85rotarypower

So, some of you that have posted in my other thread about my 79 RX-7 may remember that I was having issues with a high RPM stumble. Well, the problem still exists and I am at a loss for the most part about what it could be.

To recap what I have done:

-checked for proper function of all emission control devices (all good)
-rebuilt the carb with a good kit
-converted points ignition to electronic ignition (known good ignitors and new coils)
-checked anything else that could possibly cause the problem

The only things I haven't suspected yet are the ignition wires and spark plugs as they both look alright. The problem seems to show up as a stumbling and loss of power at 5500rpm and above, getting so bad by 7000rpm that the engine almost doesn't make it that far. One thing I have noticed besides is a puff of smoke (blue/white) out the back of the car when I shift after the engine having stumbled. I'm thinking of switching out the plugs and wires, unfortunately I don't have plugs here. I have a known good set of wires though, but have never seen plug wires cause this problem and only once seen plugs cause a similar problem when the ceramic insulator is cracked.

Besides this one problem, the engine runs absolutely beautifully without even the slightest hint of a stumble anywhere else in the RPM range under any throttle conditions.

Oneiros

are you running vacuum or mechanical secondaries? could be a transition problem. also check that all solenoids are functioning correctly, some require 12v+ when the car is off but 0v when the car is on, etc.

85rotarypower

I'm still running vacuum secondaries and have checked the diaphram for holes and whatnot. Everything seemed good and at lower RPM's the transition feels good and smooth. I'll double check the emission solenoids and whatnot today I think, just to be sure. The haynes manual lays out these checks fairly well, so it should not take long to do it again.

I've also decided to bit the bullet and just replace the spark plugs suspecting they are arcing through the isolator at high internal pressures. This is a phenomenon that I have come across before, can't really recall in what engine exactly, but I'm sure it was a piston engine with old plugs. The plugs looked good in that engine but we were seeing ignition breakdown at high RPM under high engine load. The plugs in this car were replaced a good amount of time ago, but were never ran much since the car was parked for many years before I got it.

Now that I think of it, I have pulled a n00b move and haven't checked resistance on the plug wires. Will also check that today and hopefully I have enough wires still left in my good batch to replace them all if needed.

85rotarypower

Alright, I checked plugs and wires and have basically ruled out any possibility of ignition problems. I've pretty much gone through the whole ignition system at this point with no change in the stumbling. So back to fuelling issues I guess.

I've double checked that all the emission solenoids are working correctly, along with a few of the non emission parts. One thing I noticed today is that there is a distinct miss holding the engine at 3000rpm or lower while standing still and as stated the stumble shows up even under free revving to anything above 5000rpm now. There is a remote possibility that there is a clogged circuit within the carb limiting the fuel flow within the carb causing a lean condition at high RPM (kinda scary). I blew air through EVERY circuit and jet/air bleed/emulsion tube within the carb though and all seemed to be free flowing. I am at a loss here.

Also something to note, I can't recall what the fuel mileage should be on a mostly stock 79 (free flowing muffler and K&N drop in), but at this point I'm only getting about 350km per tank, putting in about 40L for that distance. This may be normal, or it may be high consumption. like i said, I can't recall what it should get.

Colbskee

check your fuel filter. My car did this a while ago and that was the fix.

-Gamah

iTrader: (1)

are the secondaires hooked up?

85rotarypower

Fuel filter is fine and I have checked pump flow and it is flowing plenty according the the haynes specifications.

Secondaries are hooked up and seem to be working just fine, will confirm I think by removing the linkage for them temporarily and note any changes to the acceleration.

j9fd3s

iTrader: (3)

what did the wires OHM out at? new NGK wires will be about 7-7.5K ohms per meter (which is how long they are, happily. the spec is 16k ohms or less. it actually makes a HUGE difference, i'll skip the long explanation, but after about 5,000 rpms the voltage available to fire the plug is going down, so the wire resistance is important, lower is better.

the other thing is the fuel filter. its also really easy to check the fuel pump, there is a volume test, takes like 30 seconds

85rotarypower

Forgot to mention it, I did a compression test today as well, turned up >100psi on all faces for both rotors, if memory serves me right, this is very good. Also, plug wires show 5000ohms or less, 8mm NGK wires in excellent shape.

