I have a 70 340 all new. new holly 750. new msd coil dist. cap wires 6al box .but The number 1 cyl. is very lean and the number 3 is very rich all the rest are ok . has a miss when cold runs good after it warms up . I have rechecked everything it all looks good and has good compression after a hundred miles or so the number 3 plug will foul and number 1 looks new . removed the intake today to check for leaks looks good . dont know what to do next.
340
- Thread starter duster51
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nm9stheham
Well-Known Member
Parts? Stock type heads? Cam? Valve job? Need more info on this 'new' engine.
BTW, for a dual plane manifold, #1 and #3 are fed from different halves of the manifold. So do you have a dual-plane? One side could be rich and the other lean...
Got any plug pix to share? Sure the rich plug is not oil? What are you doing with the engine just before you take a plug reading?
BTW, for a dual plane manifold, #1 and #3 are fed from different halves of the manifold. So do you have a dual-plane? One side could be rich and the other lean...
Got any plug pix to share? Sure the rich plug is not oil? What are you doing with the engine just before you take a plug reading?
nm9stheham
Well-Known Member
OK. All I can say at this point is : Got any plug pix to share? Sure the rich plug is not oil? What are you doing with the engine just before you take a plug reading?
It is not oil and I checked compression 150 on #1 160 on #3. I put the intake back on today looked at the plugs again 1 and 7 are lean 3 and 5 are rich 2 4 6 8 all look good. I have checked everything I can think of and talked to my engine builder he has been building engines for 50 years and is very good and he don't what to do next.
dano
Evil Handy Man
Have you swapped plug wires to see if it moves i.e. bad wires.
nm9stheham
Well-Known Member
Well, 1 and 7 are on the same side of the carb, and 3 and 5 are on the other side so something seems mismatched between the 2 sides. Nothing guarantees all cylinders will get the same on a given side so perhaps the side that feeds 1, 7, 4, and 6 is overall lean and 1 and 7 are getting less than 4 and 6. The other half of the carb feeds 3, 5, 2, and 8, and 3 and 5 are right under the carb and are getting more than 2 and 8, with that whole side a bit rich.
The reason to ask about what you are doing before taking the plug readings is that you will see the coloration on the plugs corresponding to what the engine/carb were last doing. So if you go WOT and then cruise back to the garage and read plugs, then you will see the mixture that the plugs saw for cruise and idle, not WOT. If you want WOT readings, go WOT a few times and quick succession, and then close the throttle and cut the ignition at the same time, drift to a stop, and pull plugs by the road for a true WOT readings.
Since we don't know how you are doing this plug procedure, it is hard to say what carb circuit the plug readings are reflecting. If a lot of idle just before plug readings, then it might be just the 2 sides' idle mixture screws being set a bit differently.
The carb may have something mismatched internally between sides.
BTW, is the engine getting fully hot? Not sure where 'K' is, but the Air Gap is going to be cold-natured and the temps are getting cooler these days. Perhaps the cool air on the non heated AirGap is causing idle and cruise mixture mismatches.
The reason to ask about what you are doing before taking the plug readings is that you will see the coloration on the plugs corresponding to what the engine/carb were last doing. So if you go WOT and then cruise back to the garage and read plugs, then you will see the mixture that the plugs saw for cruise and idle, not WOT. If you want WOT readings, go WOT a few times and quick succession, and then close the throttle and cut the ignition at the same time, drift to a stop, and pull plugs by the road for a true WOT readings.
Since we don't know how you are doing this plug procedure, it is hard to say what carb circuit the plug readings are reflecting. If a lot of idle just before plug readings, then it might be just the 2 sides' idle mixture screws being set a bit differently.
The carb may have something mismatched internally between sides.
BTW, is the engine getting fully hot? Not sure where 'K' is, but the Air Gap is going to be cold-natured and the temps are getting cooler these days. Perhaps the cool air on the non heated AirGap is causing idle and cruise mixture mismatches.
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