When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
In the process I've replaced the exhaust with a new complete Racing Beat Street Port system. I've rebuilt the stock Nikki, had the fuel tank cleaned by a local radiator shop, replaced the fuel pump, replaced all the soft fuel lines, and replaced all of the vacuum lines under the hood. Prior to doing this work the car would start and run, but it had a 'dead spot' from about 1300 RPM to 2000RPM where the engine would just try to die if you lingered anywhere in that range. After all this, it starts and seems to run fine (note - my definition of 'run' here is all just sitting in the shop, not actually moving) except the idle is a little high.
I'm trying to adjust the carb using the instructions from Sterling (as pointed to in the FAQ) and I am unable to adjust the idle below about 1100 RPM. If I adjust it any lower the engine will stutter and die. If you feather the gas pedal and give it a little gas you can keep it running, but with an unattended pedal it will die. With idle set at about 1100RPM it will sit there and idle fine "forever".
According to the instructions it suggests that if you can't get it to idle smoothly below 1000RPM that it's likely an electrical issue. So, I'm potentially looking for pointers as to what to look for there also... Early in the restore process I did replace the plugs and wires (ignition wires as well). I have not touched the distributor or the coils.
I also built a compression tester (following twisted rotor's open source design) and both rotors report really good numbers.
I'm basically unsure at this point whether I'm chasing an ignition issue (it doesn't *seem* like it) or a carb adjustment issue. When I rebuilt the carb I was extremely careful to set the Mixture screw back to exactly where it was when I took the carb off the car - so I'm a little leery to mess with that, but if I could get the car idling under 1000 to begin his instructions, I would go ahead and follow them.
Thanks in advance for any and all suggestions/thoughts...
Yeah, maybe I didn't describe it well... The issue isn't that I can't adjust the idle below 1100RPM, but that when I do the engine will die (as if it's not getting enough gas) unless I work the gas pedal...
So it doesn't want to run well (or for very long) at 1000RPM and below without additional input from the gas pedal. To me this seems like it just needs "more" on the idle adjustment screw (just holding the throttle open a tiny bit more) - but I can't seem to find a sweet-spot that is less than 1000, but not so low it stalls out...
This doesn't sound like a vacuum issue to me - I've seen in the past where having a vac line open caused it to idle between 2 and 3K RPM.
Sterling's FAQ (https://www.rx7club.com/1st-generati...e-carb-310927/) though does have this in it:
"If you are experiencing stumbling during idle no matter how many times you adjust it, and if you can hear it as you slowly rev the engine, then this is most likely an ignition problem."
That's why I asked about what the ignition problem might be -- note, I've not touched the timing in the last 30 years. Haven't verified it either, but I would be very surprised if that was the issue (as I mentioned, haven't touched the distributor, other than to replace the wires).
Got it, thanks for the clarification. I had a small vacuum leak that would let me idle at 1100, but not below that. If you're totally missing a hose or something, the yeah you'll idle at 2-3k rpm.
Regarding the ignition, a stumble to me means inconsistent idle. For example you could get it to idle at 700rpm, but every 1 or 2 seconds the idle dips to like 400 rpm. I also had this problem when a coil failed and was arcing. But i could still get my idle below 1000rpm when i had this ignition issue.
Regarding finding vacuum leaks, you may have already checked these, but the few that took me the longest time to diagnose were the following:
Vacuum line that runs to brake booster.
carb wasn't tightened enough on the phenolic spacer. Was able to eliminate a leak by tightening each nut by about 1/8 of a turn.
the infamous line that runs from the passenger side along the firewall right above the exhaust manifold/air pipes.
Hmmm.... So I went back out this afternoon and tried to find any possible vacuum leaks. I sprayed carb cleaner around the ends of all (or at least most) of the vac lines and around the baseplate (at least what I could get to) -- no discernable change in the way the engine sounded with any of it...
I did a bunch more testing and basically it's still what I mentioned earlier, adjusting the idle to below about 1100 RPM and the engine will die without some attention being given to the gas pedal. Above 1100RPM and it will idle basically forever. I did discover that placing load on the engine (either physical - letting out the clutch (out of gear), vacuum (stepping on the brakes), or electrical (opening the window, also stepping on the brakes lights the tail lights) will try to bog it down and lower the RPMs, then it'll die (unless you compensate with the gas pedal).
I pulled the soft line from the brake booster to hard line on the firewall and put another piece of hose on the hardline and clamped it off -- thinking that should remove the vaccum load from pressing the brakes (seemed like a good diea at the time?) Anyway - pressing the brakes still results in the engine bogging down - so I suspect it's more the electrical load than the vacuum load...
