Misfire and Mechanical Tach reads Different that Aim Tach

[85258]

I have a new to me RF94. It has an older Aim Mychron install. The RPM on the Aim Reads approx 3% to 4% lower than the mechanical tach. While it does not sound like much When the Mechanical Tach reads 6500 to Aim reads ~6250.

The lead for the Aim is installed on the coil to dist wire and some time the aim acts funny and the rpm justs at low speed.

What should I be looking for? At the last race I shifted at 6500 on the Mech tach as I did not want to over rev the engine.

Also as an aside, my number 4 cylinder is misfiring. The plug is brown/black and you can here the miss fire at speed. Not sure if it is related to the tach issue. I have changed the cap and wires but no difference. I am doing a leak down and compression test tonight.

[Daryl DeArman]

Disconnect the AIM system and see if the miss goes away...If it doesn't, it wasn't related.

As too the accuracy of the tach...I'd hook it up to a motor (non-variable speed drill for example) with a known rpm. Non-variable speed electrical motor speed is dictated bu their physical properties and the fact that our electricity in USA is 60Hz.

Perhaps your tach has a multiplier (gears) in the tach housing, perhaps in the drive adapter or maybe just (simply) the face calibrated to read twice the actual input rpm. Cam rpm is 50% crank rpm. With an 1740rpm drill you are looking for 3480 rpms on the tach. 3% high would be almost 100 rpms high.

[Roger Eade]

are good for giving you a idication of revs, and unless it has been overhauled and checked, in the not to distance past, it maybe right, more often its not, electronic tach also can be wrong, more often than not, they are correct, they are counting the field coil pulses, I have sent both types to a well known repair business here in Christchurch, they have told me more than once, Mechical tac's are a guide only some are a lot better than others and will be within 20 rpm, but often they are slow to react to a fast reving engine, you could check your RPM by useing a Snap ON timing light and compare readings.
Roger

[Dave Harmison]

This may sound like over-kill (and might be) but I calibrate my mechanical tach against a revoluiton counter and a stop watch. You chuck this little gizmo in your drill motor, time how long it takes to turn say 2000 revolutions (20 turns), divide and get your actual rpm.

I found this axample by searching on "revolution counter".

I don't use electronic for the mis-fire reason stated.

http://cgi.ebay.com/Starrett-RPM-Rev...QQcmdZViewItem

[Rick Kirchner]

Is the mechanical tach a "clockwork" tack like a smiths? If so, the needle moves in discrete jumps and like roger said, is only a rough estimate (although it gets better the higher the revs).

After my winter re-build my AIM tach and shift lights stopped working properly. The only thing changed was the condenser. I also had a misfire once at high RPM, spent four weekends chasing it down, and it was the condenser.

Change the condenser (try two or three of them - and buy them from different suppliers to make sure you don't get three parts from the same manugfacturer and lot, it's not like they are precision parts) and see what happens.

If the misfire is only in #4 and youve been thru the wires and cap, I'd change the plug and if that doesn't fix it I'd suspect a fuel problem.

Does the carb have dump tubes? Have the dump tubes been moved around? When was the last time the engine was on a dyno? That's the only way to set the dump tubes accurately.

[85258]

I ran one session with Aim not connected and the engine still was missing, so it is not that.

I believe the miss is more of a problem than I anticipated (or hoped)
When I was at the track, the #2 and #3plugs were nice tan colour, #1 was more brown and 4 was brown/black. So I switched #4 into #3 and I had the same result, the #4 cylinder turned the plug black and the #3 had a tan plug. So I have ruled out a bad plug. I did not check the plugs in the last 3 sessions. When I pulled them last night to do a compression test, #1 was brown/black, 2 and 3 were tan and 4 was brown/black (more then #3). See picutre

I ran two comprssion test, the result were consistent between the two tests as follows
#1 - 150psi
#2 - 175
#3 - 165
#4 - 170

So the miss might not be related to the tach problem as their are compression issues with the engine. I am going do a leak down tonight to see if I can pin point it.


What is the condenser?

What are dump tubes?

This engine is a used motor that I purchased thru an apex speed WTB post back in June. I was told it has 5 races on it. We Installed it and ran one weekend with it. It was missing and noticably down on power. I was 8mph slower on the front straight than with my first motor. I only bought this car in mid May and the first engine had a major failure of the #1 bearing in 25mins of track time. The bearing had disintegrated completely and buggered the crank and filing the engine with filings, wrecking the cam too.

Isn't racing a great hobby!!!

[gord leach]

Isn't racing a great hobby!!!

myself i go to work to relax.

get it figured out Jarred, i'm tired of beating all the guy's i normally race with and need somebody new to play with

[rickb99]

What kind of ignition system are you using? #4 looks pretty fouled but dry. Are you running NKG8's or 9's? Have you tried just putting a new set of plugs in? I'd wonder about carburator jets. But I don't know enough to know if I should wonder about that.