I must have posted at the same time as you j9fd3s. Answers to your question about the filter and pump above.

85rotarypower

Alright, I've gone through the whole ignition system, not the problem. i've narrowed it down to a carb issue I think. I rechecked the float levels in the carb, front is definitely at about half way on the sight glass, rear looks high, hard to tell though. I'm just wondering if one float bowl being high could cause these issues I am seeing.

To top things off, the problem is in fact getting worse now. Stumbling now begins between 4500 and 5000rpm and my part throttle performance has suffered a bit. I really need to get this problem figured out before the weekend as I am planning to attend a meet and cruise this weekend.

PercentSevenC

I had some spark plug wires that were leaking around the boots that caused exactly the symptoms you're describing.

85rotarypower

I tried a different set of spark plug wires and the problem didn't change. I've pretty much gone through the whole ignition system at this point with nothing improving. Like I said in my last post, to this point, I have checked pretty much everything. I determined that all emission controls are working fine, secondaries are opening, everything is hooked up right on the carb, and checked fuel volume and made sure the return line was free. The only odd thing at this point is a possible high rear float bowl level and I can't imagine it would affect the carb this much, though anything is possible.

I'm going to take the air horn off tomorrow after work and double check that everything is proper with the floats. I installed new needles and seats when I rebuilt the carb, but didn't touch the float adjustment. They looked alright when I visually inspected them, both sitting at about the same position when the air horn was inverted and I already knew the front float was fine. Maybe a bad oversight on my part. Will find out tomorrow.

In regards to the carb. I completely cleaned the whole thing out, blew air through every passage and jet/air bleed in the thing. Everything was free flowing and looked quite clean.

Oneiros

Disconnect the vac secondary linkage on the front of the carb and wire tie the linkage tabs on the back to get mech secondaries. And see if the symptoms change. This gives you a better troubleshooting platform than vac secondaries.

If you are getting the same stumble when secondaries are more than partially open, a secondary main fuel or air jet is blocked. If the symptoms are rpm related irrespective of throttle position it's more likely going to be outside of the carb, or perhaps in the emissions system.

pratch

Yet another possibility...

When my fuel pump was getting killed by the factory computer, the only symptoms were at high RPM. A bad signal was being sent, and the fuel pump was only getting cut under those conditions.

Changed the power source and installed a kill switch - problem completely went away.

If you find yourself without anything else to check, run some test wiring and give it 12v with ignition on. If your problem goes away, kill switch or do the jumper trick on the comp.

85rotarypower

Alrighty, think I may have hit something here. I double checked fuel bowls, rear is in fact fine after messing around with a mirror to see it properly. Checked ignition yet again and confirmed that advance is working properly. Here is an odd one though that I can't seem to find any information about in the haynes or FSM. The vacuum solenoid on the carb doesn't show up for any checks in either manual. I've tested for proper function of the ACV which this solenoid is connected to and it passes air out the upper pipe when its supposed to. Interesting thing though, if I unplug the solenoid electrical connector or vacuum hose at idle, the engine dies almost instantly and can't be restarted easily.

This is confusing to me since every diagram I have seen of the ACV shows it has no direct interaction with the intake air or fuel system in a way that it could cause the engine to die. Also, I noted that my stumbling issue at free revving also shows up when the ACV "opens" at approx. 1700rpm. Is this normal operation for the ACV?

Oneiros, I an reluctant to think that the secondaries aren't opening at all on this car. There is just far too much power to be operating only on primary barrels. Even with the stumbling issues, I can leave a 10 foot black strip behind me shifting to second, this is not possible with only primaries working (I know from experience).

85rotarypower

Ok, scratch what I said about the ACV at this point. Found exactly what I was looking at. Was on two pages that stuck together in my haynes lol. Its the anti-afterburn valve and seems to be working fine actually, unless it happens to be opening at high RPM somehow. Will disconnect the vacuum hose and plug at the ACV to confirm. If that doesn't solve it, then I may take the ACV off and check for leakage in the gasket.

