Stuttering At Speed

[Steve Simmons]

Ok, I'm open to some outside of the box thinking on this one. The supercharged MGB runs great but after running at high speed, say 75+ MPH / 4K+ RPM for more than 30 seconds or so, it begins to stutter as if it has lost two cylinders. It still has power but runs rough until I slow down and let it recover. I was thinking it was a fuel delivery problem but the AF meter says the mixture is still good. Unless I'm not thinking clearly, a good AF reading should indicate an adequate amount of fuel.

Assuming the coil and condenser are both good, what else could cause this? Plug wires are new and high quality. Maybe a spark plug overheating?!? The engine was run successfully on a dyno before installation.

Next step is to go buy new plugs and swap in another known good condenser.

[tannyo]

I had stuttering with mine and it turned out to be a bad condenser. Stewart said that there were a batch of bad condensers for the 45D.

[gorms68]

What about a weak ground?

[Steve Simmons]

It isn't acting like a weak condenser, and this one has proven itself good although anything is possible. The thing is though, a condenser wouldn't cool off so fast just by slowing down a bit. I suspected the coil but the same applies.

Bad ground could possibly cause overheating electrics I suppose, but again it recovers very quickly so I just don't think so. Sure feels like a fuel issue but it can't be. Hmmm...

[malcolmr18zoy]

Maybe it's running too rich. What do the spark plugs look like?
Malcolm

[Steve Simmons]

I was going to pull them earlier but I can't find the socket! AF meter says it's ok but you can't always trust those things either I suppose. I'll find the socket tomorrow. I'm done for tonight.

[Stewart]

Did you notice if the a/f ratio went richer when it started misfiring?

I know your engine is quite a bit hotter than mine but are you running the bp7es plugs that moss supplied? I know with mine it ran better once I switched back to the bp6es.

[malcolmr18zoy]

Yes, Stewart has a good point, some softer plugs may help.
Malcolm

[Stewart]

I also wouldn't rule out a bad set of wires as well. I have had a new set go bad shortly after installation. The last and only time I had a coil go bad (this last Thursday and if I was wrong I'm sure it will rear its ugly head when I make it past my 50 mile tow limit tomorrow) it was an intermittent miss at all rpms the engine felt like it was down a cylinder or 2 but you could still power out of it. When I had a fuel delivery issue last year it would start by dropping a cylinder and any increase in throttle just made it worse.

[Steve Simmons]

Plugs are still bp7es but they look good. If anything they look a bit too perfect (yes I went back out and found the socket). I haven't seen tan-ish grey-ish plugs since the 80's which leads me to believe they are either running too lean, too hot, or both. I've richened it up a bit more and we'll see how it goes tomorrow.

[Stewart]

How did it run on the way back?

[Steve Simmons]

It ran beautifully, although the AF swings continue. But I have a new theory on the high speed problem. I noticed some noise that sounded like belt slip when I wound the RPMs up during acceleration. Unless I'm mistaken, if the belt is slipping at higher speeds then that means the blower would be slowing down and fuel delivery would suffer. Air delivery would also be reduced so the AF meter might still register OK even though the engine is starving a bit.

I won't have time to open the bonnet until this weekend though.

[Jimmy]

Steve, you don't need an expensive blower, slipping or not, to achieve those symptoms.
My Beater B has been doing pretty much the same thing for months now.

[RodH]

I realize this is an older post...
Not sure if you ever got this completely sorted, but, I'll clammer in anyhow...

I'm was running the eaton m45 s/c on my Bugeye (ford zetec engine, though) has the same problem.

I found the heat generated from the m45 at speed (or at boost) and temperature was occasionally blowing the spark out. I confirmed this with 4 hours on the dyno.

What worked for me, reducing the spark plug gap from .050 to .035. Reduces mileage a bit, but elliminated the missing.

of course, what really made the big differnence was dumping the M45 for a Sprintex Twin Screw S/C, whooohooo,

Rod

[Steve Simmons]

Thanks for the note, Rod. I haven't touched the car in over a week because of a busy work schedule but I plan to get back on it this weekend if possible. I'm actually thinking that the belt may be slipping at higher RPMs, and that may be a cause of my problems. The spark gap is already below 35, as I find that about the maximum acceptable gap on an MGB. The fuel ratio is still all over the place as well but not quite as bad now that I've removed the bypass valve and done a bit more tuning.

[RodH]

Since you mention the bypass valve, I replaced mine with an adjustable bypass/blowoff valve from these guys:

http://www.turbosmartusa.com/product/" onclick="window.open(this.href);return false;

It works a lot better than the eaton one. Requires some puttering and tinkering skills to install though.

Sorting is the best part of a performance set-up, cause your wife can't tell you to slow down.

Rod