Zenith balance

JelmerE12-E28

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Good news! I worked on it again this evening and now it runs way better! I backed of the throttle screws (like Steve described in his tread) and turned in the idle mixture screws and backed them out again 2,5 turns. I also backed out the smaller screws next to the idle mixture screws 2 turnes.

Than I've put on the adapters and synchrometers and started adjusting with the idle mixture screws. Weird thing is that I could back out the rear one completly whitout affecting the vacuum! So I turned them in completly and started from there. When I had the vacuum equal in both it ran pretty smooth and at about 700-800 rpm. Than I re-connected the throttle linkage on the rear carburetor and adjusted it to be equal again. After that I turned up de the rpm's with the screw on the throttle linkage and adjusted the linkage to make vacuum equal again. Than checked idle again and higher rpm's again. It ran beter and beter! I was short on time so had to quit but I'm happy with the result so far. Between the adjusting I blipped the throttle some times to see if there where any differences.

After all this I've put the aircleaner back on and drove home. I drives more smooth and the fluctution is almost gone now. Also, when it gets back to idle again (When entering a corner or when waiting to make a turn) it goes right away to the 700-800 where it is set instead of some hesitation or dropping to 400-500 rpm and than climbing to 1000 rpm.

Now it is time to make small adjustments! Is it wise to hook up both the vacuum gauge to the ports under the throttle blade and the synchrometers in the adapters over the carbs?
 

Stevehose

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The single screw on the linkage should not be used for final idle adjustment, that is used for holding the revs up for checking the timing, then it is backed off so that it doesnt make contact or affect idle. Your idle should be set by each indvidual carb butterfly screw (after proper idle mixture) so that the idle is what you want and drawing equal air on the carbs.

Mixture screws should be adjusted with the butterflies as closed as possible while maintaining idle smooth but not higher than 800 or so, the book says that final adustments are made with mixture screws but that is for emissions reasons and it worked better for me to set the butterflies to run at 800, adjust mixtures so it idles highest, if it goes higer than 800 then close the butterfly some and repeat the process. Once idle mixtures are dialed in then raise each butterfly screw equally to desired idle speed and that the carbs are still balanced. Check balance in rev range (do your meters have the bleed hole in the side for higher rpm reading?)

if you are not getting response from the mixture screw then you either have a blockage or vacuum leak.

Backfire is a lean condition, check for thoroghly for vacuum leaks before setting idle mixture etc.
 

JelmerE12-E28

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Thanks Steve,

I didn't use the single screw on the linkage for adjusting idle, only to raise the rpm's. After that I backed it out completely because the spring is missing.

It responded to the mixture screws but not as much as I would expect. Like you explained they had to be backed out by 2½ turn but this was to much so I tried 1 turn. I also backed out the smal screws next to the idle mixture screws by 2 turns, it responded to this pretty good.
When I have time again I'm going to work on it again.
 

JelmerE12-E28

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After a week is drives pretty good but the stumble is still present. I did a search on "accelerator pump" and came across some treads which told me to replace the accelerator pump skirts. In my rebuild kit they where not present. It also had the wrong top gasket so I ordered 2 new ones from BMW. Could this cause any trouble, the wrong gaskets?
 
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