WHY low vacuum on freshly rebuilt motor?!

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Did you readjust the carb when you increased initial? If not, you're not solving or getting to the root issue. What is the current idle speed?

Is the carb dripping fuel in from the boosters at all?

That should pull ~11" and idle at 800-850.

If you reach in and twist the distributor CCW and it picks up RPM, the initial wasn't right before. Set it at 20* and see if it kicks back on the starter, if not, then tune the carb. If it does, pull out 2* to 18* and see if it kicksback. If not, then tune it. Find the max point the starter can handle.

That cam is NOT big at all. I have a 340 with a 284/528 mechanical, tight lashed at .020/.022 and it idles right at 750-800 with 11-12" of vacuum. It has 24* initial on it and fire at a flick of the key.

Since you know it seems to run OK at 36 total, find the happy initial place and do some basic math to get to your 36 number.
 
Get it tuned first, think about cams later.
Get someone involved who really does know something about engines.

If you don't want to tune your engine that other new cam will probably be a dog aswell.
 
pulling the timing cover is 8 bolts and a balancer pull. Probably 30 minutes and you can rest assured that the cam is timed. Then start messin......could be a tooth off and never be the wiser. Pop it unless you got a piston stop and a degreed balancer and a timing wheel and a dial mic and a cam card and......
 
IMO, the cylinder pressure is a little low for that cam and a true 9.5:1. It's worth checking the cam timing...
 
Nothing wrong with the cam choice. How was the motor broke In? Lobes go flat?
What oil did you use on the break in and what procedure?

Get the ignition tuning right first then like cracked keeps saying then tune the carb. If you mess with the timing at all, you gotta go after the carb tuning again No getting around it.

At this point I would make 2 guesses, cam is going flat or the cam timing is off from not being degree'd in properly along with the standard timing and carb tuning.
 
moper said:
IMO, the cylinder pressure is a little low for that cam and a true 9.5:1. It's worth checking the cam timing...
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I'm not sure if those numbers are that far off or not. These are obviously ballpark as we don't know the true measurements, but did a quick calc with that cam and the numbers provided and came up with 145lbs. and a dynamic ratio of 7.45.

I'm concerned about the fact that, in your words, it's a dog from top to bottom. Yes the vacuum number is a little low, but at some point in the rpm range it should hit pretty good regardless. I ran the Mopar version of that cam with a stock converter, 3.23's and a good timing curve. Was just soft enough to not blow the tires off, but from 2800ish or so it pulled hard all the way to 6500. How much run time is on this engine? is your oil pressure good?
 
I had a 340 that was a dog off the line...sounds like same issue you have...I had the engine rebuilt by a local shop and had them put adjustable rockers on, they had them cranked way too tight... Once I readjusted properly. it came right to life. Completely different engine. Not sure if you have adjustable rockers or not. Just a thought.
 
............that cam is ground 4* advanced, if u have a tc set with adv slots did ur buddy adv the crank gear 4*..........kim...........
 
Yes to the adjustable rockers. The carb was taken off and put on a 396 Chevy. Vacum 15 inches ran like a sewing machine. I have ran the motor with vacvum advance. Also put one of those orange revinator box'e with no advance hook up. They insist they run better with no advance . I will try cranking up the initial and get it as high as I can on the starter. What seems to be weierd is the motor idles fairly smoooth cold and when it warms up with the choke open it starts to run very rough. Have been through coils /dist/ ignition boxes ballast/ wires / differnt carbs etc. Still the same . Its probably the cam timing . Although the guy that assembled the motor runs mopar at the track for 30 m+ years. This i\s why I took the motor initially to him.
 
The motor and cam was broke in properly on a dyno. I have tore cams up before. Proper lube and oil additive . Inner springs removed etc. Cam is good on the lobes and proper break in.
 
