Just finished a new 2 liter engine build with Webcam for carbs installed.
New Raby heads, Hoffman Eng, are used. New 44IDF Webers installed with cross bar linkage. Valve lash to spec and timing is spot on. Carbs are synched. Continue to hear a slight popping from only the pass side carb at partial throttle positions. Car starts and idles with plenty of power thru acceleration, but who knows I haven't driven one in over 25 years. It's a hoot to drive except for the Issue. Played with jetting to no avail. Any Carb guru's out there able to provide input to solve this dilemma.
This is interesting since I am having the same problem. Also popping on deceleration. I had this problem coming home from rt66. I overhauled the right carb and problem went away. After about a month, the problem reappeared. I then got off my ass and overhauled the left carb and problem still exists. After 13 years of trouble free operations (except for a few clogged idle jets easily fixed) I am faced with popping and back firing. Otherwise it seems to run great. Sorry if I highjacked this thread.
I struggled with my carbs lately and just got them running better than EVER just yesterday.
No popping, no backfiring on engine braking (I thought that was just 'normal' until now) and it starts immediately on the first crank. And I have more power than ever.
A few things I learned:
1 - For the lean idle setting: Follow the instructions exactly -
A - get the engine a little warm with the linkage still hooked up , THEN Disconnet the linkage
B - NO MORE than 1/2 turn in from the adjuster touching the arm
if you cannot get it to idle like this (like a tractor) then go up on the idle jet before moving on.
C - once you get it to BARELY idle, check that both sides are pulling the same with your flow meter and adjust the higher one down (unless it dies).
2 - once you have it running decent and warmed up - spray carb cleaner around the bases of the carbs and where the manifolds exit the head looking for intake leaks. The engine will stumble/die if you locate a leak. FIX THE LEAKS.
3 - Synchronize at idle and at ~2500 RPM (or as close to as high as your flow meter can handle)
4 - If you use pump gas with ethanol find different gas or use Sta-Bil to eliminate the ethanol.
I was getting crusty deposits in the float bowls and clogging the jets until I switched to AV gas.
This only took ONE DAY after a complete rebuild with the ethanol 91 gas in CO.
Filters will NOT help this - the gas will absorb water while sitting in the float bowls, then evaporate leaving behind the deposits.
5 - Get a AFR sensor. I have been using the innovate unit I bought WITH gauge for $152 on ebay. It has been working fine for ~5 years now.
6 - The BIGGEST ONE - Measure the output of the accel pump jets. I did this by taking a small cap for a hypo-needle and wrapping a wire around it to reach down in the carb. I prime the pump jets, then stick the 'measuring cup' down in the throttle body and give the acccel jet pump one full pump. These caps held ~0.5mL and was exactly how much my "good" pumps were pumping.
The other side was only pumping 1/5 of this.
I took the accel pumps apart, cleaned them AGAIN and swapped all parts from one side to the other and nearly gave up UNTIL.
I switched the pump diaphragm from one side to the other and the crappy pumping moved sides.
I dug through my stash of carb parts where I had ~5 sets of accel pump diaphragms and found that I had mixed & matched sets.
They were NOT all the same even though the y looked like it.
I matched up a diaphragm with the "good one", installed it and then my pumps pumped the same from side to side.
Now the car runs like it never has before.
I decided to check the idle jets one more time. Even after rebuilding the both carbs, I found the idle jets on the right carb hopelessly clogged. Compressed air wouldn't dislodge the clogs, so I used a small strand of copper wire to unclog the jets. Runs great now. When trouble shooting a problem, I always (most times) check the easiest thing first. Idle jets are easy to remove (I've had lots of practice) so I usually go there first. The car seems to be running good now but I'll take it on a drive to make sure. "Check the idle jets"! I'm thinking, fuel injection might be nice.
98% of the time it's a linkage issue, both carbs are not opening at the same time/rate, they are going out of sync as soon as they come off the idle stops, etc.
See it all the time.
To make sure mine opened all the way to WOT and opened at the same rate, I had my wife push down on the pedal while I watched the butterflies move. When nothing moved at all, I realized she was pushing on the brake pedal. I guess it was my fault. I told her to slowly push down on the pedal on the "right".
To me, cable or not, you still have one carb opening slightly before the other. I go to great pains to make sure the throttles are opening at the same time and rate. Number one with a crossbar or center pull is to make sure the arms are as close as you can get to 90 degrees to the throttle shafts. Then there must be zero slop in the linkage. I do not use a linkage with one CCW threaded shaft I feel this causes more problems then it solves. I use high quality rod ends that alone cost me $50 for 4 .
I sync the carbs with one arm off, I never have to sync at any other position, because again I feel it creates more problems then it solves. Then I carefully bend the one arm (that's off for sync) till it goes in like butter, absolute zero effect on the sync/linkage. If I'm going out of sync at higher RPM then it's time to start over, as I still have a linkage issue.
Also I've found I have the exact same linkage sync issues with programmable EFI and ITB's, you can't blame that on jets.
One of the reasons I like the center pull linkage is I can shape the arms (to get my 90 degrees to the throttle shafts) to help eliminate the geometry issues caused by the offset of the carbs. With a slight bit of slack on the throttle cable engine expansion is not a problem. Even if you get the geometry perfect on a hex bar linkage, the bar itself and bushings tend to twist.
The T4 has a huge intake port offset compared to the T1. The only crossbar linkage that I have any use for is the Gene Berg unit for the T1, unfortunately you can't adapt it for type 4 use.
BTW the /6 has way less linkage/sync sensitivity, it acts more like two 3 cylinder engines ganged together, where as the /4 is very susceptible to even the slightest issue.
Good info- some news tricks as I'm fighting the popping etc on deceleration primarily. We get the sh**ty alcohol infested gas in AZ over half the year so in goes the Marine Stabil to help fight that problem at least a little...
The Tangerine linkage from Chris does help some with adjustments though!
Here's my carb tuning procedure from a March 03 post:
Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.
Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns (both sides).
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest rpm, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way (from baseline setting), you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.
That's a rough, five minute draft of my carb tuning proceedure, hope it helps!
If it goes good it should take about 45 minutes, if not about three years.
Ed Mayo told me 20 years ago that about half of carb problems are ignition.
PK
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