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
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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.
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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