Loss of Power > 5K RPM under load

[jrusso07]

Hi - I am new to the club and need help and advice with a 72 240Z problem.

The car is stock - except that the emission controls have been removed and the auto tranny has been replaced with the period correct 4 spd. It also has AC. The car idles and runs like a champ untill I really get on it. So if I am driving along at say 60 MPH and I stomp on it, the engine misses or hesitates at about 5000 RPM. Like it is has a governor on it. I susepected ignition problems or perhaps fuel...blockage somewhere.

I tried from a dead stop and in first gear it pulls hard all the way up to the yellow line. Then in second, it chokes at around 5000. It is interesting that once it chokes (stummples, has no power, gas pedal has no effect), it stays that way until the revs come back down (say run at 2000 or 3000 RPM) for a short period (about a minute or 2). It doesn't stall and after an "episode" it idles smooth as silk.

I just recently installed rebuilt carbs, new plugs (NGK), new wires (NGK), new points and condesors (it is a dual point left over from the AT), new cap and rotor, a new "fireball" coil. A manifold vacuum test checks out fine. She runs smooth and strong at idle and I can rev it all day long without a problem when the engine is not under load.

I check the carb floats, reset the dwell on the points, check the plug gap and wires, the connections to the dizzy have been cleaned and are secure. I put back the stock coil (thinking maybe the dizzy was arcing). The fuel filter has been replaced. I haven't checked the screens in the carb banjo bolts but they are freshly rebuilt...by a third party. Timing is set at 17 deg BTDC. This (I belive) is the non-smog setting (from a spec sheet for a 71) vs 5 deg BTDC. This is where the manifold vacuum is at it best.

Also, I tried running on just the advance points in the dual point dizzy...no help here.

The exhaust is new - a new Y pipe from the manifold, a ANSA center pipe (free flow) and a OEM muffler on the rear (all brand new). So I don't think the exhaust is clogged...

My last guess is to check the (mechanical) fuel pump...any one know the best way to do this?

Any ideas on what else to check - or a method on how to isolate the problem?

An electrical connection?...something in the tranny or rear differential that is locking up or really loading down...that's a long shot!

Thanks!

72 Triumph Gt6

70 Datsun SRL311 Roadster

72 Datsun 240Z (HS30)

[nwcubsman]

Check your valves. If you are crapping out at higher RPM, starts to stumble, it sounds like an issue I had. Also, it sputtered after cold starting and had trouble until it could get up to choked RPM. Valves were too tight!

[jrusso07]

Hi Bryan - thanks for the reply. I checked the valve lash (hot) - 0.010 intack and 0.012 exhaust. All checked out ok. I will check them cold (.008 intake and 0.010 exhaust) tomorrow

Is this what you mean by too tight - the lash is not set correctly?

I also found that 4, 5 & 6 cylinders don't seam to provide any power - removing the spark plug wires does nothing. That carb (the rear one) I can't lean out. So I am guessing I have either a carb problem, or a intake manifold leak (I get a intermittent high pitch whistle but I have not been able to isolate it - it comes and goes). Worst case I have some cylinder to cylinder leakage at the head gasket - oil and water are fine...

[Mat M]

It may just be that your mechanical fuel pump just can't keep up with demand. Sounds like stomping on it in first gear takes most of the fuel out of the floats, and while mashing it in 2nd, all the gas gets used up and the pump isn't making it.

It's easy to rebuild the pump. The main thing you need are all the gaskets intact. Oh, yeah, there is a small rubber boot inside where the pump arm goes through, DO NOT try to pry it out of it's seat. Those old rubber boots are fragile! There are two one-way valves that may need cleaning, or it may be possible that the pump arm just needs to be repositioned on the cam. Hope that helps.

Have you tried to set your mixture screws on your SUs? Tighten both screws all the way, and then back out 2 1/2 turns and go from there.

