Proform 750 double pumper issues

[Paul G]

I brought home a used proform 750 double pumper. You would think I should know better than bring home a used carb, I am a glutton for punishment I guess. Disassembled the carb, cleaned out all the passages with carb cleaner spray, checked the jet sizes front and rear, squirters front and rear, everything was correct. Replaced the primary accel pump diaphragm since it was old and hard. New blue gaskets and it should be ready to go, but not.

Jets: main 74 sec 80
Squirters both 31
main air bleeds: #31 front and rear
Idle air bleeds: 70 front and rear
Idle fuel restrictor measures out about .031"

Base engine timing at 15°  

Transition slots are squared up front and rear, all four mixture screws set to 1 1/2 turns out to start.

Put it on the 383 and started it up. Set floats to just shy of center of the glass. Warmed up the engine till the choke came off. Felt strong till the choke came off, then the idle was rough. Adjusted the mixture screws for highest vacuum, they ended about 1/4 to 1/2 turn out to get smooth idle and highest vacuum. Made about 18" of vacuum.

Upon blipping the throttle, it would backfire through the carb and stumble almost every time. Lean pop right? Richened the idle mixtures to all 1 turn out and the backfire was mostly gone, but the stumble was still there. Swapped the primary squirter to a 35 I had, just a little less stumble. Upsized the main jets to 76, slightly better, maybe.  

Changed the spark plugs, swapped out the coils. Made no difference

What would create the need for idle mixture screws to want only 1/4 turn out for highest vacuum and smoothest idle?

I gave up and put the Holley 600 vac sec back on. It has none of those symptoms. Smooth idle at 12", blip the throttle with no hesitation and no backfiring.

I am stumped? What else can I check/do on the Proform?

[cdr]

1

[XH29N0G]

First off, I'll start with the qualifier that I do not know what the ideal set up should be for your particular engine.  I would consider contacting folks who might know a good starting point and assume that is where you are starting from.  From what I see, the set up looks like a good starting point, but maybe someone will have a better idea with that carburetor.  I'll list some thoughts below in case they help and then wait for others to pipe in.

I thought the IFR was what determined where the idle mixture screws were set.  The two determine the mixture at idle and then at cruise. My guess is that both also depend on the manifold vacuum under those conditions.  [EDIT] I just had a thought.  The IFR I am using (different carb) is considerably larger 036, which if I understand correctly would only make your problem worse (you would need to close the idle mixture screws even more).  This made me wonder if the idle mixture screws, or the places where they seat, might be damaged and letting in more fuel than you want at idle for any given number of turns in or out.  My guess is that this still won't get the cruise vs idle AFR change for a given vacuum/IFR, but it might make it difficult to easily set.

I have ben using an A/F gauge as an assist on this and it helps.  I'm not disputing that someone who has a better understanding cannot do it without, but I found it helpful. I also found that I needed to set for highest vacuum and then open 1/8 to 1/4 turn (richer) to solve lean issues.

[c00nhunterjoe]

Is your throttle "blip" primary only or primary and secondary? What is the rest of the engine build as i cant recall what is in that 383. 1st thoughts are if this is a wide open blip of a 750 double pumper on a low compression cammed 383.... well... a 750 is alot of volume for a tame 383, lets start there and wait to see the rest of the engine and how the blip is performed.

[Paul G]

Is your throttle "blip" primary only or primary and secondary? What is the rest of the engine build as i cant recall what is in that 383. 1st thoughts are if this is a wide open blip of a 750 double pumper on a low compression cammed 383.... well... a 750 is alot of volume for a tame 383, lets start there and wait to see the rest of the engine and how the blip is performed.

The engine is in the 73 Charger 4 speed car I picked up. The engine is a 68 383. 

I dont know the internals of the engine, but I do think it might be a little tired. There is no PCV on it right now, but it does smoke a little out of the breather on the valve cover when fully warmed up. With the 600 on it, it runs well, idles smooth, no mis or backfire, not lumpy like a big cam. The lifters are a little noisy. It has stamped steel rocker arms, the last 3 digits of the head is 346.

