weber 48 dcoe car stumbling at 3000rpm
 

Hello fellow rotarians,

I just purchased a '85 GSL-SE converted to a side draft weber 48 dcoe. The engine is a 6 port 13b with about 80k miles on it. The car idles and drives fine. But when it reaches about 3000rpm, it stumbles and would not accelerate any more. The guy I bought it from said that he thinks it needs timing belt adjusted since he installed a Gilmer pulleys on the car and the new wide pulley is not marked. I am not an expert but I dought that this is the reason for stumbling, since I read some info here that stumbling is due to carburetor jets and tube sizes. I live in CO at 7500 ft and the air here is thin.

Could you please help me diagnose and fix the issue. Much appreciated.

rade m.

Is it a DCOE or a downdraft, can't be both. What intake is being used and whats being done with the 2 extra ports? Wired open? Too many unknowns to help much. Could be a lot of things.Some pics might make things clearer.

there are many things to check, but you should pull the jets out and see what is in there. particularly the emulsion tube, most carbs are sold with an F11 tube and that is just not applicable to a rotary engine

Sorry its a side draft 48 DCOE. I ll check the ports tomorrow and take some pictures and post here. Is it possible that timing being off could cause stumbling, or is carburetor the only thing to check here?

The story I was told is that the car ran fine and after upgrading the pulley with Gilmer kit that came from AUS, the car started to stumble. He then upgraded ignition to DLI DFIS but of course this didn't fix stumbling. The car starts and idles fine. Is the carburetor the only thing that could cause stumbling?

I don't understand how the Gilmer conversion could be responsible. Unless it is adjusted really really tight or something. Fuel pressure okay? Clean fuel filter?
I don't have experience with 6 port motors but does it have the correct 6 port intake manifold? Did he install the new ignition correctly?

Got any pictures? We love pictures.

So more info is good. I'll assume its the RacingBeat upper intake on the stock 6 port intake for the 13B. I still wonder whats being done with the extra ports but yeah we need pics of the setup and you need to make sure the fuel delivery is to spec for pressure and flow before even messing with the carb. The timing can be checked by looking thru the flywheel inspection hole and looking at the dizzy, at least you can determine TDC and mark that on the pulley and go from there. Oh and I just saw you live high up, so at some point you may need to tweak the carb setup.

Altitude is a good point...
Did the previous owner live up high near you? If he lived at a much lower elevation that might explain it and point you (and us) in the right direction.

Thank you for all the replies. Much appreciated. Attached are the pictures of the engine. The Gilmer belt is not too tight. Not sure about the fuel filter and when was it changed last time, but I ll replace it regardless, it wont hurt. The previous owner lives at sea level and I live at 7000 ft. He admitted stumbling was present after I asked him about after purchasing the car. Looking forward to working this out with your help. I REALLY like the setup of this '85 13B RX7 and want to get it working properly. Thank you so much guys.
https://cimg4.ibsrv.net/gimg/www.rx7...844176822f.jpg
https://cimg5.ibsrv.net/gimg/www.rx7...e96f857ce7.jpg
https://cimg6.ibsrv.net/gimg/www.rx7...9c0d7e93d8.jpg
https://cimg7.ibsrv.net/gimg/www.rx7...c701f780d8.jpg
https://cimg8.ibsrv.net/gimg/www.rx7...ce0e37556c.jpg
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besides the carb and ignition setup, i also see alt upgrade, either fc or fd?

7000' wow. Yeah, you absolutely need to rejet that carb for the thin air where you live. I used to live on the West side of Oregon and took some road trips to see friends in Colorado and Oklahoma. Edelbrock carbed big(ish) cam stroker sbc. Truck ran awesome in the Oregon hills but could barely idle going over the Rockies. I'd have to adjust my idle screws and change metering rods to keep it running strong.

There might be a separate issue that he was dealing with, but a 7000' elevation def requires carb adjustments. Buy some Weber books and google topics like "carb adjustment for high altitude". Same thing happens to any low-land old-school carbed engine.

The factory Nikki carb had circuits to richen the mix at high altitude but performance carbs like yours, or IDA's or Eddys and Holley do not. If your gonna drive an antique car with a carburetor, your gonna learn how to work on a carburetor.

I like the strut bar and firewall bracing. The guy made some nice mods.

