Weber jetting question

[Laldog]

Before I go way in over my head and start tearing apart my carburetors I have a general question.

Does the 3.5 engine require a different sized jetting than the jetting for a 3.0 engine?

Short story long:
Not long after I bought my coupe, (15 or so years ago) I did an engine swap, replacing the stock 3.0 engine with an engine from a fuel injected 535 (year unknown at present.) The transplanted engine was fitted with an electric fuel pump, and the Weber carburetors from the original engine.

I've never been really satisfied with my carb adjustment, it's been running rich and I've experienced backfiring from from the exhaust, popping etc, etc, etc.

Does the 3.5 require different sized jetting, or is it all specific to each individual carburetor?
Any sage advice and related experiences would be greatly appreciated.

Thanks in advance.

 

[HB Chris]

Backfiring indicates timing is a bit too far advanced. Make mark with a marker on dizzy and head and turn it CW a little bit. You can always return to where it was.

 

[Stevehose]

Backfiring from the exhaust means it is rich. Have you adjusted the mix screws and synched the carbs? We are talking downdrafts, yes? Which model?

 

[JFENG]

Suggest you have an experienced Weber 32/36 specialist check out and adjust your carbs (after the ignition is throughly checked and given a thumbs up). These carbs are used widely on a variety of old cars. The 1st generation of formula Ford race cars use the 32/36 and vintage race shop that work on those will have the expertise and jets needed to setup and tune yiur carbs. Some will have dumps to really optimized the setup for you.

 

[Mike Goble]

Does the 3.5 engine require a different sized jetting than the jetting for a 3.0 engine?

Not really. Carburetors work off the air that is drawn through the venturi, more air through the venturi means more fuel through the jets. I had 32/36 Webers on my 3.0l and they worked just fine on my 3.5l. I now have a carburetor from a 5.7l motor on my 3.5l and it works just fine with the stock jetting.

 

[Dick Steinkamp]

In addition to the above...

Valves not completely sealing (adjusted too tight and/or worn) will also cause popping through the exhaust.

The carburetors not synchronized with each other will cause popping



Which Weber Carburetors do you have?

Jetting should be the same in both carburetors.

I'd start with a valve adjustment and compression check.

If the car still runs the same, I'd do an electrical tune up (plugs, points, condenser, rotor, cap) and set the dwell and timing by the book.

If the car still runs the same, I'd remove the carbs, disassemble and clean them (it sounds like it has been more than 15 years...maybe lots more?), note all the jet sizes but don't change any, reassemble the carbs with new kits.

Synchronize the carbs and set the choke, fast idle, hot idle, and idle mixture (idle mixture actually controls more than the mixture at idle). The process for doing this is on this site and more on the internet (below are links pertaining to the Weber 32/36 DGEV which are the most common Webers on M30's, but yours may be different)...

Google Search

Google Search

 

[bluecoupe30!]

in my experience, once valves are perfectly adjusted, ignition is set correctly, these 32/36s can be tuned quite easily. I say this now, but it has been a long learning experience. In my case, car does sit a lot and fuel gums up the carbs over time. I found the mixture screw on one was not even functioning. Once I cleaned it out, chased the thread, carb could actually be set according to the widely available instructions for doing so. Final turning of those mixture screws has to be done with engine "warm", which means hot, and it is very easy to burn your hands trying to make the minute and matching turns on each carb to achieve balance. But if everything is clean it is so much easier. Good luck.

 

[Ohmess]

I think Dick's suggestion of starting with a valve adjustment and electrical tune up is a good one. These are not expensive and you won't get your carburetors set correctly if these are wrong.

That said, it seems like these carburetors have been in use, without a rebuild, for more than 15 years. As I posted here - https://e9coupe.com/forum/threads/korman-triple-carb-conversion.43268/post-380180 - carburetors require periodic maintenance. It is likely that your carburetors should be rebuilt even if the more basic steps improve your car's performance.

 

[JFENG]

backfiring from from the exhaust, popping etc, etc, etc.

Did the original 3.0 setup do this as well, or is it only after you swapped in the 3.5 liter motor?

While I hesitate to say this before you are 100% sure about your ignition and carb tune,…. Ill offer a few cents of opinion:

popping from the exhaust is sometimes caused by a combination of an overly rich carb tune (putting unburnt fuel into the exhaust system), and an air leak in the exhaust system (which supplies oxygen so the exhaust gases can explode).

Basic test: accelerate up to say 3500 or 4000 rpm. Leave the car in gear and take your foot off the throttle, letting the car “engine brake.”

if the backfiring and popping ONLY happens when you do this but never when you are on the throttle, then start by checking for air leaks in the exhaust plumbing.

 

[JFENG]

very easy to burn your hands trying to make the minute and matching turns on each carb to achieve balance

You need one of these, if you plan to play around with the 33/36 and 38/38 carbs. This tool gives easier access to both the idle mixture and the cold idle screws and keeps your hands away from the hot engine.

 

[bluecoupe30!]