When I open the throttle and hold it, maybe call it 25%, the engine gains rpm till the squirter shot stops, then it bogs and backfires. When I open the throttle far enough to activate the secondary squirters, say 50% the same thing happens. When the primary squirters stop it bogs and as the secondary squirters stop it backfires.

I cleaned the plugs before I started the car. They looked new, 10 years ago, little or no run time. After my trials with the Proform 750 I pulled all the plugs again. They all were black and sooted up.

First thoughts are the problem is in the transition circuit. But is it too rich, or a lean bog?

[Paul G]

Here is a video of the start up.

https://youtu.be/KUTLeCC8xjs

[Paul G]

I have an old hacked up 3310 in the cabinet. I might try putting the primary metering block on the Proform, if it fits, and see what happens.

[c00nhunterjoe]

If you are holding constant throttle at any position and it pops and backfires after the pump shot then you either have a metering block or main body plugged most likely. You could try uppingthe main jets for giggles. Im trying to recall what i had in my 383 but im pretty sure it was more then 74s. I would stick 80s in the primary and see what it does.

[XH29N0G]

I am sure those who know have better resources and would be interested in learning of them.  My thinking about jetting and air bleeds came from this sheet"  http://www.quickfueltechnology.com.au/content/Quick%20Fuel%20Carb%20Size%20with%20details-AUST.pdf  I realize they are carb specific, and was assuming the proform and qft were the same main body in using this.

Keep up posted on the progress

[Paul G]

Yesterday I disassembled the Proform and took a real close look at the transition passages in both the metering blocks and the main body. They are clean as a whistle. Same for the air bleed passages. 

I didnt put it back on the car and start it. Holiday yesterday, the car is too loud, gave my neighbors a break.

Because of the spark plugs being so black and a little wet, overly rich right? And because of the mixture screws only being 1/4 turn out, could it be drowning the engine in fuel? I slipped a small wire, .017", in all four of the IFR's. See what that does with the mixture screws next time I run the carb on the engine.

If you are holding constant throttle at any position and it pops and backfires after the pump shot then you either have a metering block or main body plugged most likely. You could try upping the main jets for giggles. I'm trying to recall what i had in my 383 but im pretty sure it was more then 74s. I would stick 80s in the primary and see what it does.

I had a set of 76 jets, largest I have, tried them in the primary side, made little to no difference. I have much smaller jets I could try. That's going in the opposite direction, problem will get better, or worse? I am sure I could borrow a set of 80's from friends.

BTW, That start up video is with the 600 on it.

[XH29N0G]

I may be way off base here, but was under the impression that the IFR set the cruise AFR and then the idle mixture screws would be used to reduce the fuel at idle further so it could maintain an AFR within range.  My guess is that if you shrink the IFR, you will end up starving the cruise circuit even more. 

The problem with opening up the IFR, however is (unless I am mistaken) that it will require you to close down the idle mixture screws further and you are already only 1/4 turn out.  The 1/4 turn out made me wonder if there could be something wither with the conical tips of the idle mixture screws or with the hole they seat into, that is making them work incorrectly.

If this doesn't make sense, or if someone corrects this, ignore it.  Others know way more about this than me and I may be missing something like the jets on the main circuit or the air bleeds that could be a better solution.

[Paul G]

The IFR is the idle feed (fuel) restriction. This determines how much fuel can pass from the main well to the idle and transition circuits. By reducing the flow, with a piece of wire, it should allow the mixture screws to come out some. And, reduce the amount of fuel flow available to the transition circuit. The IFR does effect AFR at very low throttle opening, as does the mixture screws. At very low throttle opening, low speed cruising, the engine is still running on the idle circuit and the transition slot.

Once the throttle blades are open past the transition slot and air flow through the venturi is sufficient, fuel will flow through the boosters. Booster flow amount is controlled by the main jets.

That's if I got this carburetor stuff down right.

[torqueflite]

The front and rear idle mixture screws dont have to be set the same ..
I would try the rear just cracked open about an 1/8 or less and then work on tuning the primary side .
Transition slots in the front squared up and then use the rear throttle blade adjustment to help set idle speed
you may not need the wire.

