Rebuilt Carb, all bog except idle

[Kjetil]

Hi

I rebuilt my second carb this winter with Cliff's HP kit. It was going on a Chevrolet Performance 454HO crate in my 1975 Monte Carlo. Cliff recommended to use the second recipe for the idle system. Besides that I modified the secondary side slightly. So the specs are:

Carb specs:

# 17080202
Choke removed and plugged.

Idle tube: 0,039"
Idle down channel: 0,059"
Lower idle air bleed: 0,07"
Upper idle air bleed: 0,07"
Idle screw holes have been opened up

Accelerator discharge holes: 0,031"

Main jets: 77
Rods: 44

Float level: 1/4"

Secondary POE well restriction: 0,039"
Secondary POE restriction: 0,059"
Secondary tube restriction: 0,036"

Secondary flaps are notched, and the spring tension is 3/4 turns. Choke pulloff is new from Cliff and unmodified.

Engine specs:

Compression ratio: 8.75:1
Camshaft duration (@.050 in) :211 degree intake / 230 degree exhaust
Initial timing is at 8-9 degrees BTDC
Edelbrock RPM fuel pump
Mr Gasket Spreadbore to Squarebore adapter

The Problem
The car idles nicely with Idle screws about 4,5 turns out. If they are less than 3 turns the engine dies. If I rev the engine at Idle in park before it's completely warmed up, the response is crisp and no bog. When driving even the slightest throttle makes it bog. It doesnt go away up through the rpms. The only situation it doesn't bog is when applying a feather's weight to the pedal. But then it's probably drawing mostly from the idle circuit?

At WOT throttle it still bogs, but slightly less than at part throttle. Tried to adjust the the APT. It was actually all the way down after the rebuild. Then I set it 3,5turns up, then 4,5 and it seemed to improve a bit, but really hard to tell. Overall the engine feels choked. Also tried to physically block off the secondary flaps, but that seemed to worsen the problem.

The engine has been running hot during the test drives. Goes up to 220F and stays there. I have a suspicion that the gauge reading is off, but for now I assume it is correct. If I rev the engine again in park after a drive, the response is really slow, like 1 second after the throttle is twisted.

Some comments that might be relevant:
- the carb has been turned upside down after it was reassembled (goofball friend was visiting)
- when reving in park I observed the secondary flaps opening slightly
- I've got and external filter in addition to the internal fuel filter

Any suggestions on the issue?

[Kjetil]

By the way, I also noticed fuel around the edge of the air horn gasket. Could leaks here cause the problems? I did sand the underside of the horn when rebuilding the carb, and used the thickest gasket in the HP kit.

[429bbf]

if i had to guess id say your acc pump is sticking. if you shut it off with it warm and look down inside is it getting any pump shot. could have pulled the cup off . rare with chiffs stuff but it happened to me with a napa kit in less than 20 minutes. let us know what you find.

[Cliff Ruggles]

Did you set the APT using our "tip-in" procedure?

Did you verify accl pump shot with any throttle movement right off idle?.....Cliff

[Kjetil]

Did you set the APT using our "tip-in" procedure?

Did you verify accl pump shot with any throttle movement right off idle?.....Cliff

Thanks for the replies.

I didn't do the complete tip in. Reved it up to over 2000rpm and locked it with the screw. Then I held my hand over the top to see if the RPM increased, which it did not. I was a little worried about the temperature at this point, so I  aborted the tip-in without pulling a vacuum hose. 

Regarding accelerator pump shot it is a big squirt with a quick movement, and almost a solid beam with a slow movement. The latter also seems to go all over the place, but mostly into the barrels. I'm afraid I oversized the holes here. But could the allround bog be caused by the pump?

[429bbf]

maybe its to much fuel (you say it runs fine when its cold needing more fuel ) try moving the acc. pump to the out side hole (less fuel )and check your timing when its warm . maybe something is sticking in the distributor when it heats up. just a couple of suggestions.let us know what you find.

[429bbf]

one other thought i had a pickup act similar to that. the steel fuel line touched the side of the engine and the fuel boiled before it got to the carb . i took a 3ft chunk of hose and ran it out to the fender well and back to the carb to see if it was the problem. well problem solved.i rerouted the steel line and insulated it. ran good after that.

