Carb Assistance

[Bob Chadwick]

I have been having a high speed miss on my car and the suggestion is that the problem is carb related, specifically a lean condition that is causing a high speed lean surge during highway runs of any length of time.

I have found the number on the carb (17056208) and determined that it translates to a post 1975 Rochester carb (170) manufactured in 1976 (56) for 49 states (2) for Chevy (0) for use in an automatic (8). There are some other numbers on it that indicate when it was manufactured exactly but I left them at home and will post them later.

I had the original carb replaced two years ago and was told that I was getting a matching carb but I appear to have gotten a carb for a 1976 car.

I'm unsure from the perspective of jetting, hangers, rods and tension spring wind up whether or not there is any difference between this carb and what would have come on a 1977. Lars Grimsrud has written a technical paper entitled "How to Tune a Q-Jet" that assists with identifying your carb and it's characteristics. Unfortunately this carb is not listed on the chart included on page 16 of the report so I can't determine the carb characteristics. Does anyone have information on this carb? An Internet search suggests that it came off a 1976 Chevy C10 truck with a 350.

It would appear that the correct carb for a 77 a/c Vette with an automatic is either 17057204 or 17057228. Can someone help with the correct selection.

Thanks.

 

[photovette]

1977 L-48 w/air + auto 17057204

1977 L-82 w/air + auto 17057228

1976 L-48 n/air + auto 17056208

Data from the Corvette Restoration Guide 1968-1982, Richard Prince

 

[Bob Chadwick]

Thanks Photovette. Appears I should have 17057204.

Where did you pull up this info and do you have the differences between the 76 non air and the correct 77?

 

[photovette]

This looks helpful:

http://www.recarbco.com/technical/rochester/qjet.html

My '77 w/air + auto + 48s had the 17057204

 

[Bob Chadwick]

Same car as mine.

Interesting site. I'll need to take a look at it tonight. Thanks for the help.

 

[motorman]

I have been having a high speed miss on my car and the suggestion is that the problem is carb related, specifically a lean condition that is causing a high speed lean surge during highway runs of any length of time.

I have found the number on the carb (17056208) and determined that it translates to a post 1975 Rochester carb (170) manufactured in 1976 (56) for 49 states (2) for Chevy (0) for use in an automatic (8). There are some other numbers on it that indicate when it was manufactured exactly but I left them at home and will post them later.

I had the original carb replaced two years ago and was told that I was getting a matching carb but I appear to have gotten a carb for a 1976 car.

I'm unsure from the perspective of jetting, hangers, rods and tension spring wind up whether or not there is any difference between this carb and what would have come on a 1977. Lars Grimsrud has written a technical paper entitled "How to Tune a Q-Jet" that assists with identifying your carb and it's characteristics. Unfortunately this carb is not listed on the chart included on page 16 of the report so I can't determine the carb characteristics. Does anyone have information on this carb? An Internet search suggests that it came off a 1976 Chevy C10 truck with a 350.

It would appear that the correct carb for a 77 a/c Vette with an automatic is either 17057204 or 17057228. Can someone help with the correct selection.

Thanks.

17056208 1976 C-10 chevy truck,350 cu in light duty with auto trans and federal emissions. pri jets #77,pri metering rods #48, secondary metering rod tip diameters .057. 17057228 corvette only,auto,A/C,federal emissions. 17057204 350 cu in chevelle,monte carlo,nova and corvette, A/C,auto federal emission.the 228 and the 204 have the same secondary metering rods .057 diameter. the pri metering in the 228 is 53 rods and 77 jets and the 204 has 52 rods and 77 jets. the air bleeds could be the difference in the primary jetting of the 228,204 vs the 208

 

[photovette]

You're welcome--it's why I love this site--you can almost always get an accurate answer to any question in minutes--I'm glad I knew the answer to one for a change.

 

[Bob Chadwick]

Thanks as well motorman. Appears that the primary jets are the same size on both carbs but the primary rods appear smaller with a #48 for the carb in the car v. a #52 for the carb that should have been in the car.

The chart I have shows the secondary metering rods as being CH for the secondary rods. You indicate that the secondary metering rod tip diameters are .057 on the 76 carb in the car. Do you know what this translates into in the letter size?

The chart also lists spring and Jet area at cruise and WOT. What does this information mean and do I need it. Or is just a function of the jet and rod variables?

Thanks.

Bob

 

[motorman]

Thanks as well motorman. Appears that the primary jets are the same size on both carbs but the primary rods appear smaller with a #48 for the carb in the car v. a #52 for the carb that should have been in the car.

The chart I have shows the secondary metering rods as being CH for the secondary rods. You indicate that the secondary metering rod tip diameters are .057 on the 76 carb in the car. Do you know what this translates into in the letter size?

The chart also lists spring and Jet area at cruise and WOT. What does this information mean and do I need it. Or is just a function of the jet and rod variables?

Thanks.