[Paul G]

So today I tried putting the Proform on my 72 Charger 360. It has a Holley 670 SA that works very well. And it has an AF ratio gauge in the car. 

The Proform 750 still had the .017 wires in the IFR's, I jetted down to 66's in the front and 70's in the back. Changed both squirters to 25's. Got it to idle smooth around 13 AFR with the screws out almost 1 turn. I ran the old 750 on this engine set up similar to this. The proform had the same issues bogging and backfiring on this engine. But with the AFR I could see it was going off the scale lean as soon as the shooter was done. Not getting any fuel from the transition slot?

Next, I got the front metering block off the old 750 in the cabinet and put it on the Proform. Jetted it the same with 66's. No wires in the IFR in this block, still had them in the rear metering block. Started the car and adjusted the idle mix to around 13 again. Low and behold.... no bog and no backfire. But it was too lean everywhere. Tried to drive the car, but it was just to lean. But no backfiring. It would stumble till the main jets pulled in, then it would run in the 17's AFR.

Next, changed the jets in the old metering block to 72's, left the 70's in the rear. Switched the squirter's back to the original 31's. Started it up and now I could drive the car. Just a little off idle lean stumble, but no backfiring. Cruise in the 14's, WOT in the 12's. No point tuning it any further, I think I found the problem.

So I have a problem with that Proform metering block I guess. Time to call Proform and see whats up.     

[c00nhunterjoe]

You said your meter shows lean, but the plugs are black?

[Paul G]

Yes. Could be the constant shot from the squirters without any real run time.

[cdr]

You said your meter shows lean, but the plugs are black?

lean misfires can make a plug black

[Paul G]

The metering blocks are on there way to Proform.

I took a good look at the TB side of the metering blocks. There is a  channel in the metering block that feeds fuel to both the mixture screw and the transition slot, "B" in the pic below. There is a hole in the gasket right at the bottom of the channel that allows fuel from the slot to to enter the main body and feed the transition slot.

It is at "m" in this pic. (M) Idle transfer slot discharge: Idle fuel exits the metering block to deliver fuel to the transfer slot.

http://enthusiastnetwork.s3.amazonaws.com/uploads/sites/21/2008/07/ccrp_0807_05a_z-carburetors_basics_guide-metering_block.jpg


At this site.

https://www.hotrod.com/articles/ccrp-0807-holley-carburetors-basics-guide/

The hole in the gasket lined up perfectly with the hole in the main body, but this hole in the gasket obstructed the opening in the channel by a lot, maybe 40%. Probably why I was not getting much if any fuel from the transition slot. Both metering blocks were like this. The Holley metering block was not, the hole in the gasket was dead center over the channel.

The proform tech said he wanted the blocks and will send me new ones.

This is why the guy I bought this carb from said they could not get it to work right, no kidding. They thought it was to big for the small block Chevy they had it on.

[XH29N0G]

Paul,  Thanks for posting.  This makes a lot of sense. Looking forward to seeing how all works with the carburetor after this is resolved. 

[c00nhunterjoe]

Did you take pictures of the blocks by chance?

[Paul G]

I did not.

[metallicareload99]

... Set floats to just shy of center of the glass...

Forgive my ignorance of ProForm carbs, but is this the recommended float height?  On a Holley this would be too high IMHO, although my Holleys never seemed too sensitive to float hight

[Paul G]

Instructions for a Holley double pumper.

