Twin H 308

Billys438

My 308 motor has good compression, no audible vacuum leaks, new plugs. ignition but my mechanic says it seems to be only running on the 3 front cylinders. put your hand over front carb and its really strong intake. hand over back carb (top) and intake seems really weak...anybody have any ideas as to why?

HoustonHudsonHornet

I was actually going to post my own post about what is "normal" compression PSI on the 308.  Can I ask what your compression readings were on all the cylinders?  Mine were:
1:  90
2:  95
3:  110
4:  105
5:  90
6:  95

It seems to run well enough, I'm just curious how it compares.

Ol racer

Billys438 said:

My 308 motor has good compression, no audible vacuum leaks, new plugs. ignition but my mechanic says it seems to be only running on the 3 front cylinders. put your hand over front carb and its really strong intake. hand over back carb (top) and intake seems really weak...anybody have any ideas as to why?

Suggest to check if your Balance Tube is Plugged...

Billys438

Ol racer said:

Billys438 said:

My 308 motor has good compression, no audible vacuum leaks, new plugs. ignition but my mechanic says it seems to be only running on the 3 front cylinders. put your hand over front carb and its really strong intake. hand over back carb (top) and intake seems really weak...anybody have any ideas as to why?

Suggest to check if your Balance Tube is Plugged...

...the balance tube is connected to the fuel pump...there should vacuum there, correct?

Billys438

If I disconnect the tube from the fuel pump, should there be vacuum on the pump side and tube side?

Hudson_in_Seattle

HoustonHudsonHornet said:

I was actually going to post my own post about what is "normal" compression PSI on the 308.  Can I ask what your compression readings were on all the cylinders?  Mine were:
1:  90
2:  95
3:  110
4:  105
5:  90
6:  95

It seems to run well enough, I'm just curious how it compares.

i think what's most important is the variation of compression over the cylinders - also, i prefer to have the compression checked when the engine is warm - like your #'s, my #'s to date show cylinders 3 and 4 as the highest compression - i wonder if that is because they run the hottest, so they warm up more/cool down less than the other cylinders when they check is done?

squirrel

Are the throttle plates synchronized?

Jay_G

Yes, there should be vacuum at the balance tube if you disconnect from the double action pump.  If you disconnect the balance tube connection and run the car the double action pump should also pull vacuum from the wipers and push the air to the balance tube.

If your carbs are not pulling the same amount of air your engine will run like crap.  Especially if the balance tube is plugged.  First thing to do is to make sure the carbs are some what set up the same.  The throttle plates should be closed at the same time and fully open at the same time.  These are not progressive carbs.  The chokes should be opening at the same rate and at the same time.

Best thing to do is look at the manual about syncing the carbs and get yourself an air flow measuring device.  Does not matter which one just as long as it fits the carbs.  Then make sure the carbs flow the same air at the same time (RPMs). 

As stated above a compression check is the first place to start to insure you do not have a dead cylinder or two. 

Jay

53hud54

According to the mechanical procedures manual compression should be 100 minimum but does not say what good compression is, we are going to check mine this weekend, just putting new head gasket on, and I'll post here where mine is as we just found out it was recently rebuilt by the looks inside but I had no idea.

Max162

Hudson_in_Seattle said:

HoustonHudsonHornet said:

I was actually going to post my own post about what is "normal" compression PSI on the 308.  Can I ask what your compression readings were on all the cylinders?  Mine were:
1:  90
2:  95
3:  110
4:  105
5:  90
6:  95

It seems to run well enough, I'm just curious how it compares.

i think what's most important is the variation of compression over the cylinders - also, i prefer to have the compression checked when the engine is warm - like your #'s, my #'s to date show cylinders 3 and 4 as the highest compression - i wonder if that is because they run the hottest, so they warm up more/cool down less than the other cylinders when they check is done?

I don't know if does affect the results, but when doing a compression test, remember that the engine has six cylinders in line : temperature wise, the cylinders near the radiator might be cooler than the one that are far in the back. 

Ol racer

Check if the Tube connecting the two Carbs is Plugged by unbolting from the side of the Intake.

super-six

Have you checked vacuum with a gauge? Intake manifolds can rust through from the runners to the heat chamber, hidden from sight. It will cause this symptom, but also usually a rough idle due to the vacuum leak.

Billys438

Thats the thing, it idles really smooth,no shaking or anything...I will check tube, then check the carb plates..I know the choke plates, when cold are both evenly closed..I know you can adjust the idle and fuel mix at carbs but done anything about adjusting air intake..how is that done?

Ol racer

Super-Six is Spot on. Look underneath the Intake at the small round soft plugs. They sometimes rot or develop a crack right under the carb pockets. Its not easy to see so try using a flexible mirror. If so it can be patched or repaired.

Billys438

OK..but I repaired my Twin H intake manifold as it was rotted in the back corner. A specialty welder (cast iron) welded a plate underneath, do you think they might have covered over them? or do you have to take the carbs off and look down the manifold?

Glowplug

I grabbed some pictures that S Maas posted showing the cleaning of a TWIN H manifold.
There are three plugs under each carb plenum that are shown in the picture.  Note the red arrow point in the top side picture/

Billys438

Ohhh...OK I will check this..what are the plugs made of and where would I get one if needed?

Billys438

Thats where it was rotted out so my guy welded a plate over that area...was running great for the last year....

Billys438

Anybody know where I can get a good Twin H manifold if needed?

JasonNC

You can get the plugs from NAPA.  Somewhere on this forum the size is posted.