Re: Re: My M is sick

Posted by Scott Price on March 12, 2003 at 16:00:07 from (64.22.197.91):

In Reply to: Re: My M is sick posted by Ty on March 09, 2003 at 20:00:50:

Where is Paul Harvey? The rest of the story! I think you have found the problem. When you said that you changed the manifold, you forgot to mention the non-stock configuration of the new manifold! This Manifold flows much better for both intake and exhaust. So, you are starving for air, which results in the uneven air/fuel mixture between the cylinders that I mentioned earlier. This means that the majority of the air/fuel mixture is probably taking the path of least resistance into the center two cylinders causing them to be too rich and foul plugs. The outer two cylinders are getting a leaner mixture, due to the longer runners and not fouling.

What has happened here is that you significantly upgraded the performance of one component in the system without upgrading and/or tuning the other components to match it. You helped things out with the electronic ignition providing a hotter spark, but you still need to fine tune the rest of the system. In addition to providing more air to the intake with a lower restriction air cleaner, you will probably need to adjust or possibly even re-jet the carburetor to mix enough fuel with all that air. Once you can supply enough air and fuel, at the proper mixture, to the intake, the burn rate in all four cylinders should equalize. At that point you may not need hotter plugs, and you should be able to use the same heat range in all cylinders, because they will all have the same air/fuel ratio to burn. In other applications, at this point you would than need to be concerned that you have enough exhaust flow to get rid of spent gases without restriction. You already have the free-flowing exhaust manifold, and most tractor mufflers are relatively low restriction, so hopefully this will not be a problem.

Some basic rules to consider:
--Intake flow rate matched to exhaust flow rate, meaning increase intake flow and you need to increase eshaust flow.
--When you increase intake/eshaust flow, you must increase carburation fuel delivery to maintain correct air/fuel ratio.
--When you do the above items, if you end up with an air/fuel ratio that is slightly different thatn stock, you may have to adjust intensity of spark, heat range of plugs, timing, or a combination of these to get proper combustion.
--Modifications like this may cause a seemingly good stock engine to suddeny seem like it needs an overhaul, because you have now increased combustion temperatures, pressures, and Horspower, without changing condition of rings, valves, etc. If you started with a good engine, you should be Ok, but if it was marginal, this could push it over the edge.

I have been down this road before myself.

I would go back to Denny's and ask their advice on carburation, ignition, and air intake changes for your particular engine. They really know their stuff, and should be able to help you match your components.

Good Luck--Keep us posted.

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