You need one of these, if you plan to play around with the 33/36 and 38/38 carbs. This tool gives easier access to both the idle mixture and the cold idle screws and keeps your hands away from the hot engine.

I made up some custom tool that worked for me, but what is this one called? And where is it sold?

 

[JFENG]

It’s an old school carb adjusting tool. Most often sold new by motorcycle shops,

Pilot Screw Adj. Tool - Motion Pro

Motion Pro - High quality cables, tools and controls for motorcycles, ATVs, snowmobiles and personal watercraft.

www.motionpro.com

They regularly turn up on eBay for about $20. I bought one like the eBay link below w a 1-4” drive head so I can change tips for different jobs. I have a magnetic 1/4” drive socket that I use with hex drive screwdriver bits.

search for eBay item
266316062755

John

 

[Laldog]

Backfiring from the exhaust means it is rich. Have you adjusted the mix screws and synched the carbs? We are talking downdrafts, yes? Which model?

Thank you. I have Weber 32/36 DGAV (-03 B) downdrafts.
Yes I have adjusted the mix screws as per the Weber instructions. I read somewhere (and I'm thinking it was Pierce Manifolds) that said backfiring from the exhaust indicates a LEAN condition.
I'm certainly not disagreeing, and I hope you're right because it certainly smells like I'm running things way too rich (in an effort to cut down on the backfiring.
I'll give it a go as soon as it stops raining.
Thanks

Before I go way in over my head and start tearing apart my carburetors I have a general question.

Does the 3.5 engine require a different sized jetting than the jetting for a 3.0 engine?

Short story long:
Not long after I bought my coupe, (15 or so years ago) I did an engine swap, replacing the stock 3.0 engine with an engine from a fuel injected 535 (year unknown at present.) The transplanted engine was fitted with an electric fuel pump, and the Weber carburetors from the original engine.

I've never been really satisfied with my carb adjustment, it's been running rich and I've experienced backfiring from from the exhaust, popping etc, etc, etc.

Does the 3.5 require different sized jetting, or is it all specific to each individual carburetor?
Any sage advice and related experiences would be greatly appreciated.

Thanks in advance.

Thanks to all who shared their tips and advice with my carb tuning "dilemma".

I finally had a chance to spend some quality time with my coupe and implement some of the recommendations you all suggested.

I started with turning my distributor back (Clock Wise) to retard my timing, but that didn't seem to have any impact on the backfiring, popping etc.
I re-set it back to my previous setting (which I had adjusted last summer with strobe light, etc.)

Next I re-set my carbs, following the Pierce Manifold instruction sheet CARBURETOR SET UP AND LEAN BEST IDLE ADJUSTMENT (https://www.piercemanifolds.com/category_s/313.htm).
This was definitely the solution. No more backfire and popping out of the exhaust.
Car runs smooth and no coughing, wheezing or stumbling.
(I also somehow managed to avoid burning my hands, fingers or forearms that I was thoughtfully warned about)

I realize now that my carbs were running WAY too rich, indicated by excessive fuel consumption, and strong gas smell. I read somewhere that backfiring out the exhaust was caused by a lean condition, so I set my carbs to run excessively rich to get rid of the backfire.
I guess we know how that worked out.

Further complicating the situation was a hole in my muffler. I don't want to say that was the cause of the backfire, but I'm sure it didn't help.
A new exhaust from FitzBitz was put in a month or so ago, but it's only been this week that I've been able to work on my car.

Again thank you all so much for your help.

(next up: window motor replacement )

 

[Stevehose]

Pierce Manifolds) that said backfiring from the exhaust indicates a LEAN condition.

Usually backfire out of the carbs indicates lean, out the exhaust indicates rich. But I suppose anything's possible with these carbs after 50 years!

 

[Dick Steinkamp]

At least according to this web site, both rich AND lean mixtures can cause backfiring through the exhaust...

What Makes an Engine Backfire? | Beaverton, OR

If your engine has been making loud noises that have you suspecting a backfire, we'll tell you five things that most commonly are the reason.

www.carrsubaru.com

1. Lean Air/Fuel Mixture​

Not having enough fuel added to the engine can also cause this problem! See, if there isn't enough fuel mixed with the air in the engine, the resulting mixture will actually combust more slowly -- meaning, when the exhaust valves open, there will still be unburned gasoline ready to get dumped into the exhaust. This problem could be due to clogged fuel injectors, a failing fuel pump or even a pressure leak elsewhere in the fuel system.

2. Rich Air/Fuel Mixture​

Not only does your engine need to fire its spark and close its valves at the right time, it also needs to burn the right ratio of air to fuel. If too much fuel is added to the engine, it may not all burn up before the exhaust valves open -- letting unburned gasoline into the red-hot exhaust headers, where it can combust and lead to a backfire.

 

[Laldog]

Sorry for the confusion. It was definitely a rich mixture that was causing backfire out of my exhaust.

Car is running great now, no backfire, less smell.

Fingers crossed. Something always seems to pop up.

Again, thank you all for your help!