HELP WITH CARB SYNCH AFTER CARB REBUILD

I just did a major (timing belts & tensioners, accessory belts, carb rebuild and richen main jets). After putting everything together and synching the carbs, the revs do not drop like they should. I blip the throttle to about 3k and it takes about 10 seconds to drop back down to 900 rpm. I don't think I have a vacuum leak and each throat seems to be measuring the same. The linkage and cable are not binding. Anyone have any suggestions?

Thanks

Henry

 

Henry
This condition seems to be somewhat common as indicated by the number of threads which relate to this problem in one form or another. I have a simular situation in that when I first run the car (79) it works fine but after a half hour run the idle does not drop like it should and hanges up around 12K. I might ask if your idle worked OK before you rebuilt the carbs, are your R2 points removed, and was your engine completely warmed up before you synchronized the carbs? You might take the car out for a good 5-10 mile run and then resynchronize. Believe me I'll be watching this thread to see if anyone has an answer. I sometimes wonder if this is not just the nature of the beast.

 

Check for air leaks in base gaskets or lead plugs, idle jets could be clogged or loose and the butterflys do not return to idle position due to sticking of shaft or free play in the control joints/linkages.

Could also be lean or out of tune.

 

Check the advance on your distributors to be sure that one or both of the mechanical advances aren't sticking. If the throttle plates are closing nice and quickly and the engine speed isn't dropping with it, you probably have a leak someplace or something funky with the ignition.

Birdman

 

Chuck, I assume you mean 1200 RPM, not 12K!!

It is common for the idle speed to be a little higher when the car is warm than when cold. You should adjust the idle when fully warmed up and IN THEORY the cold idle device (if you still have it) will compensate when cold. Mine was removed by the PO so I just have to deal with a very slow idle when cold. If your car is synched well, it will idle perfectly well when stone cold, just very SLOOOOOOWLY. When mine is cold it just idles putt-putt-putt like a lawnmower!

However, if you blip the throttle and the idle speed does not come back down afterwards, as Henry describes, something is not right.

Birdman

 

Henry,

If you are still having carb problems, let me know. I can run over there and give you a quick hand.

 

Birdman
That makes sense and when I get a chance I'll do the adjustment again...got to fix my accelerator cable as it is frayed pretty bad. I must have caused it to drag on the threads of the length adjusting device after I synchronized tha carbs. BTW the 12K was not a misprint my car has been running pretty good after I rebuilt the carbs. LOL. Have a safe and happy Thanksgiving.
Chuck

 

I'm thinking it has got to be a vacuum leak. Think I'm gonna pull the carbs off and check.

The ignition is fine. I have an Electromotive DIS.

Matt - Where 'bouts are you located? If you'd like you can PM or email me.

Chuck - that's pretty funny, if you can rebuild carbs and get the engine to rev to 12k, then you are an amazing tuner.

Thanks guys. I hope you all had a great Turkey day.

 

Valencia

 

Hmmmm, assuming ignition is dead right, here's my approach:
to 'make rpm', engine needs air + gas. When the throttle is opened ('blipped'?), all of the butterflies open together feeding more air and gas. To bring the arpm down, ALL of the throttle plates in ALL four carbs must come to the idle closed position at the same time. I would take the air cleaner off (8 minutes) and have someone 'blip' the throttle while you actually watch to see if all 8 butterflies return. Even just one carb binding open perhaps due to the intercarb linkages would keep rpm high. If you are certain all eight butterflies are returning equally (and this is important) to the individually adjusted pre-set idle closed position, the answer pretty much has to be an air leak. Common places include at the gasket fiber pad between the carb and the manidold and in some cabs at the butterfly shaft bearings.
I may be wrong, but it sure sounds like a linkage bind from here if the ignition is right. That said, to pull the carbs off, reinstall with new gaskets, reset idle and sync with the individual carbs, then synch the response with the linkage will probably fix the problem with the carbs
Good luck and let us know, could be some lessons learned for all of us
!

