carb tuning issues

[droppedbrd]

engine idles fine ( a little rough but its probably the cam)

runs great at W.O.T.

falls on its face when "maintaining speed"

if I mess with the choke (force it to be closed all the time) it smooths out but runs WAYYYYY rich


help?

 

[ludwig]

You are changing the air/fuel mix when you force the choke closed. It smooths out because you have richened the mix. Thus, you must be running lean. You need to adjust the fuel/air mix screw. You could just back it out some (counter clockwise) and see if that helps. It should because it is making the mix richer.

You could do it a little more scientifiic though. Get a vacuum guage (+/-$18.00). Connect it to a vacuum source close to the carb base and, with the engine running in Park or neutral, see how much vacuum you are pulling. Now turn the fuel mix screw in until the engine stumbles. Now watch the vacuum guage and back the screw out slowly until you get the max vacuum. Just to be sure, back the screw out and see how much the vacuum falls off. Now turn it back in until you have max vacuum again; should be 18 or more lbs.

You can also use the vacuum guage to tune it. Loosen the distributor retainer and turn the dist forward or backwards until you get max vacuum. It should be at least 18 lbs and can go up from there. When you run it afterward and it starts to knock on hard acceleration or with your foot in it going uphill, get out and turn it back (clockwise) just a touch.

For the real deal tuning, you need a dwell tach and a timing light, but the vacuum guage will help you with the fuel mix and getting the advance in the right place.

 

[ludwig]

Of course the vacuum will not be 18# if you are running a non-stock cam with more overlap, might be lower, but you want max vaccum anyway.

 

[CZLN6]

Howdy back DB:

Please remind me. What engine, cam and carb are you running? What distributor/ignition? Auto or manual trans? Elevation? Are the carb and distributor compatible?

Things to check in priority order-
1st- check for and eliminate any vacuum leak(s).
2nd- check the float level for a too low setting
3rd- check the initial spark advance setting

Any of the above can cause a lean condition, especially at steady speed/high vacuum situations. Vacuum leaks are a no brainer but too low a float setting or too little initial advance have to be considered and eliminated as the cause before going on.

Ludwig's suggestions with a vacuum guage are good. A low reading may also indicate a vacuum leak, late ignition, high elevation, non-stock cam overlap.

Since your engine idles fine and then falls on its face at highway cruise speeds. I'm guessing it is not the low speed idle screw setting, but a problem in the vacuum advance system. Increasing the richness of the idle air screw will help to richen at idle and transition, but won't have any effect at highway cruise speeds. Have you changed the main jet(s)?

Help us to help you by giving as many details of your circumstances as possible.

Waiting to hear back.

Adios, David

 

[gtm1086]

What carb are you running. Sounds like it is lean

 

[droppedbrd]

its a 200 with a mildly ported 250 head, clifford dual header, dsII dist. and msd 6a w/ 90's mustang coil, dont remember the specs on the cam (don't think I was ever told what they were) but it wasn't anything too radical. its a T-5 conversion manual trans. My car is at a buddy's shop at the moment because I had a small mishap with the water pump pulley and his shop was the closest place to flat-tow it (so as far as carb and such I have no way to check at the moment)

I was talking to my dad the other day and he said it sounded like a float issue too

 

[droppedbrd]

aparently its an 1100 carb

 

[droppedbrd]

oh yeah elevation 500 ft. I'm in Portland Oregon and its a '65 block

 

[gtm1086]

Sounds like it is the stock carb. When was the last time it was pulled apart and cleaned. Could be a float issue.

Now I have tried quite a few different carb set up's and I feel the best to date has been getting the adapter to fit a two barrel Holley and then get a new 350 cfm Holley. Baxter auto parts has the carb in stock. Probably not the adapter though.

 

[ludwig]

aparently its an 1100 carb

He says its an 1100.

 

[droppedbrd]

its a rebuilt carb from baxters... maybe 8 mo old (been running this way since I put it on)