Hey, thanks for clarifying that John. My thoughts were yer going to have higher intake vacuum @ 100mph than at 30 mph. Buy what do I know?. Ha, ha.
Who said anything about throttle settings? U?

 

That is lean popping...and should not be described as back firing, which is Always under power. Lean popping is from air leaks in the exhaust, not the carb, and happens at or near idle throttle And high rpm. Now...”spitting out the carb” would be lean mixture, so that is a carb issue.

Back firing out the tailpipe under power is always ignition. Period.

 

You got it backwards.

 

Have either of you ever put a vac gauge on a bike? Or any engine?

Here's the deal, and if you think way back you may remember reading it somewhere. Consider the engine an air pump. The faster(more RPM) you run the pump the more air it moves. Given the size of the intake passages don't change how does it get more air thru? By creating a bigger differential between the intake and ambient atmospheric pressure. Now ambient atmospheric doesn't change(disregarding elevation changes) so a drop in the intake manifold vacuum is the only thing that can affect air volume increase. An engine spinning at 4000 rpm is displacing more air than one spinning at 1000 RPM. Therefore the intake vacuum is greater at any given throttle setting at 4000RPM than at 1000 RPM.

In fact an intake leak can lean out the engine causing lower top speed. It just so happens that I have a vacuum gauge on one of my bikes.

 

So, I noticed the carb gasket that goes on the air filter backing plate was in shambles, so I replaced it yesterday hoping it was just sucking too much air. That was not the case. I have gotten side tracked the last few months with many other projects, but the Harley is coming back up on the list. I still need to clean the carb, and I havent replaced the plug wires. I replaced the coil because my old coil was physically broken. The problem feels like loss of spark which seems like it should be a timing issue. I've attached a picture below of what I think may be the problem, but I'm not sure. I haven't traced it back yet.

 

You got it backwards. An engine makes the most intake vacuum with the throttles closed and the engine rpm coming down. (Like using engine compression to slow down.) This is your highest vacuum reading in the intake tract. (And yes, I've had manifold vac/pressure gauges on my Pete's and Freightliners since 1970's. Even in a turbo'd car in the 1980's.)
You're right about a 4000rpm engine breathing more than a corresponding engine at 1000 rpm. No need to comment on the rest of your post.
Just trust me when I tells ya, the lower the rpm, the higher the vacuum reading.

 

Yes. In fact, I'm a pilot and aircraft mechanic. We use manifold pressure gauges in concert with tachs to determine power output and expected fuel consumption.

OK, I get where this guy is coming from. He's saying that if (for example) you go to WOT at 1000 RPM, the vacuum will increase as engine speed increases. (Manifold pressure decreases.) This is, in fact, correct. However, the change under WOT is insignificant ( a few in. hg. at most) compared to the change between idle throttle and WOT (~15 in. hg.). In that case. the pressure goes up and the vacuum down. Dramatically.

What I was saying is that, as engine power (at a particular RPM) increases, so does manifold pressure. And, any intake leak is going to be dramatically more noticeable under high vacuum conditions, as when the throttle is closed, and becomes insignificant in proportion to how far the throttle is opened. Bottom line: intake leaks are most noticeable at idle and diminish in effect as you open the throttle. TMSAISTI!

 

Ok this is getting boring. Like a pissing contest. Mincing words. We all know knowitallnorty has the most facts in his head. I mean he was a riding instructor for crying out loud. He just can’t make sense of the jumble in his head.

And jumpinjohosefatsjohn, was alright at one time. I think he’s been hanging aroun knowitall too long.