Webers wont hold cruise rpm

[saltshaker]

Between 16-1900 rpm when cruising it surges or won't maintain a constant rpm. All other ranges work fine, it accelerates great but with all the different combinations i have tried it won't go away... Lean spike on lm2. I have tried 40 & 45 chokes,
60,65,70 idle jets, holders ranging from 100,105,110,115,120&125
F11&F2 etubes always the same problem. My best combo
428
48ida's
40 chokes
65 idle / 120 holder
160 main/175 air
70 pump jet( can't get smaller) 45 exhaust
18 int/38 total in @2600
Any thoughts?
Thinking about adding 3rd progression hole ...
Tuned with lm2
Jon

[Gaz64]

Jon,

You have mentioned rpm, how much throttle?

If about 10%, I'd say go for the 3rd progression hole modification.

There is a thread on here about it.

[zrayr]

Have you tried the richer emulsion tubes such as the F 8 or F 7 ?

Z

[saltshaker]

No, I haven't tried a f7 etube. I'm thinking this is in the idle circuit and maybe the relationship between the progression holes and butterfly. I have removed the main stack so to just run on the idle circuit. I would think the benefit of the etube would come in much later. Other possibility is timing issue....or twisted throttle shaft on one carb. 4 centered barrels are sync'd
Jon

[jhirasak]

Have you checked your right bank to left bank sync as you slowly increase your rpms through the range where you have a problem using your idle stop screw?

What emulsion tube are you currently using?

John

[saltshaker]

Both sides are sync'd
F11

[jhirasak]

Since you have indicated a lean surge in the rpm range of concern, I would recommend that you stepwise increase the richness of the idle circuit by incrementally decreasing the size of your idle jet holder. This might partially correct your problem. If it still exists after you have gotten to the smallest size of idle jet holder, I would look to add the 3rd progression hole.

Just as a point of reference, I had to decrease the size of my idle jet holder to 100 to get rid of an off idle driveablity problems. My idle jet size is currently 70.

John

[saltshaker]

I have tried these but not with a 120 or larger holder. With a 115 or smaller the exhaust smells of gas and fouls the plugs. Going to try them again with a larger idle jet holder
Jon

[zrayr]

Originally Posted by jhirasak

"......If it still exists after you have gotten to the smallest size of idle jet holder, I would look to add the 3rd progression

Additionally I'd be trying some richer emulsion tubes before breaking out the drills.

Re. The issue of a couple of cylinders not responding to the idle mixture screw being turned completely closed:

This is often just a matter of the throttle plates not in alignment and the throttle shaft needs a little tweek to set matters right. however, from the description of when the drivability issue happens, this misalignment, and only idling on 6 cylinders has nothing to do with the stated problem. Of course if there is some blockage, that's different. But with a minor throttle plate difference that causes a cylinder not to idle, the engine does just fine once the throttle is opened slightly.

Originally Posted by saltshaker

"........ I would think the benefit of the etube would come in much later. Other possibility is timing issue....or twisted throttle shaft on one carb. 4 centered barrels are sync'd
Jon

I'm doubting timing has anything to do with this. As far as one of the throttle plates lagging behind its mate, check the sync on ALL eight holes. If the dead ones are pulling much less vacuum than the others, you definitely have a little throttle shaft tweeting to do. It takes very little pressure to set things right if that's the case.

In my limited experience, changing an emulsin tube can make a huge difference when the throttle is barely open, just as it can when the throttle is more open.

Z.

[Rick Parker]

Jon: Just a thought a little out of the box....With the carbs syncd and idle set. How far do you you have each idle mixture screw open? And can you kill each cylinder by screwing each one closed indepedently? If there are cylinders that dont respond to the idle mixture screw it's possible the port is plugged or the edge of the throttle plate is not exposing the port to manifold vacuum. Id be using a 100 idle holder and a 60 or 65 jet.
Shouldn't require more than that. Also what are you cam specs.

[saltshaker]

Rick, when syncing carbs cylinders 4&5 are off from the other 6(FE Motor) with regards to shutting down a cylinder with the mixture screw there is a cylinder that doesn't shut down. Would you blow air in the passage to clear it? Comp cam 282s 110cl 236@.50 577lift
Jon

[carmine]

hello Jon

Does it surge in 5th gear overdrive at cruise ?

[jhirasak]

Jon,

I think that Rick might have identified the source of your problem. If cylinders 4 and 5 cannot be made to drop out by closing their mixture screws, the carbs feeding those cylinders are very suspect in having blockage in their idle passage and/or progression holes. You will have to dismount the carbs and remove the progression hole inspection cover to check for debris or incorrect position of the butterfly. If there were debris in this area, it cannot be blown out while the carb is mounted on the engine. You will have to blow from inside the throat at the progression holes to remove any debris that might have gotten trapped there.

While you have the carb off the engine, shine a light at the butterfly from the manifold side of the carb and look through the progression hole inspection port. There should not be any visible light until you start to open the butterfly. You can also check to make sure that both throats uncover the progression holes at the same time.

John

PS By the way, my carbs all have three progression holes.

[carmine]

hello Jon

Does it surge in 5th gear overdrive at cruise ?

Carmine

[saltshaker]

Only around that rpm range
Jon

[Gaz64]

Jon,

Sounds like a majority of your problem is twisted shafts, generally from tightening the lever nuts.

Take all carbs off, back out the idle speed screws and observe the throttle plate balance one carb at time.

You'll quickly see what's going on.

It then becomes a matter of a twist and adjustment of blades within the bores.

Everything to do with throttle control should be on the lever end, nothing on the other end. I also run my carbs with all the speed screws resting of the linkage. I run centre mounted heim jointed linkage, my carbs have stayed perfectly balanced since I fitted them.

[saltshaker]

Got the carbs synced and fix the problem with fuel drippingin the barrel after the motor was shut off. Problem was that the idle jets wern't seated and fuel was going around them. Also noticed on several barrels that the idle mixture screw doesn't shut down the motor when screwed in. I was suggested that on those mixture screws that the spring be taken off to see if they could now shut tje fuel supply off. Going to try that tomorrow...
Last, one barrel isn't working on the idle circuit.... Pulled te
He carb apart and the passage where the fuel goes from the well to the idle jet is clogged. I ran out of gas last week....going to fix that tomorrow and hopefully it runs much better!
Jon

[saltshaker]

The plug on #6 is wht, the idle circuit is rough up to 2k and smooths out.
Rebuilt the carb over5/6 and still have the problem
Jon