63 R2 Avanti with auto trans stumbles. Need some good troubleshooting advice

[edpjr]

My car has been fully tuned. It has a T-bow Mallory electronic distributor, and the carb was rebuilt with a special T-bow R2 kit. The fuel pump was tested and it produces 7.5 psi normally, and 11 psi with the supercharger line connected. The volume flowrate is about a pint in 15 sec. The timing is set at 24 degrees @1600 RPM. The gas octane is about 100 average. The problem is that the car encounters a "dead spot" around 40 mph (it's pretty fast up till that point). Then it recovers and starts to run pretty good up to about 70 mph before wanting to top out around 80-90 mph. There's no leaking gas and the exhaust fumes don't seem heavy on unburnt fuel or smoke.

So, can anybody please give me advice or speculation as to why this car with good compression, rebuilt carb, excellent fuel pump, new elec dist and high voltage coil, all tune-up parts replaced, and new low-restrictive mufflers won't run like an Avanti's supposed too??? I'm at my wit's end. Thank you, Thank you for any suggestions.

[StudeRich]

What about the air gap in the Malory Dist.?

[edpjr]

Can you elaborate please?

[41 Frank]

There is nothing to adjust in the Mallory distributor it is plug and play.

Let me rephrase that , the only thing that is adjustable is the vacuum advance and you should not have to deal with that. I have one and it runs fine without adjusting it. I did adjust the rate of advance for racing but you should have no need to deal with that. The advance curve from the factory is closer to the R-1 distributor than the R-2. But it should run ok for everyday driving.

[JDP]

My car has been fully tuned. It has a T-bow Mallory electronic distributor, and the carb was rebuilt with a special T-bow R2 kit. The fuel pump was tested and it produces 7.5 psi normally, and 11 psi with the supercharger line connected. The volume flowrate is about a pint in 15 sec. The timing is set at 24 degrees @1600 RPM. The gas octane is about 100 average. The problem is that the car encounters a "dead spot" around 40 mph (it's pretty fast up till that point). Then it recovers and starts to run pretty good up to about 70 mph before wanting to top out around 80-90 mph. There's no leaking gas and the exhaust fumes don't seem heavy on unburnt fuel or smoke.

So, can anybody please give me advice or speculation as to why this car with good compression, rebuilt carb, excellent fuel pump, new elec dist and high voltage coil, all tune-up parts replaced, and new low-restrictive mufflers won't run like an Avanti's supposed too??? I'm at my wit's end. Thank you, Thank you for any suggestions.

I've had issues with the springs under the metering rod pistons. There are a half dozen different rate springs depending on vaccum/boost. It is also possible the rods/jets have been swappped at some point over the years.

[jclary]

Just a couple of thoughts from here...check to make sure that your gas cap allows the tank to breathe properly as you get on the gas. Could it be that the cap is performing partially but not enough to keep up? Also make sure that you don't have a dragging brake somewhere.

[bige]

Since this seems to be the dominant thread I will post my suggestion from the identical post in the Technical section with some additional thoughts..

Rather than using MPH as a reference point it would help if you noted the rpm at the point of the bog. It's possible that it occurs at the same rpm just at different speeds because of gear changes. Also is it a bog (complete loss of power) or a flattening ( steady rpm no increase in power )?

I have an idea what it might be but before I suggest a permanent mod to your carb try disconnecting the linkage that allows the secondaries to open. They actually open via spring pressure but there is linkage to keep them closed until the primaries are opened far enough. Doing so will eliminate or confirm the secondary fuel circuit, air valve, jetting etc as an issue.

I suggest this because it has been noted that blower boost pressure can keep the secondaries from opening or cause them to flap open and closed. It's real and happened on my Edelbrock conversion. I made changes that eliminated it and the improvement was instantaneous and obvious but it requires a non-reversible mod to the linkage that I wouldn't suggest doing to and original R2 Carb. However, if you still get the same bog with the secondaries out of the picture then we look elsewhere.

I also run without a choke plate so maybe before you do anything adjust the choke thermostat so it locks the choke open or wire it open as it can also close partially under boost and along with restricting airflow it will keep the secondaries from opening.

The other possibility is blower belt slippage occurring at higher RPM or aloss of boost through a leak where the carb hat meets the carb or through a bad pcv valve allowing boost to the crankcase while the blower is working its hardest. Removing the belts and testing with the hose to the hat removed will narrow it down to loss of boost pressure or secondary throttle plate issues but won't tell you which one is actually the problem so start at the carb linkage. If it still happens then leave the linkage off and move to the belts.

ErnieR

[edpjr]

I bought the R2 spring kit from Sunset.

There is nothing to adjust in the Mallory distributor it is plug and play.

Let me rephrase that , the only thing that is adjustable is the vacuum advance and you should not have to deal with that. I have one and it runs fine without adjusting it. I did adjust the rate of advance for racing but you should have no need to deal with that. The advance curve from the factory is closer to the R-1 distributor than the R-2. But it should run ok for everyday driving.

[edpjr]

It has new matched set of Gates belts. The S/C boost is high enough to levitate the chrome cover if not bolted down. The car has an aftermarket electric choke. I have a NOS brass PCV valve and hoses. I can't measure the RPMs easily since the factory Tach finally died completely. We used a remote tach when doing settings.

