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Author Topic:   Motor rev problem HELP!
5kracing
Dirt Newbie

Total posts: 2
posted July 11, 2005 12:07 AM  
I have a problem with a motor not wanting to rev, maybe someone can help? I have a 383 bored .40 over (so actually a 385), 6inch rods, KB dome pistons,rotating assembly fully balanced, Midwest motorsporst head 76cc with 1.94/1.50 valves, roller tip rockers, the cam is rated from 3500-6250. The carb i have on now is a rochester 2 barrel 500 cfm (the one out of speedway). It won't rap any higher than 5500 rpm, it is timed at 36 deg, i know it should rap higher than that and quicker. I have changed: Coil, module,plugs, plug wires, fuel pump, carb, put .100 over push rods in it. if you need any more info just ask, thanks for the help.

btennant
Dirt Full Roller

Total posts: 28
posted July 26, 2005 05:28 PM  
hey that little motor that should isnt getting enough air. Take your streetcar out and disable the four barrell youll get the same thing. Or its just dead lean in wich case you would notice a pretty red glow from the headers or manifolds after a rece in the evening. Also the engine will run a little hotter after runnin hard on a lean condition

theslowguy
Dirt Full Roller

Total posts: 30
posted August 03, 2005 06:56 PM  
if you look at the simple math of it, a 383 with a volumetric efficiency of 75% will be using 968 cfm of air. if you've only got a 500cfm carb, it just can't sustain it.

ryan
Dirt Forum Champ
Total posts: 776
posted August 04, 2005 09:25 PM  
Is your air cleaner clean. That will make it slow to rev been there done that.

iowa_dirt_track_girl
Dirt Freak

Total posts: 461
posted August 04, 2005 10:18 PM  
Dont let em fool ya there are guys out there that run the 350 carb and turn their motors 7000

Valve spring could be your culpret even though you might have put new ones on that dont meant that they are any good

so check for coil bind and check you valve springs.

unless your timming is really way off that wont keep you from turning that few of ropms heck we have had ours at 50 degrees and still run like a rapped ape (but not for long)
check you timming mark make sure its right bring 1 up on TDC and look at your mark and make sure its at zero. could be your ring has slipped a bit and you dont have enough timming in it


BOMBER #67
Dirt Newbie

Total posts: 2
posted August 25, 2005 03:44 PM  
"theslowguy" what math equation are you using to come up with that cfm???????????

BOMBER #67
Dirt Newbie

Total posts: 2
posted August 25, 2005 03:55 PM  
CI=383
RPM=5500
VE=75%
So take 383/2=191.5
Take RPM =5500/1728=3.183

So we take (191.5x3.183)/75%=457.15 CFM required at 5500RPM

Using the same formula at 6500 RPM you need 540 cfm

racerman707
Dirt Freak

Total posts: 374
posted August 29, 2005 04:30 PM  
I'm with the girl. Make sure of the valve springs.

Blackgold
Dirt Full Roller

Total posts: 71
posted August 29, 2005 09:01 PM  
I think that you would gain some if you had the 2.02/1.60 valves. That should gain 500-700 rpm. Also, I would try another carb. A friend tried that speedway carb and never did get it to run right. He contacted the co. that is selling it to Speedway and they had him send it to them and he still wasn't satisfied with it. May have just been a lemon, as I know others that like that carb. I ran a similar 383 on my Desktop Dyno and it started falling off pretty fast above 5000 rpm with the small valves and 500cfm carb. It's starving for air. I know some are running them a lot higher than that, but I can't see how they are pulling well with the HP curve falling off like it does. JMO

5kracing
Dirt Newbie

Total posts: 2
posted September 12, 2005 09:01 PM  
I got my problem fixed. It was a combination of things, first off the the push rods were to short, had to put .100 rods in it. And second off the carb i got from speedway was a lemon, It was puttin more fuel down one side of the carb than the other, and when you would rev it up the would only run on 4 cylinders. I didn't figure the carb would be bad since it was brand new, but i guess sometimes you get a lemon.

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