Oneiros

OK sure but comparing data from mechanical secondaries to vacuum secondaries will give you some helpful information in finding out where the problem lies. For instance if you floor it at say 4000RPM with mech secondaries, does it still stumble in the same manner. This would indicate a problem with secondary main circuit.

If however the symptoms remain the same regardless of throttle position with mech secondaries then you would have to look outside the carb for the source of the problem, e.g. emissions solenoids.

85rotarypower

Thats a good point, I may try that yet, depending on time. I already have a bit of an idea of how it reacts to throttle position since I can keep it out of the secondaries and it still stumbles.

85rotarypower

Messed around some more today, tested all components of the emission system, all looks good there. Unplugged the emission computer, no change to the stumbling. Did a quick and dirty but functional mechanical secondary mod to confirm the stumble exists even while on on the primaries, still there without secondary barrels open. I reconfirmed pump flow to be at least 1600cc/minute (haynes calls for 1100cc/min or more). Reconfirmed float levels at idle, half way up the sight glass on both. I made absolutely sure all jets and air bleeds were clear when rebuilding the carb. As far as I can tell, every part of this fuel and ignition system is functioning just fine but obviously something is still not right.

85rotarypower

Another day, still the problem has not been located. I pulled the ACV off and checked for sections of the gasket that may be missing. Nothing... Messed around with the ignition timing, confirmed proper advance of leading and trailing, all is good there. Checked again for vacuum leaks, nothing. Found my inline fuel pressure gauge, put that on and revved the crap out of it, pressure holds steady at 4.5psi the whole time even when the stumbling starts. Only thing left is an internal problem with the carb. I'm thinking I'll pull the carb off tomorrow and do a basic look over of it. All looked good when I rebuilt it, but maybe I missed something.

j9fd3s

iTrader: (3)

how are the cap and rotor? voltage at the coils? in my experience a miss or stumble @WOT and high rpm is usually an ignition problem, with usually being the key word.

but the wires are good, plugs i assume have been changed in this process (they can look ok, but be bad). cap and rotor? i had decent ones on my car AND an MSD, but it liked the new cap and rotor still.

coil voltage is out there, but coils work from charge time and voltage, so if voltage is low to the coils you loose ignition power.... my car actually has quite a bit of voltage drop, the alternator is right there @13.6, but the coils only see mid 12's

in my experience a fuel problem will either make the engine stop running because its lean, or stop running because its too rich. a little lean or rich of optimum changes power, but usually still runs ok.

not that i'm right, but just what i've seen

85rotarypower

I agree with your observations about the fuel mixture usually changing power a bit if its rich or lean, but nothing to the extent of what i'm seeing here. This is why I'm so reluctant to think this is directly a carb problem. I experienced both ends of the fuelling spectrum while tuning the Edelbrock on my 82 4 years ago and I must say, even with air fuel ratios at the extremes on my wideband, the engine never at any point reacted as this engine is.

This being said, I am out of ideas. I have used 3 different sets of plugs in this engine with absolutely no change, changed all the plug wires, even ran a grounded probe around the ignition system all with no change in how the engine ran. I've advanced the timing, retarded the timing, disconnected and capped vacuum advance, all with no change to the high RPM stumble. I ran propane around every possible source of a vacuum leak to detect something, anything and nothing showed up. Power at the ignition coils is just under 12V with the engine off (never tested running, probably should).

Going back to the carb, I know for a fact everything was clean when I put it back together. I know the fuel pump is not cutting out, I know the fuel pressure is holding fine and the float bowls are both holding steady at half way up the sight glass.

I feel like I have exhausted every possibility. Something is definately not right of course, but I just can't seem to find it. About the only thing I have not checked is for a grounding problem from the engine to the battery. This is the only other thing I could think of causing complications with the ignition.

84stock

iTrader: (5)

You must get awful excited reading your haynes manuals....

85rotarypower

lol, yeah I guess so.

j9fd3s

iTrader: (3)

i would add a ground, but i doubt that will do anything....

what's the exhaust like?