madkaw

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Datsun P79 cylinder head mods

Thank God we got a second page, it takes forever for my phone to scroll that first page- I am 99% positive a have an ignition problem= Megasquirt. When plugs 4,5,,6 look carboned on triple carbs, then it's probably ignition. I've always ran the 6 with good results.

That sucks- so much a grinder wouldn't fix?

Fuel pressure gauges reads steady at idle. Checked the filters/screens when checking floats. I'm not in town right now, but I have decided that first thing I'm going to do is readjust valves. Bouncy idle, low vacuum reading , miss, it's text book tight valve lash-and I know my valves are tight. I went beyond specs to quiet bad rockers basically, and maybe it's tightened up more to the point that the valve(s) is not sealing.

That's the first thing I jumped on.Everything is holding. This issue is worse and effects all parameters. Basically it's running like poop! It acts like a slight miss, just enough to hear and notice in performance. AFR's are not stable which is easiest to see at idle and it causes the idle to sound like a galloping 3/4 cam. For now I am going to readjust my valves to stock specs to eliminate that. Maybe an internal fuel leak that is allowing some fuel to bypass normal metered paths and causing the roughness. I am going to recheck the needle/seat assemblies. I also eliminated the brake booster and PCV by plugging them at the vacuum log.

As far as the dizzy - go with what you can get away with. I think there should be a number stamped on the pull rod of the vacuum advance. Double that number to get advance. My ZX only had 17 degrees vacuum. I didn't run vacuum, just static at 19 + mechanical 17=36 total. The vacuum advance units seem to be trouble in a lot of these old units.

Nice job- that will make a big difference in the way the engine looks

Had a long day of getting my hands smelling like fuel. I pulled my float covers off and sure enough my settings were off. Probably a case of not properly adjusting the float within the parameters of the adjustment screw. There is only about 5mm of adjustment externally but the float has to be set so you can get 2.5 either direction. Anyway it wasn't off much and didn't help my issue so I moved on for now. Plugs looked good. I checked timing on my MS and timing light- that was good. Sprayed starter fluid everywhere looking for vacuum leaks an couldn't find any. Checked vacuum at idle and it was 12. I can't ever recall measuring this before so I can't compare, but it seems low. I measured at my vacuum log . Checked valve clearances and some are tight, but I knew that because I set them that way- might need to undo that. It's been that way for many months with no issue. My issue seems to be getting worse- maybe it will just break so I can figure it out. Sounds like a miss and my AFRs are bouncing around at idle Compression check next

Chasing my tail lately I believe. My 'studder' is back(randomly) and seems to be directly tied to a rough idle I intermittently get. These carbs use to idle at 750rpm -rock solid, but now everyonce in a while the idle bops around. I no longer have a working tach, but my idle speed hasn't changed much, but just gets erratic and shows super lean . It seems when my idle gets crappy my transition issue re-appears. I guess it's time to start from square one. Tomorrow I'm pulling the float covers off and start double checking all the settings and looking for anything that would cause a float issue. Yes I am focused on the floats because it is one of the few adjustments that can effect all parameters of operation. Not to say I don't have something else going on --and I will check. The mikuni manual eludes to checking for carbon build up of butterflies or pilot screws, but these carbs are clean.

Wow- Rebello can't believe these engines made it out of the factory with the close piston/head clearance . Just having a hard time believing this. I guess the extra clearance would be the safe route. And I am not questioning Rebello s engine building capability, but to prefer an open chambered design head for piston clearance doesn't quite make sense.

Mitch, thanks for the filter advice. Not sure how the main air jet change leaned you out. The smaller jet should have richened you up a bit at WOT and brought the main in sooner. The cruise is ALL pilot to a certain point. I haven't read anything about the idle needing to be 1000rpm. I have been able to keep it around the 750 rpm range pretty consistantly. It gets a little lopey sometimes, but still idles well. I have noticed that hot starts are easier with a rich idle .

