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#1
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carb experts... diagnose this!
1987 Dakota 3.9 Carter(?) 2bbl carb:
While cold (choke closed) the engine accelerates well. While warm (choke open) the engine accelerates poorly. Idles poorly cold or warm (except at high idle, then fine). The carb is clean and all orifaces I could see through were clean (inside and out). It also wants to die sometimes when I come to a stop. The float level is correct, the needle and seat are new. The exhaust is good, new headers (no heat riser butterfly) and backpressure checks out fine. EGR valve moves correctly while revving the engine, not checked beyond this. My ideas for possible culprits: Vaccum leak (not sure where). Bad distrubutor. Bad fuel pump (supposedly gave fluxuating pressure between 6-7 psi). Bad timing chain. So... where should I look for the problem? Any experience out there with this? I appreciate any help! |
#2
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sounds like a vac leak to me .....I have found on a few small block and v6 that the intake gaskets get sucked in to the intake ports, causing vac to be sucked from the lifter valley...Lee...
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#3
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quality control... hello! I suppose I could find this with a can of carb cleaner/starting fluid/etc. Has this occured on an engine that has never been taken apart? What type of gaskets does this happen with? What type should be used to prevent this? Thanks for the input. I never would have considered that, but it makes sense. *edit* I just noticed you said pulled vaccum from the lifter valley. How would you find that? *edit* |
#4
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I'm not sure how you would test for that kind of leak..other than take the intake off and look.... if it is torn or sucked in replace it with a felpro intake gasket , it has a medal core and wont get sucked in... most stock gaskets are paper and when soaked with gas and oil they give up....Lee.....
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#5
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I have noticed before, if I pull the PCV valve, nothing comes out of the hole. This seemed strange, especially given the miles on the motor. I suppose if ventilation was being handled by an intake leak, that would explain why nothing comes out of the PCV hole! Maybe I could confirm this by revving the motor a bit and spraying a little juice in the PCV hole. This doesn't sound like a great idea, but might it work?
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#6
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I have noticed you said headers. Key word being "NEW"
Headers lean out the mix. Richen up the carb to gain back the correct air/fuel ratio. Thats why it runs well when the choke is closed. (IMO) When the choke plate is closed, the air/fuel ratio is closer to what it should be. A rich ratio. Even more so because it's cold. In the old days, Muscle car era, when you added headers, the ratio leaned out to where we set it up today and it caused instant power not only because of the headers but also due to a better ratio of air and fuel. I could be wrong. Lee's idea is very good. To check for the vacumm leak, use what ever spray is fine with you. WD-40, Crab cleaner, NOT starting fluid. Thats, ah, ethol or somthing like that. Go's boom. Right? ....Right Spray around the base of the carb and around the edge of the intake manifold and head, intakemanifold and block. If the idle changes, you have found the problem. Fel-Pro and Mr. Gasket Ultra seal gaskets are the best that I have used so far. Factory use's the cheapest that does the job. Chrysler or any other car company will do this. |
#7
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The headers are new (Edelbrock), but the problem is old. It behaved the same before rebuilt carb and before headers. I will try to find the leak as suggested and post what I find. Thanks a bunch!
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#8
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shes lean for sure,the headers probably made the issue even worse.If you cant get jets use number drill bit to determine size and drill out the next size to .002 bigger.While drilled jets arent the best they may be your only choice and theres lots of them in the scrap yard.
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#9
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Another way to go richer is to increase the float level a little so that there is more gas in the bowl. But another fix might just be to get a bigger carb to take advantage of the better breathing you get from the headers. Is it a Carter BBD? Find one that came off a 318 and compromise between the size of jets in yours and the jets in the one from the 318. There may even be a timing adjust that will help here. You will just have to experiment until you get right, but that is the fun part, right?
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#10
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Use a spray can of WD-40 and spray the Butterfly shaft at ends of carb with engine warm. Old carbs wear here and develope vacume leaks sometimes. Just thinking out loud.
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#11
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Tarrbabe, I like when you think out loud.
If it's a 318, go get a 360 carb.... |
#12
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UPDATE:
Last night I wiggled and sprayed every vaccum connection on the manifold and carb. No leaks anywhere. The carb shaft is sealed on one end and doesn't leak on the other. I am still suspicious that nothing comes out of the crankcase breather or pcv holes. This poor idle and stalling condition acts like it may be temperature related. The truck has a 180 deg. thermostat and cools VERY well. When it is very warm out (~80 deg) the truck idles perfectly, but still accelerates as if it was running lean. When it is cold out, the truck will not idle well, even if it is warmed up. There must be some electronic piece that helps the carb control the mixture, because it uses an oxygen sensor in the exhaust. It also has what is called an oxygen feedback valve in the carb, but I am not sure what it does. The truck has every emmisions piece in place, but the air pump has been seized for a long time. I would believe it if I was told that this (no air pump) contributes to the lean condition under power (due to location of oxygen sensor), but it should have nothing to do with the idle. Maybe it needs a hotter thermostat? I also know the ported vaccum switch(?) on the manifold is temperature controlled, but am not sure what it does. Maybe this has something to do with it. Also, this truck was bought in Phoenix and is now living in Albuqerque over 5000 ft in altitude. This should make the mixture rich, not lean... right? I'm confused! Any suggestions? Thanks a bunch for your patience and interest. |
#13
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Carb Problem?
Sounds like you have dig a little deeper. One other thing to check is to pull the EGR valve and check to be sure it is clean under it and can actually re-circulate the air. Most likely not your problem but should be checked. I pulled the one off my 81 Cordoba and it was full of gas. Carbon had build up and stopped it totally. I still haven't figured out how the gas got in there. I pulled the intake and cleaned it out and rebuilt the carb and it fixed my poor idle and dying. You mentioned you didn't have any blow-back out the pcv. That could be normal if you are going by what you see. Did you place a finger over the hole and see if there was any pressure build-up? I have a 91 Dakota with the 2.5 and 167,000 on it and there is nothing to be seen with the oil filler off but if I place my hand over it, then some pressure will start to build so that may be normal. You may neet to try another carb and if that don't help then you will need to check the intake gasket to head seal. Good luck.
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