I've been thru this a bunch of times on this forum but here goes again. The engine was running good with a little stumble above 4500, timing was at 7-8 degrees and I couldn't get it to advance any further so I left it. I did a minor valve adjust cold then hot with only some very minor adjustments needed to some of the valves both hot and cold. Today, engine runs like shyte, stumbles above 3500 rpm and backfires on decel and the BCDD is now hanging up holding the idle at 1200 rpm and drops if you tap the accelerator. WTF is going on? This all sounds like timing to me, but need some other input. Timing has not been checked after valve adjust, was done just prior to the adjust. The crank pulley was rebuilt by the guy up in Oregon(got CRS syndrome right now). Thanks :mad:

Mike

Everyone is going to want to know what you did the first time to try and remedy this.

Now my buddy kind of went thru the same thing with his 75 and it was the dizzy. Basically the plastic guts of the dizzy fell apart which screwed with the timing. After he fixed that it was a different animal

Did you dbl, chk. to see if the timing marks were in fact aligned when #1 cylinder was on TDC, power stroke? There can a number of reasons for the marks not being aligned but this easy check will let you know. If they don't line up you can re-notch the pulley in the proper location. This could also explain why it won't advance more than 7 degrees.

The dizzy is a off a 79 and to be honest I checked it over but didn't really get into the guts of it. I sorta suspect the vacuum advance diaphragm is not in great shape, but I'm not sure if that would account for the problem, although it might.

Mark, I checked the timing marks at TDC and had to remark the crank pulley, but I also think the timing was off by maybe 1 tooth, but the last time I redid the oil pump and dizzy I couldn't quite get it to line up, after trying 1 tooth in each direction, its almost like its off by 1/2 a tooth.

Edited March 15, 201411 yr by mjr45

I would think that a timing light and some time would shed some light on the problem.

Zed, right now I'm gonna sit down and cool off until Monday then I'll be checking with the light before I do much of anything else. Its just very irritating to be so close to a great running car and not be able to get there. Oh well, I'll just what turns up.

OK I've rechecked the timing and its still at 7-8°, which is not perfect I know, so why is it suddenly missing and stumbling >3500RPM and why is it backfiring on deceleration having nothing more than a minor valve adjustment? Prior to this it would stumble at about 4800RPM and no backfiring. Today I'm going to put the engine at TDC and check exactly where the dizzy is at #1 and go from there.

OK I've rechecked the timing and its still at 7-8°, which is not perfect I know, so why is it suddenly missing and stumbling >3500RPM and why is it backfiring on deceleration having nothing more than a minor valve adjustment? Prior to this it would stumble at about 4800RPM and no backfiring. Today I'm going to put the engine at TDC and check exactly where the dizzy is at #1 and go from there.

mjr,

OK, right up front, my disclaimer is I know nothing about your problem. I am going to recite what my mechanic found when my '71 was having similar, but

not nearly as violent, a problem. Just some occasional missfiring, backfiring, jerking, etc. I don't know how similar your '75 dizzy is to my '71, but, there is

a white plastic insulator for the electric wiring that attaches to the top edge of my dizzy just below the distributor cap mounting. He found that this insulator had cracked and generally degraded from time and heat to the point where it allowed some electrical arcing causing my poor engine performance.

If you have a similar insulator, it may be worth checking, FWIW.

Dan

Edited March 17, 201411 yr by AZ-240z

Have you checked timing at full advance? If the distributor is not operating as intended, it will have unforseen side effects.

I did a minor valve adjust cold then hot with only some very minor adjustments needed to some of the valves both hot and cold. Today, engine runs like shyte, stumbles above 3500 rpm and backfires on decel and the BCDD is now hanging up holding the idle at 1200 rpm and drops if you tap the accelerator. WTF is going on?

How soon after the valve adjustment did the problem happen? If it was immediate, then it was probably something you moved when you took the valve cover off, or you didn't get the adjustment right. As for the throttle hang, that doesn't sound like BCDD if it goes away with a tap on the throttle. More like you got a plug wire stuck in the linkage, or broke the return spring, or something. Next time it hangs, leave it idling high, open the hood and see what's stuck.

You've kind of taken a shotgun approach to possible causes, with BCDD and timing. If it ran good before, then a before and after focus might be better. Look at what you touched during the valve adjustment process.

Zed, will do, the problem was not really immediate, but showed up after about 7-12 miles of driving. The valves were adjusted cold then hot, they had been adjusted cold several months ago and this time there only only a couple that were off by 1-3 thousandth's with both the cold and hot adjust. I'll check to make sure there is nothing hanging and then take another look at the valve adjustment(cold) and go from there. Thanks.

Captain, no it hasn't been checked at full advance, but I'll get to it.

Mike

7-12 miles might be something has come loose. Kind of sounds like a vaccum leak, with the high idle, the backfire and the lack of power at high RPM (fuel mix off). Maybe check all potential leak spots, like the PCV hose from the valve cover, and the valve cover gasket, etc. Check for hose splits also, the old hoses get brittle.