Carb problems/Holley 350

[Ronn]

Ok, I had problems before and posted about it and got some fixes from it. I put the 350 back on the old engine which is still in the car with the clifford adapter but this time I have a 7.5 PV, the correct bowl gasket with a hole for the Accel pump..(the one I had did'nt have one so I made one but it leaked back into the bowl ) It has the 50 cc diaphram and stock 280 nozzles. I think anyways.

Well, the problems I had before were it would bog out if I so much as tried anything more than 1/3 throttle at low rpms or off the line. It would just die out like fuel was cut off or it was flooded very rapidly. So I tried different jet sizes and power valves with no change in problem. Also noticed was the fact the air fuel screws have to be all the way in for it to run smooth. Even one turn out roughened the idle. Thats a problem as well.


Ok, well now that the correct PV is in, and the bogging out was due to lack of accel pump pumpage and thats fixed..I can stomp it now and she goes but...I still have to have the air fuel screws all the way in. Is this a rich condition? Oh, and there's this kind of whistle coming from the carb almost like air being sucked in a tiny hole..But I can't find any! I only hear it when accelerating. The fact that the screws have to be all the way in is just like what my lawn tractor was doing..come to find out that the carb had a huge crack in it. I fixed that and now it runs right with the air fuel about 3 turns out and it will run rough in both directions instead of one which means the mixture adjust is working.

Why I mention my lawn tractor is that it sounds like theres a leak in the holley but I can't find any. The bowl vent tube that sticks straight up, it has vacuum on it. It increases with engine speed and if I cover it the engine shuts off. Is that normal? Do I need to down size my jets to get correct air fuel mix? : :

 

[wsa111]

Ronn, If you can screw the idle screws all the way in and the engine still runs you have a blown power valve.

I used a 59 main jet, 8.5 power valve, .031 accerator discharge nozzle with an orange cam in the #2 position.

If you are using a hotter camshaft use direct manifold vacuum to the distributor, not the ported vacuum only available with the 7448 carb.
You don't need the 50cc accerator pump. Good luck. William

 

[Ronn]

But the PV is brand new. I mean less than 2 days old now. The idle adjustment has been like that since I got the carb. I put in smaller jets and it sux. Even less power. Right now as it sits, the RBS 1 bbl beats this holley hands down. I should be feeling a a marked improvement with the 2 bbl but it's in reverse. I could take the PV out and check it out but I doubt it's blown. I mean how does that happen anyway?

 

[66200i6]

A 350 CFM carb should be more than enough unless there is a demand for an above average airflow needed to run. Modifications?

Check down the throat of the carb and look at the position of the throttle plate. There is a defined dimension for clearance from the edge of the throttle plate to the wall of the venturi. This clearance is needed for correct activation of the idle circuit. If you see a larger gap than defined, the airflow past the idle activation holes in the throat of the venturi is not high enough to activate that circuit. If that is the case, then you are probably idling with the main jet. That would explain why you are able to completely shut off the flow of fuel to the idle circuit and still have the engine running.

I have only seen this happen once before. The CFM rating of the carb was below that which was standard for that application.

Good luck, Ric.

 

[Ronn]

So if theres more clearance or Gap than should be, then maybe I should try backing off on the idle screw and then adjust the air fuel screws out?

William, the PV is ok...now I'm stumped.

I have no mods done to this engine. It is a basically stock 73 250 with D2 spark, K+N, MSD Coil, accel 8.8 wires and indexed plugs. What I'm trying to do is get this carb as good as it can get so when I install the fresh built 250 I have almost done, there won't be as much tuning needed. Everything works on this carb except whats stated. Timing advance works correctly also. It just runs like a dog past half throttle. It may be the clifford adapter causing it to run like that.

Whats baffling to me is that the RBS carb has a wider, much wider throat than the butterfly. The butterfly measures 1.75" and matches the log hole, the throat is almost 2 1/4" wide. Where the holley is smaller at the top of the throat by quite a bit. Backwards they are. I drilled the RBS jet out to a 90 a few months back and noticed a nice difference. Why is it with this holley with 2 75 jets, runs like a dog? I had 120's in there yesterday and it ran better...so how is it William that your modified to the hilt engine only needs 59's? God, I don't want to know how mine would run with those. Right now, seems the bigger I go the better..but gas mileage sux worse lol. I need to do something before I scrap this whole idea.

