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750 can. I rebuilt carb (found chunk casting inside). Have on SB 400. Runs great until after driving car for about 15 mins, then it will not hold idle for its life! Have hard time even moving car when does this cause just keeps dying. Left me almost stranded. Cannot figure out what heck cause this. It will not do it when I start car back up next day, run fine until at some point will just no longer idle after running for period after engine heated up. About to throw carb in garbage can and get Holley 750.
Appreciate any help with this.
 

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If you have an older original Barry Grant Speed Demon carburetor which from what little information you posted sounds like you do but I could be wrong but you mentioned a chunk of metal in the casting. What exact carb do you have and can you find the part number on it which I believe is around the air base filter area on top of the main body if its an older model before Holley bought them out. I listed link below if its an older model.

Back in the day when Barry Grant was on there last leg they had a lot of issues with quality with the metering blocks or base plates having casting flash or metal grindings being inside them and causing there carburetors not to run right and having troubles like your experiencing. Folks had to remove casting flash in the metering blocks or had to replace the carb itself or the base plate had to have the casting flash removed. I guess on the assembly line they had a problem where stuff was being machined on the line and being drilled, grinded, and shaped etc that the left over material was falling into the lines below and getting into the carburetor parts before they were assembled and also some other issues before they found out and fixed that problem but they went out shortly after that as summit quit carrying them along with Jegs because of so many returns.

Also post the rest of your engine specs and as much info as you can so we can help maybe trouble shoot this more and help you out.

When tuning the pump shot part of the carburetor how I do it is test it as is in different conditions from slow off idle opening of the throttle and also quick off idle throttle opening to different degrees of opening and different driving conditions but before you want to mess with any of this you have to make sure your butterfly to transfer slot setup is within spec and not showing to much or to little on the front and also having enough of the secondary side set correctly in relation to the front.

You also want to have your idle circuit to not be to rich or to lean and you idle mixture screws set as good as possible to what your engine needs and is using an O2 air fuel ratio gauge don't always look for the magic number of 14 .5 idle etc as with bigger cams and more overlap the more the idle circuit will need to be richer vs a bone stock mild camshaft.

The thing with the shooter size is the bigger the number the bigger the shot will be but be across less throttle duration and the smaller the number the lessor of a shot but will have more duration across throttle travel.

As far as pump cams goes and on there profile if you look at a chart, take for instance the orange pump cam it will give a bigger shot with less throttle duration of the shot profile. Now looking at say the pink pump cam its profile will give lesser of a shot but move the profile of the shot across more duration of throttle opening vs the orange pump cam.

I always start off with first going up a 4 size difference on the pump shot size and see how it does and if a lean stumble was there and the bigger pump shot got rid of most of it but not all then I proceed to use a different pump cam that gives a bigger faster shot with less throttle travel to see if it gets rid of it and also to make sure its nice and smooth and not to much.

Its a trial and error and with experience it becomes a lot easier but you have to make sure your timing curve is dead on and your carb calibrated correctly or you will end up with a bunch of band aid cover ups to make it run.

 

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The pump cam in the number one position will activate quicker and more instantly vs the number two position which will activate slightly later in the throttle opening and that has some to do with how high your idle is. The number one position works best with a 900 rpm and below and the number works with a 1000 rpm and higher since with a higher idle if you keep it on the number one position you have already used some of your pump cams profile but its not etched in stone but only a starting point to fine tune.

For the double pumper the secondary side the same principal applies on the number one vs the number two position but not such as much on your primary idle setting as that is main just the primary side. The number two position on the double pumper is just to really fine tune the timing of the shot in relation to shooter size.

Looking at the chart below for example the 17cc of the white pump cam on number one will be a quicker shot with less throttle duration vs the number two shot which will start off with a slightly lesser shot but will have a slight more duration of the throttle opening.


Cams from lightest to the heaviest :

cam color #1 #2

white 17cc - 19.5cc

blue 18cc - 20cc

red 18.5cc - 20cc

orange 19cc - 24.5cc

black 19cc - 18cc

green 24cc - 30cc

pink 30cc - 37.5cc
 
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