Just throwing out what I tried in case it prompts any suggestions from those in the know...
Towards the end of my session, I did notice this:
It was difficult to get a good pic of, but it looks like I'm getting fluid (gas?) leaking at the main body gasket (between the main body and the throttle body).
That is right under the OMP lines, but I saw no evidence that I was getting leaking from those and it really seemed like the fluid was at the gasket line and below, not coming from above it.
I used the new gasket from the rebuild kit when I rebuilt the carb - but I just went back and checked, and there is no torque spec for those bolts (that I found) in the carb manual. Maybe I didn't crank them down hard enough?
I hate the thought of having to pull the carb again, but unless anyone points out something more obvious that I'm missing, I think I'm going to have to do just that... sigh...
I used the new gasket from the rebuild kit when I rebuilt the carb - but I just went back and checked, and there is no torque spec for those bolts (that I found) in the carb manual. Maybe I didn't crank them down hard enough?
I hate the thought of having to pull the carb again, but unless anyone points out something more obvious that I'm missing, I think I'm going to have to do just that... sigh...
Uh oh, you mean the paper gasket that goes between the phenolic and the carb? I thought we were supposed to chuck that one in the bin! My gut still says vacuum leak, but I'll let the real experts chime in since I am a newb to all of this as well.
Well, progress has been made... I went to the shop today with the intent of pulling the carb to tighten the bolts holding the main body to the throttle body. Once I had the air cleaner off I noticed I had somewhat reasonable access to the offending bolts (well, three of them at least). So I went ahead and tried to tighten them - the one in the pic above did tighten up a tiny bit (maybe an 1/8 of a turn) the others that I could get to (2) would not tighten any further. Hoping this may have taken care of the issue I decided to warm it up and see how it ran. Instead of putting the air cleaner back on I plugged the two large lines into the air cleaner, and the one small vac line to the rat's nest. The car warmed up fine, but stayed at about 1500 RPM idle. I was able to bring the idle down with the idle adjust screw (it had to come out quite a bit). In fact I got it down to 1K RPM without it stalling out - so I went ahead and did the mixture tuning procedure. I was able to get the car running smoothly at 750RPM. Thinking tightening that bolt must have fixed the problem I put the air cleaner back on. After that it was back to its old tricks - dying at idle...
Interesting, so I pulled the air cleaner again, plugged the 3 hoses and verified the car idled fine again. Pulling the plug from the large hose on the passenger side of the air cleaner had no ill effects. Pulling the small hose had no (or negligible effect). Pulling the large plug on the driver side caused the car to immediately die - and tons of air was pulled into the hose...
Well, well, looks like I may have a smoking gun... That large hose goes to the shutter valve. Once I searched the forum for shutter valve issues, I found several examples that match my symptoms exactly - really poor idle under about 1500 RPM. Not sure yet whether I've got a bad valve, a bad valve control on the rats nest, misrouted vac lines on the rats nest, possibly a large vac line on the firewall side of the carb that isn't connected (not sure what this is, but saw a mention of it the forum causing shutter valve issues).
At least I've got a solid lead on what to look for at this point. And on the plus side, the car ran well with air cleaner off and all the lines plugged
One other possibility since you rebuilt the carb is that the gasket between the throttle body and main body might be in backwards. I have experienced this twice. I don't remember if it resulted in poor low idle, since it was in a race car that was set for a higher idle anyway, but it probably did. Also with it backwards, the vacuum signal to open the secondary throttle valves will be blocked, resulting in the secondaries never opening, leaving you with only 60% of the power you are supposed to have. The gasket you refer to above is the one I am talking about. The attached picture shows the correct and incorrect orientation of the gasket. You should be able to tell whether or not it is in backward without pulling the main body off by looking for the gasket peeking out between the joint. Correct orientation. Note the location of the vacuum secondary actuator Incorrect orientation. Note open vacuum channel.
Thanks for the pointer on the gasket -- I will try to verify the gasket is in there correctly when I get back into the shop. I know I spent a lot of time trying to ensure that everything went back together on the carb the way it was supposed to, but I wouldn't put it past me making that mistake!
That will be the plan if I find it's actually the valve that's bad. But I'm going to go through all the vac lines again to make sure I didn't route something incorrectly when I replaced them... Then troubleshoot to see if I can verify the valve is toast before just bypassing it.
Didn't get into the shop today, hopefully tomorrow.
