85rotarypower

Some may remember my thread about 2 years ago about a bad stumbling issue my 79 GS has when taken above 5000 RPM (https://www.rx7club.com/1st-generati...500rpm-971718/). At the time it didn't seem fuel related so I focused more on ignition than carb, but I did still check the carb and fuel system quite a bit.

A bit of back story of what I have done since the last thread. I rebuilt the engine with all new soft seals and new Atkins apex seals through the winter of 2012-2013. Engine was in surprisingly good condition internally, all hard seals besides the apex seals measured out to almost new specs. I replaced all the soft seals, apex seals, and the stat gear and rotor bearings as well for good measure. The engine started up and ran excellent first try. After break in, where I didn't go over 4000 rpm, there was no sign of oil getting past the o-rings.

Unfortunately, my high RPM stumbling continues to be a problem. When I first took the engine up to 5000+ after the rebuild, it performed fairly well, stumbled a bit though. Wasn't too happy about that to be honest. Took the car out for a track event after getting about 3000 km on the engine, well broke in and it performed quite well. Had a bit of stumbling issues on my way to the track, but that seemed to clear up once I got out on the track. The stumbling was a bit on and off throughout the year though, quite odd really.

Fast forward to this year. I have driven the car a fair bit with the high RPM stumble as bad as it was before the engine rebuild. I gave up on it being an ignition problem as I have been over the entire ignition system many times. The only parts I haven't checked are the ignitors and I haven't changed the cap and rotor. I have one spare ignitor and will try it tomorrow likely. I'm also pretty sure this isn't a fuel problem.

See, I recently decided to take the plunge and swap out the carb for a nice 2 barrel sidedraft. I picked up a nice condition Dellorto 48 DHLA, RB prepped and the matching wrap around intake manifold. I confirmed the carb is clean and it has all the original RB spec 12a components inside. All being roughly equal, this carb should run quite well on my engine. I just finally got it all installed today.

Out for my first drive after getting the idle mixture sorted it seemed to run fairly well, but to my dismay, going over 5500 RPM results in a massive amount of stumbling regardless of throttle position, the same problem as with the stock Nikki, but amplified a bit, but it might just seem amplified since I can seem to hear it from the intake a bit as well now. The high RPM mixture could be out a bit, but I suspect something more since, again, I never experienced anything like this while tuning the Edelbrock on my old 82. It doesn't feel like lean or rich mixture.

Thinking about it now, the sound is similar to one of my old daily drivers. It had an ignition cut traction control system on it which in some ways sounds very similar to what my 7 is going through. I have replaced the spark plugs with BUR8EQ all around, as per the 82 12A requirement. The plugs aren't actually more than 100 km old. Same spark plug wires as before, which tested fine. New coils when I converted to electronic ignition and removed ballast resistors.

Sorry for the long post, but anyone have any ideas? Somehow I feel this is something simple that I am missing or refuse to believe to be the problem.

JDuncan

iTrader: (3)

If you haven't changed the cap and rotor, I'd do that first. I'd also swap the coils around, if you haven't already tried that, to see if the symptom follows a coil.

On the non ignition side, what kind of fuel pump? When was the last fuel filter change. On a car I had a long time ago, I'd swear a fuel filter change netted me 20hp at the seat of the pants. Might want to check on that stuff if you haven't already.

85rotarypower

I haven't replaced the cap and rotor and plan to order it tomorrow. The fuel pump is an inline Walbro 255 equivalent running through a bypass style fuel pressure regulator. Fuel filter is new. The persistence of the problem regardless of throttle position indicates this isn't a fuel problem but rather an ignition problem.

The coils are fairly new, replaced both when I converted to the electronic distributor. I can try swapping them around to see if there is any difference.

JDuncan

iTrader: (3)

What is the fuel pressure? When you say Walbro 255 equivalent, I immediately think of a pump for a fuel injected vehicle with high pressure. You should probably be running between 4 and 5 psi for fuel pressure with the carb you are running.

85rotarypower

Fuel pressure is set to 3.5 psi indicated. I used a bypass style regulator to allow the high pressure pump to be regulated to a very low pressure without burning out the pump.