[85258]

I am using the 8s. Not sure what I should use. Most guys up here are running stock champion plugs because NGK has not sold these plugs in Canada for 15 years or so. This engine came from Texas wtih the 8s and the original motor was from Phoenix and it had the 9s.

I cleaned up fouled #4 with brake clean and stuck it into #3 and it came out tan. The plug that came from #3 and into #4 fouled. So it is not the plugs.

What impact would the carb have if it is fouling #1 and #4, both on the longer intake runners?


Also, there is lower compression in hole #1.

[Rick Kirchner]

Dump tubes are small copper tubes that richen and even out the mixture at WOT. Look down the carb. If there are a couple of tubes sticking into the airflow you have dump tubes. The engine builder sets the position of the tubes on the dyno using EGT.

The condenser is actually a capacitor (condenser being the archaic term that was never stricken from the automotive vocabulary) that keeps the arching across the points to a minimum so that the coil actually sees a full pulse across the low voltage side.

AP9FS and AP8FS are available at incredibly competitive prices from WWW.sparkplugs.com

You need to run those compression and leak down tests hot. The compression test is so dependent on cranking speed and other variables that it's only a gross look.

Just for grins and to be complete I'd check the valve lash, especially on #4.

Turn the motor over so that you can look at the backsides of the valves and top of the piston in #4. All cruddy? If so you might have a valve stem seal leaking, but from the pic I'd expect more fuel than oil. So, at speed you are either delivering too much fuel, not enough air, or not lighting the mixture. But if it's a miss and relegated only to #4 it's pretty hard for it to not be electrical in nature.

[rickb99]

pretty much same story

It does look spark related though. Two things to try.
1.
Try switching the cylinders 4 and 3 plug wires around. BOTH at the coil and on the plugs. See if the problem moves to #3. i.e. bad plug wire.
2.
If you have a buddy around, try changing distributor caps. A VERY slight almost invisible crack in the cap can cause a high RPM miss on one cylinder.

Reason I brought up the carburator and plug heat range is (unless it's the lighting). Plugs 2,3 look pretty good. But 1,4 look a bit sooty.

If this engine was professionally built, have you tried contacting the builder?

[85258]

Guys, the help is great! I am heading to the garage for a leak down test and to check the valve lash.


It is not the cap and wires as I change both out and had the same results.

The engine is a Farley. I have not contacted them yet. I am not running the carb that came with this engine. I was not able to remove the air filter from the carb as one of the studs spins when I try to take the nut off and the bottom of the stud is inside the filter. I could not install my intake tube and body work with the air cleaner in the carb (My air cleaner is mount by my helmet). So I am running the carb that came with my car (that engine was an Ivey).

Can the carb run rich but only affect #1 and #4?

[rickb99]

I don't know. I do know Farley and Ivey spend a LOT of time (say your money) on the dyno tuning the carburation to match their engine. You could also get in to different porting between the two and other things.

But there are many (as in most) people here who know way more then I do. Also, are you sure it's always the number 4 that misfires? Could be 1,3,4 at any given moment. If you look at 1,3,4, you could almost say Hmmm.. Might it be the coil or coil wire?

I'd say it might be wise to talk to Farley at this point. At least they could tell you how to get the air filter off the carb. Brut force may be in order to get that matched carb back on before you chase the snake much farther.

BTW, you didn't mention the ignition setup. Standard, Petronix, MSD, etc.

Oh and, might the misfire cause the AIM dash to read lower RPM??

[Rick Kirchner]

I'm going to go out on a limb here and make a prediction.

1) miss is due to bad condenser
2) plugs are not due to misfire, but mis-matched carb to engine.

[85258]

This is the Intake lobe (rear valvle) on #1 as taken from the passenger side (right side)

I got 80 mins of time on this motor before it was screwed. Maybe I will make 100 mins on the next one...

So what now?

Can I just take the head off or should the whole engine could out for a inspection/cleaning?


I never bothered to measure the valve lash as it was late.

[Charles Warner]

Call the engine builder and discuss the issue. He should be willing to at least examine the engine to see if there is any other damage and you should be able to come to an agreement on the cost to fix, depending on the failure mode.

[Roger Eade]

Looks like it is blocked, cam lobe looks like it has not been getting oil.
Roger

[85258]

Took the car to a local engine builder.

He looked at and thought the same as Roger, no oil on the lobe. The valve was set way too tight, less than .003.

So we are removing the head for inspecting and installing a new cam. And hopefully good to go!

Still no resolution on the missing and rpm difference with the micron.

[Steve Demeter]

Make sure to check / replace the srpay bar to insure that all the holes are open.

Make sure that whatever you put on has no gunk inside it that could get loose and plug the holes.