Saw your pm.
Appears obvious your carb is way too lean,strongly suspect it wasn’t right at sea level either.
The statement DFI was added to help stumbling is a red flag. DFI done properly is a proper performance improvement Installing it on a car with a driveability problem in hopes of a”fix” never really happens and often the install is fraught with its own problems.
Don’t assume anything with your new to you car.
You will need to go thru each system to determine all is as it should be and 100%operational.
On an original 100% stock car this is known as baselining.
Essentailly what you can’t prove to be recent/new gets renewed.
Cap,rotor,wires,plugs,air filter,fuel filter.

You should check charging system voltage at battery.
You should check actual fuel pressure.
Pull carburetor and look at emulsion tube,slow and main jet selection. You won’t know what you need to correct afr til you know what you have. Accept the fact you’ll be into carb several times before you arrive at appropriate jetting for your altitude.
What spark plugs are in your engine?
Post closeup pics of DFI conversion?

Before removing carb,check and set ignition base timing,be sure centrifugal and vacuum advance work properly. They all all have an input on starting,driveability,fuel economy,power.

Is that a rack and pinion swap i spy as well?

The crossmember makes me think it's the RE-Speed kit.

Somebody put some work into this car. New owner needs to become a Weber master so he can tame those mountains!

Yes, it is a RE-Speed R&P kit. The car has tons of mods, too many to list, but after reading GSLSEforme reply, I am embarking on checking timing and TDC, regulate fuel pressure to 4.5 psi and carburetor re-jetting. After reading several threads on 48 dcoe tunning for rx7 13b 6 port and since I have no clue what jets and tubes are in the 48 dcoe at the moment, I will start with the baseline setting from member AirwalkerRX7 for a 6 port 13b, compare and go from there:

Weber 48 dcoe on a 6 port 13b:

42mm Choke
180 Main Jet
160 Air corrector Jet
65F9 IdleJet
55 Accelerator Pump Jet
300 needle/seat.
F7 Emulsion Tubes
0 Accelerator Pump Discharge Valve

Noticed, vacuum ports on both dizzies are left open. Does it matter? If it does, where should they go to? Reminder the issue is stumbling at 3000rpm and I live at 7000 ft.

thanks,

stumbling with a weber is usually caused by the transition between the idle circuit and the main circuit. usually the idle circuit is tapering off before the main circuit is starting. solution is to either get the mains started earlier or make the idle circuit work to a higher rpm. the change in altitude would make any existing problem worse.

step one is to pull your jet stacks out (its under the little cover with the wing nut) and see what you have. with an DCOE you have some tuning options that an IDA doesn't, which is nice.

primarily we're looking for the emulsion tube number

It's fine, you won't be able to use vacuum advance anyway because there's no port on the DCOE for it. It might run better with the dizzy advanced a little, but get it running well first.

Just to confirm, the engine stops revving at 3000rpm? Not just stumble and keep going at a slower pace? Only time the engine hit a wall to me was when I took out the main jets just to see where the idle transition happens.

https://cimg7.ibsrv.net/gimg/www.rx7...f165e7d9c0.jpg
There are vacuum take off ports on the DCOE, use these adaptors available from your carb shop. I highly recommend the Vac advance for the street.

Emulsion tubes are F11, idle jets are 65F9. Air bypass screws are closed in all the way. The previous owner said the car runs too rich, I pulled out and cleaned the plugs, they were charcoaled (see pic). I need to make it run leaner. Found this Low Speed Tuning Circuit guide and I am going to follow this first, unless someone here has a better idea.
https://cimg5.ibsrv.net/gimg/www.rx7...49b89d7698.jpg
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Replace plugs very few people can clean them acceptably. Put in BUR8EQ14 in leading and trailing. Buy a couple sets,you'll need them.A proper DFI should be able to fire that 2nd gen plug,yours appears to have some issues.
I suggested several things for you to check,have you done them? Post actual numbers. Likely you have both ignition and fuel issues. To be able to assist you in further direction need answers to questions in previous post. Your car has a lot of desirable mods,becoming a little more clear some are not working well together.

