Weber experts needed

[gjz30075]

Hi all,
Having a bit a problem trying to solve some uneven running. Symptoms are this: immediately off idle, the engine burbles and runs rough and it'll stay this way until about 3k rpm. It shows up with no load, ie, just sitting and slowly revving it, or surging and burbling at a steady throttle on the road at something like 40 -50 mph. Idle is perfect and good hard acceleration is smooth, because it gets through this small throttle open transition period quickly.

Engine is a stroker (1720cc) with Webers, type 151.
Weber setup:
idle - 45F9
Emulsion tube - F11
Main jet - 130
air corrector - 170
idle mixture screws are out 2 1/2 turns

I think the float level specs are different than 18's but I'm not sure so I don't know what the float level should be.
Also, there is a brass plug right next to the idle mixture screw on each throttle body, as seen in the pic(to the immediate left, in this particular case). What is it and why is it there?

Anyone have any ideas as to how to eliminate this uneven running at small, partial throttle?

[john.p.clegg]

Try

http://groups.yahoo.com/group/sidedraft_central/

John

[gjz30075]

Thanks John. I just joined and am awaiting the secret handshake code for total acceptance.

I forgot to mention that the chokes are 33's. I'm still running points and they are fairly new, along with the plugs, wires, cap and rotor. Rotor is the new kind that is supposed to eliminate the rivet coming loose. So, ignition-wise, I think I'm good. Timing is at 12 degrees initial (don't know total), distributor bushings are good and the advance weights are loose.
Thanks,
Greg Z

[twincamman]

okkkkay ----check the thackery washers are correctly gaped -----check to see if the rubber seals [carb to spacers ]are not kinked or blown out in a small area ----causing a small intake leak -----check both float arms are the correct distance from the body -----I assume both carbs are jetted identically and correctly --and the carbs are balanced correctly --and the idle screws are 1 1\4 turns out when hot after a spirited run ---buy a unysin -for 30 bucks and keep it handy --some read chicken entrails -use colour tune --use voo doo dolls --use tubes inserted in various orifices of the body and all other forms of crude instruments dreamed up by man to match the carbs --but the unisyn works for me --ed

[gjz30075]

Ed, thackerys are good. The balance seems to be the issue, I think. The only way I could get a balance between the two carbs was to play with the air bleeds, which aren't found on earlier, made in Italy, webers.

I can watch the throttle plates uncover the transistion holes and they all look to be covered/uncovered equally. The holes look pretty rough and I wondered if I should/could run a drill bit through them to get rid of some flash casting around the holes. Could the rough casting be part of the problem?

Greg Z

[twincamman]

NO NON NON NO NICT NYET NEVER ---- you cant undo that -----and you cant set dual carbs up by ear or sight OR ANY OTHER BODY PART ----LISTEN read my lips ---USE A UNYSIN-- --ED

[gjz30075]

Ed, balance was done with a unisyn.

[twincamman]

if the balance was done with that instrument then the problem is in the distributor ----timing or dwell or condenser - plug - plug gap dist cap -dist weights or springs --worn shaft bearing ----my mantra is once the carbs are set and the plug reads good is ----carburettor is French for leave it alone cause the problem is elsewhere ----ed

[lotusanglia1965]

try 50F9 idle jets,souds to me like too weak till it comes onto the mains at about 3000 revs.
your idle mix could be ok,but over two turns out might indicate idle jet too small.
the pump jets will mask weakness under acceleration,but not steady/cruising/light throttle.
do you know anyone with an LM1 wide band lambda reader?

Martin

[gjz30075]

Martin, I've been reading the Sidedraft forum over at Yahoo and all indications point to needed a Lambda reader to really make things right.

However, running by the seat of my pants for now, I'll try the richer idle jet.

Thanks,
Greg Z

[trw99]

There is a wealth of really useful info on the forum at this site: http://www.innovatemotorsports.com/index.html

Look out especially for posts by type26owner. Keith use to post on here and knew a good deal about tuning Webers.

Tim

[gjz30075]

Thanks Tim. I remember Keith's postings so I'll add that forum to my list.

Greg Z
'72 Sprint

[Bruce Crowthorne]

as well as VooDoo entrails and such ....
Try a ColourTune. It is only any good for the idle and progression, but that gives a good start
Bruce

[bill308]

Greg Z,

I'm not conviced you have the air flows properly equalized. Your difficulty occurs off idle up 3K rpm. I assume this is under no load conditions?

If so, the engine doesn't need much throttle opening to maintain 3k rpm. It's likely you're on the progression ciruit the entire time.

Is the roughness worse in the off idle-2k rpm range or the 2k-3k range?

If the idle-2k is worse or the same, I think you have an off idle air balance issue.

Balance is very important in the off idle-2k rpm range where you run at small throttle openings, and less important at larger throttle openings.

You state you have a good idle so what may be happening is that as you come off the throttle stop(s), the linkage becomes loaded in a different manner and there is likely enough flexibility in the linkage to change the butterfly openings a little bit and ultimatly the air flow.

When I get serious about adjusting the linkage to overcome off idle balance issues, I balance the setup with the throttle assemblies off the throttle stop(s) and the throttle cable loaded as it would be on the road. Once balance is achieved off idle, return to idle allow the throttle to rest back on the stop(s).

Balance at idle is important, but so is off idle balance, especially up to maybe 2.5k rpm.

Air balance screws on the Weber 151's, or any other carb with this feature, are normally all closed. They are used for balancing bore airflow within a carb, not carb-to-carb. If there is a mismatch in flow between the 2-bores of a carb, open the lower flowing bore's air bleed enough to equalize flows and lock it in place. There should never be an occasion where both air bleeds on a carb are open simultaneously. Mark the open air bleed for future reference. Balance carb-to-carb using the linkage adjusment screw.

I not sure it matters much which balance you try to optimize first. It always seems that one adjustment affects another a bit so I go over everything multiple times before I'm happy.

Now you're ready to dial in the idle mixtures. A color tune works well on the twin cam as one can directly view the flame color. I've found it best to be well into the orange range at idle on the pump fuel I use.

I'd be a good idea to plot your advance curve if you ever get the chance but I think you at least need to know the total advance you are getting.

Bill

[gjz30075]

Bill, you really hit it on the head here. After intensely perusing both sidedraft and innovate forums, I went back to my carbs and dialed in the balance more closely than ever. I didn't know the function of the air bleeds up to now but they required adjustment to really equalize each carb. I sighted down the progression holes to be certain I had them covered up and then equalized, again, between the carbs, using my trusty unisyn.

I then adjusted the mixtures. A big difference now with the smoothness and really nice progression up to 3k, without a load. A quick jab of the throttle with no hesitation or gurgle.

Like I said, no load so far. I'll be able to get it out this weekend for the true test.

As far as a previously good idle, I think I had several problems masking each other, ie, two wrongs making a right.