Two rear cylinders not Working?

[cruzoropeza]

Hi guys hopefully someone can give me some ideas.
If I put my hand onto of rear carb and I'm getting no vacuum too very little compared to front carb, I'm pretty much running on two cylinders.

Checked sparkplugs/wires
Compression is the ruffly the same in all four cylinders
Carbs seem to be working
I also checked valve adjustment

What else can I check or recheck thanks in advance.

[Stekay]

Intake manifold leak, lack of valve lift? Even an exhaust blockage on those two cylinders could cause that symptom I'd think.

[audiocontr]

Idle adjustment screws and butterfly adjustments? Just sound like the carbs are out of sync.

[cruzoropeza]

I'm going to try to switch carbs Front to back to rule them out. Manifold leak is also a good idea,

[Pinzgauer Pete]

before you waste time moving carbs around check for a intake manifold leak could be the manifold itself or the gasket. I seriously doubt the carbs have any relevance to the issue

[cruzoropeza]

Sounds good I was going to check manifold first, it's gotta be something that affects both rear cylinders witch point to manifold.

[cruzoropeza]

Is there an easy way of checking manifold for leaks?

[Jimm391730]

Is there an easy way of checking manifold for leaks?

Spray carb cleaner around the manifold joints while the engine is running; any change up or down in RPM indicates a leak. Idle may go down if the liquid blocks off the crack, or rpms may go up as the carb cleaner acts as fuel and burns in the cylinders.

The difference in carb suction is only an issue at idle; this is what the carb sync tool is for, and will give you a smoother idle. Once you start opening the butterflys by stepping on the gas they pretty much balance out, both getting good airflow (I once had a carb linkage fall off on the highway, and it was a very noticable drop in power where I couldn't go more than about 40mph - you'll really know if you are only running on two cylinders).

[spandit]

What do they adjust when syncing the carbs?

[cruzoropeza]

O I know I'm only running on two cylinders because when I remove rear spark plug wires it runs exactly the same way

[Jimm391730]

What do they adjust when syncing the carbs?

There is a little screw at the base that pushes the butterfly shaft lever; this is the same lever that the linkage pushes. The screws set the stop for the lever, and this sets the idle RPM. The carb syncing tool is used to ensure that both carbs are drawing the same amount of air, in other words they are both open the same amount. As little as a half a turn of the screw will unbalance the carbs - it is pretty sensitive.

I have done the "shade tree mechanic" method of cuping my hand over the top of the carb(s) and detecting the airflow through the opening in my thumb and index finger (think of making an "OK" symbol with your hand); aim for equal airflow through both carbs. Then increase/decrease both screws evenly to set the idle rpm, then recheck the balance. But the sycronizers from EI and other suppliers are much more accurate and easy to use.

[audiocontr]

Hmmm, have you run a compression check?

In regards to syncing, I've been successful by doing the following after reading the sdp site instructions:

Purchase a sync tool. They are relatively cheap at ei and help a bit. I was 4 points off between carbs and no matter what I did to one carb, it seemed to raise or lower the other carb, never allowing me to close that gap. I needed to create a baseline.

Remove the accelerator linkages to the carbs. Do not let them rotate at all. It will help later. Carefully turn in all 4 of the idle/air screws at the base of the carbs (drivers side). Some are internal, some are external, but all can be reached by a long thin flat head. Turn them clockwise until they won't turn anymore. Do it gently and don't force them. Now rotate them all counter clockwise 1.5 turns.

The accelerator linkages have stop screws on the external portion of the butterfly shaft. Some are at the bottom, some at the top. Turn them out past the point of contact and close the butterflies manually. Most will close completely naturally. Others will require slight pressure to close. Keep this location in mind and reattach the acceleration linkages so that they close the butterflies completely. It will take some attempts and half turns to find that perfect spot where both have equal tension.

Now turn the throttle stop screws back in until they make contact with the stop plate. Turn them one half turn further. try to start the engine. If your lucky it will start, and the carbs should be very close to synched. I was not lucky, I needed to rotate the set screws a turn or so, and I also rotated the idle screws .5 turns in order to raise the rpms to ~900. The trick is doing the adjustments to the both carbs equally. If you turn one screw, turn the other the same amount. Now my carbs are synched exactly the same on the meter.

[Jimm391730]

I would recommend that the throttle linkages NOT be attached while the carbs are syncronized; then connect one linkage to one carb and then check to see if the other linkage fits neatly on the ball, or rotate the ball end on the linkage to lengthen or shorten it until it can be attached to the second carb without pushing or pulling on the carb lever. Don't assume the linkages are correct! And as the carbs wear, one may need a longer/shorter linkage than the other to be set properly.

[cruzoropeza]

I did do a compression test

[Jim LaGuardia]

I did do a compression test

So what were the results?
If 2 cyls not firing and you have spark and compression, pull the top off of the carb of the affected cyls and see if there is gas in it.
If the float needle is blocked or stuck it will prevent fuel flow.