OK, first of many questions - what is going on with my MAP?

spandit · Post by **spandit** » Tue Apr 19, 2011 3:37 pm

I've just logged a short run of my engine but am very confused about the kPa readings. As expected, when the engine is not running, I get a value of around 100kPa (I've trimmed the zero rpm values from the attached graph). The values then sit at roughly 70kPa but DROP as the rpm increases - this is counter intuitive to me...

Post by **brentp** » Tue Apr 19, 2011 4:47 pm

Where do you have your MAP sensor connected on your manifold?

spandit · Post by **spandit** » Tue Apr 19, 2011 5:19 pm

I've tee'd into the brake servo feed. Between the T and the servo there is a non-return valve. I've temporarily disconnected the servo and turned the valve around to block it off

I'm using a 2m length of silicone vacuum hose which I would have thought was flexible enough to damp the readings

Starter motor has just fallen off too so can't do any more testing for a bit

Post by **brentp** » Tue Apr 19, 2011 5:39 pm

How does it it relate to general engine load? as in, how wide the throttle is open as you sweep through the RPM band?

spandit · Post by **spandit** » Tue Apr 19, 2011 9:02 pm

This was a static test. To tweak the throttle you have to stand directly in line with the fanbelt. Considering I've had problems with the nuts falling off, it's not a very comfortable feeling! Once I get it wired up properly I'll take it for a spin & see if anything differs

cng1 · Post by **cng1** » Tue Apr 19, 2011 10:18 pm

Your graph there looks entirely normal for a static test. As your revs climb the engine is trying to suck harder against the (almost completely) closed throttle. The harder it is sucking, the more it is throttled the lower the air pressure you get in the manifold.

Your load line is a little spikey. If you want to smooth it out get hold of a baby fuel filter (a 99 pence mini moto filter is fine) to use as a damping champer or alternatively push a MIG welding tip into the vacuum hose to act as a restrictor.

spandit · Post by **spandit** » Wed Apr 20, 2011 6:31 am

Thanks, Chris. I've got to spend today fabricating a new starter motor mount as the old one has perished (yes, for some unknown reason they hang a huge 24V starter motor off a rubber mount) but once I get it running again I'll try that

spandit · Post by **spandit** » Sun Apr 24, 2011 5:48 pm

OK, after fabricating and installing a new starter mount, I set about wiring this thing in a bit better. It's still not perfect and the HT leads are a bit tight, but she runs (and seeing as I installed a relay... stops again too!)

As suggested by Chris, I pushed a mig welding tip into the hose to act as a restrictor. At idle the readings are a little stabilised (I'm going to buy a fuel filter tomorrow to see if that helps) but with increased RPM, they're all over the place again. The flat patch in the middle of the graph is where I squeezed the hose shut for a few seconds.

I also connected a timing gun and ran the engine on EDIS alone. It said my timing was 15 degrees so I corrected that in the software. However, I noticed that the timing gun was saying about 40 degrees when the laptop was showing me 18-20... perhaps it's a crappy timing gun because the engine is starting better than it ever did in the past!

Hopefully I'll be able to take it for a drive later and see what's what.

cng1 · Post by **cng1** » Sun Apr 24, 2011 6:36 pm

Can you post a zoomed in plot over the course of say 15 seconds of data at higher RPM where it is a bit lumpy? I've an inkling that we're chasing ghosts here so even better would be a plot from a short sample from a real drive where I suspect things will look just fine.

spandit · Post by **spandit** » Sun Apr 24, 2011 7:07 pm

You might be right, Chris, let's hope so. Bit nervous about taking her for a drive as engine bits not easy to access once seats are back in (it's a forward control vehicle) but hopefully putting the distributor back in should be a relativly easy side-of-the-road job once you can see it

cng1 · Post by **cng1** » Sun Apr 24, 2011 7:24 pm

That graph is definitely a bit lumpier than I would like but that may just be a symptom of you running the carbs in an area that they wouldn't see in real use and thus delivering a mixture that is a somewhat off.

spandit · Post by **spandit** » Sun Apr 24, 2011 7:27 pm

I'll see if I can log some road data tomorrow as realised due to the bank holiday I won't be able to buy a fuel filter

spandit · Post by **spandit** » Sun Apr 24, 2011 9:50 pm

spandit wrote: I also connected a timing gun and ran the engine on EDIS alone. It said my timing was 15 degrees so I corrected that in the software. However, I noticed that the timing gun was saying about 40 degrees when the laptop was showing me 18-20... perhaps it's a crappy timing gun because the engine is starting better than it ever did in the past!

Just realised - wasted spark - probably doubling the timing readings on the gun... means my installation wasn't so far out!