 

Russ a point of concern here. NEVER look into the carbs while blipping the throttles. All you need is a short blowback and you'll have a face full of fire. I've seen a few of these in my time and the results are horrific. So if you need to look into the carbs to verify anything when the engine is running then take a mirror and hold it over the carb and look through the mirror when checking.

 

Everytime this happened to me I would richen the idle mixture jets. Then the idle will go up. Now back the idle speed screws (2 of them) out so you get the idle back down to 1000-1200rpm.

 

Yes - Steve is correct - I neglected to say the engine should be off of a common bathroom hand mirror can be used; I never can see down them directly because I cannot stretch over that far, anyway
Thanks!

 

Once I have the carbs off and am able to take a look I'll report back.

Thanks for the help guys.

Matt - I'll keep you posted.

 

ok. I am busy during the week but I could run out there no problem on the weekend.

 

I am not very familiar with DCNF downdraft carbs, so I don't know if the following technique that works on sidedraft DCOEs wil work on the DCNFs as well.

You can see the location of the throttle plate relative to the progression circuit holes (there are 4 on a DCOE) by removing the progression hole inspection cover and shining a flashlight down the carb throat. You should be able to see all but one of the holes lit by the flashlight. The last (furthest downstream) hole is just covered by the edge of the throttle plate. If there are more holes covered OR a different number of holes covered across the carbs, then the throttle lever should be adjusted to ensure that all throttle plates are closed, i.e. same number and maximum number of holes uncovered.

All this can be done withOUT running the car.

I think the progression holes is just above the idle mixture screw on a DCNF. What I don't know is if there is an inspection cover that screws out, letting you see the holes. Could be an idiosyncrasy of sidedrafts.

This is a link to instructions for the procedure in a DCOE. Hope this helps.
(560KB Word document)
http://home.earthlink.net/~fchat/data/Carburetor%20adjustment%20on%20a%20365%20GTC4.doc

 

I recently fixed a 246 that had had someone "rebuild" the carbs.

I noticed it had poor throttle synch, and when I went to synch it, I noticed the spindle nuts were loose making synch impossible, and when I tightened them, the carbs bound up due to mis aligned throttle plates.

When re fitting the throttle plates, tighten both spindle end nuts BEFORE setting the butterfly into the choke. This could cause the throttle to hang, making idle unstable.

 

Are you running cats in your exhaust? then don't listen to the guy that said take it out for 5~10 mile and then adjust. Good way to melt your exhaust\cats, start fire, (I've seen it) if they're out of whack!(The carbs)A hard run or "Clearing it out" so to speak can get your exhaust red hot and glowing. BTW, fat chance you can "clear out" a fouled Ferrari plug by reving the sh%# out of it so if you see any black smoke while your "synchronizing" your carbs solve the prob first, change plugs and go back to it. Patience is the key here; don't get frustrated if you plan on doing this yourself. It's a learning experience!

Carbs on a 308 should ONLY be approached by a professional who knows WTF he's doing!!!!!! Not a shadetree mechanic, or "lets try this approach". There are too many things to set in a proper order. Too many things to check before installing them.

I read a lot of good advice here:

I would start here:

Quote- 308GTS
" Everytime this happened to me I would richen the idle mixture jets. Then the idle will go up. Now back the idle speed screws (2 of them) out so you get the idle back down to 1000-1200rpm. " Probably a different problem but something to check if you didn't reset your adjustments back where they were before. (They should have been ok b4 right?)


Quote- Steve King
"Russ a point of concern here. NEVER look into the carbs while blipping the throttles. All you need is a short blowback and you'll have a face full of fire. I've seen a few of these in my time and the results are horrific. So if you need to look into the carbs to verify anything when the engine is running then take a mirror and hold it over the carb and look through the mirror when checking. "
HaHa.......True, I can picture it in my mind. Use a 5\16~3\8 rubber hose to listen (My 20yrs. of carb sync\tuning secret) or a mirror like snj5 suggested.

