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Posted By Discussion Topic: Engine with low vacuum -- page: 1 2 3     LTim1947 02-14-2012 @ 12:07 PM reply profile send p.m. Member Posts: 11 Joined: Feb 2012 My engine was rebuilt a few years ago and the car was wrecked with only 500 miles on the rebuilt. I have gone through the whole engine and confirmed all is good. The distributor was struck in the wreck and required a new cap rotor,condenser and points - all of these are fine. Problem is the engine starts and idles just fine, however it will not accelerate. It has dual Stromburg 97 carbs Finned alum heads etc and is 12 volt. I only have 3 lbs of vacuum and suspect this is why it will not acceletate. The compresion is equal on all cylinders. I need suggestions on what I might do to regain vacuum. This message was edited by LTim1947 on 2-14-12 @ 12:15 PM Pauls39 02-14-2012 @ 2:23 PM reply profile send p.m. Member Posts: 81 Joined: Oct 2009 I am surprized that an engine with vacuum that low will even run! You likely have a leak in the distributor housing or in the line from the manifold, caused by the accident. Those aluminum housings bend and crack easily. Your mechanical advance may also be stuck which would further kill the performance. It is possible your exhaust is bent or plugged from the wreck which would also cause a low vacuum reading, but my bet would be on a leak up front on the distributor vacuum brake. Pauls39 02-14-2012 @ 2:23 PM reply profile send p.m. Member Posts: 81 Joined: Oct 2009 I am surprized that an engine with vacuum that low will even run! You likely have a leak in the distributor housing or in the line from the manifold, caused by the accident. Those aluminum housings bend and crack easily. Your mechanical advance may also be stuck which would further kill the performance. It is possible your exhaust is bent or plugged from the wreck which would also cause a low vacuum reading, but my bet would be on a leak up front on the distributor vacuum brake. supereal 02-14-2012 @ 7:42 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 If the car was hit hard enough to break the distributor, I'd suspect that the intake manifold is broken, causing the loss of vacuum. If the leak is in the vacuum line to the distributor, the engine would run well, except it would idle roughly. Fiddling with the timing won't raise the vacuum that much. Old Henry 02-15-2012 @ 7:14 AM reply profile send p.m. Senior Posts: 738 Joined: Apr 2010 My vote would be for plugged exhaust. Old Henry (The older I get, the better old looks.) supereal 02-15-2012 @ 7:57 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Another possibility is that when the front of the engine was damaged, the timing gear was damaged, causing an out of time situation.timimg gears tend to be fragile, particularly the fiber variety. Being just a tooth off would allow idling, but change valve timing enough to affect compression enough to produce the problem you report. TomO 02-15-2012 @ 8:15 AM reply profile send p.m. Senior Posts: 7253 Joined: Oct 2009 Post your compression readings. It will help to know if they are equal, but low or if they are normal. To isolate the problem, Drop the exhaust manifolds, or headers and take a reading. Then try removing the carburetors one at a time and block off the opening. Then try replacing the intake manifold with a stock manifold. Then remove the timing gear cover and examine the timing gear. We could guess all day, but it is better to try one thing at a time to isolate the problem. If a step does not make a change, restore the engine to the previous condition. Tom LTim1947 02-15-2012 @ 10:41 AM reply profile send p.m. Member Posts: 11 Joined: Feb 2012 Wow,thanks for the response. Most of your suggestions have been checked, but I still have the problem. 1. The exhaust is new, as are the headers and mufflers. The output of the exhaust is uniform, both sides. 2. The timing gear is new and aluminum and flawless. 3. Timing gear cover is without cracks or blemishes. 4. The low vacuum is measured at the base of the first carborator. I think this rules out any timing issues. 5. Compression on all cylinders is balanced and 60 lbs. 6. The intake manifold has been set on a surface plate and does not rock. It appears to be without cracks or problems. This could have a crack that I haven't detected. 7. Being that the vacuum is created at the base of the carburetor, do you think I could have a venturie problem with both carborators? I appreciate your response. LJT supereal 02-15-2012 @ 12:20 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 Was your engine ever run with the current dual carbs? Lack of vacuum and poor performance off idle can be the result of a mismatch between the CFM (cubic feet per minute) rating of the carb(s)and the requirement of the engine. In short, it means that there can be too much opening to the intake manifold. Most flatheads run best with a single carb rated at less than 300 CFM. If you put two of these on the engine, it has the effect of quadrupling the venturi area (square law) and lowering the vacuum at the intake. The engine would idle, as it is fed by the air bleeds, but off idle, the engine usually coughs and dies. You can test this situation by uncoupling the throttle of one carb. Stock flathead intake manifolds are divided, and feed four cylinders from each carb throat. Many multicarb manifolds have a "plenum" that feeds all cylinders. Setting up a multicarb installation can be very difficult. Sometimes progressive linkage can help. We prefer a four barrel carb with a CFM front venturi rating of about stock. The rear barrels don't open until the engine is wound up fast enough to produce necessary vacuum, allowing good low speed performance. If you are using a non stock cam, the installation becomes even more critical. Admittedly, multiple carbs look "cool", but unless the car is used for racing or other high speed driving, they are often a real headache. LTim1947 02-15-2012 @ 1:24 PM reply profile send p.m. Member Posts: 11 Joined: Feb 2012 Thank you your response it is along with my line of thinking. I plan to block off one carb to see if this gives new clues. This car ran well prior to the wreck, with all parts being the same I have not looked for anything but the obvious. The fan bracket waw damaged in the wreck and could have sustained damage to the threaded hole in the manifold,I havent checked that. Thanks Lee Taylor Az LJT

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