I verified (as well as I could with the carb still on the car) that it doesn't look like I flipped the gasket Carl mentioned above.
Then I started verifying the vacuum lines related to the shutter valve. Everything looked correct there as well.
Then I noticed that one of the clamps (that I put on all the vac lines I replaced) on one of the lines going into the baseplate of the carb looked like it was rubbing against the top of the little block that connects the coasting valve actuator arm to the shutter valve. I did some more inspection and poking and while maybe it was close it didn't seem to actually be dragging on the little block. Then I realized that an ear from the clamp on the vac port just forward of that one was actually pressing against the end of that same block - It was, in effect, holding the coasting valve (and shutter valve) about half open.
These aren't the offending clamps, but just an example of the kind of clamp I'm talking about:
A few seconds with a long aluminum rod with a slot in the end and I was able to turn the clamps about 90 degrees, so their ears weren't interfering with the operating rod anymore. And viola, the shutter valve is now working as expected - yay!
Fixing this does seem to have had an effect on my idle (it was actually now lower than I set it a few days ago), so I plan to readjust it and the mixture screw. But for now I just turned the idle up a bit and then took the car for a run down the driveway and back (about a half mile each way) - the car ran reasonably. It still feels like it's kinda stiff and sleepy, but I'll try to get it out on the road later today to see how it runs once the cobwebs are shaken off it.
Thank you guys for the input -- just having someone to talk to and bounce ideas off of is a big help.
Just to close out the previous line of discovery - the shutter valve control arm on the coaster valve getting hung up on the vacuum line clamps was the majority of the previous issue. After correcting that, the car seemed to run ok but I realized I was missing fast idle with the choke. It turns out that I had let the hot-start arm on the carb throttle assy "flip-over" so the fast idle arm couldn't do it's thing. While trying to determine that though I also realized that I didn't reassemble the sub-return spring correctly when I rebuilt the carb. I tried to correct that on the car and I ended up bending some of the parts on the throttle arm assembly -- fun. Anyway, I pulled the carb off the car again and managed to fix the parts I bent, realize the fast-idle issue (correct that) and while I had the carb off the car I split the throttle body from the main body - just to 100% verify I had the gasket Carl pointed out above installed correctly. Good thing I did that, because it was exactly as he described, in upside down. So that is now fixed as well. So at the end of all that I have, what I believe is, a solid understanding of the throttle arm assembly and a carb that when back on the car runs well except for one nagging issue...
The car is now exhibiting a significant stutter/stumble as it approaches 2000 RPM - unless you pass through it with some "intent" (give it enough gas to push through). If you just slowly raise the RPMs it will do fine until it hits 2000, then it will start to stumble and if you just hold there the RPMs will drop and the car will stall. This happens both in the shop (out of gear) and on the road (in gear). I've hooked a vacuum meter up and it's definitely experiencing a vacuum drop as it goes through this stumble.
When it stumbles the shutter valve does engage, but it seems to be as a result of the stumble, not causing the stumble. I wanted to verify this so I created a test rig so I could see the activity on the solenoid control lines. It goes between the control plug and the solenoid and the lamp turns on when the solenoid is activated:
I did tests with the lamp on each solenoid in turn - looking for a solid relationship between the solenoids engaging and the problem. The shutter control solenoid was (as I suspected) firing after the stumble caused the RPMs to drop - I think its engaging then cause the RPMs to drop even more and is what leads to the engine dying, but I'm pretty confident it's not the initial cause of the stumble.
The grey solenoid (switching) looks to be tightly correlated to the stumble. On the way up to 2K RPM the solenoid fires once very briefly and there is a corresponding pulse of loss of vacuum. As 2000 RPMs are reached the solenoid engages again and the vacuum drops (from about 500mmHg to ~400mmHg) and the stumble begins. It looks so much like an obvious cause of the stumble -- however, if I disable the grey solenoid completely (unplug it) the car behaves exactly the same way. So it can't be the cause, but more likely a result of the stumble... I have verified the vac line fed by this solenoid when activated does hold a vacuum. BTW the Air Control Valve (which this solenoid controls) has been removed (along with the air pump) - it's covered over with a 1/4" aluminum blocking plate with 2 layers of gasket paper between the plate and the manifold.
At the onset of the stumble I can hear what sounds like a vacuum hiss begin somewhere near the top of the carb - but I can't zero in on exactly where it's coming from (it could even be from down in the the horns of the carb).
I am at the end of what I can think to test/try for this problem -- any suggestions would be greatly appreciated.