[beandip]

If you plan on running hi revs , change to a electronic ignition and get away from the points. You wont be sorry . I am running a '79ZX ignition and am VARY satisfied. Have you balanced the carbs ? You said that you cannot lean out the rear carb? Some thing is out of whack there. Check the float level , it sounds like maby it is set to high and could be allowing raw gas flowing at idle and or low rpm. Set the balance then the mixture. How many miles on the engine since rebuilt ? Do check the fuelpump for both pressure and flow .. And you can remove the banjo filters.

[Sailor Bob]

It reminds me of a fuel line obstruction - it revs high in first as you develope pseudo vacum in the line and in second (when you're still throttling) it starts to bog (as you push the throttle). When you release she idles OK? I had a similar problem in a high end boat (200 hp but no guts???) turned out that the manufacturer don't clean the tank and the line was partially blocked with debris. The more I throttled the slower it went, when throttle released it idled good.

Just a thought.

[jrusso07]

Thanks for all the suggestions. I have tried repeatedly to balance the carbs. The air flow (unisyn) is perfectly balanced on the carbs at idle and at 3K. I used a colour tune to set the mixture. The front carb sets right up. Just dial it in and it is solid. I can't get "blue" (lean) at all on the rear carb - even with the mixture nut all the way in (lean). I does react (negatively) to an over rich setting. I checked the float level and the needle setting - both right on.... Also, I can run the the engine on the front carb all day - but only a few seconds on the rear... I am going to put an old (decent) carb on the rear after I check the fuel pump and line for obstructions.

Based on your inputs I am settled that the rear carb is not set up right or it has a obstruction in it's fuel supply. Note that the rear cylinders don't contribute to any RPM change when the plugs are removed.

Oh, two other observations - two things happend almost simulataneously - the temp sensor became intermittent (gauge checks out ok when lead is shorted to ground) and the oil pressure at running revs is almost at the top of the scale (where it was normally at about 70% of full scale)

Thanks again for the fine advice!

I am thinking electronic ignition - but I don't want to give up the tach..I think

More to follow.

[Mat M]

I am running the ZX dizzy electronic ignition; I have not had to give up my tach.

Have you tried lifting your carb pistons with your thumbs to see if there is too much of a lifting resistance (on th erear carb) when the throttle is applied? Is it possible to have the carbs sync'ed and yet to have one lazy piston?

[jrusso07]

Mat - I was under the impression that the ZX electronic dizzy wouldn't support the tach. I guess I shouldn't belive everything I hear!

I checked a bunch of things tonight - valve lash - good both hot and cold. Timing, fuel pump, banjo bolt fuel filters and the piston resistance. All "pass". The pistons both drop with a nice clunk, no wear on the needle, both pistons rise simultaneously with thottle. I swwapped out the rear piston and dome from a old (good) carb - no difference.

I am really stumped...

This weekend I will take it from the top and do the tune up checks and tune up procedure - step by step (inch by inch)

Thanks everyone for your help!

[Arne]

Mat - I was under the impression that the ZX electronic dizzy wouldn't support the tach. I guess I shouldn't belive everything I hear!

Popular misconception, I think partly due to comments on Bryan Little's page. If you ask around, you'll find there are a fair number of people here running the ZX ignition with the stock 240Z tach. I'm one of them.

[a7dz]

I just bought a new ZX dizzy for my 70 and will follow the instructions to modify the coil wiring and remove the resister. Everyone I know runs this set up with great reviews.

[beandip]

I am running the stock 240 tach as well with the ZX ignition. back to the carb problem . What about the shut off valve on the float on the rear carb ? could it be flooding the rear carb and there by adding fuel so you cannot adjust the needle to lean it out ? It is not wise to swap parts with these SUs , you might get by with a 4 screw but don't do it with a 3 screw. The domes are fitted at the factory and should not be interchanged . Since you have another rear carb , give it a try , swap out the needle so those are the same. I also have a color tune but find that it indicates proper mixture when actually it is lean . It is a great tool though. When I set the mixture with it , I get a lean pop when I blip the throttle. I find the color tune to be a good indicator at 2500 to 3K when trying different needles.