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Old 06-11-2006, 02:32 PM
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blower idle rpm problem

Hi everybody,

my friend just bough a truck with a 6-71 blower. he bough this truck from an old man...the guy never built up this motor and nobody know what this motor is made with. The only thing we know is:

- 396 big block chevy cubic inches, probably bored to 40 thousand.
- hydraulic (sealed power cam) with those spec:
cam number CS 175R
Intake Exaust
duration at 0.50 222 235
duration 306 322
lift 500 505
lobe center 110 120 ???

- 2 vacuum 650 holley carburetor
- 6-71 blower (probably 15 or little more years old)
- a very old point distributor with a accel external coil
- We see oval port head
- And the most bizarre thing, the upper and lower blower pulley have 35 teeth, so it's mean this blower have 1:1 ratio

Our problem is: When we start the blower when the motor is cold or hot..we are not able to let him stay at idle, the rpm goes up from 900 rpm to 1800 at neutral position and he down to 900 and up to 1800 rpm....we where thinking about a vacuum leak problem...so we where replacing all the gasket of this blower....We where setting the initial timming to 20 degre and total at 32 at 3000 rpm.....

WHEN the truck is in drive position and drive it around the street, we are not able to stay the idle at 900 rpm...he always want to die...and he make allot of backfire and after the backfire...the motor dead at 0 rpm....I know it's not a timming problem we set it at 32 degre total...

After this, we where restarting the motor and he made the same problem..the only difference is when we put the rpm more than 1800, the motor goes really weel, no up and down rpm..and the throttle respond very well...but if we dont touch the gaz pedal, the rpm goes up and down again...

What is the problem...we really need help fast....

DO YOU THINK THAT THE POWER VALVE OF BOTH CARBURETOR HAD BLOW???

We really dont know what to do...and we have no blower shop around here where we can asked question (we live in quebec, canada).....

Plz...anybody have comment, advice or trick...or question they can ask to us to solve this problem...

ANOTHER QUESTION, WHAT HAPPEN TO A MOTOR IF THE BLOWER RATIO IS 1:1, DOES IT MEAN THAT THE BLOWER DOESN'T WORK WELL, i MEAN, THE BLOWER GIVE TO THE MOTOR NOT ENOUGHT BOOST OR AIR TO MAKE BIG POWER...WHAT IS UNDERDRIVE???

If I put a 38 lower pulley to make 6 % underdrive, what will be the psi...this motor will be take this psi (if this motor have a stock bottom end)...

Can it be an electrical problem?? I mean not engouh spark....or volt to the spark plug?

Can it be a fuel pump problem..this motor have a mechanical holley fuel pump...can it be the problem at low rpm..i mean the pump dont give enough fuel.....

Another thing, the truck have a th400 transmission with a stock stall...this is good or not??? We have no more information on the cam.....

Can it be a intake gasket problem..i mean if air goes by the intake gasket, is this possible to be the problem....

Heeeeeeeeeellllllllpppppppppppppppppppppp usssssssss

Stephane

nobody can help me???? I really need help and faster as possible.... The guy who have the truck/blower become nervous....

Stephane

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Last edited by Ratoflic; 06-12-2006 at 06:02 AM. Reason: add information
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Old 06-30-2006, 07:35 PM
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I was out of town and missed this post 2 weeks ago. I was just digging to see what was back here.
If you don't get answers in 5 days or so, just add a post to your thread and get it back to the top of the list.

The first thing that I would do is to disable the vacuum secondaries on each carburetor. stop them from opening, as a test. Just put a piece of wire on the linkage to hold them closed. It will not hurt anything.
They might be opening irregularly causing the surge problem.

If this helps you might need stonger springs in the vacuum cans to delay the opening.

The lean backfire could be caused by the vacuum secondaries popping open, since the secondaries do not have accelerator pumps.

Also I would reduce the timing to 28* for a while until things get worked out.

Blower ratio is the amount that the blower turns compared to the engine crankshaft. 1/1 is ok. faster is overdrive, slower is underdrive. The best ratio to be determined by testing and trial and error involving induction temperatures, boost pressure, etc. A larger lower pulley will make it over-drive.

You won't hurt the bottom end with 6 psi. You might run into detonation due to insufficient octane for the boost amount before the bottom end is hurt.

Just keep asking, and somebody out here will try to help you.

Last edited by xntrik; 06-30-2006 at 08:01 PM.
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Old 07-01-2006, 12:58 AM
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It is difficult at best to trouble shoot your combination from here but I also think that backing down the timing to 28 will help. Try to have it all in by 2600 rpm or so. I don't think I would mess with increasing the blower speed until you have a benchmark tune up, if anything, I'd slow it down until I was out of the woods with it somewhat.
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Old 07-01-2006, 04:36 AM
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Its called a lean surge. Fatten the idle mixture up and check egts. The fuel will actually surge back and forth in the lines once it gets started.
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Old 07-01-2006, 10:12 AM
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blower idle rpm problems

good advice from everyone so far, one thing you have got to do is get some fire in that motor, an old points distibutor is not going to do it, at the very least get an HEI with a 50,000 volt coil or better yet set up a crank trigger system with an msd box and coil, the best thing would be a mag system, of course we can only guess with out being there but for sure with the irregular idle either the fuel or the fire isn't consistant, i have messed with blowers for a lot of years and the best fuel system for either street or strip is the enderle injected systems, good luck
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Old 07-01-2006, 10:51 AM
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Quote:
Originally Posted by blownalkybbc
good advice from everyone so far, one thing you have got to do is get some fire in that motor, an old points distibutor is not going to do it, at the very least get an HEI with a 50,000 volt coil or better yet set up a crank trigger system with an msd box and coil, the best thing would be a mag system, of course we can only guess with out being there but for sure with the irregular idle either the fuel or the fire isn't consistant, i have messed with blowers for a lot of years and the best fuel system for either street or strip is the enderle injected systems, good luck

The points distributor is probably not causing the regular surging problem.

The interesting thing about ignition systems is that if there is enough juice to fire the spark plugs, then that is all the coil will put out. Since yours fires well above 2000 rpm when the boost is increasing, it is OK voltage wise. That does not mean there might not be a timing problem or an errant spark causing a misfire or backfire.

But do upgrade to an aftermarket distributor when you can. I don't think that any HEI will clear the blower. I would avoid magnetos. When you upgrade ignition systems, do it as A SYSTEM... get all the components including plug wires from the same manufacturer. I would go MSD.
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Old 07-01-2006, 11:31 AM
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msd box 6 al

Quote:
Originally Posted by xntrik
The points distributor is probably not causing the regular surging problem.

The interesting thing about ignition systems is that if there is enough juice to fire the spark plugs, then that is all the coil will put out. Since yours fires well above 2000 rpm when the boost is increasing, it is OK voltage wise. That does not mean there might not be a timing problem or an errant spark causing a misfire or backfire.

But do upgrade to an aftermarket distributor when you can. I don't think that any HEI will clear the blower. I would avoid magnetos. When you upgrade ignition systems, do it as A SYSTEM... get all the components including plug wires from the same manufacturer. I would go MSD.
i would get a msd 6al box for the distb. then i would get a fuel regultor on it and set timming to 25 deg total with the coldest plugs i could find
and i would run a 36 slug on the bottom run it at 5 % under
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Old 12-01-2008, 03:31 AM
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The lean / rich surging is quite common on roots type superchargers running carbs. Make sure you have correct air bleeds, and power valves for your specific combo. If, correct it will Idle normal.
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