Help with QF HR-750

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JamesRR

JamesRR

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Recently slapped a quick fuel hr 750 mechanical secondaries carb on my 340. Pulls like a mule but is super rich at idle. Blows a tiny bit of smoke. Rings are healthy. I was having a problem with a hanging idle but I found this to be that I had the idle adjustment screw in too far. Backing it off helped the problem but I cant get my idle high enough for the cam. I changed the air bleeds up a few sizes but the sweet spot for my idle mixture is only about 1/2-3/4 turns out and its still rich. Not pulling a ton of vacuum at idle. around 5in but this cam is pretty big for a small block. Curious if I should go higher with the air bleeds to get the mixture screws within normal range? too far in idle goes down too far out idle goes down. is this just too much carb? anything helps. Thank you :banghead:

James
 
I haven't kept up with "those carbs" but I assume they still use a power valve? Have you looked at that?

As Crackedback would say, and I'm sure he will.........where is the timing?

How 'bout float level? Is it pulling / dripping fuel off the boosters?
 
67Dart273 said:
I haven't kept up with "those carbs" but I assume they still use a power valve? Have you looked at that?

As Crackedback would say, and I'm sure he will.........where is the timing?

How 'bout float level? Is it pulling / dripping fuel off the boosters?
Click to expand...

They do have a power valve. What do I look for there? Timing is about 14 initial all in at 36 around 2800 or so. Does not appear to be dripping from the boosters.

J
 
Your going to have to reduce the size of the idle feed restrictors. There are 2 per metering block and they are on top. Where do you live? Reason I ask is because elevation plays a big role in what size the idle feed restrictors need to be. For example I bought a quick fuel super street series 680cfm vaccum secondary carb back in july for my 360 magnum and I bolted it on and was fouling plugs and loading up at idle real bad. Even with my timing being at 16 degrees initial. I live 4000 feet above sea level, I ended up going down 4 sizes on my idle feed restrictors and that took care of my plug fouling problem and it idles real good now.
 
pjc360 said:
Your going to have to reduce the size of the idle feed restrictors. There are 2 per metering block and they are on top. Where do you live? Reason I ask is because elevation plays a big role in what size the idle feed restrictors need to be. For example I bought a quick fuel super street series 680cfm vaccum secondary carb back in july for my 360 magnum and I bolted it on and was fouling plugs and loading up at idle real bad. Even with my timing being at 16 degrees initial. I live 4000 feet above sea level, I ended up going down 4 sizes on my idle feed restrictors and that took care of my plug fouling problem and it idles real good now.
Click to expand...

some one correct me if I`m wrong, but going to bigger idle restictions will let it flow more gas, instead of less. think of it as a vent--?bob
 
pjc360 said:
Your going to have to reduce the size of the idle feed restrictors. There are 2 per metering block and they are on top. Where do you live? Reason I ask is because elevation plays a big role in what size the idle feed restrictors need to be. For example I bought a quick fuel super street series 680cfm vaccum secondary carb back in july for my 360 magnum and I bolted it on and was fouling plugs and loading up at idle real bad. Even with my timing being at 16 degrees initial. I live 4000 feet above sea level, I ended up going down 4 sizes on my idle feed restrictors and that took care of my plug fouling problem and it idles real good now.
Click to expand...

Thanks. The idle feed restriction is in the metering block, yes? .033" is the stock size? What should I put in there? Also... having already bumped my air bleeds up significantly should I jet them back down before trying the idle feed restriction? Thank you for the help.

James
 
Your carb cam with a spec card. What power valve does it say it has in it? If its a 4.5 or more you will have to change it out for something less. Its not the air bleeds or anything like that, if your only pulling 5" vacuum with a 6.5 or 4.5 PV, it OPEN dumping fuel in at idle.
 
I would start by getting the timing up to about 20° initial (maybe even a little more) and 36° total. The carb is definitely not too big for a 340.
 
MRL Performance said:
Your carb cam with a spec card. What power valve does it say it has in it? If its a 4.5 or more you will have to change it out for something less. Its not the air bleeds or anything like that, if your only pulling 5" vacuum with a 6.5 or 4.5 PV, it OPEN dumping fuel in at idle.
Click to expand...

My card says 6.5 power valve. If im only pulling 5" I need something like a 2.5 valve? Is that needed in both bowls? Thank you
 
JamesRR said:
My card says 6.5 power valve. If im only pulling 5" I need something like a 2.5 valve? Is that needed in both bowls? Thank you
Click to expand...

Most carbs just have one power valve on the primary side.....but.....that is a pretty low vacuum reading for a relatively small cam. I would double check for vacuum leaks and increase the initial timing before spending money on any parts...JMO.
 
flyfish said:
Most carbs just have one power valve on the primary side.....but.....that is a pretty low vacuum reading for a relatively small cam. I would double check for vacuum leaks and increase the initial timing before spending money on any parts...JMO.
Click to expand...

