wj,
A sure method of pinpointing the cause of low compression in an individual cylinder is to perform a leakdown test. This involves special equipment which applies compressed air to each cylinder through the spark plug holes. As the air leaks out of the cylinder, a gauge shows how fast it is leaking out. If the air is coming out the exhaust, you have a bad exhaust valve; if it's coming up through the carbs, bad intake valve, and if it's leaking into the crankcase (you can check by removing the oil cap and listening while the air leaks out) it's bad rings. How many miles does the engine have on it? If mileage is over 125,000 or so, valve job (and timing chain, etc.) time is at hand anyway. You can save a lot on a valve job if you remove and reinstall the head yourself, if you feel comfortable doing this. If you've never done it on an overhead cam engine before, you should enlist the help of someone who has, along with your Haynes manual. If the rest of the car is rust free and in good condition, and if you like the car, it would be worth it to do the valve job (if needed) AND put the Webers on it. Then you'd have a great running, reliable, unique car.
Colin

By now I get the overwhelming idea to replace these Zeniths! That's on the "to do" list, but a little further down the financial road. Have made some improvements that I should have checked from the get-go. Firstly, there was a crack along the brake vacuum assist line so I replaced it. Secondly, I found a small vacuum at the base of each carb. Problem here was only 3 gaskets in the rebuild kit and 4 possible places to put them (between the spacers and heat shield from the throttle body to intake manifold), so I made my own - cured the leak even if it's unorthodox. Thirdly, and I don't know how, the timing was too far retarded by about 2 degrees. Fourthly, rechecked the carb covers and the front one was just slightly (I was picky here) off-true, so I re-sanded. Fifthly, balanced the carbs yet again. The above brought me to about a 75% improvement with much better acceleration, but still sluggish past a certain acceleration point, some backfiring, and poor fuel economy. I'm going to check the valve clearances this weekend and, depending on what I find, do a compression test (probably will test compression anyway just so I can sleep at night). I feel that I'm almost there and I couldn't have kept with it without all of the support you guys have given - I'll keep you posted!

WJ,

Way to hang in there. Before too long you will have earned you're PHD in Zenith Carburetor Engineering.

Keep on pluggin',
Larry

wj,
While I am all for ditching the Zeniths, if the Webers aren't in the budget right now, there is no reason why it shouldn't be possible to get the Zeniths to function properly for now. It sounds like your experimentation has paid off with substantial improvement in how the car runs. Keep experimenting with small changes in the timing and carb settings.
Something that occurred to me was that if the engine idles smoothly, it's unlikely that you have a burned valve. A burned valve would manifest itself as a regular (not erratic) misfire at all engine speeds, accompanied by a "chuff, chuff, chuff" sound at the exhaust each time the affected cylinder misfired.
If the car runs normally until the gas pedal is about halfway down, then starts to act up, the secondary sides of the carbs could be lean, or not opening for some reason. Once the throttle is about halfway depressed, the vacuum-operated secondaries should start to open. If the engine acts up at all engine speeds above idle, this is probably not the problem. Keep working on it, and sooner or later you'll solve this riddle.
Colin

I spoke with a buddy of mine and he questioned the float levels. I replaced the fuel inlet valve washer with the same thickness as the original when I rebuilt the carbs, but I'm thinking that with the changes I've made that I may have to re-measure. Problem is that Haynes gives a lousy procedure: "Measure the distance between what is the bottom of the float when in its normal 'in car' attitude and the surface of the jet block below the float...The distance should be between 0.827 and 0.906 in". Correct me if I'm wrong, but if you measure from the bottom of the float to the top of the float, that's more than 1", not to mention the additional distance to the jet block surface when it's in its "in car attitude" - there's no way to measure less than 1" when going from the float bottom to the jet block surface when including the height of the float in the measurement. Should I be reading this as "top" of the float to the jet block surface?
Speaking of Haynes, another problem, this time with the timimg specs. The timing table (pg. 92) shows 3 asterisks after the idle speed degree, but doesn't include a key for what these indicate as it does for 1 and 2 asterisks. Is this with vacuum hose or without, or does this mean something completely different? Did I miss the explanation somewhere?
Further, and I asked this in the tech forum before but am still confused, the manual gives 2 specs for the 250/8 and the 250C - the car is a 250C but the identification plate names it a 250/8. The previous response in this forum to my original question told me to go by engine number, and mine is the 130, but both of these models have the 130 engine according to Haynes. So, once again, is it 4 degrees ATDC or TDC? I know, I know - it's time to invest in the CD tech manual!

I would think that if its supposed to be 4 atdc and you have it set to 4 btdc you would get a substantial amount of ping.
-my 2 cents

-Larry

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03/83 300D
07/73 280




[This message has been edited by Larry Delor (edited 02-26-2000).]

Well, I currently have it set to 4 ATDC, but am wondering if it really should be set at TDC. I suppose the easiest way to find out would be to set it at TDC and see what happens, but was just wondering if anyone knows for sure the correct timing. At this point, I'm willing to try anything and everything!

She works!! I adjusted the valve clearances and all of the intake valves were about .002" off. Next, on a hunch, I messed with the timing and I advanced it to about 20 BTDC and she purrs, fantastic acceleration, like she's a brand new car! I tried more retarded settings and 20 seems to be it. Why on earth would the timing need to be advanced so far? I checked the cam timimg mark while I was adjusting the valves and it was right-on at TDC for the #1 cylinder and not much play in the chain. Does someone have a good reason for such a difference from the 4 ATDC recommended setting?
I'm so glad it's finally over! Thanks to everyone for your input - I couldn't have stuck with it without your support. Now, what else can I work on.....

wj,
I wouldn't worry about where the timing is set. Just set it to where the car runs well, doesn't ping on acceleration, and starts readily when hot. Note the setting for future tune ups. 1971 was during the early stages of emissions requirements, and manufacturers (all of them) went to all kinds of bizarre lengths to make their cars comply. Some of this included retarding timing. Glad you got it running well!
Colin