Holley 750dp transfer slot problem

[godsmack635]

Hello all- I have a Holley 4150 750dp that I've been trying to get tuned in. I removed the edelbrock 600cfm and have been running this new one for a month now. I rebuilt the 750 carb with the kit I picked up and had it set with stock jets and power valve. After some trial and error I'm now running 72 jets primary and 82 secondary. Not far off from stock with a 4.5 power valve as I have between 8 and 9" of vacuum at idle. The idle mixture screws are about 1 full turn out from being bottomed out. This is where I had my higheset vacuum at idle. Both float levels are correct. The engine is a 383 with Eddy performer intake, 2 1/2" hedman headers, stock heads with a pretty decent cam. I do not have the specs on the cam but it sounds pretty lumpy. Initial timing is at 24* and total at 36* running an MSD 6AL setup. I bought this carb used on CL and it seems to be in great shape other than a few helicoils I've noticed in the main body. Car was running pretty good until I stripped a float bowl screw. I took the carb off and after fixing my problem I noticed the transfer slots on the primary and secondary plates were set up different. The primary was exposed about .080 versus the secondary side being completely covered. When I had the carb mounted with this setup it seemed to run way nicer at idle. After setting the transfer slots both primary and secondary to .025-.030 each. Now it runs like crap. I've read somewhere that both slots should be evened out like this but its running like it hates it. The carb now runs really rich at idle and won't even stay idling in gear. Do I have to drill holes in my throttle blades or should I put the transfer slots back to where they were? Only reason I adjusted the slots from where they were is because I thought it might help my off idle stumble. thinking I should have left them the way they were. Any suggestions or help would be great. Thanks!

 

[MikeyT]

Did you try and adjust the air fuel screws again?

Mikey

 

[Meep-Meep]

Put the secondaries back where they were. You want to set that little stop screw just so the blades don't stick and that's about it. Do all of your idling off the primary bores. Also that 750 is more of a race carb then a street carb but it seems you have a pretty hot cam so I expect it to do well in that application.

 

[godsmack635]

I did adjust the mixture screws again but I didn't get any results or rather it still wouldn't idle. When you say to set the stop screw on the secondary blades so that they don't stick, what do you mean? So the secondary blades do NOT sit flat? Slightly open is that correct? not necessarily exposing any of the transfer slot on the seconary side though?

 

[bm02tj]

off idle stumble is to little or too much squirt from acc pump
adjust or use different pump cam to suit motor

 

[godsmack635]

I may try a different pump cam or the same cam but different location for the screw. As of now I have a 37 nozzle up front and a 35 in the rear. I don't think it's the nozzle and I did check both primary and secondary pump arms with a .015" feeler gauge at WOT and both still had rebound in them. Also, the instant I move the throttle linkage the primary nozzle gives a shot of fuel. the feeler gauge has some drag to it between the arm and bolt in the idle position. I do not think it's the acc pump arms. Still need to figure out my idle situation.

 

[nm9stheham]

Put the secondaries back where they were. You want to set that little stop screw just so the blades don't stick and that's about it. Do all of your idling off the primary bores. Also that 750 is more of a race carb then a street carb but it seems you have a pretty hot cam so I expect it to do well in that application.

Check your manifold vacuum; it'll probaly be pretty low. And then do exactly as suggested above and I bet you're vacuum will be a lot higher. Leave the primaries as you recently adjusted them.

 

[Gpuller]

Now that you've closed your secondary's the idle screws do nothing? Screwed in or our there is no change? Ways around that is to bump your secondaries open a bit or drill holes in the primaries.

 

[GTX MATT]

Why does it run like crap now, does it idle lower? Or just rougher? Or what? Both transfer slots should be exposed only until they are square (around .030). If you need any more air than that you should then drill the throttle blades. .080 is a lot of exposure, were the mixture screws very sensitive? Is this a 2 or 4 corner idle carb? Which part number? Choke?

Also your jet spread is a little extreme IMO, what was the stock jetting?

 

[coloradodave]

If you take off the carb and look at the bottom of the baseplate you will see a recessed screw on the passenger side of the base that controls where the secondary blades sit. Back the screw out until the blades are completely closed, that is, the screw is not contacting the stop tang. At that point, loosen all 4 of the screws that attach the blades to the throttle shaft, and while applying a little pressure to close the blades further, assuring that the blades are centered in the bores, tighten the 4 screws. Then turn the stop screw in until it contacts the stop tang and add 1/8 turn more. This is just to make sure the blades don't stick in their bore, although with a double pumper there is mechanical linkage that should open them anyway. Set the primaries so that you can see a tiny bit of the transfer slot and you should be good to go. I'm sure you checked, but it is also possible to use the incorrect gasket on the metering blocks and power valve or to install the P/V with the gasket slightly off center. Another thing to check.