I checked there are 0.8 mm bore or 3 mm bore adapters. How do you tell which one fits 48 dcoe?

the idle jets are probably ok, although it is worth going through the process. generally its best to start with idle and work your way up. if you have a place with no traffic, put it back together without the main jet stack and go drive. you will only have power until about 25% throttle, so keep that in mind.

doing this the carb will be only running on the idle jet stack. the next test is to put the main stack back and put it in about 4th or 5th (an rpm lower than the stumble), and floor it. if you have everything right it'll go, if its wrong it will run on the accel pump and then either stop running or have a delay and eventually go.

these two tests tell you where the idle circuit works, and where the main circuit works, and once you know that its easy to make changes. the F11 e tube is out to lunch, but you could probably tune around it. the E tubes go by cylinder size, and the 13B wants an F7 or F8 (can't find the damn chart).

once its in the ball park you can try different air correctors, smaller = main coming on sooner. i've been meaning to 3rd print some blank ones, and you could just drill it out until the car ran right, measure it and order the one you need

oh i get my stuff from here https://www.piercemanifolds.com/default.asp they have a ton of stuff in stock, and they are local to me so i get next day service, which is cool.

Great stuff, let me get back to you with the results. Just ordered F7 e tubes, thanks.

You want the smaller ones for the Vac advance 0.8mm

OK here comes the report and its a good news. First this is what jets and e tubes I have in the car at the moment:

Main jet: .200
Air correction: .180
Idle jets: 65F9
e tubes: F11

I ordered a set of tubes and jets: .180 and .170 main, .170 and .160 air correction and F7 e tubes. I also checked the mixture screws and they were 1/2 turn out. I set them to 1 turn out because I know the car was running too rich.

Now driving test. After taking main jets out, the idle circuit goes to about 3500 rpm and would not go further in any gear. After I put back the main jets the car starting working better. I can get to 5000 rpm in 3rd and I could floor it in 4th and 5th and it would still accelerate. In other words the car drives more like it should with current jets and e tubes. In fact it probably never ran better. But I am sure it should run even better, so with the new jets and e tubes I will do another test and report back. Th AFG was varying between 10 and 16. I cannot explain why the car now drives so much better, pass 3000 rpm with no sweat. I will upload a driving video to youtube and post here. Thank you all but specially j9, you rock :)

Screwing out the idle mixture from 1/2turn to 1 turn would make the idle richer, not leaner.
It is important to know on which rpm ranges the AFR are showing 10 and when they are showing 16.

16 - slightly lean - on the idle circuit while cruising is OK, not on the main circuit on WOT.
10 is way too rich, regardless of where it happens.

sweet! i went through this a few years ago and found a really good paper by a guy called Keith Francks, its a really good step by step way to tune the carb, we start with the float level, and then idle and wor our way up, and at the end you know what does what, and the car runs great, i'll post it.

for AFR's we can shoot for about 12-13:1 at idle, whatever its happiest with. 1500-3500 cruise, can be 13.5-15:1, and then wide open we're looking for about 12.5-13.2:1

https://www.rx7club.com/1st-gen-gene.../#post12417710

my method comes from that paper...

Here is the driving video. I apologize for the wobble:
The AFR at idle was 11-12. It fluctuates during idle as well as driving. While driving it goes between 12-16. Its within the bulk park. It keeps on changing during driving. I am awaiting for my new set of jets and e tubes and once I put them in I will send an update here. thanks

So far done this:

Replaced oil, filter, plugs (BR8EQ) cap and rotor. Drove the car, noticed no improvement, but I am ok I did this for piece of mind.

2nd I replaced e tubes and jets on 48 DCOE:

Before After

Main jet: .200 .180
Air correction: .180 .160
Idle jets: 65F9 same
e tubes: F11 F7

3rd I reverted the mixture screws to 1/2 turn out from 1 turn out because I want the car to run leaner not richer.

Have not driven the car yet, coz the weather is crapy in CO right now, but I will drive it soon and post here.

Still to check:

1. TDC and distributor timing, adjust if needed. I do have timing strobe light, but I haven't used it yet. I know there are instructions, but if anyone has a HOW TO do this on a RX7 please post here. Also I need to mark the Gilmer pulley when rotors are in TDC. Not sure how to do this. If anyone has a procedure pls post here. Thanks

2. Measure RPM for AFR measurements, the best I can. This is not easy coz it fluctuates quite a bit during normal driving but I ll try.

I have few other new bits and pieces for the carburetor, but I am not replacing anything else until I see some improvement with new e tubes and jets.

back soon, cheers

I don't know if this person is on here but RAD potential on YouTube just posted a video on how to find TDC.

Hi thanks for posting the video. I watched it and its no brainer. One thing I am not sure about, he said find ANY apex seal through T2 plug hole on the rear rotor to begin with and mark the pulley. How can that be as there are three APEX seals on the rotor and they are 120 deg apart. How could it not matter which seal you start with? Could someone explain this please.