I sprayed and checked for vacuum leaks. No rpm change when sprayed. I believe this is my cam.

http://www.summitracing.com/parts/cca-20-672-4?seid=srese1&gclid=COrRo-X8q70CFafm7AodIEEAog

My problem is I have low idle because if I turn in the idle speed screw too much it is exposing too much of the transfer slot and thus not running off of the idle circuit. Vacuum is very good in all other ranges. Is a .508 small in a small block street car?

best,
James
 
flyfish said:
Most carbs just have one power valve on the primary side.....but.....that is a pretty low vacuum reading for a relatively small cam. I would double check for vacuum leaks and increase the initial timing before spending money on any parts...JMO.
Click to expand...

20 seems like a lot for a street driven car? Please school me if I am wrong.

J
 
JamesRR said:
I sprayed and checked for vacuum leaks. No rpm change when sprayed. I believe this is my cam.

http://www.summitracing.com/parts/cca-20-672-4?seid=srese1&gclid=COrRo-X8q70CFafm7AodIEEAog

My problem is I have low idle because if I turn in the idle speed screw too much it is exposing too much of the transfer slot and thus not running off of the idle circuit. Vacuum is very good in all other ranges. Is a .508 small in a small block street car?

best,
James
Click to expand...

I had the old version of that cam in a 360 years ago. I had a hard time getting it to idle well too. I finally got tired of messing with it and locked out the timing at 34 degrees...after that it worked great and was very responsive. You are correct, In a 340 street car that cam is on the big side (my bad, I was thinking you had a 4" crank for some reason).... I have a tendency to see things a little differently because I race my street car a LOT, and I put up with my car not having the best street manors for the sake of power.

Oddly enough, my cam has the same duration as yours, but it is in a stroked small block and it is a little too mild for me....but I'll be fixing that soon enough. My motor pulls 12" of vacuum at idle with the timing locked at 34°. I'm not saying it is right or wrong the way I have my car set, but it works for me. It makes the car behave well enough that I can drive all over on the street and lay down mid 10 second runs at the track.
 
Adding more initial timing won't hurt anything. Eventually if you add enough it might get hard to start with a stock starter (like over 25° initial), but that's it.
 
Street car or not, the cam will be a bigger factor in initial timing than anything else. It's not stock so throw all the normal thinking out the window.

My 508 cammed 340 is locked out at 36*. Mini starters handle that easily. Older ones can have some cranking issues with the weak wire sizing.

IMO, no way will that run well with less than 20* initial, unless it's a 12.5:1 compression engine. Most I've fixed are in the 24+ range.

Before you spend one penny on power valves, buy a one or a pair of power valve plugs. Put them in the blocks as necessary and get the idle right. It will take that entire circuit out of play, still rich, it's not the PV's.... Then you can grab a vacuum reading and make a guess at what PV to use. I have a car that pull around 8" at idle with a 10.5 power valve, idles fine. Just don't drive the car around without the PV's!
 
flyfish said:
Adding more initial timing won't hurt anything. Eventually if you add enough it might get hard to start with a stock starter (like over 25° initial), but that's it.
Click to expand...

I have a newer starter and comp is 10.5:1
 
crackedback said:
Street car or not, the cam will be a bigger factor in initial timing than anything else. It's not stock so throw all the normal thinking out the window.

My 508 cammed 340 is locked out at 36*. Mini starters handle that easily. Older ones can have some cranking issues with the weak wire sizing.

IMO, no way will that run well with less than 20* initial, unless it's a 12.5:1 compression engine. Most I've fixed are in the 24+ range.

Before you spend one penny on power valves, buy a one or a pair of power valve plugs. Put them in the blocks as necessary and get the idle right. It will take that entire circuit out of play, still rich, it's not the PV's.... Then you can grab a vacuum reading and make a guess at what PV to use. I have a car that pull around 8" at idle with a 10.5 power valve, idles fine. Just don't drive the car around without the PV's!
Click to expand...

I believe I'll try the 20* of initial. I think my total is all in at 36.

J
 
You'll have to alter the mechanical advance to get the total right after you figure out your initial timing setting.
 
JamesRR said:
I have a newer starter and comp is 10.5:1
Click to expand...

You should be fine then. I have 10.8:1 compression with my car and the timing locked out at 34° and a newer "mini" starter....works great.
 
If it's a MSD pro billet, I have 10 and 14* bushings for them.

Mopar perf, depending on age will require welding or some work with a torks head and drill bits.

Find the point where the starter kicks back when hot/at running temps. That's your initial timing limit.
 
Get initial set, THEN, worry about total.

The bushing shouldn't matter getting the idle set properly.
 
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