Please help, hesitation around 1500-2000 rpm
Please help, hesitation around 1500-2000 rpm
I did my searches and most of what I came up with was for higher rpm hesitations so heres my issue. First off
-1983 Mazda Rx7
-Stock for the most part, MSD blaster 2 coils, no cat.
-Just tried to do the water pump but was wrong one so put old one back on, just
did the cap and rotor to.
-Did not hesitate before I did the cap and rotor, put old cap and rotor back on
and still hesitates. I can make it hesitate when at idle in driveway, not moving, by slowly giving it gas and raising the rpm to 2000, I can hold it there but it starts to slowly fall, when I try to give it a little gas it drops but doesnt die it picks back up and revs like normal. If im moving/under load it will die if I dont work it smooth enough, around 1500-2000. The coils have been on for 2 days with no issues, problem just started happening today after I did the cap and rotor and still does it after I put old cap and rotor back on.
hope someone c an point me in the right direction, went thru and checked my vac lines and all looks good, from the few posts I did find with a low rpm hesitation kinda point to the accelerator pump, does that sound like a possibility?
-1983 Mazda Rx7
-Stock for the most part, MSD blaster 2 coils, no cat.
-Just tried to do the water pump but was wrong one so put old one back on, just
did the cap and rotor to.
-Did not hesitate before I did the cap and rotor, put old cap and rotor back on
and still hesitates. I can make it hesitate when at idle in driveway, not moving, by slowly giving it gas and raising the rpm to 2000, I can hold it there but it starts to slowly fall, when I try to give it a little gas it drops but doesnt die it picks back up and revs like normal. If im moving/under load it will die if I dont work it smooth enough, around 1500-2000. The coils have been on for 2 days with no issues, problem just started happening today after I did the cap and rotor and still does it after I put old cap and rotor back on.
hope someone c an point me in the right direction, went thru and checked my vac lines and all looks good, from the few posts I did find with a low rpm hesitation kinda point to the accelerator pump, does that sound like a possibility?
edit- what about a choke in need of attention, I do kinda have to fiddle with it to idle smooth in choke, but it does retract and hold itself out.
Last edited by aragami78; Dec 26, 2010 at 06:56 PM.
Joined: Jun 2008
Posts: 8,376
Likes: 30
From: Chino Hills, CA
If it happens after full warm-up with the choke fully open, it's not choke-related.
If it happens when accelerating slowly, it's not the accel pump. Accel pump really comes into critical play mainly for fast throttle openings, to get past vacuum lag.
Watch your float bowl levels as you accelerate the engine; if they stay steady, your pump and filter are okay. If not (if the drop noticeably) you've likely got a plugged-up filter or maybe a weak pump.
There's a specific circuit in the carb that covers the range between idle and a couple thousand RPMs, called the transfer circuit. Consists of a specific (small) vertical port in the primary throats, and associated jets and bleeds. Gumming or plugging there can cause your symptom.
Try using some carb cleaner in the throat around the throttle plates, and some Seafoam through the gas tank. may help.
A small vacuum leak can cause this, too, since the carb is most highly sensitive to mix at idle and at transition; We can adjust for idle mix, but not transition, so sometimes a small leak gets tuned out of the idle equation but shows up in the transition phase.
Your description of it slowly failing as you hold 2000 rpm leans me toward fuel filter or other inlet restriction, which should show up in falling bowl levels.
If it happens when accelerating slowly, it's not the accel pump. Accel pump really comes into critical play mainly for fast throttle openings, to get past vacuum lag.
Watch your float bowl levels as you accelerate the engine; if they stay steady, your pump and filter are okay. If not (if the drop noticeably) you've likely got a plugged-up filter or maybe a weak pump.
There's a specific circuit in the carb that covers the range between idle and a couple thousand RPMs, called the transfer circuit. Consists of a specific (small) vertical port in the primary throats, and associated jets and bleeds. Gumming or plugging there can cause your symptom.
Try using some carb cleaner in the throat around the throttle plates, and some Seafoam through the gas tank. may help.
A small vacuum leak can cause this, too, since the carb is most highly sensitive to mix at idle and at transition; We can adjust for idle mix, but not transition, so sometimes a small leak gets tuned out of the idle equation but shows up in the transition phase.
Your description of it slowly failing as you hold 2000 rpm leans me toward fuel filter or other inlet restriction, which should show up in falling bowl levels.
If it happens after full warm-up with the choke fully open, it's not choke-related.
If it happens when accelerating slowly, it's not the accel pump. Accel pump really comes into critical play mainly for fast throttle openings, to get past vacuum lag.
Watch your float bowl levels as you accelerate the engine; if they stay steady, your pump and filter are okay. If not (if the drop noticeably) you've likely got a plugged-up filter or maybe a weak pump.
There's a specific circuit in the carb that covers the range between idle and a couple thousand RPMs, called the transfer circuit. Consists of a specific (small) vertical port in the primary throats, and associated jets and bleeds. Gumming or plugging there can cause your symptom.
Try using some carb cleaner in the throat around the throttle plates, and some Seafoam through the gas tank. may help.