JDuncan

iTrader: (3)

I thought the sidedrafts preferred closed to 5 psi for good operation. I know the downdraft RE kit I used to run did. Might check on that as well. If the pressure can't keep the bowls filled, that could cause the fuel to not keep up in the higher revs.

When you were having the problems with the Nikki, was it with the same pump and regulator?

ghost1000

iTrader: (1)

Sounds like something is loose in the ignition, possibly effected by vibration. Swap your leading and trailing coil, wires and swap the igniters.Make sure the tips of the spark plugs are tight. Dbl check timing.

The engine will run smooth as silk if the trailing is disconnected so swaping the parts will help you eliminate problems.

DivinDriver

iTrader: (15)

If the cap/rotor change doesn't fix it, I'd lean toward dizzy issues.

If you are still running the original points ignition, I'd do a points and condenser swap, as you may be getting point bounce or a poorly charging condenser, both of which become more of a problem as RPM increases.

If you've converted to electronic ignition, check the pickup gap and bearing plate operation, make sure all stays within spec.

Dizzy shaft play would be good to check, too, as would both vacuum and mechanical advance mechanisms.

85rotarypower

Thanks for the tips guys.

JDuncan, I have checked gruntled.com, a site about Dellorto carbs on rotaries, and it suggests 3.5 psi. Multiple posts around here as well seem to float around this number. With the Nikki, I was running the stock pump which was confirmed to operate at 4.5 psi dead headed. It was a brand new pump that I swapped out during the carb changeover since the original decided to fail last year. Flow of the new pump was confirmed to exceed the lower limit as specified by the haynes manual which was 1300 cc/min minimum flow. Even the original pump put out 1600 cc/min before it failed and the problem still existed.

ghost1000, I have swapped the ignitors around today and even installed the spare I had and nothing changed. New cap and rotor will be ordered tomorrow, hopefully next day delivery.

DivinDriver, I have never checked the pickup gap on the electronic distributor. The only shaft play is a bit of end play, which is within spec. Mechanical advance is working fine, no vacuum advance with the Dellorto though, but i confirmed it was working fine with the Nikki when it was on there.

JDuncan

iTrader: (3)

You have a spare ignitor. If you have an extra coil, you might consider doing the DLIDFIS mod. This would take the cap and rotor out of the equation for the leading side. With the pump change happening at the time of the new carb, it moves me away from it and back towards the ignition side due to the issue existing prior to that pump.

I'm sure you don't, but make sure you don't have the vacuum advance ports on the distributor plugged. IIRC, that can cause issues.

DivinDriver

iTrader: (15)

Do you get adequate advance at higher RPMs without the vacuum advance? I've never tried running without it myself.

85rotarypower

The vacuum advance is only effective in low throttle situations to my knowledge and has little effect when under full throttle since vacuum is 0 or near 0. I followed the directions in the Racing Beat Dellorto installation manual as found on gruntled.com. That states to disconnect the vacuum advance and set to 2° BTDC leading and 20° ATDC trailing base timing at idle. It seems to idle best right around there and to the best of my knowledge it should result in about 14° total advance on the leading accounting for approximately 12° mechanical advance as specified by the FSM.

I checked the pickup coil air gap and it is .024". Haynes specifies a range of .020" to .035".

85rotarypower

I don't have a spare coil, used my only spare for a 6V to 12V conversion on a historic tractor I just recently sold. I did have the vacuum advance ports plugged so maybe it wasn't reaching full advance?

JDuncan

iTrader: (3)

I'll let an old post from ages ago explain: https://www.rx7club.com/1st-generati...6/#post3525846

However, re-reading that old thread makes me pretty certain this has nothing to do with your issue.

pinoyremix

I recently had a carter fuel pump installed this year, after having my dell supplied by a gsl se pump for the last 6 yrs, it never really ran right and I had my Mallory fpr dialed down to 3 psi, after installing the carter fuel pump it runs so much better, I'm still running with my Mallory fpr so I can have a fuel return line. I did a full gsl se fuel system swap 6 yrs ago. It's definitely the fuel pump

85rotarypower

This is odd. Did you GSL-SE pump simply not supply enough fuel for the carb? According to theory, as long as enough flow is available, a given restriction, in this case the needle valve in the carb that meters fuel flow into the fuel bowl, will produce a constant pressure. If flow is reduced or the restriction lessened, the pressure will reduce. If the flow is increased or the restriction increased, pressure will rise. A pressure regulator is exactly that, it regulates pressure to a set level.

