does it have comparable cylinder heads on both banks? Is the left hand exhaust manifold and/or crossover pipe clear of debris? The difference in compression from side to side could be caused by one of these conditions.
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| cliftford | -- 09-30-2015 @ 8:46 PM |
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With the high steady vacuum reading at idle, I too would question the accuracy of your compression test. Is your guage in good condition? Did the engine crank over at about the same speed for all cylinders? As stated it is highly unlikely that you would get the same reading on all 4 cylinders in one bank.
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| MG | -- 09-30-2015 @ 9:09 PM |
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@tube - You said: "forgot to say that I have 5 lbs fuel pump pressure" - I think 5 lbs. is too much pressure. Should be about 3 lbs. or less, I think.... This message was edited by MG on 9-30-15 @ 9:09 PM
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| ken ct. | -- 09-30-2015 @ 9:41 PM |
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Thanks MG I was going to say that's about twice the recommended pressure. Its a wonder the carbs not pukeing it out all over the place.Way too much pressure. ken ct. Is this an electric pump or one of those "new" China made mech. pumps sold by the big box boys. Have too heavy a spring under the diaphragm causing high pressure And a diaphragm made out of recycled condoms or some other garbage and will deteriate shortly with this corn gas. ken ct. This message was edited by ken ct. on 9-30-15 @ 9:49 PM
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| len47merc | -- 10-01-2015 @ 3:33 AM |
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Assuming the compression test was performed correctly (likely as it would be extraordinarily coincidental to get it 'wrong' on 4 adjacent cylinders), and the head nut torque is within spec, and no coolant is in the oil, then carcrazy hit it for me - the heads must be different.. Someone must've changed one in the past and installed the wrong one, OR - ??? - perhaps the wrong head gasket thickness was used on one side (doubtful but possible). Either one of these or the driver's side exhaust is partialy clogged by debris (heat riser baffle, if stiill there stuck closed -?) What markings are on each head? Doubtful a manifold leak would affect only 4 cylinders and/or affect them so consistently. Could cause a miss at rpm though. The fuel pressure is definitely too high. Steve
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| tubetype | -- 10-01-2015 @ 10:59 AM |
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lets see if I can answer ken and car crazy . pressure taken with hand held tester. all plugs in except cly under test. allowed pressure TDC 4 times with starter turning engine over. cylinder tested 60 +-5 lbs on weak side and 90 +_ 5 lbs on cylinder 1-4. Elderblock heads, intake and carb. according to past owner. engine was overhauled 10 years ago. bored to 60 over. I have run this car for over a year and never a problem until this. same oil for past year and still full and clean. BUD
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| tubetype | -- 10-01-2015 @ 11:03 AM |
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lets see if I can answer ken and car crazy . pressure taken with hand held tester. all plugs in except cly under test. allowed pressure TDC 4 times with starter turning engine over. cylinder tested 60 +-5 lbs on weak side and 90 +_ 5 lbs on cylinder 1-4. Elderblock heads, intake and carb. according to past owner. engine was overhauled 10 years ago. bored to 60 over. I have run this car for over a year and never a problem until this. same oil for past year and still full and clean. BUD
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| tubetype | -- 10-01-2015 @ 11:07 AM |
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Carb is a large 4 barrel elder block (webber) 750 cfm. according to mfg this carb needs 4-5 lbs pressure. but because you raised the question I will check it again. BUD
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| len47merc | -- 10-01-2015 @ 11:53 AM |
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Well I guess the plot thickens now that we know it is not a stock flathead and has a 4 bbl Edelbrock 750 monster carb - likely a double pumper too 'eh? Plus custom heads, intake, surely ignition, ... Retract my comment about fuel pressure being too high - have no idea what that beast requires. Have to admit my paradigm previously had me wrapped all around this being a stock flathead so will bow out on this one tubetype. Good luck! Steve This message was edited by len47merc on 10-1-15 @ 1:43 PM
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| cliftford | -- 10-01-2015 @ 12:00 PM |
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I have always done compression tests with all plugs out. That's how I was taught to do it 60 years ago. You might try it and see what readings you get.
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| MG | -- 10-01-2015 @ 12:05 PM |
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What Len said....it being a "stock flathead" etc... Never mind....
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| ken ct. | -- 10-01-2015 @ 1:58 PM |
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I agree after all these posts he tells us hes got a 750 cmf carb on there , normal 4 barrel carb on a flathead is around 390 cmf. Trouble is his way over kill carb on there tooooooooo big. See the hot rod guys on the Hamb section. ken ct
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| ken ct. | -- 10-01-2015 @ 2:55 PM |
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I agree after all these posts he tells us hes got a 750 cmf carb on there , normal 4 barrel carb on a flathead is around 390 cmf. Trouble is his way over kill carb on there tooooooooo big. See the hot rod guys on the Hamb section. ken ct
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