https://static.speedwaymotors.com/pdf/42576750.pdf


FLOAT LEVEL ADJUSTMENT:
NOTE: Do not try to remove the sight glasses. It takes special tools to install these and it is not recommended that they be
removed.
Primary and secondary float adjustments are set at the factory, but variations in fuel pressure could cause a change in these
settings. To aid in adjustment of the float levels, clear sight glasses are installed from the factory. The following procedure
shows how to make these adjustments:
1. Start the vehicle.
2. Observe the sight glass for the fuel level. If none is seen, the level is too low. If it is higher than the middle of the sight
window, it is too high. NOTE: A properly set float level will have the fuel level located at the middle or slightly below the
middle of the sight window, as shown by the line in Figure 12.
3. To adjust, shut down the engine.
4. Loosen the lock screw on top of the fuel bowl just enough to allow you to turn the adjusting nut. Hold the screw in position
with the screwdriver.
5. Using a 5/8" wrench, turn the adjusting nut in the appropriate direction: Clockwise to lower fuel level and
counterclockwise to raise the fuel level.
NOTE: Many customers attempt to adjust the float level down by turning the adjusting nut clockwise, only to see the
fuel level rise through the sight glass. This is due to the float being pushed down into the fuel, therefore
displacing the fuel to a higher level. You will find it easier to adjust the float levels, if you rev the engine
slightly between adjustments to evacuate the fuel.
6. Turn the adjusting nut in increments of 1/4 of a rotation.
7. Retighten the lock screw.
8. Restart the vehicle and observe the sight window.
9. Repeat steps 1 through 8 as necessary.

[metallicareload99]

Instructions for a Holley double pumper.

https://static.speedwaymotors.com/pdf/42576750.pdf


FLOAT LEVEL ADJUSTMENT:
NOTE: Do not try to remove the sight glasses. It takes special tools to install these and it is not recommended that they be
removed.
Primary and secondary float adjustments are set at the factory, but variations in fuel pressure could cause a change in these
settings. To aid in adjustment of the float levels, clear sight glasses are installed from the factory. The following procedure
shows how to make these adjustments:
1. Start the vehicle.
2. Observe the sight glass for the fuel level. If none is seen, the level is too low. If it is higher than the middle of the sight
window, it is too high. NOTE: A properly set float level will have the fuel level located at the middle or slightly below the
middle of the sight window, as shown by the line in Figure 12.
3. To adjust, shut down the engine.
4. Loosen the lock screw on top of the fuel bowl just enough to allow you to turn the adjusting nut. Hold the screw in position
with the screwdriver.
5. Using a 5/8" wrench, turn the adjusting nut in the appropriate direction: Clockwise to lower fuel level and
counterclockwise to raise the fuel level.
NOTE: Many customers attempt to adjust the float level down by turning the adjusting nut clockwise, only to see the
fuel level rise through the sight glass. This is due to the float being pushed down into the fuel, therefore
displacing the fuel to a higher level. You will find it easier to adjust the float levels, if you rev the engine
slightly between adjustments to evacuate the fuel.
6. Turn the adjusting nut in increments of 1/4 of a rotation.
7. Retighten the lock screw.
8. Restart the vehicle and observe the sight window.
9. Repeat steps 1 through 8 as necessary.

Good to know.  I've only had clapped out, used regular Holleys, so obviously that doesn't apply in this case.  It will be interesting to see what Pro Form has to say

[Paul G]

I got the metering blocks back from Proform. They are new blocks. I took some pics of the gasket. The first 2 pics are the secondary block, the 3rd and 4th pics are the primary block, last is the main body. The holes you can see are the idle fuel hole on the outside and the transition slot hole on the inside. Then the same gasket on the main body. The holes are perfectly centered over the holes on the main body. See what you think. Is the gasket restricting the transition slot in the metering block enough to cause a problem?

[XH29N0G]

I think it probably is fine as long as the restriction is not smaller than the holes on things like the IFR and other metering orifices.  My guess is that it is perfectly fine, but I suppose the holes could also be enlarged. 

I will be very interested to see how this turns out.  My guess is you found the solution, and that was also probably the reason for the carburetor being sold.

[Paul G]

I put the proform back on my my 72 with the AFR gauge. I can see what the carb is doing on the 72. The new metering blocks from Proform are on the carb. Adjusted idle mixture and idle speed. Stumble and backfire issues are gone. Drove the car, no issues. The carb was running very rich but I am not surprised. It is too much carb for a 360.

Not sure what was wrong with those original metering blocks? Possibly they were not the right blocks for this carb. Maybe they were supposed to be on a smaller carb or something?