Can tight valves result in a flat spot?

Personally I would not thing tight valves would cause an issue mid rev range.
More information about the engine and condition would help are we to assme that this is a freshly rebuilt engine?

 

There is much discussion of tuning tweaks for the midrange 308 carb stumble in the 308/328 section.

 

what idle jets are you running and how far out are the mix screws?

 

A flat spot is best tuned out (as much as they can be) by using a smaller air corrector and larger main jet. This brings the main jet in a bit sooner. Keep using your 55 idles only swap to 140 mains and 190 air correctors. Additionally, make certain the motor is seeing at least 30 degrees timing at 3-3500rpm ish. These 2v motors love tons of timing at about that range but it's difficult to do with stock distributors

 

IF the valves were tight they would be tight all the time especially once they warm up they would expand and be tight all the time. A stumble can only be caused by 3 things air, fuel, or electrical. Fuel and air would mostly be caused by the carbs. Check the distributor, wires or if electronic it could be defective. I am betting on the carbs.

 

NO.If it has a stumble it is one of two things, fuel or ignition. Check the advance in the distributor for correct operation as well as plug wire/spark plugs for correct resistance. Same for the coil & coil wire. Fuel, look at accel pump as well as blocked passages for mid rpm transitions. Same for fuel delivery.Clogged filters or low fuel pressure can augment your problem.

 

Try bypassing the O2 sensor and put an onboard Fuel air mixture gauge attached to it or them if you have two... Then you can adjust the jets based upon RPM and load while driving. I use this method on 300SL's to do tuning instead of dyno runs.

 

i had thought (and i am no expert), that the issue with the mid range stumble was the spot in between the idle circuit and the main jet circuit kicking in, and that by giving a little more fuel through the idle circuit (either through the mix screws or the idle jets, or both), that provided just enough fuel during that transition. On my 308, I can find the stumble nearly exactly down to the 1/4 turn of the mix screws (in one 1/4 stumble, out 1/4 no stumble). By using 58 idles and just the right mix screw setting I can get the stumble out without being stinking f'ing rich. With this setting, I can pass CA smog, so it cant be that rich. This of course flies in the face of the "best lean run" method of setting idle, the trade off being rich at idle, but good through the transition.

Again, i am not a carb expert, this is just what I thought I had read and worked for me.

 

This is my preferred approach to do first when sorting the carbs. Generally if the best idle is achieved with the idle screws out 1.5 turns the idle jets are right. If they are less than 1.5 turns then the idle jets are to rich and if they are more they are to lean. Usually a 1/4 turn richer of the idle screws is what is needed to get rid of popping on the overrun. Generally it does not effect the engine stumbling (but it is possible). The idle jets usually effect stumble the most (assuming it is a light throttle stumble). Occasionally you have to get the main jets to kick in earlier which I normally do with an emulsion tube change. Changing emulsion tubes should be one of the last things you try though. Before touching the carbs make sure the ignition system is perfect, I fix most carb issues by addressing the ignition system. Also note, a hemi head breaths well but has fairly poor combustion characteristics. Typically they like lots of advance. In general 42 degrees total advance is fairly typical for an engine with a hemi head. Retarded ignition timing can also cause a stumble.

Cheers Jim

 

I have been thinking about this, I would check the float levels on the carbs. The height of the fuel in the bowls has aneffect on how easily the fuel is drawn into the jets and if there is a week mixture at the transfer from the idle circuit to the mains this is a possible cause of the flat spot/slow transfer.
There are some great Weber tuning manuals available.
This book in particular http://www.ebay.com.au/itm/like/191219375427?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=107&chn=ps&ul_ref=http%253A%252F%252Frover.ebay.com%253A80%252Frover%252F1%252F705-139619-5960-0%252F2%253Fipn%253Dicep%2526icep_id%253D62%2526mtid%253D1797%2526kwid%253D1%2526crlp%253D40593704497_573061%2526icep_item_id%253D191219375427%2526itemid%253D191219375427%2526icep_meta_categ_id%253D131090%2526icep_etrs%253DN%2526icep_epid%253D-999%2526icep_ctlg%253D-999%2526icep_cond%253DNew%2526targetid%253D78459356017%2526rpc%253D0.42%2526rpc_upld_id%253D45358%2526device%253Dt%2526icep_cbt%253Dn%2526icep_msku_flag%253Dn%2526adtype%253Dpla%2526srcrot%253D705-139619-5960-0%2526rvr_id%253D811219348129
Cheers
Graeme

 

I've always had better luck selecting a larger main as it allows the main circuit to come in bit sooner as well as allowing the idle jets to reach up in the rpm range a bit further thus creating more of an overlap effect for the two circuits. Whereas only changing the idle jets only allows that circuit to last a touch longer in the rpm range. 308 motors with this modern fuel need more main jet especially above 5000rpm so it's win win every time I've tuned one of these. Just my experience anyway.

Re timing with these hemi heads. I've never seen more than 38-39 degrees dialed into one of those motors in stock form at 6k rpm, and that was with rich jetting. Detonation occurred with any more.

 

As I recall, the valve adjustment is a preliminary step, as you are trying to balance all the individual cylinders before you then balance each individual carb barrel. this is mentioned in a number of Weber tuning books.

 

David,
It's pretty easy to pull the jets out and read the #'s on them. Knowing where it is now is key to what you want to try next.
Joe

 

Your problem is most likely your spark plugs you need to select a much colder plug go to a reputable supplier and ask for plugs that are of a much colder range