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 Post subject: Unilite Dist. Question
 Post Posted: Sun Apr 22, 2007 4:13 pm 
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Ok Guys, My buddy has a hot rod model a with a 351C in it. It's got a stumble just off idle and really don't think it's the carb as he had one built for it and really works well.... Now if you look at a timing lite it retards just off idle and really don't start to advance till about 1200 to about 1300+ rpm. This is a mechanical advance and there is spring and weight adjustments under the top plate....any ideas??????? Thanks

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 Post Posted: Sun Apr 22, 2007 5:18 pm 
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thats differant ..... does it stumble and retard every time you bring the throttle up ... regardless of how slow / fast you give it throttle ?

I would start off by locking out the mechanical advance at 0 and see if it still retards / stumbles


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 Post Posted: Sun Apr 22, 2007 5:43 pm 
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Have not waked the throttle to see what it does but just a slight hit on the gas and it stumbles every time and timing retards...

We bumped the timing up to 14 and still does it just not as bad...

When I say that it retards I mean that we had it set at 10* and it would retard to 3 to 5*...What Im thinking that it should advance when you hit the throttle soft or hard...... :-k

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 Post Posted: Sun Apr 22, 2007 7:16 pm 
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check the distributor gear and shaft. The gear may have a bunch of wear on it. If he put a bronze gear on it the gear may not be located properly. There may also be a problem with the timing chain or cam thrust plate.

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 Post Posted: Sun Apr 22, 2007 7:25 pm 
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gearhead1011 wrote:
check the distributor gear and shaft. The gear may have a bunch of wear on it. If he put a bronze gear on it the gear may not be located properly. There may also be a problem with the timing chain or cam thrust plate.



Chuck this is all new/rebuilt engine...Dist has cast/steel gear not bronze

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 Post Posted: Sun Apr 22, 2007 8:08 pm 
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Barry when it stumbles the RPM will go down, correct? If RPM goes down then there is a chance that so will timing, depending on how it's setup.

I'd make sure that the idle mix is right as it does affect off idle performance. If that is ok then I'd work on the pump shot to see if that will help. If it's a Holley then you can change the position of the cam to make it come in faster and if that doesn't work you can put a larger squirter in.

What you are telling me indicates that it's fuel not ignition.

Oh, and I never liked those distributors. Junk, IMHO! =;

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 Post Posted: Sun Apr 22, 2007 8:31 pm 
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A stumble can be caused by slow ignition timing also. Generally the engine needs more initial and you limit the amount of mechanical advance to get the correct total. If you have much cam in an engine, you'll need to get the initial up around 16-20º.

It may be a combination of accelerator pump and initial ignition timing.

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 Post Posted: Sun Apr 22, 2007 8:34 pm 
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Don't really know if It can be up that high and run on pump gas???

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 Post Posted: Sun Apr 22, 2007 8:48 pm 
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check dist gear end play, when you hit gas gear and shaft may be moving and taking up play. i usualy set mine to .005 end play. try a gm hei in place of unilite and see what happens. if you need i can send you my back up dist to try.

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 Post Posted: Sun Apr 22, 2007 9:15 pm 
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Beretta wrote:
Don't really know if It can be up that high and run on pump gas???


At idle there is no load, so it shouldn't be a problem. Just so the total isn't too much under load. Is there a vacuum advance? Where is the initial and total now?

My 383 ran locked at 38º w/ pump gas 10.8:1 comp.

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 Post Posted: Mon Apr 23, 2007 6:03 am 
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tonyy358 wrote:
check dist gear end play, when you hit gas gear and shaft may be moving and taking up play. i usualy set mine to .005 end play. try a gm hei in place of unilite and see what happens. if you need i can send you my back up dist to try.



Tony its a 351C O:)

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 Post Posted: Mon Apr 23, 2007 6:05 am 
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Rick360 wrote:
Beretta wrote:
Don't really know if It can be up that high and run on pump gas???


At idle there is no load, so it shouldn't be a problem. Just so the total isn't too much under load. Is there a vacuum advance? Where is the initial and total now?

My 383 ran locked at 38º w/ pump gas 10.8:1 comp.

Rick



Right now it's at 15* and total at 30*.....including idle setting


Should I take some out of mech adv and add to idle setting...To much will make it crank hard hot......

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 Post Posted: Mon Apr 23, 2007 6:32 am 
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Ken0069 wrote:
Barry when it stumbles the RPM will go down, correct? If RPM goes down then there is a chance that so will timing, depending on how it's setup.

I'd make sure that the idle mix is right as it does affect off idle performance. If that is ok then I'd work on the pump shot to see if that will help. If it's a Holley then you can change the position of the cam to make it come in faster and if that doesn't work you can put a larger squirter in.

What you are telling me indicates that it's fuel not ignition.

Oh, and I never liked those distributors. Junk, IMHO! =;



Ken, I really don't think its carb totally. we changed cams and nozzles but still not much better.......

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 Post subject:
 Post Posted: Mon Apr 23, 2007 12:31 pm 
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Beretta wrote:
Have not waked the throttle to see what it does but just a slight hit on the gas and it stumbles every time and timing retards...

We bumped the timing up to 14 and still does it just not as bad...

When I say that it retards I mean that we had it set at 10* and it would retard to 3 to 5*...What Im thinking that it should advance when you hit the throttle soft or hard...... :-k


Checking end play like Tony suggested would be a good idea.

It shouldn't move around like that, unless the mechanical advance is already in the picture and it's never on the real initial.

Is there a vacuum advance? This could retard timing when throttle hit, depending on how its connected.

Just thinking out loud here.

Rick


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 Post Posted: Mon Apr 23, 2007 4:12 pm 
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Beretta wrote:
gearhead1011 wrote:
check the distributor gear and shaft. The gear may have a bunch of wear on it. If he put a bronze gear on it the gear may not be located properly. There may also be a problem with the timing chain or cam thrust plate.



Chuck this is all new/rebuilt engine...Dist has cast/steel gear not bronze

There can still be a problem with the cam walking. Some of the older timing gears required using a spacer between the cam and cam sprocket. The newer style gear has the spacer made on it. I have seen where the spacer had been used with the new style sprocket and it allows the cam to walk about 1/8 ". As ken said if the engine stumbles and the advance is in at all the lower rpm will cause the timing to drop. Weak ignition may be causing the problem too. Possibly using a resistor where you shouldn't or a coil that's not right for the rest of the ignition.

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