Whisper

I got fed up with trying to troubleshoot low idle, and frankly I have nothing left to replace aside from the wiring harness, and so I forced the throttle body slightly open at idle with the throttle stop screw. So now it idles around 800 RPM, but it's still all lumpy and it misses and shakes like a **** in a repeating pattern. And it's not very smooth when I drive it, either. It smoothes out a bit between 3K and 4K RPM, I don't know if it gets better above that, because I don't go above 4K, since I'm only beginning the break-in.

So I know a rebuilt engine will have crap compression, and mine does - it's quite uneven between front and rear rotors and different rotor faces, but can I realistically expect the idle and roughness when driving to improve enough to make it run smooth? How soon? I think have about 70 miles on the motor now... Should I just keep driving it until I have at least 1000 miles before passing judgement on the whole thing?

19-r-e-d-79

iTrader: (1)

explain the rebuild. what did you use new? how were your housings? and irons? i just rebuilt my 12a Bridge port and used everything new except the housings and irons and it runs great. it was smooth right at first start. i am at about 1100 miles now and she runs awesome!!!

Jeezus

iTrader: (4)

1. Timing

2. Was it really rebuilt?

Whisper

It was a Pineapple rebuild. New housings. Not sure about irons, but I imagine they were in fair shape, if used. Rob said compression should even out with break-in as the seals seat to the housings, but I've no idea if it should be running this rough because of slightly uneven compression.

Yes, timing is spot on.

It has bits of idling where it idles quite smoothly for a second, and then it'll jerk again. I can feel it with my hand on the shifter, or on the engine. It's like THUMP THUMP THUMP smooth THUMP smooth THUMP THUMP THUMP THUMP smooth THUMP, pretty rhythmic miss overall, with occasional smooth bits. Almost as if the engine makes a few turns, then misses, then makes a few turns, then misses.

Would it make sense to get a proper compression test at this point, like with a Mazda tester? Or should I drive it some more and see if it gets better?

19-r-e-d-79

iTrader: (1)

if it has new housings and apex seals there will be no break in period for the compression to even out as the apex seals are already even matched with the housings. you will still have to do the break in proses on the motor but it will not affect the compression.

it sounds like a vacuum issue >

was it ported or left stock??

DarrenTRS

iTrader: (1)

sounds like a vacuum leak to me, I had an extremely small leak between my Dellorto and the upper Intake manifold and it had me stumped for almost a month and I had the same kind of symptom with the car sounding like it was ported but idling low/normal, light throttle caused a heavy missfire and stumble and overall drove like crap.

t_g_farrell

iTrader: (1)

We don't know enough about your cars setup to really help.

I could be vacuum leak, timing set too aggressively, ignition issue.

So what mods exist? 12a or 13B? Carb or injection because you say
throttle body but it all sounds like a carbed system to me. Maybe you
meant carb butterfly.

DivinDriver

iTrader: (15)

I can only offer this:

I've rebuilt three times for my SA; two times with original housings & plates (lapped) & once with new housings and refurb'ed plates as the originals were finally worn past usefulness. New hard and soft seals each time.

I've never had issues with neither smoothness nor compression after a rebuild.

That said, this sounds a little familiar:
I've had a similar-sounding issue with my current setup for a while, and have never solved it. In my case, though, is it slight, it is most noticeable during warm-up, it goes away when off idle, and it doesn't seem to impact performance. Didn't even cause issues with passing smog.

My current working hypothesis is that it is ignition based, as I can sometimes see it as jitter in the timing light.

Here's a video I made some time back, trying to explain to others here what my symptom sounded and felt like:



Is yours similar, or worse, or...?

thunkrd

iTrader: (1)

rx7s don't like idling, they are crying for more rpms

Whisper

It's 13B fuel injected, with mild street porting, and no, it's not a vacuum leak.

So the general consensus is that it should be smooth from the get-go. Alright, I see. Thanks.

t_g_farrell

iTrader: (1)

You need a new ignition setup and a an FB dizzy and it will be all fixed up for you
Mine sounded like that for a long time before the ignition started crapping out
when it got hot. I did a new ignition (see the TLIDFIS link in my sig) and now
it runs fine, not missing at idle anymore. The SA dizzy plus the distance to the
ignitors spells M-I-S-F-I-R-E when anything starts to go.

So back to the OP issue, tell us your mods and what not.

