pyrohamish Posted September 17, 2011 Report Posted September 17, 2011 I have a 4k in a sierra and i'm trying to sort out the advance. Not sure if this is normal or not but. With the advance vaccuum line on the advance port(i think) the dizzy will go from 8deg down to 0deg for just less than a second then it will come back up past 8 deg and up to whatever.. I havn't tested with a timing light but with it on the manifold it still seems to happen The carb has literally nothing else attached, no emissions or anything only things connected are choke cable, accel cable and advance line, every other port is blocked off. Also this is only testing while sitting in the driveway under no load. Quote
Evan G Posted September 17, 2011 Report Posted September 17, 2011 have you set the timing with the vac advance disconnected? Quote
pyrohamish Posted September 17, 2011 Author Report Posted September 17, 2011 yes it only happens when i press the accel straight hard to the floor fast, it wont do it if i accel a bit slower Quote
altezzaclub Posted September 17, 2011 Report Posted September 17, 2011 (edited) So... You are telling us it has vacuum at idle, which is where you set it at 8deg. Then when you boot it the inlet vacuum collapses, so the dizzy advance goes back to zero deg, then catches up to 8deg again as the throttle closes and vacuum reforms. So that port is not closed off by the throttle plate at idle, or you wouldn't get a reduction in advance. It is a manifold vac port, not a 'ported' vacuum port. Also, it seems you've set the timing with the vac line on, so that initial 8deg was vacuum induced. Can you check that?? Is there only one inlet on the vac advance diaphram on the dizzy?? So you cannot get vac retarding?? If you give it 8deg with the vac line off, then put the vac line on it will idle at 16deg, falling back to 8deg when you boot it. That would be better than setting it at 8deg idle and falling back to zero. You'll probably have to reduce the idle speed at the throttle screw. However, it is working as it should. When you boot it it retards to give you the intitial acceleration you need, and as you lift off it advances again, & it is advanced at idle for a clean burn. You definitely need a timing light and preferably a vacuum guage for this. Edited September 17, 2011 by altezzaclub Quote
Jackson Posted September 18, 2011 Report Posted September 18, 2011 I have a 4k in a sierra and i'm trying to sort out the advance. G'day mate, I have the same setup and am trying to figure out the carb myself, The last few weeks I have been poking around inside the carb. yes it only happens when i press the accel straight hard to the floor fast, it wont do it if i accel a bit slower One thing I have found while poking about is that the cluster of ,I think 4, ports on the side of the carb. I think all of these ports come out inside the primary barrel just above the butterfly however on of them is slightly slot so that it is both above and bellow the butterfly. I guess this allows the dizzy to get vacumn at both idle and when the butterfly is open. So at idle, butterfly closed, all the vacumn is in the manifold while at full throttle, butterfly open, vacumn in manifold goes to 0 (I think thats what Altezza was trying to say in another thread?). When you vacumn test them I know that it is steady (about 21 whatever units) when idling but if you rev it sharply then needle jumps about but definitely heads to zero. I have never tried but would be interesting to see what happens to the vacumn if revs are steadily increased. Is there only one inlet on the vac advance diaphram on the dizzy?? So you cannot get vac retarding?? Sorry to change the subject but is this what the second outlet is on an electronic dizzy? One side advances and one side retards? Quote
altezzaclub Posted September 18, 2011 Report Posted September 18, 2011 One side advances and one side retards? On the K motor one, I don't think so Jackson. They both advance on mine, and I don't know why there are two unless the 5K or 7K or whatever forkhoist they came off had two different ports on the carb. Some motors do have a vac retard however. If you get your hands on a vac guage make sure you go for a drive with it hooked up. The vac behaviour you get driving is quite different to the empty revs when you sit in neutral. Quote
Evan G Posted September 18, 2011 Report Posted September 18, 2011 (edited) i was told 5k electronic dissys had a vac advance and retard (apprently for easier starting) Edited September 18, 2011 by Evan G Quote
Jackson Posted September 18, 2011 Report Posted September 18, 2011 Fair enough. It was just an interesting idea. I don't know what my dizzy is off. I pulled it off a 3K I have lying around and it has two outlets on the dizzy. So you just T the two together? Quote
pyrohamish Posted September 18, 2011 Author Report Posted September 18, 2011 will have a look in prob a couple of weeks when i get time to spend on the project car Quote
pyrohamish Posted September 18, 2011 Author Report Posted September 18, 2011 Fair enough. It was just an interesting idea. I don't know what my dizzy is off. I pulled it off a 3K I have lying around and it has two outlets on the dizzy. So you just T the two together? I would get a timing light and see which port advances and which port retards when vacuum is applied then block the retarding port Quote
altezzaclub Posted September 18, 2011 Report Posted September 18, 2011 Both of mine advance, and I just use the 'front' one. If you T-junct the line you should get more advance as each outlet has its own diaphram. Quote
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