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Just got my first rotary and super psyched about it. She's a gorgeous 1976 RX5. Anyway, been trying to make sure she's running in the best shape possible, but I don't know what's wrong, or if I'm reading my tools wrong, or if I just have no clue what I'm doing.
Firstly, it seemed like she was running rich and you could definitely smell it from the exhaust and it would often backfire as the engine slowed down anywhere form 3-5k RPM (driving and stationary). Also, I could never get the idle below 1-1.1k according to the tach. She has a Racing Beat Holley carb. Played around with the idle mixture and idle speed screws but couldn't get idle below 1-1.1k. Checked the timing and the leading was set between 5-10 BTDC with trailing set to match it. I also noticed that the RPM on the timing light reads about 400-500rpm above what reads on the tach, so I'm not sure which one is correct, because of the tach is right then that means my idle isn't going below 1400-1500RPM. I do have the # of cylinders set to 4 on the timing light as advised in other posts I read. So, I looked up to see if Racing Beat suggested any specific timing for their carb, and it says for the 1976 13B to use stock timing, which according to my manual is 5 ATDC leading and 25 ATDC trailing. However, when I changed the timing, I had to richen the mixture to keep it running. The lowest RPM I could get it to with the stock timing is 1700RPM on timing light (1200-1300RPM on tach), with the idle mixture screws both 2 turns out, and idle speed screw backed out all the way.
Current plugs are NGK BUR9EQ and BUR7EQ. All wires and spark plugs are good. I'm still waiting for my rotary compression tester, but I went through the formula with a standard tester (90psi compression, 90psi consistent puffs from both front and rear rotors, counted 228 cranking RPM, 3202' of elevation) and the normalized reading came out to 7.26 bar which should be within good compression.
I'm kind of confused on what to try next, as I don't even know which readings are correct, if either (since I can't even get below 1k on the lowest reading), and it seemed to be able to run at a lower idle and less rich on timing other than what was suggested by Racing Beat. I don't believe it's ported, as the person I had purchased it from also had another RX5 which he did mention to me was rebuilt and ported, but I can always double check to see if he has any history.
you said it's a RB Holley, so my next questions are: (1.) are you sure?, and (2.) are you sure it's the right one (i.e.- not one meant for a bridge port or something)?
next, you'll probably want to make sure there are no vacuum leaks.
what fuel pump and regulator are you using? also, what is your exhaust setup?
I did notice after I posted that the stock settings I had read on the timing chart (the one I was looking at was the one you provided) was for stock intake. I did adjust it to the 3*BTC and 13*ATC suggested but barely made a difference. It might be slightly advanced as the RPM's according to the timing light weren't going below 1700 when I spent some time on it today.
I'm pretty sure it's the RB as the intake and plate both look like the one provided by RB. I don't know how I'd be able to tell which application the carb is for though since that's all internal differences right? I have some pictures of it here.
As far as vacuum leaks go, I tested around the carb base and the intake but found nothing. The only vacuum ports I could find to the carb was the one running to the brake booster and the one ported vacuum port that I have plugged. Speaking of which, I don't even know how to test vacuum accurately as even though I'm able to get a vacuum reading on the brake booster line, it seems to hold the vacuum until it's released, meaning it'll hold the highest vacuum until I unplug the gauge. I also did check to make sure the accelerator pump was backed off enough to not engage on idle.
Fuel pump and regulator are both Mallory. It holds a steady 4.5-5psi of pressure while running. Exhaust looked to be 2.25" pipe and the muffler must be a generic one since all I got were the numbers "04y7 80035365" which neither corresponded to anything in a Google search.
Is there a way to check if everything in the ignition system is working/getting enough voltage? All the wires and and plugs test good as I mentioned, and it's getting spark on all ends, but I'm not sure how I would check to make sure that the spark is consistently hot enough and/or if it starts to fall off. I did notice the last time I changed the timing (to the 3*BTC and 13*ATC) that when I revved the engine and hit 2.5k-3k on the tach, it would start to miss and upon decelerating have a lot of backfire. When the timing was originally on the 5*-10*BTC on both leading and trailing as it was when I first got the car, this was not happening other than slight decel backfiring from 5-6k. But in this timing, I still couldn't get below 1.1k on the tach, and regardless of timing, any time I do get the engine to go below 1.1k it would start to shake violently as it got slower and eventually just shut itself off. This and the current midrange RPM misfires would make me think it might possibly be an ignition issue?
I'll try to take a video tomorrow with her current timing, try to get the idle as low as possible and compare the idle on timing light with the tach and so you can hear it as well.
RB holley for sure. RB modified the throttle cable connection point to be pulled from the bottom. and from my experience, that little flapper thing over the hole on top of the primary bowl denotes this is a 465cfm carb. later version tho. hex head screws for mounting the bowls instead of slotted ones of the earlier version. can't say what jetting it has as i've never looked to see.
Hi rxtasy3, thanks for clarifying that. Wasn't too worried that it wasn't an RB but good to know for sure.
I did get a video of it running. Even though I hadn't changed anything from the previous day since I only had time to get a video, it did run a lot worse. I had to richen it up a little and turn the idle in slightly to keep it running but it still didn't like it.
Ignition timing: Leading 3* BTC and Trailing 13*ATC per Racing Beat's recommendation on a stock 1976 13B w/ Holley carb
Spark Plugs: NGK BUR9EQ and BUR7EQ
Idle Mixture: 2 turns out both sides
Idle Speed: 1 turn in
I also found 2 other warm idle clips I had recorded back when it was the original settings with timing for both leading and trailing plugs the same at somewhere between 5*BTC and 10*BTC.
i wanted to confirm you had a genuine RB Holley because i wanted to be sure we were all on the same page. a lot of people i used to know that had Holleys on their Rx-7s either got generic Holleys or bought RB Holleys meant for big ports (thinking it would give them more power). either way, i now know that that's skating uphill.
i wish i had more concrete info for you, but the work that my brother and i did on his Holley (generic) was years ago, and i did not yet have the foresight to get a journal to document what we did. one thing i do remember is playing around with fuel pressure while trying to get it to idle right. his actually didn't like the range yours is in, it downright hated 4.5 psi. i think we ran something like 6 psi. another thing that may or may not be a big deal is your vacuum source for the distributor. keep that in mind. you may also want to invest in something to measure AFRs.
Appreciate it man, I'll take a look at those and see if adjusting the fuel pressure makes a difference. Though it'd be weird for low fuel pressure to allow it to run right. Hopefully I can figure it out. In regards to timing, how would you play with the split for leading and trailing safely, as I read it could be bad for your engine if you get it excessively wrong, and I feel I need to play around with that since it ran better with equal leading and trailing timing as opposed to the recommended timing.
03-15-22, 09:18 PM