Whisper

It's almost the same, actually, except it never smoothes out as much as yours does. I'm also thinking ignition or possibly bad wiring/ground somewhere. I just wanted to know if it's a problem due to the engine not being broken in fully, but sounds like it's not. Gonna get a new dizzy and swap in a spare harness. Nothing else left to replace.

j9fd3s

iTrader: (3)

this is not right. i have had one like this, but it cleared up in 15-20 minutes, and ran fine for the next decade.

every other engine, especially the new housing ones, start right up and run great, usually despite something else in the EFI/ignition being wrong/unhappy. (the PP fired right up and idled fine with timing a tooth off and the wrong jets in the carb!)

so run it in the happy spot (3-4k you say?) and if it doesn't get better quickly then something it wrong

Kentetsu

Make sure the engine has good grounding, otherwise spark has noplace to go.

Have you already replaced the spark plug wires? If not, try that. If you have already done that, then check the routing to make sure that they don't get any "cross talk".




.

Whisper

Yeah, I'm gonna get a proper compression test done soon, to be sure.

dj55b

iTrader: (2)

How did you check for vacuum? Can you list everything you replaced? Were they also new or old parts?

Whisper

Like I said, dizzy and wiring are the only things I've not replaced, everything else is either new/rebuilt parts or known good used parts that tested within spec. All new vacuum lines, cleaned tested vacuum rack, all new manifold gaskets, injector seals, etc; checked with carb cleaner, propane, and smoke.

So it's either ignition/wiring or the engine itself.

DivinDriver

iTrader: (15)

The lack of definite rhythm is what has made me think ignition; much easier to make a case for intermittent electrical than intermittent mechanical. 80's like mine are very prone to crossfire, too, because of the remoted ignitors.

When you say "crap compression," how crappy is it? Got numbers?

gsl-se addict

iTrader: (3)

Is this a GSL-SE or something else? Stock ECU?

1. Check the vac switch (buried under the firewall corner on passenger side, near washer bottle and stuff). If left unpluged (electrically or vac line), the car can run rough. It is an easy one to forget as you don't really see the thing.

2. In that same area, check the vac lines around the blue/grey solenoids. I've seen lines pop off or a tee break and sometimes hard to see.

3. For vac leaks, be sure to check around all manifold gaskets, ACV, BAC, etc.

4. On the compression, did you use a piston tester to see the differences on the faces and front/back or are you just going by sound cranking the car with the plugs out?

Whisper

1. Vac switch was ok when I checked it a month ago or so.

2. No vac lines there anymore, because the solenoids are gone, as I got rid of the ACV. Now it's just a direct line from vac switch to the LIM.

3. Couldn't find any leaks at manifolds. Mating surfaces are cleaned and prepped, gaskets are new. Shouldn't really be leaking anything.

4. I used a piston compression tester, which I know isn't accurate. With valve depressed: front rotor 70, 70, 80; rear rotor 80, 80, 90. These are approximate, mind you. And with valve released max compression readings: front rotor ~110; rear rotor ~120. That was a while ago, though. Maybe it's different now.

Whisper

So I tried to explain earlier in the other thread, but it wasn't very clear, and a picture is worth a thousand words, and a video has thousands of pictures, so just watch.

I don't even know what's causing this. Both leading plugs are firing consistently, at least based on the in-line testers I hooked up, and both rotors are combusting, because I can make it run even rougher by pulling either one of the leading plug wires. Disconnecting the trailing doesn't seem to make any difference. Also if you watch carefully, it kind of bounces and misses even when revving, although when driving I can't feel it and the car makes pretty good power overall.

Is it possible my injectors are just randomly misfiring?

Also my compression looks like it's kind of uneven between rotor faces, and max is maybe 10psi lower than it was before...

http://www.youtube.com/watch?v=sGj_7HOLRJU

rxtasy3

iTrader: (2)

when using one of these testers, that valve core should be removed and note the highest reading for each rotor face. readings for each rotor face should be higher than that, even with just 70 miles on it. i'd be curious as to what the afr readings is.

Whisper

Well, piston testers are only so accurate for rotary even with valve removed, plus there's probably some margin of error in the tester itself. I don't know the afr, I don't have a wideband, but I imagine it's running a bit rich, but maybe not, I pulled the plugs and they look ok, kind of normal. If you watch the video, you can see where the gauge bounces to on each compression.

Hopefully I'll have time to take it to Mazda this week and get a compression test done correctly, because I just can't seem to get a good idea of what's going on.

DivinDriver

iTrader: (15)

The visuals do help gauge the degree of roughness - - your symptoms are clearly more severe that what I've been dealing with.

Looks like I've accidentally introduced a new measurement - the water bottle scale!

Whisper

Heh, yeah. Vibrations don't really show up that well on the video, and water works good for that.