Mazda B2000 Troubleshooting help

I did a little research around this forum and could not locate any info on this issue.

Purchased the truck from a friend (original owner) 6 months ago. Truck has 110,000 original miles.

One day is started idling rough, so I inspected the vacuum lines and associated system components. No issues noted.

I then removed and disassembled the carburetor. Cleaned all components: jets, float bowl, springs, linkages, etc. During that time, I noticed a discrepancy with the accelerator pump o-ring. Replaced the oring with a similar oring. Re-assembled the unit and installed it back on the engine. The truck ran the same.

I then purchased a weber carb kit. I installed the carb and plugged all open vacuum lines. The truck ran the same.

I messed around with the timing and was able to get the truck to idle good, however, when I snap the throttle, the engine backfires at higher rpm.

I then replaced the distributor, cap, and rotor (all replaced at different time periods) and the truck ran the same at the end of the day.

I checked and tinkered with the timing many times, including top dead timed the engine, and could not make it run better.

I removed the belt from the vacuum pump, and the truck ran the same.

Compression is the same in all four cylinders.

Checked the spark on each wire, no issues noted.

Re-inspected all vacuum lines for obstructions, no issues noted.

I disassembled and inspected the three way modulating valve, air control valve (cannister), and vacuum amplifier - no issues noted.

I changed the gas, and the truck ran the same.

Items replaced in the last month include the following:
-Alternator
-Coil
-Starter and solenoid
-Fuel Filter
-Carburetor
-Air Filter
-Both belts
-Dist
-Cap
-Rotor
-Air Injector Manifold

Plugs and wires replaced with new shortly before I took ownership of the truck.

The engine idles perfectly, but once you quickly advance the RPMs while parked, the engine backfires. If you slowly advance the RPM while parked, the engine will not backfire.

If you drive faster than 5 mpg, the engine will surely backfire.

I have two questions:

1. Any possibility the timing belt can be a contributing variable?
2. Any other troubleshooting that can be done at this point?

Any help is appreciated.

Thanks,

Rudy

well my dood,If i say something you already know bro just keep reading just trying to help, Low fuel pressure can be caused by a failing fuel pump or fuel injector too or an old ass fuel filter,fuel blows into the exhaust system, then the shit pools and collects at a low spot in the exhaust pipes.Causing a backfire, dealing with RPM doubt it,but its there.If you said you got the timing right then chance for the engine to produce a spark before the intake valves have become fully closed isnt as high, but then again we all make mistakes shit might be out of time, and your sparking on that valve causing a backfire and a increase in RPM may happen.And also like i said above the fuel, like most mazda folk we dont run our catalytic converter, and yea that too can cause a back fire, but wont cause it to run shitty so rule that out....,umm...and youve ran it and sprayed brake fluid or some type over your vacuume lines to check for leaks? and none? ALSO is the backfire from ENGINE, or EXHAUST i know you said engine but just making sure...umm..its not during deceleration so its not the main converter,im almost certain its a vacuum line, or your runnin to lean and the carb has a plugged jet?...didnt catch the year of your truck maybe a fuel pressure regulator,that will give you all the symptons you claimed...damn i think im stumped lolz sorry but thats my 2 cents! good luck broseph!

guess you figured it out good luck!

lol, sorry for the lack of response.

No, I haven't figured it out.

The backfire is from the engine (flame and pop seen from the carb).

Please note that the carb is new from Weber/Redline, so a clogged jet from service time is unlikely. A clogged jet from debris or other material from manufacturing processes is possibly (but unlikely in my mind)

At one point, I removed the vacuum belt and the engine ran the same.

I did spray some brake cleaner around the carb spacers/adapter plates and noted an increase in engine rpm, so i added some RTV and the problem went away.

Rudy

yeah there is alot of issues that cause a backfire but the main cause is a lean issue. If thats the case which it sounds like it is, you may have a air leak somewhere in your carb. spray some ether around the carb, intake, etc. if you get a idle up you have a leak. I had a backfire with a old car, i chased it forever and it ended up being a intake gasket leak on the bottom side. good luck