You said "I don't think I have a vacuum leak", Make sure first. (Just check the hoses if your confident about the base gaskets) On the 308 any air bypass will totally mess up your idle and give you this problem. These cars were imported jetted leaner than hell to pass emissions. Just a little more air and the idle jumps up.

 

OK, I've got a confession to make...when I rebuilt one of the carbs I broke a one of the "tangs" that holds the float pivot pin in place. I J.B. Welded it, but I guess it is not resistant to gasoline. I opened up the carb to find the float kind of "dangling". Does anyone know if this could be soldered, sweated, or welded back? Anyone know of any adhesive that is resistant to gasoline? I would guess that this was flooding that carb and causing my problems (if not, it sure couldn't help).

Thanks

Henry

 

Now you tell us!

In a circumstance such as yours, Seal-All Glue is OK to use in carb floats...
http://www.totalmotorcycle.com/reviews/SealAllGlueReview.htm

 

Most excellent.

 

Carb bodys are made of a pot-metal alloy hence your broken tang. You can't solder\weld it.

 

Bryan,

Your suggestion of removing the brass caps covering the progression holes on DCOE's to judge the relative position of the butterfly valves is quite observant on your part and works pretty well. I have used this technique to make rough adjustments on DCOE's fitted to a Lotus Elan. Final balancing is done with a 4-bank mercury manometer system. Unfortunately, there is not a similar cover on DCNF's.

My 78 308, even with new DCNF's fitted has, and I would guess many others have, a problem idling below 1000 rpm if their carbs are truely synchronized (again, I use a multi bank mercury manometer system to measure and equalize manifold vacuums).

There are at least three reasons why this can happen:

1. Excessive ignition advance at idle and just off idle, will tend to make the engine idle faster. The factory uses R1 and R2 point sets to effect 3-degrees ATDC ignition timing at idle for 78-79 308's. If the idle point set is disabled or removed, you are stuck with something on the order of 8-degrees BTDC or more advance if one wishes to maintain proper advance at speed. This additional 11-degree advance (the difference between 3-ATDC and 8-BTDC) will cause the engine to idle faster at idle unless you can compensate by decreasing the throttle opening a sufficient amount.

2. The distributor advance mechanism may be sticking or the springs may be weak preventing the advance from returning to it's proper idle setting. My Electromotive ignition system eliminates this problem but available adjustment range limits require I still run about 8-10-degrees of initial advance, so idling below about 1000 rpm is iffy when fully warmed up.

3. DCNF's can only be closed down so much. Eventually, you reach the point where the throttles are no longer sitting against the idle stop screws, but against the inner wall of the carb throat. If one wants to maintain perfect synchronization between all throats (highly desireable), one has to adjust all throats to the highest flowing throat when it is fully closed. Inevitabley, this leads to higher idle speeds than may be desirable. Air leaks from the throttle shaft bearings or manifold gaskets will exacerbate the problem. One can cheat a little here, at the expense of giving up perfect synchronization, by closing down those carb throats which still have a little adjustment in them. I personally don't like this solution as is affects off idle response in a detrimental way.

I hope this helps.

Bill

 

Bill
This posting is pure gold! I've been fooling with my 308 DCNF carbs for months and this post and another one you made on 10/13/05, concerning "Cylinder #8 Insanity" confirms what I've suspected all along. As a novice I've been stumbling along not being able to get the fine tune and idle speed I wanted. Now I understand why and can take a different approach to synchronizing. This info should be understood by anyone attempting to tune their Webers and a copy of your post s/b framed and mounted on the garge wall. Thanks a bunch 'cause it sure helps.

 

OK, I think I got it. I was able to fix my float problem (I hope). I synched the carbs yesterday and all seems good. The revs seemed to drop faster, but still were hanging up a little. I then opened up the idle mixture screws more and voila. The thing now idles solid at about 1k without any "spitting" or popping. The flows in all 8 throats are now equal. A couple things though, the idle mixture screws are all out now around 5 turns out. Also, the idle screws are backed out completely. The linkage rest on the body not on the screws, so I cannot drop the rpms any more. Do any of you think that this is a concern?