Ok - one more bit of info... After getting the carb back on the car I noticed that the coasting/shutter valve would sometimes not disengage when the car got back down to idle (after revving). Since I had done quite a bit of fiddling with the throttle arm assembly I figured there was a possibility the throttle sensor needing adjusting. So I ran through the procedure in the FSM to test the throttle sensor (the 2 lamp test jig plugged into the "Connector of Check" (as it's called in the wiring diagram)) and sure enough the two lamps did not turn on at the same time -- so I did adjust the throttle sensor to get the two lamps the turn on the same time (as described in the FSM procedure). This did correct the issue with the coasting/shutter valve not disengaging - so I believe it was done correctly and needed to be done. I mention it because it is something I futzed with that could have some bearing on the current issue.
One more thing I thought of - all of the above is with the choke off. If the choke lever is pulled (even just a little bit) then the stumbling (and the grey solenoid triggering) does not occur...
I've spent the last week trying to diagnose what could be the issue with the carb on the car. I've replaced the gasket on the Air Valve cover plate (no air valve because air pump removed and it's got headers) - no affect. I've tested the throttle sensor - the resistance changes smoothly over the entire range of motion (from center tap to either side of the pot). I've pulled the vac lines from both the Relief and Switching solenoids, plugged the vac lines, and unplugged the solenoids from the ECU - no change. The Switching solenoid firing at the same time it begins to stumble felt too tightly coupled to ignore, but even with everything associated with it disconnected the stumble persists..
So anyway, I've decided I have to go ahead and pull the carb again and go through it one more time - maybe something will jump out that I reassembled wrong?
My reading of the carb manual leads me to suspect it's something wonky with the low speed (transfer) circuit which is supposed to help the transition from idle to the mains operating. I can confirm that during the stumble very little fuel appears to be being pulled from the main nozzles and after pushing through the stumble (above 2.5K RPM) that there is definite flow from the mains. So everything seems to point to an issue with the slow jet or air bleed. Anyway, hopefully something jumps out when I dig into it...
Ok - I broke down and bought an ultrasonic cleaner. I cleaned every part of the carb in it -- after cleaning sprayed the parts with brake cleaner (avoiding vacuum diaphragms) then blew off with compressed air. All air passages were blown out and verified they pass air. Carb was reassembled:
I found no gaskets incorrectly installed. All jets and air bleeds appeared to be open prior to the additional cleaning.
I just put the carb back on the car, hooked everything back up as original (except the air cleaner is off, and the one vac line going to the Idle Compensator (in the air cleaner) is plugged).
The car exhibits the exact same symptoms as before - a severe stumble at 2000 RPM when giving it gentle throttle.
Here's a video trying to capture the stumble:
Anyway - more good news to report. I think I've tracked down the cause of the stumble. I'm pretty certain at this point that it was being caused by the #2 Anti-Afterburn Valve (or the line from that to the intake manifold). I had found a couple of ancient posts with similar symptoms and their problem was a leak in the large vac line that goes from the back of the intake manifold to that valve - apparently that hose gets hot and can develop a tear. That doesn't seem to be my issue (the line seems fine) - but when I remove the valve completely, plug the two small vac lines going to it, and plug the nipple on the intake manifold the car no longer exhibits the stumble. yay!
I haven't found any good description of exactly what is going on inside the valve, but neither of the small nipples will hold a vacuum. I suspect this is expected for the rearward facing nipple (it seems to be ported directly to the large rearward facing nipple, but the forward facing small nipple can be pumped up (with a vac) but it just bleeds away almost immediately. This doesn't seem normal for most of the vac valves I've tested, but perhaps that's normal for this one, I don't know. The important thing is that with the valve removed the stumble (and associated vac loss) is gone...
Since I don't know whether the original valve is really broken, and just the replacement hose from the manifold to the valve is $40, and I don't even find a source for "new" valves -- I'm probably just going to leave it off. I had hoped to keep the carb as stock as possible, but if I go without this valve, I might just lose the shutter valve as well. And now I'm considering whether I should just go with a full stripped nikki to keep everything as simple as possible...
I decided to give the valve and it's associated hoses a nice warm bath in the ultrasonic cleaner. After cleaning the large hoses seemed to have been quite rejuvenated - not just clean, but the rubber was more flexible and the ends tightened up quite a bit. I verified the problem was still gone, then put the valve (and hoses) back on the car and the problem was still gone. So it seems it was probably the hose from the manifold to the valve all along. Since my goal has been to keep the car as relatively stock as possible I've decided to just leave the cleaned valve on and call the carb complete at this point.
07-26-21, 05:27 PM