Since this seems to be the dominant thread I will post my suggestion from the identical post in the Technical section with some additional thoughts..

Rather than using MPH as a reference point it would help if you noted the rpm at the point of the bog. It's possible that it occurs at the same rpm just at different speeds because of gear changes. Also is it a bog (complete loss of power) or a flattening ( steady rpm no increase in power )?

I have an idea what it might be but before I suggest a permanent mod to your carb try disconnecting the linkage that allows the secondaries to open. They actually open via spring pressure but there is linkage to keep them closed until the primaries are opened far enough. Doing so will eliminate or confirm the secondary fuel circuit, air valve, jetting etc as an issue.

I suggest this because it has been noted that blower boost pressure can keep the secondaries from opening or cause them to flap open and closed. It's real and happened on my Edelbrock conversion. I made changes that eliminated it and the improvement was instantaneous and obvious but it requires a non-reversible mod to the linkage that I wouldn't suggest doing to and original R2 Carb. However, if you still get the same bog with the secondaries out of the picture then we look elsewhere.

I also run without a choke plate so maybe before you do anything adjust the choke thermostat so it locks the choke open or wire it open as it can also close partially under boost and along with restricting airflow it will keep the secondaries from opening.

The other possibility is blower belt slippage occurring at higher RPM or aloss of boost through a leak where the carb hat meets the carb or through a bad pcv valve allowing boost to the crankcase while the blower is working its hardest. Removing the belts and testing with the hose to the hat removed will narrow it down to loss of boost pressure or secondary throttle plate issues but won't tell you which one is actually the problem so start at the carb linkage. If it still happens then leave the linkage off and move to the belts.

ErnieR

[edpjr]

Refurbed gas tank, new lines, and new chrome gas cap (same as original). The brakes have been checked recently and are in excellent condition.

Just a couple of thoughts from here...check to make sure that your gas cap allows the tank to breathe properly as you get on the gas. Could it be that the cap is performing partially but not enough to keep up? Also make sure that you don't have a dragging brake somewhere.

[bige]

New or old choke doesn't matter if it's being forced close under boost and the belts don't have to be old to slip, ask any of the guys who race these things.

RPM, is an important piece of info while you're trying to solve your problem might be worth it to buy an inexpensive aftermarket tach and temporarily mount it where you can see it.

If you aren't watching RPMs how do you know you aren't just over revving the engine to the point of valve float?

Good luck.

[edpjr]

We'll definitely check out the choke and the belts theories. We can certainly remove the belts and and the hose to the carb and give it a try. That should sure eliminate a boost issue. As far as revving the engine too high - could that occur with the car in drive and simply giving it gas around 70 mph? Thanks for all the suggestions.

New or old choke doesn't matter if it's being forced close under boost and the belts don't have to be old to slip, ask any of the guys who race these things.

RPM, is an important piece of info while you're trying to solve your problem might be worth it to buy an inexpensive aftermarket tach and temporarily mount it where you can see it.

If you aren't watching RPMs how do you know you aren't just over revving the engine to the point of valve float?

Good luck.

[bige]

Well I have a little more insight now...you have a flat spot on part throttle acceleration would that be a fair description? I was thinking you are at full throttle and the engine bogs at 40 mph, recovers and you continue to accelerate until it loses power again at 80 mph.

If what you are feeling doesn't occur at full throttle then disregard all that I've said so far even though all of the above can occur it sounds like there is another issue.

JDP has you on the right track. The accel rods are pulled down by vaccuum and raise up as vacuum disappears momentarily as the carb opens to accelerate. You could have springs that are too weak keeping the rods from moving quickly or far enough to richen the mixture. Get an Edelbrock Tuning kit for the 600 CFM carbs 1406 style and experiment with stronger springs and maybe even a rod change. Easy to do and you don't have to remove the carb.

Even though your carb is rebuilt by a pro it still could require tuning. Even when the cars where new dealers had access to different rods and springs to cure driveability issues.

Stronger spring first and then if it helps but isn't perfect also do a rod change. If you get to that point you will need to mic the rod that's in there unless it's already been replaced with an Edelbrock piece because the originals weren't marked with the sizes like the new ones. There's a cross reference somewhere but it's just as easy to mic them. You want to go smaller on the thin end. The thick part of the rod sits in the jet at idle and at cruise and the thin part when accelerating or at full throttle. It's no big deal if you are a little smaller on the thick end also but avoid big changes like from 75 to 65.

ErnieR

We'll definitely check out the choke and the belts theories. We can certainly remove the belts and and the hose to the carb and give it a try. That should sure eliminate a boost issue. As far as revving the engine too high - could that occur with the car in drive and simply giving it gas around 70 mph? Thanks for all the suggestions.

[unimogjohn]

I had a similar problem, it would stumble at 40 mph and then recover somewhat. I did not have the distributer clamped tight enough. You might just want to do a quick check.

[edpjr]

Definitely worth checking out. The dist hold-down bolt is not exactly easy to get to even with the special distributor wrench. Thanks.

I had a similar problem, it would stumble at 40 mph and then recover somewhat. I did not have the distributer clamped tight enough. You might just want to do a quick check.