What seems to be weierd is the motor idles fairly smoooth cold and when it warms up with the choke open it starts to run very rough.
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chasduster said:
I had a 340 that was a dog off the line...sounds like same issue you have...I had the engine rebuilt by a local shop and had them put adjustable rockers on, they had them cranked way too tight... Once I readjusted properly. it came right to life. Completely different engine. Not sure if you have adjustable rockers or not. Just a thought.
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Could Chaz be on to something here, when you say it runs OK cold, but runs rough after it warms up? Could the pushrods expand that much after they get warm? Were they adjusted hot or cold?
 
lucahemi said:
The motor and cam was broke in properly on a dyno. I have tore cams up before. Proper lube and oil additive . Inner springs removed etc. Cam is good on the lobes and proper break in.
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How did it run on the Dyno? If there were problems it would have shown up there for damn sure. You should know where it makes power and where it drops off. Now that it is home and in the car running bad makes no sense. The Dyno guy should have given you the timing info etc , tuned the carb adjusted the valves and everything so you could just set it up like he did and be pretty damn close. If not the dyno guys a moron.
 
I think you have vacuum leak as you say smooth with choke on and gets rough when it opens up. Also check rockers and make sure not to tight. I would also make sure timming marks are right by checking TDC.
 
I have the dyno specs. I agree . Unfortunatly he used his carb . And I have no idea. I have had 3 different carbs on this motor. Same results. The expansion is why I bought adjustable rockers to see if that was a change . Same results
 
Mad Dart said:
How did it run on the Dyno? If there were problems it would have shown up there for damn sure. You should know where it makes power and where it drops off. Now that it is home and in the car running bad makes no sense. The Dyno guy should have given you the timing info etc , tuned the carb adjusted the valves and everything so you could just set it up like he did and be pretty damn close. If not the dyno guys a moron.
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xactly'. no full pulls after break in? no sheet? :wack:

otherwise what changed from then to now?
 
What type fuel system are you running? Have you looked at the plugs to see if your running lean?
 
lucahemi said:
I have the dyno specs. I agree . Unfortunatly he used his carb . And I have no idea. I have had 3 different carbs on this motor. Same results. The expansion is why I bought adjustable rockers to see if that was a change . Same results
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Something doesn't add up. If you set the timing exactly like it was on the Dyno "assuming it ran correctly" and tune your carb from there, it shouldn't take more than 20 minutes to have it tuned and screaming through the gears.:burnout:
 
Guys I will post the dyno sheets. I have a good freind is a old school muscle car mechanic and he is running out of ideas . The poor vacum is drving me crazty. Its hard to get a carb to idle with poor vacum
 
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Make sure the firing order is correct. Wouldn't hurt to run the valves AFTER you get the initial up and reset the carb if your issue persist.

Just because a carb works well on a different engine doesn't mean it will be happy on yours. What if the other car has 5 psi of fuel pressure and yours has 10? That's going to cause BIG issues.
 
lucahemi said:
Guys I will post the dyno sheets. I have a good freind is a old school muscle car mechanic and he is running out of ideas . The poor vacum is drving me crazty. Its hard to get a carb to idle with poor vacum
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Your worrying too much about the vacuum number. Yes it's a few inches low, but keep in mind, race engines run in the 3/4" range. Yes they make there power up high, but they make power somewhere. If you have no power anywhere, there must be a mechanical issue somewhere. Are you having popping, backfiring, chugging problems when you drive it? Maybe we all just don't understand what a dog is............If Allen Johnson got out of his Pro Stocker and got into a 10sec. car, he would probably think it was a dog......:grin:.
 
Verify you are checking it at MANIFOLD VACUUM (from a port below the throttle plates or in the manifold). Low vacuum will indicate mechanical problems. Check your valve lash both hot & cold. Make sure heat expansion isn't causing the valves to hang or open too soon. Is the cam degreed correctly? Is the mechanical advance working correctly in the distributor? Any restrictions in the exhaust? Old school guys are great, but you need someone with good diagnostic skills not someone guessing & throwing parts at it.
 
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