A small vacuum leak can cause this, too, since the carb is most highly sensitive to mix at idle and at transition; We can adjust for idle mix, but not transition, so sometimes a small leak gets tuned out of the idle equation but shows up in the transition phase.
Your description of it slowly failing as you hold 2000 rpm leans me toward fuel filter or other inlet restriction, which should show up in falling bowl levels.
If it happens when accelerating slowly, it's not the accel pump. Accel pump really comes into critical play mainly for fast throttle openings, to get past vacuum lag.
Watch your float bowl levels as you accelerate the engine; if they stay steady, your pump and filter are okay. If not (if the drop noticeably) you've likely got a plugged-up filter or maybe a weak pump.
There's a specific circuit in the carb that covers the range between idle and a couple thousand RPMs, called the transfer circuit. Consists of a specific (small) vertical port in the primary throats, and associated jets and bleeds. Gumming or plugging there can cause your symptom.
Try using some carb cleaner in the throat around the throttle plates, and some Seafoam through the gas tank. may help.
A small vacuum leak can cause this, too, since the carb is most highly sensitive to mix at idle and at transition; We can adjust for idle mix, but not transition, so sometimes a small leak gets tuned out of the idle equation but shows up in the transition phase.
Your description of it slowly failing as you hold 2000 rpm leans me toward fuel filter or other inlet restriction, which should show up in falling bowl levels.
edit- and yes this happens at operating temp and while warming up in driveway, but not real bad at idle, and not bad when im not under load, its kinda easy to get around it as long as I dump the clutch at around 2000 rpm. If im under load when I hit 1500-2000 rpm it will stumble bad and sometimes die.
Last edited by aragami78; Dec 27, 2010 at 03:35 PM.
Well thank you DivinDriver, looks like I owe you a couple drinks next time im down in that area, Sprayed the hell out of the carb with carb cleaner, splashed some seafoam on it and let it sit for 10 min and threw what was lefft of seafoam in gas tank. Started it up and no stumble at 2000 cold or warm yay, still some issues to deal with like droping my tank and giving it a good cleaning and rebuilding the carb and new fuel pump. But for now my car wont bog in middle of intersection so im happy. Thanks again DivinDriver.
Just to close up this thread seeing as I fixed the issue, It appears my transfer circuit in the carb was blocked or clogged. Spraying a good amount of carb/choke cleaner and sprinkling some seafoam in the carb and letting it sit and soak, and pouring rest of seafoam in the gas tank did the trick and cleared up the blockage. Rx7 ran great on the way home today, no stumbles, backfires or bogging and accelerated really smooth all the way to 6500 rpm with alot of pull, felt freaking great. Thanks a ton DivinDriver I think you nailed it with the transfer circuit, I owe ya one 
And thanks to you to 74RX4 never hurts to go back over the obvious points that go overlooked because you assume its right. Again cars running great thanks guys, later.
Randy
And thanks to you to 74RX4 never hurts to go back over the obvious points that go overlooked because you assume its right. Again cars running great thanks guys, later.Randy
Trending Topics
Joined: Oct 2005
Posts: 1,329
Likes: 8
From: SF Bay Area, California
I have a 1982 all stock engine wise.
Symptoms:
I have been experiencing this similar issue as well. It does not occur during warm up with choke on but when the engine is warmed up and the choke is off. Idling seems to not be an issue.
When I rev the engine above 2500 rpm it will perform as it should. I have tested this so far by revving 3000-6500 rpm variously for a minute. Although, there is only a slight hesitation when I let go completely off the throttle and the engine starts coming back to idle and then I press the throttle again.
When I rev the engine around 1000-2500 rpm I can feel the hesitations so I let off or feather the throttle until the engine starts to rev back up again.
The engine can come close to cutting out or it just dies completely. When it dies, I will have to wait a minute or so before the engine will turn over (probably from flooding).
What I have tried so far:
I have tried spraying copious amounts of carb cleaner down the carb. Revving seemed to be a tad bit smoother. I also added a small amount of Seafoam along with the carb cleaner which blew out the familiar smoke when I started the engine.
Sadly, I this did not work for me unlike the OP, hence why I am making this post.
Also I can confirm it is not a fuel issue because I see the fuel bowl window sights are at the usual level as well as the fuel filter being full of golden colored gas. I also revved the engine above 3000 rpm for a minute as mentioned earlier without issues (except a slight delay when completely letting off the gas pedal then back on... more on this below), and this also confirms it is not an ignition issue.
I have also looked at the anti-afterburn hose and did not notice any cracks.
My thoughts:
I believe this minor hesitation is due to the engine using the idle circuit when it's coming back to idle. I believe the bad hesitation occurring at 1000-2500 rpm is due to the idle circuit being used in this lower rpm range (correct me if I am wrong please), thus leading me to believe it is a small vacuum leak.
Or it can be a carb issue. Accelerator pump? Anti afterburn valve? Shutter valve? Rats nest? Hoses??