Bob

the CH= .057 diameter. a high speed miss is usually a lean condition and you should be sightly rich. make sure your secondary is opening the whole way. some Q jets had a air valve stop to prevent the secondary from opening the all the way,90 degrees, but i do not know for sure if that is your case. the air valve can be made to open the whole way by filling the air valve stop shorter. make a full throttle run with rew plugs,make a engine kill at speed and check the plugs. real white means lean and dark plugs mean rich or the ignition is breaking down

 

[Hib Halverson]

Plug runs at WOT are not going to help with finding a part-throttle miss.

In reading this thread it seems the carb on the engine now is reasonably close in calibration to the one used previously, so, unless the "new" carb has a problem that's restrictiong fuel flow at part throttle, I'd look elsewhere for this miss.

I can't remember if a 77 has EGR, but, if it does, I'd make sure that system is working properly. After that, I'd check the engine's ignition system. Sometimes, part throttle cruise mixtures are the hardest to light because, even when the carb is calibrated right, they are pretty lean.

 

[Bob Chadwick]

Hib,

The car had an EGR, which was removed when the stock manifold was replaced with the Edelbrock Performer, 2101 as I recall. Could removing the EGR and not capping everything off correctly have started this problem. The original rough running at idle and at speed began when the manifold was replaced, though the high speed problem has gotten worse since the advance was increased.

I've spent quite a bit of time with the ignition and I'm reasonably certain that it is firing correctly and with a cruise advance of 54 degrees, I've been told that this is right on for the small block.

Bob

 

[motorman]

Hib,

The car had an EGR, which was removed when the stock manifold was replaced with the Edelbrock Performer, 2101 as I recall. Could removing the EGR and not capping everything off correctly have started this problem. The original rough running at idle and at speed began when the manifold was replaced, though the high speed problem has gotten worse since the advance was increased.

I've spent quite a bit of time with the ignition and I'm reasonably certain that it is firing correctly and with a cruise advance of 54 degrees, I've been told that this is right on for the small block.

Bob

since you have a HEI i would make sure the wire are all the way into the cap and make sure the metal ground strap is in the distributor under the coil cover. also check to make sure the rotor is not burned thru. check the spark color as it should be bright blue/white not orange.

 

[motorman]

Plug runs at WOT are not going to help with finding a part-throttle miss.

In reading this thread it seems the carb on the engine now is reasonably close in calibration to the one used previously, so, unless the "new" carb has a problem that's restrictiong fuel flow at part throttle, I'd look elsewhere for this miss.

I can't remember if a 77 has EGR, but, if it does, I'd make sure that system is working properly. After that, I'd check the engine's ignition system. Sometimes, part throttle cruise mixtures are the hardest to light because, even when the carb is calibrated right, they are pretty lean.

his first post says "high speed miss"

 

[Bob Chadwick]

since you have a HEI i would make sure the wire are all the way into the cap and make sure the metal ground strap is in the distributor under the coil cover. also check to make sure the rotor is not burned thru. check the spark color as it should be bright blue/white not orange.

I need to pull the new cap, coil and rotor that came with the MSD distributor and put the old one back on to see if I have a problem with that. I want to run it this weekend and make sure I haven't made the problem go away with some of the shielding that I have done on the pickup wires.

 

[Bob Chadwick]

his first post says "high speed miss"

Hopefully I haven't mislead. By high speed I mean freeway speeds of 65 - 70 mph, hey, that's high speed for the old 77 with an auto 3 speed, where the throttle is constant at what I understand to be about 1/2 to 3/4 open. The miss does not go away with WOT and only comes up after cruising for awhile. Once you go back to idle for a short period of time it's like nothing has happened.

 

[MoeJr]

The carb you have in the car is slightly richer than the stock one. I would definitely check the EGR system. if the car still has the stock exhaust I would check for a plugged cat. converter. I had a 77 L48 Auto and AC car just like your and at 70 on the highway my pedal was not even half way to the floor. Also may wanna check ignition module if you have not done so already. Heat effects them alot when they start getting old.

Also have you tried putting the stock HEI back in?

:bar

 

[Bob Chadwick]

Moe,

The EGR went out the window with the Performer. What would I check?

The cats are new.

Ignition module is new.

I may ultimately end up with the distributor out but I haven't gotten there yet.

Am I correct that this carb is richer because of the smaller primary rods, which allow more fuel to flow by blocking less of the jet?

Bob

 

[motorman]

Hopefully I haven't mislead. By high speed I mean freeway speeds of 65 - 70 mph, hey, that's high speed for the old 77 with an auto 3 speed, where the throttle is constant at what I understand to be about 1/2 to 3/4 open. The miss does not go away with WOT and only comes up after cruising for awhile. Once you go back to idle for a short period of time it's like nothing has happened.

try opening the idle mixture screws several turns to richen up the carb because the idle feed continues to fuction at any speed. did you check your float level and fuel pressure because low fuel level in the float bowl will cause a lean condition.

 

[Bob Chadwick]

Fuel pressure and float level are both on the list. I don't know anything about this carb and was concentrating on what I know first. I've just about exhausted the electrical and am trying to figure out how the Q'jet works.

 

[MoeJr]

Yes the smaller primary rods make that carb slightly richer.

The more I look at your problem the more I think ultimately it will be the MSD system.

Moe