Here comes the driving update after re-jetting:

- AFR at idle is between 10.5-12.0
- AFR at cruising speed 65-70 mph is 10 and is solid.
- 1st 2nd 3rd 4th its about 10-10.5

Driving:

Re-jetting definitely helped with stumble. Now I can get in 1st 2nd 3rd pass 4000 RPM. Before it wouldnt go pass 3000 RPM. But it doesn't accelerate quite as much as I would like in 2nd, once its pass 3000 RPM, between say 3000 and 4000 RPM. It starts to accelerate again in 3rd 4th and 5th. I wonder how timing is set, thats the next thing I will try. Mark TDC on Gilmer pulley and check the distributor position, play with it a bit try retard it then advance it a bit and see the difference. I feel I am very close even though right now the car drives nicely just not quite at 100%.

with 1 rotation of the e-shaft, the rotor turns 1/3 of a rotation. for each apex seal that u align with the plug hole, the shaft is in the same position within that 360 degrees of rotation.

Still way too rich.
Buy a good set of jets, and go test them on the road (starting with getting idle afrs right).
Now that you saw it is quite easy to change jets in a Weber, just play with it.

How do I correct the AFR? Mixture screw? Timing? What else? Whats are correct AFRs?

the idle mixture is the idle mixture screw, everything else is done with the jets.

think of it like this, everything from the tip of the exhaust to the engine to the air filter dictates the air flow, and your job as a tuner is just to give the thing the right amount of fuel, and a spark

I understand the theory, I need instructions or links to instructions for AFR correction. Too lean do this, too rich do this and correct AFR readings. Could you guys with good tuned in webers post your AFR readings here.

ah ok,

at idle, just turn the screws until it idles the best, and then maybe 1/8 turn leaner. this should be about 12.2-12.5:1 AFR. ideally this is with the idle mixtrue screws out somewhere around 1/2 turn. if the idle mixture screws are more than a turn out its telling you you need the next size bigger jet (someday), and if they are in most of the way its telling you the idle jet is a bit rich.

from 1250rpm to about 3500rpm cruising, AFR can be 13.5-14.5 or even a bit leaner.

at high load/Wide Open throttle you're looking for 12.5-13.2:1 give or take. the factory is about 12.2 and it gets a little leaner (12.8) and then goes rich and by redline its in the 11's.

hope that helps!

Thats a good info, thank you. Also I saw your post #28 with AFR numbers. Now I know what AFR should be. The mixture is currently 1 turn out, I will put it to 1/2 turn out to make it leaner. More driving tests tonight and I ll do new AFR readings and post.

I do have a good set of jets and e tubes. I went one size smaller on main and air jets _and_ I went from F11 to F7 e tube. I may need to go back to bigger jets with F7. That could be what it needs. I ll know when I re-measure AFR.

That makes sense. I ll go ahead and check TDC and mark my Gilmer pulley and play with +/- distributer position. thank you

Could you pls give some links where to get those adapters from. Also where on carburetor to install them and how to connect them to the dizzies.

I get allot of carb parts from Eurocarb in the UK. takes a couple weeks for the parts to arrive , but always good quality and correct. There are two blank plugs just above the throttle plate, remove them. This is where they go,
https://cimg0.ibsrv.net/gimg/www.rx7...ca9345ae5e.jpg
then hook vac lines to dist advance for L and T. This is my Dell but should be similar to your Weber

New AFR numbers:


Main jet .180 (down from .200)

Air jet: .160 (down from .180)

e tube F7 (instead of F11)

Mixture screw 1/2 turn out.
AFR:

IDLE: 12.5-14.2

4th and 5th CRUISING @1500-2500 RPM 10.5-11.2

WOT: 11.2-12.8

Idle looks ok, but the rest still seems too rich. Would putting bigger main and air jets, say .200 and .180 back in, with F7 e tubes make it richer or leaner? I am guessing richer (more fuel for the same air). Also I do have one size smaller jets: main .170 and air .150. Should I try that combination?

Also I am finding the car would not rev much above 4000 RPM. Should it? Otherwise it drives well and it sounds like a race car :)

try the 170 jet and see what it does, it should lean out the WOT, make one change at a time.

As he said, one change at a time... or you will get lost.

OK, but the doc says that MAIN and AIR jets need to be changed together. So if I put .170 for main, I also need to put .150 for air. There has to be .020 difference between them according to the doc.