Weber IDF's surging?

Fuel Supply & Ignition Systems

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mlrailguy
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Weber IDF's surging?

Post by mlrailguy » Thu Jun 22, 2017 5:26 pm

Trying to get my carbs dialed in but having heck of a time. When it starts, will idle aprx 750 rpm, then after a few seconds, will speed up to 3000 rpm for a few seconds ,then come back down. I need to figure this out before I can get the idle mixture set. What could possibly cause this?

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FJCamper
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Re: Weber IDF's surging?

Post by FJCamper » Fri Jun 23, 2017 10:40 am

A common cause of what you're experiencing is a vac leak on one carb, and your timing set to an initial high advance.

The leak causes the idle rpm's to go up, and the idle rpm's cause the timing advance to kick in.

Obviously, you'd search for the vac leak and retard the timing a little to fix that.

Another, very common cause of this in the carb linkage itself. In this case, there is a slight adjustment on one or both carbs linkage that wants to press the throttle open. The throttle return springs will try to keep the throttle closed at idle, but can slowly lose the fight and the throttle opens. And the timing advances.

Take both throttle down arms off once you start and idle the engine. If the rpm increase does not happen, readjust your throttle linkage.

FJC

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Fri Jun 23, 2017 3:54 pm

I had both arms off. Would a bad regulator cause this? Have both carbs torn down and ready to go back on, going thru all the bench settings first, so hopefully will be good again

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FJCamper
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Re: Weber IDF's surging?

Post by FJCamper » Fri Jun 23, 2017 6:09 pm

A fuel pressure regulator that allows momentary flooding out of the float bowl so that the fuel level will rise around the emulsion tubes will cause irregular idle, up and down, and if you have your distributor advance set hair trigger to kick in with just a couple of hundred extra RPM's, it could cause a surge.

I don't know what distributor you have, but we run 009's, and 10 degree initial advance with about a 900 to 1000 rpm idle. a 200 rpm surge would kick our advance in and suddenly we're at 1500 and climbing.

I can't be more precise because so few 009's are precise.

There is also the outside possibility you have a worn throttle shaft bushing causing a vac leak, or maybe a mixture adjustment screw with damaged threads.

I'd also go looking for a failing O-ring seal on one or more idle jet carriers.

FJC

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Fri Jun 23, 2017 9:35 pm

got her back together. Found my floats were way off. Blew all the ports out in both directions. Did all the bench settings. She idles now, pretty decently, but the passenger side carb sits at 10 on the synchrometer and driver side at 6. Can't get the high one to come down to match the low one. I DON"T want to mess with idle speed screw yet, correct? I'm running the Pertronix distributor and timing is around 10 degrees advanced right now. Going to recheck valve lash in the morning. Running the FK8 cam and 006 clearance.

66brm
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Re: Weber IDF's surging?

Post by 66brm » Sat Jun 24, 2017 2:05 am

Yes you need to adjust the idle speed screws before the linkage is connected to even out the flow through each throat and set your idle speed, then set your idle mixtures, then recheck idle speed. Then you can hook up the linkage, if the idle speed or quality changes then adjust the linkage only

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Mon Jun 26, 2017 12:10 pm

OK, I have both arms disconnected. The idle screws are both at 1/2 turn from contact with linkage. One carb is at 11, the other at 6. I couldn't get the 1rst one to come down. Do I turn idle screw back out, or mess with mixture screws?

66brm
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Re: Weber IDF's surging?

Post by 66brm » Tue Jun 27, 2017 8:28 am

You shouldn't play with mixtures till you figure out the airflow issue, bypasses fully closed, wind out the idle speed screw till the airflow matches the lower one. Have you adjusted the valves recently?

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Tue Jun 27, 2017 9:19 am

Yes, just double checked the valves. I have wound out the idle screw all the way, on high flowing carb, does not come down much. So I need to wind other 1 in, to bring it up to 1rst one?

66brm
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Re: Weber IDF's surging?

Post by 66brm » Wed Jun 28, 2017 12:53 am

Pull the high flowing carb and shine a light through it with the butterflies fully closed, there is too much air getting past, are the blades fully closing?

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Piledriver
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Re: Weber IDF's surging?

Post by Piledriver » Wed Jun 28, 2017 2:01 am

May have a bent shaft... are both holes the same in each carb?
Relatively easy to fix and pretty common.

If thts good and the flows are still wildly out, time to pull them and do some "sesame street" troubleshooting.
("one of these things is not like the other...")
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Wed Jun 28, 2017 8:39 am

Well, I finally got them pretty close, but I'm not happy yet. Just ordered the master rebuild kits from CB Performance. Will put all new stuff in and start over again. My mistake was trying to adjust mixture screws first. They were actually sounding pretty good, then I carefully aligned the linkage. Once it was bolted up, it would just die. I guess my return spring was too tight?

66brm
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Re: Weber IDF's surging?

Post by 66brm » Thu Jun 29, 2017 12:13 am

ALWAYS adjust the idle speed screws first and with the linkage off, then any changes to the idle speed are due to linkage, mixtures or timing. Make sure when you put them back together that you don't overtighten the shafts nuts or you will crush the bores and end up with throttle plates that don't fully return

mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Thu Jul 06, 2017 10:59 am

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Got them rebuilt, and running 100% better. Still need to get accelerator pumps adjusted. Not pumping till throttle is wide open. Assuming I can adjust nut in to tighten spring?
Also noticed that arm on backside of one carb had gotten bent and was not letting throttle plates close completely, couldn't figure out why the rpm's wouldn't come back down after a ride. Also the other carb had a loose nut on throttle arm. It was just enough to cause problems.
Any way, back to timing. Still learning. I've been told to advance it as far as it will go, and still restart easily. Also, I am running the Flamethrower billet distributor. I don't know anything about advance curves. It came with a lot of springs and tabs for setting up. I have all the stock settings still
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mlrailguy
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Re: Weber IDF's surging?

Post by mlrailguy » Thu Jul 06, 2017 11:30 am

Don't know why all the same pics? I only submitted 3

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