idle problem with long duration cam

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joes68340s

joes68340s

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Hi, I have a 68 Barracuda with 340. Built years ago with a 305 duration magnum cam. It will only idle at 950 rpm or more in gear. It has a 3500 stall convertor 4.56 gears. I have played with different carbs and done the little air tricks but still wont idle below 950. The car runs good low 12s in the quarter. But I would like a more drivable car that still has the same performance. I still wonder if it needs more convertor as it does seem to pull done in gear. Engine is out I have thought of a new cam. Any Ideas Thanks
 
2 questions What are the little air tricks you used. Why do you think that 950 rpm in gear is necessarily bad with that camshaft? It might be a touch high, but its not really out of the ballpark for that size motor.

don
 
I realize its not that off. But it seems like it could idle a little better. I am wondering if it needs more convertor as it does tend to pull down the idle to much when I put it in gear. It actually idles more at 1000rpm. Depending on weather.I have played with opening the rear throttle blades some, so I dont uncover the slots. And also added small holes on the primary blades. No change. Have gone back to a stock setup on my 750 double pumper.
 
What are the rest of the specs on the motor? Did you ever take a cranking compression test on it before? 950 is a little high, which is why it pulls on the convertor. Also, what is your full timing curve? not just total, but initial and centrifical? Im assuming the vaccum advnace is already not in use. What about power valves and/or idle mix screws? When was the carb last rebuilt?
 
Moper is right about why it pulls down when in drive. Also that is a long cam..you need 10.0 min. compression with that cam. You didn't mention yours. That is the first thing I think about when we complain about drivability with long cams. And what vacuum is it pulling in drive at idle? If less than 6 or 7..you don't have enough compression (for good carb signal). If you use that cam with your combo..might want to consider a vacuum sec. 3310 Holley. It WILL improve drivability. Terry.
 
The engine has close to 11.1 compression. Vaccum in gear is about 10. cranking compression 165-170. full advance off idle. car runs great at track, I just would like it to be a little better on the street. Would like to find a smaller cam but not loose much power. I have run 750 vaccum. no difference.
 
11 to 1, 10 inches of vacuum, 170lbs cranking...you should have excellent drivability. No you wouldn't see any improvemnet with a vac. carb over mech sec's. with 11.0 to 1. Should be strong everywhere. Rumblefish is right. If you seek better idle quality..try one of the new shorter, hi lift cams. (less overlap). I like them because they work well with ported heads and pull good down low. 305 is long for the street. Keep us informed as to your progress. Terry.
 
joes68340s said:
full advance off idle.

That's your problem. It's common in race cars..Not street cars that need to perform well at idle/low rpm too. You need to limit the initial advance to like 18* or so. Then you can use the carb adjusments to calm things down. That is why the advances were used on production engines. You dont need that much timing at idle.
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I think you have something there Moper. VERY good point. I have 15 initial and 35 total and all in @ 1800 (no vacuum advance). That is what my combo likes. Other than too much advance too soon, That engine should have wonderful manners on the street with a rough idle (lope) but decent throttle response. I believe problem should be solved.
 
headsbikesmopars said:
I think you have something there Moper. VERY good point. I have 15 initial and 35 total and all in @ 1800 (no vacuum advance). That is what my combo likes. Other than too much advance too soon, That engine should have wonderful manners on the street with a rough idle (lope) but decent throttle response. I believe problem should be solved.
Click to expand...


think soo too!

i have a 340 with simmilar compresion and a solid roller cam with a totalduration that is about the same as your totalduration and i have adjusted my timing by ading more initialadvance until the idle rpm stops rising and that happens at about 20degrees of timing have tried more and less but it seams to like that best:)
i am sure that someone will corect mee if that is a bad way of adjusting it;)
 
Thanks I will go after tuning the advance curve when I put the engine back in. I actually avoided it before do to the constant running and idling. Noise, I have Bitchy neighbors. Though while the engine is out I wouldnt mind putting a tamer cam in and drop the power band down a little. This cam is rated at 3500 to 6800.
 
i don't think 950 to 1000 is all that out of line.. especially with a big cam like that... did any of those carbs have 4 corner idle ?? i just put a proform 750 on my dart with 4 corner idle and man did it make a difference.. rpm barley drops at all when i put it in gear.
 
Yes the carbs have four corner idle. I think it really is an advance issue. I have been thinking about installing the mopar 284/528 solid cam. As my heads are already setup with comp dual springs. The solid cam would drop the rpm range from 3500-6800 to 2600-6800. I like that better.
 
That .528 solid is EXCELLENT cam for street and strip. Much better idle and vacuum. Wonderful drivability and still sounds good with a lope. I changed to that cam from the 292/508 hyd, WOW, the difference was amazing. Vacuum went from 4" to 11". Overlap is 76 degrees (292/508), and 60 degrees (284/528). Throttle response was great and the car dropped into the 11's from the low 12's. I know it is a dinasour..but it is still a good one. You will like it. Good luck and let us know how you like it. The follow ups are very valuable to all the rest of us. So please advise and thanks. Terry.
 
Also don't forget to check some of the fast ramp cams made for mopars. Comp, Crane, and I think Hughes (?) have cams that take advantage of the large diameter lifters. I have a wicked dirt track custom grind from Comp and it pulls HARD. As for the vacuum advance... Get an advance can that is adjustable and work it from there. The other thing to check in your timing is that the mechanical advance isn't starting to tip in at your higher idle speed, that can screw up drivability also.

Revhendo.
 
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