SU carbs and high revs

[240ZMan]

This weekend I received the ZTherapy video on tuning the SU carbs (I have 3 screw round tops on a 73 240 with an E31 head). I followed his procedure to do the tuneup, just short of taking the bodies off the manifold. From 1000 rpm to about 4800 rpm all is great now. But at idle if the engine isn't fully warm (and I mean from driving, not just as indicated by the temp gauge) there is some hesitation when first cracking the throttles a small amount at idle. This was never there before.

Also, when accelerating at full throttle, just before 5k rpm the power falls off. I don't mean gradually, it's almost as if there is a rev limiter it's that sudden. Feels like it's not getting enough fuel. I have both a mechanical and electric fuel pump and have changed all the fuel line hoses and filter as well (hoses needed changing anyway - I think one piece was the original 30 year old stuff!). And prior to me tuning the carbs the sudden drop off didn't exist.

The two main changes I made with the carbs apart were to reset the floats (both were a bit more than the .55" recommended in the video) and to set the needles with the shoulder flush with the piston (They weren't even close).

I've checked the plugs and they look really good, the best they've looked in a long time. I have them gapped to 0.040 as I'm running a pertronix ignition with their 40kV coil.

So I'm looking for some suggestions on the power falling off a cliff at around 4800 rpm.

Thanks in advance.

Daniel

[Victor Laury]

Well, something is wrong. How's that for good info?

Before you fiddle with anything, have a friend step on the throttle pedal, whilst you confirm that the throttle is opening completely.

I know that seems simplistic. But, It happens. More often on cable pull throttles like the roadster. Someone will say their car is no good after 4800 or so, and it turns out their throttle is only opening 3/4 on the stop.

Since youv'e just swapped the whole system out, perhaps the linkages are not in adjustment for the new set. SU on a stock engine should sing to 6,200

[Daniel]

Also make sure the chokes are indeed turning fully on/off and the needle seats are pulling back to the correct flush position on the carb bottom.Maybe your new carb settings would like a small timing change.

[240ZMan]

Good suggestion to check the throttles opening fully. I checked this about a month ago and did have to adjust the linkage a bit but I'll check it again this evening. I did check that the chokes are all the way up several times this weekend so I don't think that is my issue this time.

What's got me puzzled the most is that prior to me tuning the carbs I could pull past 5k. The power was falling off, but it didn't just drop off a cliff.

One more thing, I live at 6500 feet elevation. Of course, that hasn't changed since the tune up either ;-).

[240ZMan]

Just got back and tried an experiment on a deserted road. Instead of upshifting when the power fell off, I just left it in gear and held my foot down. The revs stop climbing right around 5k and just stays there as if there was a rev limiter. When I did the same thing in 2nd gear, the same thing happened at the same RPMs. Power up that point is good, not like it was starving for fuel. Besides, if the fuel supply couldn't keep up, I'd expect the power to drop off more gradually, and after a sustained WOT. Not at all what I'm experiencing.

Any other ideas?

[inline6]

It's been my experience that the timing and mixture adjustment are inextricably linked. What are you setting the timing at? Try advancing your timing about 7 degrees for kicks and see if it changes anything.

Also, you say that you adjusted the float level. Su's are very sensitive to this level. Are you positive that you set in exactly how he described it in the video? The reference point is unusual in my opinion.

I set the level by cutting a small nail to the specified length. I usually miss it a bit on the long side and then grind the end to the exact measurement. The Nissan factory manual for 1972 states 14-15mm or .5512 to.5906 inches from the recessed, flat area inside the float cap to the top of the flat, metal part of the float. I modify the nail to be at 14.5mm

Next, use needle nose pliers to hold the nail in place and then lift the float up to pinch the nail vertically in place, then blow through the fuel inlet tube. It is correctly adjusted when I have to put some effort into blowing through the tube, and can just barely hear the air squeeking past.

Garrett

[2ManyZs]

I recently crewed for my old race car.. OK, it's a 280 but it too would not rev past a certain RPM not matter what we did... ended up replacing the cap and rotor and the problem disappeared...:stupid: One thing we shouldn't have overlooked to begin with.....

I agree also it could be a static timing, or vacuum advance problem as well....or, it could be the fuel pickup in the tank is getting clogged up with junk in the tank.

Could be a combination of things, but wouldn't hurt to verify them all...

[240ZMan]

If anyone is still with me, here's the latest:

Per the suggestions above, I've gone back and checked that my timing is still solid at about 7 degrees at idle (which is all I can do before I get pinging), that my throttles are opening all the way, and I used inline6's technique to set my float level (which meant that I raised the level a little more than 1/8" on each one). Also swapped the old distributor cap and roter back in. None of this has had any affect on pulling past 5k (with one exception below), but the good news is she runs great from idle up to that point:-)

So here's some new inputs: I was worried I was not getting enough fuel. Although I have both a mechanical and electric fuel pump, I usually leave the electric off as it's rather noisy (I put a switch in case I need it). I wondered if perhaps the mechanical was somehow restricting the flow so I routed the fuel line to bypass it and run entirely on the electric. That made no difference.

However, I took a run with the air cleaner cover off (base plate still on) and found that the power now didn't fall off until 5500 rpm and it was more gradual, although I think still too steep. Also, while running at >5k at wot, I heard a few loud pops, almost like backfires, until I upshifted. It was always between 5000 and 5500 rpm. Just to refresh, I have an early aircleaner on my 3 screw SUs with the 73 intake manifold and an E31 head. The rest is a stock 73 240. And the airfilter is pretty new and looks clean.

I put the air cleaner back on and went out to get a burger. On the way back I experimented with turning the mixtures a full turn richer and a full turn leaner. Neither helped, but immediately after I set them back to normal, when pulling back onto the road at WOT in first gear, the power didn't fall off until after 6k rpm. It only happened that once, and then 2nd gear gave me the usual "rev limiter" at 5k. But clearly there is something intermittent here.

One last thing, as I was adjusting the float levels, I notice that rear needle valve stuck once. I took it apart and the sides of the needle show some wear that appears to correspond to the size of the hole in the seat. The front is fine and doesn't show any wear.

So, what you think? I'm leaning towards buying a rebuild kit from ZTherapy to get to a "known state", but hate the thought of spending the money if it isn't necessary.

Sorry this has dragged on so long, but as you can probably tell, I'm pretty stumped!

[indyzprez]

Your on the right track it's a fuel supply issue, do the kits. It will eliminate another variable.

[Alan Pugh]

You say in your first post that you set the needles flush with the pistons because they weren't even close but you haven't said where they were before that.

It seems to me to be on the lean side at high revs.

If the needles were too high in the piston and you have lowered them this will give you a leaner mixture..

Something else to look at before you spend more money.

Alan.

[inline6]

"On the way back I experimented with turning the mixtures a full turn richer and a full turn leaner. Neither helped..."

Neither helped? Was there a noticable difference in engine behavior? When I put mine a full turn leaner from where they run, about all my car will do is idle. Hitting the throttle just makes it hesitate and backfire through the intakes?

A full turn richer just makes my car start fouling plugs and it loses a little bit of power-- by the way.

What needles are you running? The original N-27's?

Garrett

[240ZMan]

Good question Garret: when I went richer it didn't want to idle well and I didn't notice any other difference. Going a turn leaner didn't make any difference that I noticed at all.

I'm running whatever needles were in the carbs when I bought the car with the 72 roundtops. I'm going on the assumption they are stock. Is there some way I can tell?