Primary holes drilled

[Omie01]

My father and I both have 71-73 Mustangs with 351 C's. When I rebuilt my engine I had Pro Systems build my carb for me and it works GREAT!! I tried to get him to do the same when he built his engine but he refused to go that route. Instead he bought a Quick Fuel 750 DP. We got it running ok, but when he first starts it it blows blue smoke that smells like gas until it's fully warmed up. then it runs ok and the smoke clears. Is this just a matter of drilling the front throttle plates? Is it drawing too much fuel when it's cold? Everything else works fine once it's warm. Thanks!! PS: My Pro Systems carb has the primaries drilled.

 

[TommyK]

Does the QF carb have a choke on it? If it does that is the likely cause of the rich on start up condition. If it cleans up after it warms up and the transition slot exposure is correct putting holes in the butterflies isn't going to do anything for you.

 

[jpaz]

Maybe its from the fuel leaking into the manifold after sitting overnight? My QF is doing this, but I don’t see any blue smoke when I first start it. I can’t see the exhaust at first start up though. I’ll have to get a video of it to see.

I always thought that the holes drilled in throttle plates were to get more air in for idling. If his engine is running really rich at idle, and it can’t be adjusted with the mixture screws, then they may need the holes.

 

[Stanglover]

My father and I both have 71-73 Mustangs with 351 C's. When I rebuilt my engine I had Pro Systems build my carb for me and it works GREAT!! I tried to get him to do the same when he built his engine but he refused to go that route. Instead he bought a Quick Fuel 750 DP. We got it running ok, but when he first starts it it blows blue smoke that smells like gas until it's fully warmed up. then it runs ok and the smoke clears. Is this just a matter of drilling the front throttle plates? Is it drawing too much fuel when it's cold? Everything else works fine once it's warm. Thanks!! PS: My Pro Systems carb has the primaries drilled.

 That's interesting that the Pro Systems carb has drilled primary plate. did they do it because they knew something many don't, or because you asked them to do it?

 As for the QF 750 DP, it may not be the case it needs plates drilled. I am NOT knowledgeable with DP carbs, but if it smokes and belches gas smell until it's warmed up, I have to think it's more choke related, if they have chokes that is!! In my experience and you've maybe read post by me about drilling my primary plates to solve an over rich idle issue, an over rich idle will not be affected by whether it's warmed up or not. If the motor is not getting enough air with the primary plates set exactly where they need to be, i.e. the transfer slots are "square" if looked at from underneath, then there may be a need for drilling. BUT if you do it, it's not easy to go back without buying new plates. Do NOT forget that if you remove the screws, you will need to loc-tie them or lightly peen the screw so they don't come loose. Engines don't like eating screws!

Best bet is to take the carb to a tuning specialist who knows what he's doing. The hole size is somewhat critical.

EDIT: Reading back on another post about power valves, it occurred to me that there might be other reasons for smell and smoke on start up. You might want to find out what P/V is in it and the inches of vacuum (Hg) the motor is pulling. Check out that post, Timing Frustration, post #223 (or thereabouts)

 

[Omie01]

I gave Pro Systems a complete spec sheet of every component to my engine and they built a carb for me, so they chose to drill them. I'm pretty sure his QF has the 6.5 hg power valve, and yes his carb has a choke, while mine does not. For whatever reason he refuses to take it to a performance shop for tuning,"must be old age stubborness" LOL!! I'm just worried he is going to mess up his new engine over a carb issue! We set his choke to let off really fast, and to be honest I have not seen the car in over a year so I'm not sure if he got it worked out or not.

 

[Stanglover]

I gave Pro Systems a complete spec sheet of every component to my engine and they built a carb for me, so they chose to drill them. I'm pretty sure his QF has the 6.5 hg power valve, and yes his carb has a choke, while mine does not. For whatever reason he refuses to take it to a performance shop for tuning,"must be old age stubborness" LOL!! I'm just worried he is going to mess up his new engine over a carb issue! We set his choke to let off really fast, and to be honest I have not seen the car in over a year so I'm not sure if he got it worked out or not.

 Omie01, that's interesting the Pro System carb came with drill primary plates from the factory. It does somewhat go to prove the theory why my tuner guy did it to my carb. which has been a controversial subject.

Maybe before more comments can be made on your Dad's carb situation, can you either go see it or ask him what if anything he has done? There's nothing wrong with taking it to someone more knowledgeable and getting it tuned properly.

 

[Hemikiller]

If it's blowing *blue* smoke on startup, that says to me you have oil getting into the cylinders somehow. Could either be valve seals or a leaking intake manifold. The restriction induced by the choke flap could pull oil from under the intake ports, then it clears up once the choke opens.

Has he tried starting the car from cold, but blocking the choke open?

 

[Stanglover]

If it's blowing *blue* smoke on startup, that says to me you have oil getting into the cylinders somehow. Could either be valve seals or a leaking intake manifold. The restriction induced by the choke flap could pull oil from under the intake ports, then it clears up once the choke opens.

Has he tried starting the car from cold, but blocking the choke open?

"Blue smoke" I missed that, just saw smoke.

 

[Carolina_Mountain_Mustangs]

Might be dumb question. Do you have PCV valve and working? Did you put guides in the heads when apart?

 

[Omie01]

I actually over torqued an intake once and split the paper gaskets and sucked oil into my intake so I know what that looks and smells like. I don't think its that, might be something to look into though, it VERY much smells like a rich mixture, very gas smelling so I'm sure for whatever reason its running rich when cold. And yes, the heads were rebuilt by a reputable place and they did the guides, and he has Ferrea valves. I won't be down there until Thanksgiving week this year so I will report on it then if he doesn't take it in. I was just wondering if most of you were drilling your primaries or not. Thanks!!

 

[Stanglover]

I was just wondering if most of you were drilling your primaries or not. Thanks!!

The simple answer to that would be no. Just about any time I've mentioned doing that, I get blasted. However I can without hesitation, tell you this worked on MY engine and set-up. The gas smell was so bad that it got into the house (attached garage). I had to leave the car out in the driveway until it cooled off and most of the gas smell gone.

I have done no other changes other than upping the squirter. I would have to physically check to see what number it is now as it's not listed on my receipt. I did try changing the cam, the cam position,  changing the primary jets, but these changes only made it worse, so I went back to factory cam and setting and jets.

What is important is to make sure you can slip a .015" feeler gauge between the pump arm and adjuster screw. This will be tight, i.e. no looseness between arm and screw. This is to make sure the diaphragm is NOT being stretched to its max. You must be able to see the arm move downward and return. Hope that makes sense.

With the timing set at 14 initial and 20 on the crank, total 34 mech, no vac, it pulls and runs great now, no over richness at idle cold or hot, no smell. However, be warned, if you drill too big, you can't go back other than to replace the plates and start over. 3/32" worked on mine.

This of course refers to a Holley S/A 670.