 

[64SF]

Had a very similar problem ended up drilling the throttle plates and it solved my problems. Drilled each of them 1/8".

 

[godsmack635]

I agree my jets are a bit spread out as far as 72 and 82 but I am still in the tuning phase. I'm still trying to get this thing to idle again. When I say it runs like crap I mean it runs really rough and really rich. I had both transfer slots set to square and it runs really rough now. I made sure when installing my 4.5 PV that the gasket was centered so I'm sure it's installed correctly. I am using the "re-usable" red metering block gaskets that came with the rebuild kit and there are no leaks or drips. I will set the secondary blades as suggested above and see how it runs. I think in this setup though I will be idling on the primary circuit rather than the idle circuit. As a friend if mine told me just give the engine what it likes. Also the carb is 4 corner idle setup model 4779-6 no choke. I have yet to pull the carb back off to reset the secondary blades more in the closed position.

 

[GTX MATT]

Stupid question but did you tune the secondary idle mixture screws? Or did you just set them all the same? You are going to need a different adjustment on the secondaries than the primaries.

 

[godsmack635]

I have all the idle mixture screws set the same. The secondary throttle blades have only about .030 of the transfer slots exposed while the primary transfer slots are larger now that I've had to increase my idle speed. I'm unsure of exactly how much I only know I've had to turn in the curb idle screw on the primaries to get it to idle at all in park. What kind of adjustment do I need to make if my primary transfer slots are now more exposed than the secondaries? lean out the secondaries?

 

[Meep-Meep]

I did adjust the mixture screws again but I didn't get any results or rather it still wouldn't idle. When you say to set the stop screw on the secondary blades so that they don't stick, what do you mean? So the secondary blades do NOT sit flat? Slightly open is that correct? not necessarily exposing any of the transfer slot on the seconary side though?

The adjustment procedure is what Colorado Dave said. You don't want to see a significant gap between the blades and the bore. Typically no transfer slot exposed on the secondaries but there is usually a tiny idle discharge hole.

 

[godsmack635]

If I set my secondary blades to eliminate the transfer slot then my primaries will have an over exposed transfer slot and be running on the primary circuit rather than the idle circuit. It actually ran smoothly this way before but I was under the impression that setting up my idle this way was incorrect and that I'm supposed to have both transfer slots exposed and no more than .025-.030 exposed on the primaries

 

[nm9stheham]

Whe you say 'lean out the secondaries' in post 14, are you talking secondary jets or the secondary idle mixture screws? One idea behind the primary and secondary being active at idle is so that you can find where the secondary idle mixture adjustments want to be, so when the progressive (transfer) slots open, the mixture will be close where it needs to be in the secondary transfer action. But if it don't work for your engine, it don't work.

I would start with the secondary idle mixture screws at the same point as the primaries, even though the secondary slots are pretty well closed. Then tune your secondary main jets as you normally would, keeping in mind that they will have some effect on the secondary idle mixture and you may have to re-rouch the secondary idle mixture screws to keep the light throttle and transition action smooth. Don't set the main jets for good idle; set the main jets as you normally would (for good moderate to heavy cruise running, etc.) and then adjust the idle mixtures to compensate for main jet changes to the idle mixture.

Hope that makes some sense! I am not sure I answered your questions.....

 

[Meep-Meep]

If I set my secondary blades to eliminate the transfer slot then my primaries will have an over exposed transfer slot and be running on the primary circuit rather than the idle circuit. It actually ran smoothly this way before but I was under the impression that setting up my idle this way was incorrect and that I'm supposed to have both transfer slots exposed and no more than .025-.030 exposed on the primaries

Then it seems to me you need more initial timing. This will allow you to back the idle stop screw down and close some of the transfer slot. A 750 DP should have no issue at all keeping a moderate performance big block idling. With a hot cam that has a fair amount of overlap (108 LSA) you will need at least 15 deg BTDC. Don't forget to limit the centrifugal to 38 deg total.

 

[godsmack635]

Before I pulled the carburetor I was running at 24* initial and 36* total. Advancing it a little more may help. I am still waiting to throw this all back together and see where I'm at.

 

[Cornpatch MO]

No one has mentioned what your idle RPM is set at. If your cam demands a higher idle RPM, wouldn't that put you out of the idle circuit? .........................MO