[Kjetil]

I'll try the accelerator pump tip 429bbf. But since the problem sustains through the rpms I doubt it's the root cause. maybe it'll fix the reving while hot problem.

My fuel line passes directly above the heater hose, so it does get quite warm.

This weekend I'll do a more thorough vacuum leak check by spraying starting fluid around the base, and take the carb off for a visual inspection.

Thanks

[Kjetil]

Worked on the issue this weekend.

First I did a check for vacuum leaks by spraying starting fluid around the carburetor. No indications of rpm increase from this. Then I hooked up a vacuum gauge and got a reading of -15in at idle. I concluded there were no leaks judging by this.

Next I took the carburetor off the engine, and separated the air horn from the main body. The fuel level in the bowl seemed very high, but I verified that the float was at 1/4" as the recipes recommend. The gasket was soaked in fuel, and even the top of the splash guard was wet. Inspected the power piston and rods, which had a nice smooth stroke through the jets.

I put it together again and looked at the accelerator pump shot. Fuel was literary pouring through the bores when it sprayed, dripping down on the floor. This must be way overkill. I put the carb back on the car and hooked the linkage to the outermost hole on the pump lever just to see if it got better. There was a slight improvement, but still bog all over the place. Again I noticed that the air horn gasket was soaked in fuel.

Then it struck me that this behavior was very similar to the time my old carb flooded. So I started looking into float settings. Cliff recommends 1/4" in the recipes, but I read an article by Lars Grimsrud which suggested a float level of 0.375-0.4" for late model carbs. I took the carb apart again and set the float level to 0.4". 

This made a huge difference, and now the bog i the higher RPMs was as good as gone. I did the break in procedure for my engine which involves runs up to 4000rpm at 50% and  100% throttle, and the car ran pretty good with a mighty roar when the secondaries opened. I also noticed that the air horn gasket was now pretty dry compared to earlier. 

But there is still a bog at part throttle, and the engine feels weak down there even though it should generate close to 450ft/lbs below 2000rpm. Tried to adjust the APT in both directions from a base of 3.5 turns, but there was no improvement in either case. Bumped the initial up to 12 degrees and got the vacuum up to 16-17in at 700rpm, which I reckon is pretty good?

Could it be that the float has to go even lower, together with a downsizing of the accelerator pump discharge holes?



 

[Kjetil]

Here's a clip of the accelerator pump test:

http://www.youtube.com/watch?v=2mvYnV0plyI

[Cliff Ruggles]

Fuel level and float settings are directly related to fuel pressure.  Most late model fuel pumps had high pressure, and they used a return system to keep fuel in circulation.  This accounts for the lower float settings listed in the build specs for many later units.


"Bog" is most often a lean condition.  If you have a large MAB carburetor, this is not uncommon even with huge jets and small metering rods in them. 

Timing is equally as important as fuel delivery.  If the ignition timing isn't enough for the engine it can't burn a lean mixture and a "bog" or lack or power will result......Cliff

[Kjetil]

Thanks Cliff.

It is a MAB carbuertor, but I thought your choice of jets (77) and rods(44) for the setup would account for it. I did order air bleed inserts from you as well. Would it be possible to size them so I still can run the same jet/rod combo, while getting a slightly richer mixture?

Gonna try to resize the accelerator pump holes and do the tip in again next week.

[Cliff Ruggles]

I've set up hundreds of the large MAB carburetors with 76 or 77 jets and 44 metering rods.  Never had one yet that didn't run fine everyplace.  I still like going to the smaller MAB's and much smaller jetting.  Dropping back to .070" MAB's and 72 or 73 jets with the 44 rods is a good place to be.  The smaller MAB's richen things up clear across the load/speed range without needing huge jets.  Less efficient engines will like this deal, especially those with big cams in them, relatively low compression, and poor low speed characteristics.

The accl pump is only there to cover up a potential lean condition with quick throttle movements.  Might want to try disconnecting it in association with tuning the main jets, metering rods and airbleeds to take the additional fuel out of the equation.

I seldom open up the accl pump discharge holes past about .032", .026-.028" are fine for most applications.  Really hard leaving cars with excellent power to weight ratios (drag strip cars) may enjoy holes up to about .036".......Cliff