Check for loose connections especially around the dizzy and follow Carls advice

I haven't forgotten the accel pumps and I have chased those tiny cotter pins on the rods too many times. That is one of the hardest adjustments to do on these carbs while mounted on the engine. The accel pumps seem to be doing there job, and I wonder even about going a smaller size yet. but this doesn't have effect on the transition issue I am dealing with. My issues comes in high vacuum, mid rpm range ,light tip-in, moderate loading. It goes beyond the expectations of accel pumps. Changing the bleed pipes had the most significant change, but not perfect. I am chasing perfection-which will not happen, but still fun yet. These carbs were about brand new when I got them. Idle is kept in the 750 range. I did check throttle plates by measuring the opening and setting all alike.

So I have the 70's pilots in there now and I watch my AFR's hang right in the beefy section on 13eens. At 75mph I was probably hanging around 13.2-13-5 at best. I was also watching my gas gauge thinking it was going to plumit at anytime. What happened was un eventful as the engine purred as good as it ever has and the gas gauge didn't move much different from before. I made a round trip to work which is 50 miles each way 80% highway. I tweeked the pilots to 1&3/4 turns out. That gives me 14.5-15 AFR idle. I'm telling ya-the engine/carbs love fuel. It just sounds better and runs better. Putting around town the engine responds quickly and decisively. Now look at my plugs. I have done nothing with these plugs since I installed my megasquirt. These plugs have seen everything from flooding carb to len mixtures. So after last nights run a pulled number 3. It is way better then I anticipated. The blackness at the top of the threads is dry soot and I always seem to have that. My tan line goes all the way down the plug! I might still turn in the pilots another 1/8.

You need to verify by checking plugs too. Are the rear three sooty or fouled?

So you are advanced now already? What dizzy model are you running and what's your static timing

Guess I sounded snarky there - sorry. Maybe not the best dizzy and they are all different as far as mechanical advance. How much timing all in? Might consider a ZX dizzy or electronic ignition .

To Hell with stock specs - you aren't running a stock motor. Any pinging you might have had might have been from vacuum advance. Unplug it and play around with advance. If you don't have a timing light- you shouldn't own a Z

Looks great

Didn't you say it ran better without the vacuum advance? Now without it you won't get the ignition advance you normally get at cruise with light throttle advances. Not sure what numbers you are running but you might want to advance your timing with the vacuum part disconnected and see how it does.

Been the float route already. The Mikunis can be adjusted with a screw from outside the carb . It didn't help the lean spot on transition no matter how high the level. I haven't revisited that option now that I have done the bleed pipes. It might add a fine tune option. I think there is a limit to how precise the tune is with carbs. If I didn't have a wideband to stare at I would probably be happy with a powerful rich mixture. Now I'm obsessed with my AFRs and my Z :0

Your running a " stage 2" cam also right ? I have a complete set of jets too Did a long drive with the 70's in there today. With the bleed holes plugged and the 70's I noticed my AFRs get near 12ish at 75mph - eek. So I went in an 1/8 turn and that bought me a whole point. Now a steady 13- 13.2 at high speed cruise. With the 67.5's I achieved 15 at that speed, but put up with the "bobble" in transition. Carbs=compromise

Mitch, Using bpr6 and running T bleed pipe. I get no back fire on decel and never have with all my settings. You are not going to get any better AFRs going with smaller main jets. Your 390 gearing is going to put you in the 13 range because of rpm . I see you have bumped up the pilots. Surprised your not up with me in the 70 range with your L 28. Bigger pilots will tan those plugs up and might get rid of your back fire. Timing might also help with decel? What rpm is the backfire? My MS has timing way high at low load- maybe 45 degrees, but I didn't backfire with the ZX dizzy either

Newly rebuilt engine with loud tapping noise even at idle?!!?! - Page 3 - Nissan L6 Forum - HybridZ