Poor fuel economy and flat power sounds like its going dead rich. And black smoke or soot out the back when it happens?

My thought as well. One more cause. The Mikuni uses a dual main jet for the primary barrel. One ordinary jet of about half required size, and a second ecu contolled jet to make up the required extra fuel. This second jet may have failed open. Really sooty tailpipe and black smoke and a strong gas smell at the tailpipe is indicative of a very rich mixture. Plugs will be black carboned, a soft deposit on insulator and electrodes. Pull a plug and check. Is the idle smooth and no gas smell at tailpipe at idle? If so, raise the engine speed to around 3000-3500 and smell again (run fast).

A couple more things to check for are a plugged up cat-converter and/or a defective depression chamber (Vacuum Secondary Actuator). Both of which have cause poor power and bad fuel economy at various times in my buddy's Montero.

Plugged cat will cause loss of power, but the downshift with some power return does not sound like a plugged cat - the increased gas flow at higher rpms will just jam it up worse. If the depression chamber is bad, the secondary just won't open, and you lose no fuel that way. You may get bad mpg due to trying to accel without enough air and fuel, leading to very long accel times in a lower gear, but not as bad as the carb running dead rich.

wow lots of great info here, let me try and answer al the questions.

ok, exhaust smell seems to be normal, I guess the problem does not happen only when i hit the highway, in town on a free way, say if i step it down to pass some one or if there is a long hill, it seems to load up more and more as it gos up the hill.


I do not have a cat converter any more.

and slower speeds and just basic city driving it seems fine.

I did change the secondary opener with a known good one.

when it is idling and warm i do not see gas pooling on the throttle plate.



Thanks for all the help guys, just wish i could figure this out.

The gas will not pool on the throttle plate. You will see very rapidly evaporating spots of gas. You will have a gsssy smell at the tailpipe.

Pull one plug and look at it. It should have light grayish tan deposits. If it is very white, it is too lean. If it has black sooty deposits, it's too rich.

Test spark quality. Pull a plug wire and crank it and see if you have a strong blue white spark that jumps about a quarter inch (we are testing the coil and the rest of the secondary ignition circuit).

Test the depression chamber to see if the swap in is holding vacuum.

Run a can of seafoam thru the fuel system.

Test for a dragging brake. The dragger will be noticably hotter. One of those nifty remote temp guns is a good buy for testing this. If you buy one, use it on all the exhaust runners near the head. A cooler one indicates misfire.

Also, as noted above, check the distributor's vacuum and centrifugal advance mechanisms.

John B.

Very good point, JB.

I have an FSM for the 1987 Raider feedback carb

well the truck is not running at the moment i got the rad out getting a upgraded core in it.

but it is not the ignition system, full new plugs, wires, cap, upgraded coil.

Now the distributor i am not sure, i did test the vacuum advance and it does work correctly so i am unsure what it could be.

The plugs never looked black or sooty, i have had them out a few times looking...

I would say the rich running only ever really happens under load.