grey beard Posted August 28, 2009 Report Share Posted August 28, 2009 My fresh engine runs and starts just fine. It idles like all of us want our engines to idle. But at cruising speed on the highway, it bucks and jerks. After checking several things, I discovered it smooths right out and runs very nice if I pull the choke out about 3/4-inch. This tells me the engine is running too lean on the main metering circuit. I thought about this a little and then disconnected the vacuum source for my PCV system. Now it runs smooth at cruise speed on the highway, just like it should. This means the small amount of vacuum hijacked by the PCV is making the carb run lean eenough to affect performance. I can go to a larger man metering jet or drill the old one oversize, and this will likel8y fix the problem. My question to you is this: have any of you had this problem? How did you fix it? Does anyone know how large the jet must be on a Carter BB to overcome this situation? Thanks for your time. Quote Link to comment Share on other sites More sharing options...
John-T-53 Posted August 28, 2009 Report Share Posted August 28, 2009 Dave, I'll assume your PCV valve is brand new and works properly. Does it rattle if you shake it? PCV valves should only open during acceleration (low vacuum condition). If you're cruising down the road at a fixed speed, the valve should be closed unless the RPMs are high. I've also been told they work best when oriented vertically. I'm about to install a PCV system on my motor, but haven't come up with a design yet. Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted August 28, 2009 Report Share Posted August 28, 2009 My fresh engine runs and starts just fine. It idles like all of us want our engines to idle. But at cruising speed on the highway, it bucks and jerks. After checking several things, I discovered it smooths right out and runs very nice if I pull the choke out about 3/4-inch. Now it runs smooth at cruise speed on the highway, just like it should. This means the small amount of vacuum hijacked by the PCV is making the carb run lean eenough to affect performance. Thanks for your time. Is a PCV sized to cubic inches....or for the amount of vaccum the engine produces? I guess what I am asking is do you need to find a specific valve to match your application, say like a modern engine of similar size? Just a thought. Quote Link to comment Share on other sites More sharing options...
grey beard Posted August 28, 2009 Author Report Share Posted August 28, 2009 The PCV system I am using came off of a military Power Wagon. I purchased it from Vintage Power Wagon - the only vendor I know who handles these things. The valve itself is plumbed as it was from the factory, and is indeed in a vertical position. The valve was made for our flathead engines. The valve operates at idle speeds - I can hear when it opens and closes by the noise it makes. It also is open at cruise speed, because vacuum is lower then at idle. This fact is borne out by the change in engine operation with the PCV disconnected. My question is how to overcome the unbalancing of the carburetor when the system is in use. Runs fine without it, but bucks and surges at steady road speed, cruising with the PCV hooked up. Bigger jet? How much bigger? What say ye, gentlemen? Quote Link to comment Share on other sites More sharing options...
TodFitch Posted August 28, 2009 Report Share Posted August 28, 2009 Is a PCV sized to cubic inches....or for the amount of vaccum the engine produces? I guess what I am asking is do you need to find a specific valve to match your application, say like a modern engine of similar size? Just a thought. I believe that is true: The PCV should be sized based on the displacement of the engine. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted August 28, 2009 Report Share Posted August 28, 2009 With the original draft tube system the crankcase had ample ventilation when the vehicle was at "speed". The PCV system works well at idle but in your case it does not work at speed. So devise a way to revert to the original system when at speed and kill the engine vacuum requirement when at speed. Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted August 29, 2009 Report Share Posted August 29, 2009 I believe that is true: The PCV should be sized based on the displacement of the engine. Thanks original forum Tod:DI am used to seeing one size fits all in the help section when I pass by it[one size fits none IMO]and go on my merry way. Quote Link to comment Share on other sites More sharing options...
MBF Posted August 29, 2009 Report Share Posted August 29, 2009 GB-the opening size on the valve is geared towards specific engine displacement. Since I was building my system from scratch I used an old one from a slant 6 which was too big and caused a lean running situation. I switched to one for a 4 cyl Ford Escort. Works a lot better at idle. I wonder of the military vehicles equipped with these had different carburator jetting to compensate for the new vacuum "leak". Mike Quote Link to comment Share on other sites More sharing options...