Non-return style regulators will simply cut off flow on the low side to keep the pressure stable, increasing pressure on the high side until your pump relieves over its internal bypass, or if no bypass, shuts the pump down. A return style regulator, IMO a superior option that offers many benefits, will maintain a set pressure without increasing high side (supply) pressure to maintain the low side pressure.

So, for one pump to work better than the other, the other pump must have been faulty in some way. It didn't provide enough flow which allowed the overall pressure to drop below the set point, or the regulator couldn't provide enough return flow to keep the low side pressure at the set point which caused overall system pressure to rise. I could see this being the case if you reused the original 1/4" return line from the 12a engine. In my case I have oversized my return line (3/8"), even modified the tank for a larger return port, to prevent any possibility of overpressurizing the system due to return line restriction. A return style pressure regulator only regulates as low as your return line pressure loss due to restriction.

pinoyremix

The stock pump wasn't providing enough fuel so a buddy and I installed the full gslse fuel system including the hard lines and tank because he had a set lying around. I think the stock gslse pump pushes 39 psi? I can't explain it all I know is that now that I have a lower psi pump 4-6 psi everything runs better, maybe even though I had the Mallory set at 3 psi it was unable to fully lower the pjs actually or maybe to much flow of fuel, not sure.

ghost1000

iTrader: (1)

Timing is so trick on these cars. The leading mark on the main pully is 5*after TDC

Its will run better at 5-10 BTDC which is 10-15mm clockwise away from the factory mark. Once its running good you can advance it more by feel but you will start to get a choppy idle.

EFI fuel pump is fine watch you fuel psi as the problem occurs I doubt you will notice anything. Areomotive makes the best fuel pressure regulatores on the planet but Mallory is also a good regulator.

sundeezy

I agree with Ghost, I had a similar problem with my 85 GS. I would recommend pulling the distributor gear out and moving it around in either direction and trying to time the car by 'feel'. It will take several tries to get it close, but once the timing is close you can use the adjustment slot to fine tune it. However, I would recommend if you decide to go this route that when you try starting the car, if it doesn't fire up right away do not keep cranking it as it will put a huge strain on your starter, just pull the distributor out again, adjust it, and try again.

DivinDriver

iTrader: (15)

Beg to differ; on a 12A, the leading mark is @ 0* TDC, not before or after. Trailing mark is 20* later.

13B's used 5* & 20* ATDC, but the OP is talking about a '79 w/a 12A.

85rotarypower

Well I know I have said this before, but I think I have found the problem. I replaced the distributor cap and rotor, no change. Swapped the ignition coils, no change. So I thought, why not just throw the Nikki back on to make sure the problem still exists with the Nikki. Sure enough, its still there, but not nearly as bad as with the Dellorto. So I decided to play around a bit with the fuel pressure to see what would happen. I had set it to 4.5 psi indicated, the same pressure that I measured off the stock pump, when I put the Nikki back on. As a test, I dropped the pressure to 2.5 psi indicated. Sure enough, this nearly eliminated the problem.

So, it looks like this problem is fuel related after all. My guess is the float levels on the Nikki were changing a bit over time which would alleviate the problem or make it worse depending on the level. Chances are they are at the high side right now which might be why reducing the pressure helped since the fuel bowls didn't fill as much with less pressure pushing the needles open.

Onto the Dellorto, I will have to rebuild it completely I guess to make sure everything is clean and good. I had another look at the needle and seat on it and I suspect it is too small since it has 1.5 stamped on it, suggesting it may be a 150 needle and seat which is too small so I could have been draining the fuel bowl too far even under half throttle. I have read the 225 will work alright since the 300 RB used is no longer available but the 350 is too big.

JDuncan

iTrader: (3)

Glad you are making headway.

Jeff20B

iTrader: (13)

The Nikki wants 2.5psi.

13x

interesting as I have just started hitting a 5k wall with one of my cars and we may have similar fuel reg ... will go there first .... and if it works this is why places like these forums help each other brap brap