1966 Mack Engine Trouble

[gmbelt]

I have a 1966 Mack C-95 semi-open cab pumper with a Mack ENF707C gas engine and Zenith 29D-series carburetor. When shifting through the gears and accelerating, there is a loud popping sound almost like a "back-fire" that we have isolated as coming from the area of the carburetor in the engine compartment. The air breather hose is not presently connected to the carburetor (new one on order) and I'm wondering if this could be the cause of the popping sound. If not, is their an engine or carburetor adjustment I should be looking to make? Where else might I look?

Many thanks in advance for your help.

Gary M. Belt

Manassas, VA

[Greg Rider]

First thing that you need to check is your timing. Make sure that it is advancing as RPM's increase. There are springs inside under the cap that control the advance rate. Next thing to consider is that the carb is probably due for a rebuild. It sounds like your accelerator pump in the carb isn't working right. It is there to give a "boost" of fuel when you hit the throttle. The popping noise would be present with or without the hose in place. It would probably just be muffled. Don't run without your air-cleaner hooked up! My truck makes a popping noise when it is cold and I try to take off. After she warms up it is fine. I run my carb a bit on the rich side cause I don't want a possible lean condition. Your carb may be running ''fat" and causing some of that. Check the easy things first, timing, then look into the springs, then go after the carb.

[HK Trucking]

Don't run without your air-cleaner hooked up!

Yes, or the next step will be a complete engine overhaul.

[gmbelt]

First thing that you need to check is your timing. Make sure that it is advancing as RPM's increase. There are springs inside under the cap that control the advance rate. Next thing to consider is that the carb is probably due for a rebuild. It sounds like your accelerator pump in the carb isn't working right. It is there to give a "boost" of fuel when you hit the throttle. The popping noise would be present with or without the hose in place. It would probably just be muffled. Don't run without your air-cleaner hooked up! My truck makes a popping noise when it is cold and I try to take off. After she warms up it is fine. I run my carb a bit on the rich side cause I don't want a possible lean condition. Your carb may be running ''fat" and causing some of that. Check the easy things first, timing, then look into the springs, then go after the carb.

Many thanks for your help. Singer Associates (premier fire apparatus repair facility in Manassas, VA) did some work on the truck to get it running, but I don't think 33 year old gas fire truck engines is theri fortay. They service most of the fire departments in the Northern Virginia area and they are all front line (diesel) pieces. Any suggetions as to where I might turn to find someone who could work on the engine correctly? I'm a fire truck buff (I have two Mack's), but I don't know a spark plug from a lug nut.

Thanks again,

Gary

Manassas, VA