Frank Elder Posted August 29, 2009 Report Share Posted August 29, 2009 GB-the opening size on the valve is geared towards specific engine displacement. Since I was building my system from scratch I used an old one from a slant 6 which was too big and caused a lean running situation. I switched to one for a 4 cyl Ford Escort. Works a lot better at idle. I wonder of the military vehicles equipped with these had different carburator jetting to compensate for the new vacuum "leak". Mike So about a 3.9 liter engine PCV for a 230 then? Quote Link to comment Share on other sites More sharing options...
grey beard Posted August 30, 2009 Author Report Share Posted August 30, 2009 Thanks, Mike. I'll give it a try. My guess is that my own valve from VPW was likely from a bigger cid truck engine - and mebby jetted to compensate for it. Do you suppose that swapping to a smaller CID valve makes more sense than opening up the main metering jet? Just curious . . . Quote Link to comment Share on other sites More sharing options...
MBF Posted August 30, 2009 Report Share Posted August 30, 2009 Not sure GB. I haven't pulled my plugs to check since I went to the smaller diameter valve, but I'm coming up on 1000 miles on this oil change and the oil is still amazingly clean looking so I gotta believe it's doing the job. If you find a jet kit for this carb let me know! I didn't know there were different sizes available. Quote Link to comment Share on other sites More sharing options...
grey beard Posted August 31, 2009 Author Report Share Posted August 31, 2009 I have several used Carter BB cores that could be rebuilt. I'll see what jets these have in them. If nothing is larger, I may risk drilling one out if I cannot find a pcv valve for an engine of this size. Problem is, the pcv valve I must replace is brass, and threaded on each end. To replace it, I'll have to replumb the thing with rubber hose, like the more modern stuff. Quote Link to comment Share on other sites More sharing options...
grey beard Posted September 4, 2009 Author Report Share Posted September 4, 2009 I have three core carburetors of various MoPar flathead parentage. Pulled the main jet out of the three of 'em. Each jet was marked "256". No two of them were the same size bore. The largest of the three would pass a #54 drill bit. It is my thinking that I must rejet my carburetor to overcome the vacuum drawn by the PCV system. In spite of whatever size engine my PVC valve was made for, my carburetor was not jetted for ANY pcv system at all. Therefore, I've gotta' compensate for this change to the induction system somehow. I was sorta' hoping someone who has a PCV system already installed had invented this particular wheel already, so that I could stand on his shoulders, so to speak, as it were, to coin a phrase. Howsomever, it appears no one else has had this problem, or else no one is admitting they do - go figure. Mebby they just drive with their chokes pulled out a tad on cruise, like i do lately - great for fuel economy, too. Still open to any local wisdom on this issue. Thanks Quote Link to comment Share on other sites More sharing options...
grey beard Posted September 14, 2009 Author Report Share Posted September 14, 2009 My Pilothouse has really been running lousy. Today I removed my PCV valve - a big honkin brass affair that came with my system when I purchased it from VPW. I was surprised to see that it screwed apart. Sooooo, I took it apart and was surprised to see no rubber valve at all - just a small piece of metal and a spring. This thing basically amounted to a 3/8-inch air passage vacuum leak in my intaie, all this time. No wonder I had to run with the choke out to get any performance. Well, went to CarQuest, cause' they're closest, and susbsequentially picked out a smallish valve made fror "light Dodge trucks", their part #76-2549. Cost me all of $7 bukkz. Plumbed it up with some gas line hose and a few fittings, and what a difference! Runs like a compeltely different vehicle. PCV systems are wonderful on these old flatheads. They're just the berries for keeping a crankcase clean. But they have to be able to meter vacuum at different throttle positions. The system I purchased was from a military truck of unknown size. I just never even thought of pulling the valve apart to inspect it. Now I'm glad I did. 1 Quote Link to comment Share on other sites More sharing options...
Jim Roach Posted September 14, 2009 Report Share Posted September 14, 2009 Dave, Glad to see things worked out for you. One of these days I'll get around to addressing my brake problem. We'll have lunch. Jim Roach Quote Link to comment Share on other sites More sharing options...
MBF Posted September 15, 2009 Report Share Posted September 15, 2009 Hmmmmm GB. Sometimes if ya want stuff done right ya just gotta do it yourself. Glad you found the solution. Bet your system looks a lot like mine now! Mike Quote Link to comment Share on other sites More sharing options...
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