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  #1 (permalink)  
Old 11-19-2001, 09:23 AM
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Default Rich Running

Hi. I'm looking for a little help on a bowtie problem I have - I hope you guys can help.

I have a 383 chevy crane 284, Edelbrock RPM Setup, running 10:1
I'm using a 650 holley vac sec. mallory dual point vac adv. timimg 10deg

my problem is that I can't get the engine to run well at low throttle openings, it runs rich, sputtering until I get to 3000rpm, then eventually clears with acceleration, I can see a black deposit in the exhaust pipes, and whilst stationary, if I hold the throttle open at 2000 rpm I can feel lots of hesitation & stumbling

If pressing on hard, it runs well enough, until I drop revs.
I get a lot of popping in the sidepipes with any decelaration, and I also get high RPM at tickover perhaps 2000 if I get off the gas. this sometimes takes 10 - 20 seconds to get down again. this is not a stuck throttle. or a choke issue

It seems to do this at most RPM, with gentle throttle, but runs better with more throttle, it sems not to want to stay at part throttle

I've changed the power valve, jets etc, plugs, timing but to no avail.

It has ben sugggested that I look at the exhaust, which has a 13/4 primaries going into a 4 inch collector, then into a 2 inch side pipe. the suggestion being that I might be getting back pressuer at low rpm which is causing the block to run rich

I would be grateful for any help - I feel a little stuck

ps most cobra reps over here in the UK run bowties, and mostly SBC


steve
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  #2 (permalink)  
Old 11-19-2001, 09:59 AM
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Steve,

What size primary jets are you running?
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Old 11-20-2001, 02:52 AM
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Mike

Originally the carb came with 64 from Holley, now at 62,

I have gone smaller than that, but found that the fuel mix was wrong at higher revs if I went lower, and the power dropped off.

Steve
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Old 11-20-2001, 05:55 AM
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Steve,

Your primary jet size seems to be fine and I would not at all suspect your header size and 2" sidepipes to be causing the problem. Have you looked down the primary throttle bores with the engine idling to see if the fuel is properly atomizing or in fact dropping out in globules?

I'm assuming that you have your float levels correctly set and that your vacuum advance is connected to a port above the throttle plates. I'm also assuming you have checked compression in all cylinders (and it's adequate) and that your dual points are correctly mounted and set for proper dwell angle. With all that said and assumed correct, I would probably disassemble the primary fuel side of the carb and look for dirt or some kind of obstruction in any of the internal passage ways. At the very least you should attempt to blow out the passages with compressed air.

Sadly, I had a similar type problem with a 780 cfm vac secondary Holley many years ago and the only remedy in the final analysis was to swap it out for a 650 DP, which worked fine.

Good luck,

Mike
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Old 11-20-2001, 08:53 AM
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Mike thanks for the advice,

This is the only carb car I've had, and to be honest fiddling with the carb is unknown territory & leaves me a bit nervous, but I guess I'll buy a good holley rebuild book from amazon & give it all a good strip & clean.

I do hope the stainless steel headers & sidepipes are not the root of my problem, they were expensive & I don't want to go hacking them around.

The engine is a fresh build with good compression etc, and the builder will loan me an alternate carb off his dyno engine this weekend to try, I have my fingers crossed

Steve

I'll let you know how I fare
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Old 11-20-2001, 09:13 AM
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Steve,

Having a known "good" carb to put on and try is absolutely the best bit of diagnosis you can employ. That should go a long way toward pinpointing whether your problem is purely carb related or not.

Again, good luck and let me know the results.

Mike

BTW, don't be intimidated by Holley carbs, they're rather simplistic in their design and easy to work on. Only a few things to watch out for like the seal washers and check balls associated with the nozzles and things of that nature.
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Old 11-20-2001, 10:43 AM
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Steve,
Are you sure it's a rich condition?

Try raising the floats to where the fuel level is near the top of the sight plug. (clear plugs are a big help here if you don't have them) Some carbs respond very well to this. The difference between a mid level and a high level can be unbelievable. This is quick and easy to try before taking things apart.

Good luck,
Dave
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Old 11-21-2001, 11:09 PM
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Steve,
I know you said you changed the power valves but sometimes the power valves are not the right size for your engine. You need to check how much vacuum you are pulling at idle. If it is say..12
lbs then you need a power valve thats a 6.5. You can find out which part number power valve you need by calling the Holly 800 number or go to their site. You can generally tell if it is a power valve thats bad by the smell of gas all the time while you're driving.
I hope this helps...

Hersh
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Old 11-22-2001, 04:51 AM
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Gentlemen, thank you all for your suggestions.

I will try each in turn this weekend, and hope to get to the bottom of this soon - it's getting a little "hairy" driving everywhere at 3/4 throttle!

would a vacuum leak resulting in a drop in vacuum result in the power valve opening much of the time giving me the problem I have?

wish me luck & happy blastin
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Old 02-25-2002, 09:39 PM
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Steve,
Some of the simptoms you describe could be hinting at an ignition timing problem. Make sure that your vacume and mechanical timing are combining with your static (10') for a maximum of 40 at about 3k rpm. The mechanical advance should have a mechanical stop ring to limit the total advance. The vacume advance needs to be curbed also. Aftermarket vacume advances are adjustable.
Good Luck...JJ
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Old 02-26-2002, 02:49 AM
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Jeff thanks for the suggestion, and you were right, There was a problem with the ignition and advance.

I took ages trying to sort this problem, and after a problem with a noisy pilot bearing on the end of the gearbox shaft, eventually pulled the block (I needed to for the bearing) and took the block along to the engine dyno guy. along with all the movable electic paraphanalia, fuel bits etc. First thing was a base line run, which showed the block running very rich, then leaning out at the top end, first thing he did was rip off the fuel regulator & fitted a replacement, then fitted lighter springs to the holley vac secs, which sorted variance, then on to the ignition, which he set by dwell angle, and vac advance, which was set up at 43 degrees, but there was a lot of discussion here as it was set back to 40 and lost power. we eventually decided that the pointer may not be accurate as TDC, cam alignment etc were all spot on.
The advance was also coming on too aggressively, and this was adjusted with a allen key in the vac chamber. We eventually got 390 hp, 440 torque.

Then we put on the headers supplied with the kit, pulled again, and lost 6 hp over the dyno optimised headers, which was considered OK as my headers did not have a collector (the collector is on the muffler)and may not scavenge properly. we then added sidepipes and lost 30hp. the problem here was we couldn't vent these through the dyno exhaust, and the dyno guy felt that the exhaust was recirculating within the dyno chamber, and reducing the o2 available, causing the drop in performance.

I am not sure at the moment if I have a problem here or not, but the block is running good numbers, not leaning or running rich, I guess I'll have to wait till I get it back together before I know
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