Thanks for the help as I'm out to try your idea Ric.

 

[wsa111]

Ronn, Have you checked your float level??
Have you checked your fuel pressure & volume of the fuel pump. These holleys prefer 6# to 8# of fuel pressure.
Check these. If the above is correct can you swap a buddys holley to check & see if you have a warped metering plate or god knows?
There is no way in the world you should require over a 63 jet. They come stock with a 61. the carb is jettted for a V-8 engine thats why you can run leaner with the smaller engines. William

 

[Stubby]

I think you should look at the butterfly setting in relation to the off idle transfer slot. You will probably find that they are open too far.
Do you have the pcv valve hooked up? If this is an emissions type carb it gets some of its idle air from the crankcase via the pcv valve. I have seen a lot of guys eliminate the pcv on perf. applications and you can end up with an engine with less man. vacume trying to pull more idle air thru less throtle opening. The butterflys end up so far open that you usualy end up with a low rpm lean sag that can be partialy covered by richening everything.

 

[Ronn]

Well, it's late but I will address both of yalls ideas. I did run the idle screw all the way out and to my surprise the idle did'nt go below 650! If I tried that with the RBS the engine would shut off before the screw was all the way out. Also after doing so, the idle mixture screw problem did'nt change. I'm sure about float level, I have a stock fuel pump, and the carb is absolutely clean. 125 lbs of compressed air made sure of that. I mean it ran like this before, then I re-re-rebuilt it and re-re-cleaned it only to have the same problem.

Hopefully I will get off work on time tommorow to check it out further.

 

[wsa111]

Ronn, If you know that the float level is correct & the fuel pressure is correct, its time to move on to another carburetor for a comparison. Something is F-uped somewhere.

Hope you find your problem?

My 350 or 500 carbs rock. good idle & great drivability. I hope you find your problem. William

 

[LEROY POLL]

HELLO RONN

.....I HAVE READ THE POSTS AND WONDERED IF YOU WOULD LIKE ANOTHER TRY AT LOOKING AT WHAT'S GOING ON.

.....SOUNDS LIKE TO ME THAT YOU MUST HAVE AN AIR LEAK.

.....REASON ONE....MOST MOTORS WILL NOT RUN AT ALL WITH OUT THE IDLE SYSTEM WORKING. YOURS IS CLOSED OFF....! YOU MUST BE GETTING AIR SOME WHERE.

.....REASON TWO....THE JET SIZE IS TOO BIG. MOST 350'S WILL RUN WITH JETS OF #57 to #61'S. YOU SAID YOU CAN ONLY RUN WELL WITH #65'S. WHEN TOO RICH AT IDLE AN AIR LEAK WILL NOT SHOWUP VERY WELL. TURNING DOWN THE IDLE SCREW TO CLOSE THE BUTTERFLIES TIGHT AND THEN BACKING OUT THE IDLE AIR ADJUST SCREWS TWO TURNS WILL SHOW THE LEAK UP MORE.

....REASON THREE....THE SOUND OF THE WHISLING WOULD BE MOST LIKELY THE SOUND OF AIR AND FUEL NOT MIXED AND BEING PULLED INTO A LARGE HOLE AROUND THE BUTTERFIES AT LOW SPEED. THE SOUND WILL SOUND LIKE AIR WITH A GURGLING SOUND ALSO.

.....REASON FOUR....THIS MOTOR SHOULD IDLE DOWN TO 500 RPM AND BE SMOOTH. 600 RPM IF YOU HAVE A AUTO TRANNY. YOU SHOULD BE ABLE TO GET THE MOTER TO IDLE JUST LIKE THE SMALL ONE BARREL WITH LITTLE DIFFERANCE.

..... LOOK FOR LEAKS AT THE BASE OF THE CARB. AND AT THE MANIFOLD. CHECK FOR VACUUM HOSES THAT ARE OFF OR CRACKED.
THE LOG MANIFOLD WAS GOOD FOR SOMETHING....NOT MANY LEAKS.

.....LOST OF LUCK....LOL

LIVE IN GRACE

LEROY POLL