Video clip (click):
Video clip commentary:
At 4 and 7 seconds in, you notice the hesitation as the rpms drop. Then the engine picks back up and I rev above 3000 rpm where there is only a slight hesitation if you can see/hear around 20-25 seconds into the clip. Then at 35 seconds in, I try to rev around 1000-2500 rpm range and try to hold the rpm there. The engine tends to drop speed then pick up and fluctuate as you can see. Then I will rev it again and try to hold a constant engine speed and the symptoms happen again. At 1 minute in, the engine almost dies but then regains itself. Note: every time you see the rpms drop after this point, it is not because I let go of the gas pedal, it is because of the engine hesitating. Then I try to rev it from 2500rpm to a higher rpm thats out of the idle circuit and then you see the engine dies.
Symptoms:
I have been experiencing this similar issue as well. It does not occur during warm up with choke on but when the engine is warmed up and the choke is off. Idling seems to not be an issue.
When I rev the engine above 2500 rpm it will perform as it should. I have tested this so far by revving 3000-6500 rpm variously for a minute. Although, there is only a slight hesitation when I let go completely off the throttle and the engine starts coming back to idle and then I press the throttle again.
When I rev the engine around 1000-2500 rpm I can feel the hesitations so I let off or feather the throttle until the engine starts to rev back up again.
The engine can come close to cutting out or it just dies completely. When it dies, I will have to wait a minute or so before the engine will turn over (probably from flooding).
What I have tried so far:
I have tried spraying copious amounts of carb cleaner down the carb. Revving seemed to be a tad bit smoother. I also added a small amount of Seafoam along with the carb cleaner which blew out the familiar smoke when I started the engine.
Sadly, I this did not work for me unlike the OP, hence why I am making this post.Also I can confirm it is not a fuel issue because I see the fuel bowl window sights are at the usual level as well as the fuel filter being full of golden colored gas. I also revved the engine above 3000 rpm for a minute as mentioned earlier without issues (except a slight delay when completely letting off the gas pedal then back on... more on this below), and this also confirms it is not an ignition issue.
I have also looked at the anti-afterburn hose and did not notice any cracks.
My thoughts:
I believe this minor hesitation is due to the engine using the idle circuit when it's coming back to idle. I believe the bad hesitation occurring at 1000-2500 rpm is due to the idle circuit being used in this lower rpm range (correct me if I am wrong please), thus leading me to believe it is a small vacuum leak.
Or it can be a carb issue. Accelerator pump? Anti afterburn valve? Shutter valve? Rats nest? Hoses??
Video clip (click):
Video clip commentary:
At 4 and 7 seconds in, you notice the hesitation as the rpms drop. Then the engine picks back up and I rev above 3000 rpm where there is only a slight hesitation if you can see/hear around 20-25 seconds into the clip. Then at 35 seconds in, I try to rev around 1000-2500 rpm range and try to hold the rpm there. The engine tends to drop speed then pick up and fluctuate as you can see. Then I will rev it again and try to hold a constant engine speed and the symptoms happen again. At 1 minute in, the engine almost dies but then regains itself. Note: every time you see the rpms drop after this point, it is not because I let go of the gas pedal, it is because of the engine hesitating. Then I try to rev it from 2500rpm to a higher rpm thats out of the idle circuit and then you see the engine dies.
Joined: Oct 2005
Posts: 1,329
Likes: 8
From: SF Bay Area, California
I did some more diagnosing this past weekend. It's been a while and I've been feeling bad for neglecting my 7. 
I tried adjusting the carb idle speed and air/fuel mix per Sterling instructions you can find in the FAQs. This made no difference. Though, I do not believe that I have a vacuum leak anymore since it can idle at 500 RPMs pretty smoothly.
I removed the lower spark plugs right after it stalled one time and the cause is due to flooding from too much fuel being uncombustible.
I also tried swapping the ACV that is fairly new with another spare one I have. The symptoms still persisted so then I swapped it back.
Perhaps after reading through this thread again, I have the same problem with the "transition circuit" in my carb as the OP stated and I need to do a thorough cleaning. I do recall running out of gas shortly before this problem occurred.
BTW my stock 12A sounded and smelled awesome without the air cleaner assembly off and revving it.
I tried adjusting the carb idle speed and air/fuel mix per Sterling instructions you can find in the FAQs. This made no difference. Though, I do not believe that I have a vacuum leak anymore since it can idle at 500 RPMs pretty smoothly.
I removed the lower spark plugs right after it stalled one time and the cause is due to flooding from too much fuel being uncombustible.
I also tried swapping the ACV that is fairly new with another spare one I have. The symptoms still persisted so then I swapped it back.
Perhaps after reading through this thread again, I have the same problem with the "transition circuit" in my carb as the OP stated and I need to do a thorough cleaning. I do recall running out of gas shortly before this problem occurred.
BTW my stock 12A sounded and smelled awesome without the air cleaner assembly off and revving it.
Thread
Thread Starter
Forum
Replies
Last Post
LongDuck
1st Generation Specific (1979-1985)
12
Oct 7, 2015 08:12 PM
eplusz
General Rotary Tech Support
15
Oct 7, 2015 04:04 PM
